modern isotope ratio mass spectrometry

I. T. PLATZNER

VOLl'ME 145 IN CHEMICAL ANALYSIS \ i*ries of Monograph* an Aiuh l i a ! I ntaistiy and its Appiwatîonw

Vor» Eililm

3 ADVANCED ISOTOPE BATIÓ MASS SPliCTIlOMETRY I MAGNETIC ISOTOPE RATIO MASS SPECTROMETERS

Il ÇUADRUPOLE IS

B PRECISION AND ACCURACY IN ISOTOPE RATIO

PARTI INSTRUMENTATION

Chapters 1 and 2 will be devoted to a brief discussion on the history of mass spectrometry, oriented particularly toward the development of isotope abundance determinations. The reader will be taken for a rapid journey through the milestones of the pioneering works which laid down the foundations to one of the more precise and accurate methods of physical measurement. In terms of these criteria, it may be safely claimed that isotope ratio and isotopic mass determinations are second only to laser wavelength quantification.

The next three chapters will address in detail the latest achievements in instrumentation for modern isotope ratio mass spectrometry. They were contributed by leading scientists, describing the state of the art of 'classical' magnetic sector, inductively coupled plasma and quadrupole IRMS.

Chapter 6 deals with special purpose instruments, by which ratio determina-tions are accomplished on particularly difficult samples using a choice of sophisticated techniques and, in particular, specially custom-tailored large mass spectrometers.

CHAPTER 1

HISTORICAL ISOTOPE RATIO MASS SPECTROMETERS

The highly interesting and important scientific discipline of isotope ratio mass spectrometry emerged when Sir J.J. Thomson used his positive ray parabola mass spectrographs [1-3] to discover that neon is a mixture of two isotopes, 20Ne and 22Ne, rather than a single species. The existence of a positively charged stream of particles in a gas subject to an electrical discharge was demonstrated earlier, in 1886, by Goldstein [4], and soon thereafter Wien [5] showed that this stream is deflected in a magnetic field. These observations provided the physical basis for the parabola mass spectrograph. Thomson made enormous contributions also in other areas of mass spectroscopy. He replaced the photodetection plates with an electrical detection system, thereby inventing the first mass spectrometer, studied positive as well as negative ions, observed multiply charged ions and metastable transitions, and suggested the existence of ion-molecule reactions. Thomson won the 1906 Nobel Prize and is considered as the father of mass spectroscopy.

Thomson's pioneering work was continued by Aston, who set out to confirm or disprove the existence of neon isotopes. Aston improved Thomson's instru-ment and named it the 'mass spectrograph'. He confirmed the earlier work on neon isotopes, and also discovered the third neon isotope 21Ne [6]. Aston devoted his life to building ever more improved and precise mass spectrographs and discovered 212 of the 287 naturally occurring isotopes. Aston also measured the masses of these isotopes with a precision of 0.1 %, determined their abundances and calculated the atomic weights of the elements. By his mass spectrographic studies, Aston observed that the isotopes do not have integral masses, but rather they are characterized by a mass defect [7]. This mass defect, later designated by Aston as the packing fraction [8], is related to the binding energy of the nucleón; the lower the packing fraction, the greater the binding energy. For his outstanding achievements, Aston was awarded the Nobel Prize in chemistry in 1922.

In 1918 Dempster [9] published details of his 180° magnetic sector mass spectrometer with a permanent magnetic field. The samples were ionized by either electron impact or thermal ionization, the sample being heated directly on a platinum ribbon. The accelerated ions were deflected to a fixed electrometer

4 HISTORICAL ISOTOPE RATIO MASS SPECTROMETERS

detector. The ion trajectories in a magnetic field are described by the classical equation

m/z = B2R2/2V (1)

where B is the magnetic field intensity, R is the radius of curvature of the ions in the magnetic field, and V is the accelerating potential.

At fixed B and R, m/z = c(\/V), therefore a mass scan could be achieved by varying the acceleration energy, and ions of a given energy arrived at the detector at a given time. Dempster used three slits along the ion trajectory: the ion source and the detector slits, both adjustable, and a fixed slit at the middle of the deflection path. An ion beam emerging from the source and passing through the slit diverges upon entering the magnetic analyzer. However, the magnetic field has the property of refocusing the beam at the focal plane in which the detector slit is located. This property is termed direction focusing, and will be illustrated in the next paragraphs. The fixed slit served to reduce the number of stray ions and electrons reaching the detector. Compared with Aston's mass spectrograph, the Dempster mass spectrometer performed better in abundance measurements but could not be used for precise mass determinations.

Before the second generation of isotope ratio mass spectrometers made their appearance in the early years of World War II, contributing to the extraordinary breakthrough in isotope ratio mass spectrometry, several very important works had been published in the mid 1930's. All of them related to the fundamental properties of ion motion in non-linear magnetic and electrical fields and the consequent construction of the double focusing mass spectrograph. Although these instruments did not have immediate applications in isotopic ratio measure-ments, but rather contributed to isotopic mass determinations, detection of rare isotopes and low level impurity analysis in solids, in modern isotope ratio mass spectrometry they comprise the core of highly sophisticated IRMS systems.

Herzog [10] solved the equations of the direction focusing properties of magnetic and electrostatic sectors. Taking the important case of the symmetrical 60° magnetic sector analyzer with the entering and exiting ion beams at an angle of 90° and with the entrance slit located at a distance of 1.7321/?m from the entrance of the magnet, the separated ion beam of a particular m/z value will converge at the same focal distance of 1.7321/?m from the magnet exit face, where Rm is the magnet radius. In principle, the solutions for 90° and 180° magnetic sectors analyzers also predict equal focal lengths from both sides of the magnet, but these will be progressively shorter for 60°, 90° and 180° sectors.

A radial electrostatic analyzer also reveals focusing properties on charged particle beams. For the case of a symmetrical 31°50' analyzer, the focal lengths are 1.707Äe from either side of its edges, where Re is the radius of the electrostatic analyzer. Ion trajectories in this analyzer are described by

Re = 2V/E (2)

HISTORICAL ISOTOPE RATIO MASS SPECTROMETERS 5

O = source M = magnet C = collector S, = entrance slit S, = exit slit

Figure 1.1. Focusing of a divergent ion beam by a 60° magnetic sector analyzer. (Reproduced by permission of Prentice-Hall, New York, from R.W. Kiser, Introduction to Mass Spectrometry and its Applications, 1965, p. 49)

Figure 1.2. Focusing of a divergent ion beam by an electrostatic analyzer. (Reproduced by permission of Prentice-Hall, New York, from R.W. Kiser, Introduction to Mass Spectrometry and its Applications, 1965, p. 51)

6 HISTORICAL ISOTOPE RATIO MASS SPECTROMETERS

where V is the ion accelerating potential preceding the analyzer and E is the electrostatic field. It is immediately evident that the electrostatic analyzer is not a mass analyzer but rather a velocity analyzer. Schematic diagrams of a 60° magnetic sector analyzer and a 31°50' electrostatic analyzer are given in Figures 1.1 and 1.2 respectively.

The coupling of an electrostatic and a magnetic analyzer in such a way that the exit focal point of the first coincides exactly with the entrance focal point of the second confers on the system its double focusing characteristics. In simple terms, the electrostatic analyzer (ESA) focuses all the ions with equal kinetic energy but different m/z values at the entrance focal point of the magnetic analyzer (MA), which then separates the ion beam according to its m/z values. In the second analyzer an energy homogeneous ion beam is analyzed, therefore the whole analyzer system demonstrates high mass resolving power. The double focusing analyzer has at least three slits, one at the entrance focal point of the ESA, the second at the mutual foci of the ESA and the MA, and the third at the focal point on the exit side of the MA. Additional slits are used to prevent interference from stray charged particles.

The first double focusing mass spectrograph was built in 1935 by Dempster [11]. A 90° ESA and a 180° MA were used, achieving an approximate resolving power (RP) of 7000. This instrument was soon followed by the Bainbridge-Jordan [12] and the Mattauch-Herzog [10, 13, 14] double focusing mass spectrographs. The first had a 127°17' ESU, a 60° MA, and an approximate. RP of 7000, and the second had a 31°50' ESU, a 90° MA and an approximate RP of 3000.

An excellent review, covering in detail the early period of mass spectrometry, was presented by Beynon and Morgan [15], The Development of Mass Spectroscopy: An Historical Account.

CHAPTER

2

SECOND GENERATION ISOTOPE RATIO MASS SPECTROMETERS

The first magnetic sector mass spectrometer dedicated to isotope ratio deter-minations in gases was a 60° magnetic analyzer instrument designed by Nier [16]. This mass spectrometer was a predecessor of many diverse instruments, dedicated to thermal ionization, laser source and other ionization technique IRMS, organic mass spectrometers, instruments used to determine physical parameters or for high temperature studies of solids, and others which were commercially built around this or the 90° analyzer. The earlier mass spectrometers used permanent magnets and accelerating potential mass scanning. Later, constant potential acceleration and magnetic mass scanning were introduced. This option reduced the mass discrimination effect introduced by the electron multiplier detector. The precision of isotopic ratio determina-tions was significantly improved by Nier [17, 18], who incorporated the dual collector system for simultaneous measurement of the ion currents of two isotopes. Further progress was made by McKinney et al. [19] and by Wanless and Thode [20], who introduced the dual gas inlet system for the alternate admission of sample and standard into the mass spectrometer. Reynolds [21] developed the static isotope ratio mass spectrometer for analyses of very small gas samples. Wright et al. [22] and Carr et al. [23] discussed dynamic versus static mass spectrometry, concluding that the latter technique is about three orders of magnitude more sensitive because the static mass spectrometer acts as its own sample reservoir, whereas in a dynamic mass spectrometer the major part of the sample is lost via the pumps without contributing to the measurement.

Holmes and Morrell [24] were the first to utilize the separation power of a chromatographic column by direct coupling between a gas Chromatograph and a mass spectrometer. Sweeley et al. [25] applied this important development for isotopic ratio measurements. Sano et al. [26] introduced the sample combustion step into the IRM-GC-MS technique, demonstrating the determination of 13C in labeled drug metabolites. Soon, work by Matthews and Hayes [27] followed, applying the technique for N and C ratio determinations in amino acids. Their publication contains a good review on the development of the method, in-cluding the insertion of a combustion furnace for converting the sample to C02, N2 and H 2 0 and, in some cases, the conversion of H 2 0 to H2. A more detailed discussion on the IRM-GC-MS technique is given in Chapter 9, Section 7.1.2.

8 SECOND GENERATION ISOTOPE RATIO MASS SPECTROMETERS

The single focusing magnetic mass spectrometer, mainly with 90° and 60° sectors and with normal incident ion beam entrance and exit angles has been the instrument most frequently used for IRMS. This instrumental concept was manufactured up to the late 1970s. Over the years much progress has been made on virtually all the peripheral components around the analyzer magnet. Electronic design was revolutionized by the change from vacuum tubes to solid state transistors, improving the stability of electronic systems; new ion detection components were invented and the ranges of existing measuring devices were extended. The use of microprocessors and partial computer control has been initiated together with automatic data acquisition systems. Improve-ments in vacuum system components, ion source designs, and ion optics have resulted in very reliable instruments. The annoying times referred to by F. W. Aston in 1941 in his book Mass Spectra and Isotopes [8a] have fortunately gone for ever:

'The mass spectrometer behaves at times in the most capricious and unaccountable manner.... When by good fortune all is well the arrangement is capable of good performance. Thus after a favorable setting of the apparatus, six elements were successfully analyzed in as many working days. On the other hand, after dismantling became imperative and it had to be cleaned and rebuilt, exactly as before as far as one could tell, no results of any value were obtained during weeks of work.'

At the beginning of the 1980s, newly designed mass spectrometers were launched by the two major manufacturers: Vacuum Generators in England and Finnigan MAT in Germany. Briefly, these instruments were developed for fully automatic analyses of large numbers of gaseous or solid samples, with com-puter operated data reduction systems able to function without operator intervention. New magnet shapes, doubling the ion beam dispersion relative to the same size conventional magnets, and adjustable multiple collector Faraday cup detection systems for simultaneous measurement of up to nine ion beams were introduced. Advanced pumping systems for the ion source compartment and ion flight tube were also installed. In consequence, the precision and accuracy of isotope ratio measurements were significantly improved and measurement times were markedly shortened.

It would be unforgivable, before ending this, chapter, not to recall once more the late Professor A.O. Nier, a leading scientist, who had the greatest impact on the development of isotope ratio mass spectrometry besides his many other contributions in this field. He influenced this scientific discipline for over half a century from the late 1930s. The 60° sector mass spectrometer already mentioned, followed by the first absolute isotopic abundance determination of an element (argon), the discovery of four new isotopes, the measurement of isotopic ratios in uranium, lead, strontium and many other elements, and always performing highly accurate isotopic ratio measurements were only a few of his

REFERENCES FOR CHAPTERS 1 AND 2 9

direct contributions. Laying down the basis of Ar/K, U/Pb and Pb/Pb geochronology, developing methods for carbon isotope enrichment, the first separation of minute quantities of uranium isotopes, development of the portable helium leak detector, development of a continuous gas impurity monitoring system comprising more than 100 mass spectrometers (so responding to the mass spectrometry challenges of the Manhattan Project), building miniature mass spectrometers for space research and large high resolution double focusing instruments for atomic mass determinations, and solving the chemists', elemental oxygen, vs. the physicists', 1 60 based, atomic weight scale dispute by proposing 12C as the common base for the two scales; these are all examples of his diverse scientific activities firmly related to his original strongly instrumental orientation. Several of these activities will be described in more detail throughout this book. As a point of curiosity, Professor A.O. Nier was probably the greatest contribution of the American Great Depression to the physical sciences. We shall never know what electrical engineering lost by being unable to offer him a position when he graduated in 1931. Professor Nier's retrospective views, regarding his activities during those years have been recorded in a personal interview given in 1992 [28]. He published another article, Some Reminiscences of Mass Spectrometry and the Manhattan Project, describing the World War II years, several years ago [29]. Recently, J.R. De Laeter, in a lecture entitled 'Dedication to Alfred O.C. Nier', paid a tribute to the 'Father of modern mass spectrometry' [30].

Excellent reviews describing the very early days of mass spectrometry, and also covering the second epoch of IRMS, have been presented by Svec [31]: Mass spectroscopy—ways and means, a historical prospectus, and by Matsuo [32]: High performance sector mass spectrometers: past and present.

REFERENCES FOR CHAPTERS 1 AND 2

[1] J.J. Thomson, Philos. Mag., 20, 752 (1910); 21, 225 (1911). [2] J.J. Thomson, Philos. Mag., 24, 209 (1912). [3] J.J. Thomson, Rays of Positive Electricity and Their Application to Chemical

Analyses, Longman, Green and Co., London, 1913. [4] E. Goldstein, Bed. Ber., 39, 691 (1886). [5] W. Wien, Wied. Ann., 65, 440 (1898); Ann. Phys., 8, 244 (1902). [6] F.W. Aston, Philos. Mag., 38, 707, 709 (1919); 39, 449 (1920). [7] F.W. Aston, Philos. Mag., 45, 934 (1923). [8] (a) F.W. Aston, Mass Spectra and Isotopes, 2nd edn., Edward Arnold and Co.,

London, 1942; (b) Proc. R. Soc. (London), A216, 511 (1930). [9] A.J. Dempster, Phys. Rev., 11, 316 (1918).

[10] R. Herzog, Z. Phys., 89, 447 (1934). [11] A.J. Dempster, Proc. Am. Philos. Soc, 75, 755 (1935). [12] K.T. Bainbridge and E.B. Jordan, Chem. Rev., 50, 282 (1936). [13] J. Mattauch and R. Herzog, Z. Phys., 89, 786 (1934).

10 SECOND GENERATION ISOTOPE RATIO MASS SPECTROMETERS

[14] J. Mattauch, Phys. Rev., 50, 617, 1089 (1936). [15] J.H. Beynon and R.P. Morgan, Int. J. Mass Spectrom. Ion Phys., 27, 1 (1978). [16] A.O. Nier, Rev. Sei. Instrum., 11, 212 (1940). [17] A.O. Nier, E.P. Ney and M.G. Inghram, Rev. Sei. Instrum., 18, 294 (1947). [18] A.O. Nier, Rev. Sei. Instrum., 18, 398 (1947). [19] CR. McKinney, J.M. McCrea, S. Epstein, H.A. Allen and H C Urey, Rev. Sei.

Instrum., 21, 724 (1950). [20] R.K. Wanless and H.G. Thode, J. Sei. Instrum., 30, 395 (1953). [21] J.H. Reynolds, Rev. Sei. Instrum., 27, 928 (1956). [22] LP. Wright, N.J. McNaughton, A.E. Fallick, L.R. Gardiner and CT. Pillinger, J.

Phys. E: Sei. Instrum., 16, 497 (1983). [23] R.H. Carr, LP. Wright, A.W. Joines and CT. Pillinger, J. Phys. E: Sei. Instrum., 19,

798 (1986). [24] J.C Holmes and RA. Morrell, Appl. Spectrosc, 11, 86 (1957). [25] C.C. Sweeley, W.H.Elliott, I.Fries and R.Ryhage, Anal. Chem.,3$, 1549(1966). [26] M. Sano, Y. Yotsui, H. Abe and S. Sasaki, Biomed. Mass Spectrom., 3, 1 (1976). [27] D.E. Matthews and J.M. Hayes, Anal. Chem., 50, 1465 (1978). [28] M.A. Grayson, J. Am. Soc. Mass Spectrom., 3, 685 (1992). [29] A.O. Nier, J. Chem. Educ, 66, 385 (1989). [30] J.R. De Laeter, Int. J. Mass Spectrom. Ion Processes, 146/147, xvii (1995). [31] H.J. Svec, Int. J. Mass Spectrom. Ion Processes, 66, 3 (1985). [32] T. Matsuo, Mass Spectrom. Rev., 8, 203 (1989).

CHAPTER

ADVANCED ISOTOPE RATIO MASS SPECTROMETRY I: MAGNETIC ISOTOPE RATIO MASS SPECTROMETERS

K. HABFAST Bremen, Germany

3.1 INTRODUCTION 11 3.2 ION OPTICS 14

3.2.1 Magnetic Sector Optics 14 3.2.2 Special Devices 21

3.3 ION SOURCES 22 3.3.1 Electron Impact Sources 22 3.3.2 Thermal Ionization Sources 26 3.3.3 Other Sources 32

3.4 ION COLLECTORS 33 3.4.1 Multiple Faraday Collectors 34 3.4.2 Secondary Electron Multipliers 40

3.5 SAMPLE INLET SYSTEMS 46 3.5.1 Viscous Flow Inlet Systems 47 3.5.2 Continuous Flow Inlet Systems 50

3.6 SAMPLE PREPARATION DEVICES 52 3.6.1 Bulk Sample Isotope Analysis 55 3.6.2 Compound Specific Isotope Analysis 57 3.6.3 Isotope Ratios of Light Gas Mixtures 61 3.6.4 Sample Preparation of Water 62 3.6.5 Sample Preparation of Carbonates 63 3.6.6 Sample Preparation by Laser Ablation 64

3.7 COMMERCIAL INSTRUMENTATION 64 3.7.1 Typical Instrument Configurations 64 3.7.2 Basic Data Evaluation 69 3.7.3 Specifications 71

REFERENCES 77

3.1 INTRODUCTION

The vast majority of applications in the field of isotope ratio mass spectrometry is found in the measurement and interpretation of natural variations of isotope ratios in geological and biological systems. This means that an isotope ratio mass spectrometer must be a system capable of measuring isotope ratios of a

Modern Isotope Ratio Mass Spectrometry Edited by I. T. Platzner © 1997 John Wiley & Sons Ltd

12 ADVANCED ISOTOPE RATIO MASS SPECTROMETRY I

large variety of samples over a wide range of values with the highest possible precision and accuracy. Relative measuring errors in the range 20-50 ppm (parts per million) are required (and routinely achieved) or, for instance: ^ C / 1 2 C « 0.011 200 ±0.000 000 5. Only a minor, albeit important, number of applications deals with the determination of artificially produced isotope ratios, such as isotope dilution and tracer methods or energy related nuclear research problems.

The requirement for such high precision has resulted in a separate branch of mass spectrometer systems and measuring methods that is quite distinct from all other mass spectrometer design principles; for example, quadrupole or time-of-flight mass spectrometers. A special isotope ratio nomenclature has also been developed.

In fact, isotope ratio mass spectrometers and the associated sample preparation and measuring methods are so highly specialized and so demanding that special training and much experience is required from everybody working successfully in this fascinating field.

It is well known that a wrong value may be measured with high precision and that such a precise result may be mistaken as also being highly accurate. Systematic deviations from true values always originate from improper sample and data handling or from inappropriate operation of the mass spectrometer. This is why users of isotope ratio mass spectrometers should have some basic understanding of the design principles of an isotope ratio measuring system as a whole: for a critical assessment of the results, the system must be transparent to the user.

It is the intention of this chapter on isotope ratio mass spectrometer hardware to help the day-to-day user understand the complex machinery and to become knowledgeable and critical with the goal of achieving correct results.

Although most applications of isotope ratio measurements are developed in an academic, mainly research oriented environment, and still relatively few in commercial organizations, the development, production and selling of the hardware are, to a very high degree, performed by a small number of speciali-zed commercial companies. Demanding competition in this transparent market (where 'everybody knows everybody' for each specialized application area) has raised the state of the art and the reliability of isotope ratio mass spectrometers and associated equipment from all manufacturers to a very high level while system prices have been reduced drastically. The virtually total commercializa-tion in the field of hardware, however, is also the reason why the state of the art in instrumentation is no longer represented by traditional scientific literature. Instead, the state of the art can be found only in the manufacturers' published documentation which, unfortunately, is a mixture of facts and sales promotion in many cases.

Basic design principles of the mass spectrometer itself have not seen major changes during the past decade, whereas sample preparation systems and

INTRODUCTION 13

automation have experienced considerable improvements and innovations in the same period.

For most users, inexperienced in the design of a mass spectrometer, it is extremely difficult to understand the differences between the systems of the (currently three) major manufacturers. In addition, manufacturers are highly successful in hiding such (real) differences by producing considerable com-mercial 'noise'. While such a situation is by no means unusual nowadays, the author nevertheless hopes that this chapter may help a new user to make the right decision in purchasing an instrument for his specific needs.

The chapter starts with a description of the usual ion optical systems: single focusing, low resolution magnetic sector optics applying so-called stigmatic focusing are used almost exclusively. Such ion optics require ion beams of low energy spread. In isotope mass spectrometry they are produced in two types of ion source, which are described in Section 3. For gas samples, electron impact type ion sources are chosen, whereas for solid samples thermal ionization on hot surfaces is applied.

A major reason for the achievement of highly precise results is the simultaneous collection of all relevant ion beams in a multiple Faraday collector system (without scanning the ion optics). Such collector systems are almost exclusively applied for high precision isotopic measurements.

In order to achieve not only precise, but also accurate (and reproducible), results, it is necessary to calibrate the mass spectrometer each time by measuring a standard sample together with or shortly before or after the sample in question under virtually identical conditions. For radiogenic isotope systems in geochronology and geochemistry, which mostly exist as inorganic, solid samples and which are handled by thermal ionization, it is preferred to calculate the 'absolute' isotope ratio in relation to a known (or agreed) internal standard ratio of the same element. This guarantees identical conditions for sample and standard and is therefore straightforward and relatively simple.

The majority of isotopic samples, however, cannot be measured by thermal ionization, nor does there exist an internal standard. The important isotopes in life sciences, traditionally called 'stable isotopes' (13C/12C, f5N/14N, 1 8 0 / 1 6 0 , 2H/1H, 34S/32S), occur in a broad variety of chemical compounds in gaseous, liquid or solid form. For a sample/standard comparison, it would apparently be vain to search for an appropriate sample introduction and ionization method that could handle all different samples with similar quality and/or to provide a suitable external standard for each sample that behaves identically to the sample during introduction and ionization. The way out of this dilemma is straight-forward but by no means simple.

Each sample is converted into an appropriate elementary gas (C02, N2, 0 2 , H2, S02) before measurement in the mass spectrometer. Once in gaseous form, it is easy to compare such a chemically modified 'sample' in a sufficiently fast sequence with a corresponding external standard gas, using the same


introduction (and ionization) method for all samples, i.e., a suitable dual inlet system. These systems are described in Section 5.

This unavoidable methodological detour has generated a broad selection of sample preparation systems for each kind of sample, the only purpose of which appears to be to convert a sample into the appropriate light gas without changing the isotope ratio. Such systems are described in Section 6.

Last but not least, the properties and qualities of an isotope ratio mass spectrometer can be measured and described as 'specifications'. Therefore, Section 7 of this chapter contains a brief definition of the relevant instrument specifications and a description of how to measure them.

3.2 ION OPTICS

An ion optical system for a high precision isotope ratio mass spectro-meter should primarily fulfill two requirements. First, its lateral mass dis-persion must be at least so large that three or more separate Faraday type ion collectors for the proper detection of ion beams with a minimum mass difference of 1 dalton can be placed in the focal plane of the system. Second, its optical transmission should be high (i.e. near 100%), stable and independent of mass.

Other requirements which are known to be important in other areas of mass spectrometry (such as high resolving power, high mass range, energy focusing, scanning speed and image errors) are of minor importance.

A magnetic sector type ion optical system [1-6] is the perfect and inexpensive solution for such requirements.

3.2.1 Magnetic Sector Optics

The principles of a magnet sector instrument are as follows. If a charged particle of mass m (Da) and charge n x e(n = 1 .. .A/)(C) is accelerated by passing a potential difference of V (V), i.e. by passing an electric field, it assumes the velocity v (a vector!) of size

v = 1.39 x 1 0 6 W — (cm/s) (1)

along the direction of the electric field. If such a particle enters a magnetic field H of size H (T) with its speed vector v perpendicular to H, the so-called Lorentzian force

ÎC = vxH (2)

is induced. The direction of this force is perpendicular to v and H.

ION OPTICS 15

Hence, as long as H is constant in space, the particle is deflected into a circular path, Lorentzian force and centrifugal force being equal. The radius rm of the path is given by

rm = 0.01436 x ~ ^ (cm) (3)

A singly charged l 2 C0 2 (44 Da) ion, accelerated by 3 kV, will therefore assume a speed of « 1.15 x 107 cm/s (or 414000 km/h) and it will be deflected to a circular path of 12 cm by a magnetic field of 0.3367 T.

Based on such simple facts, a magnetic field shaped like a prism (i.e. a magnetic sector) has three important features [6],

First, it acts like an optical lens for ions. Radially diverging ions in front of the prism are refocused after having left the prism (Figure 3.1(a)). The prism has a radial image distance (exit slit) /^ for a given object distance (entrance slit) C

Second, an image (b") of the entrance slit (b') for the ions is produced in the focal plane of the system. Like any other optical system, a magnetic sector field has a lateral enlargement factor Gm = b"/b' (Figure 3.1(a)).

(a)

(b)

(c)

focal plane

Figure 3.1. (a) Radial focusing and radial enlargement of a magnetic sector (oblique beam entrance); (b) mass dispersion and focal plane of a magnetic sector (normal beam entrance); (c) axial focusing of a magnetic sector with normal (e'm = e'¿ = 0) beam entrance and exit


Third, owing to the mass dependence of the radius of movement, the magnetic sector focuses ions of different mass to different locations on the focal plane of the system (Figure 3.1(b)). This lateral separation of two beams is called mass dispersion D = \/2Am/mKm, where is Km is the dispersion coefficient and Am/m is the relative mass difference of the ions of two adjacent beams which are separated. Both Gm and Km are dependent solely on the geometry (deflection angle (f>, radius rm, entrance and exit angle e'm, e^ and slit distance /¿) of the system.

As an example, for a symmetric system (l'm — l^ = lm) and for normal beam entrance (e'm = 0), Km and Gm are calculated as Gm = — 1, Km = 2rm (i.e. independent of 4>m), and

/m = r m ! + £ ^ (4)

e.g. l'm = rm for cpm = 90°. The overall ion separating power of an ion optical system is called the

'resolution' or 'resolving power' of the system. It is measured as follows (Figure 3.2).

If an ion beam of width b (in the image plane) is scanned across an exit slit of width i A and measured in a Faraday cup, a trapezoidal 'peak' is produced which has the width b + 5A- For real life beams, the corners of the peak are more or less rounded. The less so, the better. In the case of b < 5A, this peak has a flat top region of width 5A — b, and for b > 5A, the flat top region of the peak will be b — 5A- For b = 5A, no flat top region is observed. For high precision isotope measurements, 5A is always chosen to be at least 2b. This guarantees a stable signal also if the magnetic field or the acceleration voltage is subject to (small) fluctuations. A second ion beam of one mass unit (1 Da) lower mass must be spatially separated from the first ion beam by a distance of at least b + 5A in order not to overlap with the first peak. This condition defines the resolving power.

l~D=K^!-i i m

~ i b r ~it>r (®1 f®l ~~*~

— I I I ' Beam Profile ksAH

* 1 Exit Slit

bsd>b u-S.+b- i

Registered Peaks

Figure 3.2. Registered peaks and mass resolution

ION OPTICS 17

As the distance of two adjacent peaks in a magnetic prism depends on the relative mass difference Am/m of the peaks, the condition of a full separation of two peaks is fulfilled only up to a certain mass «JR.

mR/Am = «(for Am = I) (5)

and R is therefore called the resolving power. Typical values for R range from 80 to 200 for stable isotope instruments and up to « 500 for thermal ionization machines. As the condition 'full separation' is not well defined numerically, real life resolutions are defined for two adjacent beams of equal height which produce a valley of 10% between their corresponding peaks.

A particle beam always has a divergence (i.e. an opening angle) in two directions. Hence it would be desirable to have an optical system that also focuses in two directions. As can be seen from Figure 3.1(c), a radially focusing sector system with normal beam entrance has no axial focusing power for particles traveling out of the middle plane of the system. There is no magnetic force that could deflect them back to the middle plane. By a simple trick, however, focusing in the axial direction can be achieved without sacrificing radial focusing [7].

The entrance angle of the beam is made oblique (e'm ^ 0) relative to the front plane of the pole pieces (Figure 3.3(a)). As shown in Figure 3.3(b), out-of-plane particles now move parallel to the poles within the magnetic field and are refocused to the middle plane behind the magnetic prism. This useful feature stems from the fact that the magnet possesses a magnetic fringing field which extends over the physical borders of the pole pieces. The field lines of this magnetic stray field are rounded (Figure 3.4) and the field vector H therefore has a (small) component Hx for all locations in space which are out of the middle plane of the system. This component Hx of H is perpendicular to the main field Hz and to the front plane of the pole shoes, and it is parallel to the

26.5 26.5

< 90

a

Z I I (b)

Figure 3.3. (a) Radial and (b) axial focusing of a magnetic sector with oblique (e'm = e¡í, = 26.5°) beam entrance and exit


"4— (HF)(V)

H.J.V /

& / y

/ H m K K TK

I

H„±V

Figure 3.4. Magnetic forces for radial and axial focusing in a magnetic sector

middle plane of the system. Depending on the location above or below the middle plane, Hx points towards or away from the front of the poles.

On the other hand, the speed vector v of an ion diverging off the middle plane, as related to the magnet's coordinate system, has a component vy (due to the oblique entrance) in addition to its main component vx and to its component Pj (due to the divergence). As exemplified in Figure 3.4, vx is perpendicular to Hz and thus induces the force Ky which is responsible for the radial deflection.

Furthermore, vy _is perpendicular to Hx. For particles flying above the middle plane, the force Kz is induced which points in the same direction as the homogeneous magnetic field, i.e. it is antiparallel to vz. This force is the origin of the axial deflection of the beam. This deflection is repeated during passage of the stray field at the exit of the magnet. For particles flying on the other side of the middle plane, the Hx component of the stray field points away from the pole pieces. Hence, the corresponding force Kz again deflects the beam towards the middle axis.

Apparently, the axial focusing power strongly depends on the entrance and exit angles of the beam, i.e on vy. In fact, for an entrance and exit angle of 26.5°, the particle's speed component vz is annihilated by Kz at the entrance and adversely added again at the exit. Hence, in between the pole pieces, the ion moves parallel to the middle plane and the axial focal distances become equal and are also equal to the radial focal distances. Apparently, this is stigmatic focusing and results in the maximum possible ion optical transmission.

An additional side effect results from the fact that the dispersion coefficient of such a (symmetric) stigmatic focusing magnetic prism is doubled to ^m = 4rm. This means that the system will show the same mass resolving

ION OPTICS 19

power as a normal entrance system with double the radius. This feature significantly reduces both the size and the cost of the magnet.

Virtually all modern precision isotope ratio mass spectrometers are based on either a symmetric or an asymmetric stigmatically focusing ion optical prism. Typical dimensions and parameters are as follows: For 'stable isotope systems' which must process SO \ ions as the heaviest species, a 3 kV acceleration results in a 12 cm radius for gases from H2 (0.1310 T) to S 0 2 (0.5243 T). The lateral distance of two adjacent ion beams would be 8.6 mm ( 2 8 N j / 2 9 N j ) , 5.5 mm (1 2C02/1 3C02) or 3.8 mm ( "SCfe/^SOa). This is sufficient to mount two Faraday cups for two adjacent beams. Magnetic deflection angles are chosen between 60° and 125°. For 'solid isotope' systems using thermal ionization, much larger dimensions are chosen, because sufficient dispersion for higher mass isotopes (e.g. 238U) is required. The acceleration voltage is normally 10 kV and the system's physical radius is around 30 cm. Therefore the required magnetic field is between 0.1172 T (6Li) and 0.7385 T (238U), whereas the beam distances are 10 cm for 7Li/6Li and 0.255 cm for 2 3 6U/2 3 5U, for example.

Figure 3.5 gives schematic presentations of four ion optical systems from three commercial mass spectrometer companies. These four systems (including

(a) 44 45 46 CO,

Magnet

Ion Source

Figure 3.5. Ion optics of commercial isotope ratio mass spectrometers; (a) Finnigan MAT type DELTAp,us; (b) Europa Scientific type 2020; (c) Micromass type Optima; (d) Finnigan MAT type MAT 262


75.30 76.63 80.50

82.98 90.0

\ mass 34

(c)

mass 40

mass 28 \ m a s s 32 mass 29

Multiple Collector

Retarding Lens and SEM

(d)

r - p - - - : ? K « » >

Ion Source

Figure 3.5. (continued)

some minor variations) are the basis of at least 80% of all isotope ratio mass spectrometers bought by users in the past 15 years.

A special feature of the systems for stable isotope applications is the separate collection facility for 'H and 2H at a smaller (or larger) radius. This allows a reduction (or enlargement) of the cup distance for the hydrogen/deuterium double collector.

One of the systems (Figure 3.5(b)) has a deflection angle of 120°. This has no particular optical advantage compared with the others except that it has a smaller footprint for the same radius [8].

ION OPTICS 21

As can be seen from Figure 3.1(b), the focal plane of a standard symmetric, stigmatic focusing system is far of the perpendicular to the main beam axis (30°). Apparently this requires relatively large radial dimensions and wider movements per mass unit for a variable multiple Faraday cup collector because the cups must be moved along the focal plane.

Therefore, a system is offered by one of the commercial companies in which the focal plane is bent to a near 90° angle against the main beam axis [11]. This is achieved by a slightly round shape of the pole pieces of the magnetic prism and has no measurable side effects on the other performance specifications; nor on the other hand, does it result, in a simpler multiple collector. Such a fancy design variation is a good example of the many attempts at (non-essential) commercial differentiation, not transparent to a 'normal' user.

3.2.2 Special Devices In order to achieve accurate isotope ratio results, each specific ion collector out of the multiple set of collectors should measure just one ionic species, i.e. the ions of the isotope in question. In principle, however, so-called isobaric inter-ferences cannot be prevented. Trace constituents of odd compounds in an impure sample, the residual gas pressure of the mass spectrometer's vacuum system and the outgassing of system components located in the vacuum give rise to ion beams at mass positions where the isotopic ions in question are to be measured.

In principle, isobaric interferences can be separated from the ions to be measured by using an ion optical system with high resolving power (e.g. using double focusing). High resolution, however, either reduces the sensitivity and/or requires enormous system dimensions (at least 10-20 times larger) and has been applied up to now only for a very special application [9, 10].

Otherwise, for most systems, a high capacity differentially pumped and bakeable vacuum system in which only selected and suitable materials (stainless steel, quartz, special ceramics, gold seals, etc.) are used is consi-dered a basic requirement to reduce isobaric interferences. Indeed, in the vast majority of applications a well designed vacuum system can prevent the risk of wrong results due to interference. Nevertheless, each user is bound to check a system regularly for sufficient purity of the baseline spectrum. Moreover, any sample inlet or sample preparation system must fulfill similar requirements with respect to the possible production of isobaric inter-ferences.

Another source of error is the cross-talk of the ion beam falling into a certain cup with another cup next to it [21]. This phenomenon is (historically and unluckily) called 'abundance sensitivity'. It defines that part of a large ion beam (falling into a certain collector) which cross talks into a collector one mass unit apart. In a typical system, as used in thermal ionization mass spectrometry,


abundance sensitivity is « 2 x ÎO-6 at the low mass side and « 2 x 10~7 at the high mass side of a 238U peak.

For the majority of applications such a small disturbance can be tolerated. However, in two relatively widespread applications, namely the measurement of extremely large isotope ratios in 'thermal ionization mass spectrometry' (e.g. 232Th/230Th « 500000 : 1) or in 'compound-specific stable isotope ratio mass spectrometry' (GC-IRMS) for the measurement of H/D ratios (« 10000 : 1) in the presence of high amounts of 4He carrier gas ions which give rise to cross talk into the HD (mass 3) collector, it is necessary to counteract this by suitable ion optical means. Such peak tails, which may extend over several mass units, are produced mainly by the scattering of ions at the molecules of the residual gas in the vacuum system. Scattered ions mainly lose energy and, to a lesser degree, they change their flight direction. Hence peak tails are strongly asymmetrical versus the low mass side of the peak [23] because a simple magnetic prism will focus a lower than nominal energy ion at the same place as a lower mass ion. This is beneficial for the measurement of the so-called stable isotope ratios (I3C/12C, 15N/14N, ) because the less abundant isotope is heavier than the main isotope. For carrier gas assisted measurements of D/H and for most of the solids isotope applications (e.g. 230Th/232Th), this condition is not fulfilled. Apart from the use of a relatively large dispersion, an energy or mass filter in front of the disturbed collector which sorts out 'wrong' ions is the appropriate ion optical solution for the problem. Technical solutions range from complicated multiple (magnetic/electric, magnetic/magnetic) sector optics [12-20] down to simple retardation lenses [22,23].

3.3 ION SOURCES

An ion source for an isotope ratio mass spectrometer must show high sensitivity (or low sample consumption), high stability and low energy spread of the ion beam and it must produce average ion currents of larger than 10 I0 A for results of highest precision. Since the very beginning of isotope mass spectrometry, electron impact and thermal ionization sources for gases and solids, res-pectively, have proven to be perfectly suited for this purpose.

3.3.1 Electron Impact Sources All modern electron impact ion sources are based more or less on a design proposed by A.O. Nier [24, 25] in 1947 (Figure 3.6). The gases are fed into the so-called ionization volume which is traversed by an ionizing electron beam and which, for the sake of gas tightness, preferably has only three small open-ings: two for the entrance and exit, respectively, of the electron beam and one for the exit of the ions. Gas tightness is desirable for low sample consumption.

ION SOURCES 23

VE, Electron energy Gas Inlet

Electron current ie

Ionization volume Pusher

Filament

| ' ' Ion current i M l / I I I W/ \\l

• VI - J

/ft M\

Drawout plates

Einzel lens

Beam defining slit

Figure 3.6. Schematics of an electron impact ion source

The ions are extracted out of the ionization volume by application of a lateral electric 'draw out' field and/or by a so-called repeller plate, which results in the same effect, and are then accelerated further (3 kV is usual) and electrically focused in two directions before they pass through the entrance slit of the ion optics. Focusing can be achieved by electric immersion lenses that use stepwise acceleration in zones of field inhomogeneity or by 'Einzel' lenses, which function by a suitable combination of deceleration and acceleration of the ions.

Electron impact ionization is characterized by the following simple relationships [26].

For atoms:

A + e~—•An+ + (n + l)e-

and for molecules:

AB + e~—>AB+ + 2e~ (ionization)

(6)

(7)

or —>A+ + B° + 2e (ionization and dissociation)

and is given quantitatively by the following relationship:

i = Jliep (i = EQPQ) (8)

The ion current i is proportional to the gas pressure p and to the size i'e and length / of the ionizing electron beam. The factor J (ionization constant) summarizes ionization specific parameters which depend on the isotopic species. This simple linear relationship (i = EQPQ) is a prerequisite for accurate


measurements of isotope ratios. The observed isotope ratio Ry of two isotopic species (i, j) is given by the ratio of the number of atoms (N¡, Nj). In a gas, this is given by the ratio of the gas pressures (p¡, pj). Hence

* = £ = * = ** (9) Nj pj ij

EQ is called the 'source pressure sensitivity' and is usually defined in (A/mbar). In practice, source pressure sensitivity is very difficult to measure, because the pressure /?Q in the ion source is normally not known. It is therefore sometimes replaced by the 'system pressure sensitivity' EP, where the ion current is related to the pressure in the vacuum system near the pump (pp), which is measured anyway to monitor the residual gas pressure pp in the whole vacuum system: / = Eppp. Although this kind of pressure sensitivity has the advantage of being accessible for a measurement, it cannot be used for comparison of the sensitivities of two different ion sources (or two types of mass spectrometer) because its actual value depends on the gas conductivity between the ion source volume and the position in the system at which the pressure is measured. This gas conductivity is apparently different for each type of mass spectrometer. Another disadvantage of this type of definition of ion source sensitivity is that it does not contain direct information on sample consumption, a value of high practical interest.

On the other hand, the ion current can be directly related to the inflowing amount of gas, or to the rate of sample consumption As/At:

i = EM As/At (10) This so-called 'molar sensitivity, EM is usually measured in (As/mol) and is accessible to direct measurement. EM and £p are simply related by £M — RTLqpEp. Where LQP is the molecular gas conductance (ml/s) between the ionization volume and the location in the system where the system pressure PP is measured. R is the universal gas constant (e.g. 83134 ml mbar/mol K). A numeric equation for a typical system is, for instance,

£M(As/mol) « 400£/>(Ambar1) (11) Instead of being reported in (As/mol_1), the molar sensitivity is often given by an equivalent value EMM [molecules/ion] by measuring the number of gas molecules which are required to detect one ion at the output of the mass spectrometer. Typical values range from 1500 to 500 molecules per ion.

As a rule of thumb: a sensitivity EMM of 1 molecule per ion corresponds to a molar sensitivity EM of « 105 As/mol:

EM x êMM = 105 (12) It is important to note that all real life values for source sensitivity are based on the measurement of the ion current through the entrance and exit slits of the ion

ION SOURCES 25

optical system. They therefore, include, a reduction in sensitivity by the optical transmission of the ion acceleration and separating systems.

Ion acceleration is governed by Liouville's theorem (preservation of phase space), which is given in its (approximate and simplified) formulation for ion beams by

x\ot\\/V\ = xiOL2\ñh. (13) The width *2 and the opening angle a2 (aperture) of an ion beam at electric potential V2 are predetermined by their corresponding values x\, a\, V\. They depend on the acceleration voltage, which is just the difference of the potentials V\ (ion production) and V2 (exit slit). The higher the acceleration voltage, the narrower the beams (in x and a) that can be produced. No focusing system can do better than allowed by this condition.

It is desirable that the majority of the ions produced should leave the narrow source exit slit, and it is hence apparent that all ions need to be produced in a volume with small lateral dimensions to fulfill Liouville's criterion. In most ion sources this is achieved by focusing the ionizing electron beam by use of a longitudinal magnetic field (i.e. in line with the main speed vector of the electrons).

Diverging electrons in a longitudinal magnetic field fly on a spiral path around the axis of the field, owing to the radial component of their speed. A typical example: an electron flying with a speed of 100 eVand with an angle of « 6°, relative to the axis of the magnetic field (10~2 T), moves on a spiral path of about 1 mm radius. Its rotation time is around 50 ns and it advances, for one rotation, by about 3 mm along the field axis. In a 1 cm long ionization volume it thus performs somewhat more than three revolutions and its path length in the ionization box is thus « 20 mm. The focusing of the electron beam therefore has two favorable effects. First, it keeps the lateral dimensions of the electron beam, and hence of the direct ionization volume, limited to the diameter of the spiral (as required for a good exit slit focus) and, second, the path length of the electrons is larger by a factor of up to 2 and hence more ions are produced for a given gas pressure. Furthermore, the small lateral dimensions of the ionization space result in a small energy spread of the extracted ions.

However, the magnetic field does not have only positive effects. It represents, with still relatively slow ions in the ionization volume, a small 'mass spectrometer' of its own. The unavoidable result is that the number of ions leaving the source in a given direction depends on the mass of the species.

Another very important and sometimes very disturbing effect is called (self-)interference. The production of ions causes a certain positive space charge within the ionization volume, which is larger, the slower the positively charged ions are. Although the number of (negatively charged) electrons in the volume is much higher than the number of ions, the electrons do not compensate the positive space charge because they are much faster. The result


is that the space charge reduces the drawing out field or, in other words, certain ions prevent other ions from being drawn out of the source. In combination with the mass discriminating magnetic field in the source, this effect results in the dependence of the measured isotope ratio on the gas pressure and/or the electron current itself. Furthermore, in applications that use a carrier gas to feed the isotopic sample into the ion source, the space charge of the carrier gas ions may have a large effect on the measured isotope ratio of the sample (cross interference). Such effects are summarized under 'isotopic non-linearity' and should not be mixed up with the co-existent ordinary ion current - gas pressure non-linearity of one species alone.

Another source of non-linearity must be seen in ion-molecule reactions in the ionization volume, e.g. H20J + C02—»HCOj + OH". The reaction H J + H2—>Hj + H is of perticular concern for the accurate measurement of the (low) abundance of HD (at mass 3).

As the numbers of both H J ions and H2 molecules depend linearly on H2 gas pressure (PH2). the H^ ion current is proportional to the square of PH2 and hence to the square of [Hj]. Therefore the current ratio i(3)/i(2), as measured at masses 3 and 2, respectively, is given by

¿(3) ¿(2) -

HD+ + Hj HD+ k[H¿}2

H2 Hj H2

? - $ • " »

or n n + ;Ci\

(14) k is called the H3 factor' and must be known for an accurate measurement of the deuterium abundance in H2 gas. The actual size of k is determined mainly by the residence time of the H2 ions in the ionization volume.

As can be easily understood, the design of the physical layout of a suit-able ion source and the choice of the different operating parameters is a dif-ficult task; it is always an empirically found compromise and instrument manufacturers try to keep it secret. Additionally, important specifications can be maximized easily at the cost of others (e.g., sensitivity versus isotopic linearity or sensitivity versus ion beam stability). It is therefore important to demand all important specifications at the same time for the same source parameters.

3.3.2 Thermal Ionization Sources All modern thermal ionization sources are modifications, more or less, of a (multi-filament) design published in 1953 [27, 28, 46, 47]. The isotopic samples are prepared as a solution of a salt (chloride, nitrate, etc) or an oxide. They are loaded in p.% to ng quantities onto a ribbon (filament) of rhenium or tungsten

ION SOURCES 27

rrC^Tî t/

^ öDn ^ ^

Figure 3.7. Common filament types for thermal ionization sources: (A) Single filament, (B) multiple filament

and dried. They are then vaporized, hit a hot surface of rhenium or tungsten and are ionized on that surface.

Virtually all such salts have a very low vapor pressure and must be heated to show a useful vapor pressure and a suitable vaporization rate, which is usually in the order of pg s"1. Evaporation temperatures are in the range 500-2500 K. The evaporated species in the vapor phase can be molecules of the originally loaded species, dimers or trimers of this salt, or monoatomic or polyatomic dissociation products.

Depending on the layout of the ion source (see Figure 3.7B), a smaller or larger part of the evaporated molecules or atoms hits the ribbon next to the evaporation ribbon and is adsorbed on this surface. After a very short residence time on the surface the particles are desorbed, partially as molecular or atomic ions (positive and/or negative), the remainder as neutral species.

In special design versions of a thermal ion source [29, 30] the particles are not evaporated onto the hot surface. Instead, they reside on the surface from the beginning and are thus desorbed as ions or neutrals (Figure 3.7(A)).

The ions are accelerated into the ion optical system in a way very similar to that described for electron impact sources. The surface area in which the ions are produced is small and their energy spread is also small. Hence thermal ionization sources offer favorable conditions for Liouville's theorem and for good focusing in a magnetic sector.

All commercial thermal ionization sources are equipped with rotatable sample magazines (turrets). Without such a magazine, it would be necessary to break the vacuum of the ion source and to evacuate it again for each loading of a new sample (Figure 3.8). In order to save time and to improve vacuum conditions, up to 21 filament holders of different types (single, double or triple filament) are mounted in a sample magazine and are then transported under vacuum, one after the other, by a carousel-like movement on top of the ion acceleration lenses.


1

Figure 3.8. Thermal ionization ion source with a sample turret for 13 filaments (Finnigan MAT 262)

The principle of thermal ionization is simple [41-44]. If a neutral atom approaches a (hot) surface, the Fermi levels of this atom and of the surface metal are equalized. The discrete energy terms of the atom are getting broader. The width of this energy band depends on the temperature, on the work function of the surface material, on the ionization potential / of the (adsorbed) atom and on the distance. As a consequence, below a critical distance of the atoms from the surface, electrons can be exchanged between the atom and the surface: The state of the particle (ion or atom?) is undeterminable at this place and can only be given, on the basis of Fermi statistics, as the probability P, whether the valence band of the particle contains an electron or not.

By a further energy transfer from the hot surface to the particle near to the surface, the particle is evaporated back into vacuum. Its status as ion or atom depends solely on the above probability at the critical distance. This is quantitatively described by the Saha-Langmuir equation, which defines the degree of ionization a (the ratio of ions and neutrals leaving the surface) as

N+

No 8+ — exp 80

c(>-/) ' kT

£ go — — exp 1.16 x 10 4 ( * - / ) ' (15)

where g+/go is the ratio of the so-called statistical weights of ion and atom which, as an example, equals 1/2 for alkali metals; the atom can exist in two states (parallel or antiparallel spin of the valence electron), the ion has only one

ION SOURCES 29

state; <j> is the work function of the surface (V); I is the ionization potential (V) of the ionized particle; and T is the temperature (K).

For practical applications, it is interesting to know the so-called coefficient of ionization ß = N+/N, which defines the ratio of ionized particles leaving the surface relative to the total number of particles hitting the surface. As N = No + N+

ß = a / ( l+a) , (16) or, if <p — I > 0, then a > l , and ß « 1, i.e. all atoms are ionized. In an analogous way, the Saha-Langmuir equation also applies to the production of negative ions where electrons are transferred from the surface to the particle (e.g. for chlorine, fluorine, etc. and their salts).

In real life, the Saha-Langmuir equation can be used only as a rough guideline for the design of experiments for thermally produced ions. The above simple equation is valid only for an extremely pure, layer-free homogeneous (monocrystalline) surface [43], provided that the atoms approaching the surface in their ground state are in full equilibrium with the hot surface during their residence time on the surface. There are many reasons, however, why these conditions are not fulfilled in practice.

1. The surface is inhomogeneous (polycrystalline) and, in consequence, the work function is locally different. This may produce lateral electric fields that diminish or enlarge the actual work function of the surface.

2. Neutral particles approaching the hot surface are elastically reflected at the surface. In particular, this may be the case if molecules of metal salts or oxides are evaporated.

3. At the temperatures required to generate a suitable evaporation rate, the sample's particles may no longer be in their ground state. Molecules dissociate during evaporation or evaporate in polymeric states. Also, more than one species of ions may be generated in cases where molecules are evaporated (e.g. U+ and UO+ from U02), or molecules may undergo dissociation on the hot surface before or after ionization. Even chemical reactions [33-37] between species at the hot surface are observed. For instance oxides may be reduced in a reducing environment (e.g. at a carbon layer [33, 34] on the hot surface).

4. The evaporation rate from the evaporation filament is too high and, as a result, overloads the ionization filament with a (mono)layer of molecules, thus changing the work function. For example, a monolayer of oxygen on tungsten increases the work function by 1.9 eV, and a 40% monolayer of cesium reduces the work function of tungsten by 2.4 eV.

Additionally, if the sample consists of more than one compound having different vapor pressure and/or different ionization energies, the more volatile and easier to ionize species will in most cases prevent the production of enough


ions with sufficient stability from the remaining species. It has become common practice, therefore, to purify the samples carefully before loading, even if this sometimes appears to be a troublesome process.

The separation of evaporation and ionization temperatures in the multiple filament source is to be considered a big advantage. All important parameters are under nearly separate control and allow a relatively easy tuning of the ion source to optimum conditions for the experienced user, despite the difficulties mentioned. There is also much freedom in the choice of the sample's chemical species. Hence most applications are performed using the multiple filament source. Nevertheless, inexperienced users are sometimes confronted with seemingly strange effects.

The evaporation filament is located relatively close to the ionization filament. Thus the ionization filament still has some influence on the evaporation tem-perature by radiation heating (and vice versa). As a consequence, an increase of the ionization temperature which is expected to increase the ionization rate may, in fact, reduce the ion current owing to a reduction of the work function by overloading the ionization filament with neutral particles.

This slight, but not negligible, interdependence of evaporation and ionization temperatures for multiple filament sources is apparently an unavoidable inherent feature of the single filament source [29, 30]. Evaporation and ionization temperatures are always the same. Therefore there are only very few practical examples where a single filament source (although it is the classical thermal ionization device) has advantages over a multiple filament source.

A series of recipes to enhance ionization efficiency and to ease operation has been proposed [33-37], especially for single filament sources. These range from an (electroplated) layer of platinum or rhenium [38-40], in a sandwich-like structure on the (single) filament, with the sample in between, to the creation of a special (mostly porous) ceramic [31,32] or oxide layer on the surface. Such layers store the sample in the bulk of the layer and reduce the evaporation rate, while at the same time showing a relatively high work function.

Single filament techniques for very small samples of lead (silica gel layer [32]) or uranium (rhenium sandwich) are good examples of excellent and easy to operate ion sources.

One special effect deserves special attention: the fractionation of the sample with time. This time-dependent bias must be corrected properly. Fractionation correction [51-58] is one of the most important tasks in thermal ionization mass spectrometry.

During evaporation, particles of lighter mass are preferentially evaporated from the filament and hence the observed isotope ratio is not the true isotope ratio. It is lighter at the beginning and gets isotopically heavier in the course of evaporation (Rayleigh distillation [48-50, 52, 55]). This simple process only seems to produce easily understandable results. In virtually all practical cases, the various evaporating (multiple) species with frequently very different masses

ION SOURCES 31

* *; ¿*í*> s , .

*

«o**̂ \.n • »,

• •-•> s Ï S»? %#. • ^ ^ Ñ * * 34HTM

# * y ^

M ^ PB mm. -3SÄ

Figure 3.9. REM picture (3400:1) of a Nd nitrate sample on a Re filament. Recrystalhzation, generation of whiskers and indications of a Nd /Re alloy are visible

are not fully known. Their relative proportion may even change with time [55]. The evaporated upper layer of the sample may not be in a (rapid) mixture equilibrium with the bulk of the sample. The sample may change its chemical state with time or temperature and evaporate in different form at different times of the measurement.

Moreover, the sample can change its physical state. It can, for instance, recrystallize or 'explode' in a short burst. In order to illustrate such (and other) difficulties, Figure 3.9 shows a REM picture of the remaining part of a Nd nitrate sample on the evaporation filament after a successful (i.e. precise and accurate) measurement.

In summary: fractionation correction cannot in principle be correctly achieved by application of a theoretically derived expression alone, because most details in the source that could be used as a basis for a complete theory are unknown. Hence only some more or less empirical rules to correct for fractionation have been proposed [51, 53, 57, 58] and are widely used.


After this short description of artifacts, it will be easily understood that the thermal production of stable and intense ion beams which accurately represent the isotope ratio in their ion current ratio is an art (sometimes even a black art) in itself and requires much empirical knowledge. For each element in question, a special 'recipe' must be worked out, and this requires a lot of experience. Therefore it is good practice also to report the applied techniques and tricks when presenting ratio results.

The question remains why such a relatively untransparent ion source has been used for decades and is still being used in the vast majority of applications for solid isotopes. There are many good reasons: first, its high selectivity (up to 10~9) for metals (and rare earths) over such interfering species as hydrocarbons or light gases, and its ability also to produce negative ions [45] with equal selectivity (e.g. for isotope dilution measurements). It definitely produces the purest ion beams with only very minor isobaric interferences, and it can be finely tuned to an optimum performance (intense and highly stable ion beams) for one selected element by the proper choice of all operating parameters. Second, the ionization efficiency for most metals is higher as compared with other ionization methods and is virtually independent of the mass for a given isotopic system.

The sensitivity of a thermal ion source (or better, of a thermal ionization mass spectrometer) is usually given by the number of sample atoms evaporated from one of the filaments in order to produce one ion at the output of the mass spectrometer. Typical values (which already include the ion optical transmis-sion of the mass spectrometer) range, for instance, from 20 : 1 (or 5 % overall efficiency) for Pb in a single filament silica gel environment, down to less than 0.01% for Hf in a triple filament source. All important elements, such as Sr, Nd, U and Th, range between these limits. As described above, values are heavily dependent on the empirical art of handling the respective isotope in the ion source, considering many experimental details.

In summary, sample consumption, as compared with other ion sources, is very low. Moreover, ion beams with very low energy spread are produced. Last but not least, users have learned over the years how properly to operate thermal ion sources for selected atomic species (mainly for geochemical applications) and how to correct for unavoidable side effects such as fractionation or remaining isobaric interferences.

3.3.3 Other Sources

For quantitative simultaneous multiple element mixture analysis from one sample, the thermal ion source is unsuitable owing to the exponential nature of the Saha-Langmuir equation and must be replaced by an inductively coupled plasma source [59-63], which, however, requires double focusing in some applications for the separation of the many isobaric interferences. This ion

ION COLLECTORS 33

source has just begun to produce interesting results for isotopic applications. The main advantage for multiple or single element isotope analysis is that less complicated purification of the sometimes complex sample is required. Beam stability, on the other hand, is much worse as compared with the thermal ion source. In combination with a multiple collector [64, 65], to compensate for beam instabilities and with the assumption of negligible interferences, this approach could well become a routine method for isotope ratio determination of solid samples within the next few years.

Extremely high selectivity (up to 10~14) and thus better signal to noise ratio and much lower limits of detection have been achieved by (multiple) photon resonance ionization [66-71] for selected atom species. The high costs of the necessary lasers and the extremely difficult operating procedures have up to now limited this interesting method to a few specialized laboratories.

3.4 ION COLLECTORS

After ionization and separation, the final objective in isotope ratio determina-tions is to measure the ratio(s) of two (or more) ion currents as accurately and precisely as possible [72]. This process is subject to statistical laws, as an ion current is a sequence of particles which arrive at the ion collector at statistically distributed time intervals, following Poisson statistics [73, 74].

Hence, if N particles arriving at the collector are counted, the mean square error of the counting result is o2

N = N and the relative measuring error aN/N = \/\[Ñ. This is in principle the lower limit of the precision with which an ion current can be measured.

N ions arriving within t seconds correspond to an ion current of i = Nqe/t(A) (qe — 1.6 x 10~19 C). Hence the lowest possible relative error ay (in %) for the measurement of an ion current i during time t is

ar = 4 x l O " 8 / ^ (17)

Counting the ions is the direct way of measuring ion currents as long as the ion counter is fast enough to resolve the small time intervals between the ions arriving at the collector. The practical limit for ion counting is reached at about 5 x 106 ions s ' (or 8 x 10 l3 A). At such an ion current, approximately 10% of the ions arrive at time intervals of < 20 ns (or 50 MHz), and the minimum pulse width is « 10-15 ns.

For higher ion currents, which are normally used in isotopic applications, the total charge per unit time (i.e. the ion current) instead of the number of ions is measured directly by using an ion collector that serves as a sink (for negative ions) or as a source (for positive ions) for charge compensating electrons, which are then measured as an ordinary electric current. In practice, this is achieved by feeding the electron current i through a high ohmic resistor R (108-10n Í2) and


by measuring the voltage U across this resistor. Hence i — U/R. The direct charging of a (small) capacitor (capacity C, charge Q) and the measurement of the voltage U across the capacitor (U = Q/C) has also been proposed.

Two ion currents which are measured sequentially or simultaneously are required to compute the observed isotope ratio. The observed ratio does not, however, represent the true isotope abundance in the sample and must be calibrated or corrected accordingly.

3.4.1 Multiple Faraday Collectors

For singly charged ions, an ion collector must be able precisely to neutralize each incoming positive or negative ion by delivering or accepting, respectively, exactly one electron per singly charged ion. In fact, this is not an easy task. First, a charged particle with an energy of several keV sputters the collector, i.e. it releases (mostly more than one) charged secondary particles (positive and/or negative ions and/or electrons) from the conducting surfaces of the collector. This adulterates the result of the ion current measurement if these particles are allowed to leave the collector.

• j ' iV i ' iV ' '

Figure 3.10. Schematics of a Faraday-type ion collector behind an ion exit slit. i/s is a (negative) shielding voltage to prevent secondary electrons from exiting the cup

ION COLLECTORS 35

Second, ions can be reflected at the surface without getting rid of their charge, for instance, if the surface is charged as a consequence of an insulating layer produced by a long ion bombardment.

In summary, an accurate ion collector must be a perfect black body for ions. The so-called cup efficiency must be equal to unity as precisely as possible. Therefore, an ion collector is preferably designed as a relatively large closed box with only a small opening for the entrance of ions (Figures 3.10 and 3.11). Such a box is called a Faraday cup. Additionally, a charged electrode or a small magnetic field is used to reflect or to deflect, respectively, charged secondary particles, i.e. to keep them inside the cup. The body of the cup is frequently inclined relative to the main ion beam axis, and/or the width to length ratio of the cup is made as small as possible, in order to prevent reflected or sputtered particles from leaving the cup. The inner surfaces of the cups are often coated with a material of low sputter rate, e.g. porous carbon.

>

• •,

C

'

•

Figure 3.11. One of eight Faraday-type ion collectors from a variable multiple ion collector system


The ratio of two isotopic ion currents, as emitted by the ion source, is always constant at a given time, even if the sum of the beam currents (total current) changes. However, if the total current is drifting, and if the two currents representing the ratio are not measured at the same time, the computed ratio of the currents is too large or too small for a declining or increasing total current and must be corrected for draft. This effect can be avoided comple-tely if the two (or more) ion currents are measured at the same time. Therefore, virtually all precision isotope measurements are performed by using sets of Faraday cups [91-96], one for each ion species in question. However, each ratio measured in this way is biased by the measuring resistors and by the cup efficiencies of the relevant collectors and does not necessarily reflect the accurate isotope ratio. Two methods are used to overcome such difficulties.

First, each sample measurement is related to that of an (external) standard sample which is measured in the very same cups and with the same inlet system. This method corrects not only for all collector biases, but also for other methodological biases which might, for instance, originate in the inlet system. However, the final result is a relative deviation of one ratio from another, not an absolute isotope ratio. This is the origin of the '¿-notation' for isotope ratios.

Second, for absolute ratio determinations with an internal standard ratio, frequently a sequential multiple ion collector measuring scheme can be found in which each beam is measered not only in one cup, but also in all other cups which are used for measurement of the isotope pattern. This requires jumping of the magnetic field ('dynamic multicollection').

Such a scheme [96] is given in Figure 3.12 as a simplified example for a dual jump double collector measurement of the 8 7Sr/8 6Sr ratio with the 86Sr/88Sr ratio as internal standard ratio. Two ratios (masses mi and m2)

rx=F— and r2 = F— ^ = 7 ^ - (18) '87 '88 \ J2K2/

are measured one after the other in a dual collector, f\ and f2 being the respective cup efficiencies and Rl? R2 the measuring resistors for the cups. This method assumes that no Rb is left in the sample. Owing to fractionation, we have the observed ratio ro of two ion currents (i'i, i2) in general

h h2 - = robs = 7 r - 7 = ii nx

h\ and h2 being the isotopic abundances in the sample at any time. This relationship holds, independent of any assumption of a certain

'fractionation law' (power, exponential or Rayleigh). The unsolved problem,

(19)

ION COLLECTORS

86 87 88

86 87 88

FT/„- R87/ai

Figure 3.12. Dual collector dual jump scheme for strontium to compensate for cup deficiencies

however, remains the question of which masses actually evaporate. These may not necessarily be the atomic masses of the species.

The 'true' ratio is the ratio in the sample before evaporation starts. For the exponential fractionation law [57] as an example, two observed ratios (7obsi ) r0bs2 and 71,72 respectively) of a sample are related to each other and to their true ratios (/?oi, R02) as follows

7obsl _ /7obs2V"12

^01 V ^02 / (20)

Using equations (18), (19) and /?03(86/88) = R0i(86/87) • /?02(87/88); 73 = 71 • 72 one finally obtains


As g « 0.5, the influence of the (not precisely known) 'cup factor' F is reduced by a factor of approximately 180 in this case. For the linear approximation (g = 0.5) the cup factor is fully cancelled and the well known classic fractionation correction formula is obtained:

(¿) - fik (22)

However, such a linearly corrected result differs from the exponentially corrected result by 17 ppm. Fractionation correction by application of the so-called 'power law' [53] leads to a difference of more then 30 ppm. In any case, the results obtained by the different correction formula are also dependent upon the total amount of sample used up for the measurement as long as the sample does not evaporate in accordance with the applied law.

In view of the fact that, on modern solid sample instruments strontium isotope ratios are routinely measured with 10-15 ppm precision, it is clearly important to apply the appropriate fractionation correction in order to prevent a systematic error of more than 33 ppm.

The same holds true for all other dynamic multiple collector schemes that have been proposed, e.g. a triple collector triple pump scheme for determining the normalized, bias-free 143Nd/144Nd ratio with , 4 4Nd/1 4 6Nd as the internal standard ratio. Therefore, the results of very high precision (< 10 ppm) obtained by applying such dynamic methods cannot be honestly compared concerning the 'true' ratio if the way in which fractionation has been corrected is not explicitly known. In other words, the job is not completed just by cancelling the cup biases. If no internal standard is available, or if transparent fractionation correction independent of any cup efficiency compensation scheme is important, static multicollection (one collector for each isotope in question) is preferably used. However, both the resistor biases and the cup efficiencies should be determined in separate measurements.

These static schemes with permanent simultaneous recording of the ion currents do have the advantage that the measuring times can be shorter for a given precision. This is important for small samples. The resistor ratios are measured simply by feeding a constant external current through each resistor [75] and precise measurement of the voltages across the resistors. This has become a standard method of sufficient precision (< 5 ppm).

The determination of cup efficiency ratios, however, is not yet a mature method, nor has it been applied so far by the majority of users of solid isotope mass spectrometers. Most users just believe the manufacturers' specifications, which are based on not always transparent measurements. Or they assume the non-existence of wrong cup efficiencies because they measure, by chance, the 'true' ratio, whatever that means. Such users get nervous (and helpless) only if the measured ratios show a long term drift. This indeed points to an instability

ION COLLECTORS 39

of the cup efficiencies, but does not give any information about the true values for the cup efficiency before or after the drift has been observed.

In fact, it has been shown [88, 89] that cup efficiency ratios can be determined in a relatively complicated dynamic 'multiple jump, multiple configuration' multi-collector experiment, in which all required isotope ratios are assumed to be known with sufficient accuracy: With the N-fold measurement (Äj, R2,... ,RN) of one given (and 'known') ratio (e.g. 143Nd/l44Nd) in N different triple collector sets out of an array of N collectors, and with the application of one of the fractionation laws, N — 1 cup efficiency ratios (fi/fj) can be computed from N — 1 simultaneous equations, which have, given the power law for fractionation correction as an example, the following (complex) form

(fi/f,)=R2-R21-R^-R¡-R;1

(/2//4) = ( j

Another elaborate and time consuming method is to measure one single, extremely stable ion beam in all relevant cups. The ion current ratio of two cups equals unity in this case (at least after appropriate drift correction), and the cup efficiency ratio can thus be calculated, provided the resistor ratio is known. An important advantage of this method is that there is no need for the assumption of a particular fractionation law.

With the described procedures, a precision of 1 ±(15-50 ppm) for the cup efficiency ratio has been achieved. The determination of the cup efficiency itself, given the required precision, is practically impossible. Hence, no serious specification for the efficiency of a single cup can be expected from any manufacturer.

In summary, for dynamic multiple collector measurements, any cup effici-ency effects can be virtually suppressed, whereas for static measurements, cup efficiency effects have a direct influence on the final result. Proper fractionation correction, on the other hand, can be easily applied for both methods.

Another handicap of the multiple collector method is that the lateral distance of the ion beam in the focal plane depends on the mass of the ions. This is the origin of the so-called variable multiple collector in which a set of (generally eight) collectors can be moved along the focal plane, their distances being precisely tuned to each particular isotope pattern. However, for dynamic multi-collector measurements, the collectors can only be adjusted to average distances which are a compromise between the different (mass dependent) beam dis-tances. Although representing a relatively complex device, such collectors have become standard equipment for thermal ionization mass spectrometers.

In gas isotope applications where the number of different isotopic species is limited to a few gases with relatively small masses, two simpler solutions are preferred. First, the use of six separate ideal Faraday cups in correspondingly fixed distances can cover all needs to measure the isotopes of C02, N2 and S02. Second, as the resistor and cup biases are cancelled anyway by computing the


28

44

64

29

45

66

30

46 CO,

SO,

Figure 3.13. Universal triple collector scheme, including two wide cups, for a stable isotope ratio mass spectrometer

relative difference of the ratios, a set of only three collectors (not necessarily all ideal cups) is a tolerable compromise [93, 94] to measure the above gases (Figure 3.13), because it is permissible to assume that the relative behavior of two imperfect collectors (concerning the release of secondary particles) does not change in the short time span between measurement of the sample and the standard.

3.4.2 Secondary Electron Multipliers For the measurement of small ion currents, and/or in order to count individual ions, secondary electron multipliers are required. It is virtually impossible to detect directly the arrival of one ion at a Faraday cup or to resolve the signals of a small number of ions from the baseline noise of an ordinary high ohmic input current amplifier.

The mean noise voltage í/N (V) across a high ohmic resistor is generally given by

Us = 7.4 x l O - ' V r R A / (24) where T is the absolute temperature, A / is the bandwidth of the amplifier and R is the size of the resistor.

For R = 10n Í2, T = 300 K and A / = 10 Hz, the noise UN at the amplifier output would be « 128 uV. An ion current of 1.3 x 10"15 A (or about 8000 ions s-1) flowing through the 10" Cl resistor will therefore produce a signal as

ION COLLECTORS 41

high as the amplifier noise, i.e. the signal to noise ratio equals 1 at this current. In other words, noise-free amplification for the direct, precise analog measure-ment of currents smaller than » 10~14 A is required.

On the other hand, the arrival of a single ion with its charge of Q = 1.6 x 10 l9 C will, for instance, momentarily load up the (small) capacity of the collector (C « 10 pF) to a potential V = Q/C « 1.6 x 10~8 V. If the next ion which may arrive after 0.1 ps is to be detected as another pulse, the cup's capacity must be discharged within this short period. This would be done by a resistor of ~ 104 il. An electronic pulse counter requires at least 100 mV to be triggered. Hence, single ion pulses at the collector must be amplified 106 to 107-fold before they can be counted. Suitable electronic amplifiers for this purpose are not available. A bandwidth of at least 10 MHz would be needed. Hence the noise at the output (which equals amplification x input resistor noise) would reach several volts, and a single ion's pulse would not be detectable within this noise.

The way out of this (analog or digital) dilemma is the direct, virtually noise-less amplification of the charge [76-81]. This is achieved by a device consisting of several dynodes, as shown in Figure 3.14. A potential of several hundred volts is applied between each of the dynodes. A charged particle released at the surface of a dynode is thus accelerated to the following dynode, and so on. An incoming ion may release an average of a electrons at the first ion-electron conversion dynode, and each electron may then release an average of ß electrons at each of the following electron-electron conversion dynodes. Then the amplification factor of this device would be

A = aßN (25) where N being the number of electron-electron dynodes. If, for instance, a = 3, ß = 2.5 and N = 16, then A « 4.5 x 108.

Apparently, this device offers an amplification of the requested order of magnitude and is inherently noiseless. Only very rarely may electrons produced by field effect tunnelling or by cosmic rays be released by chance at one of the first dynodes, giving rise to occasional fake pulses (dark current). The device delivers either enough current for an ordinary analog current measurement of small ion currents using resistors of the order of 106S7, or it produces sufficiently high output pulses for triggering a counter, a and ß are average numbers, because the number of electrons actually released for each incoming particle is distributed over a relatively broad range following Poisson statistics. Basically, the probability W„ that n particles are released by one incoming particle is given by

W„ = e " ^ (26) n\

7 being the average number of secondary particles produced per incoming particle.


f

Í

Conversion dynode

Secondary dynods

Electron collector

To amplifier

Figure 3.14. Schematics of a secondary electron multiplier (SEM) with one conversion electrode and 10 dynodes

Such a Poisson distribution [73, 74, 80] for the first stage (ion-electron conversion) is shown in Figure 3.15(a) for 7 = 6 (i.e. one ion produces an average of six electrons). As can be seen, up to 14 secondary electrons may be produced by one primary ion.

ION COLLECTORS 43

0.1

0.05

0.01 (a)

—

— r=6

0 2 4 6 8 10 12 14 16

Number n of electrons

0.16

0.12

V 0.08 a

0.04

(b) 0. 04 0.

0

a=8.3;ß=2

08 0.

=7.9; 0=2.7

7 ^ /

12 0.

-y

16 0. 20

aß

Threshold

Figure 3.15. (a) The Poisson distribution at the first dynode of a SEM shows the probability Wn that n electrons per incoming ion are produced; (b) The composite Poisson distribution of a SEM near the counter threshold


However, there also is a finite probability that no ion is produced (n = 0 and 0! = 1). The first dynode thus has an efficiency that is smaller than one: •n = 1 — e"a , or if, for instance, an average of three electrons is produced per ion, r] — 0.95. This means that at least 5% of the incoming ions do not produce an electron that could be further multiplied in the following stages. This disadvantage must be reflected especially in analog measurements [90], as the conversion coefficient a is always a function of the energy of the arriving ions and, hence, indirectly a function of their mass.

In principle, all other dynodes behave similarly to the first dynode. Therefore the output from the last dynode is a composite Poisson distribution. For the purpose of ion counting, all electrons originating from one ion should appear at the final stage within a time interval of 5-15 ns. This is achieved by a suitable geometry of the dynodes. The resulting short electron current pulse is fed into a low ohmic resistor ( « 100O) and thus produces short voltage pulses with a pulse height distribution at this resistor corresponding to the composite Poisson distribution. These pulses are further amplified, if necessary, before they finally trigger a counter. A high frequency amplifier is required which should not significantly enlarge the pulse width. Because of the Poisson distribution, very small pulses do still occur at the output of the amplifier, albeit with a relatively small probability. As each counter has a lower threshold to be triggered, however, some pulses will get lost at this point.

The counting efficiency is therefore further reduced by the sum of all pro-babilities Pk(n) that pulses below a certain threshold are found.

m m

i i

where Pk(n) is a function of a and ß. The threshold cannot be shifted to a very small size because more and more fake pulses of low intensity are recorded in such a case.

Figure 3.15(b) shows the very lower end of the ideal composite Poisson [80] distribution without fake pulses at the last dynode for two different ion/electron conversion coefficients (a = 7.9 and 8.3, respectively). In order to give a clear idea of the process, the axes of the graph are related in a suitable way to the average number of electrons in one pulse, and a counter threshold is shown at such a pulse height that approximately 99% of all pulses pass the threshold. It can be seen that a 5% change in a will change the measured counting rate and hence the overall counting efficiency, by ¡=s 1.0%. This is 50 times lower than with analog measurements, but must nevertheless be taken into consideration for very precise measurements.

Moreover, it has been observed that the conversion coefficient ß of most electron multipliers depends slightly on the total number of ions arriving at a dynode within a certain time span. This is true, in particular, for large electron

ION COLLECTORS 45

clouds, i.e. at the last two or three dynodes. Such observations are summarized under the term 'activation effects'.

In summary, also, the seemingly simple process of ion counting shows a more or less pronounced bias. If an isotope ratio is measured in a multiple collector experiment by using an ion counter together with one or more Faraday cups, the measured ratio can be biased, as shown, by several percent and it may, in addition, depend slightly on the counting rate, i.e. it will show non-linearity (up to 0.5%) depending on the particular design. In other words, the counter must be calibrated over a certain dynamic range by measuring the same ion beam in the Faraday cup and in the counter, or by performing a 'reverse' isotope experiment, i.e. by measuring (and normalizing) a known isotope ratio and using this knowledge to determine the bias. Because of well-known aging effects, especially at the first dynode, such calibrations must be repeated within relatively short time intervals to compensate for medium and long term drift of the counting efficiency. Each incoming ion produces an output pulse of a certain length (typically « 25 ns). During this short period after the arrival of one ion, the system cannot distinguish (detect) the arrival of a second ion. This period is called the 'dead time'. The time distance of the arrival of two ions for a given ion current is not constant, but is distributed according to Poisson statistics. Therefore the counter gradually loses more and more ions for an increasing ion current. This high current non-linearity can be corrected by the following equation, which is derived from Poisson statistics [85-87]

r = (1 - rr)R0 (28) where r is the observed counting rate, R0 is the true counting rate, and r is the dead time.

At very small ion currents (< 10~17 A) the so-called dark current is of concern. For a modern multiplier, however, the rate of these fake pulses, is less than two per minute for a not too small counter threshold, corresponding to an ion current of 2.6 x 10~21 A.

Another strange fact deserves mention here (by way of curiosity). For small counting rates, the standard deviation of the ratio of two Poisson-distributed entities (e.g. of two ion currents) is mathematically not defined [74]. Therefore the error statistics of low current isotope ratios are questionable, at least theoretically.

In earlier years, when the technical data for secondary electron multipliers that are standard today (amplification, stability, noise, linearity) were not yet available, a somewhat different device, the so-called Daly detector [82, 83], was proposed, and is still being used. Ions at the output of the ion optics are acce-lerated by about 30 kV to a relatively large, massive electrode. Such energies result in a relatively high ion/electron conversion rate, virtually independent of the mass and kind of the ions. The electrons released from the conversion electrode are accelerated and hit a (metallized) grounded scintillator. There they

4 6 ADVANCED ISOTOPE RATIO MASS SPECTROMETRY I

are converted to photons which are then detected by an ordinary photomultiplier (photon/electron converter) and fed into a counter. In this relatively complicated multiple conversion device, too, pulse height distribution and its consequences are characterized by Poisson statistics, as with the (simpler) secondary electron multiplier.

Except for a dark current (baseline noise) typically up to 10 times higher than that of a modern ion-electron multiplier, the Daly detector remains an acceptable preamplifier for isotopic measurements. No general rule with respect to the comparative advantages or disadvantages [87] as related to linearity or stability or aging of the two devices can be given. However, it is relatively easy, and is recommended, to test and to compare each individual brand offered by the manufacturers by the same test methods in order to reach a conclusion independent of the manufacturers' specifications.

3.5 SAMPLE INLET SYSTEMS

Thermal ionization mass spectrometers use carousel-type multiple sample introduction systems which are an integral part of the ion source, as described in Section 3.3.2, on thermal ion sources (Figure 3.8).

In stable isotope ratio mass spectrometry, automatic sample preparation devices as described in Chapter 6 convert the samples from their broad range of original chemical forms into a small range of elementary gases (C02, N2, H2, S02, 02) while preserving the original isotope information. These gases are then available in clean form or as a mixture, mostly near or at atmospheric pressure. They are introduced into the gas inlet system, which then feeds the gases into the ion source.

In the case of a gas mixture, it is absolutely necessary to separate the gases from each other before their introduction into the ion source. Only clean gases guarantee measurements free from interference. The most disturbing inter-ferences are isobaric interferences. For instance, C02 introduced together with N2 is partially dissociated and CO + ions are generated which, in turn, disturb a N2 measurement at masses 28/29. Ion-molecule reactions as described in Section 3.3.1 are another source of isobaric interference.

The gas inlet system reduces the sample gas flow in the ion source to a value such that the vacuum system can maintain a sample gas pressure of at least W4

to 10-6 Torr in the ion source, while the basic vacuum (without sample inflow) is 10-9 Torr or better.

The most important feature of a gas inlet system must be that a sample gas and a standard gas (given the same chemical species) can be introduced into the ion source in a very equal way. In other words, if a certain adulteration of the isotope ratio during the process of introduction cannot be prevented, this adulteration should at least be exactly the same for sample and standard.

SAMPLE INLET SYSTEMS 47

Another requirement of such a system is that the isotope ratio remains constant during the time of introduction.

Two types of gas inlet system are in common use: the so-called 'viscous' gas flow and the 'continuous' gas flow inlet systems. Although these commonly used names give a perceptive description of the functionality, both systems nevertheless do have viscous gas flow and none has discontinuous gas flow. The main differences are these.

In the viscous flow system, the major part of the sample is used to ensure proper non-fractionating flow conditions over a long measuring period. Under such conditions, however, only 10% of a sample is typically consumed by the measurement.

In the continuous flow system, a carrier gas that does not interfere with the sample gas maintains proper flow conditions. The sample is transported like a bubble in this stream of carrier gas. In this way, up to 100% of the sample is available for the measurement, but only within a relatively short period.

Considering the fact that sample consumption (amount of sample into the ion source per unit time), which is approximately the same in both methods, determines the size of the final output signal of the mass spectrometer, this description of the principal differences of the two inlet methods apparently shows their main features.

The 'viscous' flow system allows long measurement times at stable con-ditions and thus offers measurements of utmost precision and accuracy, but relatively large samples are required.

The 'continuous' flow system, on the other hand, offers lower precision (5-50 times less) but allows the measurement of much smaller samples (by a factor of 100-1000).

The smallest sample that can be used in a viscous flow system is inherently predetermined by the principle of introduction of the sample, whereas, with the continuous flow systems, the lower limit for sample size is rather given by the overall signal to noise ratio of the entire system.

3.5.1 Viscous Flow Inlet Systems

A (classical) viscous flow inlet system always consists of two identical sub-systems [98-100], one for the sample gas and one for the standard gas. Figure 3.16(a) shows the (simplified) scheme of a typical system.

Behind an inlet port for the sample and standard containers, a variety of valves and dosing volumes (1-5 cm3) for partitioning a large sample is arranged. Finally, the partitioned sample or standard is stored in a variable storage volume VSA or VsT (variable range « 2 - 1 0 0 cm3, pressure 20-1000 mbar). In most design variants, this volume is actually a metallic bellows, varied in size by a motor, that allows automatic adjustment of the gas pressure. The sample or standard is fed into the ion source through a 60-100 cm long


To on Source

t Changeover TTwwr V valve

-tXHHXr-=HXH

(a) To Waste Pump

-CXH=

> m .

(b)

"V-T -TV.

^ r

Figure 3.16. (a) Schematics of a dual viscous flow gas inlet system. SA/ST: sample and standard gas storage volumes; M: motor for automatic variation of the volume (b) Viscous flow through a capillary. The isotopic enrichment factor in the capillary is shown. Py. gas storage pressure; PQ. gas pressure in the ion source; PE: gas pressure in front of the molecular quench; Lv: viscous gas conductivity; LM< LP: molecular gas conductivities

capillary (i.d. « 0.2 mm). For gas pressures larger than s=s 20 mbar (i.e. for a minimum of « 2 pmol of gas in 2 ml), this leads to viscous-type gas flow because the mean free path length of the gas molecules is sufficiently smaller than the capillary's dimensions. The capillary is constricted at its end, thus limiting the gas flow to the required size. In contrast to the capillary, the con-striction has a lateral dimension which is comparable with or smaller than the mean free path length of the streaming molecules. Hence, molecular flow conditions will be observed through this restriction.

Both sample and standard capillary are connected to the so-sailed 'change-over' valve, which is in turn, connected to the ion source and to a vacuum pump. This changeover valve actually consists of four valves, and thus offers the arbitrary connection of the sample and the standard, respectively, to the ion


source while the standard or the sample, respectively, is connected to the vacuum pump. Apparently, this scheme ensures that the gas flow is not interrupted in either of the systems and that, finally, exactly the same amount of gas has left both storage volumes, provided that the flow rates have been equal. For tuning flow rates, the gas pressure is variable. To achieve the same flow rate in both capillaries at the same gas pressure, the capillary constriations are adjustable. Such a symmetrical dual inlet system offers at least the guarantee that sample and standard are handled in the very same way.

Figure 3.16(b) shows schematically the detailed flow conditions on the way from the storage volume Vs (pressure Pv) through the capillary (viscous gas conductance Lv, pressure PE at the end of the capillary) and the constriction (molecular gas conductance LM) to the ion source (source pressure PQ) and, finally, to the pump (molecular gas conductance LP). Viscous gas flow is proportional to the difference of the squares of the pressure at the beginning and at the end of the capillary, whereas molecular gas flow is linearly related to the pressure difference across the constriction. This allows computation of the final pressure PQ of the gas in the ion source depending on the different parameters of the entire gas flow system:

LM + LF ' M

+ / > 2 _ L M 2Lv 2Ly

Source pressure (and hence ion current) and sample pressure in the storage volume are related non-linearly.

Another feature of viscous gas flow through the capillary [100] is that the flow rate does not depend on the mass of the isotopic species; it does, however, depend on the gas species. Molecular gas flow through the constriction, on the other hand is inversely proportional to the square of the mass M of the isotopic species. As the amount of gas flowing into the capillary and out of the constriction is always the same, the unavoidable consequence of this different behavior is that the isotope ratio of a gas (i.e. the pressure ratio PE2/PE, of the two species) at the end of the capillary (in front of the constriction) is different from the ratio Pv2/P\, of the same gas in the storage volume [101]:

** = MX»H (30) PE, VM, PVl

V ' In front of the quench the gas is isotopically heavier. The constriction itself has a higher conductance for the lighter component of the isotopic sample, but this is also the case for the (molecular flow) transport of the gas from the source to the pump. Therefore, the pressure ratio of the isotopic species in the source is the same as the ratio in front of the quench, but it is different from the pressure ratio of the sample in the storage volume.

As long as the mass independent viscous flow is maintained in the capillary, this biased ratio remains constant and is independent of the gas pressure in the


storage volume. Unavoidably, the enriched gas at the end of the capillary gives rise to a concentration gradient along the capillary. By back diffusion, this gradient can reach the storage volume provided that the forward sample flow speed through the capillary is smaller than the average back diffusion speed. At this point, mixed viscous/molecular flow conditions produce an undesired fractionation of the sample.

For a typical set of viscous flow capillary and molecular flow restriction as used in isotope ratio mass spectrometers, the lowest tolerable forward flow is reached at about 20 Torr sample pressure. This sets the limits for the size of the sample in the storage volume. The practical lower limits may be shown with a so-called 'small volume' inlet system, as an example. Such a system consists of a small gas storage container (e.g. « 120 pi) connected to the inlet capillary. Part of this container (e.g. a cylindrical volume of 10 mm x 2 mm diam. or w 30 pi) can be cooled to liquid N2 temperature in order to freeze a small amount of C02 into the volume.

Taking into account that the inlet capillary (1000 mm x 0.2 mm diam., or « 30 pi) must also be filled with gas (total volume, therefore, « 150 pi), 40 mbar gas pressure will be built up, for example, if « 250 nmol of C02 are frozen into the container and then warmed to room temperature. By feeding the gas into the ion source, the pressure will decay exponentially. Given a sensitivity of the mass spectrometer of 1000 molecules per ion and an average flow rate of 0.5 nmol s_1 through the capillary, an average ion current of « 5x 10~8 A would thus be observed over a period of » 4 min until the fractionation limit of 20 mbar is reached (half of the gas has then been consumed).

3.5.2 Continuous Flow Inlet Systems As mentioned earlier, continuous flow inlet systems [97, 102-116] use a carrier gas to transport the sample into the ion source while samples are injected, in some way, into the carrier gas stream. In fact, continuous flow inlet systems were first used to couple a mass spectrometer to a gas Chromatograph for the analysis of organic sample mixtures.

For isotopic applications also, gas chromatographs are the main source for samples in a carrier gas stream and are coupled to a mass spectrometer in one of two ways. Either the mass spectrometer is coupled to the output of a (packed column) gas Chromatograph for light gases (C02, N2, etc.), which delivers the separated products of a bulk sample preparation process (mostly a combustion) in a 30-100 ml min-1 He stream (BSIA: bulk sample isotope analysis [102-106]), or the separated products of a sample mixture in the 3-10 ml min-1 He stream at the output of a (capillary column) gas Chromatograph or of a liquid gas Chromatograph are fed through an oxidation and/or reduction reactor before their reaction products are introduced into the mass spectrometer (CSIA: compound specific isotope analysis [107-116]).


(a) t

Ü o to ¡on source ID-

Standard gas

fr

^

to ¡on source 3 -

(b) He

Figure 3.17. (a) 'Schematics of a simple continuous open split inlet (b) Schematics of a continuous flow standard gas inlet. The standard gas capillary can be moved in front of the sniffing capillary in order to mix the standard gas with the helium carrier gas

In both cases, the carrier gas flow of the gas Chromatograph is larger than the mass spectrometer can usually accept for proper linear operation. The heart of each continuous flow inlet system therefore, is the so-called 'open split' coupling device by which only a part of the sample-containing carrier gas stream is fed into the ion source (Figure 3.17(a)). An open split device is also the right place or device at which to add the standard gas to the carrier gas, in order to ensure an equal introduction process for both standard and sample (Figure 3.17(b)).

In detail, a viscous flow capillary (e.g. 0.1 mm i.d., 1000 mm length) limits the carrier gas flow to «0 .3 -0 .5 ml min- 1 and it 'sniffs', with its front end, at the carrier gas stream. The sniffing area is designed such that other atmospheric gases around will be swept away by the main carrier gas stream and thus will not enter the capillary. On the other hand, the standard gas is mixed upstream into the carrier gas. Thus, it will enter the mass spectrometer in the very same way as the sample gas.

The ratio of the incoming flow to the gas flow into the ion source is called the 'split ratio'. As the flow into the mass spectrometer is always in a relatively narrow range, the actually observed split ratio varies over a broad range (e.g. 2:1 for CSIA to 500:1 for BSIA, see Section 3.1.6), depending on the carrier gas flow of the different preparation systems which are connected on-line to the mass spectrometer via the open split device. Needless to say, there exist several


design variants of the described principle from different manufacturers. There also exist differences in the design of the open split as used for BSIA or CSIA. They are described in the following section.

3.6 SAMPLE PREPARATION DEVICES

This section deals with the conversion of samples from their original physical/ chemical state in nature into a form measurable by a mass spectrometer [102— 164].

For most applications of isotopes in geochemistry and geochronology, and for other 'solid' isotope applications, sample preparation is a manual and mostly a complex chemical separation process, preferably performed under clean room conditions. Even the mass spectrometer itself is often placed near or within the clean room. Preparation methods are traditionally carried over from laboratory to laboratory and from generation to generation of students, and the automation of sample processing has not been developed further than the usual clean chemistry treatment, including ion chromatography. Hence there has been little room for special automatic sample preparation devices up to now. Quite in contrast, all or at least a large part of sample preparation for most applications of 'stable isotopes' in the life sciences or geochemistry can be and is done automatically nowadays. Suitable devices are offered by all mass spectrometer manufacturers.

As already mentioned, the sole purpose of such a sample preparation device is to convert a (high number of) sample(s) into a corresponding elementary gas, e.g. C02 or CO for 13C and/or 180, N2 for 15N, H2 for deuterium, and S02 or SF6 for 34S, while precisely preserving the isotopic composition of the original sample.

In most cases, sample preparation is done in two steps. As the compounds to be analyzed are virtually always part of a more complex system (such as soil, sediment, atmospheric air, sea water, plant/microorganism/animal tissue or other biological material, e.g. blood or urine) or because they are not directly suited for further processing (like amino acids in a gas Chromatograph), the first step always is to preprocess and/or to isolate the compound(s) in question by all kinds of physical and/or chemical treatment (extraction, dissolution, separation, selection, derivatization, etc.). The preprocessed samples can still be complex mixtures of single compounds.

The second step is the (automatic) conversion of the preprocessed samples into the corresponding gas and the introduction of the sample and the standard into the mass spectrometer.

This section summarizes the most often used recent (automatic) methods of sample preparation for stable isotope applications (C, N, O, H, S), after suitable preprocessing if this is necessary. With these methods, high sample throughput

SAMPLE PREPARATION DEVICES 53

is achieved, but the costs per sample are moderate as compared with more traditional manual or semi-automatic methods like combustion in sealed quartz tubes [117-123] followed by cryogenic gas cleaning, reduction of water over uranium (or zinc) [140-144] or the famous Rittenberg method [124-127]. Such methods are mentioned but are not described in detail nor recommended. Their use is complicated, expensive and may well produce inaccurate and/or imprecise results in the hands of an inexperienced user.

For automatic systems, the knowledge of how to prepare a sample is an integral part of the design and of the operating software which easily produces lots of data. This data, however, can only be interpreted properly, especially in case of a malfunction, if the user has sufficient knowledge of the working principles of the robotic sample handler. Automatic sample preparation has one definite advantage, however. After establishing the detailed operational pro-cedures and parameters, often in a carefully controlled trial and error process, the results may be considered much more reproducible as compared with manual methods. If the measuring procedures are calibrated regularly by processing standards in the same way as the samples, then the results will be not only precise but also reliably accurate. This was proven in the past decade in many cases by a careful comparison [121] of the results obtained with classical manual or semi-automatic methods on the one hand and with fully automatic preparation robots on the other.

Nevertheless, mostly for financial reasons (i.e. because a modern continuous flow inlet mass spectrometer or an automatic preparation device is not avail-able), manual or semi-automatic methods are still used by many laboratories and are still recommended as 'reference' methods [135-137]. Other reasons for such conservatism might be that most early pioneering work was done by using these manual, 'classical' methods, or because such methods are (seemingly) less dependent on the particular design principles of the different manufac-turers. It would be desirable for most important automatic preparation methods to be internationally standardized by defining suitable rules (or control experi-ments) for the manufacturers. For some important isotopic systems, automatic sample preparation devices have not so far been offered or proved to be reliable. In such a case or, similarly, if the selling of a certain system will not pay back the manufacturer's costs, users are more or less left alone with the problems and must invent their own solution in this still extremely dynamic field.

A rough rule of thumb can be given for the choice of an appropriate pre-paration system. If the required precision of the ratio measurement (in 6-notation) is below 0.05%o (or 0.5%o for H/D), then a preparation method must be selected (or developed) that produces enough elementary gas for a classical viscous double inlet system (i.e. at least 0.5 pmol of C02). For less demanding applications and/or if only very small amounts of sample (< 0.1 pmol) are available, one of the very flexible carrier gas based preparation devices may or must be used together with a continuous flow inlet system.

Table 3.1. Sample preparation methods LO

N O H

Most organic and inorganic compounds, solid or liquid

(*) Sealed tube combustion (*) Sealed tube combustion

Mixture of light (trace) gases

Water

Carbonates

Silicates

Elemental analyzer or laser assisted bulk combustion (BSIA, cont. flow)

Gas chromatographic or liquid chromatographic separation of mixtures and combustion (CSIA)

Preconcentration and/or GC/separation and/or GC/combustion

Single or common acid bath (phosphoric acid)

Laser ablation (batch) Laser ablation (cont. flow)

Elemental analyzis bulk combustion (BSIA, cont. flow)

Gas chromatographic or liquid chromatographic separation of mixtures and combustion (CSIA)

Preconcentration and/or GC/separation and/or GC/combustion

(*) Sealed Ni tube preparation

(*) Pyrolysis in a carbon surplus environment ( - C O )

(*) Reaction with HgCl2 [Pyrolysis in a carbon

surplus environment, cont. flow)]

[Gas chromatographic separation of mixtures and pyrolysis (CSIA)]

[CSIA incl. pyrolysis]

Pyrolysis in a carbon environment (—* CO) [cont. flow or (*)]

Equilibration (H20)

Single or common acid bath (phosphoric acid)

Laser ablation (batch) Laser ablation (cont. flow) Chemistry assisted (F)

laser ablation

Reduction (U, Zn, Cr) (online or (*))

Equilibration (H20+Pt)

Key: (*), off-line method; [...], a method not yet mature.


Table 3.1 gives a summary of the most important preparation methods and devices for the different stable isotopes as a guideline for the main design principles described in this section.

3.6.1 Bulk Sample Isotope Analysis (BSIA) Bulk combustion can be applied for most solid or liquid organic or inorganic samples for the measurement of 13C, 15N or 34S. The main criterion for a 'suitable' sample is that combustion must be complete at combustion tem-peratures of about 1050 C. The reaction products are C02, N2 or S02 (and H20) and the isotopic species are in thermal equilibrium at such temperatures. The resulting gas, therefore, represents the average isotopic composition of the (single or composite) sample. After combustion, the gases must be purified and/ or separated from each other before being introduced into the mass spectro-meter.

For simple manual batch processing of C and N, sealed quartz tubes [117— 120] filled with a mixture of the sample and CuO as oxygen donor are heated to at least 800°C for several hours. After slow cooling, C02 and N2 are cryogenically separated from each other and from all other reaction products and are then measured using a dual viscous gas inlet system.

Manual batch preparation of 180 has also been proposed [128-134], but is difficult and tedious and does not work for all substance classes. The sample is reduced to CO in a sealed quartz tube [128] in the presence of HgCl2 or Hg(CN)2 or in a nickel tube [131]. If any (interfering) N2 is present in the resulting gas mixture, CO cannot be measured directly and must be converted into C02 by an electric discharge in a carbon surplus environment or by reacting with I205 [133]. The disadvantage in the latter case is that the 1 60/1 80 ratio of the I205 must be accurately known.

Automatic bulk combustion [102-106] is performed using an elemental analyzer (CNS analyzer or ANSCA: automatic nitrogen, sulfur and carbon analyzer). Solid samples are loaded in small tin or silver capsules. Liquid samples are dried in the capsules or are loaded together with a suitable absorbent (Figure 3.18(a)). For small sample loads (e.g. smaller than 50 pg C), it is very important to use wholly carbon-free capsules to prevent high blanks.

Up to 200 capsules are introduced into the combustion reactor (C) one after the other and flash combusted in an oxygen/helium stream, using a carousel type sample magazine. The resulting pulse of reaction gases is first transported through a reduction reactor (R) (removal of 0 2 and conversion of nitrous oxides to N2) and then through a drying tube to remove water (Mg Perchlorate). After this, they are fed into a packed column gas Chromatograph (P) which separates the remaining reaction products into pulses of pure gases (see Figure 3.18(a)). These pass through an open split device and are thus introduced into the mass spectrometer. Usually, the combustion reactor C is filled with CuO, Cr203,


Autosampler CO,

Open Split

(a) Elemental analyzer

He Standard gas

\ , \C0 2 N 2

JULr-

Mass Spectrometer

Sample:

From EA

/ / '

(b)

~zrz.> Standard gas

d/\oU to ¡on source

X \

\ He (Dilution)

Figure 3.18. (a) Elemental analyzer (ANSCA) and open split inlet. The simultaneous measurement of two gases (N2, C02) is shown schematically together with the gas dilution capabilities of the special open split inlet, as shown in (b); (b) As soon as C02 flows into the open split, a capillary with a pure He flow is moved in front of the sniffing capillary and dilutes the C02 concentration without fractionation

C03O4, Ag and quartz wool and the reduction reactor R contains Cu. The reagents are used up by sample combustion and must be replaced (about every 2000th sample for the combustion reactor) or regenerated (about every 400th sample for the reduction reactor). The temperature of the flash combustion is as high as 1800°C. Occasionally, CH4 not fully separated from C 0 2 is observed at the output of the GC, together with minor amounts of N 2 0 that pass through the reduction reactor. If 15N is to be measured, the 0 2 gas in the carrier stream which is used to support the combustion must be free from traces of N2.

Figure 3.18(b) shows schematically the design variant of a universal open split device, whose purpose is not only to reduce the elemental analyzer's carrier (and sample) gas flow. It is also used to introduce multiple standard gases (C02, and/or N2, and/or S02) during periods when no sample gas pulse is present. For this purpose, a second capillary delivering a small flow of standard gas can be moved up and down in the carrier gas stream, thus adding standard gas to the gas mixture, which then enters the mass spectrometer whenever the exit opening of the standard capillary is positioned upstream of the entrance opening into the mass spectrometer.


In most samples of natural origin, the C content is at least one order of magnitude higher than the N content. Hence a suitable amount of sample to produce enough N2 gas for a precise measurement of the N isotopes results in a much higher C 0 2 gas pulse. If both C and N are to be measured from the same sample, this C 0 2 may overload the ion source and thus prevent the proper determination of the C isotopes. Therefore, in this particular open split, an additional He flow can be arbitrarily added during the time a C 0 2 pulse is passing in order to dilute the He /C0 2 mixture. This has the same effect as if the split ratio were variable.

In order to adapt the ion source to different sample flow conditions, another possibility is to switch one or more appropriate source operating parameters (e.g. the ionizing electron current) to different values for C 0 2 and N2. This may be disadvantageous, however, because ion source conditions should be kept constant whenever possible, as discussed earlier in this chapter.

The average carrier gas flow in a typical elemental analyzer is 80-120 ml min~ ', and the gas flow into the mass spectrometer, as determined by a typical open split, is only 0.5 ml min '. Hence, on average, less than 1% of the sample is actually fed into the ion source for the measurement (split ratio > 100:1). This limits the minimum sample size for this type of sample handling to about 5 pg C or N (for 0.2-0.3%o precision). The average sample size for an easy and uncritical measuring process (<0.2%o) would be about 20-100 ug C or N. This is much less than is normally used for sealed tube combustion (some mg of C or N), but much more than is necessary for compound specific isotope analysis (max. 10-100 ng of C or N), as described further below.

Whereas the measurement of bulk 13C, 15N and 34S by automatic combustion has become a routine method in the last few years, the automatic determination of bulk 1 80 is by no means a mature process yet. Automatic analysis of the bulk 1 80 (and) 13C content of carbon- and oxygen containing samples was performed only recently [138, 139] in a modified elemental analyzer where the combustion reactor was replaced by a pyrolytic reactor (at 1080°C) to produce CO from O-containing organic samples. The reduction furnace is bypassed. If only the 1 80 content is to be measured, it has been shown that an increase of the pyrolysis temperature to 1300°C in a carbon surplus environment considerably broadens the range of compound classes that can be measured. Even water works fine. Reactors producing CO at 1000 °C and higher should not be built using quartz tubes because the CO would react with quartz and exchange I 8 0 , thus producing huge memory effects. Instead, a vitreous carbon furnace is chosen.

3.6.2 Compound Specific Isotope Analysis (CSIA)

For compound specific isotope analysis [107-116], the sequence of processes is reversed as compared to bulk sample isotope analysis. Mixtures of samples are first separated in a gas or liquid Chromatograph and then burnt (GC combustion


Oxidation Reactor NiO / CuO / Pt / °C

injector X-piece

US to FID or

closed backflush

GC Capillary

double Reduction Reactor T-piece Cu/600 °C

Mass Spectrometer

u X3 *1

O: He

He vent-

Water Separator

Liquid Nitrogen Trap for CO.

Open Spirt

Assembly

T

-5 k -i k

Ref

Open Split

Assembly

Reference Gas Inlet Assembly

Figure 3.19. Schematics of a system for compound specific isotope analysis (CSIA or GC combustion) with solvent dilution (valve 1 open), split to a FID, reference gas inlet (valve 4 open), and oxidation reactor regeneration (valve 3 open)

or LC combustion). The 13C and 15N of separated compounds in complex mixtures have been very successfully measured in this way. Mature devices are available from all manufacturers. Some 1 8 0 and 2H measurements have been published [138], but the devices for both isotopes are still in prototype status and not yet offered by all manufacturers.

Figure 3.19 shows the schematic diagram of a CSIA device [113] for C and N. From the end of the capillary column the separated compounds are flushed through a combustion furnace (alumina, 0.5 mm i.d., 300 mm long) which is filled with CuO, NiO and Pt and operated at « 950 °C. NiO and CuO are used as oxygen sources for quantitative combustion. A reduction reactor filled with Cu at 600 °C follows. Nitrogen oxides as by-products of the combustion are converted to N2 and any surplus oxygen is prevented from entering the mass spectrometer.

Water is removed by feeding the combustion products through a short capillary tube made of hygroscopic material (Nafion). As always, any C 0 2 must be quantitatively removed for the measurement of 15N. This is achieved by passing the gases through liquid a N2 trap. An open split (Figure 3.14(a)) with a typical split ratio of 3:1 (1.5 ml inflow, 0.5 ml min- 1 out) connects the combustion device to the mass spectrometer. Some design variants remove the water by cooling the inlet capillary to - 9 0 °C or below. This method, however,


has the disadvantage that obstruction of the narrow capillary by frozen water will occasionally prevent a measurement. Moreover, every time the capillary is warmed up, the water leaving the capillary may contaminate the ion source.

A state-of-the-art combustion device provides for a separate connection to a FID (for parallel real time registration of the original gas chromatogram or for the measurement of scanned mass spectra by a traditional GC/MS system via a heated transfer line) and for means to regenerate the combustion reactor automatically if its oxygen reservoir is used up. Solvent dilution is also necessary to prevent useless consumption of oxygen in the combustion furnace and/or to prevent any passage of sample through the combustion system.

Referencing of the samples to an external standard gas [114] can be done in full analogy to dual inlet system methods either by use of a switchable second open split (Figure 3.19(b)), which delivers a constant flow of C 0 2 and/or N2 reference gas in a stream of helium, or by introducing a clean gas standard through the changeover valve, if available. However, there are two important differences. First, the sample flow in a GC peak is by no means constant. Instead, it varies from zero up to a maximum and back to zero. Moreover, the average gas pressure in the ion source is not equal for sample and standard. Therefore, the highest possible ratio linearity of the ion source (e.g. 0.01%o nA~') is mandatory to achieve correct results. Second, sample/standard comparisons cannot be made arbitrarily as with the viscous flow dual inlet. Instead, a region in the gas chromatogram where no peak arrives must be chosen for the measurement of the standard. This might sometimes be difficult for a complex gas chromatogram and possibly can be done only once at the very beginning of the gas chromatogram before any sample peak arrives. However, the time between standard and sample measurements in a lengthy gas chromatogram might become too long. The way out of this dilemma is to sacrifice some peaks in the gas chromatogram by diverting them, using the solvent diverter, and to introduce a standard gas instead, via a second open split for standard gases.

A completely different way of standardization [114] is to add one or more standard samples to the sample mixture, to burn them in the same way as the sample, and to select these standards according to the condition that they elute just in between the peaks of the sample mixture. The gas Chromatograph is preferably equipped with an on-column injector for liquids (solutions) and with a split injector for gases. Improper injection is a major source of measuring errors. For instance, deposition onto the column of a part of the sample during injection may give rise to isotopic fractionation. Hence, at least a reproducible injection yield is an absolute requirement.

Another source of systematic and statistical errors may arise from derivatized samples if the yield of derivatization (dilution of the 13C/12C ratio) is not constant. For most GC peaks, the isotopically heavier fraction of a substance elutes somewhat faster than the corresponding light fraction (average time


difference « 50 ms). Therefore the isotope ratio of a GC peak must be computed from the fully integrated peak areas of the combustion products, using exactly the same time window after having shifted the lighter peak into the time position of the heavier peak during data evaluation. Sometimes, curve fitting [164] is applied before the ratios of the peak areas are computed.

A real time display of the instantaneous isotope ratio of a GC peak typically shows an S-shape with the heavier ratio at the ascending or the lighter ratio at the descending peak side [97]. This side effect has been used successfully to detect underlying unresolved GC peaks [152].

The 1 3C/ C ratios of samples that cannot be injected into a gas Chromatograph or that do not pass through a gas Chromatograph without being decomposed can be measured using a liquid Chromatograph (HPLC) [115, 116]. However, effluents at the output of the HPLC apparatus cannot be handled as easily as their vaporized counterparts in the He stream of a capillary gas Chromatograph. Instead, they are transported into a combustion oven by a moving nickel wire, as known from the earliest LC coupling experiments in organic mixture analysis when particle beam techniques were not yet available (see Figure 3.20). The loading capacity of the wire is limited, and typically only about 10% of the sample from LC can be deposited on the NiO surface of the wire.

The loaded wire passes through a drying device for evaporation of the solvent. For the complete removal of the solvent (e.g. water/methanol), 150°C is sufficient. The sample is left on the wire as a solvent-free coating. However,

LC Pump

Eluent Splitter

y Injector

IRMS

Ö-Open Split

Coating Drying Stage Assembly

Combustion Reactor

He ) Cleaning

Oven

Feed Spool

Collect, Spool

Figure 3.20. Schematics of a liquid Chromatograph combustion system


part of the sample may already have been evaporated at this stage if the chosen drying temperature was too high. The wire then enters a combustion furnace. Helium gas flushing the furnace carries the combustion products to the ion source via an open split. The wire speed is typically 7 cm s~\ and the com-bustion temperature is between 800 °C and 1000°C. In existing commercial (breadboard) devices, the length of the wire between feed and collect spools is « 1 m. There is still much room left for improvement and for gaining experience, before such devices can be used as routinely as today's GC combustion devices.

3.6.3 Isotope Ratios of Light Gas Mixtures

The determination of 13C and 15N in trace gas samples (e.g. C0 2 , N 2 0 or CH4 in atmospheric air, for global change research problems) is an excellent example of the very low limits of detection that can be achieved by the continuous flow technique combined with efficient separation methods. Whereas about 70 1 of air are needed to prepare enough clean CH4 or N 2 0 for a classical, dual inlet measurement [156], the corresponding continuous flow method [157] requires only 100 ml of air containing the trace gases (atmospheric CH4 and N 2 0 at 1.7 ppm and 300 ppb, respectively). The experimental strategy (Figure 3.21) is as follows. The trace gases of interest are pre-concentrated by cryogenic and/or chemical trapping in a relatively high flow of He carrier gas. In some cases, a given trace gas is first converted to another gas which can be better trapped or absorbed. For instance, CH4 is burnt to C 0 2 before being trapped. This selective integration-by-trapping technique allows a very high dynamic range as compared to other separation techniques,

Vent

Pretreatment

Trapping

6-port value

c Hem

^=^

Separation (GC) to ¡on source

Cryofocus

Figure 3.21. Selective preconcentration of trace gases in a dual flow rate continuous flow inlet system


such as single step quantitative gas chromatography. However, it does not produce sufficiently clean gases for direct isotopic analysis.

Given a suitable selection of trapped gases, the dynamic range of a (capillary) gas Chromatograph is sufficiently high to further separate the small amount of trapped gas mixture without overloading the column. After trapping in the high flow path, a 6-port valve switches the system to low flow operation. In this way, the gases are refrozen on a cryofocus device, e.g. a liquid N2-cooled injector. Heating the cryofocus 'injects' the samples into the GC. After GC, the separated components are fed into the mass spectrometer via an open split. Such a two stage, dual flow rate system has the advantage that the high gas flow in the pre-concentrator is able to transport relatively large amounts of gas to the traps in a short time, and the second stage can be optimized for the flow conditions required for high resolution GC and a high open split ratio.

The dual flow rate principle can also be applied for less demanding applic-ations with a smaller dynamic range of gas concentrations in a simpler way. At the (high flow) output of a packed column gas Chromatograph, the component of interest is trapped or absorbed completely [158]. Afterwards it is introduced into the ion source via a suitable low flow open split device. In this way, the loss of most of the sample in a (direct) high inflow open split can be avoided. The dynamic range of such a post-concentration method is smaller by orders of magnitude as compared with the pre-concentration techniques described above.

It is important to note that the operational details and the particular design of a suitable pre-concentration/separation (or separation/post-concentration) device may be completely different for each particular trace gas. However, the individual components of such systems are basically the same.

3.6.4 Sample Preparation of Water Reversible solvation of C02 (or H2) in water leads to the exchange of oxygen (or hydrogen) between the gaseous and the liquid phase, with a temperature dependent isotope specific equilibrium constant [165-167]. Therefore, if C02 or H2 (at atmospheric pressure) is stored together with water (with a vast molar surplus of water) at constant temperature, the 180 or 2H isotopes in C02 or H2 will, after due time, assume the isotopic composition of their corresponding species in the water [145-149], while the isotope ratio in the gas, as compared with that in the liquid, is biased by the isotope dependent equilibrium constant, which is 1.0412 for C02 and 3.7 for H2. H2 equilibration must be supported by the addition of a suitable (hydrophobic) Pt catalyst. Based on this simple equilibrium exchange reaction, automatic sample preparation devices have been developed. For the measurement of C02 and/or H2, classical viscous flow inlet systems are used. For equilibration, a minimum of « 0.5 ml (typical 3-5 ml) of water and 5 ml (typical 50 ml) of gas (0.5-2 atm) are put into small flasks and shaken at 25 °C for several hours (C02) or for several minutes (Pt-assisted H2


exchange), before the particular gas is introduced (via a water removing cold trap) into the inlet system. Typical reproducibility for measurement of the l 8 0 / 1 6 0 ratio is 0.04%o (¿-notation), whereas for D/H, considering the strongly temperature dependent H2/H20 equilibrium ( « 6%o per °C) results of approxi-mately ±0.5%o at the best are obtained. This method is not suited for all applications because the required amount of H 2 0 sample is comparatively high.

H2/H20 equilibration has also been applied in a continuous flow method to determine the (approximate) D content of urine or body water [151].

As already mentioned in the section on continuous flow systems, nl amounts of water for isotopic analysis of 1 8 0 can be converted successfully to CO by pyrolysis [138, 139] in a C surplus environment and measured automatically at masses 28 and 30. A maximum of about 2 pi of water is sufficient to produce enough hydrogen to be introduced via a classical viscous dual inlet system. For this purpose, the water is reduced under clean vacuum conditions with U, Zn or Cr as reducing agent. The devices proposed for this purpose [141-144] are mostly operated manually or semi-automatically. With U or Zn as reducing agent this is the traditional method to prepare H2 from H20, but it has particular disadvantages. Uranium produces relatively large memory effects. Zinc does not work at all if the reducing agent is not selected properly. Chromium was proposed only recently and appears to be the best choice.

3.6.5 Sample Preparation of Carbonates

The classical method to measure 13C and/or 1 80 in CaC03 is to convert the calcite into C 0 2 using water-free H3P04 (with excess P205) at 75 °C [155, 159]. C0 2 is then measured via a classical viscous flow inlet system at masses 44/45 (l3C) and 44/46 (180). This process has been automated, applying two different approaches.

First, in the 'common acid bath' method, the calcite samples (minimum 50 pg) are loaded into small boats and introduced into an evacuated bath of H3P04, using a sample carousel for up to 50 boats. The rapidly evolving C 0 2 is cryogenically freed from water and introduced into the mass spectrometer via a so-called cold finger inlet system, as described in Section 3.5.

In the second method, the calcite samples (minimum 10 pg) are loaded into small reaction vessels, and some drops of H3P04 are added to each vessel after evacuation. This 'individual acid' method includes a carousel type magazine for the reaction vessels, which are moved sequentially to a combination of a pumping port and an injection valve for the acid. Subsequent gas handling and measurement are as in the first method.

In general, the single acid bath method delivers better long-term reproduci-bility of the results (0.02-0.04%o) for both ,3C and 1 80 and does not show memory effects. This method also allows the preparation of dolomites and even the slowly reacting siderites.


Whereas it is possible with a 'single acid' device to produce enough C 0 2 for the classical viscous inlet from less than one small foraminifera, the com-bination of laser ablation and continuous flow principles allows the measurement of even smaller amounts of calcite for the purpose of isotope ratio mapping.

3.6.6 Sample Preparation by Laser Ablation

An infrared laser beam is focused onto the mineral's surface. Its energy is absorbed almost completely and thus heats up a very small volume sufficiently to decompose, for instance, CaC03 into CaO and C 0 2 [161-163], Similarly, silicates or oxides can be handled for the preparation of 1 8 0 . In a short flash, a small crater is formed in the surface and a short gas pulse is produced. In the first devices built for laser ablation, several such gas pulses were frozen in a small volume cold trap. Afterwards, the gas was cryogenically purified before its measurement via a classical dual inlet system. In a more recent device, a single gas pulse is fed directly into a capillary column by a stream of He carrier gas and is cleaned up by chromatographic separation. Then it enters the mass spectrometer via an open split interface. For laser ablation, it is important to use a laser frequency which is resonantly absorbed in the mineral, in order to achieve a sufficiently high temperature for reproducible fractionation and fast isotopic equilibration of the decomposition products (CaO and C02).

In another proposal, the tedious and dangerous classical batch process of 1 8 0 production (CO) from silicates (using fluorine chemistry) has been significantly simplified by laser ablation of the silicate in a local fluorine gas environment [171]. Other chemistry-assisted laser ablation methods for extremely small sample sizes (e.g. for isotope ratio mapping in minerals), combined with a continuous flow inlet, are in the pipeline [172] and can probably be realized by virtually any user as soon as some suitable building blocks (lasers, GC columns, ablation chambers, universal open split inlets, etc.) are available to customize the device, like a puzzle, for each particular problem. As an example, single grains of pollen might be burnt to C 0 2 in an oxygen surplus environment by one laser shot, chromatographically cleaned, and introduced via an open split device.

3.7 COMMERCIAL INSTRUMENTATION

3.7.1 Typical Instrument Configurations

Configurations of commercial isotope ratio mass spectrometers for stable isotopes (i.e. the particular combination of sample inlet systems, ion optical systems, ion collectors and data systems for a given application) have been

COMMERCIAL INSTRUMENTATION 65

almost standardized in recent years. There is virtually no difference between the manufacturers, all of whom offer modular systems that can be easily tailored to the needs of the user. The old historical saying for stable isotope measurements that it is necessary to use one dedicated instrument for each individual gas (C02, N2, etc.) is no longer valid. With a modern mass spectrometer system, it is possible to use one 'basic system' (ion source, ion optics, multiple collector) for all kinds of inlet systems and for virtually all the gases usually measured. Although in many configurations several inlet systems are connected in parallel to the basic system, only one of them is used at a time for measurements. A configuration with too many inlet systems is therefore rather uneconomical, although (ironically) such systems are often chosen in cases of too limited a budget. Hence, the old rule 'one instrument per isotope species' is definitely to be modified: one major inlet system or automatic sample preparation method per 'basic unit', for economical and high sample throughput.

On this understanding, if both CSIA and BSIA are to be applied to many samples (a not unusual situation in most life science applications of stable isotopes), it is strongly recommended to purchase two 'basic units' together with a gas Chromatograph and an elemental analyzer, respectively. One of the two 'method-dedicated' systems may now easily be configured in addition with a dual viscous inlet system if samples that have been prepared off-line must be measured occasionally.

On the other hand, it would be unwise to use a mass spectrometer that is connected on-line to an expensive, high precision single acid carbonate preparation system (reproducibility « 0.02%o) for the measurement of off-line prepared samples of questionable purity which might contaminate the ion source.

A typical modern 'basic system' for stable isotope applications has a magnetic radius of 9-12 cm and a resolution of 90-110. It has a mass range from 2 (H2) to 66 (S02) and the ion source uses an ion acceleration voltage of 3-4 kV. Its vacuum system is capable of achieving a residual gas pressure of < 10~8 mbar and it is able to cope with a helium carrier gas flow of at least 0.5 ml min- 1 (differential pumping) if one of the continuous flow inlets is used. Its abundance sensitivity is in the region of 10~5 (or is appropriately corrected by software) and its peak flatness with a universal triple collector is at least 10~4. All the associated electronic components (power supplies, etc.) show a high level of reliability. In contrast to earlier years, nowadays they are a point of discussion only rarely, if ever.

An adequate dual inlet system has a non-leaking changeover valve and a separate vacuum and waste pump. The two viscous gas flows can be adjusted for equality and the gas pressure in the motor-driven variable gas storage volumes (2-100 ml) can be measured. A suitable continuous flow inlet system should be able to introduce a sample and also a standard gas.


Electron impact ion sources of (modified) Nier type are used exclusively. Two important features must be mentioned especially: sensitivity (typically 1000 molecules per ion) and ratio linearity (typically 0.01%o nA_1), also under He carrier gas flow. The H3 factor for hydrogen measurements should not exceed 15 ppm nA-1 with a stability of at least 0.02 ppm n A 1 h_1. The typical universal collector system consists of three collectors, one center collector with a narrow slit and two collectors with wider slits. The required measuring resi-stors should be switchable (or easily exchangeable) if certain combinations of gases are to be measured (e.g. C02 and S02). Alternatively, more than three collec-tors may be used with their resistors tailored to the particular gas. The most important features of the associated ion current amplifiers are low and temperature independent baseline drift (10-20 pV h_1) and high linearity (« 10 6).

The described set of basic modules, or the 'basic configuration' which is that of virtually all modern stable isotope ratio instruments (with or without a dual viscous flow inlet and/or a continuous flow inlet), is usually connected to one or more of the following, commercially available sample inlet and/or preparation systems: (a) a multiple gas sample inlet port (including tube cracker) for the unattended

measurement of several (up to 30) samples, prepared off-line in suitable sample flasks,

(b) an elemental analyzer (for C, N and S) with multiple sample magazine, for unattended BSIA (Figure 3.22);

(c) a capillary gas Chromatograph with a combustion system for C and N (CSIA, Figure 3.23), together with a selection of injectors (mostly an on-column injector) and an appropriate autosampler. GC resolution, which is determined not by the GC column alone but by the entire combustion system, should be as high as possible.

(d) multiple flask (up to 100) water equilibration systems for C02 only or for C02 and H2. High precision systems (ss 0.03%o) have dual viscous inlets. Less demanding specifications (0.5-5%o, e.g. for medical/clinical use) can be realized with a high sample throughput continuous flow inlet system;

(e) a carbonate preparation system, using either the common acid bath method (long term reproducibility typically « 0.06%o) or the individual acid bath method (typically 0.03%o).

(f) pre-column or post-column continuous flow concentrators, using dual flow switches for isotope analysis of trace gases, possibly together with a suitable gas separating device.

Currently, there is a lack of commercially available automatic preparation systems for hydrogen- and oxygen-containing samples (other than H20). However, such systems should be commercially available soon.

Isotope ratio mass spectrometer configurations (Figure 3.24) with a thermal ion source are even more standardized. They use 8-10 kV acceleration with a


^«^

Figure 3.22. Desktop elemental analyzer/mass spectrometer system type 2020 ANCA (Europa Scientific, Crewe, UK)

mass range of 6-300 Da (magnetic radius «25 cm). Resolving power is between 400 and 500 and peak flatness is at least 10~4 within a mass range of 0.3 Da. Negative and positive ions can be measured. The residual gas pressure in the analyzer is normally below 10~9 mbar (ion getter pumps), and abundance sensitivity (without a retarding potential filter) is 2 x 10-6. Eight variable Faraday collectors and one fixed (center) collector are becoming the standard collector configuration. Ion beams through the slit in front of the center collector can be deflected at least to both a SEM (or a Daly detector) including ion counting and a Faraday collector. A retarding potential filter (10-9

abundance sensitivity) often forms the third channel behind the center slit. All ion sources contain a 13-21 sample turret.

For all associated electronics, extremely high stability is required, and is also achieved routinely (< 10 ppm). In particular, low baseline drift of the (up to nine) Faraday cup amplifiers («5-10 pV h_1) is important. Modern com-mercial thermal isotope ratio mass spectrometers undoubtedly represent the most precise quantitative mass spectrometer systems ever built. For instance, an inter-sample (external) relative precision of 4-8 ppm for the I43Nd/144Nd ratio


• G • O

O lit!

4 * « Ä 4 -

•m&

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Figure 3.23. Gas chromatograpn/combustion/mass spectrometer system with auto-sampler, Type DELTAplus (Finnigan MAT, Bremen, Germany)

has been achieved on both current manufacturers' instruments (Finnigan MAT and Micromass).

A typical modern data system is a Pentium-based PC with 12-16 MByte of memory, a 1-2 GByte hard disk and a suitable graphic printer (inkjet or laser). The data system not only performs all necessary data evaluation and pre-sentation on screen or printer; it also has virtually complete control over the mass spectrometer in two respects: First, all measurements are performed fully automatically without user intervention (sample injection and sample handling, peak switching, data acquisition, etc.) and, second, the mass spectrometer's operational parameters (e.g. precise peak center, ionization filament tempera-ture or ion source focus) are kept at an optimum.


'

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i mé *

t . •

«

Figure 3.24. Thermal ionization mass spectrometer with a nine-cup multiple collector, a Daly detector and a retarding potential filter, Type Sector 54 (Micromass, Wythenshawe, UK)

Some of the commercial MS data systems also perform automatic quality checks and error diagnostics and can regularly and automatically measure the most important specifications according to a standardized scheme. Other important features of a modern data system include archiving, editing and retrieval of acquired data and of sets of operational parameters in a suitable data management system and, last but not least, network capabilities for the exchange of data and, possibly, for connection to an on-line support facility.

3.7.2 Basic Data Evaluation

An isotope ratio mass spectrometer delivers data in the form of voltages or counted numbers of ions. Such raw data must be converted into analytically useful values and in virtually all cases, they must be manipulated, by numerical


corrections before they can be used, together with other relevant parameters, to compute the final analytical result [153, 154].

Historically, all kinds of instrumental artifacts, such as cross interference in the changeover valve [182], or inadequate abundance sensitivity or memory effects [183], required a correction of the raw data before any further data handling. With a modern mass spectrometer, most of these corrections are no longer of any concern. Others are still important [179], however, and some new methods became necessary [164] with the introduction of continuous flow techniques.

These corrections are performed routinely by the data systems which are part of every modern mass spectrometer, and the majority of users are surprisingly uncritical of the computing methods built into the systems by the different manufacturers. They concentrate instead on a user friendly interface. Un-fortunately, there is no general agreement or standardization among the manufacturers as to how these basic data manipulations are to be done. In fact, in some cases data manipulation methods are misused as commercial issues instead of being coordinated with the aim of achieving comparable and correct results. On the other hand, in many cases there is no consensus among users, either, over the best way to perform such corrections.

Relatively simple issues, such as how to subtract the electronic and analytical biases or how to compensate for interfering elements (e.g. a 87Rb contribution to a 87Sr ion current) are of concern in this respect, as well as more complex mathematical methods, e.g., (GC) peak deconvolution for continuous flow data evaluation.

Another seemingly simple problem has, in fact, not yet been solved with sufficient clarity: the definition and computation of errors in commercial mass spectrometry data systems. Two examples may illustrate the case:

(a) For stable isotope data acquisition, a sample (ratio rSA) and a standard (ratio rSx) are measured n times (n ss 10) one after the other. The set of data obtained with this procedure consists of n pairs (rSA, rST) of data which are used to compute a mean ¿-value [1000 (rSA—rST)/rST] and its standard error cr„-i.

Some data systems use the set of data twice, because they compare a certain rSA value, not only with its corresponding rST value, but also with its precursor r$r- This gives (2n — 1) ¿-results for the computation of the mean and its errors, even though only n pairs of data were acquired.

(b) The electronic baseline, e.g. in a thermal ionization mass spectrometer, is never exactly zero, and therefore must be subtracted from the measured height of the peaks before an isotope ratio can be computed. Correspond-ingly, multiple baselines must be considered for multiple collector measurements.

As the baseline is overlaid by a considerable noise voltage (« 20 pV for the usual bandwidth of the amplifiers), baseline subtraction is a major


contribution to error propagation (especially if only short integration periods are applied) and must be taken into account. How this is done in practice or how the data acquisition scheme should be designed for optimal results is different for each manufacturer.

There are many other examples of such unsolved low level problems and uncoordinated solutions which mostly lie in a grey area and are probably detected only by clever and interested users if, for instance, analytical results are inconsistent in an extreme situation, e.g. if the result is near the limit of detection. This occurs regularly if extremely small samples are measured or if the highest possible precision and accuracy are required.

Also, at the second level of correction and/or data manipulation which deals with ion corrections for stable isotope measurements and with fractionation correction (using internal or external standards) for thermal ionization measure-ments, no general consensus among users and manufacturers has been reached up to now, although such corrected data are input data for the computation of the final analytical result.

Although the problem of ion corrections [153-155] (and stable isotope fractionation [178]) may be considered to be finally solved [184], but is not yet implemented and documented in all software packages, fractionation correction of thermally produced ions remains an open issue. In fact, all efforts to find the 'correct' way to correct for fractionation seem to be vain, because, so far, in no practical case have all relevant parameters of the evaporation process except the masses ever been considered important for a given experiment. This appears to be the chief stumbling block in obtaining highly precise, accurate and compar-able results (< 10 ppm) in measurements of a given sample by different users.

For the highest level of data manipulation, i.e. for computation of the final analytical result, not much help is offered by any current commercial data system; nor have users reached to a general consensus on how certain methods should be handled computationally. The standardization of complete analytical methods, e.g. the analytical calibration of measured (and mostly biased) results against a standard sample [114, 176-178, 155, 168-170], is of concern in this respect, as is the seemingly simple question of how to subtract the always present analytical blank, including appropriate error propagation methods [73, 85, 86, 179-181, 185].

Nevertheless, the situation is not too bad, although the results of published collaborative tests [e.g. 173-175] (whereby the same samples are measured in many laboratories), show that much room is left for improvement.

3.7.3 Specifications

Instrument specifications help to answer the question of whether an analytical problem can be solved using a given mass spectrometer. For instance, the molar


sensitivity of the ion source (together with the expected analytical blank) gives at least a rough estimate of the amount of sample required for a certain experiment or, as another example, the known overall system behavior (ion source stability, baseline stability, magnetic field stability, GC resolution, etc.) may have a considerable influence on the design and layout of the experimental strategy for solving a given analytical problem. Furthermore, regular verifi-cation of the basic specifications is a necessary step towards good analytical quality and, last but not least, specifications are important factors in the complex decision process when acquiring a commercial instrument. Unfor-tunately, the (simple) rules as to specifications should be defined and measured are by no means standardized or coordinated among manufacturers. The result is that the published 'specifications' of different manufacturers often cannot be compared directly. General rules are therefore presented in this chapter on how to measure (or to verify) the most important instrument specifications.

3.7.3.1 Thermal Ionization Mass Spectrometers Sensitivity: To determine sensitivity, one measures the number of atoms (or molecules) for certain isotopic species needed to register one ion at the collector. This is done by loading a known amount (n moles) of sample (pure standard samples are generally used for this purpose) onto the filament and by integrating the resulting ion current/time function f(i, t) until the sample is completely evaporated.

The ion current/time integral gives the total charge (the total number of ions) registered for the loaded number n of atoms or molecules. Therefore:

. . . „ nMF .„„, Sensitivity 5 = . NJ (31 )

where M = 6.02 x 1023 and F = 1.6 x 10~19 [A s]. Typical values are Su = 700 atoms per ion (uranium) and SSr = 10 atoms per

ion (strontium). This parameter depends heavily on the process of loading the sample and thus should be used with due scepticism.

Such a measurement can also be used to determine the so-called overall transmission of the mass spectrometer, which is defined as the percentage of ions reaching the collector as compared with the number of ions leaving the ionization filament. For this purpose, a very small Cs sample (with 100% ionization efficiency, if evaporated very gently) is used.

Peak flatness: Flat peak tops are needed to compensate for small fluctuations of the magnetic and/or the electric field of the sector optics during a long high precision experiment, and to compensate for the small radial differences in mass dispersion in dynamic multiple collector experiments. Peak flatness is


defined as the percentage deviation from the ideal, horizontal flatness of a peak, as related to total peak height, and is given for certain mass ranges, e.g. AM = ±0.15 Da.

Peak flatness is not easy to measure: A low noise ion current of high stability and, preferably, very low time drift is generated, and a minimum of three measurements (A, B, C) is performed at the flat area of a peak by slight variation of the magnetic field: A and C on the left and right side ( i.e. — 1/2AM and + 1/2AM, respectively, from the center) and B in the center of the flat area. A large number of back and forth measurements (ABC, CBA, ABC, CBA, . . . ) is performed. The ' ß ' measurements of two consecutive adjacent triplets are used to correct the A and C values of each triplet for a possible ion current drift, and then AIB and CIB are computed for each (drift-corrected) triplet of values. These values approach a value of 1 with increased flatness of the peak. The averages (a, c) of the many AIB and CIB values and the average (b) of all B values are used to compute peak flatness, F = 100(ö — c)/b (%).

Cup efficiency: The two methods (single collector and multiple collector) for the measurement of cup efficiency ratios have already been discussed in Section 3.4.1. If the single beam method is used, the measurement resembles the method described for the measurement of peak flatness, including accurate drift correction. An ion beam which is as stable as possible, or which shows only slight linear drift, is repeatedly measured in two cups A and B. The baseline is also measured. Two consecutive measurements on cup A are used to drift-correct the measurement on cup B. Then the ratio of the drift-corrected voltage measured at cup B to the voltage at cup A equals the ratio (RB/BVÍRA/A), R and/ being the measuring resistor and the cup efficiency, respectively. The described experiments for both peak flatness and cup efficiency are equivalent to the consecutive single collector measurement of two ion beams in one cup, i.e. the classical way to measure isotope ratios without a multiple collector. Hence it is apparent that the precision of the peak flatness and the cup efficiency measurements cannot be better than the precision of an isotope ratio measured on a single collector in the old classical way by peak jumping. This precision is, at best, in the region of ± (10—15) ppm. For details of the multiple beam calibration method, see [88, 89].

Recording system specification: Baseline specifications [baseline noise (pV) and baseline drift (pV h 1 ) ] of the DC amplifiers are the most important parameters for Faraday cup measurements. Both are simply determined by measuring the electronic baseline repeatedly (e.g. 30 times within 4 min), i.e. with an integration time of 8 s per measurement, and repeating such a group of measurements 15 times within 1 h. By means of a linear regression (and its variance) through all values, one obtains the desired results (e.g. 20 pVand 5 pV h~'). The noise depends on the square of the integration time (time constant) and should therefore be measured for at least three different time constants.


For secondary electron multipliers, the analog and counting conversion bias, the linearity for small counted ion currents and the dead time of the counter for linearity correction at high ion currents must be known. Bias and small current linearity can be measured with sufficient accuracy by using a suitable three isotope system with two components of high (and known) abundance and one isotopic component with a known trace concentration, e.g. a U-500 standard sample. The two highly abundant species are measured in two Faraday cups of a multiple collector, and the trace component is simultaneously counted in the SEM that is normally located behind the center cup of the multiple collector. Data evaluation resembles a static triple collector measurement. However, instead of computing the normalized isotope ratio, the SEM bias is obtained by comparing the known isotope ratio with the measured and the fractionation corrected ratio.

Similarly, the low current linearity is simply determined by varying the ion current of the sample and computing a corresponding bias for each beam intensity. Dead time measurements and corrections are described by Hayes and co-workers [85, 86].

Internal and external reproducibility and accuracy: In a thermal ionization measurement, each sample is in the ion source long enough for a high quality test for internal reproducibility to be made. It is usual to measure at least 10 blocks of 10 ratio measurements each to compute the internal reproducibility (or precision) as the standard deviation (CT#-I) of the 100 single values after appropriate low level corrections (baseline drift, interfering elements). Typical error values are 10-30 ppm for Nd.

External precision is similarly measured using at least 10 independent samples of equal size and quality and by applying a full loading and heating procedure separately for each sample. The standard error ON-I of the mean of the results of the 10 samples runs is defined as the external reproducibility. A typical value is 15 ppm (2 x standard error).

The test for external reproducibility should be used at the same time to check the accuracy of the ratio result, at least for such internally normalizable elements as Sr or Nd, which should not exceed ± 15 ppm. For this purpose, standard samples with known or, at least, with agreed isotopic composition can be obtained from several sources. For such elements as Pb or U, which must be normalized externally (with much less accuracy as compared with Sr or Nd), a so-called system calibration checks the overall linearity and the external reproducibility (each « 2 x 10~4) of the system. For this purpose, standards of known isotopic composition over a wide range are available.

3.7.3.2 Stable Isotope Mass Spectrometers Sensitivity The experimentally most useful parameter from the several definitions for the sensitivity of a mass spectrometer is the so-called


molar sensitivity, as described in Section 3.3.1. It is measured in a two step procedure.

First, the ion current/pressure relationship for a viscous gas flow inlet as described in Section 3.5.1 is established. Second, a small volume (max. 500 pi) is connected to the viscous flow capillary and filled with (C02) gas. It is important to measure the size of the small volume in a separate (gas expansion) experiment instead of calculating its size from its dimensions, and to include the volume of the capillary itself, as described in Section 3.5.1. During the flow of the gas into the ion source, the pressure in the small volume decays exponentially and the consumed amount of gas can be computed at any time by using the known ion current/pressure relationship and the known size of the small volume. The amount of gas flowing to the ion source within, for instance, 5 min is then related to the current-time integration for the same time span at the output of the (mass 44) Faraday cup. Thus, molar sensitivity in A s mol- 1

and/or in molecules per ion can be computed. The slope of the decay function (ion current versus time) of this experiment

apparently depends on the ion current itself, and thus represents the dependence of the ion current on the inflowing amount of gas. The knowledge of this dependence is important for the production of a suitable viscous flow cali-bration capillary of known flow rate which is then used to determine ion source sensitivity under continuous flow conditions. For instance, with a 0.025 mm i.d., 2 m long capillary, a N \ ion current of approx. 30 nA is obtained for a source sensitivity of 1000 molecules per ion if air is introduced through the capillary. If an ion current of size i is produced by a flow rate of/ = An/At (nmol s_1) into the source, the molar sensitivity Em is simply given by Em = i/f (A s nmol 1 ) .

It is worth mentioning here that the minimum amount of sample (e.g. in nmol) required to achieve a given precision (internal reproducibility) might be a useful piece of information for the planning of an experiment. However, this has nothing to do with the 'sensitivity' of the system, as defined above, although at least one manufacturer still occasionally specifies mass spectrometer 'sensitiv-ity' in pg. Such a number can at best be a rough estimate for the limit of detection which may be expected in a certain experiment, as long as the true variation of the analytical blank is not known.

Ratio linearity: For both viscous flow inlet and continuous flow, the ratio linearity LR is simply determined by measuring an isotope ratio rx or r2 with two ion currents of size i\ or i2, respectively:

L R = ^ ^ ( n A - 1 ) (32) 1*2 - t\)

With a dual viscous inlet system, the same gas is used in both the sample and the standard gas storage volume, and the pressure in one of the volumes is tuned to produce either the same ion current as the other (i\) or an ion current


differeng by some nA (i2). Similarly, for continuous flow devices, ratio linearity (under carrier gas conditions) is measured by simply introducing two different amounts of sample, one after the other.

//3 factor. The mass 3/mass 2 ratio is measured over a certain, not too small range of mass 2 ion current (e.g. 3-8 nA). The ratio, plotted versus the mass 2 ion current, is a straight line (see Section 3.3.1). The slope of this line (measured in nA' 1 or ppm nA- 1) is the H3 factor. If a slightly curved line is observed, the gas inflow through the capillary is probably not viscous.

Such a measurement can be fully automated and takes no more than 10 min. It is repeated over a period of 3 h with 3 or 4 measurements per hour in order to obtain the important value for the stability of the H3 factor with time. A typical value is 10 ppm nA - 1 , with a stability of 0.02 ppm nA - 1 h_ 1 .

Electronic baseline stability and system stability: These values are measured as described in Section 3.3.2 on thermal instruments.

Important method-specific tests: For each particular sample preparation or sample inlet system, at least two tests should be performed.

(1) The external reproducibility of the overall system (inlet system plus mass spectrometer) is determined by measuring not less than 10 independent and equal aliquots of a sample of known, standard-like quality. The mean value of all measured results and the standard deviation are computed. This allows a much better assessment of the device's quality than the standard error of the mean which is sometimes specified.

Such a test can also be used (and is recommended) to check the accuracy of a method if a certified standard is used as the sample. However, it must be noted that it is extremely important to check whether the particular working standard that is always needed for calibration (¿-notation) has been independently and correctly intercalibrated against one of the primary standards, e.g. VPDP (for 13C) or VSMOW (for D and I 8 0) .

(2) Weighed aliquots of a known sample as above, ranging in weight over at least one order of magnitude, are measured, preferably not in a sequence sorted by weight. The isotope ratio obtained for all samples should be constant, i.e. independent of the size of the sample after blank correction (i.e. the system should be linear). A similar test also allows the assessment of the blank and of the limit of detection and it also answers the general question as to how much sample is required to achieve a certain reproducibility.

If necessary, the internal reproducibility should be assessed by repeatedly measuring the very same sample. This test is meaningful only for viscous flow (dual) inlet methods. For continuous flow methods it is normally not performed.

REFERENCES 77

Internal reproducibility is also computed as the 'standard deviation' of the mean ((TJV-I)-

For a GC combustion device, peak resolution and peak tailing of the GC instrument and of the combustion system as a whole may be tested by the introduction of a mixture of phytane, pristane and the corresponding C i 7 and Ci8 n-alkanes. Two doublets are abserved which should be fully separated.

Finally, it is important to note that these tests, once performed, do not release a user from repeating them at regular time intervals and, in particular, from intercalibrating the complete system with other laboratories.

REFERENCES

Many of the papers on mass spectrometry hardware cited here no longer represent the current state of the art at the time of publication of this review. This can be obtained in detail from the mass spectrometry companies. Nevertheless, these publications, many of which are historical milestones, were selected for two reasons:

(a) the instruments described in these papers are the solid basis on which modem commercial mass spectrometry is undoubtedly founded, and (b) they contain many theoretical aspects or principles which are independent of the current state of the art.

As a whole, including the many additional references in each of the cited papers, this selection should give a hopefully complete overview of the literature on isotope ratio mass spectrometer hardware for the past 50 years.

[ 1 ] L. Kevin, Mass Spectrometry, in Advances in Electronics and Ion Physics, Vol. Ill, L. Marton (Ed.), Academic Press, New York, 1956.

[2] H. Hintenberger and L. A. König, in Advances in Mass Spectrometry, J.D. Waldron (Ed.), Pergamon Press, 1959, p. 16.

[3] A. Septier (Ed.), Applied Charged Particle Optics, Vols A, B and C, Academic Press, New York, 1980.

[4] H. Nakabushi, T. Sakurai and H. Matsuda, Int. J. Mass Spectrom. Ion Processes, 52,319(1983).

[5] H. Nakabushi, T. Sakurai and H. Matsuda, Int. J. Mass Spectrom. Ion Processes 55, 291 (1983).

[6] H. Wollnik, Optics of Charged Particles, Academic Press, New York, 1986. [7] W.G. Cross, Rev. Sei. Instrum., 22, 717 (1951). [8] S.J. Prasser, Int. J. Mass Spectrom. Ion Phys, US, 241 (1993). [9] W. Compston, I.S. Williams and S.W.J. Clement, 30th Annu. Conf. Am. Soc. Mass

Spectrom., 1982, p. 593. [10] W. Compston, I.S. Williams and C. Meyer, J. Geophys. Res., 89, B525 (1984). [11] J.S. Cottrell et al. (Inventors), Eur. Patent Appl. 82306491.0 (1982). [12] F.A. White and T.L. Collins, Appl. Spectrosc, 8, 169 (1954). [13] F.A. White, EM. Rourke and J.C. Sheffield, Appl. Spectrosc, 12, 46 (1958). [14] L.A. Dietz, Rev. Sei. Instrum., 30, 235 (1959). [15] L.A. Dietz, Rev. Sei. Instrum., 31, 1229 (1960).


[16] H.W. Wilson, R. Munro, R.W.D. Hardy and N.R. Daly, Nucl. Instrum. Methods, 13, 269 (1961).

[17] CR. Lagergren and J.J. Stoffel(s), Int. J. Mass Spectrom. Ion Phys., 3,429 (1970). [18] D.H. Smith, W.H. Christie, H.S. McKown, R.L. Walter and G.R. Hertel, Int. J.

Mass Spectrom. Ion Phys., 10, 343 (1972). [19] R.N. Gall, N.S. Pliss and A.P. Shcherbakov, in Advances in Mass Spectrometry,

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(1992). [21] EG. Ruedenauer, Rev. Sei. Instrum., 44, 1487 (1970). [22] N.J. Freeman and N.R. Daly, J. Sei. Instrum., 44, 956 (1967). [23] K. Habfast and H.-J. Laue, Los Alamos National Laboratory Report, LA-12522-

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CHAPTER

ADVANCED ISOTOPE RATIO MASS SPECTROMETRY II: ISOTOPE RATIO MEASUREMENT BY MULTIPLE COLLECTOR INDUCTIVELY COUPLED PLASMA

MASS SPECTROMETRY ANDREW J. WÄLDER

VG Elemental, Winsford, Cheshire, UK

4.1 INTRODUCTION 83 4.2 MULTIPLE COLLECTOR-INDUCTIVELY COUPLED PLASMA

MASS SPECTROMETRY 84 4.3 THE IMPORTANCE OF ABUNDANCE SENSITIVITY FOR THE

DETERMINATION OF MINOR ISOTOPIC ABUNDANCE 87 4.4 SAMPLE TRANSMISSION 89 4.5 MASS BIAS AND THE MEASUREMENT OF ISOTOPIC

RATIOS 90 4.6 MEASUREMENT OF THE ISOTOPIC COMPOSITION OF

ENVIRONMENTAL LEAD SOLUTIONS 96 4.7 MEASUREMENT OF THE ISOTOPIC COMPOSITION OF

GEOLOGICAL SOLUTIONS 97 4.8 THE ISOTOPIC RATIO MEASUREMENT OF URANIUM 99 4.9 MC-ICP-MS FOR THE DETERMINATION OF ATOMIC

WEIGHTS 103 4.10 DIRECT ISOTOPIC ANALYSIS OF SOLID SAMPLES BY

LASER ABLATION MC-ICP-MS 103 4.11 CONCLUSIONS 107 4.12 ACKNOWLEDGEMENTS 107 REFERENCES 107

4.1 INTRODUCTION

The measurement of highly precise and highly accurate isotopic ratios is of paramount importance to industrial and academic workers within nuclear, geological, environmental and health-related industries. The nuclear industry requires isotopic ratio measurements in a number of key areas, for example, to monitor the isotopic composition of fuel during manufacture, to monitor its sub-sequent burn-up within the reactor, to quantify the levels of secondary fission products, and for environmental monitoring within the plant and surrounding

Modern Isotope Ratio Mass Spectrometry Edited by I. T. Platzner © 1997 John Wiley & Sons Ltd

84 ADVANCED ISOTOPE RATIO MASS SPECTROMETRY II

areas. The geological community utilize isotope ratios to determine the origin and evolution of terrestrial and extra terrestrial bodies and as an aid in the exploration for metals and natural fuels. The isotopic signatures of elements contained within the host rock contribute to an understanding of the relative and absolute ages of the strata. Hence reasoned assumptions can be made related to the formation, composition and possible exploitable yield of the rock. Environmental control has developed into a major industry over the last 10 years. An increasing number of studies demand isotopic information to identify and quantify the source and magnitude of elemental pollutants. Biomedical health studies routinely utilize isotopic information as a part of tracer studies to assess body function and to determine the level of bodily contamination.

The choice of analytical instrumentation for accurate and precise measure-ment of the isotopic composition of solutions has, over the last 20 years, been restricted to thermal ionization mass spectrometry (TIMS). This type of instrumentation is generally considered as the benchmark technique for isotope ratio measurements. The latest generation of these instruments is equipped with multiple detectors that allow isotopic measurements to an accuracy and pre-cision of within 0.002%. This technique does, however, have its disadvantages. Analysis is complicated by labour intensive sample preparation, tedious sample handling protocols, long analysis periods and inflexible analytical routines. To produce a sample in sufficiently pure form requires considerable preparation and separation chemistry. A volume of this pure sample must be deposited onto a metallic filament and evaporated to dryness, and each metallic filament containing a single sample must then be loaded into the source vacuum chamber of the mass spectrometer. This chamber must then be evacuated to a pressure of less than 1 x 10~7 mbar. A gradually increasing electric current is then passed through the filament to vaporize and ionize the sample, and the resulting ionic cloud is then accelerated into the mass spectrometer for isotope ratio analysis. Because there is a maximum temperature to which a metallic filament can be raised the range of elements that can be analyzed by this technique is limited.

As within any analytical area, there is an ongoing requirement to improve the range and quality of isotope ratio measurements and thereby allow a more detailed, conclusive and widespread interpretation of their meaning. Inevitably, any one piece of scientific instrumentation will ultimately approach its measurement capability. Further analytical opportunities and improvements will then require the development of a new type of measurement device.

4.2 MULTIPLE COLLECTOR-INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

An inductively coupled plasma (ICP) ion source has considerable advantages over a thermal ion source. It can tolerate relatively high levels of sample

MULTIPLE COLLECTOR-INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY 85

Nine Farada Detectors

Daly detector

Pulse counting photomultiplier

High mass Faraday

Electromagnet

Ion path

Differential pumping apertures

Electro static Filter

Exit lens M Electrostatic analyzer

Entrance lens

Figure 4.1. MC-ICP-MS instrument layout

Beam shaping^ lenses

-Inductively coupled plasma

-Sampling cone -Skimmer cone

impurities, it allows immediate sample analysis, and it can efficiently ionize over 95% of the elements in the Periodic Table. ICP source mass spectrometers (MS) equipped with a quadrupole mass filter and a single multiplier detector are widely used for generating elemental fingerprint information. Their use as isotope ratio instruments is also well established. However, such measurements are limited by the necessity for sequential measurement of isotopic abundance. Analytical precision is therefore limited by the inherent instability of the ion signal generated by the ICP source. The accuracy of the isotopic measurement is also limited by this instability, and is further complicated by mass bias effects exhibited by the ion source, the quadrupole mass filter and the multiplier detector.

In an attempt to quicken, simplify and greatly extend the range of isotopic ratio measurements and their applications, a multiple collector- inductively coupled plasma mass spectrometer (MC-ICP-MS) was conceived. This instru-ment comprises an ICP ion source, a double focusing magnetic sector mass analyzer, and a detection system equipped with nine Faraday detectors and a single ion counting Daly detector. A detailed description of the instrumentation is given by Wälder and Freedman [1]. This instrument is manufactured by VG Elemental and is marketed under the trade name of the VG Plasma 54.

The major components of this new mass spectrometry system are illustrated in Figure 4.1. Samples in solution form are generally contained within a test tube positioned in an auto-sampling rack. Solution is drawn from the test tube by means of a peristaltic pump and is directed to some form of nebulization system. The nebulizer converts the liquid into an aerosol which is directed into the central channel of the ICP. The high temperature within the plasma


desolvates, dissociates and ionizes the constituents of the aerosol. The resultant ions pass through two small circular orifices, known as the sample and the skimmer cone, and these separate the atmospheric plasma from the vacuum chamber of the mass spectrometer. These orifices are held at approximately 6000 V, and therefore impart this potential to the ions as they travel through them. The ions then undergo some beam shaping to convert their circular profile to a rectangular shape and are directed into a small electrostatic analyzer. This compensates for the relatively large ion energy distribution typical of an ICP ion beam and is essential to produce flat top peak shapes. The ions are then accelerated into an electromagnet where they undergo separation into their isotopic components. Each isotope is simultaneously focused into one of nine Faraday detectors, which, via its amplification circuitry gives an ion current proportional to the isotopic abundance.

The use of multiple Faraday detectors allows a simultaneous isotopic measurement, which eliminates the effects of signal intensity variations and allows highly precise isotopic ratio measurements. The combination of the electrostatic analyzer with the electromagnet creates a double focusing mass spectrometer giving characteristic flat top peak shapes. This allows highly accurate quantification of isotopic abundance.

The MC-ICP-MS detector system comprises nine Faraday detectors posi-tioned side by side, perpendicular to the optical axis of the mass spectrometer. Each detector is independently adjustable in this plane to allow the simul-taneous isotopic detection of most elements. The electromagnet will focus an isotope at 540/mass (mm) distance from its neighbor. Thus, the Faraday detec-tors must be positioned approximately 2.2 mm apart to measure simultaneously the isotopes of uranium and 6.3 mm apart to measure simultaneously the isotopes of strontium. The maximum separation of the conventional Faraday detector array is approximately 10% of mass. This maximum width of isotopic detection has been considerably increased with the inclusion of an additional Faraday detector positioned on the high mass side of the flight tube (Figure 4.1). Use of this detector allows the simultaneous measurement of uranium and lead. Initial measurements utilizing this detector have been discussed by Halliday et al. [2].

Typical MC-ICP-MS peak shapes characteristic of a double focusing instrument are shown in Figure 4.2; this represents a magnet scan between masses 143.7 and 144.3 and illustrates the simultaneous detection of all the isotopes of neodymium.

Because of its excellent linearity, stability and reproducibility, the Faraday detector is the detector of choice when isotopic ratio measurements of high precision and high accuracy are required. It is not, however, the most sensitive of detection devices. Thermionic noise within the amplification circuitry contributes a baseline noise level equivalent to approximately 10000 ions per second. This poses no problems when large ion beams are being measured but it

THE DETERMINATION OF MINOR ISOTOPIC ABUNDANCE 87

Faraday Detector

H4 H3 H2 H1 Ax L1 L2 L3 L4

Ion beam Intensity Max. Scale (Amps)

1.0E-11 1.0E-11 1.0E-11 1.0E-11 1.0E-11 1.0E-11 1.0E-11 1.0E-11 1.0E-11

143 70 Mass 144 30

Figure 4.2. MC-ICP-MS magnet scan illustrating simultaneous detection of neo-dymium isotopes

does prevent the measurement of very small ion currents. In recognition of this, a Daly ion counting detector is mounted behind the Faraday array and is used for the measurement of very small ion currents. The Daly system works by attracting the positive sample ions onto a highly polished aluminum knob held at — 20 kV. Each ion that hits the knob releases approximately six electrons, which are repelled from the knob by the —20 kV potential. The electrons interact with a scintillator device and release photons, which are recorded with a pulse counting photomultiplier. The Daly system has a background count of less than 20 counts per minute. It is therefore used simultaneously with one or more Faraday detectors to measure isotopes of very small abundance.

4.3 THE IMPORTANCE OF ABUNDANCE SENSITIVITY FOR THE DETERMINATION OF MINOR ISOTOPIC ABUNDANCE

Abundance sensitivity is an important consideration in isotope ratio mass spectrometry. Ions travelling toward the detector system will undergo elastic and inelastic collisions with residual gas molecules in the mass spectrometer. The number of these collisions and hence the magnitude of their effect is directly dependent upon the operating pressure of the vacuum system. Ions that are involved in collision processes will arrive at the detector at a different position to that predicted. The net effect of these collisions is a pressure tail extending either side of the true mass position. The effects of pressure tailing become critical when measuring very high isotopic ratios, i.e. when measuring a minor isotope which is adjacent to a major isotope. If the operating pressure is


too high, the tail associated with the major isotope will contribute to the minor isotopic measurement, resulting in a false isotopic measurement.

As samples are introduced into the MC-ICP-MS at atmospheric pressure, several stages of differential pumping are required sequentially to reduce the analyzer pressure to give an acceptable level of abundance sensitivity. Essentially, the MC-ICP-MS is divided into four separate chambers, each chamber being linked to the next via a small aperture. Ions enter the mass spectrometer through a circular orifice 1.0 mm in diameter known as the sample cone, and several mm behind this cone is another circular orifice of 0.5 mm diameter known as the skimmer cone. The region between these two cones is evacuated by a large rotary vane pump and operates at a pressure of » 1 mbar. The second chamber houses the beam shaping lenses and is pumped by a 600 1 s~' turbomolecular pump, this region operates at a pressure of about 2.0 x 10~4

mbar. Ions exit this chamber through a rectangular slit 1.0 mm wide and 10 mm high. The third chamber houses a lens stack used to direct the ions into the electrostatic analyzer; it is pumped by a 360 1 s_ 1 turbomolecular pump and operates at a pressure of about 3 x 10~8 mbar. Ions exit this chamber through a slit 0.3 mm wide and 10 mm high. The fourth and final chamber houses the elec-trostatic analyzer, a lens stack to direct the ions into the electromagnet, the flight tube and the multiple detector system. This region is pumped by two 3601 s_1 turbomolecular pumps and operates at a pressure of less than 5 x 10~9 mbar.

Abundance sensitivity is generally expressed as the contribution of the 238U isotope to the baseline at mass 237. A conventional thermal ionization mass spectrometer achieves an abundance sensitivity of less than 2 ppm at mass 237. This means that the magnitude of pressure tailing at mass 237 is less that 2 ppm of the ion signal at mass 238. Through the use of differential pumping stages and the sequential reduction of operating pressure, the MC-ICP-MS system is able to deliver an equivalent performance. A magnet scan using the Daly detector between masses 233.5 and 237.5 is shown in Figure 4.3. The tailing

1.0E-15

Ion beam Intensity

Max. Scale (Amps)

233.50 Mass 237.50

Figure 4.3. MC-ICP-MS abundance sensitivity scan (NIST U010 reference material)

I 2 3 4 U

^

"\l 2 3 6 u

237^00,

SAMPLE TRANSMISSION 89

1.0E-15

Ion beam Intensity

Max. Scale (Amps)

233.50 Mass 237.50

Figure 4.4. MC-ICP-MS abundance sensitivity scan with rear electrostatic filter (NIST U010 reference material)

from the 238U peak is clearly seen and is quantified at approximately 2 ppm in this example.

An abundance sensitivity of 2 ppm is acceptable for the vast majority of isotopic ratio determinations. There are, however, requirements to measure isotopic ratios of greater than 100 000. To do this accurately, the level of abund-ance sensitivity must be further reduced. This can be accomplished through the use of an electrostatic filter positioned between the main Faraday array and the Daly detector. The filter will remove ions that are not of the correct mass and hence result in significantly reduced abundance sensitivity. A schematic diagram of this filter illustrating its connection to the main analyzer is shown in Figure 4.1. A magnet scan between masses 233.5 and 237.5 through the additional electrostatic filter and using the Daly detector is shown in Figure 4.4. Use of this electrostatic filter is able to reduce the abundance sensitivity to less than 200 ppb.

4.4 SAMPLE TRANSMISSION

The level of isotopic ratio precision obtained by MC-ICP-MS is dependent purely upon ion counting laws. Hence the quantity of sample available and the transmission of the mass spectrometer are critical in determining the quality of the measurement. As sample quantities are often limited, it is important to maximize ion transport into and through the mass spectrometer. Conventional nebulization processes return 99% of the solution to the drain pipe, and « 1 % of the solution reaches the plasma as fine droplets several pm in diameter. About 1% of these droplets are able to pass through the sample and skimmer cones and enter the mass spectrometer. Ions that are able to pass this stage will reach the detectors.

2 3 4 u

J 1

2 3 6 U 2 3 6 u

1 1 237.00 1

9 0 ADVANCED ISOTOPE RATIO MASS SPECTROMETRY II

The quantity of sample used during a sample measurement will depend upon the concentration of the solution, the rate at which the sample is delivered to the nebulizer and the measurement time. A solution containing an element at a concentration of 1 pg ml - 1 will yield a total ion current of (1-10) x 10~n A at the detectors. Sample delivery is usually restricted to 0.3 ml min- 1. Sample analysis time is determined by the level of measurement precision required and typically varies between 2 and 20 min per sample. High precision isotope ratio analysis with conventional nebulization systems therefore utilizes approxi-mately 1 pg of sample per analysis.

Several alternative sample introduction systems that increase the efficiency of sample usage are available. One such device is the Mistral desolvating nebulizer. This system increases the amount of sample ionized in the plasma by removing most of the water from the nebulized aerosol cloud. This is achieved by directing the aerosol through a sequential heating and cooling process. The net effect of this desolvation is an increase in ion transmission by a factor of 10-20, and the use of this unit thus allows the analysis of much smaller sample quantities. Its use in conjunction with MC-ICP-MS has been described by Walder et al. [3]. This publication details high precision isotope ratio analysis of uranium, lead and neodymium reference materials utilizing 30-300 ng of sample.

More recent experimentation utilizing a micro concentric nebulizer (MCN) has reduced sample requirements still further. High precision isotope ratio analysis is now possible with 5-50 ng of sample. This trend towards lower and lower sample quantities and the goal of pg levels of analysis will soon be realized as sensitivity enhancements achieved with conventional ICP-MS over the last 10 years are applied to MC-ICP-MS.

4.5 MASS BIAS AND THE MEASUREMENT OF ISOTOPIC RATIOS

As with all mass spectrometry systems, ions that enter the MC-ICP-MS experience mass bias effects, and this bias favors the transmission of the heavier isotope into the mass spectrometer. This phenomenon is controlled entirely by processes occurring in the source interface region, and the dominant cause is believed to be space charge effects in the region of the skimmer cone [4]. As the double focusing mass analyzer and the Faraday detectors do not exhibit mass bias, studies of these effects are interesting for their own sake, as this instrument provides a unique opportunity to qualify and quantify plasma source mass discrimination. As fresh sample is continuously introduced into the ICP source, mass discrimination is time independent; this greatly simplifies the correction of mass bias and hence the accurate quantification of isotopic abundance. This contrasts with TIMS, which exhibits rather complicated time dependent mass fractionation effects.

MASS BIAS AND THE MEASUREMENT OF ISOTOPIC RATIOS 91

4.5.1 Internal Mass Bias Correction Mass discrimination must of course be determined and corrected for if an accurate isotope ratio measurement is to result. Measurements involving internal normalization routines are widely used by isotope scientists using TIMS. Internal normalization can be applied only to elements that contain stable (non-radioactive) pairs of isotopes and thus have a constant universal value. In the case of neodymium, it is generally accepted that the stable 146Nd/ 144Nd ratio equals 0.7219. Comparison of the measured l46Nd/I44Nd value with the reference figure of 0.7219 is used to determine the magnitude of mass bias. This mass bias figure is then applied to the isotopic ratio measurement of 1 4 2 N d / 1 4 4 N d ; 1 4 3 N d / 1 4 4 N d . 1 4 5 N d / 1 4 4 N d ; 1 4 8 N d / 1 4 4 N d ) a n d 1 5 < > N d / 1 4 4 N d

MC-ICP-MS experiments with neodymium and hafnium isotope reference materials utilizing internal normalization procedures have demonstrated the accuracy of standard mathematical models in predicting mass bias [2, 5J. Broadly speaking, a linear relationship (Eq. (1)) can be accurately applied to the third decimal place, a power law relationship (Eq. (2)) is good to the fourth decimal place, and an exponential relationship (Eq. (3)) can accurately predict and correct for mass fractionation to the fifth decimal place.

ßtrue/Äobs = 1 + A m e l i n (1)

Rtme/Robs = (1 + Cpow) (2)

Ätrue/Äobs = exp(Ameexp) (3)

JMC (Johnson Matthey Corporation) neodymium reference solution is widely used by geologists as a reference material for isotopic analysis. Its isotopic composition is generally accepted as l42Nd/,44Nd = 1.141820, 143Nd/144Nd = 0.511830, I45Nd/,44Nd = 0.348410, 148Nd/144Nd = 0.241578 and 150Nd/I44Nd = 0.236418. The accuracy of the exponential equation in determining and correcting for mass discrimination, and indeed the accuracy of the MC-ICP-MS measurement system itself, can be illustrated through the analysis of this solution.

The nine MC-ICP-MS Faraday detectors are referred to as L4, L3, L2, LI, Axial, HI, H2, H3 and H4 (this reflects their relative positioning about the optical axis). For a typical neodymium analysis the Faradays would be positioned such that 142Nd would be measured in detector L2, I43Nd in LI, l44Nd in Axial, 145Nd in HI, 146Nd in H2, 147Sm in H3, and 148Nd in H4. (The analysis of ,47Sm is required to allow a 144Sm correction to 144Nd). This analy-tical configuration is acceptable for routine conventional analysis, but experimental trials aimed at quantifying the quality of the exponential law in correcting mass bias require a more thorough investigation. It is vital for the isotopic measurement to be independent of the combination of Faraday detectors that is selected for analysis. To check this, the isotopic composition of


Table 4.1. Faraday detector configurations for neodymium analysis

Faraday L4 L3 L2 LI Axial

Configuration 1 142Nd 143Nd Configuration 2 142Nd 143Nd 144Nd Configurations 142Nd 143Nd l44Nd 145Nd

HI H2 H3

1 4 4 N d 1 4 5 N d 1 4 6 N d

1 4 5 N d 1 4 6 N d 1 4 7 S m

146Nd 147Sm 148Nd

H4 147Sm 148Nd 150Nd

Table 4.2. JMC neodymium analysis, detector configuration 1

Sample 142Nd/144Nd 143Nd/,44Nd 145Nd/144Nd

1 2 3 4 5 6 Mean %RSD Ref. value

1.141815 (0.0011) 1.141813 (0.0010) 1.141813 (0.0013) 1.141826(0.0010) 1.141853 (0.0013) 1.141844(0.0014) 1.141827 0.0015 1.141820

0.511811 (0.0009) 0.511818 (0.0010) 0.511816(0.0011) 0.511808(0.0010) 0.511835 (0.0011) 0.511824(0.0011) 0.511819 0.0019 0.511830

0.348419(0.0012) 0.348413(0.0011) 0.348405(0.0011) 0.348412 (0.0009) 0.348403 (0.0010) 0.348412 (0.0009) 0348411 0.0017 0.348410


Sample 2Nd/ , 4 4Nd 3Nd/144Nd 5Nd/144Nd 148 Nd/l 4 4Nd

1 1.141849(0.0011) 2 1.141843 (0.0010) 3 1.141819 (0.0013) 4 1.141798 (0.0010) 5 1.141820 (0.0013) 6 1.141824 (0.0014) Mean 1.141826 %RSD 0.0016 Ref. 1.141820 value

0.511820(0.0009) 0.511839(0.0010) 0.511839(0.0011) 0.511823(0.0010) 0.511846(0.0011) 0.511828(0.0011)

0.511833 0.0020 0.511830

0.348408 (0.0012) 0.348415(0.0011) 0.348422(0.0011) 0.348421 (0.0009) 0.348420 (0.0010) 0.348416 (0.0009)

0.348417 0.0015 0.3 48410

0.241573 (0.0016) 0.241578 (0.0021) 0.241570(0.0018) 0.241588 (0.0019) 0.241585 (0.0018) 0.241581 (0.0020)

0.241579 0.0029 0.241578

neodymium is determined in three different collector configurations. If the same isotopic result is obtained in all three configurations, the mass spectrometry system and the correction procedures can be argued to be accurate. The three configurations chosen for study are shown in Table 4 .1 . Six aliquots of neodymium were analyzed in each configuration. Each aliquot was analyzed for 500 s, this comprising 100 measurements each of 5 s duration. The results of each analysis for the three configurations are shown in Tables 4.2, 4.3 and 4.4.


Sample 142Nd/144Nd 143Nd/,44Nd ,45Nd/144Nd 148Nd/144Nd 150Nd/144Nd

1 2 3 4 5 6 Mean %RSD Ref. Value

1.141881 (0.0011) 1.141776 (0.0010) 1.141812(0.0013) 1.141846 (0.0010) 1.141802(0.0013) 1.141776 (0.0014)

1.141815 0.0036 1.141820

0.511842(0.0009) 0.511816(0.0010) 0.511820(0.0011) 0.511830(0.0010) 0.511828 (0.0011) 0.511824(0.0011)

0.511827 0.0018 0.511830

0.348411 (0.0012) 0.348404(0.0011) 0.348407(0.0011) 0.348405 (0.0009) 0.348419 (0.0010) 0.348414 (0.0009)

0.348410 0.0017 0.348410

0.241588 (0.0016) 0.241599 (0.0021) 0.241589 (0.0018) 0.241583 (0.0019) 0.241588 (0.0018) 0.241585 (0.0020)

0.241589 0.0023 0.241578

0.236383 (0.0026) 0.236404 (0.0026) 0.236421 (0.0025) 0.236393 (0.0023) 0.236398 (0.0026) 0.236382 (0.0022)

0.236397 0.0062 0.236418


The isotope ratio result for each aliquot is shown with its associated internal measurement precision (percentage standard error, %SE). The external reproducibility is illustrated as the mean value for the six aliquots and its error is displayed as a percentage relative standard deviation (%RSD).

The MC-ICP-MS instrument is able to measure the isotopic composition of neodymium to an internal precision of approximately 0.001% SE. The external reproducibility of six such analyses is approximately 0.002% RSD. The isotopic measurements for the three differing configurations agree with each other and agree with the accepted values to the fifth decimal place. These results validate the measurement system and the selection of the exponential law in determining and correcting for mass bias. They also demonstrate the high quality of isotope ratio precision and accuracy that is achievable with this type of instrumentation.

The range of elements that can be analyzed by TIMS is restricted by the method of ionization. Some elements are ionized very efficiently, whereas some cannot be ionized at all; the remaining elements lie somewhere between these two extremes. An ICP ion source is particularly constant in its ability to ionize most elements to the same degree. Hence MC-ICP-MS analysis of most elements in the Periodic Table will be of similar accuracy and precision to that reported for neodymium in Tables 4.2, 4.3 and 4.4.

4.5.2 External Mass Bias Correction It is not possible to utilize an internal normalization procedure to correct mass bias for all elements. For example, three isotopes of lead (206Pb, 207Pb and 208Pb) are radiogenic decay products and therefore vary widely in nature. It is therefore not appropriate to assume a constant ratio. This characteristic complicates the measurement of lead isotope ratios by TIMS and necessitates the analysis of a lead reference material prior to the analysis of the sample. Once quantified, the mass bias figure can be applied. This technique obviously lengthens, complicates and limits the measurement procedure.

A unique feature of MC-ICP-MS is that, over a limited mass range, bias is independent of elemental species and its magnitude can be calculated purely from mass difference. Thus, the mass bias figure exhibited by the isotopes of thallium (203T1 and 205T1) is equivalent to the mass bias exhibited by the isotopes of lead. Prior to analysis by MC-ICP-MS, lead samples are doped with a similar concentration of thallium, and a simultaneous measurement of all isotopes 203T1, 204Pb, 205T1, 206Pb, 207Pb and 208Pb is then made [5]. Comparison of the measured thallium ratio with its true stable ratio is used as a calculation of mass bias. This mass bias figure is then applied to correct the measurements for the isotopes of lead. In effect, thallium provides an internal normalization for lead isotopic measurement..

There are numerous pairs of isotopes throughout the Periodic Table that overlap in mass and can thus be treated in a similar way to the lead/thallium

MASS BIAS AND THE MEASUREMENT OF ISOTOPIC RATIOS 95

system described above. Examples are: palladium and silver; cadmium and silver; tin and antimony; antimony and tellurium, neodymium and europium; tungsten and rhenium; osmium and iridium; platinum and iridium; mercury and thallium.

This particular feature of mass bias can be further extended to correct for isobaric interferences. The presence of significant quantities of samarium in a solution of neodymium will severely complicate a TIMS measurement of the 143Nd/144Nd isotopic pair because of the 144Sm interference on 144Nd. This effectively means that all traces of Sm must be removed from the neodymium sample prior to any analysis. This obviously adds considerable preparation and separation time to the measurement process. Such a situation poses no such complication for MC-ICP-MS.

A graphical representation of a neodymium and samarium mixture showing the isobaric interference of 144Sm with 144Nd is presented in Figure 4.5. An accurate MC-ICP-MS determination of the 143Nd:144Nd isotope ratio can be realised by the following methodology. A simultaneous measurement of 142Nd, 143Nd, 144Nd + 144Sm, 145Nd, 146Nd, 147Sm and ,48Nd is made. Mass bias is calculated by comparing the measured 146Nd/144Nd ratio with the geological reference value of 0.7219. The magnitude of mass 147 is used to predict the 144Sm contribution assuming that the 147Sm/144Sm ratio is equal to 4.8389 (the measured 147Sm signal must first be corrected for mass bias). The true 144Nd magnitude is then determined by subtraction, and the l43Nd/l44Nd ratio is then corrected for mass bias to reveal the true 143Nd/144Nd ratio.

Samarium

Neodymium

Atomic mass

Figure 4.5. Graphical representation of a neodymium and samarium mixture


Experiments by Walder et al. [5] measured the isotopic composition of 143Nd/144Nd in a pure neodymium reference solution and also in a mixture of the same reference solution doped with a similar concentration of samarium. Despite the presence of the 144Sm interference, the measurements revealed accurate and equivalent 143Nd/144Nd isotope ratios for both solutions. This has major implications for the analysis of solutions containing isobaric interfer-ences and for the potential analysis of solid samples directly by laser ablation.

4.6 MEASUREMENT OF THE ISOTOPIC COMPOSITION OF ENVIRONMENTAL LEAD SOLUTIONS

The power of this type of instrumentation, together with a practical implementation of the features discussed in Section 4.5.2 were illustrated in a publication by Wälder and Furuta, [6] describing the measurement of lead isotope ratios within environmental samples. Three samples were selected for measurement: pond sediment collected at the University of Tokyo, airborne particulates collected at Shinjuku, in Tokyo, and a reference solution containing 23 elemental species (Merck, USA). After collection, the environmental samples were digested in a nitric acid-perchloric acid mixture and 4 ml of hydrofluoric acid was added to remove any silica. The mixture was evaporated to dryness and 4 ml of nitric acid was added. The final weights of the solutions were adjusted to give a pond sediment containing 82 ng ml-1 of lead, an airborne particulate solution containing 45 ng ml-1 of lead and a Merck multielement standard containing 100 ng ml-1 of each component element.

Sample separation was kept to the absolute minimum, and therefore the MC-ICP-MS analysis was performed on solutions heavily contaminated by matrix species. The pond sediment contained 102000 ppb Al, 68 000 ppb Fe, 5690 ppb Ti, 8340 ppb Ca, 8270 ppb Mg, 6080 ppb Na, 1250 ppb P, 640 ppb Mn, 350 ppb Zn, 240 ppb V, 190 ppb Cu, and 81 ppb Pb. The airborne particulate solution contained 46200 ppb Al, 46600 ppb Fe, 3420 ppb Ti, 57 500 ppb Ca; 10700 ppb Mg, 14400 ppb Na, 2350 ppb P, 870 ppb Mn, 2400 ppb Zn, 120 ppb V, 610 ppb Cu, and 447 ppb Pb. The Merck multielement solution contained 23 elements (Ag, Al, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, In, K, Li, Mg, Mn, Na, Ni, Pb, Sr, TI, and Zn), each at a concentration of 100 ng ml-1.

Prior to analysis the environmental samples were doped with thallium to a similar concentration to the lead to allow a correction for mass bias. A power law (Eq. (2)) was used to predict and correct for mass bias. The environmental solutions contained significant levels of mercury, with the 204Hg isotope contributing approximately 0.5% to the measured intensity at mass 204. To correct for this interfering species, 202Hg was also measured and the contribu-tion of 204Hg was calculated by assuming a 204Hg/202Hg ratio of 0.2293.

MEASUREMENT OF THE ISOTOPIC COMPOSITION OF GEOLOGICAL SOLUTIONS 97

Table 4.5. Lead isotopic composition of pond sediment, airborne particulates and Merck multielement standard

Sample M 6Pb/2 0 7Pb 208Pb/206Pb 208Pb/2O4Pb 207Pb/204Pb

Pond sediment 1.14243±0.00089 2.1214±0.0013 37.678±0.034 15.547±0.015 Airborne particulates 1.15279±0.00030 2.1099±0.0007 37.915±0.021 15.588±0.007 Merck standard 1.22378 ± 0.00037 1.9982 ± 0.0007 38.476 ±0.021 15.734 ±0.009

Sample analysis time was kept to a minimum to quantify throughput rate. Each sample was divided into six aliquots to allow quantification of measurement reproducibility. Each aliquot was analyzed for 50s, this period comprising 10 measurements each of 5 s duration. The analysis of each aliquot consumed a few tens of ng of lead. The isotope ratios measured were 202Hg/204(Pb + Hg), 2 0 5 T 1 / 2 0 3 T l i 2 0 8 p b / 2 0 4 p b ; 207p b / 204p b < 2 0 6 p b / 2 0 4 p b i 2 0 8 p b / 2 0 6 p b ) a n d

2 0 6 p b / 2 0 7 p b

The mean values for each of the three samples based upon six aliquots of each are shown in Table 4.5. The confidence in each mean value is expressed as two standard deviations. Measurements determined for each material show a characteristic set of isotopic ratios each statistically different from the other samples.

Analysis of lead using MC-ICP-MS delivers a level of precision comparable with that obtainable by TIMS. The technique has the advantages of a much simplified preparation and measurement process, it tolerates solutions that are heavily contaminated by other matrix species, it provides a factor of 10 impro-vement in sample throughput and it allows correction for mercury interference.

4.7 MEASUREMENT OF THE ISOTOPIC COMPOSITION OF GEOLOGICAL SOLUTIONS

There are a number of elements whose isotopic signatures relay information about the age of the host rock or mineral. The isotopic signature of the element is determined by radioactive decay processes and can be related to age through the radioactive decay constant. The natural variation of isotopic value is invariably very small, and requires very precise and accurate measurement systems to quantify even very large age differences. Typical examples of such elements are neodymium and strontium, which have become benchmark dating tools in TIMS. Neodymium and strontium have become widely established by thermal ionization partly because of the ease with which they can be measured. TIMS is, however, restricted to the analysis of elements of low ionization potential. The analysis of refractory and high ionization potential elements is difficult and in many cases impossible. Use of the Plasma 54 instrument will open up new and exciting geological isotope ratio application areas through


both liquid and solid analysis. This work is just beginning, and the following paragraphs indicate some of the more exciting areas that have already become apparent.

The high temperatures of a plasma ion source makes it more suitable for the analysis of the refractory and high ionization potential elements. To illustrate this, the MC-ICP-MS was utilized to measure the isotopic composition of a hafnium reference material. Hafnium was chosen becouse it is isotopically well characterized by TIMS but is a notoriously difficult element to ionize and measure by thermal methods.

Six aliquots of JMC 475 hafnium solution at a concentration of 50 ng ml - 1

were prepared for analysis. The isotopic ratios ,76Hf/177Hf; 178Hf/,77Hf; 179Hf/ 177Hf and 180Hf/177Hf were measured. Comparison of the stable 179Hf/177Hf ratio with the established value of 0.7325 was used to determine the magnitude of mass bias and simultaneously to correct the other isotopic ratios. Acquisition times of 500 s per aliquot, this comprising 50 measurements each of 10 s duration, were used. The measurements for all six aliquots and their mean with its associated error are shown in Table 4.6 [3]. The analysis of each aliquot consumed approximately 300 ng of hafnium. Sample consumption less by a factor of « 10 than that required by TIMS.

Mean isotopic ratios of 176Hf/177Hf = 0.282198 ± 0.000010, 178Hf/177Hf = 1.46731 ±0.00007, and 180Hf/177Hf = 1.88634 ±0.00025 were determined. These values compare well with those determined by Patchett [7] using thermal ionization at 176Hf/177Hf = 0.282195 ±0.000015, 178Hf/177Hf = 1.46710 ± 0.00010, and 180Hf/177Hf = 1.88651 ±0.00012.

The analysis of hafnium isotopic ratios was extended to the measurement of hafnium in basaltic rock samples. [8] These rock samples underwent a single column separation to remove some of the matrix elements. The analysis of 50 ng ml^1 of hafnium was performed in a matrix dominated by Al, Cr, Zr and Ti at a total concentration of 0.1%. Matrix suppression effects were not observed and the level of precision and accuracy obtained were comparable with those detailed in Table 4.6.

Table 4.6. Isotopic composition of JMC 475 hafnium Aliquot

1 2 3 4 5 6

Mean 2SD

176H f /177H f

0.282194 (0.002) 0.282191 (0.002) 0.282201 (0.002) 0.282201 (0.002) 0.282203 (0.003) 0.282201 (0.002)

0.282198 0.000010

,78Hf/177Hf

1.46727 (0.001) 1.46737 (0.001) 1.46732 (0.001) 1.46731 (0.001) 1.46732 (0.001) 1.46728(0.001) 1.46731 0.00007

180H f /177H f

1.88637 (0.002) 1.88656 (0.003) 1.88620 (0.003) 1.88635 (0.002) 1.88627 (0.002) 1.88629 (0.002)

1.88634 0.00025

THE ISOTOPIC RATIO MEASUREMENT OF URANIUM 99

A recent publication by Lee and Halliday [14] has determined when the earth's metallic core and the moon were formed following the birth of the solar system. Their conclusion was based upon MC-ICP-MS measurements of tung-sten isotopic composition of moon rocks, earth rocks and meteorites. During the evolution of the earth into a planet with a heavy metal core, a partially molten rock mantle and a thin surface crust, tungsten generally sank to the planet's metallic core while hafnium became concentrated in the mantle. As 182Hf has a half-life of 9 million years and it decays to 182W, measurement of the isotopic composition of tungsten can reveal the timing of the earth's evolution. Lee and Halliday discovered identical isotopic ratios in earth rock and lunar rock but a different isotopic signature in the meteorites. This means that the evolution of the earth and the development of its core occurred after ,82Hf had decayed away. This would take at least 62 million years after the formation of the meteorites. As the isotopic composition of lunar rock matched that of earth rock, the moon must have formed after a similar time delay.

It is recognized that U5In-115Sn geochronology could be established if the isotopic composition of both In and Sn could be measured to the high levels of precision and accuracy that are required. Indium can be measured by TIMS, but it is difficult to correct for mass fractionation as In contains only two isotopes. Tin has ten isotopes and can therefore be corrected for mass fractionation; however, it is difficult to ionize because of its high ionization potential. A publication by Yi et al. [9] describes the measurement of both In and Sn by MC-ICP-MS. Both elements were shown to be efficiently ionized within the ICP source. Mass bias exhibited by In and Sn was corrected by doping the solutions with Pd and Sb, respectively. The subsequent application of isotope dilution theory demonstrated accurate In and Sn concentrations in silicates and sulfides.

An isotopic study of the transition elements, in particular germanium, will provide valuable information on the precise chronology of mechanisms for nuclear synthesis. Germanium is another element that is particularly difficult to ionize by TIMS. Initial MC-ICP-MS work by Hirata [10J used gallium to conect for the mass bias of germanium; he has demonstrated significant improvements in analytical precision compared with TIMS.

4.8 THE ISOTOPIC RATIO MEASUREMENT OF URANIUM

4.8.1 Validation of Instrument Linearity and Mass Discrimination Correction

An accurate determination of mass bias exhibited by the MC-ICP-MS instrument is of course essential for an accurate quantification of isotopic composition. Previous sections within this chapter have described the conection


of mass bias via a simultaneous normalization measurement, e.g., by use of the 203T1/205T1 ratio to correct the measured ratios of lead and ratio of the 146Nd/144Nd to correct the measured ratios of neodymium. This is a valid and acceptable methodology in the absence of any internationally certified thallium and neodymium reference solutions. However, it should be noted that the accuracy of this correction methodology depends upon the accuracy of the assumed normalization ratio.

In an attempt to quantify absolutely the accuracy of isotopic abundance measurements with the MC-ICP-MS instrument a suite of uranium isotopic reference materials was measured. This comprised a selection of reference solutions produced at the Institute of Reference Materials and Measurements (IRMM) (Rosman et al. [11]). Each reference solution consists of three uranium isotopes (233U, 235U and 238U). Each solution has a 235U/238U ratio near to unity and certified to within 0.03% (2 RSD). The 233U/238U and 233U/235U ratios vary over six orders of magnitude and are also certified to within 0.03% (2 RSD).

Part of this uranium isotopic reference suite was used to assess the linearity of the measurement system and to check the validity of the correction model for mass discrimination effects. By utilizing the one-to-one 235U/238U ratio, which is least susceptible to any non-linearity effects, it is possible to quantify the mass discrimination factor during the measurement. This experimentally determined factor is then applied to the isotope ratios 233U/235U and 233U/238U. By applying a suitable mass discrimination model, it should be possible to calculate the corrected 233U/235U and 233U/238U ratios and compare these with the certified values. If the measurement system is truly linear and the mathe-matical model is correct, measured values should agree with certified values. Details of the experimentation are given by Taylor et al. [12].

Reference solutions IRMM 072-1 to 072-8 were chosen for analysis; each solution was analysed for 100 s, the analysis comprising 20 measurements each of 5 s duration. The isotope abundance ratios 233U/235U, 233U/238U and 235U/238U were determined. The 235U/238U ratio is approximately unity for each reference sample, and this was used to determine the level of mass bias. The measured 233U/238U and 233U/235U isotopic ratios were then corrected using this figure. The 2 3 3u/ 2 3 8U and 233U/235U isotopic composition varied from approximately 1.0 for the IRMM 072-1 solution to approximately 0.01 for the IRMM 072-8 solution.

Analysis of the corrected results reveal that a linear, a power and an exponential law applied to the 233U/235U measurement (mass difference equals 2) produce data which agree with the certified values. However, analysis of the 233U/238U ratio (mass difference equals 5) shows that the linear equation is unable to correct for mass bias. The power and the exponential corrections produced data in agreement with the certified data.

Use of the IRMM suite of uranium reference materials validated the analytical performance of the instrument and demonstrated that the measure-

THE ISOTOPIC RATIO MEASUREMENT OF URANIUM 101

ment of 233u/238U and 235U/238U isotope ratios from 1.0 to 0.01 are accurate and linear within the uncertainty range of the certified material. The use of a power or an exponential law is recommended for optimum results.

4.8.2 Uranium Isotopic Analysis within the Nuclear Industry

The isotopic ratio measurement of radiogenic elements is of paramount importance within the nuclear industry. The traditional method of such isotopic determination is TIMS, and instruments equipped with multiple Faraday detectors can routinely determine the isotopic composition of the major isotopes of uranium to a precision within 0.05%. The requirements of this industry place tough demands on both the quality and the speed of isotopic measurements. Indeed, nuclear processes are often slowed or even stopped until the result of an isotopic analysis is known. Speed of analysis is thus vital, and any methodology which demonstrates an increase in sample throughput is of major advantage to any financially orientated organization.

The conversion of uranium into an isotopic mixture capable of efficient nuclear fission is performed by a complex and highly expensive procedure which enriches the element in the fissionable isotope. This process is generally performed by a centrifugal process which separates the isotopes of uranium in the gaseous hexafluoride state. Monitoring the efficacy of the process is a vitally important procedure, and this is generally performed by TIMS analysis at strategic points of the enrichment process. The presence of significant quantities of fluoride ion on a thermal filament prevents the accurate analysis of uranium isotope ratios. Isotopic ratio measurements of uranium hexafluoride by TIMS thus demand a pure uranium sample and hence the removal of the fluoride ion.

The analysis of hydrolyzed uranium hexafluoride samples by MC-ICP-MS was described by Wälder and Hodgson [13]. Three uranic samples of 235U/238U composition 1.015267, 0.0361354 and 0.0072516 were each divided into five aliquots for measurement. The measurements proved to be accurate and linear over this range and demonstrated analytical precision superior to that of TIMS. In addition, the analysis demonstrated a factor of 5 improvement in sample throughput.

The routine use of MC-ICP-MS in the nuclear industry has the potential to improve plant productivity dramatically and return a significant saving in sample analysis cost. In an attempt to quantify the throughput advantages of the MC-ICP-MS instrument over TIMS and to evaluate the analytical performance of the pulse counting Daly detector, the following analytical trial was designed. A natural uranium solution and a NIST U-050 reference solution were selected for measurement. Five aliquots of each sample were measured in an A, B, A, B . . . cyclic sequence. The major 235U and 238U isotopes were measured using two Faraday detectors, and the minor 234U and 236U isotopes were measured


Table 4.7. MC-ICP-MS analysis of natural uranium solution Sample

1 2 3 4 5 Mean %RSD TIMS

234TJ/238TJ

0.0000541 0.0000539 0.0000535 0.0000537 0.0000538 0.0000538 0.41 0.000055

235IJ/238TJ

0.007250 0.007254 0.007249 0.007254 0.007253 0.007252 0.032 0.007255

236TJ/238TJ

0.0000005 0.0000005 0.0000004 0.0000005 0.0000004 0.0000005

0

Table 4.8. MC-ICP-MS analysis of NIST U-050 reference solution Sample

1 2 3 4 5 Mean %RSD TIMS

234U/238U

0.0002933 0.0002944 0.0002947 0.0002943 0.0002946 0.0002943 0.19 0.0002939

235TJ/238IJ

0.052784 0.052769 0.052791 0.052777 0.052764 0.052777 0.021 0.052784

236TJ/238TJ

0.0005060 0.0005058 0.0005057 0.0005065 0.0005053 0.0005059 0.09 0.0005057

using the pulse counting Daly detector. As uranium lacks a stable (non-radioactive) pair of isotopes, the prior analysis of a previously characterized reference solution was required to determine the magnitude of mass bias. As mass bias is constant with time, this calibration sample needs to be measured once only.

The mass bias corrected measurements for the solutions of natural and U-050 uranium are shown in Tables 4.7 and 4.8. The data illustrate how the combination of multiple Faradays and a Daly detector allows the measurement of a wide range of isotopic ratios. Because analysis was performed in an A, B, A, B . . . cyclic sequence, the measurement of each sample was preceded by a washing cycle to remove traces of the previous solution. The accuracy of the results demonstrates that the wash cycle is effective. Measurement of these ten samples plus the calibration sample took approximately 90 min. This is approximately four times the throughput that could be achieved by TIMS. Measurements of both the major and minor isotopes are accurate to a high level of analytical precision.

DIRECT ISOTOPIC ANALYSIS OF SOLID SAMPLES BY LASER ABLATION 103

4.9 MC-ICP-MS FOR THE DETERMINATION OF ATOMIC WEIGHTS

A recent paper by Lee and Halliday [2] has exploited the high ionization efficiency of the plasma source to determine the isotopic abundance and hence the atomic weights of the high ionization potential elements molybdenum, tellurium, tin and tungsten. Because of their high ionization potentials, these elements are particularly difficult to analyze by TIMS. However, negative thermal ionization mass spectrometry (NTIMS) has been widely used to determine the isotopic composition of particular refractory metals, and has been successfully applied to tungsten. This technique exploits the higher ionization efficiency of negatively charged oxide ions relative to the positively charged elemental ions. It therefore allows higher precision measurements than those of TIMS for some elements. However, the technique is complicated by the necessity to evaluate the oxygen isotopic content of the sample material. In addition, filaments used in the ionization process may contribute isobaric interference.

The isotopic compositions of tungsten, tin, tellurium and molybdenum determined by MC-ICP-MS demonstrate excellent agreement with those determined independently by TIMS and NTIMS. However, the precision of the ICP data shows up to a 100-fold improvement. A similar improvement in the precision of atomic weight determination was also realized. The atomic weight of tungsten has been measured as 183.84162 ±0.00004, compared to the previous best reported measurement of 183.8417 ±0.0001 by Volkening etal. [15]. The atomic weight of tin was determined as 118.7105 ± 0.0002, com-pared to the best reported measurement of 118.710 ± 0.007 by Devillers et al. [16]. Similarly, the atomic weight of molybdenum was reported as 95.93101± 0.00003, compared to the previous best measurements of 95.9318 ± 0.0009 reported by Moore et al. [17].

4.10 DIRECT ISOTOPIC ANALYSIS OF SOLID SAMPLES BY LASER ABLATION MC-ICP-MS

A particular advantage of the ICP ion source is its ability to accept samples generated by a wide range of introduction techniques. Perhaps the most exciting of these techniques is laser ablation. Solid samples can be evaporated by an incident laser beam and the resulting ablated powder passed directly into the ICP for ionization and subsequent measurement. Because this technique requires none of the sample preparation and separation associated with samples in solution, massive productivity gains can be realized. In addition, an isotopic profile of the solid can be generated simply by rastering the laser beam across the surface.


4.10.1 Lead Isotope Ratio Measurement of NIST 610 Glass Reference Material

The first study involving laser ablation and MC-ICP-MS determined the lead isotopic composition of a NIST 610 glass reference material (Walder et al. [18]). NIST 610 glass contains lead at a concentration of 420 ppm and thallium at a concentration of 120 ppm. Thallium was used to correct for mass bias as described in Section 4.6. The glass also contained mercury at a significant but unqualified level. The interference of 204Hg with 204Pb was corrected by measuring the 202Hg isotope. The glass was divided into six analytical areas and an infra-red laser system operating at 1064 nm was used to ablate several craters of 40 pm diameter in each area. The total time required to prepare, load and position the sample and acquire data was approximately 4 min per area. The mean lead isotopic value for the glass reference material over six analytical occasions is given in Table 4.9.

The results obtained by MC-ICP-MS are in agreement with those of an independent solution analysis by TIMS. This confirms that the methods adopted for mass bias correction and mercury interference are equally applicable to laser ablation measurements. The glass was shown to be homogeneous in its lead isotopic composition. The use of a laser ablation for sample introduction allowed an immediate analysis with none of the dissolution, separation and sample loading required by TIMS, and massive productivity gains can thus be realized.

4.10.2 Hafnium Isotope Ratio Measurement of an Elie Ness Zircon

Valuable geological information is contained within zircons, as they are known to preserve isotopic information that reflects early events within the earth's evolution. The relatively high hafnium content of zircons has allowed a number of hafnium isotopic studies by TIMS, although analysis is difficult because of the refractory nature of hafnium and the relatively large quantities of sample needed for analysis. Spatially resolved isotope ratio analysis of hafnium has been attempted using a sensitive high resolution microprobe. Such measure-ments revealed impressive levels of precision, although major sources of un-certainty arose through interference from rare earth element hydroxides and other polyatomic species.

Table 4.9. Lead isotope ratio measurement of NIST 610 glass by laser ablation MC-ICP-MS. TIMS data from Belshaw et al. [20] 208p b /204p D 2 0 7 p b / 2 0 4 p b 2 0 8 p D / 2 0 6 p b 207p D /206p b

Mean±2SD 36.948±0.038 15.506±0.018 2.1670±0.0018 0.9096 ± 0.0008 TIMS±2SD 36.989 ±0.024 15.506 ±0.010 2.170 ±0.002 0.9095 ±0.003

DIRECT ISOTOPIC ANALYSIS OF SOLID SAMPLES BY LASER ABLATION 105

A recent study by Thirl wall and Wälder [19] utilized an ultraviolet laser system emitting at 266 nm to ablate the zircon sample into the MC-ICP-MS. The geologically interesting isotopic measurement is the ,76Hf/177Hf ratio. Unfortunately the zircon also contains significant quantities of both lutetium and ytterbium, which interfere with mass 176Hf. To correct for these inter-ferences l75Lu and 173Yb were also measured and the true 176Hf contribution was calculated by assuming 176Yb/173Yb = 0.7938 and 176Lu/,75Lu = 0.02669. The detectors of the MC-ICP-MS were therefore positioned to allow simultaneous measurement of 173Yb, ,74Hf, ,75Lu, 176(Yb + Lu + Hf), 177Hf, 178Hf and l79Hf. Mass bias was calculated by assuming ,79Hf/l77Hf = 0.7325.

The zircon crystal was divided into ten sub-areas for the analytical work. Between five and ten craters in each sub-area were used for sampling of the zircon, each crater being approximately 40 pm in diameter. This diameter was chosen as a compromise optimum between spatial resolution and ion signal. Each crater was ablated for 25 s, the period comprising five measurements each of 5 s duration. The assessment of each sub-area took approximately 5 min.

The measurements obtained for each sub-area of the zircon are summarized in Table 4.10. Each entry represents the isotopic composition of an area of approximately 0.01 mm2, and the analysis of each individual sub-area consumed approximately 10 ng of Hf. The overall reproducibility of the 178Hf/!77Hf ratio is 0.037% 2RSD with a mean of 1.46713 ± 0.00054. This value is identical with the value of 1.46710 determined by TIMS and reported by Patchett [7], Reproducibility of the 176Hf/177Hf measurement over the 10 sub-areas is equivalent to 0.018% 2RSD. The 176Yb and 176Lu interference with l76Hf is accurately conected; the presence of these isotopes does not result in a degradation of measurement precision, and hence represents no significant problem to the analysis. The 176Hf/177Hf measurement and hence the hafnium isotopic composition is measured as 0.282855 ± 0.000052 and is constant across the zircon.

Table 4.10. Hf isotope ratio measurements for 10 sub-areas of the Elie Ness zircon Sub-area 176Hf/177Hf 178Hf/177Hf Ñ

1 0.282856 ±0.000104 2 0.282822 ±0.000061 3 0.282846 ±0.000087 4 0.282868 ±0.000066 5 0.282861 ±0.000074 6 0.282849 ±0.000067 7 0.282876 ±0.000114 8 0.282857 ±0.000104 9 0.282813 ±0.000144

10 0.282906 ±0.000039 Mean ± 2 SD 0.282855 ± 0.000052

1.46706 ±0.00039 1.46733 ±0.00021 1.46694 ±0.00061 1.46721 ±0.00024 1.46761 ±0.00040 1.46734 ±0.00015 1.46713 ±0.00018 1.46714 ±0.00023 1.46689 ±0.00058 1.46663 ±0.00049 1.46713 ±0.00054

9 7 6 5 6 6 6 6 6 6


Table 4.11. Isotopic composition of tungsten by solution nebulization and laser ablation [2]

Experiment I82W/183W 184W/183^

Solution nebulization 1.8471 ± 5 2.1457 ± 4 Laser ablation NIST 611 glass 1.8470 ± 2 2.1458 ± 6

4.10.3 Tungsten Isotope Ratio Measurement

The isotopic composition of tungsten within NIST glass reference material SRM 611 has been also determined by laser ablation MC-ICP-MS (Halliday et al. [2]). This silicate reference material contains approximately 500 ppm of tungsten and similar levels of many other trace elements. A Nd-YAG laser operating at 1044 nm in Q-switched mode at a pulse rate of 10 Hz was used for the ablation process. The laser was set up to drill through the glass over a period of 8 min. Mass bias was calculated by internal normalization assuming 186W/183W = 2.000. The isotopic data is shown in Table 4.11 together with that determined by solution MC-ICP-MS. The solution was made by dissolving Cross W filament ribbon. The two sets of data are in agreement, with similar levels of measurement precision. This suggests that the laser ablation process is not responsible for any significant isotopic fractionation effects.

4.10.4 Strontium Isotope Ratio Measurement

The strontium isotopic composition of two solid samples has been determined by laser ablation and MC-ICP-MS (Christensen et al. [21]). An ultraviolet laser system was employed for sampling. The detectors of the mass spectrometer were set to collect masses 83, 85, 86, 87 and 88. Measurements were performed over 200 s, this period comprising 80-100 measurements each of 5 s duration. Ratios were normalized to 86Sr/88Sr = 0.1194 using an exponential law.

The strontium isotopic composition of a feldspar megacryst and a gastropod shell were determined by laser ablation MC-ICP-MS. Both samples had a strontium content of « 2000 ppm. The interference of 87Rb on 87Sr was corrected using the natural 85Rb/87Rb ratio of 2.593. Typical crater diameters were between 150 and 300 pm. The ablation rate was between 0.5 and 1.4 pm per second. This translates into a sample use of 100-200 pg of strontium per second, corresponding to an efficiency of 1 atom measured for every 6000 atoms ablated.

The feldspar megacryst revealed a 87Sr/86Sr ratio of 0.703106 ± 0.000022, and the gastropod shell gave a value of 0.709182 ± 22. These measured values are identical with independent TIMS measurements of 0.703117 ±0.000013 and 0.709170 ± 0.000010 respectively. The levels of precision are similar for

REFERENCES 107

both instrumental techniques. This study is the first to demonstrate the direct isotopic ratio measurements of strontium in solid samples to such high levels of analytical precision.

4.11 CONCLUSIONS

The prototype MC-ICP-MS was designed during 1992 and the first commercial systems were delivered towards the end of 1994. The launch of this instrument is perhaps one of the most exciting developments in the area of isotope ratio studies over the last few years. This instrumentation combines the capabilities of TIMS with those of an ion probe, plus its own unique abilities. Although much new and exciting work has already been done, the technique is still very much in its infancy. MC-ICP-MS will continue to develop for the analysis of both solution and solids. The technique is destined to make a considerable contribution in the area of isotope ratio measurements and will firmly establish itself as an invaluable tool in the nuclear, geological and environmental application areas.

4.12 ACKNOWLEDGEMENTS

The author acknowledges Dr Philip Freedman, who was largely responsible for the scientific design of the prototype MC-ICP-MS instrument. The skills, dedication and invaluable contribution of Mr Stephen Bloomfield, Mr Ian Bowen and Mr Andrew Entwistle in converting the prototype into a commercial unit are also gratefully acknowledged.

REFERENCES

[1] AJ. Wälder and P.A. Freedman, J. Anal. At. Spectrom., 7, 571 (1992). [2] A.N. Halliday, D.-C. Lee, J.N. Christensen, A.J. Wälder, P.A. Freedman, CE. Jones,

CM. Hall, W. Yi and D. Teagle, Int. J.Mass Spectrom.Ion Processes, 146/147, 21-33, (1995); and D.-C. Lee, and A.N. Halliday, Int. J. Mass Spectrom. Ion Processes, 146/147, 35-46 (1995).

[3] A.J. Wälder, D. Koller, N.M. Reed, R.C. Hutton, and P.J. Freedman, J. Anal. At. Spectrom., 8, 1037 (1993)

[4] G.R. Gilson, D.J. Douglas, J.E. Fulford, K.W. Halligan and S.D. Tanner, Anal. Chem., 60, 1472 (1988).

[5] A.J. Wälder, I. Platzner, and P.A. Freedman, J. Anal. At. Spectrom., 8, 19 (1993).

[6] A.J. Wälder, and N. Furuta, Anal. Sei., 9, 675 (1993). [7] P.J. Patchett, Geochim. Cosmochim. Acta, 47, 81 (1983).


[8] C. Chauvel, F. Albarede, A.J. Wälder, and D. Koller, paper presented at the 1993 Joint Meeting of the American Geophysical Union, the Mineralogical Society of America and the Geochemical Society, Baltimore, MD, May 24-28, 1993.

[9] Yi, W. A.N. Halliday, D.-C. Lee and J.N. Christensen, Geochim. Cosmochim. Acta, 59, 24, 5081 (1995).

[10] T. Hirata, personal communication 1996, Tokyo Institute of Technology, Labora-tory for Planetary Sciences, O-okayama 2-12-1, Meguro, Tokyo 152, Japan.

[11] K.J.R. Rosman, W. Lycke, R. Damen, R. Werz, F. Hendrickx, L. Traas and De P. Bièvre, Int. J. Mass Spectrom. And Ion Processes, 79, 61 (1987).

[12] P.D.P. Taylor, P. De Bièvre, A.J. Walder and A. Entwistle, /. Anal. At. Spectrom., 10, 395 (1995).

[13] A.J. Wälder, and T. Hodgson, contained in DOE-ORNL 1994 Conference on Analytical Chemistry in Energy Technology. Symposium on Applications of Inductively Coupled Plasma-Mass Spectrometry to Radionuclide Determinations, Gatlinburg, TN, October 13-14, 1994, R.W. Morrow and J.S.Crain (Eds). ASTM Spec. Tech. Publ. 1291.

[14] D.-C. Lee, and A.N. Halliday, Nature, 378, 771 (1995). [15] J. Völkening, M. Koppe, and K.G. Heumann, Int. J. Mass Spectrom. Ion Processes,

107, 361 (1991). [16] C. Devillers, T. Lecomte and R. Hagemann, Int. J. Mass Spectrom.ion Processes,

50, 205 (1983). [17] L.J. Moore, W.R. Machlan, W.R. Shields and E.L. Garner, Anal. Chem., 46, 1082

(1974). [18] A.J. Wälder, I.D. Abell, I. Platzner and P.A. Freedman, Spectrochim. Acta, Part B,

48, 397 (1993). [19] M.F. Thirlwall, and A.J. Walder, Chem. Geol. (hot. Geosci. Sect.), Ill, No. 3/4,241

(1995). [20] N.S. Beishaw, K.W. Burton, D.J. Martel, and R.K. O'Nions, presented at the

American Geophysical Union meeting, San Francisco, December 9-13, 1991. [21] J.N. Christensen, A.N. Halliday, D.-C. Lee, CM. Hall, Earth Planet. Sei. Lett., 136,

79-85 (1995).

CHAPTER 5

ADVANCED ISOTOPE MASS SPECTROMETRY III: QUADRUPOLE ISOTOPE RATIO MASS SPECTROMETRY

A. GOETZ Balzers Instruments, Liechtenstein

5.1 INTRODUCTION 5.1.1 The Principle of Quadrupole Mass Spectrometers 5.1.2 Abundance Sensitivity 5.1.3 Dynamic Range of Quadrupole Mass Spectrometers

5.2 THERMAL IONIZATION MASS SPECTROMETRY (TIMS) WITH QUADRUPOLES 5.2.1 Thermal Ionization Mass Spectrometry with a

Quadrupole Mass Spectormeter (THQ) 5.2.2 Isotope Dilution Mass Spectrometry (IDMS)

5.2.2.1 Uncertainty of IDMS and Optimization 5.2.3 Isotopic Abundance Measurements with the THQ

5.2.3.1 Bioavailability and Metabolic Studies with Stable Isotopes

5.2.3.2 Total Evaporation of Sample 5.3 ISOTOPE RATIO AND ISOTOPE ABUNDANCE

MEASUREMENTS IN GAS SAMPLES 5.3.1 Electron Impact Ionization 5.3.2 Cross-Beam Ion Source 5.3.3 Gas Inlet Systems 5.3.4 Isotopic Analysis of Different Noble Gases 5.3.5 Isotope Ratio Measurements in UFé

5.4 STABLE ISOTOPE RATIO MEASUREMENTS WITH A QMS COUPLED TO AN ELEMENTAL ANALYZER

5.5 HIGH RESOLUTION QUADRUPOLE MASS SPECTROMETRY REFERENCES

5.1 INTRODUCTION

5.1.1 The Principle of Quadrupole Mass Spectrometers The principle of quadrupole mass spectrometers (QMS) was first reported by Paul and Steimwedel in 1953 [1]. The basic design [2] of a quadrupole in

Modem Isotope Ratio Mass Spectrometry Edited by I. T. Platzner © 1997 John Wiley & Sons Ltd

109 109 111 112

113

113 114 114 118

120 121

122 122 122 123 125 126

126 130 132

110 ADVANCED ISOTOPE MASS SPECTROMETRY III

Cathode

Cathode Gas

J l L Ion source Rod system

Figure 5.1. Structure of a quadrupole analyzer Collector

* 0.3

to

0.237

0.2-

0.1

\ X/Y ; unstable

> f = const.

Y unstable unstable

stabe

0.2 0.4

Figure 5.2. Stability diagram of the quadrupole mass filter

0.6 0.7 0.6 0.8 1 <?—•

combination with an electron impact ion source is shown in Figure 5.1. Ions are separated by their mass/charge (m/z) ratio in a high frequency electrical quadrupole field. Ions generated in the ion source are injected into the separation system. In most instruments, cylindrical rod electrodes, which approximate hyperbolic surfaces with sufficient accuracy, are used. The voltage between these electrodes is a high frequency alternating component V cos ujt superimposed by a voltage U. The influence of the high frequency field causes the ions to oscillate at right angles to the axis of the field. The Mathieu differential equations describe the motions of the ions. The solutions of these equations are divided in two groups: group one, where the amplitudes of the oscillations remain limited for any length of time (stable conditions), and group two, characterized by a continuous increase in amplitude. Figure 5.2 shows the stability diagram of a quadrupole mass filter with the parameters a =

INTRODUCTION 111

QMA 400 8 mm Rod System

QMH 400-5 1..512amu

QMA 410 16 mm Rod System

QMH 400-1 1..128amu

E-11

E-12

E-13

E-14

A ,*A-

H~ r^ws^wtv^

1 2 3 4 5 6

E-12i

E-13

E-14

I [A]

n r

<\

3 4 m/e (a) (b)

Figure 5.3. Abundance sensitivity of an 8 mm (a) and a 16 mm (b) rod system

4eU'¡mr\\x>2 and q = 2eV/mr^oj2. For a given set of operational parameters V̂ U, LO, and ro , only ions of a definite mass interval pass the 'mass filter'. The amplitudes of oscillation of these ions are smaller than ro. Ions with amplitudes greater than ro are separated out. They strike the rods, are neutralized and are pumped away as gas. The resolution of a quadrupole can be varied by altering the U/V ratio. The mass scan can be effected by varying the voltage (m varies with V), and a linear mass scale is obtained.

5.1.2 Abundance Sensitivity

The abundance sensitivity is defined as the contribution of a signal at mass m to the neighbor mass m ± 1. An abundance sensitivity of 106 means a contribution of a mass to the neighbor mass of 1 ppm. Figure 5.3 shows the resolution and the abundance sensitivity of two different QMS types in the low mass range. A brief description of the two quadrupole analyzers is given in Table 5.1.

Table 5.1. Technical data of quadrupole analyzers for isotope ratio measurement

Mass range Transmission Rod system

Diameter Length Material

Mass scale stability over 8 h Radio frequency quartz stabilized

QMA 400

1-512 30

8 mm 200 mm molybdenum ± 1/64 mass unit 2.25 MHz

QMA 410

1-128 >50

16 mm 300 mm molybdenum ± 1/64 mass unit 2.05 MHz


Helium was the main gas component. The absence of mass 3 in the background was tested before He was introduced. In Figure 5.3(a), an 8mm molybdenum rod system was used. The log scale shows the separation of masses 3 and 4 and an abundance sensitivity of better than 106. The offset function of the software was used to shift the baseline and to display the noise of the electronics. The following ion currents were measured:

4He 3He Baseline

7.181 x 10~8 A 1.20 x 10"13 A 2.0 x 10~14 A

The quantitative analysis gave 1.39 ppm for 3He, a value which agrees well with published values [15]. The contribution to the neighbor mass calculated from the spectrum is smaller than 50 ppb. With a 16 mm rod system, masses 3 and 4 are completely separated and the abundance sensitivity can be calculated to better than 108 (see Figure 5.3(b)).

5.1.3 Dynamic Range of Quadrupole Mass Spectrometers The noble gas Xe in air is an excellent standard given by nature to test the dynamic range of a mass spectrometer and the quality of the vacuum system used. The spectrum in Figure 5.4 shows Xe with all isotopes in air. The measurement was made with a GAM 500 gas mass spectrometer from Balzers.

[E-12A1

3.80000 3.60000 :

3.40000:

3.20000 3.00000 2.80000:

2.60000:

2.40000:

2.20000:

2.00000:

1.80000:

1.60000:

1.40000 1.20000:

1.00000 0.80000 0.60000:

0.40000

8.50000 B.5OO00 7.50000 7.50000 6.50000

Xe: 1.6 ppb in air 5.50000 S.50000 5.50000 4.5O0O0 4.50000

Xe: 0.1 ppb in air 3.5OOO0 3.50000 2.50000

138 141 [amuj

Xe: 9 ppb in air

Xe: 1.6 ppb in air

124 126 128 130 132 134 136 138 140 [amul

Figure 5.4. Mass spectrum of Xe in air

THERMAL IONIZATION MASS SPECTROMETRY WITH QUADRUPOLES 113

Ambient air was introduced into the instrument without any enrichment. A cross-beam ion source with tungsten filaments was used for electron impact ionization. The ions were separated in a type QMA 400 quadrupole rod system (mass range 1-512) with a quartz stabilized QMH 400 RF generator and detected with a 17 stage 90° off-axis SEM and an EP 112 preamplifier. The isotope l36Xe corresponds to a concentration of about 8 ppb. Even the minor isotopes 124Xe and 126Xe can be detected. The concentrations of I24Xe and 126Xe are about 0.1 ppb. The spectrum was recorded in multiple scan mode corresponding to an integration time per peak of 50 s. For detecting such low concentrations, the analyzed air must be very clean and free from interfering Fréons and hydrocarbons and the background spectrum of the MS must be very low. The noise of the detection system should be as low as possible and contamination by interfering components in the gas inlet system must be avoided. This spectrum shows that for analytical quadrupoles it is possible to work over a dynamic range of more than 10 decades.

5.2 THERMAL IONIZATION MASS SPECTROMETRY (TIMS) WITH QUADRUPOLES

5.2.1 Thermal Ionization Mass Spectrometry with a Quadrupole Mass Spectrometer (THQ)

In TIMS, ionization of the elements takes place on the surface of a hot filament. Metals with low first ionization potential are easily ionized as positive ions, and non-metal elements with a high electron affinity readily yield negative ions. A high electron work function of the filament enhances the positive ion beam, whereas a low work function of the filament increases the yield of negative ions. To improve the ionization efficiency for elements with high first ionization potentials, special methods such as the silica gel technique are applied (see Table 5.2). Thermal ionization is discussed in detail in Chapter 7.

The THQ is a commercially available quadrupole thermal ionization mass spectrometer from Finnigan MAT with a quadrupole mass analyzer from Balzers (Type QMA 150), of mass range 500 amu. The filaments, sample

Table 5.2. Mass spectrometric techniques for positive thermal ionization with the THQ

Ion source single filament rhenium Preparation technique (a) silica gel/H3P04 elements: Cu, Zn, Cd, TI, Pb

(b) silica gel/H3B03 elements: Cr, Fe Filament temperature (a) 700 °C: TI, 950 °C: Cu, 1150 °C: Cd

1180°C:Pb, 1500°C:Zn, (b) 1200 °C: Cr, 1260 °C: Fe


magazine, and computer control and evaluation program are from the Finnigan MAT 261 high precision mass spectrometer. A detailed description of the instrument is given in [6]. The sample magazine takes up to 13 samples, which can be analyzed automatically. Single, double and triple filament techniques can be applied. The absence of a magnet sector analyzer allows fast switching between positive and negative thermal ionization. Three different detector systems are available. With the Faraday cup, ion currents from 1 0 1 4 to 10~10 A can be detected. An analog SEM works in the ion current range from 10~12 to 10~16 A. The ion counting device with a pulse discriminator and a high freque-ncy counter operates in the range 10°-107 cps. The combination of ion counting and Faraday detection provides a dynamic range of 9 orders of magnitude.

5.2.2 Isotope Dilution Mass Spectrometry (IDMS)

The principle of IDMS is explained in detail by Heumann [3,4] and Webster [5]. A known amount of a spike isotope, preferably a stable isotope with minor natural abundance, is added to the sample. Solid samples must be completely decomposed, and sample and spike isotope must be mixed. After this isotope dilution step, the element has to be isolated and the isotope ratios can be measured in a mass spectrometer (the equations for IDMS are given in Chapter 10, Section 2. It is important to note that the isolation procedure need not be complete and that loss of the element during the isolation step has no influence on the result of the analysis. In recent years, oligoelement techniques have been developed to analyze several elements from one sample. These techniques reduce cost and time and offer the possibility of using the IDMS technique also for applications in which numerous samples have to be analyzed.

5.2.2.1 Uncertainty of IDMS and Optimization

IDMS is influenced mainly by uncertainties in spike concentration, isotopic composition of the spike, isotopic composition of the sample, the instrumental factor k, and the isotope ratio measurement of the blend. Possible sources of errors in IDMS are discussed in Chapter 10, Section 2. The uncertainty of an IDMS analysis can be minimized by optimization of the spike addition [4].

IDMS is one of the most powerful analytical techniques on the basis of isotope ratio measurements for the certification of reference materials and round-robin studies. As an isotopic specific method it is also used in the IMEP (International Measurement Evaluation Program). The aim of IMEP is to test the traceability of measurement methods for field laboratories [7,8]. In some of these programs IDMS with THQ was used. Table 5.3 lists some applications of IDMS with the THQ for certification and IMEP programs.

In IMEP-2 [8], Cd in polyethylene was analyzed by 23 participants from eight countries with nine different methods. Figure 5.5 shows the results from


Table 5.3. Analyzed elements in different certification rounds of the BCR and IMEP studies with IDMS and THQ

Sample Analyzed elements Ref.

BCR 184 (Bovine Muscle) BCR 185 (Bovine Liver) BCR 186 (Pig Kidney) BCR 189 (Brown Bread) BCR 191 (Wholemeal Flour) BCR 278 (Mussel Tissue) BCR 277 (Estuarine Sediment) BCR 280 (Lake Sediment) BCR 320 (River Sediment) Polyethylene materials Synthetic and natural water

Pb, Cd, Cu, Zn Pb, Cd, Cu, Zn Pb, Cd, Cu, Zn Pb, Cd, Cu, Zn Pb, Cd, Cu, Zn Pb, Cd, Cu, Zn, Cr, Fe Pb, Cd, Cu, Zn, Cr Pb, Cd, Cu, Zn, Cr Pb, Cd, Cu, Zn, Cr Cd Cu

[10] [10] [10] [10] [10] [11] [12] [12] [12]

[8] [7]

Certified Value: (1.761 ± 0.043)mmol kg (197.9 ±4.8) mg kg"'

more experienced lab A less experienced lab.

26 13 21 06 07 09 14 08 22 10 23 15 17 19 00 02 25 05 03 12 18 20 16 24 01 04 CODE NUMBER OF LABORATORIES ARRANGED BY ASCENDING VALUES

Figure 5.5. Cd in polyethylene. Results from 24 participants in eight countries with nine different methods


Table 5.4. Total and aqua regia-soluble concentrations of Pb, Cd, Cu, Zn, and Cr in river sediment BCR 320

Methode

Total concentration Ranges of all labs Certificate IDMS (THQ) IDMS (Mag. See. 1)" IDMS (Mag. See. 2)"

Pb

32-44 42.3 ±1.6 43.4 ±0.9 44.3 ±0.4 -

Aqua regia soluble concentration Ranges of all labs IDMS (THQ)

25-34 28.2 ±0.7

Concentration (ug g ')

Cd

0.45-0.59 0.533 ±0.026 0.586 ±0.013 0.584 ±0.016 0.564 ±0.014

0.43-0.51 0.517 ±0.002

Cu

42.1-48.0 44.1 ±1.0 44.6 ±0.3 44.8 ±1.6 43.9 ±1.2

39.3-45.7 43.9 ±0.4

Zn

124-148 142 ± 3 142 ±0.6 144.7 ±0.9 -

119-133 119.1 ±0.1

Cr

127-158 138±7 131.3±1.6 --

64-79 78.8 ±0.4

" Mag. See. 1, Mag. See. 2. magnetic sector instrument labs 1 and 2

one of the four materials, arranged by ascending values. The certified value by IDMS with the THQ was (1.761 ±0.043) mmol kg"1. 85% of the results were within ± 10% of the certified value, and 77% of 'declared uncertainties' were adequate [8]. In Table 5.4 the total concentrations and the aqua regia-soluble concentrations in BCR 230 (river sediment), analyzed with a THQ, are compared with certified values, the results from other laboratories, and IDMS results from two other laboratories in which magnetic sector instruments were used. The results obtained by the three laboratories using IDMS agree very well. There is no significant difference in accuracy and precision between the results obtained with magnetic sector instruments and with the THQ. For Pb, there was a significant difference of about 14% between laboratories which used chemical pretreatment of the sample, by, for instance, DPASV (differential pulse anodic stripping voltammetry), AAS (atomic absorption), ETAAS (electrothermal AAS), ZETAAS (Zeeman ETAAS) and ICP, and those that used methods without chemical treatment: EDXRF (energy dispersive X-ray fluorescence), RPAA (radiochemical proton activation) and IDMS. Sediments sometimes show high contents of sulfate and aluminum silicates. During chemical pretreatment of the sample, coprecipitation of PbS04tnay cause loss of Pb. Incomplete digestion of Al silicates, which fix Pb, may also be a source of error. The results of IDMS are not affected by adsorption effects after the isotope dilution step (fast reaction) has taken place. As a consequence, the certification of Pb in the reference materials BCR 277, 280 and 320 is based on RPAA, EDXRF and IDMS only.

If an element is fixed in the solid matrix, no isotope exchange takes place. Therefore, IDMS can be used to differentiate between the aqua regia-soluble content and the total amount of an element. Another type of species analysis


Table 5.5. Determination of heavy metals in different samples with IDMS and detection limits of the method

Sample

Mussel tissue

Surface water of creek (cone. x l O - 3 )

Detection limit: Inorg. and org. solid samples Water samples

1% i i i i

Q

• iM

Pb

1.900 ±0.005

0.339 ±0.002

3 x 10"3

1 x 10"6

1 PPM i i

•

Cd

0.314 ±0.005

0.0033 ±0.0006

1 x 10~3

1 x 10"6

Concentration (ng g ')

Cu

9.58 ±0.18

0.777 ±0.007

2 x IO"3

9 x IO"6

1 PPB 1 PPT i • i

• H H H

i

Zn

79.4 ±0.5

2.74 ±0.01

27 x 10"3

12 x IO-6

1 PPQ i 1

| Aerosol sample!

Cr

0.78 ±0.05

0.390 ±0.003

6 x 10"3

1 x 10-6

3 (Antarctica)

Snow (Antarctica)

Groundwater

Surface water

Food

Fe

133.1 ±1 .6 116 ± 1

12 x 10"3

30 x 10"6

Soils and sludges, uncontaminated

Sediments

Contaminated soils, sludges and fly ash

Figure 5.6 Concentration ranges of lead in different sample types that can be analyzed with THQ

was shown by Goetz and Heumann [9]. In humic acid-containing water samples, the measured concentration of Cr was different with and without decomposition of the sample. Cr can be complexed by humic acids. Cr(III) has d3 electron configuration and forms kinetically stable complexes, which are inert towards isotope exchange.

The reliability of IDMS and its importance as a reference method were shown in the analysis of Cd in the reference material BCR 144 [13]. The certified value for Cd was (4.82 ± 0.97) ppm. The analysis of this RM for Cd with the THQ (the certification was already completed) gives (3.76 ± 0.07) ppm, which agrees very well with the results from another laboratory using a


magnetic sector instrument (3.83 ± 0.10 ppm). A revision was performed and the certified value was changed.

Example analyses of an organic material (BCR 278, Mussel Tissue) and surface water are given in Table 5.5. Typical detection limits for routine analysis with the THQ are also given in [11].

Figure 5.6 gives an overview of the concentration ranges of Pb in different sample types which can be analyzed with IDMS and THQ. This picture demon-strates the wide dynamic range of IDMS with the THQ of about 10 decades.

IDMS with thermal ionization is time consuming, especially the sample preparation steps. Nevertheless it is an excellent analytical tool, which makes it possible to compare measurements made with different methods and in different laboratories and countries. In comparison with ICP-MS, TIMS shows very few interferences and is very selective.

5.2.3 Isotope Abundance Measurements with the THQ

Isotope variations are very small for most elements, and therefore they do not influence the IDMS results. Exceptions are Ar, B, C, Ca, H, N, O, Pb and Sr. The error in IDMS caused by isotope variations in nature is negligible for double isotope elements when one isotope has a high and the other one a low abundance [4]. Exceptions are elements with similar isotope abundances, like Pb. Pb consists of the four isotopes 204Pb, 206Pb, 207Pb and 208Pb. Apart from 204Pb, all Pb isotopes are end products of the radioactive decay of Th and U. Table 5.6 lists the isotopic abundances of Pb in different samples, determined with the THQ. The results are not corrected for the instrumental discrimination factor. The biggest difference in the isotopic composition is shown by the phosphate ore, with 41.06% 206Pb. Phosphate ores are used for the production of fertilizers. This may explain the higher content of 206Pb in the fertilizer sample. It is important to note that even chemical compounds may show significant isotope variations. In different Pb standard solutions from Merck, for example, the isotope abundance of 206Pb varies from 24.44 to 25.95%. Lead

Table 5.6. Pb isotope abundances in selected samples analyzed with a THQ Sample 204Pb (%) 206Pb (%) 207Pb (%) 208Pb (%)

Soil (Würzburg) [4] 1.33 ±0.02 24.83 ±0.07 21.49±0.03 52.34±0.12 Sewage Sludge (BCR 145) [4] 1.35±0.03 24.93±0.07 21.57i0.12 52.15i0.18 Brown Bread (BCR 189) [14] 1.26±0.01 24.99 ±0.06 21.63 ±0.03 52.13 ±0.03 Estuarine Sediment (BCR 277) [14] 1.37±0.02 25.10±0.01 21.45 ±0.02 52.08±0.04 Fertilizer [14] 1.34 ±0.02 25.28 ±0.08 21.32 ±0.05 52.06 ±0.13 Phosphate Ore (BCR 32) [4] 1.05±0.01 41.06±0.15 17.57±0.01 40.41 ±0.06 Pb standard solution 1 (Merck) [14] 1.40 ±0.02 24.44 ±0.03 21.85 ±0.03 52.31 ±0.07 Pb standard solution 2 (Merck) [14] 1.37±0.01 25.95±0.03 21.19±0.04 51.49±0.07

http://21.57i0.12

http://52.15i0.18

THERMAL IONIZATION MASS SPECTROMETRY WITH QUADRUPOLES 1 1 9

Table 5.7. Isotope abundances of Cd analyzed with a THQ in comparison with IUPAC values

Cd isotope l06Cd 108Cd "°Cd n ' C d , l2Cd l l3Cd , l4Cd 116Cd

THQ uncorrected (%)

1.253 ±0.012 0.892 ±0.006

12.516 ±0.042 12.834 ± 0.022 24.120 ±0.025 12.238 ±0.027 28.679 ±0.048

7.470 ±0.038

IUPAC (%) [15]

1.25 (3) 0.89 (1)

12.49 (9) 12.80(6) 24.13 (11) 12.22 (6) 28.73 (21)

7.49 (9)

standard solutions are used for reverse IDMS to characterize the Pb spike solutions.

As an example of isotopic abundance, Table 5.7 shows measurements of Cd with a THQ in comparison with the IUPAC values [15]. For Cd there is no isotopic reference material available, so it is not possible to correct mass frac-tionation in the mass spectrometer. However, this correction is not necessary for IDMS if the isotopic composition of the sample, the spike and the isotope diluted samples are measured on the same instrument and under the same conditions.

Isotope mass fractionations may appear in the ion source—lighter isotopes are preferentially evaporated and thus the heavier isotopes are enriched—and during the mass separation in the quadrupole filter. The k factor covers all isotopic mass fractionation effects (see eq. 1). For CI the k factor was determined as being 1.0229 ± 0.0056. This factor includes the uncertainty of the IRM and the isotope ratio measurement.

Isotope ratio of the IRM ... lr — ( 1 )

Isotope ratio measured byMS Table 5.8 gives some examples of isotope ratio measurements [4] carried out

with a THQ and with a magnetic sector instrument. The results are identical within the uncertainties of the two instruments. The relative external standard deviation for the quadrupole instrument was 0.2-0.6% and that for the sector field instrument (CH5-TH) was 0.1-0.2%. Koppe and Heumann used a THQ for isotope ratio measurement of Mo, V, Ti, and Zr and determination of these elements in water samples [16]. Mo was measured with negative thermal ionization as MoO^ and the other elements were determined by positive thermal ionization. Kastenmayer used a THQ with an ion counting detector to analyze the isotope ratios of Sr, Nd and I [17]. For I, a synthetic mixture was used, because I has only one natural isotope, ,27I. Sample load was a few ng.

The reactions 35Cl(n,p) and 36Cl(n,p) are important for astrophysical calcula-tions, especially for nucleosynthesis of the isotope 36S [18]. Investigation of

0.2491 ±0.0004 3.133 ±0.003 1.026 ±0.001

0.1618 ±0.0002

0.2 0.1 0.1 0.1

120 ADVANCED ISOTOPE MASS SPECTROMETRY in

Table 5.8. Comparison of isotope ratio measurements with a THQ and a magnetic sector instrument

Isotope ratio Quadrupole RSD Magnetic sector RSD 10B/UB 0.2490 ±0.0004 0.2 35C1/37C1 3.130±0.007 0.2 79Br/81Br 1.024 ±0.006 0.6 127I/128I 0.1592 ±0.0005 0.3

Table 5.9. Isotopic composition of a 37CI spike with a THQ Sample No. Amount (pg) 35C1/37C1 35CI (%) 37C1 (%)

1 2 3 4 5 6 7

8 8

16 16 16 4.8 4.8

Mean 1SD RSD 0.1%

0.069 47 0.069 25 0.069 40 0.069 33 0.069 23 0.069 36 0.069 37

0.069 34 0.000 09 0.1%

6.496 6.477 6.490 6.484 6.475 6.486 6.487

6.485 0.007 0.03%

93.504 93.504 93.510 93.517 93.525 93.514 93.513

93.515 0.026

these reactions requires well defined deposits. Thermal mass spectrometry with a THQ was used to determine the isotopic composition of the targets and the chlorine concentration was measured by IDMS [19]. As an example for the measurement of the isotopic composition of a spike, the measured isotope abundances of the 37C1 spike used are listed in Table 5.9. Seven independent sample preparations with different amounts of CI do not show any difference in the isotopic abundance. The IDMS results could be used to calibrate a spectrophotometric method. Table 5.10 gives the results of the isotopic analysis of Ag36Cl. For the correction of isotope mass fractionations, the k factor was determined with the NIST reference material NaCl (see Table 5.11) and used for the correction of the isotope ratios in the targets.

A comparison between a THQ and a magnetic sector instrument for the determination of 10B abundances in reference deposits is given by Lamberty and de Bièvre [20]. The relative standard deviation (la) for 10B (three replicates) was 0.02%, and that for the magnetic sector instrument was 0.006%.

5.2.3.1 Bioavailability and Metabolic Studies with Stable Isotopes

TIMS with a THQ was used by Vieira and Yergey to study the absorption and kinetics of calcium in premature infants, pregnant and lactating women and


Table 5.10. Isotopic analysis of Ag36CI target Sample no.

1 2 3 4 5 6

Mean 1SD RSD

35C1/37C1

3.025 ±0.005 3.014 ±0.003 3.014 ±0.005 3.021 ±0.004 3.017 ±0.004 3.014 ±0.005

3.018 0.005 0.17%

36C1/37C1

0.025 40 ± 0.000 62 0.025 95 ±0.000 13 0.025 79 ±0.000 30 0.025 61 ±0.000 42 0.026 34 ± 0.000 39 0.025 91 ±0.000 31

0.025 83 0.000 32 1.2%

35C1 (%)

74.683 74.60 74.61 74.66 74.62 74.60

74.63 0.03 0.04%

36C1 (%)

0.627 0.642 0.638 0.633 0.651 0.641

0.639 0.008 1.25%

3 7ci (%:

24.69 24.75 24.75 24.71 24.73 24.75

24.73 0.003 0.12%

Table 5.11. Isotopic analysis of the NIST standard reference material 975 (NaCI) Sample no.

1 2 3 4 5 6 7

Mean 1SD RSD

35C1/37C1

3.0615 3.0537 3.0628 3.0597 3.0652 3.0628 3.0556

3.060 2 0.0044 0.14

35C1 (%)

75.379 75.331 75.386 75.368 75.401 75.386 75.343

75.371 0.026 0.035

37C1 (%)

24.621 24.668 24.614 24.632 24.599 24.614 24.657

24.629 0.026 0.11

people with disorders of calcium metabolism [21]. They also discussed automated data analysis as a function of the separation technique.

Eagles et al. [22, 23] described a method for the measurement of apparent zinc absorption in human nutrition studies. The absorption of Zn, especially in the Western diet, depends on many dietary and physiological factors and is very important for some groups of the population, such as pregnant women. For nutritionally vulnerable groups, in particular, it is unethical to use radio-nuclides, so stable isotopes are the only option. In this study, 67Zn was used as a stable isotope, and TIMS with THQ was compared with fast atom bombard-ment (FAB).

5.2.3.2 Total Evaporation of Sample

For measurements of nuclear samples and sample sizes of a few ng, a total evaporation technique for the THQ was applied by Boness et al. [24]. The total


integration of all isotope ion currents avoids the need to correct for isotopic fractionation. Examples are given for U, I, and B.

5.3 ISOTOPE RATIO AND ISOTOPE ABUNDANCE MEASUREMENTS IN GAS SAMPLES

5.3.1 Electron Impact Ionization A detailed description of electron impact ionization is given in [25] and [26].

In principle, atoms or molecules present in the gas phase are bombarded with electrons of low energy. A small proportion of the gases will be ionized, and ions with single and multiple charges are produced. The number of ions depends on the electron energy. The maximum for most gases is in the range 50-150 eV. With higher energies multiple charged ions occur. Electron impact ionization is discussed in detail in Chapter 7.

5.3.2 Cross-Beam Ion Source A schematic diagram of a cross-beam ion source used for isotope analysis is shown in Figure 5.7. The ion source is equipled with two filaments. The Wehnelt electrode bundles the electrons emitted by the cathode and the elec-trons enter the ionization space. With sufficient electron energy, a small amount

Operating mode Constant emission Emission Current 1 mA VO, QMA, GND, gen. reference potential ground V1, IONREF ion formation potential 90 V V2, CATH election acceleration voltage 70 V V3, FOCUS ion lens 20 V V4, F.AXIS field axis voltage 15 V V5, EXTRAC extraction voltage 250 V V6. DEFL I deflection voltage 300 V

u„

u 1 2

—0%

U| I

i -V 2 CATH |(

V 3 - ,

V5-—>-EXT 3ACT

, + V 1 3NR EF-

V 4 -F. AXIS

I // h*

V6-DEFLI

?\V

« * m

Figure 5.7. Cross-beam ion source for electron impact ionization

ISOTOPE RATIO AND ISOTOPE ABUNDANCE MEASUREMENTS IN GAS SAMPLES 123

of the gas in the ionization chamber is ionized, drawn out by the extraction electrode and focused into the quadrupole filter by the ion lens. To enhance sensitivity, a magnetic accessory, which concentrates the emission current, can be installed. The injection conditions into the quadrupole, the resolution and the transmission will be improved.

5.3.3 Gas inlet systems To introduce gas for isotope analysis into the ion source of a QMS with electron impact ionization, two types of gas inlet systems are used: (a) a batch gas inlet, and (b) a continuous gas inlet, one or two stage.

In general, the working pressure of the MS is much lower (typically 10- 6-10-5 mbar) than that of the gas sample (typically to 1000 mbar). Therefore a pressure reduction is necessary. Figure 5.8 shows a batch gas inlet system with a turbomolecular pumping station. The turbo makes a very low background

To MS

Figure 5.8. Batch gas inlet system with a turbomolecular pump


contribution to the samples and therefore enhances the precision of isotope ratio measurements by lowering memory effects. Different samples and calibration gases can be connected at the same time. The gaseous sample is expanded stepwise into die batch volume. The valve combination 3 and 4 can be designed as a calibration valve with a known volume (e.g 10, 100 or 1000 pi). A pressure gauge, preferably a gas type independent gauge such as a Baratron head, controls the filling pressure for calibration and analyses. From the batch volume, an orifice, a capillary or a diaphragm leak reduces the pressure to the ion source of the mass spectrometer. The flow is mass dependent and con-sequently the heavier isotopes are enriched in the batch volume, whereas the lighter isotopes increase in the ion source. Therefore, calibration and measure-ment must be performed under exactly the same pressure conditions and timing. For very precise isotope ratio measurements, symmetrical inlet systems (see Figure 5.9) are the best choice. Sample measurements can be directly compared

D-OSO—i Baratron

Exp. Vol.2

cScj

Baratron

Sample

IToMS

To pump station

v ^ Figure 5.9. Symmetrical sample inlet system for precise sample reference measure-ments

ISOTOPE RATIO AND ISOTOPE ABUNDANCE MEASUREMENTS IN GAS SAMPLES 125

with those for isotopic reference samples or laboratory standards. For IDMS and tracer experiments, in most cases, simpler versions can be used. If enough sample material is available, continuous gas inlet systems are also applicable. Isotope demixing effects are considerably lower for continuous gas inlet systems, but the gas consumption is much higher (several ml min- 1) than for batch inlet systems.

5.3.4 Isotopic Analysis of Different Noble Gases

A GAM 500 gas mass spectrometer from Balzers was used to analyze the isotopic composition of different noble gases. This instrument is air conditioned inside, with temperature stability better than ± 1 °C, resulting in excellent stability of the electronics.

The GAM 500 uses an 8 mm molybdenum rod system with a quartz stabilized RF generator. The configuration of this mass spectrometer is listed in Table 12. The instrument was operated in the MID (multiple ion detection) mode, and a Faraday detector was used. For lower isotope abundances, a 90 ° off axis SEM is available. The precision of isotope abundances for the main isotopes is 0.005-0.01 %.

Ne was analyzed in a mixture of 30% Ne in He from an air separation plant. Ar was analyzed in a mixture with N2, and some other components. The con-centration of Kr and Xe was 10% in Ar carrier gas. The results are listed in Tables 5.13-5.16 For every noble gas, six independent analyses were carried out, each analysis compirising three blocks with 20 measurements. No outlier test was applied. The results are not corrected for the instrument factor.

For batch gas analysis the GAM 400 gas mass-spectrometer is used. Typically eight samples can be analyzed automatically in one run. A software package for isotope analysis corrects for time-dependent isotopic demixing effects. Calibration and analysis are executed under the same conditions. For very precise analyses, a symmetrical inlet system is available for sample and reference measurements. This unit gives similar results to the GAM 500.

Table 5.12. GAM 500 for on-line isotope ratio and abundance measurements Mass spectrometer Balzers QMA 400, 8 mm molybdenum rod system Ion source Closed cross-beam source with electron collimation magnets Filament Tungsten Detector Faraday Pumping system Pfeifer turbo molecular pump TPU 180H with membrane pump

MD 4 (mass spectrometer) Pfeifer turbomolecular pump TPD 020 with membrane pump MD 4 (gas inlet)

Gas inlet Quartz capillary with gold-plated dosing valve Valve manifold All-metal valves Software Balzers Quadstar 421™ (Windows based)

126 ADVANCED ISOTOPE MASS SPECTROMETRY in

Table 5.13. Ne isotope abundances analyzed with the GAM 500, uncorrected Ne isotope

20Ne 21Ne 22Ne

Table 5.14.

Ar-isotope 36Ar 38 Ar 40 Ar

GAM 500, uncorrected (%)

90.548 ±0.001 0.241 ±0.002 9.211 ±0.002

RSD (%)

0.001 0.8 0.02

IUPAC (%) [15]

90.514 (31) 0.266 (5) 9.220 (29)

Ar isotope abundances analyzed with the GAM 500, uncorrected

GAM 500, uncorrected (%)

0.333 4 ±0.000 2 0.065 1 ± 0.000 1

99.601 5 ±0.000 3

RSD (%)

0.06 0.15 0.000 3

IUPAC (%) [15]

0.336 5 (6) 0.063 2 (1)

99.600 3 (6)

5.3.5 Isotope Ratio Measurements in UFö Not only noble gases but also metals can be analyzed if they form volatile compounds. Uranium, which is also analyzed by TIMS, can be measured with a gas mass spectrometer if it is converted to UFô- Isotope ratio measurements and impurity analyses of UFô with a quadrupole mass spectrometer system for corrosive gas analysis are described by Huber et al. [27]. The sample consump-tion of the gas inlet system is about 1 - 1.5 mg/h- 1 . The abundance sensitivity is a few ppm. The relative standard deviation of a single ratio measurement is better than 1E-03, and the confidence for the ratio of ratios to be within 1E-03 is better than 99%.

5.4 STABLE ISOTOPE RATIO MEASUREMENTS WITH A QMS COUPLED TO AN ELEMENTAL ANALYZER

Elemental analyzers are used for many different materials, including pure inorganic and organic compounds, agrochemicals, soil and plant material. The most important elements are sulfur, hydrogen, nitrogen and carbon. If an ele-mental analyzer is coupled with a mass spectrometer, isotopic information is available. Quadrupole mass spectrometers can be very attractive for diagnostic tests, tracer experiments, metabolic studies, and life science experiments. Goetz et al. [28] describe the combination of an elemental analyzer, the elementar Vario EL from Elementaranalyse Hanau, with quadrupole mass spectrometers from Balzers for stable isotope ratio measurements (SIRMS). The Vario EL allows handling of small and large samples. Combustion of the sample is carried out in an oven at 1200 °C with direct oxygen injection. A purge and trap

Table 5.15. Xe isotope abundances (%) with the GAM 500, uncorrected

Block no.

1 2 3 4 5 6

Mean 1SD RSD IUPAC [15]

124Xe

0.09613 0.09611 0.09517 0.09465 0.09561 0.09542

0.0955 0.0006 0.62% 0.096(1)

126Xe

0.09066 0.08986 0.08908 0.08889 0.08991 0.08915

0.0896 0.0007 0.87% 0.090(1)

128Xe

1.9191 1.9193 1.9181 1.9177 1.9187 1.9182

1.9185 0.0006 0.03% 1.919(4)

129Xe

26.4356 26.4368 26.4357 26.4387 26.4385 26.4381

26.4372 0.0014 0.005%

26.44(8)

I30Xe

4.0790 4.0790 4.0781 4.0784 4.0795 4.0787

4.0788 0.0005 0.012% 4.08(1)

131Xe

21.1795 21.1798 21.1807 21.1810 21.1803 21.1801

21.1802 0.0006 0.003%

21.18(5)

132Xe

26.8866 26.8874 26.8890 26.8881 26.8860 26.8876

26.8875 0.0011 0.004%

26.89(7)

134Xe

10.4427 10.4420 10.4430 10.4423 10.4413 10.4423

10.4423 0.0006 0.006%

10.44(2)

l36Xe

8.8707 8.8706 8.8711 8.8705 8.8701 8.8704

8.8706 0.0004 0.005% 8.87(1)


Table 5.16. Kr isotope abundances (%) with the GAM 500, uncorrected Block no.

1 2 3 4 5 6

Mean 1SD RSD IUPAC [15]

78Kr

0.3529 0.3536 0.3529 0.3546 0.3547 0.3535

0.3537 0.008 0.23 0.360(4)

80Kr

2.2730 2.2733 2.2724 2.2730 2.2734 2.2719

2.2728 0.0006 0.03 2.277(4)

82Kr

11.5638 11.5634 11.5624 11.5622 11.5614 11.5607

11.5623 0.0012 0.01

11.58(1)

83 Kr

11.5505 11.5507 11.5514 11.5509 11.5509 11.5505

11.5508 0.0004 0.003

11.52(1)

84Kr

56.8911 56.8895 56.8898 56.8814 56.8870 56.8901

56.8882 0.0037 0.007

56.96(1)

86Kr

17.3687 17.3694 17.3711 17.3722 17.3726 17.3733

17.3712 0.0019 0.01

17.30(1)

system with autocontrolled adsorption and desorption permits dynamic gas separation. The QMS, fitted with a continuous gas sampling system with a very low dead volume (two stage pressure reduction), is interfaced to the outlet of the Vario EL. The QMS operated in the MID (multiple ion detection) mode. An ARCNET interface is used for fast data exchange between the QMS controller and the computer. The isotopic content of standard samples can be analyzed with a precision of 0.01-0.05% for N and C with sample sizes of some mg. The precision, expressed in ¿-values, is in the range 0.1-0.3. In natural samples, the 13C concentration is higher than that of 15N. Detection of the gaseous component CO2 is about 20% more sensitive than that of N2. Thus, for small samples, the precision for 13C is higher.

Thanks to automatic and simple operation, a large number of samples (80 in one run, analysis time 6-12 min per sample) can be measured and, as the instrumentation is cheaper than magnetic sector instruments, this method is suitable for routine analysis.

For isotopic analysis, different methods of calibration are available: laboratory internal standards (solid sample), isotopic reference materials (solid sample), gaseous samples and gaseous reference materials. For C, the analyzed gas is CO2, that for N is N2 and that for S, S0 2 . The calculation of 15N%, 13C% and 3 4S% is shown in Chapter 11, Sections 6, 7 and 16. Especially in the determination of 15N, the blank of the analysis process must be controlled very carefully to avoid contributions from air leaks. Combustion in oxygen should not produce CO, because the molecular peak from CO is an interfering ion on mass 28 (1 2C, 60).

Table 5.17 shows the results of the determination of 13C in a limestone sample. An 8 mm rod system QMS was used and the precision was 0.01% for 13C. Table 5.18 shows the result of the determination of 15N in urea with a precision of 0.02%. Urea was also used as a reference sample. The precision in the ¿-notation for 15N was 0.24.

STABLE ISOTOPE RATIO MEASUREMENTS 129

Table 5.17. 13C analysis in a limestone sample Sample no.

1 2 3 4 5 6

Mean 1SD RSD

13C(%) uncorrected

1.087 18 1.086 92 1.086 92 1.087 05 1.087 05 1.087 17

1.087 05 0.0001 0.01%

Table 5.18. Precision of l5N (%) determination in urea (uncorrected) Sample no.

1 2 3 4 5 6 0.277 Mean 1SD RSD

15N(%) uncorrected

0.346 60 0.346 71 0.346 64 0.346 59 0.346 50 0.346 72

0.346 63 0.000 08 0.02%

6(%o) against reference urea

-0 .069 0.231 0.0346

-0 .101 - 0.372

±0.24

For measuring isotopic variations, the <5(%o) value relative to a standard is calculated according to the following formula (eq. 2):

(5(%o) = (isotope ratiosamp|e/isotope ratiOstandard - 1) x 103 (2)

Table 5.19 gives an example of the determination of the 6 (l3C) in a coal sample against a laboratory standard. The sample input was several mg.

Continuous flow (CF) analysis with SIRMS is very attractive for tracer studies. Sometimes the amount of an element which is to be analyzed for isotopic composition is low. The precision for I5N and l3C in standard samples with natural isotopic composition as a function of the sample input is summarized in Table 5.20. The precision is calculated on the basis of 5-15 replicates. For lower sample sizes, the main restriction may be the blank value. The isotopic analysis must thus be attended by careful blank determinations and tests for air leaks and interference from the combustion process.

ADVANCED ISOTOPE MASS SPECTROMETRY III

Table 5.19. Analysis of the <5(13C) %o in a coal sample against a laboratory

standard

Sample no. <5(%o)

1 2 3 4 5 6

Mean 1SD

- 27.42 - 27.04 - 2 7 . 1 7 - 2 7 . 1 3 - 2 7 . 1 9 - 26.86

-27.13 ±0.18

Table 5.20. Precision of 15N and l3C analysis in standard samples

Amount of N or C Precision 15N (%) Precision 13C (%)

10-30 pg > 100 pg > 1000 pg

0.1-0.3% 0.05-0.1 0.03-0.02

0.05-0.08 0.05-0.03 0.03-0.01

If a sufficient number of samples can be analyzed, use can be made of an outlier test (ß-test, from Dean and Dixon [29]).

Russow et al. [30] describe a similar application for which an elemental analyzer interfaced to a quadrupole from a GC-QMS system was used. They attained a precision in standard samples of 0 .2 -1% for 13C and 0.5-1.2% for 15N. In natural samples the precision was 0.8-1.9% for 13C and 0.7-2.0% for l5N. These authors discussed sample inhomogenity as a significant impact on die precision of isotope analyses in natural samples.

5.5 HIGH RESOLUTION QUADRUPOLE MASS SPECTROMETRY

High resolution mass spectrometers are needed for nuclear research and fusion experiments. Measurement of the isotope 4He in D2 is required for leak detection of vacuum fusion vessels and for 4He/D replacement experiments [31]. 4He has mass number 4.003 and D2 has mass number 4.028. The difference of 0.025 amu corresponds to a m/Am value of 100. Most quadrupoles operate at unit resolution. For the separation of 4He and D2, high resolution QMS systems are necessary. The gas mass spectrometer GAM 400

HIGH RESOLUTION QUADRUPOLE MASS SPECTROMETRY 131

Table 5.21. GAM 400 with high resolution QMS

Ion source

Filament Detector Mass spectrometer

Pumping system

Gas inlet

Valve manifold Software

Cross-beam with magnetic accessory to collimate the electron beam Tungsten Faraday and 90° off axis SEM QMA 410 (molybdenum rod system, diameter 16 mm, length 300 mm) TPU 180H with MD 4 (mass spectrometer) TPH 065 with MD 4 (gas inlet system) Two symmetrical batch inlet systems, working pressure 10"'-10-4 mbar One batch inlet system, working pressure 10"'-2000 mbar All-metal valves Balzers Quadstar 421™ (Windows based)

0.30000 0.28000 0.26000 0.24000 0.22000-0.20000 0.18000 0.16000 0.14000 0.12000 0.10000-0.08000 0.06000 0.04000 0.02000

4.003 4.028 [amu]

Figure 5.10. Separation of 3.7% He in D2 with a Balzers Quadrupole QMA 410

4.003 4.028 [amu]

3.7% He

can be equipped with a QMA 410 analyzer (molybdenum rod system, diameter 16 mm, length 300 mm) which can be operated in a high resolution mode. The mass range in the high resolution mode is 1-20 amu, and that in the low resolution mode is 1-128. The configuration of such a unit with a symmetrical batch inlet system is listed in Table 5.21.

Figure 5.10 shows the separation of 3.7% He in D2. A spectrum of 1% D2 in He is presented in Figure 5.11. Within a few seconds, the mass spectrometer can


Ion Current (10 )

99% He

99% He 0.40000

0.35000

0.30000

0.25000 -4.003 4.028 [amu]

0.20000

0.15000

0.10000

0.05000 -

- 1 — I — | — i — i — i — i — r ~ i — r ^ — I f ' "<—I

4.003 4.028 [amu]

Figure 5.11. Separation of 1% D2 in He with a Balzers Quadrupole QMA 410

be switched from high resolution to low resolution and trace impurities in the He/D2 mixture can be analyzed. An example of trace analysis in a He/D2 mixture at a sample pressure of 0.01 mbar is given in [32].

Ellefson et al. [33] showed the separation of 4He and D2 with a QMS from Extrel. They used a 9.5 mm rod diameter operated at 5.1 MHz. Hiroki et al. [31 ] developed a high resolution QMS to detect 3He in HD and 4He in D2. They achieved high resolution with zone II conditions of the Mathieu diagram in the mass range 1-9. The normal mode of the QMS invalues the first zone and has a mass range of 1-60. The high or low resolution mode could be chosen by pushing a switch. In general, an increase in frequency gives higher resolution and sensitivity, but at limited U and V the mass range will be reduced.

REFERENCES

[1] W. Paul and H. Steinwedel, Z Naturforsch. Teil, A, 8a, 448 (1953). [2] P.H. Dawson (ed) Quadrupole Mass Spectrometry and its Application, Elsevier

Scientific Publishing Co., Amsterdam-Oxford-New York (1976). [3] K.G. Heumann, Fresenius' Z. Anal. Chem., 324, 601 (1986). [4] K. G. Heumann, in Inorganic Mass Spectrometry, F. Adams, R. Gijbels and R. Van

Grieken (eds.), John Wiley, New York, 1988, p. 301.

REFERENCES 133

[5] R. K. Webster, in Methods in Geochemistry, A.A. Smales and L.R. Wager (eds.), Interscience, New York, 1960, p. 202.

[6] K. G. Heumann, W. Schindlmeier, H. Zeininger and M. Schmid, Fresenius ' Z. Anal. Chem., 320, 457 (1985).

[7] A. Lamberty, G. Lapitajs, L. Van Nevel, A. Goetz, J.R. Moody, D.E. Erdmann and P.De Bièvre, in Biological Trace Elements, G.N. Schrauzer (ed.), Humana Press, 1994, p. 571.

[8] A. Lamberty, P. De Bièvre and A. Goetz, Fresenius' J. Anal. Chem., 345, 310 (1993).

[9] A, Goetz and K.G. Heumann, Fresenius' J. Anal. Chem., 331, 123 (1988). [10] A. Goetz and K.G. Heumann, Fresenius' J. Anal. Chem., 326, 118 (1987). [11] A. Goetz and K.G. Heumann, Adv. Mass Spectrom. B, IIB, 1702 (1989). [12] A. Goetz and K.G. Heumann, Fresenius' J. Anal Chem., 332, 640 (1988). [13] A. Goetz and K.G. Heumann, Fresenius' Z. Anal. Chem., 325, 24 (1986). [14] A. Goetz and K.G. Heumann, unpublished (1989). [15] P. De Bièvre and I.L. Barnes, Int. J. Mass. Spectrom. Ion. Processes., 65, 211

(1985). [16] M. Koppe and K.G. Heumann, Fresenius' Z. Anal. Chem., 331, 118 (1988). [17] P. Kastenmayer, Fresenius' Z. Anal. Chem., 331, 205 (1988). [18] C. Wagemans and S. Druyts, Proc. Int. Symp. on Nuclear Astrophysics, Baden/

Vienna, Report MPA/P4, Max Planck-Insitut für Physik und Astrophysik, Garch-ing, Germany, 1990, p. 296.

[19] R. Eykens, A. Goetz, A. Lamberty, J. Van Gestel, J. Pauwels, C. Wagemans, S. Druyts and P. D'hondt, Nucl. Instrum. Methods Phys. Res. Sect. A, 303, 152 (1991).

[20] A. Lamberty and P. De Bievre, Int. J. Mass. Spectrom. Ion Processes, 108, 189 (1991).

[21] N. E. Vieira and A.L. Yergey, Anal. Chem., 11, 4217 (1995). [22] J. Eagles, J. Fairweather-Tait, D.E. Portwood, R. Self, A. Goetz and K.G. Heumann,

Anal. Chem., 61, 1023 (1989). [23] J. Eagles, J. Fairweather-Tait, F.A. Mellon, D.E. Portwood, R. Self, A. Goetz and

K.G. Heumann, Rapid. Commum. Mass Spectrom., 3, 6 (1989). [24] M. Boness, M. Schmidt and G. Wagner, 11th Annual Symposium on Safeguards and

Nuclear Materials Management (EUR 12193 EN), Comm. Eur. Communities, p. 535 (1989).

[25] H. Kienitz (ed.), Massenspektrometrie, Verlag Chemie, Weinheim, 1968. [26] M. R. Litzow and T.R. Spalding, Mass Spectrometry of Inorganic and Organo-

metallic Compounds, Elsevier, New York, 1973. [27] W.K. Huber, G. Rettinghaus and P. Irving, ASMS (1978). [28] A. Goetz, N. Mueller, W. Boschmann and H.J. Kupka, Simultaneous elemental

analysis by coupling an elemental analyzer with gas mass spectrometer, Pittsburgh Conference, 1995.

[29] R.B. Dean and W. J. Dixon, Anal. Chem., 23, 636 (1951). [30] R. Russow, G Schmidt and H. Faust, hot. Environ Health Stud., 31, 211 (1995). [31] S. Hiroki, T. Abe, Y Murakami, K. Yanagishita and S. Nakamura, J. Vac. Sei.

Technol, A, 12(5), 2711 (1994). [32] High resolution quadrupole mass spectrometry, Balzers Application Report BG 800

049 AE. [33] R. E. Ellefson, W.E. Moddeman and H.F Dylla, J. Vac. Sei. Technol, 18(3), 1062

(1981).

CHAPTER 6

SPECIAL PURPOSE INSTRUMENTS

6.1 ISOLAB-54 IRMS 135 6.2 SENSITIVE HIGH MASS RESOLUTION ION MICROPROBE:

SHRIMP 137 6.3 CAMECA IMS-1270 138 6.4 GLOW DISCHARGE, GD IRMS 138 6.5 HIGH ISOTOPIC ABUNDANCE SENSITIVITY IRMS 139 6.6 ACCELERATOR MASS SPECTROMETRY, AMS 142 REFERENCES 145

6.1 ISOLAB-54 IRMS

The ISOLAB-54 is a double focusing isotope ratio mass spectrometer combining the options of monitoring positive and negative ions emitted from a hot filament (thermal ionization mass spectrometry, TIMS), secondary ions produced by sputtering a sample with a primary ion beam (secondary ion mass spectrometry, SIMS), and sputtered neutrals resonantly ionized with laser radiation (sputter induced resonance ionization mass spectrometry, SIRJMS). Sputtering is achieved with a primary Ar+ ion beam and resonance ionization is effected with a frequency doubled dye laser pumped by an excimer laser.

The instrument mass analyzer consists of a 381 mm radius, 81.5° electrostatic sector followed by a 270 mm radius, 90° magnetic sector with an exit pole angle of 26.5° and by a multicollector detection system containing four Faraday cups and a microchannel ion counting system for simultaneous ion counting of up to four minor isotopes. The specially designed magnetic sector has an ion dispersion equivalent to a 540 mm radius magnet. Selectable ion source and collector slit widths allow resolving power up to 1100 for TIMS and SIMS, securing a flat top peak shape. The highest resolving power is 4500. A second electrostatic sector installed after the multicollector system is equipped with an ion counting Daly detector, allowing high sensitivity isotopic abundance measurements over large dynamic ranges. (For the definition of abundance sensitivity see the following section on multi-sector IRMS.) Instrument opera-tion and data acquisition are computer controlled. Figure 6.1 shows a block diagram of the ISOLAB-54 mass spectrometer.

The following applications to isotopic ratio measurements have been demon-strated so far [1]. In the uranium Standard Reference Material NBS U010, the

136 SPECIAL PURPOSE INSTRUMENTS

Energy slit Duop las matron ion source

LP

Oo ip Magnet H ESA

O Primary beam o focussing column

a Baffles

D' Source slit IP O u Laser Window

Source Housing ? ̂J 3 Axis sample Multi-collector manipulator i i

\ Collector slit

Sample insertion system

T.S.P. Daly silt ( Daly detector

Figure 6.1. Block diagram of the ISOLAB-54 mass spectrometer: LP., ion pump; T.P., turbomolecular pump; T.S.P, Ti sublimation pump; C.P., cryo pump. (Not all of the pumping system is shown.) (Reproduced by permission of Elsevier Science NL from J.D. England et al., Int. J. Mass Spectrom. Ion Processes, 111, 201 (1992))

234U/238U ratio was measured with TIMS to a precision of « 0.1% (2RSD). The measurement accuracy could not be determined because the NBS standards are certified only to 0.1%. Uranium gravimetric standards were prepared containing different amounts of 233U. The measurements demonstrated that the Daly detector can be used to measure isotopic ratios with precision and accuracy at the parts per thousand level. Negative TIMS was applied to measure the 187Os/f88Os and 190Os/188Os isotopic ratios, monitoring the OsO^ ion. A LaThI standard was used for thorium isotopic ratio determination by SIMS. Sample sizes of 8-100 ng gave 230Th/232Th = 6.397 x 10 6 with an external precision of 0.6% (2RSD) (n = 15). Thorium TIMS ratio measurements, made on a standard TIMS instrument, provided better external precision (by a factor of ¡=s 2), but required several hundred ng of sample. SIMS measurements for hafnium, using 150 ng aliquots of JMC-475 Hf standard yielded 176Hf/177Hf = 0.282193 ± 14 x IO"6 (2SD, n = 9), which is in very good agreement with the results discussed in Chapter 9, Section 9.72. Samples of 7 ng were measured to 0.04% precision. This represented a 200-fold reduction in sample size

SENSITIVE HIGH MASS RESOLUTION ION MICROPROBE: SHRIMP 137

requirement in favor of SIMS compared with TIMS. Isotopic ratio determina-tions of osmium and rhenium were performed with the ISOLAB-54 instrument by applying the SIMS and SIRIMS techniques.

Lyon et al. [2] used the ISOLAB-54 in the SIMS mode, measuring in situ the 1 8 0 / 1 6 0 isotopic ratios from insulating materials, mainly quartz. A primary Cs+ ion beam of (0.1-5) x l0~ 9 A was focused into a spot of 10-100 pm in diameter, producing a secondary 1 6 0~ ion current of up to 5 x 10~u A. Typically, data were collected for 10 s, recording a 1 x 10 12 A 1 6 0~ ion current and 1.3 x 105 cps of , 8 0 ~ . Twenty ratios were measured in one block. The standard deviation per measurement from counting statistics (SD) was 2.8%o and the uncertainty of the mean (SD/i/n) per block was 0.6%o. The uncertainty in the determined ratio for a single measurement of a standard and a sample was l.5%o. In situ oxygen isotopic ratios were measured with the ISOLAB-54 also in geological and extra-terrestrial materials. [3]

An ISOLAB-120 IRMS has also been constructed. This instrument, which is unique in its ultra high isotopic abundance sensitivity, will be described in a following section.

6.2 SENSITIVE HIGH MASS RESOLUTION ION MICROPROBE: SHRIMP

The SHRIMP is a high sensitivity, high resolution, double focusing secon-dary ion mass spectrometer. [4] It is constructed from a cylindrical 85° electrostatic sector analyzer, an electrostatic quadrupole lens and a homo-geneous 100 cm, 72.5° magnetic sector analyzer with non-normal ion beam angle entry.

Positive or negative primary ion beams are produced in a duoplasmatron and focused to sa 20 pm diameter, hitting the target at 45°. The secondary ions are extracted at a potential of 11 kV and focused onto the entrance slit of the secondary mass analyzer. About 45% of the secondary ions are passed by the entrance slit and about 90% of these are transmitted to the collector slit. The instrument is used mainly for U-Pb geochronology studies of zircons, monitoring the ion beams of lead isotopes, Z r 2 0 + , Zr203' , U + , ThO+ and UO+ at a mass resolving power of 7200. A 20 pm diameter primary 0~ ion beam with an intensity of 2-4 nA yields a sensitivity of 5cps/ppm Pb. The isotopic ratios are measured by cyclic stepping of the magnetic analyzer. Beam intensities are measured with an electron multiplier in the ion counting mode. The precision attained for the 207Pb/206Pb ratio (30 measurements) was l% (2RSD) for standard zircons and 0.4% (2RSD) for the 207Pb-richer lunar zircons. Mass discrimination corrections, made with zircons of known ages, revealed a fixed discrimination between the measured 2 3 2 Th l 6 0 + / 2 3 8 U 1 6 0 +


ratio and the 232Th/238U ratio in the target

(232Th/238U) = l . l l ( 2 3 2 Th 1 6 0 + / 2 3 8 U 1 6 0 + )

The uncertainty in the factor of 1.11 is 1% (2RSD). Eldridge et al. [5] used the SHRIMP for in situ determinations of the 34S/32S

isotopic ratios in various sulfur-containing minerals. A lOkV, 3 x 10~9A primary O" ion beam produced secondary S + ions which were accelerated to 11 kV. Stable 3 2 S + ion beams with intensities ranging from 7 x 104 to 3 x 105

cps were emitted from the minerals, enabling 34S/32S ratio measurements to a precision and accuracy near 0.2% (2RSD). No memory effects could be detected when materials with different 34S concentrations were analyzed repeatedly.

6.3 CAMECA IMS-1270

The Cameca IMS-1270 is a new SIMS instrument designed for isotopic ratio analysis of solids. It is equipped with cesium and oxygen ion beam guns, the latter producing O - and 02 primary ions. The mass analyzer consists of an electrostatic sector and a large 585 mm radius magnetic sector. The detection system includes a multicollector arrangement with five adjustable Faraday collectors. The ion optics are designed for high ion transmission. Experimen-tally, 100% transmission is achieved up to mass resolving power of 3400, and 85 and 30% for resolving power of 5000 and 10000 respectively. The analytical performance of the IMS-1270 was summarized by de Chambost et al. [6] In the few published studies, the instrument was used mainly for in situ U/Pb dating of the uranium-containing mineral zircon, ZrSi04 [7, 8]. Flat top peaks were obtained at a mass resolving power of 6000, resolving the interfering 94Zr 96Zr 160+ and 177Hf 29Si+ ions from 206Pb+ and the corresponding HfSi+ interference from 2 0 7Pb+. It is of interest to note that, when the sample chamber pressure was increased from < 10~8 to 4 x 10~5 Torr by oxygen admission, the mass fractionation in lead was significantly reduced (the 'oxygen flooding effect'). This was demonstrated in a well characterized Sri Lanka zircon crystal, SL13, for which the mean observed 2 0 7pb+/2 0 6Pb+ isotopic ratio determined with TIMS is 0.05923. Mean values of 0.05962 were measured using SIMS with oxygen, and 0.05789 using SIMS without oxygen flooding [8]. Also, the Pb+ ion sputtering was observed to be increased with Oj bombardment compared with O - bombardment.

6.4 GLOW DISCHARGE, GD IRMS

Glow discharge mass spectrometry (GD-MS) affords the capability of direct sampling of solid inorganic materials. Isotope ratios of several elements and

HIGH ISOTOPIC ABUNDANCE SENSITIVITY IRMS 139

also elemental concentrations can be measured in a single sample. The advantage of the technique is that extensive sample preparation, including dissolution, chemical separation and purification can be omitted. The sample is introduced into the ion source as a pressed powder pellet or a solid pin.

The glow discharge mass spectrometer used for isotope ratio measurements [9-11J is a double focusing magnetic sector instrument with reverse Nier-Johnson configuration. A Faraday cup and a pulse counting Daly detector provide a combined dynamic range above 109. Samples are atomized by glow discharge argon ion bombardment (cathodic sputtering). The neutral species which diffuse into the negative glow region may be ionized and extracted from the discharge cell for mass analysis. Directly surface sputtered ions are affected by the fall in cathode potential and re-deposited. The discharge cell is cryogenically cooled. Mass resolving power of about 500, yielding flat topped peaks, was used for isotope ratio measurements. Ion intensities were measured by magnetic peak jumping between the peak centers of the masses of interest. Ten blocks of ten ratios were collected for each measurement, readjusting the peak centering at the start of each block. Riciputi et al. [9] measured isotopic ratios in NIST standard references materials (SRMs) of Sr (in Ag and Cu matrices), Re (wrapped on a Pt wire), Ag (in Cu), Pb (pure metal) and B (in Ag); in Cu, using metal, brass and pressed powder; and in Pb and Sb at 15 ppm elemental concentration in a solid Cu matrix. External precision better than 0.03% (1RSD) has been achieved for ratio measurements in pure solid elemental samples, a precision better than 0.1% for elements present in concentrations greater than 0.5 wt%, and precision of about 1 % for elements at 10-20 ppm concentration levels. A mass bias of up to 1% was observed, and was attributed to instrumental parameters, rather than to ion source isotopic fractionation. Donohue and Petek [10] measured isotopic ratios in palladium, reporting internal precision better than 0.05%. Duckworth et al. [11] directly measured isotopic ratios of uranium in soil samples, considered as very difficult matrices for GD-MS.

6.5 HIGH ISOTOPIC ABUNDANCE SENSITIVITY IRMS

White et al. [12] and Dietz [13] showed that an isotope ratio mass spectrometer with a tandem analyzer configuration, using two magnetic sector analyzers followed by an electrostatic sector analyzer (BBE configuration) demonstrated high isotopic abundance sensitivity and high ion transmission.

Before continuing the discussion on multi-sector instruments, it is worth clarifying the concept of isotopic abundance sensitivity (or abundance sensitivity). In a simple phrase, this is the ability of a mass spectrometer to measure a minor isotope in the presence of an adjacent major isotope. Quantitatively, abundance sensitivity is defined as the ratio between the


background intensity one mass unit away from the major isotope to the ion intensity of that major isotope, or it may be said that this is the ratio between the tail intensity one mass unit apart from the major isotope to the ion intensity of the major isotope. It is common to determine this quantity by measuring the ion intensities at m/z = 237 and 238 in a close to natural uranium isotopic standard reference material.

Lagergren and Stoffels [14] constructed a three stage mass spectrometer for heavy element isotopic ratio measurements with a similar BBE design. The main objectives of this work were to achieve (a) good sensitivity in measuring the abundance of a minor isotope in the presence of an adjacent major isotope, (b) sample sensitivity, and (c) accuracy of uranium isotopic measurements. The spectrometer comprised two symmetric, 90° sector and 12 inch radius magnetic analyzers in a zero dispersion configuration, followed by a symmetric, 90°, 12 inch radius cylindrical electrostatic analyzer. A V-shaped filament thermal ionization source [15, 16], which has high ionization efficiency, high transmission and focusing lenses, was used for ion production. Ion detection was achieved with the Daly secondary emission/scintillation detector [17]. This device has virtually a 100% detection efficiency and is free from mass and energy discrimination when used with an ion counting system. Using the NBS U010 standard reference material, the instrument had shown an abundance sensitivity of 10~9. It should be noted that a modern, conventional single stage magnetic sector thermal ionization mass spectrometer with a radius of 30 cm has a typical abundance sensitivity in the order of 5 x 10"6. Sample sensitivity, expressed as ions detected per loaded atom, of 2% was obtained for a sample of 0.36 pg, or 9 x 108 atoms, of uranium. Accurate results were achieved when correct electrostatic analyzer voltages were set and hydrocarbon background was minimized.

Following 'the need to measure ever larger isotope ratios in ever smaller samples', Stoffels et al. [18] constructed an additional three stage mass spectrometer of BBE configuration. The instrument has two tandem 90° sector, 27 cm radius electromagnets with off-normal boundaries to provide vertical ion beam focusing. The cylindrical electrostatic analyzer has a radius of 38 cm and a deflection angle of 81.5°. The ion optical design provides 100% transmission without the need for intermediate focusing lenses. It also prevents ion scattering from the top and bottom of the flight tube, and in conjunction with the electrostatic analyzer the isotopic abundance sensitivity is improved to a level of 1 x 10~ u , two orders of magnitude better that of the instrument with normal magnet boundaries. A dual detector system with Faraday cups and pulse counting electron multiplier provides the instrument with a dynamic range from 10~20 to 10~3 A. Figure 6.2. shows the ion optical design of this instrument.

Recently, Belshaw et al. [19] described the ISOLAB-120, a three stage mass spectrometer with an EBE configuration, designed to achieve high resolving

HIGH ISOTOPIC ABUNDANCE SENSITIVITY IRMS 141

18 1.48R S*

Magnet 2 Magnet 1

/ 90 18 A- \ *-" 54

/ 1.48 R

S-1 Ion

Source

Ion Detector

Electric Sector

Figure 6.2. Ion optical design of high isotopic abundance sensitivity triple BBE sector mass spectrometer. (Reproduced by permission of Elsevier Science, NL from J.J. Stoffels et al., Int. J. Mass Spectrom. Ion Processes, 132, 217 (1994))

power (m/Am « 1400) and high abundance sensitivity, approaching 10~". The instrument has a forward geometry double focusing mass analyzer with a 96 cm electrostatic sector and a 60 cm radius magnetic sector. This EB configuration ends with a Faraday cup multi-collector and ion counting detection system. A removable axial cup allows coupling of a second 96 cm electrostatic sector to improve the abundance sensitivity for the axial ion beam at the final detector. The ion source is capable of producing ions by thermal and secondary ionization and by resonant and non-resonant photoionization. A simplified scheme of the ISOLAB-120 is shown in Figure 6.3.

Belshaw et al. [20] used this instrument to measure the 10Be/9Be isotopic ratio. l0Be is a non-stable isotope with a half-life of 1.6 x 106 years, which is produced by interaction of cosmic rays with the atmosphere, or with material at the earth's surface or close to it. Beryllium samples were chemically preconcentrated and analyzed by sputtering Be ions from a tantalum target with a primary 0 + ion beam. At approx. 1000°C the contribution of 9BeH+ to 10Be+ at m/z = 10 was found to be negligible, and the contribution of 1 0 B +


Principal focal plane Second

electrostatic analyser

Magnet

Energy window

First electrostatic analyser

(Faraday)

Ion-counting Daly detector Be ( B

intermediate focal plane

SIMS ionisatio (Be+)

Figure 6.3. Schematic diagram of ISOLAB-120, illustrating the simultaneous measurement of beryllium isotopes. (Reproduced by permission of Elsvier Science NL from N.S. Belshaw et al., Int. J. Mass Spectrom. Ion Processes, 142, 55 (1995))

could be corrected by measuring n B + . At this temperature both interferences accounted for less than 1 cps. The mass discrimination was estimated to be 4 ± l%u_ 1 with the boron NIST SRM-951 isotopic reference material, and the precision of the I0Be/9Be ratio measurement for ratios as low as 10~9 was 4%.

6.6 ACCELERATOR MASS SPECTROMETRY, AMS

Accelerator mass spectrometry is a technique which allows the measurement of isotopic ratios as low as 10~14 of extremely rare isotopes. AMS was initiated in 1939 by Alvarez and Cornog [21] with the measurement of 3He at natural abundances. In principle AMS can be applied to any rare stable isotope and to radiogenic isotopes with not too short half-lives. In practice, most of the applications have been with rare radionuclides of half-lives ranging from 5000 to 16 x 106 years: 10Be, 14C, 26A1, 36C1 and 129I, for which ß-counting of very small samples is not practicable. A sample containing the element of interest in the range 0.2-3 mg and as few as (2-5) xlO5 atoms of the rare isotope is sufficient to attain a ratio precision of 10%. AMS has also been applied to other nuclides, as will be discussed in the following paragraphs.

A typical isotope ratio mass spectrometer is assembled from four basic units: an ion source, an ion extraction and acceleration system operating at up to

ACCELERATOR MASS SPECTROMETRY, AMS 143

lOkeV, a mass to charge analyzer including a magnetic sector with or without an electrostatic analyzer, and an ion detection system. An AMS instrument also includes an acceleration system to achieve high-MeV ion energies. This component, together with negative ion production and electron stripping to +3 or higher positively charged atomic ions, allows the separation and measure-ment of isotopic abundance ratios far lower than is possible at keV ion energies. A cesium ion beam gun is used in most AMS work to produce intense, pA range secondary negative ion beams. These ions are then accelerated to high energies between 2 to 8 MeV and pass through a thin foil or gas absorber, where several electrons are stripped off. Molecular multiply charged ions, because of strong repulsive forces, are rapidly decomposed. If the stripper is followed by electrostatic and magnetic sector analyzers, atomic ions in the +3 and higher states can be separated and detected with gas ionization detectors or silicon barrier detectors. The isotopic ratios are measured by alternate monitoring of the major isotope ion beam with a movable Faraday collector and the ion beam of the rare isotope with one of the above mentioned detectors. Interferences from ion scattering and charge exchange reactions with residual gases can be suppressed to any desired level by adding stages of mass selection. Molecular interferences are eliminated by the stripping process and mass separation of +3 or higher charged ions. Interferences from stable atomic isobars introduces the most difficult problems for AMS. Various techniques or their combinations are used to overcome this problem (a) high quality chemical sample purification; (b) negative ion formation in the secondary ion source. I4N, 26Mg, 36Ar and 129Xe are stable isotopes isobaric with 14C, 26A1, 36C1 and 129I, but they do not form negative ions. The stable 10B and 36S do interfere with I0Be and 36C1; (c) isobaric discri-mination can also be achieved by varying the acceleration energy, in general for higher mass of the isobaric pair, higher energy is needed. Very high energies strip all the electrons from low mass nuclides; (d) absorber thickness can be adjusted to stop an interference while allowing the radio-isotope to reach the detector; (e) the gas filled magnet is a powerful isobar separation device; (f) selective ionization, such as laser resonance and thermal ionization can also separate isobars. Further details of these techniques were given by Elmore and Phillips [22]. Instrumental background in the range of 1 x 10~15 to 2 x 10~14 (rare isotope/abundant isotope) is attainable [22].

Standards with known isotopic ratios must be analyzed frequently for normalization, and samples free of the rare isotope are analyzed to measure background. Ratios are corrected for time dependent mass fractionation by measuring at least two stable isotopes in the isotopic standard, and for fractionation which arises from the stripping process and from stray magnetic fields in the accelerator by comparison with the standard. Figure 6.4 shows a schematic diagram of the AMS components.


^Negative ion source

lit-— ES analyzer ] 3-

Magnetic Pf'e a n a , y z e r acceleration

Stripper at +HV terminal

Injector Tandem accelerator

Gas-filled ionization chamber

Time-of-flight

3-E Gas-filled magnet

Velocity selector

Magnetic analyzers

ES analyzer

Detector system Positive ion analysis

Figure 6.4. Schematic diagram of AMS instrumentation. Not all of the elements shown are used at each laboratory, in particular the injector electrostatic (ES) analyzer, the velocity selector, and the gas filled magnet are not used at most sites. The positive ion ES analyzer is sometimes located immediately after the accelerator. An offset Faraday cup is sometimes used after the injector magnet to monitor the source output while the radioisotope is being counted. In several laboratories rapid mass selection in the injector is accomplished by varying the potential of an insulated magnet vacuum chamber. Offset cups are often utilized after the first positive ion magnetic analyzer so that the field in that magnet does not have to be cycled. (Reproduced by permission of the American Association for the Advancememt of Science from D. Elmore and F.M. Phillips, Science, 236, 543 (1987))

Presently, 28 laboratories around the world use AMS [23]. The most intensively studied isotope is 14C [24-26], followed by 10Be, 26A1, 36C1 and 129I [22, 23, 27]. 41Ca determination [28], traces of precious metals [29], semiconductor impurities [30], the 187Re/1870s geochronological system [31], and 1 8 60s/1 8 70s ratio measurements in samples with total osmium of 0.2 ng at 0.01 ppb concentration level [32] had also been performed. The AMS technique has been applied to earth sciences, e.g. environmental chemistry, hydrology, geochronology, glaciology, minerals exploration, sedimentology, volcanology, climatology and cosmochemistry; to anthropology and arche-ology; and to nuclide search and half-life measurements in physics. AMS was reviewed in detail by Elmore and Phillips [22] and by Litherland et al. [27] A recent review by Rucklidge [23] provides updated information on current AMS research activities.

ACCELERATOR MASS SPECTROMETRY, AMS 145

REFERENCES [ 1 ] J.D. England, A. Zindler, L.C. Reisberg, J.L. Rubenstone, V. Salters, F. Marcantonio,

B. Bourdon, H. Brueckner, P.J. Turner, S. Weaver and P. Read, Int. J. Mass Spectrom. Ion Processes, 121, 201 (1992).

[2] I.C. Lyon, J.M. Saxton, P.J. McKeever, E. Chatzitheodoridis and P. Van Lierde, Int. J. Mass Spectrom. Ion Processes, 151, 1 (1995).

[3] J.M. Saxton, I.C. Lyon and G. Turner, Analyst, 120, 1321 (1995). [4] W. Compston, I.S. Williams and C. Meyer, /. Geophys. Res., 89, B525 (1984). [5] C S . Eldridge, W. Compston, I.S. Williams and J.L. Walshe, Int. J. Mass Spectrom.

Ion Processes, 76, 65 (1987). [6] E. de Chambost, T. Rouxel, A. Pflieger and M. Schuhmacher, SIMS IX Conference,

Proceedings and Poster, Yokohama, Nov. 1993. [7] M. Schuhmacher, E. de Chambost, K.D. McKeegan and T.M. Harrison, SIMS IX

Conference Proceedings, Yokohama, Nov. 1993. [8] Investigation on Pb/U System in a Sri Lanka Zircon, M. Schuhmacher, CAMECA

Application Note, Feb. 1994; (CAMECA, 103 Blvd St-Denis, BP6, 92403 Courbevoie, France).

[9] Lee R. Riciputi, D.C. Duckworth, C M . Barshick and D.H. Smith, Int. J. Mass Spectrom. Ion Processes, 146/147, 55 (1995).

[10] D.L. Donohue and M. Petek, Anal. Chem., 63, 740 (1991). [11] D.C. Duckworth, C M . Barshick D.A. Bostick and D.H. Smith, Appl. Spectrosc,

47, 243 (1993). [12] F.A. White, EM. Rourke and J.C. Sheffield, Appl. Spectrosc, 12, 46 (1958). [13] L.A. Dietz, Rev. Sei. Instrum., 31, 1229 (1960). [14] CR. Lagergren and J.J. Stoffels, Int. J. Mass Spectrom. Ion Phys., 3, 429 (1970). [15] L.A. Dietz, Rev. Sei. Instrum., 30, 235 (1959). [16] L.A. Dietz, CF. Pachucki, J.C. Sheffield, A.B. Hance and L.A. Hanrahan, Anal.

Chem., 32, 1276 (1960). [17] N.R. Daly, Rev. Sei. Instrum., 31, 264 (1960). [18] J.J. Stoffel(s), D.R. Ells, L.A. Bond, P.A. Freedman, B.N. Tattersall and CR.

Lagergren, Int. J. Mass Spectrom. ¡on Processes, 132, 217 (1994). [19] N.S. Belshaw, R.K. O'Nions, D.J. Martel, and K.W. Burton, Chem. Geol., 112, 57

(1994). [20] N.S. Belshaw, R.K. O'Nions and F. von Blanckenburg, Int. J. Mass Spectrom. Ion

Processes, 142, 55 (1995). [21] L.W Alvarez and R. Cornog, Phys. Rev., 56, 379 and 613 (1939). [22] D. Elmore and EM. Phillips, Science, 236, 543 (1987). [23] J. Rucklidge, Analyst, 120, 1283 (1995). [24] D.J. Donahue, A.J.T. Jull and T.H. Zabel, Nucí. Instrum. Methods, Phys. Res. Sect.

B, B5, 162 (1984). [25] T.W. Linick, A.J.T. Jull, L.J. Toolin and D.J. Donahue, Radiocarbon, 28,522 ( 1986). [26] R.E.M. Hedges, Adv. Mass Spectrom., 1985A, pp. 185-194. [27] A.E. Litherland, K.W. Allen and E.T. Hall, Phil. Trans. R. Soc. London, Ser. A, 323,

1 (1987). [28] W. Henning, Z. Liu, H.F. Lucas, G.E. Thomas, H.L. Adair and W.B. Grisham, Nucl.

Instrum. Methods Phys. Res., 257A, 60 (1987). [29] J. Rucklidge et al., Can. Mineral, 20, 111 (1982). [30] J.M. Anthony, D.J. Donahue, A.J.T. Jull and T.H. Zabel, Nucl. Instrum. Methods

B10/11, 498 (1985). [31] J.M. Luck and K.K. Turekian, Science, 222, 613 (1983). [32] U. Fehn, R. Teng, D. Elmore and P.W. Kubik, Nature, 323, 707 (1986).

PART II PROCESSES AND APPLICATIONS

CHAPTER

7

ION FORMATION PROCESSES

7.1 ELECTRON IMPACT: ELECTRON BOMBARDMENT IONIZATION 150 7.1.1 Energetics of Electron Impact Processes 151

7.2 THERMAL IONIZATION 153 7.2.1 Ionization Enhancement Techniques 156 7.2.2 Isotope Ratio Analysis of Multielement Samples 157

7.3 INDUCTIVELY COUPLED PLASMA IONIZATION 158 7.4 SPARK SOURCE IONIZATION 161 7.5 GLOW DISCHARGE IONIZATION 162 7.6 RESONANCE IONIZATION 163

7.6.1 Elemental and Isotopic Selectivity 166 7.6.2 Elemental Sensitivity 166 7.6.3 Isotopic Ratio Determinations 167

7.7 SECONDARY IONIZATION 168 REFERENCES 169

Electron impact ionization and thermal ionization are the two most frequently and most successfully used ionization techniques in isotope ratio mass spectrometry when high precision and high accuracy are required. Electron impact is very efficient for ionization of materials in the gas phase, whereas thermal ionization is capable of ionizing almost all the metal elements to positive ions and a few metal oxides and nonmetals to negative ions. Inductively coupled plasma ionization is another technique which has recently gained reasonable recognition when applied for isotope ratio measurements.

Several other techniques, such as spark source ionization, glow discharge ionization, secondary ionization and multiphoton ionization, are utilized for ion formation in conjunction with isotope ratio determinations. In general their application is limited to solving specific problems, where the analytical tasks tolerate results of lower precision. In most cases the mass spectrometer is also a complex and expensive piece of instrumentation.

150 ION FORMATION PROCESSES

7.1 ELECTRON IMPACT: ELECTRON BOMBARDMENT IONIZATION

When an electron with sufficient kinetic energy hits an atom in the gas phase, probabilities exist for the following ionization processes to take place

A + e —A+ + 2e (1)

Doubly, triply, etc. atomic ions may also be formed by electron impact

A + e -» A2+ + 3e (2) A + e - + A " + + (« + l)e (3)

Under similar conditions, molecules form molecular (parent) ions

M + e - > M + + 2e (4)

or simultaneously also, fragment ions

M + e — A+ + (M - A)0 + 2e (5)

M + e -* B+ + (M - B)° + 2e (6)

In the case of more neutral fragments, the overall reaction is

M + e - ^ C + + E ( N ° ) i + 2e (7)

Ion pair formation is also known, mostly in halide molecules

MX„ + e ^ M X + _ 1 ) + X - + e (8)

Another ionization process, yielding negative ions, is electron capture (or electron attachment)

YZ + e -> YZ (9)

Dissociative electron attachment occurs when the YZ~ ion is formed with excess of internal energy

Y Z - ^ Y + Z - (10)

The efficiency of negative ion formation is much lower than that of positive ions, therefore positive ions are almost exclusively used for isotopic ratio measurements.

Positive ions of another type that accompany electron impact ionization are ion-molecule reaction (IMR) products. The most relevant IMRs involve hydrogen (or deuterium) transfer to form various triatomic ions interfering with the H/D/T isotopic analysis. They are discussed in detail in Chapter 9, Section 1. Water vapor residuals may interfere by protonated ion formation, i.e. interfering with l3C1602 in 13C/12C isotopic ratio determinations; therefore purified, H20-free samples should be always used.

ELECTRON IMPACT: ELECTRON BOMBARDMENT IONIZATION 151

7.1.1 Energetics of Electron Impact Processes

Appearance potential is the minimum energy required to produce an ion and one or more neutral fragments. Following reaction (7), assuming that the products are being in their ground state and that there is no excess kinetic energy involved in the reaction, the appearance potential of C + will be

AP(C+) = AHf(C+) + SA//f(N0),. - Ar/f(M) (11)

where the AHf are the corresponding heats of formation. When an atom, molecule or radical is ionized to a parent ion without any

dissociation, the appearance potential is the ionization potential of this species. The ionization potential is therefore defined as the minimum energy required to remove an electron from a molecule (or an atom or radical). If the initial neutral and the final ionized species are in their ground states (i.e. their lowest vibrational states), eq. (11) for the ionization process (4) will be

AP(M+) = IP(M+) =AHf(M') - AHf(M) (12)

Both the appearance and the ionization potentials are determined experimen-tally from ionization efficiency curves. These curves are plots of ion currents of a particular parent ion as a function of the ionizing electron beam energy. Figure 7.1 shows ionization efficiency curves for H 2 and CO+. It can

NO 3.0

CO

2.0 cvj

CD

1.0

I

0 50 100 150 200 Electron energy (eV)

Figure 7.1. Ionization efficiency curves for H J and CO+. (Reproduced by permission of Prentice-Hall, New York, from R.W. Kiser, Introduction to Mass Spectrometry and its Applications, 1965, p. 164)


immediately be seen that the ion current has a maximum intensity and is relatively insensitive to electron energy in the region between 50 and 100 electron-volts. It is therefore a general practice to operate electron impact ion sources at electron energies of 70 or 75 eV.

The ion beam current intensity of an electron impact ion source is given by the following expression

i+ = ßi"leO£ng (13)

where i~ is the electron current intensity penetrating the gas molecules, le is the effective length of the electron beam interacting with the sample, OE is the ionization cross section of the gas at the energy E, ng is the gas density, and ß is the efficiency of ion extraction from the ion source. The magnitudes of these parameters are in the following ranges: ß < 1; 1 x 10~6 < i~ > 100 x 10_6A; le sa 1cm; oE for He at 75 eV equals 0.38 and for larger molecules increases up to sa 15 x 10~16 cm2 atom""1 and ng sa (3-300) x 1010 molecules cm - 3 , cor-responding to a sample pressure of 10~6-10- 4 mmHg. At extreme conditions, electron impact ion sources produce ion currents of 10"7 A. Usually i+ ranges from 10~9 to 10~14 A.

A schematic diagram of a simple electron impact ion source is given in Figure 7.2 (see also Figure 3.6 in Chapter 3). Electrons are emitted from a heated filament F made from rhenium, tungsten or other suitable material. The filament is well shielded from the ionization chamber C by plate Pi and the ionization chamber block P2. The electrons are accelerated into the chamber by a potential between F and Pi through two small slits, about 1 x 3 mm in size, to produce a collimated electron beam entering C. The final electron energy is controlled by a potential drop between F and P2. The electron beam passes through C, exits via a small slit and is collected by the anode trap T. The electron beam is also held in a weak magnetic field (about 102 g), giving the electrons spiral motion. The gaseous sample is introduced into the ionization chamber via a gas inlet system ending by an opening in the chamber block, which in Figure 7.2 is perpendicular to the electron and ion beams and shown by the open circle. Between the repeller plate R and P2, a weak electric field may be produced to repel the ions from C. Ion sources without repeller plates are also in use. In this case plate P3, which is at a slightly lower potential, acts as an ion drawout plate, and the ionization chamber block and the repeller plate form a closed equipotential ionization chamber. When the ions exit C, they are accelerated by a large potential drop across plates P3 and P5. Usually P2 is maintained at 3000-8000 V and P5 is at ground potential. The electron impact ion source is a very reliable unit, producing stable ion beams with sufficient ion current intensity and moderate ion kinetic energy spread, ranging from 0.1 to (rarely) 5.0 eV The almost homogeneous ion beam energy makes this source compatible with any mass analyzer, except for quadrupole analyzers, where the applied acceleration voltage is in the region of 100 V

THERMAL IONIZATION 153

Magnetic field

R

Electron beam

P3 P4 P5

Positive ion beam Figure 7.2. Schematic diagram of an electron impact ion source

7.2 THERMAL IONIZATION

When a substance is loaded onto a metal filament surface and then heated, atomic and, in some cases, molecular ions may be emitted. The process is known as surface or thermal ionization (TI) and is used to ionize elements of low ionization potential, i.e. metallic compounds, in an isotope ratio mass spectro-meter ion source. The compound (or the metal directly) is heated in vacuum, vaporized and partially ionized. Ion sources with single, double or triple filament arrangements are used for the evaporation and ionization processes. In a single filament ion source, all the physical and chemical processes converting the solid MX compound to gas phase M + ions take place on the same filament surface. In a double or triple filament ion source, the sample filament is used to vaporize the sample and the gaseous species (atom or molecule) is then attached to the ionization filament, where it loses an electron and is released into the gas phase as a positive ion. A less frequent but still important process can also occur on the ionization filament; an atom or a group of atoms, usually a metallic oxide, gains an electron and is released as a negative ion.

With respect to the filament processes, there is no difference between the double and the triple filament ion source, except that in the latter two filaments are sample filaments, which may be used for comparison of two different samples (e.g. a standard and an unknown sample) under the same ion source conditions. The triple filament ion source was developed by Inghram and Chupka [1]. The use of this assembly (and also the double filament source) rather than the single filament allows separate control of the vaporization and ionization yields. Many samples can be vaporized at relatively low temperatures, yielding a sufficient gas phase density of neutrals to produce an

M


Ion beam Ion beam to Ion lenses MS

Sample Evaporation filament \

Filament. holder

(a)

Evaporated I Ionization Evaporated-sample ' filament sample

\ Contact

Double-filament unit

^

slit einzel lens z-deflection r-plate x-deflection shield

Ionization filament

Isolated passage Contact pin

Thermal ion source

Figure 7.3. (a) Schematic diagram of a double filament thermal ionization ion source and (b) a boat-shaped filament. Reprinted by permission of John Wiley & Sons, Inc., from Ref. [6]. Copyright 1988 John Wiley & Sons

intense ion beam at an elevated ionization filament temperature. Two important advantages are achieved by separating the vaporization and ionization proces-ses. The rate of evaporation can be kept low, maintaining a longer sample life. At the same time the ionization filament temperature can be increased to make the ionization more efficient. Furthermore, evaporation at lower temperature reduces the isotopic fractionation effect and also the rate of change of this effect, making the fractionation easier to control. White et al. [2] employed a boat-shaped (or V-shaped) single filament ion source. A strip of rhenium or tungsten is folded to form the boat shape to hold very small samples. The ionization efficiency of this filament arrangement is greater than that of a single filament ion source and almost the same as that of a triple filament ion source.


Chen and Wasserburg [3] used for picogram actinide samples a dimple type single filament ion source. Schematic diagrams of a double filament thermal ionization ion source and a boat-shaped filament are given in Figure 7.3.

Generally, metallic ions are emitted from hot filament surfaces as positive atomic ions. Recently negative ions, mostly metal oxides in the form of MO~ and non-metals with high ionization potential in the form of X^ ions, have also been used for isotope ratio determinations. The relationship between the ion yield production a, given as the ratio of number of ions to the number of vaporized atoms, and the physical parameters involved in the ionization processes was described by Langmuir and Kingdon in 1925 [4]. Negative ion production by TI was comprehensively reviewed in three publications by Kawano et al. [5] and by Heumann [6]. The following expressions were taken from Kawano [5]

a + ( M + ) M = n(M+)/n(M) = [w(M+)/w(M)]exp{[<¿> - lP(M)/kT]} (14) a~(X-)x = «(X-)/»(X) = [w(X-)/w(X)]exp{[EA(X) - <p/kT}} (15)

where n(M+) and n(M) are the numbers of ions and atoms, respectively, evaporating from unit surface area per unit time; w(M+)/w(M) is a statistical factor; IP(M) is the first ionization potential of the ionized element M in eV; and ¡p is the work function of the filament material in eV. Equation (15) is the analogous equation for negative ions, where EA(X) is the electron affinity of the ionized element in eV

It can be seen from equation (14) that positive ion production is increased for elements with low ionization potential. Similarly, an increase in filament work function and ionization temperature yields an increase in a + . A 0.1-1 pg alkali element sample ionized in a triple rhenium filament ion source can easily produce an ion beam intensity greater than 10~9 A lasting for a few hours. Equation (15) indicates that a~ is improved when the filament work function decreases. Ionization potentials of the elements are given in Appendix 1. Filament material work functions and melting points are given in Table 7.1 and electron affinities of atoms and molecules analyzed by negative thermal ionization (NTI) are shown in Table 7.2

Table 7.1. Filament material work functions and melting points Filament material

Platinum Tantalum Rhenium Tungsten Thoriated tungsten LaBô coated tungsten

Work function (eV)

5.13 4.30 4.98 4.58 2.7 2.7

Melting point (°C)

1772 2996 3180 3410


Table 7.2. Electron affinities of atoms and molecules analyzed by NTI

Atom, molecule

N02 CI B02 F Br CN I Se S Te

Electron affin

3.91 3.61 3.56 3.45 3.36 3.17 3.06 2.12 2.07 1.96

7.2.1 Ionization Enhancement Techniques

Experimental data show that alkali, alkaline earth, lanthanide and actinide elements, all with ionization potentials below 7 eV, are efficiently ionized by positive thermal ionization (PTI). The required ionization temperature may be 2100-2300CC, therefore high melting point filament materials such as tungsten and rhenium are used, rhenium being preferred because it has better mechanical properties. Elements with I P > 7 eV need special ionization techniques to increase their ionization yields. Also, very small samples of elements with lower IP, especially actinides, are treated by these techniques. All the ionization enhancement techniques are discussed in separate sections of Chapter 9, where analytical procedures for each element are described. In this section only a brief summary is presented. Cameron et al. [7] introduced the silica gel/phosphoric acid technique, which was successfully applied for chromium, iron, nickel, copper, zinc, ruthenium, palladium, silver, cadmium, tin, antimony, tellurium and lead. Substituting boric acid for phosphoric acid enhanced the ionization efficiency of chromium, iron and nickel. Recently, Huett et al. [8] studied the silica gel ion emission mechanism. Electrodeposition of actinide microsamples was used by Rokop et al. [9]. Carbonizing the sample filament with graphite slurry has been used for the ionization enhancement of actinides, vanadium, cerium and hafnium. Addition of platinum powder onto the carbonized rhenium filament increased the filament work function and improved the ionization of molybdenum. Addition of iridium and molybdenum onto a carbonized filament enhanced hafnium ion emission. Birck [10] used a Ta-H3P04-HF solution to improve radium ionization efficiency.

The resin bead technique was developed by Smith et al. [11] for isotopic ratio determinations of small, nanogram size uranium or plutonium samples. The


element is adsorbed on an anion exchange resin bead, which is loaded into a V-shaped filament. When the filament is heated, the bead is decomposed and carbonizes the inner boat surfaces, increasing the ionization efficiency in a similar way to carbon coatings on a regular flat filament in addition to the enhancement effect of the boat filament itself. The resin bead technique was also successful in analyzing such elements as molybdenum, ruthenium and technetium, which have volatile oxides in their high oxidation states. On heating, these may be reduced by the resin bead carbon to less volatile compounds, yielding more intense and stable ion beams.

Bleeding in a gas during the course of an analysis may increase the ionization efficiency. Hebeda and Schijf [12] showed that the Freon CCI2F2 enhanced the ion production of cerium, lanthanum, neodymium and, most effectively, gadolinium. The effect is attributed to an estimated increase of +0.6 eV in the work function of the ionization filament owing to coverage with Freon.

For ionization of sub-nanogram samples the 'dimple' type filament has been used, usually in conjunction with different ionization enhancement agents. Details are given in Chapter 9, Sections 9.12, 9.20, 9.90 and 9.92.

Negative ionization enhancement is achieved by reducing the ionization filament work function. As seen in Table 7.1, thoriated tungsten activated by carbon and lanthanum hexaboride-coated tungsten are favorable filament materials. Rhenium filaments coated with lanthanum nitrate or barium hydroxide also increase negative ion production. Single and double filament ion sources may be applied. Further details are given by Heumann [6] and in the sections of Chapter 9 describing the determination procedures for the individual elements listed in Table 7.2.

7.2.2 Isotope Ratio Analysis of Multielement Samples It is widely accepted in TIMS to analyze only a single, highly purified element in each sample. This practice eliminates isobaric interferences and allows a rigorous control of the vaporization and ionization conditions between samples. However, reports in the literature show that analyses of several elements loaded on the same filament are also achievable under certain conditions. The variable parameter is the temperature. The first element is analyzed at a low temperature, then the temperature is increased to a level where this element is exhausted and a further temperature increment gives rise to the ionization of the next element. Mixtures of elements with different masses have also been analyzed at the same temperature. Hilpert et al. [13] applied the isotope dilution and silica gel techniques to determine elemental concentrations in a mixture: rubidium and thallium were ionized at about 850 °C, copper at 1000 °C, lead at 1100 °C, chromium and cadmium at 1200 °C, and strontium and barium at 1500 °C. Trincherini et al. [14] analyzed uranium and plutonium with the resin bead technique, and Aggarwal et al. [15] analyzed uranium and plutonium samples


containing 238U and 238Pu with a double rhenium filament ion source. Multi-element analysis of chloride, bromide and iodide and several other cases were discussed by Heumann [6].

7J INDUCTIVELY COUPLED PLASMA IONIZATION

Inductively coupled plasma mass spectrometry (ICP-MS) has been developed as a multielement trace analysis technique [16, 17]. It is unique because of its high detection limits, high sample throughput, relatively simple sample preparation and sample introduction systems. As the detection system is a mass spectrometer with its inherent isotope ratio determination capability, this option soon became very important, significantly extending the applications of the technique. Historically, the technique was developed in the second half of the 1970s. The first commercial instruments, based on quadrupole mass analyzers, appeared in 1983, a high resolution instrument based on an electrostatic and magnetic analyzer was described in 1989 [18], and a double focusing, multiple collector instrument, aimed solely at isotope ratio determinations, was assembled in 1992 [19].

Quadrupole ICP-MS instruments, those most commonly used, have the capability to produce isotope ratio data in the precision range of 0.2-1.0%. In the mass range up to sa 80, ionic species originating from atmospheric and solvent components and the plasma gas interfere with the measurements; thus, for light elements, only selected ratios can be determined. Isotope dilution analyses may successfully cope with diese limitations. Quadrupole isotope ratio measurements are also affected by voltage settings on the lenses and may show slight time dependent drifts. Nevertheless, recent instruments show better stability, improving the ratio precision to 0.1% or in some cases even below this value. Isotope ratio measurements with quadrupole ICP-MS instruments were reviewed by Jarvis et al. [20] and the following elements were discussed: lithium, boron, magnesium, potassium, chromium, iron, copper, zinc, selenium, bromine, rhenium, osmium, lead and uranium. With high resolution instruments many interference problems are resolved, the overall sensitivity is higher and flat top peaks, which are essential for high precision data, can be obtained, but still both types of instruments use single collector data acquisition, therefore the plasma fluctuation strongly affects the ratio precision. The double focusing, multiple collector mass spectrometer allows simultaneous collection of ion beams separated by a magnetic mass analyzer. Isotopic ratio data published with this instrument between 1993 and 1995 are included in the appropriate sections of Chapter 9. They are in very good agreement with the results of TIMS, confirming that the multiple collector system efficiently eliminates the plasma instabilities. The instrument is described in details in Chapter 4 of this book.

INDUCTIVELY COUPLED PLASMA IONIZATION 159

The most common plasma used in this technique is the inductively coupled argon plasma. The ICP source and other plasma sources were reviewed by Gray [17]. A brief description of the ICP ion source is given here. The plasma torch is a quartz tube of o.d. 18 mm and about 100 mm long. A coaxial, water cooled copper excitation coil of about three turns, coupled to a RF generator, surrounds the quartz tube close to the end of the torch. An inner coaxial tube of o.d. 14 mm ending just before the coil is mounted in the outer tube. A third, central tube ending with a 1.5 mm i.d. capillary is used to introduce the sample into the plasma. Argon at a flow rate of 12-15 1 min-1 is introduced between the walls of the two quartz tubes to cool the inner surface of the outer tube. A second argon flow, the auxiliary flow of up to 1 1 min-1 passes through the inner tube, and an argon flow of 0.5-1 1 min 1 is used as the sample carrier gas. A RF power of 1-1.5 kW, usually at a frequency of 27.12 MHz, is supplied to the coil through an automatic matching network. An electron stream produced by a spark from a Tesla coil gains the energy from the RF coil and ignites an intense plasma in the opening of the torch. The temperature of the central part of the plasma where the sample is propagated through the plasma, is about 8000 K and falls to below 6000 K at a distance of 30 mm from the coil. At this point the plasma approaches the sampling cone orifice of the ion extraction interface.

Various sample introduction techniques are available for ICP-MS. The most common is pneumatic nebulization. Other techniques used for specific applica-tions are ultrasonic nebulization, electrothermal vaporization, flow injection and laser ablation [17]. In a pneumatic nebulizer the sample carrier gas flows across or along a concentric sample tube into which a sample solution is pumped by a small peristaltic pump at a typical rate of about 1 ml min-1. Droplets of various sizes, mostly about 20 pm in diameter, are formed. These are too heavy to be supported by the carrier gas and are lost on the walls of a cooled glass chamber. Only droplets 4 pm in diameter or smaller, which comprise sa 5% of the population, are carried into the plasma. Therefore, only about 2% of the sample solution is utilized for ionization, which means that almost all the sample solution flows to waste. A pneumatic nebulizer that allows recirculation of the waste solution was developed by Hulmston [21]. The ICP torch with a pneu-matic nebulizer is shown in Figure 7.4.

Alder et al. [22] performed optical measurements of the plasma temperature. They observed that the gas temperatures Tg in the central channel are about 6000 K, the excitation temperatures Texc are sa 7000-7500 K, and the ionization temperature 7¡ is about 8000 K 10 mm beyond the RF coil. At this distance a plasma electron density ne of 3 x 1015cm~3 is assumed. Tg is defined by the kinetic energy of the neutral atom population, Texc is defined by the population of energy levels of excited states and T, is defined by the population of the various ionization states. Gray [23] measured the ionization temperature for a 1.2 kW ICP ion source, also using ne = 3 x 1015cm~3. About 8000 K was observed, in reasonable agreement with Alder's data [22].


ooo 4

ooo nr 1 2

(a) 100 mm

Stage 2 (b)

Water cooled from plate

Skimmer /

Stage 1

Extraction electrode

Sampling cone O O O

OOO t

Load coil

t ICP

torch

1cm

Figure 7.4. Schematic diagram of an ICP torch and an ion extraction interface. Reprinted by permission of John Wiley & Sons, Inc., from Ref. [17]. Copyright 1988 John Wiley & Sons

Table 7.3. Ionization yields in a plasma of 8000 Ka [17]

Ionization potential (eV) Ionization yield

< 7 8 > 0.98 0.91

9 0.71

10 0.36

11 0.12

12 0.03

13 0.01

1 A plasma electron density of 3 x 1015cm 3 is assumed.

Gray [17] calculated the ionization yields in an argon plasma as a function of the elemental ionization potentials at T¡ = 8000 K. Table 7.3 summarizes the data. Elements with IP below 7 eV are completely ionized. In practice, higher ionization yields than those given in the table are obtained for elements with IP above 9 eV; this is attributed to possible lower values of ne than those assumed. Therefore, good sensitivity is achieved for elements up to 10.5 eV, and carbon, bromine and chlorine (IP 11.26, 11.86, and 12.97 eV respectively) can also be detected.

The sensitivity of quadrupole ICP-MS has improved significantly in recent years. Longerich et al. [24] reported sensitivities for an instrument installed in

SPARK SOURCE IONIZATION 161

1994 better than 500 x 106 cps per ppm for elements from La to U. Instrumental background levels were less than 10 cps for large regions in the high mass range. An early generation (1984) commercial instrument had a sensitivity for lanthanides in the region of only 1 x 106 cps per ppm. The same type of concentric pneumatic nebulizer was used in both instruments.

The process of ion extraction process from the hot atmospheric plasma into the high vacuum mass analyzing system is achieved via a water cooled extraction interface. This unit is made from a nickel plate, shaped as a shallow cone with an orifice of 0.5-1 mm diameter, aimed to sample the plasma. Behind this cone is mounted a second, sharper cone, the skimmer cone, with an orifice of 0.5-1.5 mm diameter. The spacing between the two cone tips, depending on the various systems, is from 2 to 10 mm. The interface unit confined by the two cones and their housing is pumped by a mechanical pump to about 1 Torr, removing the large gas flow that entered the sampling cone. A second vacuum stage containing the extraction electrodes of the quadrupole mass analyzing system is pumped to about 5 x 10-4 Torr, and a third stage, containing the mass analyzer and the detector, operates at a pressure of about 2 x 10~6 Torr. An ion extraction interface is described schematically in Figure 7.4.

The main advantages of an ICP ion source may be summarized as follows: (1) the source accepts samples at atmospheric pressure; (2) the high plasma temperature completely vaporizes and dissociates the

sample; (3) the ionization yield is high; in practice good sensitivity is obtained for

elements up to 10.5 eV; (4) mainly single charged ions are produced; the fraction of M2+ or MO+ ions

is 0.01 or less; (5) the ions are produced with a small energy spread; (6) the source operates at low potential, making it compatible with quadrupole

mass analyzers. The disadvantages of an ICP source are the high gas temperature and the

interferences introduced by atmospheric, solvent and carrier gas components.

7.4 SPARK SOURCE IONIZATION

Spark source ionization was developed by Dempster [25, 26] It is used as a multielement technique for the ionization of solids. Two types of ion sources are known, the radio-frequency spark source and the low voltage d.c. arc source. The RF source is utilized in almost all commercial instruments. The sample has to be prepared in a form of an electrode. The ions produced have a wide energy spread of 2-3 kV, therefore a double focusing mass analyzer, containing an electrostatic sector for velocity focusing followed by a magnetic sector for


directional focusing, must be used. Ion detection is achieved with a photoplate or electrically with a Faraday cup or an electron multiplier. The determination of isotopic ratios, mainly for isotope dilution analyses, is usually carried out using the peak jumping mode combined witb electrical detection. This mode of operation yields improved analytical precision. Isotope dilution spark source mass spectrometry (ID-SSMS) was reviewed by Confides and Niinisto [27] and briefly also by Heumann [6]. Photoplate detection is preferred for bulk, micro-sample and surface/depth analyses. Spark source mass spectrometry gained popularity in the 1960s owing to its approximately uniform elemental sen-sitivities in the ppb range, applicability for almost all the elements, and simple spectra. The method has been reviewed by Ramendik et al. [28].

7.5 GLOW DISCHARGE IONIZATION

A glow discharge is a partially ionized gas with approximately equal concentra-tions of positive and negative charges in a high density of neutral species. A simple depiction of a glow discharge plasma will be a cathode and an anode immersed in a low pressure gas, usually argon in the range of 0.1-10 Torr. An electrical field is applied across the electrodes, causing breakdown in the gas followed by motion of the charged particles toward the oppositely biased ele-ctrodes. Neutral atoms are sputtered from the sample loaded onto the cathode by energetic plasma ions, and are ionized in the plasma. Electron impact is probably the major ionization process

M + e -» M+ + 2e (16)

where M represents a sputtered atom. Electron densities of up to 1014cm~3 with energies ranging from fast to thermal have been reported [29]. Penning ioniza-tion, involving the long lifetime metastable Ar* atom at 11.55 eV, is also an important mode of ionization.

M + Ar* - + M + + A r + e (17)

Metastable densities of 10ncm~3 in glow discharges have been reported [30]. Several other ionization processes, such as charge transfer from Ar+ ions, photoionization of excited sputtered neutrals (M*) and associative ionization due to collisions between Ar* and M also occur in the glow discharge, but these are of secondary importance. No standard glow discharge ion source is available. A group of ionization devices comprises the hollow cathode ion sources. Other electrode configurations can also be used to yield glow discharge ionization. The ion source may be coupled to a magnetic or a quadrupole mass analyzer. The most important application of glow discharge mass spectrometry is in the elemental analysis of solids. A review of the method was presented by Harrison [31]. A double focusing magnetic sector glow discharge mass spectro-

RESONANCE IONIZATION 163

meter with reverse Nier-Johnson geometry was recently used for isotope ratio measurements [32,33,34].

7.6 RESONANCE IONIZATION

Resonance ionization (RI) processes are based on selective photon excitation of valence electrons in gas phase atoms, leading to ionization. The photon source is resonant laser light. During the ionization process, a photon at a particular wavelength excites the electron from a low lying energy level, usually the ground state, to a spectroscopically allowed intermediate state. Absorption of a second photon of the same or different wavelength can ionize the atom, or further excite it to a higher excited state. Hurst et al. [35] classified the RI processes into five schemes, shown in Figure 7.5. It was suggested that with these schemes all the elements except helium, fluorine, neon and possibly argon can be ionized.

© A[2o,,û)2, »e ]A+

'///À////,

A[2û)„ m-,, û>e~]A+

'///A'////,

m = « , OR a>2

® A[w,, w,e~]A*

V///, '///A

A[2mv e>,e"]A

'////¿///A

m. 7\

7T

1 ¿

2®,

® A[(B,, ù)2, o e ]A

a> =œ, O R ® ,

X

l m = < B , OR<a2

û),(ORo2)

1Z 1 ¿

2ö),

1 ¿ Figure 7.5. Classification of resonance ionization schemes for the formation of atomic ions [35] (d)\ and (w)2 and (2UJ)\ denote photons of frequency 1, 2 and frequency doubled photons, respectively. (Reproduced by permission of the American Physical Society from G.S. Hurst et al, Rev. Mod. Phys., 51, 767 (1979))


In early RI spectroscopy experiments, the free electron which was produced by multiphoton absorption was multiplied and detected as a negative charge. It was clear that mass analysis of the positive ion counterpart should provide more specific and important information on the RI process. Consequently, resonance ionization mass spectrometry (RIMS) has been developed, utilizing different instrumental configurations. Moore et al. [36] modified a single focusing magnetic sector thermal ionization mass spectrometer: laser light-transmitting windows were installed in the ion source region and an electron multiplier detection system capable of measuring pulsed ion currents was added. The excitation and ionization were achieved with a single Nd:YAG pumped tunable dye laser system. The two photon, single color resonant ionization scheme [35] was utilized, and atomization was achieved by resistively heating a purified sample loaded onto a standard filament of the thermal ionization source. Rimke et al. [37] described a RIMS instrument based on a linear time of flight spectrometer equipped with three dye lasers pumped by two copper vapor lasers. Resonant ionization was achieved by a two step excitation process followed by an ionization step via autoionization states. Hot filament

1 1 1

Tunable dye laser

1 1 1

Nd YAG laser

Auto tracker

Frequency doubler

Photodiode trigger

Box-car |

DVM

Comp

i i i

Scop

rh 3 M

LM >

3

i i

Cal.

, S . \

Ion mult.

Pre-ar np

Figure 7.6. RIMS system block diagram.[36] M.S., magnetic sector; Cal., calorimeter; DVM, digital voltmeter; Comp., computer. (Reproduced by permission of the American Chemical Society from L.J. Moore et al. Anal. Chem., 56, 2770 (1984))

RESONANCE IONIZATION

Pulsed Cu-vapor laser (30 W, 6.5 kHz)

Dichroic mirror

¿& Pulsed Cu-vapor laser (30 W, 6.5 kHz)

Dye laser 1

Dye laser 2

Dye laser 3

Lenses

Electrodes Window

Filament

Acceleration electrodes

Focussing lenses

Quartz fibres

Channel plate detector

Drift tube

Spherical mirror

Figure 7.7. A three laser time of flight RIMS system [37]. (Reproduced by permission of Springer-Verlag, Berlin, 4 Rimke et al, Mikrochin. Acta, III, 223 (1989))

atomization was applied. Schematic illustrations of these RIMS instruments are shown in Figures 7.6 and 7.7. A RIMS system consisting of a modified thermal ionization quadrupole mass spectrometer coupled to an excimer laser-pumped dye laser was described by Wunderlich et al. [38]. Regarding atomization, it should be noted that, besides the hot filament, other techniques such as graphite furnace, flame, primary ion sputtering, glow discharge and laser ablation are also in use.

High elemental/isotopic selectivity and high elemental sensitivity, originating mainly from high ionization efficiency, are properties inherent in resonance ionization. They comprise the major advantages of RIMS as an analytical


technique. Nevertheless, determination of true, unbiased isotopic ratios using RIMS, especially in multi-isotope elements, is a complex analytical task. These fundamental parameters will be briefly discussed.

7.6.1 Elemental and Isotopic Selectivity

Resonance ionization processes are characterized by the ability to ionize a single element in a multicomponent sample. This phenomenon is particularly important in reducing or eliminating isobaric interferences. However, non-resonant ionization of other species, including isobars, increases the back-ground noise. The selectivity 5 for detecting an element X in the presence of an isobaric interference Y was defined by Spiegel et al. [39] as

5={/(X + ) / [X]}/ / (Y + ) / [Y]} (18)

where ¿(X+),i'(Y+) and [X], [Y] are the corresponding ion intensities and atomic concentrations. It was observed that selectivities improve when the number of resonant steps used in the RI process is increased, since in general non-resonant ionization is less feasible with low energy photons [40].

7.6.2 Elemental Sensitivity

The sensitivity of RIMS is determined by atomization efficiency, ionization efficiency and the detection efficiency of the instrumental set-up. Generally, in any analytical technique, the sensitivity depends on low blanks, and on the capability to control the blanks and contamination and to eliminate memory effects. Background ions occasionally produced in atomization or non-resonant ionization processes may also affect sensitivity. The major and unique contribution to the sensitivity of RIMS is the high ionization efficiency. Let us consider again thermal ionization. Applying the Langmuir-Kingdon [4] equation (eq. (14)) for an element with IP = 7.6 eV evaporated from a tungsten filament (<j> =4 .6 eV) at 1500 °C, and assuming 0.5 for the statistical factor, the ionization efficiency «(M+)/«(M) will be only a diminutive 10- 1 0 . Recently Payne et al. [41] reviewed the applications of RIMS, quoting ionization efficiencies for a wide range of elements; values up to 55% were reported. Saturation of the isotopic resonance transition is a condition for high ionization efficiency. If this is achieved for the isotopes under study, the measured isotopic ratios will be constant. Saturation for an individual isotope i occurs when

F L x VA x t > 1 (19)

where FL,'<JA and t are the laser photon flux, the effective absorption cross section of the discrete transition and the photon pulse duration, respectively [42]. Consequently RIMS is able to analyze minute solid samples. Detection limits (DL) of the order of 108 atoms are normally achieved [43]. Rimke et al.

RESONANCE IONIZATION 167

[37] reported DL for plutonium and technetium less than 107 atoms, and Bushaw and Gerke [44], using a graphite atomization source, reported for 130Ba DL of about 104 atoms. Larger samples, usually in the low nanogram range, are needed for isotopic ratio determinations.

7.6.3 Isotopic Ratio Determinations A significant constraint on the application of RI for accurate and precise isotope ratio determinations is the laser-induced isotope selective effects, which may introduce variable shifts in the measured ratios compared with the true isotopic composition of the sample. Donohue et al. [45] measured the isotopic ratios of plutonium and uranium using isotopic reference materials. Their results show isotopic ratio shifts of up to 2.5%. Walker and Fassett [46] resonantly ionized rhenium and osmium, observing isotopic abundances in good agreement with those of TIMS except for the 1890s isotope, where values higher by 2-3% were consistently obtained. It was suggested that this isotope has a higher ionization efficiency relative to other osmium isotopes. In most of the RIMS studies, larger isotopic shifts, in the range from several per cent to several tens of per cent, have been observed, typically with an enhancement of the odd mass isotopes. Wunderlich and co-workers [38] comprehensively discussed the laser induced isotopic selectivity in RIMS. The variations of titanium [38] and osmium [42] isotopic ratios were studied as a function of such laser parameters as wave-length, bandwidth, power and polarization state. The isotopic selectivity was strongly dependent on laser power and wavelength, even when the laser band-width radiation was much larger than the optical isotopic shift.

Several detailed reviews describing RIMS in analytical chemistry are avail-able in the literature. Fassett and Travis [47] discussed three major areas of application: (a) noble gas measurements in environmental studies; (b) quantita-tive elemental analysis using isotope dilution mass spectrometry; and (c) analysis of solids using direct sampling. Smith et al. [43] described the RIMS instrumentation, including atomization sources, pulsed and continuous wave lasers and mass spectrometers. Isotope ratio measurements and the detection and quantification of trace elements were discussed, and a table of more than 70 elements, including actinides, which were studied by RIMS is also given. Both reviews outline the studies related to RIMS isotope ratio determinations. Recently, Payne et al. [41] reviewed the physical aspects of laser excitation and ionization in vacuum related to RIMS. Summarizing the RIMS technique, it may be stated that its major contribution to analytical chemistry is the capability to detect very low trace element concentrations. This is accomplished by taking full advantage of the high inherent elemental selectivity and sensitivity of resonance laser ionization. Furthermore, the use of the isotope dilution technique provides the quantitative options of RIMS. It should be noted that RIMS is a sophisticated technique and, as such, it is limited to a small


number of laboratories. These centers are involved in RIMS research and development programs rather than routine analytical work. For the time being RIMS is not at a stage to be considered a general method of isotope ratio determination, but is rather a technique for solving selected problems.

7.7 SECONDARY IONIZATION

Secondary ion mass spectrometry (SIMS) is based on sputtering of secondary ions from the outer layers of a sample surface. The sputtering is achieved by bombarding the surface with energetic primary ions. Three types of primary ion source are used: the duoplasmatron gas source, producing O J , 0~, N j or Ar+

ions; the surface ionization source, producing Cs+ or Rb+ ions; and the liquid metal field emission source, producing Ga+ or In+ ions. The ions are accele-rated and focused onto a selected area of the sample. Collision of the primary ions with the surface causes ion implantation, followed with re-shuffling of about 50-500 matrix atoms and emission of secondary ions and neutrals. The secondary ions are extracted into a mass spectrometer. Mass analysis is achieved with electrostatic and magnetic sector, quadrupole or time of flight mass spectrometers coupled to the secondary ion source. SIMS is recognized as a major technique of surface analysis and microstructural characterization of solids [48]. It is noted for its high sensitivity, providing surface and in-depth profiles.

At the present time, isotope ratio measurement with SIMS is not a widely used, routine technique. England et al. [49] measured the thorium 230Th/232Th, hafnium 176Hf/177Hf and rhenium isotopic ratios. 1 8 0 / 1 6 0 isotopic ratios were measured in quartz by Lyon et al. [50] and in geological and extra-terrestrial materials by Saxton et al. [51]. The 13C/12C isotopic ratio was measured by Amari et al. [52] in meteorite graphite and by Harte and Otter [53] on natural diamond surfaces. The magnesium isotopic composition was studied by Ireland et al. [54], Fahey et al. [55] and Goswami et al. [56] Graham and Valley [57] studied the sulphur 34S/32S isotopic ratio in a standard pyrite, observing internal precision of 0.3-0.5%o and reproducibility (1SD) in the range of 0.45-1.0%o, depending on the number of standard analyses (n = 6-21). The instrumental mass fractionation for the standard was typically about 20%o. Several of these publications are discussed in the sections describing analytical procedures for the related elements. Deloule et al. [58] discussed the instrumental limitations of a secondary ion mass spectrometer (Cameca ims-3f ion microprobe): (1) peak calibration and overlaps, (2) contamination, (3) instrumental mass discrimination, (4) matrix effects on mass discrimination, (5) dead time of the counting system, and (6) analyzed volume size. Examples were taken from D/H, " B / ^ B , 34S/32S and 87Sr/86Sr isotopic ratio measurements.

REFERENCES 169

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CHAPTER 8

PRECISION AND ACCURACY IN ISOTOPE RATIO DETERMINATIONS

8.1 ISOTOPIC FRACTIONATION 171 8.1.1 Isotopic Fractionation in Thermal Ionization 171

8.1.1.1 Fractionation Models 175 8.1.1.2 Fractionation Correction Techniques 178

8.1.2 Isotopic Fractionation in Inductively Coupled Plasma Ionization 181

8.1.3 Isotopic Fractionation in Gas Inlet Systems 182 8.2 STATISTICAL TREATMENT OF DATA 183

8.2.1 Random and Systematic Errors 183 8.2.2 Precision 184 8.2.3 Accuracy 186 8.2.4 Rejection of Measurements 187 8.2.5 Error Estimation in SRMs 189

REFERENCES 190

8.1 ISOTOPIC FRACTIONATION

8.1.1 Isotopic Fractionation in Thermal Ionization Isotopic ratios measured with a thermal ionization mass spectrometer are seldom the true ratios of a sample under study. Instrumental mass discrimination may arise from various sources. Ion optics, analyzer design and non-linearity in the measuring circuitry are some of them. In modern instruments, such as the multiple collector mass spectrometers commercially available since the early 1980s, these sources of mass discrimination have been practically resolved, especially when the instruments are used in the magnetic and not the ion acceleration mode of operation. The ion detection system can also introduce mass discrimination when secondary electron multipliers are used. The gain of these devices is known to be dependent on ion velocity, therefore lighter ions in a beam of equal kinetic energy ions, being accelerated to higher velocities, yield relatively more secondary electrons than do heavier ions. Faraday cup collectors are considered to show negligible mass discrimination.

In thermal ionization mass spectrometry, the main discrimination effect which may introduce low accuracy and low precision in an isotopic ratio

172 PRECISION AND ACCURACY IN ISOTOPE RATIO DETERMINATIONS

analysis is the mass dependent, and consequently also time dependent, isotopic fractionation process occurring in the ion source within the course of an analysis. The reason for this phenomenon is the mass dependent differential vaporization of the isotopes from the heated sample filament, causing the lighter isotope to vaporize more rapidly. The usual trend is that at the beginning of an analysis the observed ratio favors the lighter isotope. During the vaporization process, the higher evaporation rate of the lighter isotope leads to faster depletion of the sample in this isotope, and the ratio of light to heavy isotope decreases towards its true value and, eventually, below it. Isotopic fractionation on the sample filament is a process which must be carefully controlled; it is recognized as a major source of variable systematic errors limiting the accuracy of isotopic ratio measurements. Isotopic fractionation was already observed in the early stages of thermal ionization mass spectrometry by Brewer [1], followed by Hoff [2], Riik and Shukoliukov [3], Reutersward [4], Ordzhoni-kidse and Shutze [5], Bentley et al. [6], Habfast [7], and Shields et al. [8].

Mass fractionation parameters which have to be considered are sample size, chemical composition and purity, sample loading procedures on the filament, the material of the ionization and evaporation filaments, filament temperatures and the rate of sample heating and, finally, the time of data acquisition. Garner and co-workers studied many of these parameters very carefully, especially for uranium U/ U isotopic ratio determinations with a triple filament ion source. The measured ratio of SRM U-500 (235U/238U = 0.9997) under a well defined set of conditions was plotted against the measurement time, yielding an isotopic fractionation curve. The observations were published by De Bievre [9] and are shown in part in Figures 8.1-8.4, illustrating the mass fractionation effect of these parameters.

The deviation of the measured ratio from the true ratio Ru may be expressed by the following equation

K = Rtr/Rmeas (1)

where K is the mass fractionation factor. K is obtained when /?meas is the isotopic ratio of an isotopic standard reference material. When all the mass fractionation (and discrimination) parameters are satisfied, Rmeas = Ru and therefore K = 1. It is a common practice to calibrate an isotope mass spectrometer with a range of SRMs, such as the U-0002 to U-970 NIST (formerly NBS) uranium suite or the CBNM uranium IRMS 072/1-15 set. The K values over a wide range of R values are determined. Their spread at a certain R value defines the uncertainty of the correction factor. The overall uncertainty of Rti will be the sum of the uncertainty of K, the uncertainty of the measured sample and that of the certified SRM. Further details on this topic are given in Chapter 9, Section 92, 'Thermal Ionization of Uranium'.

The possible use of single or multiple (double or triple) filament ionization assemblies in thermal ionization mass spectrometry is a further variable which

ISOTOPIC FRACTIONATION 173

S

1.0070

P 1.0050

1.0030

1.0010

0.9990

* Conversion at 2.0A for 3 min • Conversion at 2.2A for 3 min • Conversion at 1.8A for 10 min

30 60 90 120 Time, Minutes

150 180

Figure 8.1. Uranium isotopic fractionation curve: optimum conversion of nitrate to oxide. Rtr = 0.9997. (Reproduced by permission of Heyden & Sons Ltd. from P. De Bievre, Adv. Mass Spectrom., 7A, 395 (1978)

1.0070

=> 1.0050 CO to

• D

CO

n O

1.0030

1.0010

0.9990

* 1 x 10 A Ion Current • • 3x 10~" A Ion Current

• 6x 10""A Ion Current

30 60 90 Time, Minutes

120 150 180

Figure 8.2. Uranium isotopic fractionation curve: uranyl nitrate analysis at different ion current intensities. Solid curve is reference curve for optimum conversion of nitrate to oxide. (Reproduced by permission of Heyden & Sons Ltd. from P. De Bievre, Adv. Mass Spectrom., 7A, 395 (1978))

affects isotopic fractionation. The single filament ionization procedure is used for easily ionized elements (i.e elements with low ionization potential), and in cases of elements with high ionization potentials where ionization enhancement agents are applied to improve ionization efficiency. In both cases ions are directly emitted from the sample filament. In the multiple filament ionization procedure, the sample is vaporized from the sample filament and the vapor is


1.0070-

1.0050

=3 8 ei

-a CD £ cu CO

.a O

1.0030 -

1.0010

0.9990

• • 1 pg of uranium per filament • 5 pg of uranium per filament

30 60 90 Time. Minutes

120 180

Figure 8.3. Uranium isotopic fractionation curve: 1 pg vs 5 pg of uranium per filament. (Reproduced by permission of Heyden & Sons Ltd. from P. De Bievre, Adv. Mass Spectrom., 7A, 395 (1978))

1.0070

1.0050

CD

CD CO .a O

1.0030

1.0010

0.9990

• 1 hr. degas; oxide at 1.8A * 2 hr. degas; oxide at 1.8A • 2 hr. degas; oxide at 2.0A

30 60 90 Time. Minutes

120 180

Figure 8.4. Uranium isotopic fractionation curves for uranium oxides: clean vs. extra-clean filaments. (Reproduced by permission of Heyden & Sons Ltd. from P. De Bievre, Adv. Mass Spectrom., 7A, 395 (1978))

ionized with the aid of an ionizing filament maintained at much higher temperature. The option of separate temperature adjustment of each filament provides improved control of the rate of sample vaporization and the extent of ionization, contributing significantly to the ion beam stability and overall


sample life time. The triple filament ionization procedure was developed by Inghram and Chupka [10].

8.1.1.1 Fractionation Models

Eberhardt et al. [11] studied the time dependent fractionation of the alkali metals Li, K and Rb in a single filament ion source. They observed that the isotopic fractionation proceeds according to the Rayleigh distillation law [12, 13], where the fractionation factor is (m2/m\)1'2 , the lighter isotope being preferentially vaporized from the filament. A correction procedure to convert the isotopic ratio data to 'true' values was not developed.

Kanno [14] made a theoretical study of isotopic fractionation in a thermal ion source. It was assumed that the compound MX deposited on the filament yields neutral gaseous atoms Mm? ^ and atomic ions Mt,, simultaneously with neutral molecules MX(g) and molecular ions M X t ,

M X ( s ) - M 0 g ) + X « g ) (2)

- M f o + X f o (3)

- M X ( g ) (4) - > M X £ , (5)

Further assumptions were that:

(a) The vaporization of both the atoms M° and the molecules MX obeys the Langmuir vaporization law [15], i.e. the number of moles vaporized per unit time is inversely proportional to the square root of the molecular weight.

(b) The residual compounds on the filament undergo complete and continuous mixing, without isotopic fractionation.

(c) The vapor ion source residence time is sufficiently short to prevent isotopic exchange in the gas phase.

(d) There is no isotopic effect in the ionization efficiency in the gas phase.

Kanno proposed a differential equation relating the changes in the isotopic ratio R with the fraction of sample Q remaining on the filament. Solution of the equation for R =3 9K/4 1K vs Q — 1, which is the fraction of sample evaporated, is shown in Figures 8.5 and 8.6. It has also been demonstrated that the observed isotopic ratios equal the theoretical ratios when « 63% of the sample has been vaporized, i.e. (1 — Q) « 0.63.

Moore et al. [16] proposed an isotopic fractionation model for the multiple filament thermal ion source. They discussed Kanno's model [14] and, with respect to the experience gained at the Analytical Division Laboratory of the National Bureau of Standards, they stated that the model is 'sufficient to explain qualitatively the direction of fractionation corrections for all the absolute


14-3

B 14-1 D

r - f l 0 = 13 8566 13-9 39 K

KCI/K 41 K 13-7 0-0

B 0-4 10

D 4 0 00

I I I I 13-3 0 20 40 60 80 100 Figure 8.5. Calculated fractionation curves of KCl assuming different vaporized KCl(e)/K°{g) proportions. A = 0.0, B = 0.4, C=1.0. D = 4.0, E = oo. (Reproduced by permission of Heyden & Sons Ltd. from I. J. Moore et al. Adv. Mass Spectrom., 7A, 452 (1978))

K

KCl Br

Kl

I : n ?n 40 fin nn mn

Figure 8.6. Calculated fractionation curves of vaporized atomic and various molecular potassium species. (Reproduced by permission of Heyden & Sons Ltd. from I. J. Moore et al, Adv. Mass Spectrom., 7A, 452 (1978))

isotope abundance measurements in this laboratory, using the multiple filament source'. However, it was not adequate to account for the anomalous isotopic fractionation effects, that had been observed:

(a) fractionation processes where the light/heavy isotopic ratio increases during the analysis;

(b) fractionation processes where the light/heavy isotopic ratio decreases faster than predicted by theory;


(c) analyses in which no apparent change in the isotopic ratio is observed within the measurement;

(d) variation in the observed ratio with salt form; (e) the effect of impurities on the observed isotopic ratio.

Moore et al. used the mathematical equations introduced by Kanno, expand-ing Kanno's model. They concluded that, for most of the cases studied, the vaporization of the sample in a multiple filament ion source probably leads to molecular species. Their vaporization model allows changes in the molecular composition of the vapor during the analysis, accounting for the variety of fractionation effects.

Habfast [17] assumed that the sample, which is usually loaded in the form of a salt, evaporates from the filament as binary vapors of two chemically different forms. The molecular species can either dissociate prior to ionization, the metal then being ionized, or molecular ions may be produced and dissociate into metallic ions. Isotopic effects occur predominantly in the dissociation pro-cesses, and are negligible during ionization. The model provides an explana-tion for the observed dependence of isotopic ratio on the chemical form of the loaded sample and on the ionization temperature. Figure 8.7 shows the cal-culated fractionation of a U 3 0 8 sample, which is assumed to evaporate simul-taneously to (U308)g and (U02)g.

1.006-

1.005-

1.004-

1.003-

1.002-

1.001 -

1.000-

robi/Ro

K = 3

(u3o2)9

- - T - 1 " 1

(U02)9

(U308)9 ^ (u3o2)9

K Î O . S I \ .

T 1 1 V A-q

1 • 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Figure 8.7. Computer-generated fractionation of a UjOg sample, partially converted to U02 during evaporation. Dotted curves indicate fractionation of pure U02 and U3O8. k is the ratio of evaporation rates of the two species; ßo is the originally loaded amount of sample; Q is the quantity of evaporated sample and q = Q/Qo- (Reproduced by permission of Elsevier Science NL from K. Habfast, ¡nt J. Mass Spectrom. ¡on Phys., 51, 165 (1983))


Regarding the comprehensive validity of a fractionation model, Habfast pointed out that the major difficulty in a general quantitative model is the lack of appropriate thermodynamic data about the vaporization and ionization processes of isotopic species at the non-equilibrium conditions prevailing in a thermal ionization source.

8.1.1.2 Fractionation Correction Techniques The necessity to correct for instrumental mass discrimination in isotope ratio mass spectrometry is well known, not only for thermal ionization, but also in electron impact ratio measurements in the gas phase. Nier [18] determined the natural isotopic abundance of argon, calibrating his instru-ments with a synthetic mixture prepared from almost pure, highly enriched 36Ar and 40Ar isotopes. Different isotope enrichment and isotope separation methods, developed intensively in the second World War and after, made it possible to prepare synthetic isotopic mixtures of many elements with a wide range of isotopic composition and to certify them as Standard (Isotopic) Reference Materials. Natural compounds have also been allocated, and their absolute isotopic abundance ratios were determined using the well defined isotopic composition of synthetic isotopic mixtures. The calibration procedures for isotopic SRMs are discussed in many sections of Chapter 9.

Dietz et al. [19] introduced the internal standard technique to correct for isotopic mass discrimination in thermal ionization mass spectrometry. The technique was demonstrated with the following example. A pure and calibrated mixture of 233U and 236U in approximately equal amounts is mixed with an unknown uranium sample and the 233U/236U and 235u/238U ratios are deter-mined. A correction factor, (233U/236U)meas/(233U/236U)tr, is established and used to correct the measured 235U/238U ratio. The mass differences of the two isotopic pairs should be equal and each of the pairs must be free of the other pair of isotopes. A correction factor is established for each analysis. The technique is better known as 'double spiking'. Dodson [20, 21] and Russell [22] discussed double spiking, the latter providing a mathematical formulation for the technique. Hamelin et al. [23] among others (see citation in ref. [23]), used the double spike technique for precise lead isotopic ratio measurements. They also re-evaluated various factors in the technique, including parameter optimization, choice of the double spike isotopic composition, adjustment of the amount of spike to sample, the effect of concentration uncertainty on error propagation, and the effect of sample contamination during the chemical preparation.

In the internal normalization technique, it is assumed that the element under study has three or more isotopes and that at least two of them are stable, non-radiogenic isotopes. Such a pair of isotopes has a constant isotopic ratio, which is considered invariant in nature. If this ratio has a well established value, it may


be used for mass discrimination corrections. The 'normalization ratio' is measured (together with the ratio of interest), and the ratio of the measured value to the known value provides an internal correction factor for use in correcting the ratio of interest. It is further assumed that there are no isobaric interferences in the system to interfere with one or both isotopes of the normalization ratio. Well known examples used in many laboratories are measurements of the 87Sr/86Sr and 143Nd/144Nd ratios, using 86Sr/88Sr and , 4 6Nd/I 4 4Nd or 146Nd/145Nd as normalization ratios.

Wagner et al. [24] introduced the total evaporation/integration technique combined with multiple ion collection for isotopic fractionation correction. The important feature of the technique is the evaporation of the entire sample and simultaneous integration of the ion signal from each isotope, thus essentially eliminating the isotopic fractionation effect in the evaporation process. Nanogram samples of uranium and sub-nanogram samples of plutonium and thorium were analyzed. Callis and Abernathey [25] have shown that 20 ng samples of NIST U-010, U-020, U-100, U-500 and U-900 yielded 235U/238U ratio values which deviated from the certified values by +0.010 to +0.040% (the uncertainty of the certified NIST values is ± 0.1%). A further interesting advantage of the total evaporation technique is the independence of the observed ratio on the sample size in the range 20-200 ng uranium. This behavior is in contrast to conventional ionization, where only a portion of the sample is consumed and the fractionation curve of the observed ratio vs. time depends on the sample size. Fiedler et al. [26] also studied the total evaporation technique for uranium, and recently it has been applied for uranium and plutonium isotopic ratio determinations with a thermal ionization quadrupole mass spectrometer [27]. The technique has been successfully applied for strontium by Callis and Abernathey [25] and for neodymium, samarium, gadolinium and lutetium by Dubois et al. [28].

Russell et al. [29] investigated three mathematical formulations to describe the isotopic fractionation phenomenon. Together with the already applied linear behavior, four 'laws' were proposed: the linear law, the power law, the exponential law and the Rayleigh law. All the laws can expressed in one general formula [30]

Fmeas/Ftr = è(Xmeas/Xtr)a + c (6)

where X and Y represent the isotopic ratios mxlm3 and m2/m3 and the subscripts tr and meas indicate the true and the measured ratio values. The definitions for a, b and c are the following

Linear law: a = 1 b = (m2 - m3)/(mt - m3) (7) c= 1 -b


Power law:

Exponential law:

Rayleigh law:

a = (m2— m3)/(mi - m 3 )

b=l (8) c = 0

a = ln(m2/m3)/ln(mi/m3) b=\ (9) c = 0

a = [1 - (m3/m2)1/2]/[l - (m3/mi)l/2}

b = (m3/m2)1 / 2 / (m3/m,)1 / 2 (10) c = 0

where the values of a, b and c are calculated using the nuclide masses and not the mass numbers.

It should be pointed out, that none of these 'fractionation laws' has a sound theoretical basis, and there is no way of predicting which law will be the best fit

1.01

1.00

.99

.98

.97 CO Ü w .96 CD

.95

.94

.93

.92

.91 1.00

I 1 I 1

-

J?

Exponential law if'

\ /s \ /'

F l a s h y y ; V < jr/ Linear law

A'

/ / 7 /

v /

/ i i i i

i i //

/ 3 V

U- Absolute value

-

-

' ' 1.01 1.02

44/42 Ca 1.03

Figure 8.8. 48Ca/42Ca ratio vs. ̂ Ca/^Ca ratio in a 19 h run. The exponential law and the linear law are shown by solid and broken lines respectively. (Reproduced by permission of Elsevier Science NL from S.R. Hart and A. Zindler, Int. J. Mass Spectrom. ¡on Processes, 89, 287 (1989))


to describe the experimental data. Therefore 'fractionation functions', instead of 'fractionation laws', would probably be a more appropriate term.

Hamelin et al. [23] showed that the linear law provided the best fit for their lead isotopic ratio measurements when 207Pb/204Pb or 208Pb/204Pb ratios were plotted vs. 206Pb/204Pb. Hart and Zindler [31] plotted the measured 48Ca/42Ca ratios vs. the measured 44Ca/42Ca ratio in a 19 h run. As shown in Figure 8.8, the data were fitted best by the exponential law, diverging notably from the linear law at the early and the final stages of the run. Qi-Lu and Masuda [30] showed that the 96Mo/98Mo, 97Mo/98Mo and 100Mo/98Mo ratios, when plotted vs. the 94Mo/98Mo ratio, also obey the exponential law. Kawashima et al. [32] showed that their molybdenum isotopic ratio measurements obey the power law. Further cases of fractionation corrections using expression (6) are discussed in various sections of Chapter 9.

The following expressions are also used to correct for mass discrimination [33]

Linear law: Ätr/^meas = 1 + Am x eiin (11)

Power law: Rü/Rm™ = ( 1 + epow)Am ( 12)

Exponential law: Ätr//fmeas = exp(Am x eexp) (13)

where R is the ratio of the heavier isotope m\ to the lighter isotope m2, e is the mass bias per atomic mass unit, and Am = mx - m2.

8.1.2 Isotopic Fractionation in Inductively Coupled Plasma Ionization

The mass discrimination effect has also been observed in isotopic ratio measu-rements using inductively coupled plasma mass spectrometry (ICP-MS). Several processes contribute to this phenomenon, mainly space charge effects (coulombic repulsions) in the skimmer cone region, resulting in preferential transmission of the heavier ions. In contrast to thermal ionization, this effect is independent of time, as the sample is introduced continuously into the plasma. The power law (eq. 12) was used to correct the mass discrimination in a multiple collector ICP-MS instrument [34-36]. The mass discrimination correction factor epow per u"1 is mass dependent, varying from sa 0.006 to 0.04 on going from uranium to lithium respectively. Recently, Taylor et al. [37] used synthetically prepared mixtures of uranium isotopes, certified to 3 x 10~4

levels, to assess the linearity and mass discrimination of a multiple collector ICP-MS instrument. The power and the exponential laws provided the best corrections. With the same instrument, Walder et al. used the 2 0 3 TI / 2 0 5 T /1 r a t j 0 and the internal normalization technique to correct for mass discrimination in lead isotopic ratio measurements in solutions [35] and in solids, applying laser ablation [38].


8.1.3 Isotopic Fractionation in Gas Inlet Systems

In the electron impact ionization process, a sample, whether a vaporized solid, a liquid or a gas, must be admitted into the ion source in the gaseous state. A sample inlet/handling system which isolates the sample from the outer atmosphere and allows its transfer without affecting its composition to the ion source is always terminated by a leak arrangement, which ensures an ion source pressure in the p Torr range. Also, the leak system has to produce a particle flow and consequently an ion beam which are characteristic of the sample.

Molecular flow of a gas through cylindrical tubes at low pressure was described by Smoluchowski [39]

Qm = 3810 (D3/L)(T/M)1 / 2(P, - P2) (14)

where Qm is the molecular gas flow through the tube opening (in dyne cm s_1), D is the diameter of the opening (in cm), L is the tube length (in cm), T is the absolute temperature, M is the gas molecular weight, and (Pi — P2) is the pressure gradient along the tube.

Gas flow through a circular opening in a thin plate was given by Knudsen [39]

Qm = 2860D2(77M)1/2(P1 - P2) (15)

where the notations have the same meaning as in eq. (14). These expressions are valid for long tubes, and as long as the molecular mean free path A obeys A » D. Usually \/D « 20 is sufficient. For Knudsen flow, a 0.0025 cm thick foil with one or more orifices of 0.0002 < D < 0.0005 cm may be satisfactory.

From both equations it can be seen that the rate of admission of gaseous molecules of mass M is proportional to \/y/M, therefore lighter isotopes are preferentially introduced into the ion source and the sample composition will change with time. If the gases are pumped out from the ion source at a rate proportional to l/y/M, the composition of the sample will remain the same as in the gas reservoir and isotopic fractionation at the leak may be minimized. Diffusion pumps transfer light gas more efficiently, therefore compensating at least partially for the gas flow fractionation. On the other hand, turbomolecular pumps transfer heavy gases more efficiently, therefore they act to increase the already increased light isotope density in the ion source and further contribute to fractionation.

A long capillary tube, when placed between the gas reservoir and the ion source may admit the sample at viscous flow conditions. For high sample pressures, when A « D, the gas flow is viscous and follows the Poiseuille law [39]

Qv = (irD4/256r)L)(P2-P2) (16)

STATISTICAL TREATMENT OF DATA 183

where Qv is the viscous gas flow, r¡ is the gas viscosity (in poise) and the other symbols have the meanings as in the equations above.

From eq. (16) it may be seen that, for pure viscous flow, ß v is independent of mass, therefore no isotopic fractionation can occur. When the ion source is pumped with a diffusion pump, the isotopic ratio R]/h must be corrected by a factor of y/Mh/M\. Mutually calibrated dual gas inlet systems, with successive introduction of a standard and the sample, may also be used for isotopic fractionation correction. A 15 cm long capillary with a diameter of 0.01 cm needs a sample pressure of several Torr. Various types of gas inlet system are in use, further details of which are given elsewhere [40].

8.2 STATISTICAL TREATMENT OF DATA

One of the best ways to assess the reliability of a measurement in experimental sciences is to repeat it several times and to submit the whole set of accumulated results to a statistical analysis. Experimental uncertainty can be treated statistically only if it is a random uncertainty, i.e. a random error. Another type of error is systematic uncertainty, which cannot be revealed by statistical treatment. The average or mean of a set of measurements, the standard deviation, the standard deviation of the mean and the confidence interval are the basic parameters for the estimation of random errors. In the simple formulation of statistical treatment it is assumed that the experimental data demonstrate a normal or Gaussian distribution. In its normalized form, the Gaussian distribution is bell shaped and centered on the mean value, which should also be the 'true value', provided that there are no systematic errors in the system under study and that we know what the 'true value' is and if such a value even exists. Even if the system does not exhibit any systematic errors which may shift this value, it probably would be more correct to designate the mean value as the 'best value'. Whereas the concept 'true value' is widely used in the isotope ratio mass spectrometry literature, and also quoted many times in this manuscript, its restricted meaning should be kept in mind.

8.2.1 Random and Systematic Errors

To illustrate the distinction between random and systematic errors, two simple examples will be considered. Consecutive measurements of 97.4 mm using a ruler with 1 mm divisions will produce data in the range 97.3-97.5. A sufficient number of measurements will provide an average, 'best' value, very close to 97.4 mm. On the other hand, using a bad ruler where the marked distance from 30.0 to 40.0 mm is actually only 9 mm will introduce a systematic error. The same will happen if each 10 marked millimeters in reality are only 9.7 mm. The second example is related to mass spectrometry. In a double Faraday collector


detection system, two feedback resistors have values quoted as 1.00 x 1011 and 1.00 x 1010 ft. Actually the resistance of the first resistor is 1.02 x 10u ft. Clearly a 2% systematic error in the ratio calculations will be introduced on top of the random error.

Random errors in mass spectrometry arise owing to instability and fluctuations of the various instrumental components which produce, accelerate, mass separate and monitor the separated ion beams. It is clear that these errors will be larger for less intense ion beams. Possible sources of systematic errors are isotope fractionation effects in ionization, mass discrimination effects introduced by gas inlet systems, ion source pumping systems, ion optics in the mass analyzer systems, mass discrimination effects specific to electron multipliers, linearity and gain of the electronic measurement system, and erratic spike solution preparation in quantitative analysis using the isotope dilution technique. As already mentioned, the random errors are treated with statistical tools. The systematic errors ought to be revealed by comparison with standard reference materials or standard reference measures. Our ruler had to be compared with a standard reference ruler, and the detection system tested with a standard reference current. Systematic errors in isotope ratio determina-tions are detected and corrected with isotopic standard reference materials (SRMs). It is generally accepted within the mass spectrometry community that the SRM ratio should be as close as possible to the studied ratio. Of course, well designed and properly built instruments remove, or at least much reduce, the sources of systematic errors. The different statistical parameters relevant to isotope ratio measurements will be described in the following part of this chapter.

8.2.2 Precision Standard deviation is an estimate of the uncertainty of a set of measurements. It is a measure of the precision of this determination. For a set of isotopic ratio measurements the standard deviation is defined as

SD = { [ £ , - ( * - r , - ) 2 ] / ( n - l ) } 1 / 2 (17)

where r, is a measured ratio in a separate analysis of a sample, n is the number of measured ratios in the sample, and R is the mean of n ratios, R — E r,/n for one sample.

The number n of measured ratios in a sample is a variable which is chosen by the analyst. It depends on the overall stability of the instrument, the sample preparation prior to the analysis and the behavior of the sample during the analysis. Values up to 120 are used in strontium ratio analyses. Lower values are more frequently used. It is also a common practice to divide n into blocks, for example eight blocks of 15 measurements, or six blocks of 10 determinations


for n = 60. In automatic mass spectrometers the interval between successive blocks is used for ion beam readjustment.

In most cases, depending on the extent of instrumental mass bias and isotopic fractionation effects introduced by ionization or sample admission, the mea-sured ratio is not the true ratio. Therefore rt has to be corrected to produce a ratio as close as possible to the true ratio. The various correction techniques are described in Section 8.1 of this chapter. A further advantage of the computerized data acquisition systems is the option to perform these corrections on each measured ratio in quasi real time, thus

R'^Erl/n (18)

where R ' is a mean ratio (of one sample) of the n fractionation or bias corrected individual ratios r\. To calculate the standard deviation of the corrected ratios, we use eq. (17), replacing r, by r'¡ and R by R'. This precision is frequently also called the internal standard deviation or ISD.

The standard deviation of the mean is also a quantity used by analysts. Recalling that n ratio determinations had been made and SD was calculated using eq. (17), then for large n the standard deviation of the mean is given by SD/y/n. For a small number of determinations the standard deviation of the mean requires a correction with the so-called 'Student t factor' [41] and is expressed as

t x SD/v/n (19)

The t factor is obtained from standard tables and is determined from the deg-rees of freedom DF (DF=n — 1). Other common names for the standard deviation of the mean are standard error (SE) and standard error of the mean (SEOM).

Another very frequently used term is the relative standard deviation (RSD), also called the coefficient of variation (CV) [42]. This quantity is expressed in % and is defined as

R S D = 1 0 0 X S D / ä ' (20)

The precision of a mass spectrometric procedure is given by the standard deviation of a number N of samples

S D = { [ E , ( ä - / C ; . ) 2 ] / ( A ' - 1 ) } 1 / 2 (21)

where R is the mean of the corrected means, R = HR '¡/N. R is also termed the grand mean, and this precision is also frequently called the external standard deviation, or ESD.

To establish the precision of a procedure (the ESD), it is normally sufficient to determine R' from 10-12 sample replicates (N = 10-12). Terms such as


Table 8.1. t/y/N values at 95% probability level as a function of degrees of freedom t/VÑ

9.011 2.487 1.591 1.239 1.049

N-\

6 7 8 9

10

t/s/Ñ

0.923 0.839 0.769 0.716 0.671

N- 1

11 12 13 14 15

t/s/Ñ

0.636 0.605 0.578 0.554 0.533

relative internal and relative external standard deviation (RISD and RESD) are also in use.

The confidence interval for a set of measurements with an R value is given by

R±{tx SD)/vW (22)

As / and -y/N are both constant for any given set of measurements, t/ yjN can be calculated at different probability levels. Table 8.1 gives the calculated t/y/N values at the 95% probability level as a function of (A/ — 1) degrees of freedom. Only these values are included in the table because the 95% probability level is related to the ± 2SD interval of the mean, which is most frequently used by analysts. With a Gaussian distribution of the measurements, the interval X± 2SD corresponds to 95% of the data.

It often happens that a set of measurements is repeated. One analyst can determine a particular R value at two different times, or with two different instruments, or using two slightly differing procedures; or two analysts can use the same procedure and instrument. The variance ratio, or 'F ' test can be used to compare two sets of measurements to determine whether the difference in the precision of the two sets is significant.

It is possible that, although the variances of the two sets, as determined by the 'F ' test, may not differ essentially, the two mean values may not agree. The question is whether the difference in the means is significant. The Student 'r' test, which is a simple method for determining whether two statistical values, in this case two means, differ significantly, is applied to answer this question. It should be noted that the '/' test can be used only if the ' F ' test has shown that the variances of the two measurements are essentially the same. The reader interested in further details should refer to the literature [43].

8.2.3 Accuracy

Accuracy is defined as the difference between the measured and fractionation corrected ratio of an isotopic standard reference material and the true (also called the theoretical) ratio for this standard. The accuracy of a single sample


determination will therefore be A,-= 100 x ( * ; - * „ ) / * „ (23)

where A¿ is the accuracy, in %, for a single standard analysis, /?' is now the measured and corrected ratio for the standard, and Rtr is the true ratio for the standard.

The accuracy of a mass spectrometric procedure is established when the same ratio is determined several times with the same or preferably different standards of the isotopes under study. The following example will clarify this point. As pointed out in Chapter 7, uranium NIST or CBNM SRMs are used to calibrate thermal ionization mass spectrometers. A group of 12 SRM samples with differing 235U abundances is loaded under fixed loading conditions and in an arbitrary abundance sequence into the sample magazine of the instrument: U-005, U-010, U-020, U-050, U-100, U-350, U-500, U-850, U-005, U-930, U-005 and U-970. A second group may be: U-005, U-005, U-850, U-850, U-010, U-100, U-500, U-005, U-970, U-050, U-005 and U-970. The accuracy for each group will be

A = [ S A 2 / ( A T - 1 ) ] I / 2 (24)

where A is the accuracy in % for the group of uranium standards and N is the number of samples.

Reproducible A values represent the accuracy of the procedure: in this example, the accuracy of the 235U/238U isotopic ratio measurement. For thermal ionization, values of A < 0.05% are considered to indicate high accuracy. The accuracy achievable with a multiple collector inductively coupled plasma mass spectrometer is comparable with that obtained with the same configuration of TIMS. Generally, for electron impact ionization the accuracy is better by at least one order of magnitude.

A few more thoughts about the concepts of accuracy and precision should be mentioned. Accuracy is targeting the 'true' value. On the other hand, precision provides no hint as to how close or remote a set of measurements may be from the 'true' value. Precision is a measure of the dispersal of a set of measure-ments. Precision and accuracy complement each other. High precision and high accuracy is the objective of good analytical performance. High precision associated with an incorrect mean is a situation which should be avoided.

8.2.4 Rejection of Measurements

It may happen in a ratio determination that a value appears which departs considerably from the other values in the measurement. The appearance of an outlying value is not necessarily an indication that a serious error has been made. In a small group of values it can affect the mean and the standard deviation to such an extent that it may be reasonable to conclude that the mean


Table 8.2. Chauvenet's criterion of the rejection of suspected measurements'

n

2 3 4 5 6

P

1.15 1.36 1.54 1.65 1.73

n

1 8 9 10 12

P

1.80 1.86 1.91 1.96 2.04

n

15 20 25 30 35

P

2.13 2.24 2.33 2.40 2.45

n

40 50 75 100 200

P

2.50 2.58 2.71 2.81 3.02

n

250 300 400 500 1000

P

3.09 3.14 3.21 3.29 3.48

"n = number of measurements; p = ratio of the deviation of the measurement from the mean to the standard deviation

ratio recalculated without the outlier would better represent the true value. Although there are cases where the rejection has definitive reasons, a statistical criterion of rejection is still important. Several such criteria exist, some quite complicated.

The Chauvenet rejection criterion [44,45] is a simple and easily used test. It states that a value in a group of n values shall be rejected when the magnitude of its deviation from the mean of the group is such that the probability of occurrence of all deviations that large, or larger, is less than l/2n. Table 8.2 lists the magnitude of this deviation, derived using eq. (25), in terms of multiples of the standard deviation as a function of n

P = (r'0Ut-R')/SD (25)

where p is the number of standard deviations by which the suspected outlier differs from R'. The criterion is used as follows:

(a) /? ' and SD are calculated for a set of r'. (b) The magnitude of p is determined for a suspicious r'(r'out), using eq. (25) (c) The observed magnitude of p from (b) is then compared with the value of p

for the corresponding value of n in Table 8.2. (d) If the observed p is greater than the value in the table, r 'oui may be rejected. (e) Finally, a new mean and standard deviation of n — 1 data points is

calculated.

When the rejection procedure is completed, the resulting SD will be smaller than the original one. It may also happen that, with the new R' and SD, more measurements may be considered for rejection. Most authorities agree that the Chauvenet criterion should not be applied again on the recalculated values of R' and SD. A more detailed mathematical treatment of Chauvenet's criterion can be found elsewhere [46]. An example will demonstrate the rejection procedure. In the following series of « — 6 measurements

1013,1026,1060,1009,1020,1034

should the value 1060 be rejected?


The mean of six measurements is 1027, the standard deviation is 18.5 and the calculated p value is (1060- 1Q27)/18.5 = 1.78. This is higher than 1.73 (Table 8.2), therefore the value of 1060 should be rejected. The recalculated mean and standard deviation for n = 5 are 1020.4 and 10.0 respectively.

Several other more complex rejection criteria are known: Dixon's test, Grubb's test, the coefficient of skewness test and coefficient of kurtosis test. Dybczynski [47] has compared the effectiveness of these procedures for the rejection of outlying results.

8.2.5 Error Estimation in SRMs Since the beginning of the 1960s, the Analytical Mass Spectrometry Section of the National Bureau of Standards (now the National Institute of Standards and Technology) in Washington, DC, has undertaken a long-term program of absolute isotopic abundance ratio and atomic weight determinations using thermal ionization mass spectrometry. The elements studied include silver [48, 49], chlorine [50], copper [51], bromine [52], chromium [53], magnesium [54], lead [55], boron [56], rubidium [57], uranium [58], rhenium [59], potassium [60], silicon [61-63], thallium [64], strontium [65], gallium [66], and nickel [67].

Recently the Institute for Reference Materials and Measurements of the European Commission in Geel, Belgium has also been active in the certification of isotopic reference materials. The GE/R/MS/01/94 Catalogue [68] lists the available IRMs and spike reference materials from this source: boric acid, D20, HDO, iron, lithium, silicon, UF6, uranium, plutonium, calcium and rubidium. Other laboratories and national and international agencies are also involved in issuing IRMs: the International Atomic Energy Agency in Vienna has certified deuterium in water, carbon, oxygen and isotopes of nuclear safeguard importance. An excellent review, discussing the role of isotopic reference materials and listing the IRMs available up to the early 1990s, was presented by De Bievre et al. [69].

The general approach is to prepare a set of samples of known and variable isotopic composition from nearly isotopically pure separated isotopes of an element. The samples are used to calibrate mass spectrometers for mass discrimination bias by measuring their isotopic ratios. The measured ratios are then compared with the calculated ratios and bias correction factors are derived. These factors are used to correct the raw data obtained for a sample designated to be an isotopic standard reference material, finally yielding an absolute value for a particular ratio in the standard sample. The ratio measurements are usually carried out by two operators on two mass spectrometers.

A few more details are given to demonstrate the overall uncertainty computation procedure. The original separated isotopic (spike) samples are additionally purified and their final impurity content is determined using


different trace analysis techniques. Solutions are prepared from the purified separated isotopic samples, and the concentrations of the spike and the minor isotopes are determined by spiking with known amounts of the natural element and using the isotopic dilution technique. Then a set of calibration samples is prepared by mixing weighed portions of the separated isotopic solutions, the concentration of each isotope and the isotopic ratios are calculated. As mentioned in the previous paragraph, correction factors are calculated from measured ratios vs the calculated values and finally the measured ratios of the standard sample are corrected. The uncertainty of the standard isotopic ratio is given as 'the overall limit of error based on two standard deviations of the mean and allowances for the effects of known sources of possible systematic error'. The 'overall limit of error' calculated for the absolute isotopic abundance ratio of a certified isotopic reference sample of gallium, NIST SRM 994, is given as an example [66].

The bias corrected 69Ga/ Ga isotopic ratio 1.50676 Mass spectrometric analytical error (2SD of the mean) ± 0.000070 Limits to error in chemical analysis (2SD of the mean) ± 0.000204 Error in composition of separated isotopes ±0.000174

The overall limit of error is the sum of two standard deviation limits of the random errors plus the term covering possible systematic error in the separated isotopes. The computation is as follows

{[(0.000070)2 + (0.000204)2]1/2 + 0.000174} = 0.00039

therefore 69Ga/71Ga = 1.50676 + 0.00039. A procedure certifying a natural U 3 0 8 sample as a reference material was

given by Cretella et al. [70] Instead of preparing calibration mixtures from separated and purified uranium isotopes, the NIST uranium isotopic SRMs were used. The isotopic abundance of U, U and U were calculated in percentages as 0.00529 + 0.00005, 0.719 + 0.005 and 99.276 + 0.005 respec-tively with a 95% confidence interval. The characterized sample is considered an 'in house' working standard for uranium. Secondary IRMs referred to as 'in house', 'laboratory standard' or 'working standard', are widely used in many laboratories. In general, it is recommended that the experimental results be expressed relative to a certified primary IRM. An example is given in the H/D/T section, in Chapter 9, Section 1.

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[41] J.R. Taylor, An ¡ntroduction to Error Analysis, Oxford University Press, 1982, pp. 127-130, 132.

[42] CRC Handbook of Tables for Probabilities and Statistics, 2nd edn., W.H. Beyer (ed.), Chemical Rubber Co., Cleveland, OH, 1968, p. 6.

[43] R.L. McCutchen, Statistical Treatment of Data, Methods, 1-0060 and 9-0050, ORNL 1954.

[44] Crumpler and Yoe, Chemical Computation of Error, John Wiley & Sons, New York, 1940, pp. 188-191.

[45] Worthing and Geffner, Treatment of Experimental Data, John Wiley & Sons, New York, 1944, pp. 170-171.

[46] J.R. Taylor, An introduction to Error Analysis, Oxford University Press, 1982, pp. 141-146.

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68,589(1964). [52] E.J. Catanzaro, T.J. Murphy, E.L. Gamer and W.R. Shields, J. Res. Natl. Bur. Stand.,

Sect. A, 68, 593 (1964). [53] W.R. Shields, T.J. Murphy, E.J. Catanzaro and E.L. Garner, J. Res. Natl. Bur. Stand.,

Sect. A, 70, 193 (1966). [54] E.J. Catanzaro, T.J. Murphy, E.L. Garner and W.R. Shields, J. Res. Natl. Bur. Stand.,

Sect. A, 70, 453 (1966). [55] E.J. Catanzaro, T.J. Murphy, W.R. Shields and E.L. Gamer, J. Res. Natl. Bur. Stand.,

Sect. A, 72, 261 (1968). [56] E.J. Catanzaro, C E . Champion, E.L. Garner, G. Marinenko, K.M. Sappenfield and

W.R. Shields, Natl. Bur. Stand. (U.S.), Spec. Publ. 260-17 (1969). [57] E.J. Catanzaro, T.J. Murphy, E.L. Garner and W.R. Shields, /. Res. Natl. Bur. Stand.,

Sect. A, 73, 511 (1969). [58] E.L. Garner, L.A. Machlan and W.R. Shields, Natl. Bur. Stand. (U.S.), Spec. Publ.

260-27 (1971). [59] J.W. Grämlich, T.J. Murphy, E.L. Gamer and W.R. Shields, J. Res. Natl. Bur. Stand.,

Sect. A, 77, 691 (1973). [60] EX. Garner, T.J. Murphy, J.W. Grämlich, P.J. Paulsen and I.L. Barnes, J. Res. Natl.

Bur. Stand., Sect. A, 79, 713 (1975). [61] I.L. Barnes, L.J. Moore, L.A. Machlan and T.J. Murphy, / Res. Natl. Bur. Stand.,

Sect. A, 79, 727 (1975). [62] S. Valkiers, P. De Bievre, G. Lenaers and H.S. Peiser, J. Res. NIST, 96,617 (1991). [63] P. De Bievre, S. Valkiers and H.S. Peiser, J. Res. N¡ST, 99, 201 (1994). [64] L.P. Dunstan, J.W. Grämlich, I.L. Barnes and W.C Purdy, J. Res. Natl. Bur. Stand.,

85, 1 (1980). [65] L.J. Moore, T.J. Murphy and I.L. Barnes, /. Res. Natl. Bur. Stand., 87, 1 (1982).

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[68] Catalogue of ¡sotopic Reference Materials, GE/R/MS/01/94, Institute for Refer-ence Materials and Measurements, European Commission, JRC, Geel, Belgium, December 1993.

[69] P. De Bievre, J.R. De Laeter, H.S. Peiser and W.P. Reed, Mass Spectrom. Rev., 12, 143 (1993).

[70] R.F. Cretella, R.A. Lukaszew, J.G. Matrero and R. Servant, ¡nt. J. Mass Spectrom. ¡on Processes, 98, 99 (1990).

CHAPTER 9

ISOTOPE RATIO MEASUREMENT PROCEDURES

9.1 PROTIUM, DEUTERIUM AND TRITIUM

9.2 HELIUM 9.3 LITHIUM 9.4 BERYLLIUM 9.5 BORON 9.6 CARBON 9.7 NITROGEN 9.8 OXYGEN 9.9 FLUORINE

9.10 NEON 9.11 SODIUM 9.12 MAGNESIUM 9.13 ALUMINUM 9.14 SILICON 9.15 PHOSPHORUS 9.16 SULFUR 9.17 CHLORINE 9.18 ARGON 9.19 POTASSIUM 9.20 CALCIUM 9.21 SCANDIUM 9.22 TITANIUM 9.23 VANADIUM 9.24 CHROMIUM 9.25 MANGANESE 9.26 IRON 9.27 COBALT 9.28 NICKEL 9.29 COPPER 9.30 ZINC 9.31 GALLIUM 9.32 GERMANIUM 9.33 ARSENIC 9.34 SELENIUM

197 214 216 219 220 223 229 235 242 243 244 245 249 249 252 252 255 257 258 260 267 267 269 271 272 272 274 275 275 277 278 280 282 283

9.35 BROMINE 9.36 KRYPTON 9.37 RUBIDIUM 9.38 STRONTIUM 9.39 YTTRIUM 9.40 ZIRCONIUM 9.41 NIOBIUM 9.42 MOLYBDENUM 9.43 TECHNETIUM 9.44 RUTHENIUM 9.45 RHODIUM 9.46 PALLADIUM 9.47 SILVER 9.48 CADMIUM 9.49 INDIUM 9.50 TIN 9.51 ANTIMONY 9.52 TELLURIUM 9.53 IODINE 9.54 XENON 9.55 CESIUM 9.56 BARIUM 9.57 LANTHANUM 9.58 CERIUM 9.59 PRASEODYMIUM 9.60 NEODYMIUM 9.61 PROMETHIUM 9.62 SAMARIUM 9.63 EUROPIUM 9.64 GADOLINIUM 9.65 TERBIUM 9.66 DYSPROSIUM 9.67 HOLMIUM 9.68 ERBIUM 9.69 THULIUM

284 286 287 289 290 290 292 292 295 296 298 298 300 301 303 304 306 307 310 312 313 314 315 317 320 320 326 326 328 329 333 333 334 334 335

196 ISOTOPE RATIO MEASUREMENT PROCEDURES

9.70 YTTERBIUM 9.71 LUTETIUM 9.72 HAFNIUM 9.73 TANTALUM 9.74 TUNGSTEN 9.75 RHENIUM 9.76 OSMIUM 9.77 IRIDIUM 9.78 PLATINUM 9.79 GOLD 9.80 MERCURY

336 337 338 340 340 342 344 347 348 349 349

9.81 THALLIUM 9.82 LEAD 9.83 BISMUTH 9.88 RADIUM 9.90 THORIUM 9.91 PROTACTINIUM 9.92 URANIUM 9.93 NEPTUNIUM 9.94 PLUTONIUM REFERENCES

351 353 359 359 360 362 363 364 377 387

The following chapter is intended to provide a comprehensive element-by-element review of isotope ratio determinations, starting with the very early analytical procedures, then following the progress made over the years, and finally illuminating the state of the art of this topic. The aim is to present the subject to the reader in a brief and clear way and to guide him in finding the best solution for his specific problems. The chemistry of the procedure, technical details on sample preparation and admission or loading, ionization conditions (including available enhancement techniques), details on data acquisition and attainable precision, and types of instruments are discussed. The reader and potential user will gain a good background on how to approach his analytical task. It should be pointed out that, in general, isotope ratio mass spectrometry almost always demands high purity samples, or at least qualitative and quantitative information about the impurities. Therefore a ratio analysis is in most cases preceded by extensive sample separation and purification processes, where great care must be taken to avoid isotopic cross-contamination. To include in this manuscript a full description of these processes for each measurement procedure and for each element is beyond the scope of this book. Nevertheless, brief, and in some cases more extensive, comments are always given. The full details can be found in the quoted references. The text in this chapter also provides information about the types of instruments necessary to perform the analytical task at a predetermined level of precision, sample availability, sample throughput, mass resolving power and other instrumental variables. The information given in the various sections should by no means be taken as a recommendation of a manufacturer; for this reason also, the model of the instrument is usually omitted.

A few remarks should be made about the uncertainty concept in this chapter. As a general rule, the term is quoted as in the original publication. Unfortunately, quite a wide range of nomenclature is in use and in some cases the actual meaning is not clear. In Chapter 8, Section 8.2 an attempt was made to clarify this subject. The concepts of one or two standard or relative standard

PROTIUM, DEUTERIUM AND TRITIUM 197

deviations (1SD, 2SD, 1RSD and 2RSD) are well defined. NBS/NIST defines the uncertainty of the standard isotopic ratio in a SRM as 'the overall limit of error based on two standard deviations of the mean and allowances for the effects of known sources of possible systematic error'. An example of 'overall limit of error' calculated for an abundance ratio in a certified isotopic SRM is given in Chapter 8, Section 8.2.5. The term 'precision' usually means 1SD or 1RSD. Other terms used are relative internal and relative external standard deviation (RISD and RESD), 95% confidence level, 95% confidence limit, and also such terms as reproducibility and repeatability. It is questionable whether the last two have a statistical or mathematical definition.

It must also be mentioned that performing high quality isotopic ratio measurements demands a reasonable amount of experience, skill and intuition on the part of the technical staff. Instrumental and isotopic mass bias factors have already been discussed in the previous parts and are also considered in the sections that follow. When, in a laboratory team, several personnel are involved in the same analytical task, it is also good practice to check their 'personal bias'.


The concept hydrogen in this text will be used as a general term when more than one of its isotopes is intended. Protium, deuterium and tritium will be used for H and H2, D and D2, and T and T2, respectively.

Hydrogen is the first element in the Periodic Table. It has two stable isotopes, protium and deuterium, mass numbers 1 and 2. The representative isotopic composition of natural hydrogen in water is 99.985% H and 0.015% D [1]. The third isotope of this element, tritium, with mass number 3, is radioactive with a half-life of 12.26 years, decaying to 3He by ß~ emission. Molecular hydrogen may be composed of the following atoms: H2, HD, HT, D2, DT and T2. Mass spectrometric analysis of the isotopic composition can be divided into two categories: samples containing H and D atoms, and samples also containing variable concentrations of T atoms. The main interest in D/H analyses stems from the variations of deuterium in nature: water in oceans, evaporation of water, isotopic effects in plant growth and in chemical reactions, and the study of metabolic processes with deuterium-enriched compounds in humans and animals. The importance of tritium isotopic analysis lies mainly in nuclear research and industry.

The isotopic analysis of this element is one of the more complex mass spectrometric determinations. The following problems should be considered.

(1) The preferred compound for ratio analysis is molecular hydrogen. Most samples of interest are organic compounds or H 2 0 , and to a less extent H2S and


NH3. Generally they are oxidized to water and then reduced to molecular hydrogen. Another possibility is hydrogen isotope exchange in water with hydrogen having an accurately known isotopic composition. All the reactions involved must be quantitative to avoid isotope effects, and preferably fast.

(2) The large relative mass differences between the various hydrogen molecules require sector mass analyzers especially designed for this task.

(3) Gas inlet and pumping systems may introduce isotopic fractionation when the sample is admitted into the ion source and pumped out from there. Again, dedicated inlet and pumping systems minimize these problems.

(4) Molecular hydrogen and its ions have large reaction cross sections to form triatomic (also named trimer or secondary) ions. For natural, or close to natural, molecular hydrogen the ion-molecule reaction in an ion source will be:

H+ + H2 H+ + H (1.1)

Table 9.1. Ionic species in hydrogen and helium mass spectra Mass number

2

3

4

5

6

Ion

4 He 2 +

D+

HÍ 3He+ T+ HD+ H+

4He+ HT+

D2+

H2D+

DT+ H2T + D2H +

n DÎ

Mass, amu (12C basis)

2.0013007 2.014102 2.01565

3.016030 3.016050 3.021825 3.023475

4.002600 4.023875 4.028204 4.029650

5.03005 5.03170 5.035825

6.032 6.042

Mass separation

ppm

6376 768

6.6 1913 546

5300 1075 359

328 820

1656

Resolution required

160 1220

150000 520

1820

190 980

2860

3050 1210

610

Contribution from

HD, D2, DT

HT, DT, T2

H + + H 2 "

b

c

C

C

' Ion-molecule reaction (IRM) in the ion source. * Several reactant pairs may yield this IRM product: Hf+D 2 ; H J+HD; HD++H2. c IRM products.


H 3" interfères with HD+ when deuterium is analyzed, as the ion currents of Hj and HD+ are monitored. The cross sections of ion-molecule reactions depend on a variety of experimental and physical parameters. The trimer formation is governed by a squared pressure relationship within the ion source. A large ion energy, achieved by increasing the extraction potential in the ion source, a short distance between the electron beam and extraction plates (short focal length) (both of which reduce the residence time of primary reactant ions in the ion source), and as low as possible sample pressure in the source decrease the secondary ion-molecule reaction product yields.

(5) There may be also interference from primary ions at a nominal mass. For example, D+ contributes to H J, T + to HD+, D j to HT + etc. A further complication may arise in tritium-rich H/D/T samples, where tritium decays to 3He, which contributes to HD. A high resolution mass spectrometer can resolve most isobars but not all. Helium can be removed by purification processes. Table 9.1 summarizes the various ionic species observed in the mass spectra of hydrogen and helium and the resolving power needed to distinguish between them.

A more detailed description of these parameters will be given in the following paragraphs.

9.1.1 Sample Preparation In principle, various small hydride molecules such as H20, CH4, C2Hé, NH3 and other can be used directly for ratio determinations. By far the best and most precise analytical results are obtained by converting the samples to gaseous hydrogen and introducing it into the mass spectrometer in that form. Hydrogen-containing samples appear generally as aqueous samples, hydrides or organic molecules. The quantitative conversion of the various sample types to hydrogen has been extensively described in the literature and reviewed by Wong and Klein [2]. The diagram in Figure 9.1 demonstrates the various processes in use.

Aqueous samples

Hvdrides

Combustion

1 Reduction

Equilibration with H2

Molecular hydrogen

Figure 9.1.


9.1.1.1 Metal Reduction of Aqueous Samples and Hydrides

Water samples are reduced by a hot metal to hydrogen. The two most fre-quently used metals are uranium and zinc. Iron, chromium, magnesium, manganese and tungsten have also been used. Bigeleisen et al. [3] reduced water, ammonia, phosphine and hydrogen sulfide on hot (400-700°C) uranium metal turnings previously degreased and cleaned in concentrated nitric acid. Nief and Botter [4] inserted a uranium furnace (foil 0.02 cm thick) in the gas inlet line of a mass spectrometer between the leak and the source, thereby reducing water, ammonia and hydrogen sulfide. Memory effects were avoided by heating the line to 90 °C. The use of turnings and foils rather than powder, and maintaining the uranium at elevated temperatures, is dictated by the need to avoid the formation of uranium hydride, which ex-changes rapidly with molecular hydrogen even at room temperature. Thurston [5] and Hartley [6] modified the water reduction procedure in the mass spectrometer inlet system for automatic sample preparation. The uranium reduction of water to hydrogen at elevated temperatures is described by the reaction

2H20 + U -» U0 2 + 2H2 (1.2)

The reduction of water with metallic zinc is usually performed at 400-450 °C in sealed glass ampoules. The ampoule containing 0.25 - 0.5 g clean zinc shot is filled with dry nitrogen and 1-30 mg water are introduced into it by a micro syringe. The water is frozen at —70 °C and the ampoule is evacuated and sealed off. A batch of several samples can be prepared by heating for 1 h. Glass tubes sealed at the bottom and closed by high vacuum glass/Teflon stopcocks are also used [7-9]. Recently Tanweer [10] demonstrated that accurate and highly reproducible hydrogen isotope analysis is achieved when zinc granules (0.5-2 mm) are carefully cleaned by sieving the zinc, rinsing in water, stirring in 1% nitric acid, completely drying under vacuum and then drying and heating at 300 °C under vacuum. Continuous reduction in a zinc shot furnace at 400 °C is also practiced. Wong et al. [11] recently reported the results of interlaboratory analyses of water samples enriched with deuterium. It was observed that the optimal temperature for water reduction with zinc is 475-500 °C. Lower temperatures produced erratic results. The reduction reaction is

H 2 0 + Zn -> ZnO + H2 (1.3)

On-line reduction of water to hydrogen can be achieved with a stoichiometric mixture of zinc and CaO heated at 400 °C. The CaO must be conditioned at 550 °C to ensure complete dehydration of Ca(OH)2. A 15 min period is needed for one sample preparation. The reaction is

H 2 0 + Zn + CaO -+ CaZn02 + H2 (1.4)


9.1.1.2 Carbon and Hydride Reduction of Aqueous Samples Water can be reduced to hydrogen at temperatures between 1000 and 1300°C with carbon alone or in the presence of platinum or nickel as catalyst according to the following reactions

H20 + C->CO + H2 (1.5) 2H20 + C -• CO + C02 + H2 (1.6)

Graphite or diamond is the carbon source. The reduction time is 1-2 h. Calcium hydride and lithium aluminum hydride react vigorously with water

at room temperature according to the following reactions 2H20 + CaH2 -> Ca(OH)2 + 2H2 (1.7)

4H20 + L1AIH4 -» LiAl(OH)4 + 4H2 (1.8)

9.1.1.3 Electrolytic Reduction of Aqueous Samples Bocek et al. [12] developed a microelectrolyzer capable of converting 10 mg water samples for the determination of hydrogen isotopic ratios. The large isotopic fractionation involved in this process requires complete conversion of the sample to hydrogen.

9.1.1.4 Equilibration of Aqueous Samples Fischer et al. [13] originally introduced the water-hydrogen equilibration technique for highly deuterium-enriched water samples. Thus, 5g of water and platinum oxide are introduced into a 30 ml flask and frozen at —70 °C, the flask is evacuated and hydrogen at 1 atm is added. The exchange reaction between water vapor and hydrogen at 25 °C takes 1 h and is given by

HDO + H2 - • H20 + HD (1.9) The hydrogen gas is then used for isotopic ratio analysis. Various modifications of this technique have been described [2]. Recently Horita et al. [14] described an improved equilibration method. H2 gas is equilibrated with the water sample on a hydrophobic platinum catalyst in an automatic equilibration unit connected directly to a mass spectrometer. Long term reproducibility of ±1.5%o for SD was reported. (The 6 term will be defined in Section 9.1.3). Copien et al. [15] further improved this method. The smallest water sample size was reduced from lml to 0.1ml, and the catalyst poison H2S from H2S-bearing samples was removed by treatment with AgN03 or fine granular Cu metal to form insoluble sulfides. Replicate sample reproducibility of 0.8%o (ISD) was observed. The water-hydrogen equilibration technique and the reduction of water samples


with zinc or uranium are the preferred techniques to prepare aqueous samples for D/H isotopic ratio measurements.

9.1.1.5 Combustion of Organic Samples to Water Organic samples can be converted to C02, H20 and N2 in the presence of pure oxygen by a static combustion procedure at 800-900 °C over cupric oxide and silver metal in a closed quartz vessel, or by a dynamic procedure, with circulation of the combustion products through various traps with a magnetic or mercury Toepler pump [2], Halogens and S02 form silver halides and sulfate, and copper reduces nitrogen oxides to nitrogen. At the end of the combustion the quartz tube is attached to a vacuum manifold, through an ampoule cracker, and cooled to — 70 °C, C02 and N2 are pumped off and the H20 is submitted to one of the reduction processes already discussed. In the dynamic procedure the water vapor may be cleaned in specific traps, including Pb02 to remove nitrogen oxides at 175-190 °C according to the following reactions

2NO + 2Pb02 -+ Pb(N02)2 • PbO + 1 ¡2 0 2 (1.10) 2N02 + 2Pb02 -> Pb(N03)2 + PbO + 1 ¡2 02 (1.11)

Organic material may also be converted to water by combustion with oxygen at 680 CC in the presence of platinum or platinized asbestos.

9.1.1.6 Combustion of Organic Samples to Hydrogen Organic samples can be directly converted to molecular hydrogen at elevated temperatures of 950-1250 °C over carbon, nickel or diamond-platinum mixture [2]. A sealed nickel bomb technique appears to be the most promising.

9.1.2 Hydrogen Ion-Molecule Reactions When discussing ion-molecule reactions, it is of major importance to determine the correct reacting pair of primary ion and molecule responsible of forming the corresponding secondary ion. Reducing this problem to hydrogen ion-molecule reactions (see eq. (1.1)), it may be accepted that the primary reacting ion is the molecular hydrogen ion. It is assumed that an atomic ion as a reactant would yield reaction rate constants «200-1000 times larger than the true values, and also larger than those theoretically calculated for the fastest gas phase bimolecular reactions.

We shall now briefly derive a relationship between the measured mass spectrometric quantities, i.e. primary and secondary ion current intensities and such experimental parameters as sample pressure, the distance of the ion motion, and the repeller field strength prevailing in the ion source at the time of measurement. A schematic diagram of an ion source used to study ion-


molecule reactions is shown in Figure 7.2. It is assumed that only one kind of secondary ion is formed from one kind of primary ion, and that both types of ion are extracted from the ion source, transmitted through the mass spectro-meter and collected at the ion detection system with equal efficiency. If it is further assumed that 'single collision' conditions exist in the ion source, the measured secondary ion current /s will be related to the primary measured ion current /p by the equation

/s = 7p(l - e"""0*) (1.12)

where Nm is the number density of neutral molecules in the ion source (assumed constant), Q is an appropriate average of the microscopic cross section q for the reaction producing the secondary products, and x is the distance between a plane located in the ion source where the primary ions are formed and the ion exit slit. Assuming that NmQx is small compared with unity, the expansion of the exponent term to series and retaining only the first two terms gives

h/Ip^NmQx (1.12a)

The evaluation of Q from this equation is correct to within 5% if / s / / p < 0.1. It has been observed that for many simple ion-molecule reactions of the

type X+ + Y H - ^ X H + + Y (1.13)

where H denotes a hydrogen atom and X and Y are arbitrary atoms, Q depends on the strength of the repeller field Et, being proportional to (ET)~ ' , and is independent of gas temperature. ET is assumed to be uniform in space and constant in time.

Denoting by np and ns the total number of primary and secondary ions respectively, the rate of ns formation is given by

dnjdt = knpNm (1-14)

where k is the bimolecular reaction rate constant. Combining eq. (12a) with eq. (14) yields

dns/dt = knpIs/IpQx (1.15)

By definition Is = edns/dt (1.16)

and lp = enpvp/x (1-17)

where e is the charge on the ion and vp is the average velocity of the primary ions over distance. Substitution of eqs. (16) and (17) in (15) gives

k = Qvp (1.18)


If the primary ions start their motion from rest, the final velocity vpf they acquire at the ion exit slit is given by

mpv2p{/2 = eErx (1.19)

where mp is the mass of the primary ions. It can be easily shown that

therefore Vpf — 2vn

(eEcx/2mp)l/2

and the average reaction cross section is expressed by

Q = k{2mp/eErx)x'2

In practice, the ion repeller voltage is the experimental variable

Vr = Etx

therefore eq. (22) may also by presented as

Q = k(2mp/eVr)1/2

1.20)

1.21)

1.22)

1.23)

1.24)

1.0

0.8

0.6

0.4

0.2

I

-

-

-

" ~ y

i i

S/Sr^^

r-o r "

I I

So

a s-^o

—-'—""° i i

I

o ,

I

°s °JS

r i

, 1.232 V

/ 1.848 V

y 3.08 V

- 6.16 V

^- 12.32 V

— 30.8 V

]

2000 4000 6000 H2 Pressure (Arbitrary Units)

Figure 9.2. Dependence of the H ^ / H j ratio on H2 pressure at several ion repeller voltages. (Reproduced with permission from ¡on-Molecule Reactions by E.W. McDaniel, V. Cermak, A. Dalgarno, E.E. Ferguson and L. Friedman, Wiley-

Interscience, 1970, p. 291)


u./u

0.60-

0.50-

0.40-

0.30-

0.20-

0.10-

0.00-

jS' •

* 1

/ +

y/i

1 1

l / '

- 1 • ' 0.00 0.20 0.40

V"1'2, (V)

0.60 -1/2

0.80 1.00

Figure 9.3. Dependence of the H^/Hj ratio on ion repeller voltage at constant H2 pressure, calculated from Figure 9.2 at H2 pressure 4000 (arbitrary units)

Eq. (1.12a) demonstrates the linear dependence of 7S//P on sample pressure Nm and the distance of the ion motion in the ion source x. Eq. (1.22) shows the reciprocal dependence of Q on (Er) ' . These relationships for reaction (1.1) are shown in Figures 9.2 and 9.3.

9.1.3 Hydrogen Isotopic Ratio Presentation

9.1.3.1 Deuterium Concentrations Close to Natural Isotopic Abundance. The Delta (ó) per Mil Definition

In this type of D/H isotopic ratio determination, it is common practice to measure the sample of interest against a laboratory standard (LS). The sample and the standard are alternately admitted into the ion source of the mass spec-trometer through a leak in the inlet system. Single inlet or dual inlet systems are in use; with the latter, one is used for the standard and the second for the samples. The gas is ionized in the ion source by electron impact bombardment, usually with 70 eV energy electrons, and the ratio R = HD+/H2 of the sample (RSA) and of the standard (/?LS) are measured. Corrections for background, ion-molecule reaction contributions (such as H J to HD+) and abundance sen-sitivities must be made and the results are presented in delta units, <SD, of parts per thousand (or per mil, %o) relative to the laboratory standard.

¿DSA = [(/?SA//?LS) - 1] x 1000 (1.25) Next, to derive an absolute <5 value, the observed value is normalized against

two international water standards of well known deuterium content. Craig [16]


defined the D/H ratio in Standard Mean Ocean Water (SMOW) relative to the D/H ratio in a large volume of distilled water prepared by the National Bureau of Standards, NBS (now the National Institute of Science and Technology, NIST) in the USA and designated as NBS standard reference material 1, (NBS-1). These two standards are related by

(D/H)SMOW = 1 050 x (D/H)NBS_, ( 1.26)

In addition, Craig also prepared a large quantity of water for the International Atomic Energy Agency in Vienna with the same isotopic composition as the original SMOW. This standard is now known as Vienna Standard Mean Ocean Water, or V-SMOW. As the two are identical, the name SMOW will be used here. The second international water standard is the Standard Light Antarctic Precipitation, SLAP. Both the standards, SMOW and SLAP, are available from the International Atomic Energy Agency in Vienna. The following is the normalization expression

c __ ^SA/LS — ^SMOW/LS c0 /i nn\ ÖSA/SMOW/SLAP — T 7 x 0 SLAP/SMOW \ l •Z ' )

ÖSLAP/LS - ÖSMOW/LS

where <5SA/LS, ¿SMOW/LS and ¿SLAP/LS are the ¿D values of the sample, SMOW and SLAP respectively measured relative to the laboratory standard. Wong and Klein [2] quote for the D/H ratio in SMOW (#SMOW) a value of (155.95 + 0.54) x 10~6, and for <5°LAP/SMOW a value of -428%o. This procedure standardizes the ÔD values reported in the literature and is needed when converting the 6%o values to absolute isotopic ratios

ASA = (¿ D/1000 + 1) x .RSMOW (1-28)

The absolute isotopic scale for deuterium analysis of natural waters was developed by Hagemann et al. [17] and has been adopted for the presentation of natural and close to natural D/H ratios. They reported

( D / H ) S M O W = ( 1 5 5 - 7 6 ± °-05) x l c r 6 Í1-2 9) (D/H)SLAP = (89.02 ± 0.05) x 10~6 (1.30)

¿SLAP/SMOW = (-428.50 ± 0.10)%„ (1.31)

The SD values of several other water isotopic standards, together with the 0~18O values relative to SMOW, will be summarized in Section 9.8, Table 9.13. See also the note at the end of this section.

9.1.3.2 Precision and Sample Size Requirements

Double collector mass spectrometers dedicated to natural D/H isotopic ratio determinations have the capability to measure this ratio with an instrumental


precision down to 0.1%o (2RSD and n = 10). In most cases the variability of sample preparation is greater than the instrumental limitations to precision.

Regarding the sample size requirements, Hayes et al. [ 18] have calculated a theoretical minimum sample amount of 21 nmol of hydrogen needed for a ratio determination. It was assumed that a precision of 0.1 %o is demanded, a double collector instrument is used and the mass spectrometer efficiency is 10~4 ions per molecule of gas. The calculated minimum for beam switching measurement is 22 nmol. Generally, about 20 pmol of hydrogen is required.

9.1.3.3 High Deuterium and Tritium Concentrations; Measurements at Low Mass Resolution

Mehrhoff and Humphries [19] discussed the isotope ratio analysis of hydrogen with a low resolving power (<200) mass spectrometer. Two types of samples were considered: (a) mixtures degassed from a uranium hydride bed into a stainless steel flask, containing H2, D2, T2, HD, HT, DT and 3He and with a protium concentration of about 1%; (b) mixtures occluded within thin films of erbium, scandium or titanium, containing up to 10% protium, directly outgassed into a mass spectrometer inlet system and immediately analyzed. It was assumed that both types of sample are in isotopic equilibrium and that the protium is present mainly in the form of HD and HT. A mass resolving power of 980 is required to separate between H T + and D 2 ions (see Table 9.1). This resolving power is beyond the capability of low mass resolution instruments, therefore quantitative calculations were made using the equilibrium constants of isotopic exchange reactions.

Isotopic exchange reactions are extremely slow at room temperature. Fast equilibrium can be achieved within less than 1 min at elevated temperatures with metallic matrices such as those mentioned above. The isotopic exchange reactions and their designated equilibrium constants are given in Table 9.2. Jones [20] calculated the equilibrium constants among the isotopic hydrogen molecules between 25 and 2500 K. Lasser and Powell [21 ] studied the solubility of H, D and T in palladium and calculated the equilibrium constants for the formation of gaseous HD, HT and DT species. Good agreement with the constants reported by Jones was observed. The values of the constants published by Jones are given in Table 9.3.

It should be noted that these equilibrium constants are not independent, but:

K\ = K3 x K4, K6 = K4x K5, and K2 = K3 x K5 (1.38)

Following Mehrhoff and Humphries [19], all the equilibrium constants except K5 may be used to calculate the unresolved H T + - D 2 doublet at m/z = 4. K[, K~4 and K(, are dependent on the H2 level, which is generally too small in the samples discussed. Therefore K2 and Kj, have been used.


Table 9.2. Hydrogen isotopic exchange reactions H 2 + T 2 D2 + T2 T2 + H D H2 + DT D 2 + H T H 2 + D 2

= 2HT = 2DT = HT + DT = HD + HT = HD + DT = 2HD

(*,) (K2) (K3) (K4) (Ks) (K6)

(1.32) (1.33) (1.34) (1.35) (1.36) (1.37)

Table 9.3. Equilibrium constants for isotopic equilibria T, K

2500 2000 1500 1250 1000 900 800 700 600 500 400 300 298.1 250 200 150 100 50 25

Ki

3.99 3.98 3.94 3.90 3.81 3.76 3.68 3.59 3.45 3.26 2.99 2.58 2.57

-1.948 _

0.945 0.242

-

K2

4.00 4.00 4.00 4.00 3.99 3.98 3.98 3.97 3.96 3.93 3.88 3.82 3.82 3.77 3.69 3.57 3.32 2.60 1.95

Ki

2.00 2.00 2.00 1.99 1.98 1.97 1.95 1.94 1.91 1.88 1.82 1.74 1.74 1.67 1.58 1.33 1.18 0.687 0.282

K4

1.99 1.99 1.98 1.96 1.93 1.91 1.88 1.85 1.80 1.74 1.64 1.48 1.48 -

1.24 -

0.802 0.353

-

K5

2.00 2.00 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.09 2.13 2.20 2.20 2.26 2.34 2.68 2.82 3.78 6.94

# 6

3.97 3.97 3.96 3.94 3.90 3.87 3.83 3.78 3.72 3.62 3.48 3.26 3.26 -

2.90 _

2.26 1.33 -

The calculation method for HT, based on K2 is derived from the appropriate Jones equilibrium constant

(D2) = (DT)2/(T2)tf2 (1.39) ( H T ) = / 4 - ( D 2 ) (1.40)

where I4 is the total low resolution ion intensity at m/z = 4 and K2 = 3.82 at 300 K, thus

(HT) = h - (DT)2/(T2)K-2 (1.41)

From the K3 equilibrium constant HT can also be derived

(HT) = Ar3(HD)(T2)/(DT) (1.42)


Table 9.4. Summary of HT concentration calculations using low resolving power hydrogen mass spectrometry

Uranium bed Film degassing Film degassing flask analyses direct analyses direct analyses

at ambient at900°C at600°C temperature

Samples containing 1.74 (HD) (T2) 1.99 (HD) (T2) 1.97 (HD) (T2) less tnan 50% total tritium

Samples

(DT) (DT) (DT)

^ n t ^ m n g , (DT)2 1.99 (HD) (T2) 1.97 (HD) (T2) 50-80% ' total tritium"

3.82 (T2) (DT) (DT)

Samples containing (DT)2 (DT)2 pT^2 more than ¡4 — „_„ ,„ , , h — . _ _ ,_ , ¡4 80% total tritium"

3.82 (T2) 4.00 (T2) 3.98 (T2)

¡4 is the total low resolution ion intensity at m/z = 4.

A detailed error propagation analysis has been performed on both HT expressions derived from K2 and K3 [19] following the partial derivative method described by Shoemaker and Garland [22]. It has been shown that results of better precision are obtained at lower tritium concentrations when the calculation applies the K3 constant, or the K2 constant for higher tritium concentration. Table 9.4 summarizes the observation for HT concentration calculations. The relative error of the K2 and K3 values was estimated to be 0.3%. Separate high resolution isotopic analyses also verified the calculations. The average agreement with high resolution analysis was 0.05 and 0.43% for K3 and K2 respectively. Equilibrium constants at 300 K and 900 K were used for uranium bed and foil samples respectively. The 3He interference on HD at m/z = 3 can be treated in two ways: in fresh tritium samples the total ion current is attributed solely to the HD+ ion, or alternatively, for hydrogen adsorbed on uranium or the various films, 3He may be pumped off at ambient or lower temperatures after heating and cooling the uranium bed. The contribution of atomic hydrogen ions to molecular ions at the same mass number is usually of the order of < 0.5% of the parent molecular ion. The exact value of the dissociative ionization process at particular instrumental conditions can be


experimentally determined with isotopically pure samples. Schott and Beau [23] reported the following values

H+y/Hj =0.0053 (1.43) D + / D + = 0.0040 (1.44) T + / T j = 0.0035 (1.45)

H + /HD+ = 0.0020 (1.46)

Pure HD was obtained by hydrolyzing lithium hydride with heavy water. Schott and Beau [23] discussed the problems involved in low mass resolution

hydrogen isotope ratio mass spectrometry. This mass spectrum provides five ion current signals from m/z = 2 to m/z = 6. Assuming that the ion-molecule reaction and atomic ion contributions can be evaluated with reasonable precision, two of the ion currents at m/z = 3 and 4 are still doublets of 3He-HD and D2-HT. These authors showed that a resolving power of 1000 was sufficient to obtain adequate mass separation. This was achieved with an automatic source and a slit collector changer to adjust from wide slits (0.3 and 0.9 mm) to narrow slits (0.03 and 0.07 mm). A further point favoring direct measurements is the frequent doubt as to whether an equilibrium state exists among the isotopes. A magnetic sector mass spectrometer was used with a maximum resolving power of 1000 at 10% valley definition. The gas was introduced through a molecular leak connecting the sample reservoir to the ion source. With this inlet system the isotopic mixture is depleted in the lighter isotope. It was necessary to choose a small molecular leak to avoid ion signal depletion at m/z = 2 of >20% within 1 h. To shorten the time required for magnetic scanning of the spectrum (12 min from m/z — 2 to m/z — 6 and 20 min from m/z = 2 to m/z = 3 at low and high resolution respectively), only the ion signals were scanned, and the intervals were quickly swept from the end of the low mass to the beginning of the next high mass ion position. It was possible to scan the whole hydrogen isotopic spectrum within 1 min. A Potentiometrie recorder was used to adjust the ion beam focusing and to observe the signal shapes. The data were then collected and printed out with a digital counter.

The method for quantitative determination of hydrogen molecular species in an isotopic mixture containing H, D, T and 3He atoms with a low resolution mass spectrometer can be summarized as follows.

(a) The production of triatomic ions can be minimized and controlled by proper ion source design, i.e. short focal length and low sample ion source pressure. Preliminary experiments can determine the formation yields of these species and their contributions to the actual sample.

(b) The monoatomic ions are formed in a constant ratio to the molecular ions, therefore their contributions can be accurately calculated.


(c) The 3He-HD doublet is not resolved, therefore the measured ion intensity represents a sum. HD can be removed from the sample by pumping with a titanium sublimation pump or a uranium bed, and 3He+ ion intensity then being measured again. HD intensity is determined by difference. This determination is time consuming and introduces substantial uncertainty to the HD and HT contents of the sample (see Table 9.4)

(d) The relative amounts of D2 and HT in the unresolved D2-HT doublet are calculated from the known (measured) H2, HD, DT and T2 ion intensities and the Jones equilibrium constants, following the procedure of Mehrhoff and Humphries [19].

(e) The uncertainties in the sample equilibrium state and in the equilibrium constant values may further contribute to the overall error of the analysis.

9.1.3.4 High Deuterium and Tritium Concentrations; Measurements at High Mass Resolution

Ferguson and Chastagner [24] described the development of a high resolution mass spectrometer for the accurate analysis of mixtures of hydrogen and helium isotopes. The main features of this instrument were: large (35 cm radius) 66° magnetic sector mass analyzer; short (1 cm) focal length, tightened to reduce fast pumping of hydrogen from the ion source; a 3 liter sample expansion volume followed by a gold molecular leak (leak rate approx. 0.2 cm"3 s"1 for N2); resolving power of 1300, separating H J from D + , as shown in Figure 9.4; and improved abundance sensitivity (ratio of HD+ ion intensity to its tail intensity at 3He+) of 27 500, as shown in Figure 9.5. A Faraday collector was used for data acquisition. The analyses were performed on a fixed time cycle to correct for pressure decay in the inlet manifold. A low mass discrimination, pä 0.5%, was observed when the sensitivities for H2 and D2 were determined to be 63 mV pm - 1 and linear within 0.3% for 25-100 pm Hg pressure. Triatomic ion formation was less than 0.1%.

Chastagner et al. [25] also evaluated two advanced commercial prototype mass spectrometers for accurate hydrogen isotope analysis. One was a double focusing instrument with a resolving power of 2000 at m/z — 4 and the second was a single focusing instrument with exceptionally high ion intensities for high signal to noise ratios and a resolving power of 1300. Both instruments were designed for safe operation with tritium mixtures and were computer controlled. Their performance was essentially equal; the precision and accuracy for the D/T ratio was better than 0.5% (2RSD), and the linearity and discrimination were also equal to or below this value. The performance of the two prototype mass spectrometers is summarized in Table 9.5. These instruments of the early 1980s were the first in a new generation of advanced, fully automatic mass spectrometers simultaneously developed and marketed by two manufacturers [26]. Surprisingly, they were also the first commercial instruments dedicated


Resolution = 1300

(full scale

increasing Mass

Abudance Sensitivity = 27,500 Resolution = 512

11 mV

Scale Change"

increasing Mass

Figure 9.4. Figure 9.5. Figure 9.4. Mass spectrum of H \ and D+. (Reproduced by permission of Elsevier Science from R. B. Ferguson and P. Chastagner, Int. J. Mass Spectrom. ¡on Phys., 24, 403 (1997)) Figure 9.5. Mass spectrum of HD+ and 3He+. (Reproduced by permission of Elsevier Science NL from R. B. Ferguson and P. Chastagner, ¡nt. J. Mass Spectrom. ¡on Phys., 1A, 403 (1977))

Table 9.5. Comparison between commercial prototype hydrogen mass spectrometers [25]

Double focusing mass spectrometer

Single focusing mass spectrometer

Resolving power Abundance sensitivity

3He-HD HT-D2

Ion intensity (A) SIN ratio D/T ratio precision,

2RSD (%)

2000

>100000 >100000

5 x 1 0 " 1 x 105

0.5

1300

90000 50000 1 x 10"10

2 x 105

0.5

1300

>40000 10000 1 x IO"10

2 x 105

0.5

600

40000

5 x 10~9

> 1 x 106

0.4

mainly to hydrogen isotopic ratio measurements and impurities in hydrogen. Improved ion source design, including good ionization efficiency, reduced the triatomic ion formation to levels typically less than 50 ppm of the major hydrogen isotope. This level of interference can essentially be ignored. Three


doublets, D-H2, 3He-HD and HT-D2, may be affected by intense ion signal tail contributions, limiting the detection of the minor component in the doublet, therefore the tail intensity must be minimized. Large radius magnets (increasing the ion dispersion), the use of an electrostatic analyzer following the magnetic sector, ultra-high vacuum conditions along the ion flight tube, and the introduction of baffles in the tube to reduce stray ions greatly increase the abundance sensitivity. Computer software makes it possible to jump between the ion signal tops, to assign correctly the isotopic composition to doublets at the same mass number, to correct for monoatomic ion contributions, to determine time dependent and mass dependent ion depletion (pressure decay) rates, and to calculate the 'true' ion intensities, In at zero time (i.e. the time of starting sample introduction). The mass discrimination is determined with pure calibration gases, measuring ion intensities at well defined sample pressures. Pure isotopic samples are easily available except DT. A mass discrimination factor for this molecule can be estimated as an intermediate between D2 and T2 with an uncertainty better than 0.5%. The automatic hydrogen isotope ratio mass spectrometers have an ion steering facility for each ion, enabling the relative individual ion sensitivities to be brought to within 0.5%. The final ion current used for composition calculations will be

/0 = /mease*' (1.47)

where /meas is the measured ion current corrected for all the effects above discussed, except the time dependent depletion of the sample reservoir.

Therefore, in a high resolving power (> 1300) mass spectrometer h = [H2

+] (1.48) the subscript 0 being omitted for simplicity;

/3',=:[3He+] + [T+] (1.49) hi =[3He+]-{a4 i [HT+]+f l 5 [DT+]+a6[T2

+]} (1.50) where a4\, a5 and a6 are the T + /HT + , T+/DT + and T+/TJ dissociative ionization constants respectively,

/32 = [HD+] (1.51) /4t = [HT+] (1.52) /42 = [D+] (1.53) /s = [DT+] (1.54) /6 = [T2

+] (1.55) We shall define P as:

P = [H+] + [HD+] + [HT+] + [D2+] + [DT+] + [T+] (1.56)


Assuming no isotopic effect in the ionization cross section of molecular hydrogen, P will be the abundance summation of all the molecular hydrogens

P = [H2] + [HD] + [HT] + [Da] + [DT] + [T2] (1.57)

For example, tbe atom percent or mol percent of any atom or molecule will be

T(at%) = 100 x {[HT] + [DT] + 2[T2]}/2P (1.58)

and DT (mol%) percent = 100 x [DT]/P (1.59)

To calculate the mole fraction of 3He in the mixture, the ionization cross sections must be taken in account. At 70 eV electron ionization energy, the molecular hydrogen and helium ionization cross sections are 1.01 x 10~16 and 0.38 x 10~16 cm2 respectively; therefore the abundance sum of all the species in the mixture will be

P' = P+ (1.01/0.38) x [3He+] = P + [ 3 H e ] (1.60)

and 3He (at%) = 100 x [3He]/P' (1.61)

The mol percent of a molecular hydrogen species, for example DT, in a mixture containing 3He will be

DT (mol%) = 100 x [DT]/*»' (1.62)

Note: Recently, the International Atomic Energy Agency in Vienna issued a list of hydrogen isotopic SRMs available from the Agency [543].

9.2 HELIUM

Helium is the second element in the Periodic Table. It has two stable isotopes at mass numbers 3 and 4, with relative abundance in air of 0.000137 and 99.999863% respectively [1].

Aldrich and Nier [27,28] investigated the 3He/4He natural abundance ratio in the atmosphere, gas wells and minerals. For atmospheric and well helium, the reported ratios were 1.11 x 10 6 and 1.43 x 10~7, respectively (each ±25%). The forepump of the spectrometer was replaced by a charcoal trap and the waste helium purified by the trap was allowed to re-enter the ion source. This quasi static mode of operation increased the effective instrument sensitivity by a factor of « 100. Reynolds [29] has described an all glass mass spectrometer operating under extremely clean and completely static conditions. Mamyrin et al. [30] designed a special magnetic mass spectrometer for the isotopic study of very small amounts of 3He (approx. 5 x 106 atoms) in natural helium, with a

HELIUM 215

resolving power sufficient to separate between the HD+ ions and 3He. This group reported the isotopic ratio 3He/4He = (1.399 ±0.013) x 10~6 for helium in Leningrad air. The mass spectrometer was calibrated against specially prepared calibration samples by mixing 3He and 4He isotopes. A detailed analysis of possible sources of systematic error has also been carried out [31,32]. Boulloud et al. [33] described a double focusing mass spectrometer used to determine natural helium isotopic ratios with a reproducibility better than 0.6%. Clarke et al. [34] described an inert gas extraction system coupled to a statically operated [29] double collector mass spectrometer, specially designed for high sensitivity measurements of small helium samples. The 3He+ and 4He+ ion currents were monitored with an electron multiplier and a Faraday collector respectively, and the output was fed into vibrating reed electrometers, voltage to frequency converters and, finally, into a counter. The system was used for tritium determination by measuring 3He and for re-determination of the atmospheric 3He/4He ratio. Figure 9.6 shows the mass spectrometer in schematic form. The mass spectrometer was calibrated with synthetically prepared mixtures of highly 3He-depleted helium and a pure 3He spike. Helium samples were prepared from aliquots of air. All the samples, before admission to the mass spectrometer, were mixed with high purity atmospheric neon to a He/Ne ratio identical (within ± 1%) to the atmospheric He/Ne ratio. The observed 3He/4He ratio was 1.384 x 10- 6 (± 0.4%). The 4He and 3He contents in atmospheric air were determined by Holland and Emerson [35] and Davidson and Emerson [36] respectively. Two separate experiments were carried out. Helium was pre-concentrated by passing air samples through a charcoal trap at liquid N2 temperature. The sample measurements of each

O Faraday

Johnson MM-1 Multiplier Ti 4 o o

^ Ion Ti DP Pump

Fig 9.6. Schematic diagram of He mass spectrometer. (Mass spectrum reproduced by permission of Elsevier Science NL from W. B. Clarke et al, ¡nt. J. Appl Radiât, hot., 27, 516 (1976))


isotope were compared with identical measurements of gravimetrically prepared standards of the same isotope. The contribution of HD+ background to 3He+ was eliminated by adjusting the mass spectrometer resolution (slit width), focus and repeller voltages. The near-surface contents of 4He and 3He in atmospheric air (continental USA) were determined to be 5.2204 ± 0.0041 ppm by volume and 7.27 ± 0.20 ppt by volume, respectively, establishing an isotopic ratio of 3 He / 4 He= 1.393 x 10 6. Halverson and Herzog [37] also described a dual collector static 3He/4He mass spectrometer designed to determine very low 3He concentrations. Sano et al. [38] determined the absolute atmospheric 3He/4He isotopic ratio with a high precision double collector mass spectro-meter. They reported a value of (1.343 ± 0.013) x 10"6, which is slightly lower than previous data. The discrepancy was attributed to either experimental problems or natural phenomena. Sano et al. [39] constructed a small, single focusing mass spectrometer with large incident and exit angles to measure 3He/4He ratios. The resolving power at 5% peak height was about 600, sufficient to resolve 3He from H^ and HD. The sensitivity was about 0.3 A cm"-3 STP when a secondary electron multiplier was used. This sensitivity was high enough to measure the helium isotopic ratio of terrestrial samples. The instrument had a large mass discrimination for the lighter isotope, which was determined with synthetically prepared helium samples. The system was used as a mobile laboratory for field studies related to predictions of earthquakes and volcanic eruptions.

9.3 LITHIUM

Lithium is the third element in the Periodic Table. It has two stable isotopes at mass numbers 6 and 7 with relative abundance of 7.5 and 92.5% respectively [1]. Precise isotopic analysis of this element is important in nuclear technology, the earth sciences and the biosciences.

Thermal ionization mass spectrometry is most commonly applied for lithium isotopic ratio determinations. The ionization potential (IP) of Li is 5.392 eV [40] (all IP values in Chapter 9 are taken from ref. [40]), thus this element is very efficiently ionized. Electron impact on evaporated lithium halides was also used, but this technique needed larger samples, which contaminated the ion source and caused large memory effects. Thermal ionization isotopic analysis of this element presents a few difficulties. The ionization process is intrinsically affected by isotopic fractionation. The relative mass difference between the two isotopes is about 15%, the highest for the thermally ionized elements except calcium. Furthermore, internal normalization correction is not possible in an element with only two isotopes. In such a case, to achieve good accuracy and precision is possible only by calibrating the measuring procedure against an isotopic standard, preferably composed from two pure isotopes. Several

LITHIUM 217

attempts have been made to minimize the fractionation effect: (1) the use of salts with heavy anions, such as lithium iodide. Kanno [41] has shown that for LiCl, LiBr and Lil, the fractionation effect is the lowest in the iodide; (2) design of special mass spectrometers; and (3) the ionization of such compounds as lithium fluoride and lithium metaborate, which yield the heavier polyatomic lithium ions Li2F+ and L^BOj.

Lithium isotope ratio measurements were first reported in 1934. Brewer [42] observed for 7Li/6Li under different experimental conditions ratios of 12.14 and 11.6. White and Cameron [43] obtained a ratio of 12.7. Ordshonikidse and Shutze [44] studied this discrepancy, introducing various correction factors. Their value, 12.48 + 0.02, was in very good agreement with 12.47+0.02 as measured using the total evaporation method. Spitzer and Sites [45] evaporated Lil from a tantalum ribbon at 500 °C and ionized it by electron impact, monitoring Li2I+. This method needs larger (milligram) amounts of material and demanded frequent cleaning of the ion source. Palmer [46] used a triple tungsten filament ion source and thermally ionized LiN03, observing a ratio of 12.47 + 0.01, compared with 12.49 + 0.01 as quoted by NBS for the same sample. Schütten [47] also used a triple filament ion source. A standard was loaded on one vaporization filament and the sample on the second.

Svec and Anderson [48] constructed a double Faraday collector mass spectrometer for precise assay of lithium isotopes. 10 pg of Lil yielded the best results. Typical values for the 6Li/7Li ratio of samples were 0.0820 ± 0.00011 and 0.0818 + 0.00006, and that for a standard was 0.0941 ±0.00003, (errors quoted as one standard deviation).

Because of the wide range of lithium isotopic ratios reported in the literature, owing to possible natural variations, isotopic effects in chemical reaction and instrumental factors, the necessity to calibrate the instruments against a reference material became imperative. Flesch et al. [49] prepared a primary isotopic standard with a composition close to the natural element from high purity, well characterized, separated lithium isotopes. Using this standard, 13 kg of highly purified Li2C03 have been dedicated as a secondary isotopic standard. Their 6Li/7Li ratios are 0.083656 ± 0.000003 and 0.0832 ± 0.0002 respec-tively. The secondary standard (NBS L-SVEC) is now available from NIST, Washington, DC.

Brown et al. [50] described a Knudsen cell electron impact ionization source attached to a 90° magnetic mass analyzer and a double collector system. The capability of this instrument to measure abundance ratios with a reproducible precision of « ± 4 x 1 0 4 at 95% confidence level for sample sizes of < 50 pmol LiCl has been demonstrated.

Michiels and De Bievre [51 ] prepared a primary isotopic reference standard from chemically pure and nearly isotopically pure separated lithium isotopes. With this standard the 6Li/7Li ratio of a natural lithium sample has been


determined as 0.08253 ± 0.00028 (95% confidence limits) and the lithium atomic weight was calculated. A triple rhenium filament ion source and a single focusing, single collector NBS design mass spectrometer were used. 1 pg of lithium as Lil was loaded on each sample filament. This technique has also been applied for studying between-laboratory variations involved with the atomic weight determination of lithium [52] and certifying enriched 6Li isotopic reference material [53]. Callis et al. [54], using a similar instrument, successfully analyzed approximately 0.05 pg of lithium as LiCl.

Green et al. [55] analyzed lithium fluoride, monitoring Li2F+ at masses 32 and 33 with a single collector magnetic sector mass spectrometer. The observed isotope fractionation was negligibly low. Calibrating with a primary isotopic standard, for six samples a 0.16% RSD and a 1.973% mass bias were observed [49].

Chan [56] thermally ionized Li2B407 (95% U B enriched) from a single tantalum filament and monitored Li2B02 at masses 56 and 57. Measurement of this ion decreased the fractionation. The relative standard deviation with this method was 0.12%. Xiao and Beary [57] also deposited Li2B407 on a tantalum sample filament, but used rhenium as the ionizing filament. The mass spectrometer was a NBS design 90° magnetic sector instrument with a Faraday collector. Li+ is measured instead of L^BO^, with a sensitivity three orders of magnitude higher and precision better than 0.1 %.

Datta et al. [58] performed a detailed study of the lithium-boric acid technique. They focused their work on analyzing a heavy ion, choosing Li2B02 for the following reasons: (1) to minimize the isotopic fractionation effect, and (2) to allow the use of commercially available instruments designed mainly for higher mass analysis, as the multiple collector isotope ratio instruments. These instruments show difficulty in stabilizing the magnetic field and in using the option of multiple collector analysis in the low mass range (because of their enhanced mass dispersion). It was concluded in this work that the use of boric acid minimizes but does not eliminate fractionation. Recently, Datta et al. [59] observed that when the lithium-boric acid technique is used to analyze 6Li enriched samples, the results differ for different isotopic Li2B02 ion pairs irrespective of the actual experimental errors. Guidelines are provided for selecting the appropriate ion pairs, and the analytical requirements to achieve accurate lithium isotopic analysis for any given isotopic composition are discussed.

Moriguti and Nakamura [60] used lithium phosphate and a double rhenium filament ion source to measure the 6Li/7Li isotopic ratio. In this procedure intense and stable ion beams of 8 x 10~" A were measured, and the lithium isotopic fractionation was less sensitive to the filament temperature compared with other procedures. An analytical reproducibility of 0.026% (1RSD) was reported. A summary of the analytical techniques and the results observed is given in Table 9.6.

BERYLLIUM 219

Table 9.6. Interlaboratory comparison of Li isotopic ratio measurements for the NBS L-SVEC Standard

Loaded compound

Lil Lil LiF Li2B407 Li2B407 LÍNO3+H3BO3

Measured ion

Li + Li + Li2F+ Li2BOt Li + Li 2 BO^

6Li/7Li ratio ± S D

0.0832 ±0.0002 0.08214 ±0.00008 0.08201 ±0.00013 0.08282 ±0.00010 0.082212 ±0.000019 0.08319 ±0.00009"

RSD(%)

0.24 0.09 0.16 0.12 0.023 0.01

Ref.

[49] [52] [55] [56] [57] [58]

: A representative value only, the maximum uncertainty of the result may be as high as 0.3%.

Palacz [61] used an 18 cm radius 60° extended geometry magnetic sector thermal ionization mass spectrometer with a wide flight tube to measure simultaneously the ion currents of 6Li+ and 7Li+. Internal precisions (RSD) better than 0.005% and 0.01% were obtained for 1 pg and lOng lithium samples respectively.

9.4 BERYLLIUM

Beryllium is the fourth element in the Periodic Table. It has only one stable isotope, at 9 mass units [62].

Beryllium belongs to the group of alkaline earth metals. It has a high ionization potential of 9.322 eV and cannot be thermally ionized. Nier [62] produced Be+ ions by electron impact on BeCl2.

Beryllium has one long lived ß emitting radioactive nuclide, 10Be, with a half-life of 1.6 x 106 years. This isotope is formed in the atmosphere and is subsequently incorporated in environmental materials. Early measurements of 10Be were made by ß counting [63] and accelerator mass spectrometry [64]. Recently Belshaw et al. [65] developed a new secondary ion mass spectrometry (SIMS) technique for 10Be/9Be isotopic ratio measurements. Beryllium samples were chemically preconcentrated and sputtered from Ta filaments using a primary beam of 0 + ions. 9BeH+ and , 0B+ interferences were reduced to a level below 1 cps by sputtering Be+ at about 1000 °C and using instrumental resolving power and high abundance sensitivity. The instrumental mass discrimination was evaluated using boron NIST SRM 951 isotopic standard reference material. 10Be/9Be isotopic ratios in the range between 10~2and 10-9 could be measured with a repeatability of 4% (1RSD). A diagram of the instrumental ion optics is shown in Chapter 6, Figure 6.3, and a mass spectrum showing the resolving power and the various interferences not at optimal measuring conditions is shown in Figure 9.7.


10 cp

-10 cps -10 cps

9.0122 10.013 10.0201 11.0093

Figure 9.7. Typical SIMS mass spectrum (not to scale) of Be+ ions recorded at non-optimal measuring conditions, to show the resolution from interfering ions. (Reproduced by permission of Elsevier Science NL from Int. J. Mass Spectrom. Ion Processes, 142, 55 (1995))

9.5 BORON

Boron is the fifth element in the Periodic Table. It has two stable isotopes, at mass numbers 10 and 11, with relative abundance of 19.9 and 80.1% respectively [1]. The isotopic composition of boron is variable in nature.

The early isotope ratio measurements of boron were based on electron impact on gaseous boron trifluoride. Inghram [66] reported the natural U B/ 1 0 B isotopic ratio as being 4.31, Thode et al. [67] found 4.27 and 4.42, Osberghaus [68] 4.11, Shutze [69] 4.10 and 4.25, Bentley and Hamer [70] 4.18 and Bentley [71] 4.09. BF2 is the most intense ion and is used for ratio calculations. BF3 reacts with water vapor and glass, producing such interfering ions as U B F H 2 0 + , 10BF2H+, 29SiF+ and 30SiF+; these were corrected with the 10BFH2O+, U BF 2 H + , and 2 8SiF+ ion beam intensities. Cooling the sample with dry C0 2 reduced the interferences. Trimethyl borate [B(OCH3)3] and diborane ( B 2 H ö ) were also used for isotope ratio evaluations [72].

Boron has a high ionization potential, 8.298 eV. Nevertheless, satisfactory analyses of the boron isotopes were obtained using thermal ionization. Palmer [73] used 1 pg sodium tetraborate (Na2B407) and ionized it in a single filament ion source. Na2°B02 and N a 2 ' B 0 2 ions at m/z = 88 and 89 yielded ion

BORON 221

beams of « 10 A and were monitored with an electron multiplier. A small correction for the contribution of Na2°BI 601 70+ to m/z — 89 was necessary. "B/ 1 0 B = 4.09 was reported. Spitzer and Sites [45] loaded 2 jug of boron as tetraborate onto a tantalum filament and slowly increased the temperature until the sample melted. An ion current of 5 x 10"12A was produced at 750-1000 °C and was monitored with a Faraday collector. The observed ratio was 4.099. Memory-related interferences were not observed. Goris et al. [74] and Shima [75] reported ratios of 4.00 and 4.051 respectively. De Bievre and Debus [76] prepared a boron isotopic standard reference material. A stock of 100 kg pure boric acid (H3B03), from Merck AG, Darmstadt, Germany, was allocated for this purpose and isotopically analyzed. The boric acid was neutralized to borax with pure NaOH (essentially free of boron and strontium) to 90% of the mole ratio 2NaOH:4H3B03. About 100 pg of boron were loaded onto the rhenium sample filaments of a triple filament ion source and analyzed with a 15 inch radius, 90° magnetic sector mass spectrometer. The ion intensities were measured on a plate collector and a vibrating reed electrometer with a 10" Q input resistor. The m/z = 89 ion current was corrected for the 1 70 isotopic contribution, assuming the abundance of this isotope as 0.0395 ± 0.0006%. (The corrected N a ^ ' B ^ O j ion intensity equals the measured NSL2Bí602 ion intensity less 0.00079 times the measured Na 2 °B 1 6 0 2 ion intensity.) The instrumental bias factor was determined using synthetic boric acid mixtures enriched with I0B and n B isotopes. The " B / I 0 B isotopic ratio of this reference material (designated CBNM-GEEL 011) was established as 4.0444 ± 0.0052. Catanzaro et al. [77] at the U.S. National Bureau of Standards also prepared a boron reference material with natural isotopic composition, NIST-SRM 951, with a ratio of 4.04362 ± 0.00137, and a 10B-enriched standard NIST-SRM 952, with a U B/ 1 0 B ratio of 0.053199 ±0.000032. In general, the isotopic ratios obtained in the borax measurement technique were dependent upon the reproducibility of sample loading, on sample size and sample purity, and on the Na:B ratio.

Duchateau and De Bievre [78] developed a thermal ionization procedure based on measuring the isotopic ratios of the negative B0 2 ion. In comparison with the Na2B02/borax procedure, an improved sensitivity (by a factor of 50) was observed and the ion emission was less sensitive to sample impurities. Ca(B02)2 or Ba(BOa)2 was prepared by mixing CaCl2 or BaCI2 with H3B03 and was used as the source of BO," ions. 20 ng of boron as Ba(B02)2 loaded onto a rhenium filament in a triple filament ion source produced an ion current of 2 x 1 0 " A at an ionization filament temperature of 1500 °C. This procedure was used mainly in the isotope dilution technique for quantitative determination of boron impurities. The accuracy was estimated to be in the order of 1%. Zeininger and Heumann [79,80] modified this procedure, using a mixture of sodium tetraborate with lanthanum nitrate as a source for the BO^ ions and a single rhenium or platinum filament ion source. At 1000 °C they observed an


ion intensity of 10_10-10~9 A for U B 0 2 . The internal relative standard deviation of the isotopic ratio was in the range 0.004-0.02% and the external RSD for ten samples was 0.08%. The 170 oxygen contribution of the l0B17OI6O~ ion at m/z = 43 must be corrected. The isotopic ratios were dependent on sample size. For 0.020, 1 and 10 pg boron, nB/1 0B ratios of 3.968, 4.016 and 4.049, respectively, were obtained. Klotzli [81] also applied negative thermal ionization to analyze the isotopic composition of boron. The procedure developed by Zeininger and Heumann [79,80] was used.

Spivack and Edmond [82] and Ramakumar et al. [83] developed a boron isotope ratio measurement procedure based on the thermal ionization of Cs2B407, monitoring the Cs2B02

h ion. The NIST-SRM 951 boric acid was neutralized with cesium hydroxide to produce a B:Cs mole ratio of 2 :1 . Tantalum filaments were outgassed for 20 min with a current of 2.7 A at 10~6

Torr, allowed to cool, and heated again at 2.7 A for 10 s. The filaments were then allowed to oxidize at ambient temperature and in a clean environment for 3 days before use. 1-5 pg of boron in one drop was placed on the filament using a Teflon capillary tube, dried with a current of 0.7 A for 3 min, heated at 2.0 A for 10 s, and analyzed for 30 min at 1.7 A. Cs2°B02 and Cs^BOj ions at m/z = 308 and 309 were monitored. Typically, 150 ratios in blocks of six were collected. All the data from a block were discarded if the mean deviation of the calculated ratios in the block relative to the block average was greater than 0.15%. The 170 isotopic contribution of Cs2

0B17O16O+ to m/z = 309 was corrected for. Twelve samples of NIST-SRM 951 yielded a nB/1 0B, 170-corrected isotopic ratio of 4.04558 ± 0.00033 (95% confidence limit). No significant isotopic fractionation was observed within a run. Xiao et al. [84] modified the cesium tetraborate procedure by coating the filament with graphite, and as a result a very stable ion beam could be maintained at a (l-2)xl0~11 A level for many hours. Tantalum filaments, without degassing, were coated with 3 pi graphite/ethanol (80%)/water (20%) slurry, (approx. 100 pg of graphite) and almost dried. Samples (3 pg or smaller) of SRM-951 boron samples with a B : Cs mole ratio of 1:2 (in contrast to Spivack and Edmond [82]) were loaded onto the coated filament and analyzed. Ten samples yielded a nB/1 0B isotopic ratio of 4.05037 ± 0.00022 (ISD). Recently Eisenhut et al. [85] modified Zeininger and Heumann's negative thermal ionization procedure [79,80]. Instead of lanthanum nitrate, barium hydroxide was used as an ionization enhancement agent, and the addition of magnesium chloride was found to improve the precision. The sample was ionized from a rhenium filament at a temperature of (1000 ± 50) °C. B02 ions were produced from any boron compound. The sensitivity was about 10-10 A for 100 ng samples. The observed ratios were independent of sample size in the range 0.020-2 pg boron. The TIMS isotopic ratios measurements of boron are summarized in Table 9.7.

Note: Recently, Aggarwal and Palmer [542] and Heumann et al. [544] have reviewed boron isotope analysis.

CARBON 223

Table 9.7. Thermal ionization isotopic ratios of boron Method

PTIÛ

PTI PTI NTI

PTI

PTI

NTI

Compound

Na2B407 H3B03/NaOH H3B03/Na2C03

Ion

Na 2 BOj N32BOJ Na2BOÍ

H3B03/La(N03)3B0;-

H3BO3/CSOH

H3B03/CsOH

c

CS2BO+

Cs2B02f

BO2

SRM

CBNM-011 NIST-SRM 951

NIST-SRM 951

NIST-SRM 951

NIST-SRM 951

n B / i o B

4.099 4.0444 4.0436 4.0161

4.04558

4.05037

3.9370

Error

0.0052* 0.0036* 0.0032

ISD 0.00033

2SD 0.00022

ISD 0.004

ISD

Ref.

[45] [76] [77] [79]

[82]

[84]

[85]

' PTF/NTI = positive/negative thermal ionization. b Error includes statistical uncertainties (2SD) and possible systematic errors. c Any boron compound in presence of Ba(OH>2 and MgC^.

9.6 CARBON

Carbon is the sixth element in the Periodic Table. It has two stable isotopes at mass numbers 12 and 13, with relative abundance of 98.90, and 1.10% respectively [1].

9.6.1 Sample Preparation Carbon dioxide is the carbon compound used most frequently for isotope ratio determinations.

9.6.1.1 Carbon Dioxide in the Atmosphere Carbon dioxide from the atmosphere, respired air and breath samples is prepared by collecting the sample in an evacuated container and purifying it by cryogenic distillation. Water vapor is removed by a dry ice trap, C02 is frozen in liquid N2, and all the other gases are pumped away. Automated C02 cryogenic purification systems for handling large numbers of carbon (and also nitrogen and oxygen) samples for isotopic analysis are commercially available.

9.6.1.2 Carbonates Carbonate samples are converted to COa by reaction with a concentrated mineral acid. When only the 13C/I2C ratio is to be determined, such acids as H3P04, HaS04 and HCl are suitable. In many cases the generated C02 is used


also for oxygen isotopic analysis, in which case it is recommended to use 100% H3P04 at a temperature of 25.5 °C [86] (see also Section 9.8).

9.6.1.3 Combustion of Organic Samples

Dry combustion techniques have been developed to convert organic carbon to C0 2 . An advantage of these techniques is the quantitative recovery of carbon, water and nitrogen (the last two being usable for D/H and 15N/14N ratio measurements). Combustion of organic samples in sealed quartz tubes is most commonly used (refs. [87,88] and references therein). The technique is relatively fast (up to 40 samples per day), quantitative and free from memory effects. Isotopic fractionation was not observed. In homogeneous samples a l 3C/1 2C ratio precision of 0.1-0.3%o is achieved. Quartz ampoules (20 cm x 6-10 mm OD) sealed at one end are baked at 800 -900 °C for 1 h to remove organic contamination. Then an ampoule is filled with the sample, previously baked CuO wire and silver foil, evacuated on a vacuum line, sealed off, baked again at 800-900 °C for 2 h in a furnace, and cooled to room temperature in the closed furnace. The samples in the furnace are shielded from each other by placing them in separate ceramic or Inconel tubes. CuO supplies the essential oxygen, halogens form silver halides, and S0 2 is converted to copper sulfate and silver sulfate. Copper reduces nitrogen oxides to nitrogen (at about 500 °C) and at lower temperatures is reoxidized, removing excess 0 2 . After the cooling period, the ampoule is connected to a vacuum line and broken and the C 0 2 is separated by cryogenic purification, water vapor being removed by distillation from a COa cold trap to a liquid N2 trap. Combustion in dynamic systems is also used. The organic sample is heated in an evacuated quartz tube with CuO and an excess of purified tank oxygen. The combustion products are repeatedly circulated with a Toepler pump through the furnace to ensure total combustion and through an additional furnace to remove halides and S0 2 and decompose nitrogen oxides [83]. Cold traps operated at various temperatures were also incorporated into dynamic systems to remove these impurities [18,89,90]. Finally, the COa produced is purified cryogenically as described above. Sample preparation requires 30-60 min, therefore the per day throughput of the dynamic technique is lower than that of combustion in sealed quartz tubes. The precision is between 0.1 and 0.5%o, depending to some extent on sample homogeneity. Using a dynamic system, the possibility of memory effects must be accounted for, especially when highly enriched samples are processed.

9.6.2 Carbon Isotopic Ratio Determination Nier [91] determined the isotopic composition of carbon in two limestone samples. Two 60° magnetic sector mass spectrometers were used. In one of tiiem, the gas was introduced into the ion source from a 5 1 reservoir through a

CARBON 225

molecular flow gas leak, ensuring that the gas composition in the ion source represented the sample composition. In the second instrument a viscous type leak was used, requiring a correction of the measured isotopic ratio by multiplication with the square root of the inverse mass ratio. The ion currents l 2 C I 6 OÍ and (1 3C1 60++1 2C1 701 60+) at m/z = 44 and 45 respectively were measured, and R4% was defined as

R45 = (1 3CI 60+ + 1 2 C 1 7 0 , 6 0 + ) / , 2 C 1 6 0 2f (6.1)

or /?45 = /?i3 + Rn (6.2)

where

/ ? 1 3 = 1 3 C / 1 2 C (6.3)

and

Rn = 1 7 0 1 6 0 / , 6 0 2 (6.4

Therefore the /?t3 isotopic ratio derived from eq. (6.2) needs correction for the 1 70 abundance in oxygen. At the time when Nier made his measurements, the largest known variation of the 1 8 0 / , 6 0 ratio in nature was 4%, and this appeared to be the same in limestones and the atmosphere. Thus it was arbitrarily assumed that the 1 7 0 abundance in the limestones under study should not differ by more than ± 2% from the value he reported in the same publication [91] on the isotopic abundance ratios in atmospheric oxygen, i.e. in limestones, An = 0.00075+0.000015 (Table 9.12 in this chapter). Subtracting this value from each of the measured Ä45 ratios, values of 13C/12C = 0.01117 ± 0.00003 and 0.01124 ± 0.00003 were obtained for the Chaplain Valley, New York State, USA, and Solenhofen, Bavaria, Germany limestones respectively. Instrumental mass discrimination was determined with an accurately known synthetic mixture of pure 36Ar and 40Ar isotopes.

Craig [86] performed a very extensive study on the isotopic ratio measurements of carbon (and oxygen) using mass spectrometric analysis of carbon dioxide. A McKinney-Nier type isotope ratio mass spectrometer was used, equipped with an improved vibrating reed electrometer, a dual gas inlet system and a simultaneous double collector detection system. A mercury operated valve manifold was used to adjust identical l 2 C 1 6 0 2 ion currents for the sample and the standard gas. These modifications increased the precision of ratio measurement relative to Nier's data by an order of magnitude. The Chicago carbon dioxide was used as an isotopic standard reference material. This gas was produced from crushed, but otherwise untreated, PDB calcium carbonate reacted with 100% H3P04 at 25.2 °C. PDB is a Cretaceous belemnite, Belemnitella americana, from the Peedee formation of South Carolina, USA. The 13C/12C isotopic ratio R\3 was calculated from eq. (6.2) by subtracting the


molecular 1 7 0 1 6 0 / 1 6 0 2 ratio Rn from the measured Ä45 ratio. Craig [86] discussed the average Rn value of 749 x 10~6 published by Nier [91], claiming inconsistencies between the compressed tank and the atmospheric oxygen data. The large deviations in Rn for atmospheric oxygen were attributed to sample measurements in which nitrogen was not removed. It was suggested that this ratio be corrected to 753.5 x 10~6, yielding ä) 3(PDB) =0.0112372. Carbon isotopic ratio results are usually expressed in parts per thousand (or per mil, %o) relative to a laboratory or working standard, using the common ¿-notation

¿ I 3 CLS = [(äSA/ÄLS) - 1] x 1000 (6.5)

where 7?SA and ^LS are the I 3C/1 2C ratios in the sample and in a laboratory standard, respectively. ALS may be /?I3(PDB) o r a ratio in another isotopic reference material related to PDB. The PDB standard is no longer available and has been replaced by new carbonate reference materials NBS-19, NBS-20, NBS-21 and NBS-22. However, the SC values in the literature are related to PDB, therefore measured values are converted to the PDB scale by the following relationship

¿(SA-PDB) = ¿(SA-LS) + ¿(LS-PDB) + 1 0 ^(SA-LS)^(LS-PDB) (6-6)

where ¿(SA-LS) is the corrected value on the PDB scale, S^A-hS) is t n e measured S value using reference material LS, and ¿VLS-PDB) is the value of reference material LS on the PDB scale. Values of «5'3C and ¿>180, with SD values of

Table 9.8. Carbon-13, oxygen-18 and deuterium isotopic composition in isotopic standard reference materials

Standard

NBS-16

NBS-17

NBS-18

NBS-19

NBS-20 NBS-21

NBS-22

Description

co2

COz

CaC03

CaC03

CaC03 Graphite

Oil

S( C)ST/PDB (%o)

-41 .48 -41 .60

- 4 . 4 1 - 4 . 5 1 - 5 . 0 0 - 5 . 0 3 + 1.92 + 1.95 + 1.06

-28 .10 -28 .16 -29 .81 - 29.63

6( 0)STySMOw (%o)

+ 4.12

+ 22.11

+ 7.35

+ 28.67

+ 26.67

¿>(D)ST/SMOW (%o)

-119 .0

Ref.

[96] [97] [96] [97] [96] [97] [96] [98] [96] [96]

[2] [2]

[96]

C C / ! 26

C ) P D B = (11237.2) x IO"6 [86]. (lsO/160)SMOW = (2005.20 ±0.45) x 10"6 [99]. (D/H)SM0W = (155.76 ±0.05) x 10^6 [17].

CARBON 227

several reference materials, relative to PDB and to Vienna SMOW (Standard Mean Ocean Water) are listed in Table 9.8. The SMOW reference sample is lengthily discussed in Sections 9.1 and 9.8. Craig also determined the molecular oxygen isotopic ratios in PDB, /?I7(PDB) = 759.9 x 10~6 and Äts(PDB) = (4158 ± 5) x 10~6, where

Ra - 1 8 0 1 6 0 / 1 6 0 2 (6.7)

It should be noted that it is important to use water-free COa samples and a well outbaked, low background ion source. The presence of HaO yields the ion-molecule reaction product COaH+, interfering at m/z = 45.

Fry et al. [92] described an automated analysis system capable of performing coupled isotopic C and N ratio determinations in organic or inorganic samples. The sample is burnt in a commercial elemental analyzer. The combustion products C0 2 , N2 and H 2 0 enter an all-metal automated system of cold traps, where the gases are trapped and cryogenically separated and finally admitted for analysis in an isotope ratio mass spectrometer. The system can be operated in three modes, carbon only, nitrogen only, or nitrogen and carbon ratio deter-minations. The pure sample, containing C0 2 and N2, was measured against standards. Several NIST SRMs and carbon isotopic standard NBS-21 were analyzed for their carbon isotopic composition using this method and results were compared with those by the sealed tube technique. The results are summarized in Table 9.9. The results for nitrogen are shown in Table 9.10.

Wright et al. [93] used a dual inlet, double Faraday collector, static computer controlled mass spectrometer with modified detection system electronics to measure ! 2 C/ , 3 C isotopic ratios in CD4. Ion current intensities of 12CD4 and 13CD4 at m/z = 20 and 21 respectively were measured. 23 carbon samples of 10~8 g, analyzed within one day, yielded for the 12C/13C ratio a precision of ±0.015 (ISD, best value). The precision in measuring the same laboratory standard gas for five consecutive days was ±0.024 (ISD). A zero enrichment test, alternating the laboratory standard gas between both inlets, yielded

Table 9.9. Comparison of 613C values (%) in SRMs and NBS-21 isotopic standard between Fry et al. [92] and sealed tube technique

Sealed Automated tubes C + N

Citrus leaves (SRM 1524) -27.12±0.01 -27.15 + 0.03 Bovine liver (SRM 1577) -21.58±0.01 -21.51+0.01 River sediment (SRM 1645) - 22.21 ± 0.01 - 22.20 ± 0.16 Graphite (NBS-21 ) (3) - 28.12 ± 0.05 Notes: (1) The data are related to ¿l3CPDB.

(2) The quoted errors are 1 SD. (3) See Table 9.8.


± (0.012 ± 0.25)%o. The effect of such interferences as 4 0Ar2 + or 20Ne+, resulting from atmospheric leaks, and DO+ and 13CD3H+, resulting from sample impurities, has been extensively discussed. The quantitative reduction of carbon dioxide to deuteromethane using a Ni powder catalyst was described by McNaughton et al. [94]

COa + 4D2 -> CD4 + 2D2 0 (6.7)

Excess deuteruim and DaO were removed with a Zr/Al getter pump, which did not affect CD4. The reaction vessel was constructed from quartz tubes and Pyrex valves. The vessel contained residual carbon contamination from its production. This was removed with hot deuterium. By keeping the valves at some distance from the heated reaction vessel part, the carbon blank was reduced to a negligible level. Schoell [95] described an on-line technique for combustion of methane to carbon dioxide and hydrogen. The gas was separated by gas chromatography, oxidized, and C 0 2 and H 2 0 were cryogenically trapped with liquid nitrogen. The C 0 2 was subsequently separated from HaO using dry ice-methanol cooling mixture. Water was further reduced to hydrogen as described in Section 9.1.1. Both gases were then ready for isotopic ratio determinations.

Other gaseous or liquid compounds with adequate vapor pressure which are suitable for carbon isotope ratio measurements but are much less frequently used are CO, CF4, CC14 and CSa. Carbon monoxide yields 1 2 C 1 6 0 + ions at m/z = 28 and 1 3 C 1 6 0 + , 12C170+ ions at m/z = 29. Nitrogen as sample im-purity and from the ion source background causes interference. The n O contri-bution at m/z = 29 must be corrected. The most intense ion in the CF4 mass spectrum is CFjj". Fluorine is mono-isotopic, at m/z = 19, therefore nCV¿ / 1 2CFÍ ratio measurement at m/z = 70 and 69 allows a direct determination of the l 2C/1 3C ratio. The most intense ion in CC14 is CCl^. Two ion beam pairs, 13C35Cl;f and 12C35Cl;j- at m/z = 1 1 8 and 117 and 13C35C12

37C1+ and 12C35C12

37C1+ at m/z = 120 and 119, can be used for the ratio evaluation. CS \ is the most intense ion in the CS2 mass spectrum. The ions are 1 2C3 2SJ and 13C32S2

f with 12C32S33S+ at m/z — 76 and 77 respectively, thus the latter must be corrected for the contribution of 33S.

Isotope ratio monitoring-gas chromatography-mass spectrometry (IRM-GC-MS) is an on-line method, in which an organic material of interest in the sample mixture is chromatographically separated, chemically converted to a compound suitable for isotopic analysis, then purified and admitted into a mass spectrometer for a ratio determination. For carbon-containing materials, the conversion process is usually combustion to C0 2 . Another option is chromato-graphic separation of the mixture and purification of a carbon compound already suitable for mass spectrometric ratio analysis. The method is discussed in more detail in Section 9.7 of this chapter. In recent years, instruments combining a gas Chromatograph, a sample preparation unit and an isotope ratio

NITROGEN 229

mass spectrometer with a dual inlet system, a split flight tube with a double collector system for D/H and a triple collector system for C, N, O and S have become commercially available. The precision of a 1 3C/I 2C ratio measurement in a single determination (internal precision) is quoted in the following paragraph.

9.6.2.1 Precision and sample size requirements

Double collector mass spectrometers dedicated to isotopic ratio determinations of light gases have the capability to measure the 13C/12C ratio in COa with an instrumental precision in the region of 0.01%o (2RSD and n = 12). In many cases the variability of sample preparation is greater than the instrumental limitations to precision.

Regarding the sample size requirements, Hayes et al. [89] have calculated a theoretical minimum sample amount of 0.30 nmol of carbon needed for a ratio determination. It was assumed that a precision of 0.1 %o is demanded, a double collector instrument is used and the mass spectrometer efficiency is 10 4 ions per molecule of gas. The calculated minimum sample size for beam switching measurements is 0.37 nmol. In practice, sample requirements are at least two orders of magnitude higher.

Note: Recently, the International Atomic Energy Agency in Vienna issued a list of carbon isotopic SRMs available from the Agency [543].

9.7 NITROGEN

Nitrogen is the seventh element in the Periodic Table. It has two stable isotopes at mass numbers 14 and 15, with relative abundance of 99.634 and 0.366% respectively [1].

9.7.1 Sample Preparation

The most frequently used technique of converting nitrogen-containing organic samples to molecular nitrogen for isotope ratio mass spectrometry is the Kjeldahl-Rittenberg method [100]. The organic matter is digested to yield NH3, followed by distillation into dilute HCl or H2S04 and oxidation of the ammonium salt to N2 by NaOBr or LiOBr. The technique is a multistep process with a low sample throughput of 20 samples in 24 h and a precision of 0 .1-0.5%o. A fast (4 min per sample), single step technique is the Dumas dry combustion method. This method is easily automated and combined with a mass spectrometer. Both methods were reviewed by Fiedler and Proksch [101]. Smith and Chalk [102] described a procedure for preparing nitrogen gas samples for precise nitrogen isotope ratio measurements. Ammonium sulfate


was reacted with alkaline hypobromite in an evacuated apparatus. Nevins et al. [103] described a procedure for preparation of small nitrogen samples based on dry combustion in vacuum and molecular sieve trapping of gas in a small inlet volume. Other procedures and systems were also briefly discussed. Kendall and Grim [104] proposed a single step sealed combustion tube method, replacing cryogenic purification by the use of calcium oxide for total removal of carbon dioxide and water. It was observed that cryogenically purified samples were enriched in 15N by an average of 0.1 l%o relative to samples prepared with CaO. This result is attributed to the larger amounts of residual contaminants in samples prepared without CaO.

Fry et al. [92] described a system capable of performing isotopic ratio analyses of nitrogen and/or carbon from the same organic or inorganic sample. The sample was burnt in a commercial elemental analyzer. The combustion products C0 2 , Na and HaO entered an all-metal automated system of cold traps, where the gases were trapped and cryogenically separated and finally admitted for analysis in an isotope ratio mass spectrometer. The pure C0 2 and N2 samples were measured against gaseous standards. The system can be operated in three modes, nitrogen only, carbon only, or nitrogen and carbon ratio determinations. Several standard reference materials were analyzed for their nitrogen isotopic composition using this method and results were compared with those of the sealed tube and the Kjeldahl technique. Small samples containing 3pg of nitrogen needed an average 0.5%o blank correction of the (515N value, possibly because of trace carbon monoxide contamination. Larger samples (by a factor of 4-7) did not require blank corrections. The results for nitrogen are summarized in Table 9.10. It was suggested [104] that the tested

Table 9.10. SISN values of standard reference materials and isotopic standards determined by different analytical techniques

Citrus leaves (SRM 1524)

Bovine liver (SRM 1577)

River sediment (SRM 1645)

(NILOzSO,, (IAEA N-l)

(NH4)2S04 (IAEA N-2)

Sealed tube

4.86 ±0.19

7.58 + 0.04

4.34 ±0.28

0.56 ± 0.08

20.42 ± 0.08

Kjeldahl

4.56 ±0.48

6.66 ±0.21

3.77 ± 0.69

Automated

N only

4.86 ± 0.07

7.39 ±0.08

4.21 ±0.21

0.62 ±0.04

20.27 + 0.04

C + N

4.70 + 0.01

7.48 ±0.02

4.40 ±0.07

0.56 ±0.06

20.30 + 0.25

Notes: (1) The data are related to 6,5NAIR. (2) The quoted errors are ISD. (3) IAEA N-l and N-2 are distributed by the International Atomic Energy Agency, Vienna.

NITROGEN 231

NIST SRMs may also be suitable isotope reference standards. It was recommended that interlaboratory comparisons be carried out to establish accepted <5I5N values for these SRMs.

9.7.1.1 Nitrogen Isotopic Ratio Determination

Nier [91] determined the isotopic composition of nitrogen. Two 60° magnetic sector mass spectrometers were used and the I 5N1 4N/I 4N2 ratio was monitored. Instrumental mass discrimination was determined with an accurately known synthetic mixture of pure 36Ar and 40Ar isotopes. The corrected 15N/I4N isotopic ratio in atmospheric nitrogen was established as 0.003663 ± 0.000013. Junk and Svec [105] performed an absolute determination of the isotopic composition of atmospheric nitrogen. They, too, used two magnetic sector mass spectrometers. Instrumental mass discrimination was determined with mixtures prepared by weighing nearly isotopically pure solutions with known concentrations of (14NH4)aS04 and (15NH4)aS04. Portions of these solutions were oxidized by hypobromite and the nitrogen produced was used as a standard. The 15N/14N isotopic ratio found in atmospheric nitrogen was 0.003676 ± 0.000004. Mariotti [106] studied the use of atmospheric nitrogen as a universal isotopic standard. A double collector, 6.2 cm radius, 90 ° deflection, permanent magnet mass spectrometer with a double viscous type inlet system was used. A simple nitrogen purification system was developed. Seventeen sites in France were chosen, and their nitrogen isotopic ratio in air was determined at least once, at one site twice, and in Paris nine times. All except three deter-minations (in Paris) were performed between February and April 1980, the rest in June (two) and in September (one). Arbitrarily, results from one of the Paris determinations and from two other sites, all with the same I 5N/1 4N isotopic ratio, were taken as a reference value, (laboratory standard). All the other results were expressed relative to this reference, using the common 6-notation

S15NSA = [(ÄSA/ÄLS) - 1] x 1000 (7.1)

where RSA and /?LS are the 15N/14N ratios in the sample and in a laboratory standard, respectively. It should be noted that the 15N14N+ and 14N2 ion currents at m/z — 29 and 28 are monitored. Therefore the measured ratio R29 = 15N14N/14N2 is twice the isotopic ratio RSA = 15N/I4N. The <515N value distribution was as follows

«5,5N%o - 0 . 0 4 - 0 . 0 3 - 0 . 0 2 -0 .01 0.00 +0.01 +0.02 +0 .03+0 .06 Number of

sites 5 1 6 3 3 5 1 2 1

The mean <5I5N value is -0.007%o, with a SD of 0.026%o. It was assumed that the reference contains 0.3663% 15N, as determined by Junk and Svec [105],


therefore the calculated 5N content of the mean is 0.3663o3%. Marriotti also successively sampled 10 times a commercial bottle of pure N2 and calculated r)15N relative to sample No. 1. The mean <515N was -0.004%o with a SD of 0.016%o. Further atmospheric, air nitrogen from five different global sites was sampled against the same reference as the measurements in France. The ¿15N values were very close to those obtained in France:

Vienna, Austria: — 0.02%o St. Louis, MO, USA: - 0.01%o Saskatoon, Saskatchewan, Canada: — 0.04%o Baja California, Mexico: + 0.04%o Guadeloupe, Caribbean Islands: + 0.02%o

It may be concluded from these three sets of experimental data that, for all practical purposes, the isotopic composition of nitrogen in air is universally constant.

Mariotti [107] has observed that the presence of small amounts of argon ( « 1%) in purified atmospheric nitrogen may affect the measured <515N values by a few tenths of %o. The 'argon effect', which seems to be essentially a pressure related instrumental effect, must be established and corrected for. However, there are mass spectrometers in which this effect does not exist or is negligible and may not require correction.

Wright et al. [108] described the high precision determination of nitrogen isotope ratios at sub-nanomole levels with a mass spectrometer operating in the static vacuum mode. In this mode of operation the sensitivity was about three orders of magnitude higher as compared with dynamic measurements. The basic geometry of the instrument was similar to that used by Mariotti [106] except that the data were monitored with a single Faraday cup collector by varying the accelerating voltage. For sample sizes of 0.4 nmol and larger, a precision of 0.24%o could be achieved. For smaller samples an increment in the measured 15N14N/14N2 ratio was observed. The absolute accuracy of the method, as determined with samples of known nitrogen isotopic composition, was 0.5%o. A list of available nitrogen isotopic reference standards is given in Table 9.11. Boyd et al. [109] described the preparation and purification of sub-nanomole samples for nitrogen isotopic analysis. An extensive discussion regarding interferences such as a nitrogen blank, carbon monoxide, carbon dioxide, oxygen, methane, N 2 0 , NO and N 0 2 is also given. C0 2 cracks on the hot electron-emitting filament to form CO. Oxygen reacts with carbon in the filament, also forming CO. Ionization of methane produces the primary ion CHj , which reacts further with CH4 to form C2Hj" (interfering at m/z = 29). Beaumont et al. [110] quantitatively studied the isobaric interference of CO, which cannot be resolved with the commonly used Nier type isotope ratio mass spectrometer. Carbon monoxide has a large relative contribution to the ion

NITROGEN 233

Table 9.11. Nitrogen isotopic composition of standard reference materials Standard

IAEA N-l* IAEA N-2* NGS-1 NGS-2

Description

(NH4)2S04 (NH4)2S04 gas gas

( ^/ST/AIR %o

+ 0.56 + 20.18 + 16.2 + 0.2 + 5.0 + 0.2

Ref.

[98] [98]

[112] [112]

" (l5N/,4N)AIR = 0.003676 [101]. b Distributed by the International Atomic Energy Agency, Vienna.

current at m/z — 29 due to the 1 3 C 1 6 0 + and 1 2 C 1 7 0 + ions. A calibration procedure has been developed for reliable Sl5N precision corrections of ± 0.14-±0.20%o at CO concentrations between 0.01 and 1% respectively, and for the detection of trace quantities of CO in natural N2 samples. The method is claimed to be valid for any dynamic mass spectrometer. It should be noted that pure carbon monoxide has an apparent r515N value beyond +500%o.

Mulvaney et al. [ I l l ] reported the availability of a commercial prototype instrument for automated nitrogen isotope ratio measurements in plant material, using the Rittenberg sample preparation technique in conjunction with a double collector mass spectrometer. A sample analysis rate of 100-250 per day with unattended operation for up to three days was reported. Samples containing 20-100 pg nitrogen could in most cases be analyzed for their 15N atomic abundance with one standard deviation of 0.02%o.

Kim and Craig [113] determined the 15N/14N and 1 8 0 / 1 6 0 isotopic ratios by direct injection of N 2 0 into a dual inlet, triple collector mass spectrometer without chemical conversion to Na and C0 2 . Ion currents at m/z = 44, 45 and 46 were simultaneously measured. Analytical precision of 0.05%o for both ratios was reported. The technique is advantageous for small samples, for which extensive chemistry should be avoided. Recently, Tanaka et al. [114] derived the exact equations for the ratio calculations. Corrections for interference from C0 2 contamination and eventual N 2 0 production in the ion source were also included. The S values for nitrogen and oxygen isotope abundances were obtained with internal precision better than 0.02 and 0.08%o, respectively, on samples as small as 3 pmol.

9.7.1.2 Gas Chromatography-Isotope Ratio Mass Spectrometry

It has been shown in the preceding paragraphs that a dual inlet, dual collector magnetic sector mass spectrometer has the ability to determine the nitrogen isotopic ratio with a very good precision. Furthermore, the sensitivity is improved when the system is modified to operate in the static mode. A major


disadvantage is that a separated and highly purified sample, preferably nitrogen, must be admitted into the mass spectrometer inlet system, requiring an off-line sample preparation procedure. Consequently the sample throughput is reduced, the probability of sample contamination is increased and automation of the system is limited. These drawbacks were elegantly solved when isotope ratio monitoring-gas chromatography-mass spectrometry (IRM-GC-MS) was devel-oped and utilized for light gases. In principle, the nitrogen-containing compound is carried by helium into a gas chromatographic column, where it is separated from other sample components. The pure material is then carried into a catalytic combustion furnace and oxidized on hot cupric oxide. Next, the nitrogen oxides are converted to molecular nitrogen on hot copper. Selective trapping, cryogenic cooling and pumping are used to remove unwanted combustion products such as C02 and H20. Finally, the nitrogen/helium ratio in the carrier gas is increased by removal of helium in a jet separator and the effluent is directly admitted into the mass spectrometer ion source. The IRM-GC-MS technique, including the sample combustion step, was pioneered by Sano et al. [115] in 1976, demonstrating the determination of ,3C-labelled drug metabolites. Within two years Matthews and Hayes [116] applied the technique for nitrogen (and also carbon) isotopic ratio determination in amino acids. Precision of 0.5% or better was achieved with 100 nmol samples of nitrogen. Recently Sohns et al. [117] discussed the problems involved in the GC-combustion-MS technique. Three disadvantages were noted: (a) time consum-ing sample preparation, depending on the nitrogen concentration in the sample; (b) differential adsorption of 15N14N and 14N2 on molecular sieve at liquid nitrogen temperature in the selective trapping step, and (c) column bleed of C02, which increased the interfering ion current at m/z — 29. Consequently natural gases, containing CH4 (major constituent), Na, 02, CO and C02, were analyzed by direct GC coupling to the MS instrument through a valve which allowed purging of interfering gases such as methane and carbon dioxide. Samples with Na concentrations down to 0.1% could be analyzed. A small sample size, down to 10 nmol N2, was sufficient to measure the isotopic ratio with a precision (1RSD) of 0.3-0.5%o. Using isotopic reference standards, an accuracy of ±0.5%o for r5l5N values and a reproducibility better than ±0.3%o were demonstrated. A continuous flow isotope ratio mass spectrometry techni-que (CF-IRMS), or one of its most useful modifications, automated nitrogen and carbon analysis mass spectrometry (ANCA-MS), has been described by Barrie [118]. The instrumentation consists of an autosampler, a combustion tube, a reduction tube, purification traps, a GC column, a gas flow diverter (switching between the MS ion source and the exhaust), and a triple collector magnetic sector mass spectrometer. The duration of an analysis is 4 min including sample preparation time. The best precision achieved for the 15N atomic per cent is ±0.00007, compared with ±0.000007% obtained by conventional multiple collector nitrogen isotopic ratio mass spectrometry with

OXYGEN 235

repeated measurements of the same sample gas. The difference is attributed to errors introduced by sample preparation.


Double collector mass spectrometers dedicated to isotopic ratio determinations of light gases have the capability to measure the 15N14N/I4N2 ratio with an instrumental precision in the region of 0.0 l%o (2RSD and n = 10). In many cases the variability of sample preparation is greater than the instrumental limitations to precision.

Regarding the sample size requirements, Hayes et al. [ 18] have calculated a theoretical minimum amount of 0.96 nmol of molecular nitrogen needed for a ratio determination. It was assumed that a precision of 0.1 %o is demanded, a double collector instrument is used and the mass spectrometer efficiency is 10~4

ions per molecule of gas. The calculated minimum sample size for beam switching measurements is higher by 20%. In practice, sample requirements are at least two orders of magnitude higher.

Note: Recently, the International Atomic Energy Agency in Vienna has issued a list of nitrogen isotopic SRMs available from the Agency [543].

9.8 OXYGEN

Oxygen is the eighth element in the Periodic Table. It has three stable isotopes at mass numbers 16, 17 and 18, with relative abundance of 99.762, 0.038 and 0.200% respectively [1].

There is an immense interest in measuring oxygen isotopic ratios. Because of the easier availability of the 1 80 isotope, the 1 8 0 / l 6 0 ratio is more frequently monitored. Oxygen isotopic ratio measurements are widely used in geology: for silicate minerals, hydrothermally altered rocks and limestones. Studies on deep sea oceanic sediments provide past climatic information. Isotopic changes in water are affected by evaporation and condensation rates. Labelling of organic compounds and water with 1 8 0 is a powerful tool for metabolic studies in the human body and in other mammals. Administration of doubly labelled water (D2

, 8 0 ) has been used to measure energy expenditure in humans. Applications to the detection of food adulteration have been developed. Reaction rate and mechanism studies of hydrolytic and photolytic reactions have also been carried out with 180-labelled organic compounds, or in H2

180-labelled solutions.

9.8.1 Sample Preparation

Carbon dioxide, molecular oxygen and to some extent water are suitable compounds for isotopic ratio measurements, C0 2 being the most widely used.


The various procedures for treating water, inorganic and biological samples in the mass spectrometric evaluation of the 1 8 0 / 1 6 0 isotopic ratio have been extensively described in the literature and reviewed by Wong and Klein [2]. Sample conversion to C 0 2 and monitoring of the 1 2 C 1 8 0 1 6 0 / 1 2 C 1 6 0 2 ratio at m/z = 46 and 44 is the most accurate and most frequently used technique. Introduction of molecular oxygen with monitoring of the 1 8 0 1 6 0 / 1 6 0a ratio at m/z — 34 and 32, and direct admission of water with monitoring of the H 2

1 8 0 /H 21 6 0 ratio at m/z = 20 and 18, are also practised. Details of the

sample preparation techniques are discussed briefly in the following paragraphs.

9.8.1.1 Aqueous Samples

Equilibration of Water with C02

The equilibration technique for measuring the 1 80 content in water was introduced in 1938 by Cohn and Urey [119]. The reaction is carried out at room temperature, as follows

H21 80 + C 1 6 0 2 = H 2

1 6 0 + C 1 8 0 1 6 0 (8.1)

When the equilibration period is finished, the COa is removed for mass spectrometric analysis. The 1 80 content of water is calculated according the equation given by Craig [86].

rJ18Ow = ¿,8Of + {a/k) x (¿18Of - ¿180¡) (8.2)

where ¿I8Ow, <518Of and 6180j are the 1 80 contents in water and in COa after and before equilibration, respectively; a. is the oxygen isotope fractionation factor between C 0 2 and H 2 0 , which at 25 °C has the value of 1.0414 [120]; and k is the ratio Nw/2Nco2> where Nw and 2Nco2 are the number of oxygen atoms in the water sample and in carbon dioxide, respectively. ¿1 80 is defined in the same way as SD in Section 9.1.3

5 1 8 O S A = [(äSA/KLS) - 1] x 1000 (8.3)

where ASA and /?LS are the 1 8 0 / 1 6 0 ratios in the sample and the laboratory standard, respectively. Precision in the region of 0.01%o for the 1 8 0 / 1 6 0 ratio measurements using the H 2 0 / C 0 2 equilibration technique were reported [121]. Lifson et al. [122] had shown that the enzyme carbonic anhydrase increases the rate of hydration of C0 2 , thereby reducing the equilibration period from the original 48-72 h.

Dostrovsky and Klein [123] modified Cohn and Urey's [119] original equilibration technique. The reaction was carried out over a heated platinum wire in an evacuated vessel. 1 8 0 enrichment in the water samples was in the range of 1.00-10.56 at%. By comparision with Cohn and Urey, a memory effect was observed, increasing the 1 8 0 values by 0.02-0.14 at%. Flushing the

OXYGEN 237

reaction vessel with an aliquot of the water sample before the equilibration reduced the memory effect. Harrison et al. [124] used this technique for mea-suring the ! 8 0 content in 0.5 mg water samples, using approx 0.3 ml of C0 2 . They also used for equilibration a mixture of NaHC03 and Na2S03 instead of C02 . Sprinson and Rittenberg [125] used anhydrous Na2C03. The reaction was allowed to proceed for 3 h at room temperature, then the vessel was frozen to - 7 0 °C and evacuated. Citric acid or NaHS04 was added to release C 0 2 for isotopic analysis. Recently Horita et al. [14] described an improved equilibra-tion method. COa is equilibrated with the water sample on a hydrophobic platinum catalyst in an automatic equilibration unit connected directly to a mass spectrometer. Long-term reproducibility of ±0.16%o for ¿1 80 was reported.

Direct Mass Spectrometry of Water

Majzoub and Nief [126] described isotope ratio mass spectrometers for direct 1 8 0 / l 6 0 ratio measurements in water. Ha1 80+ and H 2

1 6 0 + ion current intensities were monitored. About six sample introductions prior to actual data collection were needed to eliminate memory effects. An average precision of 0.08%o was reported for 68 measurements of a distilled water laboratory standard. Hagemann and Lohez [127] described a twin system of two mass spectrometers to measure simultaneously HD + /H 2 and H 2

1 8 0 + / H 21 6 0 + ratios.

Wong et al. [128] evaluated a commercial prototype of a twin mass spectro-meter system.

Chemical Conversions of Water

O'Niel and Epstein [129] and Bottinga and Craig [130] used a fluorination technique to convert water to oxygen

H 2 0 + BrF5 -> BrF3 + 2HF + ± 0 2 (8.4)

After cryogenic distillation, the oxygen was reacted at 700 °C with a carbon rod to yield C0 2 .

Anbar [131] used bromine to produce hypobromite, which was decomposed to oxygen and analyzed as such

H 2 0 + Br2 -» OBr + HBr (8.5) OBr - » i 0 2 + B r - (8.6)

Anbar and Guttmann [132] described the use of a 1:1 HgCl2 and Hg(CN)2 mixture to convert water to C0 2 . An evacuated and sealed ampoule containing the sample and the reactant mixture is heated to 400 CC for 2 h. After cooling, the gaseous products are transferred to a second ampoule containing zinc amalgam at -196°C, sealed and heated to 200 °C. The C0 2 is purified and analyzed.


Boyer et al. [133] used guanidine hydrochloride, which on heating with water to 260 °C decomposes according to the following reaction

NH2C=(NH)NH2 • HCl + 2H2 0 - • C0 2 + 2NH3 + NRtCl (8.7)

The procedure was re-evaluated by Dugan et al. [120], who reported a precision of 0.08%o.

Compston and Epstein [134] used graphite and iron powder to convert water to CO, followed by catalytic conversion to C 0 2 over nickel. Majzoub [135] and Holt [136] used only graphite and nickel. Hardcastle and Friedman [137], Bariac et al. [138] and Ferhi et al. [139] described the use of platinum and diamond for decomposition of water, followed by oxidation of CO with platinum electrodes at 1.5 kV. Several other procedures for conversion of water to COa using carbon and a nickel catalyst were described by Thompson and Gray [140], Brenninkmeijer and Mook [141] and Gray et al. [142]. Doering and Dorfman [143] pyrolyzed water over carbon black under nitrogen. The reaction products were bubbled through a mixture of bromine, chloroform and carbon tetrachloride at —70 °C and then through iodine pentoxide for oxidation of CO to C0 2 .

Electrolysis of Water

Bocek et al. [12] described a microelectrolyzer that converted 10 mg of water completely to H2 and 0 2 within 20 min. Techniques to convert the oxygen to C0 2 have already been mentioned [129,130].

General Remarks

The H 2 0 equilibration technique with COa, followed by 1 2 C 1 8 0 1 6 0 / 1 2 C l 6 0 2 ratio measurement, is accepted as the most reliable procedure for 1 8 0 / , 6 0 ratio determination in aqueous samples. The technique for conversion of H 2 0 to C0 2 with guanidine hydrochloride requires only small samples; no memory effects are observed and high precision can be achieved. The conversion of H 2 0 to 0 2 with BrF5 is quantitative and precise. The HaO equilibration technique with C0 2 , direct mass spectrometry of water and the conversion of H 2 0 to C0 2 with guanidine hydrochloride were recently reported as the preferred techniques in an interlaboratory analysis of water samples enriched with 1 80 [11].

9.8.1.2 Inorganic Samples and Oxygen Phosphates

Tudge [144] converted inorganic phosphates to BiP04 and released the oxygen with bromine trifluoride

3BiP04 + 8BrF3 -» 3BiF3 + 3PF5 + 4Br2 + 60 2 (8.8)

OXYGEN 239

Cohn [145], Harrison et al. [124], Bunton et al. [146], Cohn and Drysdale [147] and Anbar et al. [148] reported thermal decomposition techniques to convert phosphates to molecular oxygen, COa and an aqueous solution of metaphos-phate. The solution was then equilibrated with COa-

Harrison et al. [124] described a technique for decarboxylation of 3-phosphoglycerate. The silver salt is reacted with bromine in CC14. Excess of Br2 and of CC14 is removed at -70 °C and C02 is analyzed.

Boyer et al. [133] have shown that guanidine hydrochloride reacts with KH2P04 when heated in a sealed glass tube. COa is formed, purified and analyzed.

Sulfates Longinelli and Craig [149] and Holt [136] described techniques for converting sulfates to a mixture of COa and CO by heating the sample with carbon-containing materials (in a graphite crucible or filter paper). The CO is further converted to COa and the combined gas is analyzed.

Carbonates It is a common laboratory practice to release COa from carbonates by reaction with 100% H3P04 at 25.2 °C [86]. A two armed evacuated vessel (Rittenberg tube) is used for this purpose and the released COa is purified cryogenically. McCrea [150] showed that the oxygen isotopic ratio in COa varies depending on the acid used and probably also on the isotopic composition of water, if it is present. Quantitative release of C02 from the carbonate sample is also recom-mended.

Nitrogen Oxides Cliff and Thiemens [151] developed a thermal decomposition system on a gold catalytic surface for quantitative conversion of nitrous oxide to molecular nitrogen and oxygen for high precision mass spectrometric isotopic determina-tions. Both the 1 80/1 60 and 1 70/1 60 ratios were measured and reported as <5180 and ¿>nO, relative to Standard Mean Ocean Water (SMOW, see Section 9.8.2), with ISD of ± 0.1%o. Kim and Craig [113] determined the 15N/14N and 1 80/1 60 isotopic ratios by direct injection of N20 into a dual inlet, triple collector mass spectrometer, without chemical conversion to N2 and C02. Ion currents at m/z — 44, 45 and 46 were simultaneously measured. Analytical precision of 0.05%o for both ratios was reported. The technique is advantageous for small samples, for which extensive chemistry should be avoided. Recently Tanaka et al. [114] derived the exact equations for the ratio calculations. Corrections for interference from C02 contamination and eventual N20


production in the ion source were also included. The S values for nitrogen and oxygen isotope abundances were obtained with internal precision better than 0.02 and 0.08%o, respectively, on samples as small as 3 pmol.

Oxygen Horibe et al. [152] described two techniques to convert oxygen into C02 or H2O for 1 80/1 60 isotopic ratio measurements. Another technique for converting oxygen to C02 was developed by Holt [136].

Organic Samples

Wong and Klein [2] reviewed the various techniques of converting organic samples to COa for 1 80/1 60 isotopic ratio measurements. The authors noted that these techniques had not been sufficiently tested and none of them can be considered as a standard procedure of preparing organic samples for oxygen isotopic ratio determinations.

9.8.2 Oxygen Isotopic Ratio Determination Nier [91] determined the isotopic composition of molecular oxygen in air and compressed tank oxygen. Two 60° magnetic sector mass spectrometers were used. In one of them the gas was introduced into the ion source from a 5 1 reservoir through a molecular flow gas leak, ensuring that the gas composition in the ion source represented the sample composition. In the second instrument a viscous type leak was used, requiring correction of the measured isotopic ratio by multiplication by the square root of the inverse mass ratio. The 1 801 60/1 602 and 1 70, 60/1 60a isotopic ratios were measured. Instrumental mass discrimina-tion was determined with an accurately known synthetic mixture of pure 36Ar and 40Ar isotopes and, finally, the oxygen ratios were corrected with this factor. The results obtained with both instruments were identical. A slight but true difference in the isotopic composition was observed between air and com-pressed tank oxygen. The results for air are given in Table 9.12.

A large quantity of a water isotopic reference material, the Standard Mean Ocean Water (SMOW), has been prepared by Craig [16], who also determined its 1 80/1 60 isotopic ratio as (1993.4 + 2.5) x 10~6 and, consequently, the absolute atomic 180 abundance as 1989.5 + 2.5 ppm. This material is now available from the International Atomic Energy Agency in Vienna. It is also frequently designated as V-SMOW (V for Vienna). Baertschi [99] has also determined the absolute content of 180 in (SMOW). High grade D2

,80 of known isotopic composition was mixed with very pure H2

160 in a weight ratio yielding an 1 80/1 60 isotopic composition as close as possible to that of SMOW, using Craig's 180 content of 1989.5 ppm (atomic) as reference. After equili-

OXYGEN 241

bration with C0 2 , the synthetic mixture was compared with SMOW by measuring the 1 2C1 601 60+ and 1 2 C 1 8 0 1 6 0 + ion currents at m/z = 44 and 46. The reported ratio was (2005.2 ± 0.45) x 10~6, higher by almost 12 ppm than Craig's value. The SMOW value for the 1 7 0 / 1 6 0 ratio, as reported by Baertschi [99] quoting Craig [16,86], is 372 x 10~6. In this determination the I 2 C 1 6 0 1 6 0 + and l 2CI 7Ol 60+ ion currents at m/z = 44 and 45 are measured. The ion current at m/z = 45 must be corrected for the 1 3 C , 6 0 1 6 0 + ion contribution. Baertschi used 13C/12C = 0.0109.

Wasserburg et al. [153] determined oxygen isotopic ratios by measuring the 150Nd18O/150NdI6O and 150NdI7O/,50Nd16O ratios with a thermal ionization mass spectrometer. The data shown in Table 9.12 were not corrected for mass fractionation. Makishima et al. [154] determined oxygen isotopic ratios using a fully automatic high abundance sensitivity, double focusing, thermal ionization mass spectrometer. Mono-isotopic yttrium (89Y), praseodymium (l4,Pr) and holmium (165Ho) are suitable elements; PrO+ ion was chosen in this work. About 10 pg Pr was loaded onto sample filaments of a triple rhenium filament assembly. The total ion current was set to 6 x 10" n A. Data were collected with a Faraday collector for about 15 h, accumulating 900 ratios in blocks of 10. 140Ce16O+ and 144Nd160+ ions at m/z = 156 and 160 were monitored to correct for Ce and Nd interference, if any. Bias due to mass discrimination was not corrected. The mean ratios and the uncertainties (2SD) are given in Table 9.12. The data are in good agreement with Wasserburg et al. [153]. The difference from Nier's values [91] is attributed to mass discrimination.


Double collector mass spectrometers dedicated to 1 8 0 / 1 6 0 isotopic ratio determinations have the ability to measure this ratio with an instrumental

Table 9.12. Atomic oxygen isotopic ratios 18o/16o ! 7 0 / 1 6 0 Source

Nier [91] Craig [86] Craig [86,16]

Baertschi [99]

Wasserburg et al. [153] Makishima et al [154]

0.002044 0.002044a

0.0019934 ±0.0000025

0.00200520 ±0.00000045

0.00211 0.002129

±0.000010

0.0003745 Oj (air) 0.00037675s* O j (air) 0.000372 SMOWc

0.000387 0.0003916

±0.0000014

SMOWc

NdO+ (air) PrO+ (air)

" Value adopted from réf. [91]. b Corrected value from ref. [91] (see paragraph 9.6.2). c Standard Mean Ocean Water.


precision down to 0.03%o (2RSD and n — 10). In most cases the variability of sample preparation is greater than the instrumental limitations to precision.

Regarding the sample size requirements, Hayes et al. [18] have calculated a theoretical minimum sample amount of 1.6 nmol of molecular oxygen or carbon dioxide needed for an 1 8 0 / 1 6 0 ratio determination. The theoretical requirement for 1 7 0 / 1 6 0 ratio determination, when 0 2 is the sample gas, is 8.8 nmol. It was assumed that a precision of 0.1%o is demanded, that a double collector instrument is used and that the mass spectrometer efficiency is 10~4

ions per molecule of gas. The calculated minimum sample sizes for single collector beam switching measurements are higher by 10-20%. In practice, sample requirements are at least two orders of magnitude higher.

9.8.2.2 Deuterium and Oxygen Isotopic Water Reference Samples

In Table 9.13 several water isotopic reference materials are listed together with their deuterium and 1 8 0 isotopic composition relative to ¿>18OSMOW and #DSMOW-

Note: Recently the International Atomic Energy Agency in Vienna issued a list of nitrogen isotopic SRMs available from the Agency [543].

9.9 FLUORINE

Fluorine is the ninth element in the Periodic Table. It has only one stable isotope at mass number 19 [156].

Table 9.13. Deuterium and oxygen-18 isotopic composition in standard reference water materials

Standard Description ¿>DST/SM0W <5180ST/SMOW Ref-

SMOW"

SLAP

GISP

NBS-1 NBS-1A IAEA 302A IAEA 302B IAEA 304A IAEA 304B

Vienna Standard Mean Ocean Water Standard Light Antarctic Precipitation Greenland Ice Sheet Precipitation Water Water Water (D, 1 8 0 enriched) Water (D, I 8 0 enriched) Water (D, 1 8 0 enriched) Water (D, l s O enriched)

% 0

0.0

- 428.0

- 189.7

- 4 7 . 1 - 183.2

506.2 992.3

% 0

0.0

- 55.50

- 24.80

- 7 . 8 9 - 24.29

252.9 503.3

[155]

[155] [155]

[11] [11] [11] [11]

' (D/H)SM0W = (155.76 + 0.05) x 10~6 [17]. (180/'60)SMOW = (2005.20 ±0.45) x lO^6 [99].

NEON 243

Fluorine is a highly corrosive gas. It is not recommended to introduce it directly into a mass spectrometer. Fluorine forms several gaseous, non-corrosive fluorides such as CF4, NF3, SiF4 and SFö which may easily be applied to produce F + ions by electron impact.

9.10 NEON

Neon is the tenth element in the Periodic Table. It has three stable isotopes at mass numbers 20, 21 and 22, with relative abundance in air of 90.48, 0.27 and 9.25% respectively [1]. Any of the isotopes of neon may be produced in terrestrial rocks, ground waters and extra-terrestrial material in the presence of natural radioactivity via the following reactions:

l 7O(a,n)2 0Ne (10.1) I 80(a ,n)2 1Ne (10.2) 25Mg(n, a)22Ne (10.3)

An isotopic composition of 20Ne, 2 lNe and 22Ne in excess of 30% each was observed in an iron meteorite [157].

Precise isotope ratio analyses of atmospheric neon were reported by Nier [158], Eberhardt et al. [159], Walton and Cameron [160], Melton et al. [161] and Bottomley et al. [162]. Melton et al. [161] used two magnetic mass spectrometers to measure the isotopic abundance of argon, neon and krypton. The purpose of the work was to make direct determinations without the need to calibrate the instruments with synthetically prepared isotopic mixtures. Mass discrimination was eliminated by constructing a specially designed high transmission ion source with a wide ion exit slit. Neon from two commercial sources was used. The given values represent the average from about 50 measurements on each mass spectrometer. One measurement consisted of six scans of each isotope. The background pressure was 2 x 10~~8 Torr. Bottomley et al. [162] used a static mass spectrometer. The instrumental mass discrimina-tion was determined by analyzing standards prepared by mixing well defined amounts of separated 20Ne and 22Ne isotopes. It was observed that serious errors may be introduced in the isotopic abundance measurements when helium is present in the sample, especially if the He/Ne ratio exceeds 103. All the samples were finally purified by passing the neon through an activated charcoal (< 80 mesh) column at — 186 °C, which formed part of the mass spectrometer inlet system. The helium was pumped away and the neon was introduced into the ion source by warming the column. The neon sample size was always large enough for the instrument background at m/z = 20, 21 and 22 (40Ar2+, 20NeH+

and 1 2C1 602 +) to be negligible. The neon isotopic ratios of the quoted references are summarized in Table 9.14. It should be noted that the data


Table 9.14. Isotopic ratios of atmospheric neon

Reference 21Ne/20Ne 22Ne/20Ne Error

Nier [158], 1950 Eberhardt et al [159], 1965

0.002827 ± 0.000006 0.09703 ± 0.00040 0.002959 ±0.000022 0.10204 ±0.00083 3SE

Walton and Camerom [160], 1966 0.002935 ± 0.000058 0.10187 ± 0.00038 Melton et al [161], 1971 Bottomley et al. [162], 1984

0.00295 0.10188 0.002980 ±0.000006 0.10219 ±0.00011 ISD"

" The quoted errors include the errors of the mass spectrometric measurements and the errors in preparation of the volumetric standards used for mass discrimination corrections.

10.4

10.2

-z.

10.0

10" 10" 10' 4He/20Ne

10' 10"

Figure 9.8. Air standard Ne isotope ratios at various partial pressures of He [163] (Reproduced by permission of Nihon Gakai Jimu Senta from H. Hiyagon, Mass Spectrosc, 37, 325 (1989))

presented in this table show a chronological decrease in the 20Ne isotopic abundance with a simultaneous increase in 2 lNe and 22Ne. The effect of helium on neon isotope ratio measurements was also demonstrated by Hiyagon [163], as shown in Figure 9.8.

9.11 SODIUM

Sodium is the eleventh element in the Periodic Table. It has only one stable isotope at mass number 23 [164].

MAGNESIUM 245

Sodium has a low ionization potential of 5.139 eV and is easily thermally ionized. All the elements used as filament materials show a strong Na+ ion emission when degassed in an ion source and must be heated for several hours to reduce this ion current. Practically every sodium compound will emit Na+

ions under thermal ionization conditions.

9.12 MAGNESIUM

Magnesium is the twelfth element in the Periodic Table. It has three naturally occurring isotopes, mass numbers at 24, 25 and 26, with relative abundance of 78.99, 10.00 and 11.01% respectively [1]. Variable extraterrestrial magnesium isotopic abundance is possible owing to radiogenic 26Mg build-up from the decay of a short-lived 26A1 isotope (half-life 7.4 x 105 years).

Magnesium has no simple compounds with sufficient vapor pressure at temperatures suitable for conventional handling in a mass spectrometer. It is possible to evaporate Mg powder from a nickel oven and ionize it by electron impact [43]. Magnesium has an ionization potential of 7.646 eV, thus allowing thermal ionization.

Spitzer and Sites [45] loaded finely divided magnesium oxide onto a tungsten filament, slurried it with distilled water and dried the sample with gentle heat. After the Mg+ ion appeared, the sample was very slowly heated to an ionization temperature of 1400-1500 °C, whereby a stable ion current was achieved. Sufficiently intense impurity 23Na+ and 27A1+ ion currents may introduce tailing interferences on the 24Mg+ and 26Mg+ ion beams. Interference from previous magnesium samples was not observed. Daughtry et al. [165] im-proved the Mg+ ion emission by a factor of 10 upon mixing magnesium oxide with high purity graphite. Better ion stability was observed and a lower ionization temperature was needed. Beryllium oxide added to the mixture improved adhesion of the sample to the filament. The optimal mixture was MgO : Bea03 : C = 2 : 2 : 1; all the components were slurried in amyl acetate, loaded onto the filament in several thin layers and dried.

Catanzaro et al. [ 166] used thermal ionization to obtain absolute values for the isotopic abundance ratios of a standard reference material of magnesium, NIST-SRM 980. Two mass spectrometers were been used, both calibrated for instrumental bias with samples of known isotopic composition prepared from chemically pure and almost separated 24Mg and 26Mg isotopic samples. A triple rhenium filament ion source was used. The ionizing filament only was degassed for 30 min at 5 A and under a 300 V potential field to reduce the Na+ ion signal. Secondary electrons produced from large Na+ ion beams could affect the base-lines of Mg+ ions. The sample filaments were simply washed in alcohol; degassing these filaments increased the possibility of sample flaking off. About lOOpg of magnesium was loaded on each sample filament from an aqueous


Table 9.15. Isotopic ratios of magnesium Reference 25Mg/24Mg 26Mg/24Mg Error

Catanzaro et al. [166] 0.12663 ±0.00013 0.13932 ±0.00026 Schramm et al. [167] 0.12663* 0.139805 ±0.000013 2SD Rosman et al. [169] 0.12732 ±0.00065 0.139294 ± 0.000029 " Overall limits of error based on 95% confidence limits for the mean and allowances for effects of known sources of possible systematic error. 6 Normalized to 25Mg/24Mg [166], c 95% confidence intervals.

solution containing 5 mg ml - 1 Mg, 5 mg m l 1 U and 10% nitric acid. The sample was dried with a heat lamp followed by a slowly increasing direct current until an orange uranium oxide was formed. Uranyl nitrate served as an agent to bind the magnesium oxide to the rhenium filament. The ionization filament was held at 2100 °C, a rigorous heating pattern for the sample filaments had to be maintained, and data were collected on stable or slightly growing Mg+ ion signals at ion intensities of (4-7) x lO - 1 2 A. During the analysis period, the 24Mg/26Mg ratio decreased by « 0.5% owing to isotopic fractiona-tion. To avoid memory effects, the filament shield plate and the first two plates of the ion source were removed and cleaned between successive analyses. The resulting absolute isotopic abundance ratios are given in Table 9.15. A dramatic reduction in magnesium sample size from 100 pg to the range of 600-100 ng has been achieved by Schramm et al. [167] and Lee et al. [168], who used the silica gel/phosphoric acid ionization enhancement technique. Laboratory magnesium contamination, especially from cation exchange columns, was the limiting factor of the sample sizes which could be analyzed without significant blank correction. V-shaped, zone refined rhenium filaments were degassed in a high vacuum for 2 h at 2000 °C. The sample loading procedure was found to be extremely critical. A 2 pi drop containing the sample was heated with a current of 1.5-2 A to burn off hydrocarbons. Then the sample was fused with silica gel and phosphoric acid and the current was increased until the filament glowed a dull red for a few seconds. In the ion source the filament was baked for 6-8 h at 960 °C (pyrometer controlled) to remove most of the sodium. Then it was slowly heated for 4 - 5 h to a temperature in the range 1480-1580°C, where stable 24Mg+ ion beams of (1-2) x 10~u A were obtained. Only the 27A1+ ion, with intensity varying from 0.1 to 1.0 of that of 24Mg+, was observed in the mass spectrum within 5 mass units of the Mg isotopes. No effect of the Al+ ion on the Mg isotopic ratios was found. A fractionation correction factor a was defined as

a = (25Mg/24Mg)meas/0.12663 (12.1)

where 0.12663 is the absolute 25Mg/24Mg ratio given by Catanzaro et al. [166]

MAGNESIUM 247

and the (26Mg/24Mg)corr was obtained by

(26Mg/24Mg)corr = (26Mg/24Mg)meas/a2 (12.2)

The resulting ratio is shown in Table 9.15. Schramm et al. [167] have pointed out that their value for the 26Mg/24Mg isotopic ratio is significantly different from that reported by Catanzaro et al. [166]. No explanation has been given for this systematic difference. Lee et al. [168] in their analytical procedure slowly increased the filament temperature to « 1500°C and obtained 24Mg+ ion beams larger than 1 0 " A for a period of 2 h when 10 pg of magnesium was used. The same authors applied a direct loading technique to study the magnesium isotopic composition in single crystals of sizes down to 25 pm, which contained only 0.6 ng of Mg. The normalized 26Mg/24Mg ratio was determined with 2RSD better than 1 %. The crystal was treated with silica gel and phosphoric acid and ionized from a V-shaped rhenium filament. Comparing the direct loading technique with the conventional (sample dissolution and Mg separation) technique, the fractionation corrected 26Mg/24Mg isotopic ratios for the directly loaded samples were systematically lower than for separated samples. It was shown that this difference is related to the high 27A1 content in the directly loaded samples. 27AI+ ions scattered from the mass spectrometer wall contri-bute to the background at the 24Mg+ signal position in the spectrum. The mass spectrum of two different directly loaded samples is shown in Figure 9.9.

Rosman et al. [169] loaded magnesium samples onto rhenium filaments in a triple filament ion source, following the procedure of Catanzaro et al. [166]. Mg+ ion currents of 3 x 10"12 A were measured. The observed 26Mg/24Mg isotopic ratio of terrestrial magnesium was corrected for fractionation using 25Mg/24Mg = 0.12663. The 25Mg/24Mg ratio given in Table 9.15 is the measured value. Stegmann and Begemann [170] used terrestrial MgCl2, loading between 2 and 50 ng magnesium into V-shaped rhenium filaments and applying the silica gel/phosphoric acid technique. Total blanks from sample dissolution and silica gel were below 2 ng Mg. Scanning the mass region 10 mass units below and above magnesium during the data collection period revealed only 23Na+ and 27A1+ ions. Interferences from scattered ions were never observed. It was noted that a better fit to the 26Mg/24Mg isotopic ratio reported by Catanzaro et al. [166] could be obtained by using a correction exponent smaller than 2 (see correction equations above). A similar observation about the fractionation correction was also reported by Gray and Compston [171]. Esat et al. [172] analyzed the magnesium isotopic composition of interplanetary dust particles. The typical particle size was 10 pm, containing about 10~10 g of magnesium. A special filament type was developed, consisting of a hemispherical dimple, 200 pm in diameter, pressed into flat rhenium filaments. Prior to sample loading, the dimple was filled with » 50 pg silica gel and phosphoric acid using a micro syringe attached to a micro manipulator. The adhesion of silica gel to the rhenium filament was improved by adding trace amounts of a glucose and

00

BEAM T

OFF J2.5x10-1> I i i i i

ANORTHITE

Mg

UyWM*l»W*«»P*%fW«» * '»*" 'W*»*

"f t J

Mg

10x10"10A 'Al

BEAM OFF

_i_ _i_ _i i_ _L _i_ AMU 23.0 24.0

±

7.8x10"12A

25.0 26.0 27.0 AMU

À 3EAM

SPINEL

Mg

x10

2.9x10 A 7AI

1 iMNifttMi* |ft<Nw> <»' am itc*

BEAM OFF i _

_i i L AMU 24.0 250 26.0 27.0 AMU

Figure 9.9. Magnesium mass spectra of two directiy loaded samples. The broad hump is due to scattered Al+ ions; arrows show where the background was measured. The inset is an enlarged view of the peak top and tail of the 26Mg+ ion from the upper analysis. (Reproduced by permission of Elsevier Science NL from T. Lee et al, Geochim. Cosmochim. Acta, 41, 1477 (1977))

SILICON 249

citric acid solution. A 24Mg+ ion intensity greater than 10- 1 2 A could be sustained for more than 2 h. Between 150 and 250 individual isotopic ratios could be collected from each sample. The ionization efficiency with the dimple filament was about 10 2, compared with 10~"4 for the direct loading technique [168].

Other methods for isotopic ratio analysis of magnesium have also been developed. Ireland et al. [173] used secondary ion mass spectrometry, obtaining their data on a sensitive high resolution ion microprobe (SHRIMP) designed and constructed by Clement et al. [174] and Huneke et al. [175] Hachey et al. [176] prepared volatile Mg chelates containing l,l,l,2,2-pentafluoro-6,6-dimethylheptane-3,5-dione (PPM) and l,l,l,2,2,3,3-heptafluoro-7,7-dimethyl-octane-4,6-dione (FOD). The dimers Mg(PPM)2 and Mg(FOD)2 were intro-duced into a GC-MS instrument and ionized by electron impact with 70 eV electrons. 24Mg+, 25Mg+ and 26Mg+ ion currents were monitored [as Mg(PPM)+ at m/z = 514, 515 and 516 and [Mg(FOD)2 - 57]+ at m/z = 557, 558 and 559}.

9.13 ALUMINUM

Aluminum is the thirteenth element in the Periodic Table. It has only one stable isotope at mass number 27.

Aluminum has an ionization potential of 5.986 eV. White et al. [164] thermally ionized aluminum nitrate from a tungsten filament at a temperature of 900 °C. Spitzer and Sites [45] used finely divided aluminum oxide slurried in distilled water and also loaded it on a tungsten filament. Al+ ion current was observed in a temperature range of 1300-1600 °C.

9.14 SILICON

Silicon is the fourteenth element in the Periodic Table. It has three stable isotopes at mass numbers 28, 29 and 30, with relative abundance of 92.23, 4.67 and 3.10% respectively [1].

Early isotopic composition determinations of silicon were made by Williams and Yuster [177], Inghram [66], White and Cameron [43], Norton and Zemany [178], and Reynolds [179]. Electron impact ionization of silicon tetrafluoride was used, monitoring the SiF+ ion intensities at m/z = 85, 86 and 87. Spitzer and Sites [45] deposited barium fluorosilicate on a tantalum filament, slurried it with distilled water and dried the sample with gentle heat. The filament was heated in the ion source, decomposing the sample to SiF4. The vapor was ionized by electron impact and the SiF+ ion intensities were measured. The optimal temperature, usually below 200 °C, must be approached very slowly,


after SiF+ first appears. The BaSiFä was prepared by dissolving 1-2 mg Si02 in diluted HF, followed by precipitation with BaCl2 and centrifuging in a plastic tube.

Within the frame of a NIST project to replace the kilogram as a standard of mass with a measurable natural phenomenon, a high purity silicon crystal with high lattice perfection was chosen, and its unit cell dimensions, density and atomic weight had to be measured with uncertainties at the ppm or lower levels. A further target was to use the silicon atomic weight together with the crystal parameter measurements to redetermine the Avogadro constant. The atomic weight was calculated from the isotopic composition of the element. To achieve these objectives, Barnes et al. [180] measured the absolute isotopic abundance ratios of a high purity isotopic standard reference material, NIST SRM-990. Two magnetic, electron impact mass spectrometers were calibrated with synthetic isotopic standards prepared from chemically pure and almost isotopi-cally pure separated isotopes. The separated 28Si and 30Si isotopes consisted of mixed Si02, SiC and residues of graphite. The mixture was dissolved in sodium hydroxide and filtered. The insoluble residue was fused several times with Na2C03 and dissolved in water. Both parts were passed through a strongly acidic cation exchange column to remove sodium and other cations, combined and converted in a platinum crucible at 800 °C to pure 2 8Si02 or 30SiO2. Each of the oxides was dissolved in hydrofluoric acid and spiked with 100 ng of about 20 different metallic element isotopes, the silica matrix was removed with perchloric acid as volatile SÍF4, and the residue was analyzed for impurities by spark source mass spectrography. The silicon assay in the separated isotope solutions was determined by gravimetry of Cs2SiF6, a highly stable non-hygroscopic compound. All the purifications and analytical determinations were performed in Teflon vessels cleaned with dilute, high purity hydrochloric acid. Chemicals of low silicon content were chosen. Calibration samples were prepared by mixing weighed aliquots of the separated and assayed isotopic solutions with BaCl2 to form barium fluorosilicate. Solid BaSiFó was heated in a nickel and copper tubing vacuum system, SÍF4 was collected and introduced into the ion source, and finally SiF+ ions were monitored. The reference material was also converted to BaSiFô and then to SÍF4. Dedicated nickel-copper tubing was used for different isotopic compositions. The resulting absolute isotopic abundance ratios 28Si/30Si = 29.74320 ± 0.00747 and 29Si/30Si = 1.50598 ± 0.00086 yielded an atomic weight of 28.085526 ± 0.000056. The quoted uncertainties are overall limits of error based on 95% confidence limits for the means and allowance for the effects of known sources of possible systematic error. A study of natural 28Si/30Si ratio variations reported in literature extended the estimated uncertainty of the atomic weight of natural silicon to ± 0.00039. The isotopic ratios re-calculated to 28Si are shown in Table 9.16. Valkiers et al. [181] prepared two isotopic reference materials of silicon, CBNM-IRM-017 (Si powder) and CBNM-IRM-018 (Si02). BaSiF6 was

SILICON 251

Table 9.16. Isotopic ratios of silicon 29Si/28Si

0.0507 0.0506 0.0511 0.0513 0.0511 0.0510 0.050633 0.05069 ± 12 x 0.0507715 ± 6 6 0.05083 ± 12 x 0.0508442 ± 48 " NIST SRM-990. b CBNM-1RM-017. c CBNM-IRM-018.

10 X 10 X

-5

io-7 -5

i o -~i

30Si/28Si

0.0331 0.0331 0.0340 0.0340 0.0338 0.0335 0.033621" 0.03352 ± 10 x 0.0334889 ± 78 0.03360 ± 10 x 0.0335851 ± 66

The quoted uncertainties are two standard deviations.

10 X 10 X

- 5 * , 0 - 7 * -5 c 1 0 ' 7 c

Ref.

[177] [66] [43]

[178] [179]

[45] [180] [181] [181(b)] [181] [181(b)]

precipitated following the procedure of Barnes [180] and decomposed to SÍF4 by heating to 540 °C. The mass spectrometer was calibrated with a mixture of the three enriched silicon isotopes prepared by gravimetry. The amounts of the enriched isotopes were chosen to prepare a mixture with an isotopic composition close to that of natural silicon. An independent set of absolute measurements with an improved mass spectrometer has also been reported (see ref. [181(b)]). The results of these measurements are included in Table 9.16.

Recently, Ku et al. [ 182] proposed a three ratio measurement scheme that has been used for redetermination of the atomic weight of silicon. Silicon tetra-fluoride was used and the 28SiFj", 29SiF;¡" and 30SiF3

f ion currents at m/z — 85, 86 and 87 were monitored symmetrically around a center time with Faraday collectors. In one measurement, each ion current was measured four times in the following sequence 85, 87, 86, 85, 87, 86, 86, 87, 85, 86, 87, 85. The second and eleventh ion currents were averaged and divided by the average of the first and last ion currents to yield R\. Three ratios were calculated in this way in one run

R\ =30SiF+/28SiF3+ (14.1) Ä2=2 8SiF+/2 9SiF+ (14.2)

R3 _ 2 9 SÍF+ / j USiF3 (14.3)

From 10 runs, three mean ratios were calculated. If there are no measurement errors

/?, x R2 x A3 = 1 (14.4)


Assuming measurement errors, the above expression may be formulated as /?, X ä 2 X ä 3 = 1+É (14.5)

and 10 values for 1 + e are obtained. A typical mean 1 + e value (n = 10) extrapolated to t = 0 (commencement of sample introduction into the ion source) was 0.999998, with 1RSD of 4.8 x 10~5. It was noted that this measurement scheme can also be adopted for a triple collector detection system.

9.15 PHOSPHORUS

Phosphorus is the fifteenth element in the Periodic Table. It has only one stable isotope, at mass number 31.

Kerwin [183] heated red phosphorus and ionized it by electron impact. Polymeric P+ ions were observed, the most intense of which was P^ (100%), followed by P j (50%) and P + (16%). Negative phosphorus ions were obtained by Halmann and Platzner [184] by electron resonance capture in phosphine at electron energies of 5.8 ±0 .2 , 8.4 ± 0 . 2 and 13.4 ± 0 . 2 eV, and by ion pair formation at an energy threshold of 22.6 ± 0 . 1 eV and above. Wachsmann and Heumann [185] applied negative thermal ionization to produce POj and POj ions. A single rhenium filament ion source with a BaO filament coating yielded ion currents of 10~12 A.

9.16 SULFUR

Sulfur is the sixteenth element in the Periodic Table. It has four stable isotopes, at mass numbers 32, 33, 34 and 36, with relative abundance of 95.02, 0.75, 4.21 and 0.02% respectively [1]. The sulfur isotopic composition is variable in nature. This variance is given in terms of <534S, expressed in per mil units, and is defined as

¿34s = {[(34s/32s)sa/(34s/32sy - i} x io-3 (16.1) where ( S/ S)sa and ( 4S/3 2S)s t are the isotopic ratios of the sample and the standard. The isotopic standard is sulfur in ferrous sulfide, FeS, from the iron meteorite Canyon Diabolo, for which 34S/32S = 22.220.

Nier [186] oxidized sulfur to sulfur dioxide and ionized it by electron impact. 0 + , S 2 + , O J , S+, S 0 2 + , SO+, and SO J ions were observed in the mass spectrum. SO J is the most intense ion and was used to calculate the sulfur isotopic abundance. The oxygen isotopic composition must be known for the abundance calculation. The contribution of the various ions is given in Table 9.17. High S0 2 pressure in the ion source reduces the electron emission and

SULFUR 253

Table 9.17. Mass distribution of the SO^ ion Mass

64 65 66 67 68

3 2 S 1 6 0 1 6 0 + 3 2 S 1 6 Q 1 7 0 +

32S160180+

3 2 S 1 8 0 1 8 0 +

Ion

33S160160+ 33S1601704 3 3 S 1 6 0 1 8 Q +

3 4 S 1 6 0 1 6 0 + 3 4 S 1 6 0 1 7 0 + 34S160180+ 3 6 S 1 6 0 1 6 0 +

Note: Such ions as *S170170+, *S'7Ol80+ (x = 32, 33,34, 36) and >'S180180+ (y = 33,34, 36) have a very low probability and were omitted from the table.

increases memory effects. The samples should not contain water or nitrogen oxides. Zmbov [187] dissolved sodium sulfate in a sodium silicate solution, loaded the mixture onto a tungsten filament and dried it. Upon heating to 700 °C, SO+ and S 0 2 ions were produced at an intensity of approx. 10~n A using electron bombardment. Schutze and Zahn [188] mixed a few drops of 1 N AgN03 solution with aluminum oxide and added one drop of sodium sulfite (up to 20 pg sulfur). Silver sulfite was formed. The sample was dried at 80 °C and heated in the ion source at 120°C. The sulfite decomposed to S0 2 and yielded SO+ and SO^ ions upon electron bombardment. MacNamara and Thode [189] measured the 34S/32S isotopic ratio in S0 2 with a RSD of 0.05%, using a double collector detection system. Ion intensities at m/z = 64 and 66 [ 3 2 S , 6 0 1 6 0 + and (34Si60i60+ a n d 32 S i6 0 i8 0 + ) respectively] were monitored. The 3 3 S , 6 O n O+ contribution to m/z = 66 was not corrected. In more recent measurements, using mass spectrometers equipped with double collectors, relative standard deviations of 0.01-0.02% were routinely achieved [190,191]. Methods for preparation of sulfur dioxide for mass spectrometric analysis were described by Ricke [192] and by Holt and Engelkemeir [193].

Sulfur hexafluoride is also a suitable gas for the isotopic analysis of sulfur [194,195]. The electron impact spectrum consists of ions from S + to SFg, the most intense of which is SF J. Fluorine is monoisotopic (m/z = 19), thus only four signals are monitored at m/z= 127, 128, 129 and 131; no isobaric interference corrections are needed. Also, sulfur hexafluoride is not affected by moisture or mercury, and is not adsorbed by metal walls or glass, therefore memory effects are negligible. Gao and Thiemens [196] measured 6S values of a commercial high purity Matheson Co. SFg laboratory standard against itself with a triple collector magnetic sector mass spectrometer. By this procedure the internal errors of the mass spectrometer were established as ±0.03%o for <533S and ¿34S, and ±0.1%o for <S36S. A sulfur isotope reference material supplied by the International Atomic Energy Agency as silver sulfide was converted to SFö and used to determine the precision and accuracy of the isotopic ratio measurements. The sulfide fluorination reaction was described by Clayton and Mayeda [197], and a cryogenic method for purification of SFö was given by


Gao and Thiemens [198]. Rumble et al. [199] described the preparation of SFô for sulfur isotope ratio measurements by laser heating of sulfide minerals in the presence of fluorine. Recently, Schaefer et al. [200] measured the isotopic ratios of SF6 in several samples with RSDs of (l-5)xl0~4 for the 33S/32S and 34S/32S ratios and (3-6) xlO"2 for the 36S/32S ratio. A method of SF6 preparation for geochemical isotope analysis was described by Puchelt et al. [201]. Spitzer and Sites [45] placed arsenic trisulfide on a tantalum filament, slurried it with distilled water, dried the sample, heated it slowly in the ion source up to 500°C, and ionized it by electron impact. AsS+, As2S+, As^J , and As2Sj" ions were observed. The best data were calculated using the AsS+

ion. Arsenic is monoisotopic, with m/z — 75. Bradt et al. [202] analyzed vaporized sulfur: S+, S 2 and polymeric ions up to S ̂ were observed, the most intense signal being S2 . Hydrogen sulfide and carbon disulfide were also used for sulfur isotope analysis by electron impact ionization, but were less suitable than the above described compounds.

Paulsen and Kelly [203] developed a thermal ionization procedure for the determination of microgram quantities of sulfur. Originally the procedure was intended as an isotope dilution technique for sulfur in metals, but recently it was applied also for isotope measurements of sub-pm sulfate aerosol particles collected over the Pacific Ocean [204]. The sample [203] was dissolved in a closed system to prevent losses of volatile sulfur compounds using a mixture of HCI/HNO3 which oxidized the sulfur to sulfate. The sulfate was reduced to hydrogen sulfide and precipitated as arsenic sulfide. As2S3 was dissolved in an ammoniacal As3+ solution to yield an As/S atom ratio of 2. A portion of this solution, equivalent to 1.5 pg of sulfur, was loaded onto a rhenium filament with silica gel, and the 34S/32S ratio was measured at 950 °C as 75As34S+/75As32S+. The ionization efficiency was estimated as 0.1% and the RSD of the ratio measurement was «0.1%. The chemical blank was the major source of uncertainty at 3—80 pg sulfur sample sizes. The isotopic ratio measurements are summarized in Table 9.18.

Recently, Giesemann et al. [205] described an on-line GC-MS technique for isotopic analysis of sulfur of soil, plant and atmospheric origin. Samples are converted to BaSÛ4 or Ag2S. Then the samples are wrapped together with V2Os in tin capsules and burnt to S02 in an elemental analyzer. The gas is separated from other combustion products by gas chromatography and is injected into the ion source of a double collector mass spectrometer through a split interface. Using this method, the amount of sulfur needed is reduced to about 10 pg per analysis. Other on-line systems were described by Pichlmayer and Blochberger [206] and by Haystead [207].

Double collector mass spectrometers dedicated to isotopic ratio determina-tions of light gases have the ability to measure the 34S/32S ratio in S02 with an instrumental precision of 0.015%o (2RSD and n — 12). In many cases the variability of sample preparation is greater than the instrumental limitations to

CHLORINE 255

Table 9.18. Isotopic ratios of sulfur 33S/32S

0.0078 0.0079 0.00794442 0.0080

34 S / 32 S

0.044 0.0444 0.0449832 0.0444

36 S / 32 S

0.000166 0.00018 5.54 x 10~8

0.000143

Ref.

[186] [189] [200]

[45]

precision. Sulfur isotopes fractionate in biological, chemical and geochemical processes. Fractionation is also likely to take place during sample preparation, especially when reactions are carried out incompletely. Therefore it is critical to ensure total conversion in any preparation reaction. Moiseyev and Platzner [195] also reported partial isotopic separation of 3 2 S F ö and 3 4 S F ó in chromato-graphic columns packed with porous polymer beads.

Note: Recently the International Atomic Energy Agency in Vienna issued a list of sulfur isotopic SRMs available from the Agency [543].

9.17 CHLORINE

Chlorine is the seventeenth element in the Periodic Table. It has two stable isotopes at mass numbers 35 and 37, with relative abundance of 75.77 and 24.23% respectively [1].

The first measurements of chlorine isotopic abundance were reported by Aston [208] in 1921. Electron impact ionization of methyl chloride has been used by Owen and Schaeffer [209]. The most intense ion current at m/z — 50 (12CH3

35C1+, 12CH37C1+) and the ion currents at m/z = 52 (12CH335C1+) and

m/z = 48 (12CH 35C1+, 13C 35C1+) are used to calculate the 37C1/35C1 ratio. No variation of the chlorine isotope ratio has been observed in ten samples within the experimental error of 0.2%. In 1984, Kaufmann et al. [210] refined this method, again using methyl chloride, and improved the precision to 0.024% (ISD); they reported isotopic variation of natural chlorine in shallow and deep ground water samples, hydrothermal samples and solid salt samples. The reported results are related to the standard mean oceanic chloride (SMOC) isotopic ratio 37C1/35C1 = 0.324, with deviations for 37C1 ranging from -0 .129% to ±0.08%. Recently Long et al. [211] further increased the precision of the chlorine isotopic analysis to 0.009% using CH3CI, which was prepared by reacting AgCl with an excess of CH3I, followed by complete gas chromatographic separation between the two methyl halides. Ion currents at m/z — 50 and 52 were monitored using accelerating voltage peak jump. Taylor and Grimsrud [212] measured chlorine isotopic ratios in CH3CI". Hoering and


Parker [213] used HCl to measure chlorine isotopic ratios with a double collector mass spectrometer. No significant variations beyond 0.1% were observed in 81 natural samples. HCl was used because it could be prepared quantitatively from small samples, and on account of its simple mass spectrum. The disadvantages of gaseous HCl are its corrosive activity and long term instrumental memory effects. Spitzer and Sites [45] evaporated NaCl from a tantalum filament between 700 °C and 1400 °C and ionized it by electron impact. NaCl+, Na2Cl+, Cl+, and HC1+ ions were observed. The chlorine ratio was determined from the NaCl+ ion intensities. This method is of limited use, as after each sample the ion source had to be dismantled and cleaned to avoid memory effects.

Hayden and Lewis [214] showed that, in thermal ionization of NaCl from a single tungsten filament ion source, Na+ and NaCl+ are produced, the latter with sufficient intensity for chlorine ratio determinations. Shields et al. [215,216] determined an absolute value for the chlorine isotope ratio 35C1/37C1 = 3.1272 (+0.0079-0.0082) in a commercial sample of NaCl originally stored as NBS (NIST) Isotope Reference Sample No. 105. Negative thermal ionization was used. The uncertainties given are the overall limits of error based on 95% confidence limits for the mean, including allowances for effects of known sources of possible systematic error. Samples of known isotopic composition of nearly pure separated chlorine isotopes were prepared and used for calibration of two instruments. Chloride was deposited on the sample filaments of a triple tungsten filament ion source in the form of ammoniacal silver chloride solution with an approximate concentration of 15 mgml- 1 CI. The use of alkali chlorides gave erratic results after a few hours of operation, and frequent cleaning of the ion source and collector assembly was required. The ratio measurements were made at ion currents of 10~12 A in the magnetic scan mode. Heumann et al. [217] applied a quadrupole mass analyzer for negative thermal ionization of chlorine. For natural chlorine, 35C1/37C1 = 3.130 ±0.007 (RSD = 0.2%) was measured. This value was compared with results obtained with a magnetic sector mass spectrometer: 3.133±0.003 (RSD = 0.01%). Xiao and Zhang [218] developed a procedure for the determination of chlorine isotope ratio by positive thermal ionization mass spectrometry of cesium chloride. Cs2Cl+ is emitted from CsCl, the ionization being enhanced by the addition of graphite to the filament during the sample loading. The advantages of this method are the high ionic masses m/z = 301 and 303, which introduce only a small and reproducible fractionation effect, and the fact that Cs is monoisotopic, both superseding the use of RbCl, which was also studied by the authors. Spectro-scopically pure HCl was analyzed with a precision of 0.034% (95% confidence limit). A single filament tantalum ion source was used. The filament was coated with 3 pi of graphite slurry (about 100 pg), nearly dried and loaded with a 3-6 pg chlorine sample from a fresh solution prepared by neutralizing HCl with CS2CO3. The sample was then dried for 2 min by passing a 1.1 A current

ARGON 257

Table 9.19. Interlaboratory comparison of chlorine isotopic ratio measurements

Ionization Source Measured Ratio RSD Ref. Method" compound ion 37C1/35C1 (%)

EI CH3CI CH3C1+ 0.31955 ±0.0003 0.1 [209] EI NaCl CH3C1+ - * 0.024 [210] NTIMS NaCl/AgCl CI" 0.31977 + 0.00081 0.26 [215]

-0.00084 PTIMS HCl Cs2Cl+ 0.32010 ±0.00011 0.034 [218] " EI, Electron impact; NTIMS, Negative thermal ionization mass spectrometry; PTIMS, Positive thermal ionization mass spectrometry. b ratio not given.

through the filament and the data were collected at (5-8) x 10 2 A, five blocks of ten ratios being acquired for 1.5 h. Significant isotopic fractionation was observed in long (4-5 h) runs. The beam intensity depends strongly on the HCl to Cs2C03 ratio, with an optimum at a Cl/Cs ratio of « 2, reflected by a pH of the sample solution in the range 2-5. The measurements were not affected by adding to the sample 0.25-2.0 pg each of Mg (as sulfate), K (as nitrate), Na (as nitrate), Ca (as sulfate) and Ba (as nitrate). The reported 37C1/35C1 ratio for natural chlorine is 0.32010 ± 0.00011. A summary of the analytical techniques and the results observed is given in Table 9.19.

9.18 ARGON

Argon is the eighteenth element in the Periodic Table. It has three naturally occurring isotopes, at mass numbers 36, 38 and 40, with relative abundance in air of 0.337, 0.063 and 99.600% respectively [1].

Nier [91] determined the natural isotopic abundance of argon, using synthetic argon isotope mixtures to correct instrumental mass discrimination. Two mass spectrometers were used and argon from two different sources was studied. Atmospheric argon was obtained by passing air over hot lithium metal. The second source was spectroscopically pure, commercial argon. Almost pure, highly enriched 36Ar and ^ A r samples were prepared by thermal diffusion and used for the calibration. The commercial sample showed a slightly lower content of the two lighter argon isotopes. Melton et al. [161] used two magnetic mass spectrometers to measure the isotopic abundance of argon, neon and krypton. The purpose of the work was to make direct determinations without the need to calibrate the instruments with synthetically prepared isotopic mixtures. Mass discrimination was eliminated by constructing a specially designed high transmission ion source with a wide ion exit slit. Argon from two commercial sources was used. The given values represent the average from


Table 9.20. Isotopic abundance ratios in argon 36 Ar/40 Ar

0.003378 ±0.000006 0.003346 ±0.000006 0.00340

38 Ar/40 Ar

0.000635 ±0.000001 0.000630 ±0.000001 0.00064

Sample

Air Commercial Average of two commercial samples

Ref.

[91] [91]

[161]

about fifty measurements on each mass spectrometer. One measurement consisted of six scans of each isotope. The background pressure was 2 x 10~8

Torr. The results are presented in Table 9.20.

9.19 POTASSIUM

Potassium is the nineteenth element in the Periodic Table. It has three naturally occurring isotopes, at mass numbers 39, 40 and 41, with relative abundance of 93.2581, 0.0117 and 6.7302% respectively [1]. The 40K isotope is radioactive, decaying by ß emission to 40Ca and by electron capture to ^Ar.

Isotope ratio determinations of potassium have been performed since the mid 1930s. The early measurements were based on electron impact of vaporized potassium salts. Nier [91] vaporized metallic potassium from a small furnace and ionized it by electron impact with 7.5 and 52.5 eV electrons. Instrumental bias effects were corrected with argon standards of known isotopic composition prepared from high purity separated argon isotopes. The calibration mixture was admitted into the ion source together with potassium and analyzed at the higher electron energy. Pure potassium was analyzed at 7.5 eV. Twenty publications dealing with potassium isotope ratio measurements between 1934 and 1975 were quoted by Garner et al. [219]. Thermal ionization has been used by this group to obtain absolute values for the isotopic abundance ratios of a standard reference material of potassium, NIST-SRM 985.

Potassium has a low ionization potential of 4.341 eV. It is very efficiently thermally ionized; practically every filament material emits K+ ions in vacuum upon passing through it a direct current. It is therefore most important to use clean and properly degassed filaments for thermal ionization of this element. Garner et al. [219] used a triple tantalum filament ion source. Tantalum was selected in preference to other materials because of its low efficiency in ionizing calcium (the most abundant 40Ca isotope is isobaric with the minor 40K isotope), and the relative ease of reducing the potassium background ion current to 1 x 10~15 A. After being thoroughly washed (twice) in ultra high purity water and dried with a heat lamp in a clean air environment, the filaments were

POTASSIUM 259

degassed for 1 h at 4 A and for 1.5 h at 4.5 A. After cooling for at least 30 min, the final cleaning consisted of a series of 1 min cycles in which the filaments were alternately pulsed with a 4 A current and cooled for periods of 5-15 s. This procedure ensured a potassium background ion current of < 1 x 10~15 A. Two magnetic mass spectrometers were used, one with a 15 cm radius, 60° magnet and the second with a 30 cm radius, 90° magnet. Electronic, ion source and detector components were interchangeable. Special Teflon containers and quartz pipets were chosen for laboratory work after earlier testing, and rigorous cleaning procedures were adhered to. Either 12 of 25 pg (depending on the instrument) of potassium from a dilute hydrochloric acid of KCl were loaded, in a clean air environment, onto the sample filaments and evaporated to dryness with heat lamps and electric currents in the following sequence: 1.0 A for 10 min, 1.2 A for 5 min, 1.5 A for 5 min and, finally, cooling for 5 min. The distance of the sample filament from the edge of the ionization filament was adjusted to 0.5 mm to reduce the effects of radiant heat from the latter. During the course of the analysis, the ionization filament was maintained at 1250 °C and the sample filaments were initially adjusted to 0.6 A. After 5 and 10 min the K+ ion current was adjusted to 5 x 10~" and 1 x 10~10 A respectively. Only samples for which the ion current increased to 1.5 x 10- 1 0 and 2 x 10~10 A after 15 and 20 min were processed for ratio measurements. After 40 min, data collection commenced for 45 min in the following order: 3 9K+/ 4 1K+, 4 1 K + / 40K+, 3 9K+/ 4 1K+. Isotopic fractionation was observed within this period, denoted by the decreace in 3 9 K + / 4 I K + ratio of <0.2%. Secondary electron effects were observed close to the 40K+ ion signal base owing to the high intensity of the 39K+ ion current. This disturbance was corrected by mechanical narrowing of the 39K+ ion beam when the 40K+ ion current was monitored. After approximately every 50 analyses, the collector assembly was dismantled and cleaned. A contribution of the 39K+ tail to the 4 0K+ ion signal was detected on the 30 cm instrument, but was up to 1 x 1 0 1 5 A on the 15 cm mass spec-trometer. As significant amounts of potassium were deposited in the ion source, and even though memory from previous samples was not detected, it became good practice to remove the ion source from the mass spectrometer, dismantle it and clean it carefully. The clean source was conditioned by degassing it for at least 1 h with bare filaments and then for 90 min with a sample of approxi-mately the isotopic composition to be analyzed. The mass spectrometers were calibrated with standards of known isotopic composition, prepared by gravimetrically mixing of chemically pure, highly enriched (>99.1%) separated 39K and 41K isotopes. The absolute isotopic abundance ratios were 3 9 K/ 4 , K = 13.8566 ± 0.0063 and 40K/41K = 0.0017343 ± 0.0000061. Garner et al. [220] performed an extensive survey, measuring potassium isotopic ratios in about 70 terrestrial mineral samples from different locations and different geological origins. No major isotopic abundance variation was observed.


9.20 CALCIUM

Calcium is the twentieth element in the Periodic Table. It has six stable isotopes at mass numbers 40, 42, 43, 44, 46 and 48 with relative abundance of 96.941, 0.647, 0.135, 2.086, 0.004 and 0.187% respectively [1].

Nier [186] evaporated metallic calcium and ionized the vapor by electron impact. The intensity of 44Ca+ was 4 x 10~12 A. Calcium iodide has been evaporated in an electron impact ion source [221]. The 44Ca and 48Ca abund-ances were measured with RSDs of 0.5-1.0 and 1-2% respectively.

Although calcium has an ionization potential of 6.11 eV, its thermal ionization is difficult. This technique for isotopic ratio determinations is generally subject to instrumental fractionation effects. Calcium is particularly affected, being a low mass element with a large mass range of its isotopes (Am = 8). Also, the very low abundance of most of its isotopes, except "^Ca and ^Ca, requires production of high intensity ion beams for precise analysis. Several very good publications address this topic. Spitzer and Sites [45] placed 1 pg of calcium as CaC03 on a tantalum filament and slurried it with hydroiodic acid, dried the sample to evaporate excess of iodine, and finally increased the filament temperature to a dull red heat ( « 750 °C). The well adhering Cal2 or CaO sample was ionized at a temperature between 1400 and 1600 °C. 1 pg of calcium produced a stable Ca+ ion current of 10"10 A for more than 20 min. 40K+ interferes, and data were collected only when the 41K+ ion intensity was much lower than that of the major 40Ca+ isotope. No calcium memory effect from previous samples was noted. CaO, Ca(N03)2 or Cal2 may be used instead of CaC03. Isotopic fractionation was not corrected. Stauffer and Honda [222] also used a degassed tantalum filament, loading onto it about 10-20 pg of CaS04. They measured ion intensities of 3 x 10"12 A with an electron multiplier. A small potassium contamination was always observed, but the contribution of 40K+ to 40Ca+ was negligible. From 15 to 30 isotopic spectra were monitored. Correction for isotopic fractionation were made by using the 40Ca/48Ca isotopic ratio value of 521.71 established by Nier [186]. Coleman [223] used as a laboratory standard Johnson & Matthey 'Specpure' CaCÛ3 in a triple filament ion source (filament material not specified). CaCl2 was loaded onto the sample filaments. It was observed that only extremely pure samples yielded sufficiently intense and stable ion beams, and even the smallest traces of iron or aluminum could reduce the Ca+ ion emission by a factor of 100. The ion current during the data collection period was 10~10-10~9 A, but beam stability varied between the samples from 30 min to 6 h. Two enriched samples of CaC03 with 85% 42Ca and 90% 48Ca were used to prepare a spike solution for isotopic fractionation correction. The precision of a corrected ^ C a / ^ C a ratio was generally within 0.2%, compared with variations of up to 4% within the course of a 6 h measurement. The author normalized his results to 40Ca/44Ca = 45.70, determined by Backus et al. [224], who applied thermal

CALCIUM 261

ionization from a single tantalum filament coated with an electrolytically deposited porous platinum layer. Backus et al. [224] assumed a 3% isotopic fractionation for their 48Ca/40Ca ratio. In the work described in the next paragraph (ref. [225]), the ^ C a / ^ C a ratio of 46.480 was established for a calcium standard reference material. It was also shown that natural isotopic fractionation in four Ca reagent materials was negligible [169]. In view of these findings, Coleman's [223] results were recalculated by normalizing to 40Ca/44Ca from ref. [225]. It should be noted that a work reviewed later in this chapter demonstrated small terrestrial, lunar and meteorite variance of Ca isotopes, but in all cases the 40Ca/44Ca ratios corrected for fractionation were larger than 47.1, which does not support the low ratio reported by Backus [224].

Moore and Machlan [225] intensively studied the complexity of Ca isotopic ratio analysis. The NIST SRM 915 high purity CaC03 was used. Three single focusing magnetic mass spectrometers with identical triple rhenium filament ion sources, pyrometers and single, deep Faraday collectors were applied. Class 100 clean air hoods were used for sample preparation and loading. A very careful loading technique and isotopic analysis procedure was developed, permitting replicate ratio measurements with a 95% limit of error equal to or lower than 0.05%. Table 9.21, summarizes the time sequence of the analysis. Data were collected after a 15 min preheat period and 35 min of heating and adjusting the 40Ca+ ion current to (3.5-4) x 10 n A. Within this time interval, the 40Ca/44Ca ratio increases to a value close to 46.5 and remains stable within 0.1% for the following 40 min. Figure 9.10 demonstrates this behavior. For each sample, five ratios were measured in the following sequence: 40/44, 42/44, 43/44, 48/44, 48/44, 43/44, and so on. The very low abundance 46Ca isotope was measured at the end of each run. When a manual mass spectrometer is used, it is recommended to follow this time sequence. Automated instruments with multiple collector detection systems allow slight modifications in the data collection periods. Isotopic fractionation was corrected by mixing known quantities of the SRM 915 with a known amount of a 44Ca spike and comparing the measured 40Ca/44Ca ratio with that calculated. Calcium is one of the most abundant solid elements on earth. Special care must be taken in handling the samples to avoid contamination. By using high purity reagents and clean laboratory facilities, the blank levels could be kept below 0.1% of the sample size ( 1 pg per filament). Interfering potassium was removed from the samples by cation exchange and by preheating. An interference of unknown origin at mass number 43 was also partially removed by preheating the sample filament. Calcium nitrate, when loaded on the rhenium filament and heated in air, is converted to a mixture of calcium rhenate compounds, mostly Ca2ReÛ4 and Ca3Re2Og. Moore et al. [226] suggested that, in the ionization process, initially a high molecular weight calcium rhenate is vaporized. This substance produces Ca+ ions and, when the system reaches thermal equilibrium, a constant calcium


Table 9.21. The NIST calcium sample loading and isotopic analysis procedure [225]

Time, min Action

Sample loading 0-5 Place 1 pg of Ca (5 ul of 0.2 mg Ca ml-1) onto each of two degassed Re

sample filaments. Set drying current to 0.5 A. Tum on IR lamps. 5-10 Set drying current at 1.2 A.

10-12 Set drying current at 1.5 A. 12-15 Turn off IR and room lights. Slowly adjust current to red heat. Reduce

current slightly, maintain filaments at dull red for 3 min.

Sample preheating 0-1 Set ionizing filament to 1410 °C and sample filaments to 0.6 A. 1-5 Adjust sample filaments to 0.8-0.9 A while burning off large amounts of

K. Check for 40Ca+ at 0.9 A. 5-15 40Ca+ should increase tos¡5 x 10~12 A and then decay, followed by

stabilization and a slow growth towards the end of the preheating stage. The K+ ion beam should decay steadily within this period.

Analysis 0-1 Set ionizing filament to 1410 °C and sample filaments to 0.6 A. 5-6 Slowly adjust sample filaments to 0.9-0.95 A until a 40Ca+ ion current

of (3-4) xlO A is obtained. Focus the moderately growing ion beam to 5 x 10"12 A.

10 40Ca+ ion current should be < 10-" A. Adjust to (1.3-1.5} xlO"11 A. 15 Adjust 40Ca+ ion current to 2.5 x 10~" A. 20 Adjust ^Ca-1" ion current to 3.25 x 10 - u A and allow to stabilize at

(3.5-4)xl0-n A. 30 Take base-line data. 35 Start data collection.

isotopic ratio is observed. It was demonstrated that the value of '"'Ca/'^Ca is temperature dependent.

Rosman et al. [169] determined the calcium isotopic ratios in the Brownfield chondritic meteorite and compared them with those for terrestrial calcium. The thermal ionization procedure developed by Moore and Machlan [225] was applied. No significant natural isotopic fractionation was detected for the chondrite samples. The uncorrected results for terrestrial samples are included in Table 9.22.

Russell et al. [227] performed a very detailed study of calcium isotopic fractionation in terrestrial and extra-terrestrial samples. The aim was to make a precise determination of the isotopic composition of Ca and to identify possible variations of this composition in samples of diverse origin. The technique involing a double spike, containing 42Ca (79.58%) and 48Ca (15.280%) isotopes ( Ca/42 Ca = 0.192 ± 0.002), was applied for instrumental mass fractionation

CALCIUM 263

O 10 20 30 TIME MINUTES

Figure 9.10. Change of the ^Ca/^Ca ratio with time for SRM 915 CaC03. (Reproduced by permission of the American Chemical Society from L.J. Moore and L.A. Machlan, Anal. Chem., 44, 2291 (1972))

Table 9.22. Isotopic abundance ratios in terrestrial calcium 40Ca/44Ca

46.954 47.07* 46.8c

± 0 . 2 d

e

_ / 46.480*

±0.087 46.2663'" 47.153

±0.003 ' 46.447

±0.022*

42Ca/44Ca

0.310 0.311 0.313

±0.0015 0.3095 0.3121 0.3104

±0.0011 0.30944 0.31212

±0.00014

« C a / ^ C a

0.070 0.0704 0.0641

±0.0002 0.0619 0.0622 0.0648

±0.0009 0.06445 0.06487

±0.00003

46Ca/44Ca

0.0016 0.0016 0.00152

±0.000015 0.0016 0.0015 g 0.0017

±0.0005 0.00152 0.00152

± 0.00001

^ C a / ^ C a

0.090 0.0898 0.0893

± 0.0004 0.093 0.0914 0.0898

± 0.0006 0.09008 0.08874

± 0.00004 0.08978

± 0.00020

Ref.

[186]° [45]

[222]

[223]

[225]

[169] [227]

[229]

" Electron impact ionization. b Raw isotopic fractionation uncorrected data. c Fractionation corrected and normalized to 40Ca/48Ca = 521.71, taken from ref. [186]. d ISD, average for CaSÛ4 from three different sources. ' Normalization factor 45.70 from ref. [224]. / Normalization factor 46.480 from ref. [225]. 8 Normalization factor, data corrected for isotopic fractionation. * 95% limit of error for five determinations on NIST SRM 915. ' 2SD, mean for a CaF2 laboratory standard. * 2SD, uncorrected mean for a Ca(N03>2 laboratory standard.

46.5

46.0

to O

O

45.5

á.K o


corrections. The measured data were normalized to this ratio by applying the exponential correction law

(48Ca/42Ca)tr = (48Ca/42Ca)meas x (48/42) ' (20.1 )

where (48Ca/42Ca)tr = 0.192, (48Ca/42Ca)meas is the measured ratio and P is the calculated fractionation correction factor. The correction of other ratios is made according to

('Ca/44Ca)corr = ('Ca/44Ca)meas x (i/44)p (20.2)

where i = 42, 43, 46 or 48. Two mass spectrometric analyses were made for each sample, measurement of the unspiked sample and measurement of a mixture of the sample and the double spike. Differences in (natural) isotopic fractionation were expressed as S( Ca/ Ca), defined by

«("Ca/"«*) = [(40Ca/44Ca)corr/(40Ca/44Ca)r - 1] x 1000 (20.3) where the subscript 'corr' denotes data corrected for instrumental fractionation and STD refers to a CaF2 standard. The samples were dissolved and purified in a cation exchange column. It was noted that this procedure fractionates the Ca sample, the lighter isotopes being more strongly retained by the resin; thus the early eluted fractions were enriched in the heavier isotopes. Complete calcium elution eliminated this problem. The chemistry blanks were found to be negligible. It should be added that Kobayashi et al. [228] found that the heavier Ca isotope is preferentially retained by a strongly acidic cation exchange resin, whereas the lighter isotope is more strongly retained with a weakly acidic cation exchange resin. Russell et al. [227] loaded 5-10 pg Ca as CaCl2 or Ca(N03)2 onto an oxidized, zone refined, V-shaped tantalum filament in a single filament ion source. After degassing, these filaments showed at 1400 °C (analysis temperature) 39K+ ion currents below 2 x 10~14 A, thus there was no interference from % at m/z = 40. The more abundant isotopes were measured in one sequence, 40Ca, 42Ca, 42Ca, 44Ca, ^Ca, with a 1010O electrometer resistor (three times). The minor isotopes, 44Ca, 48Ca, 42Ca, 43Ca and again ^Ca, provided the second sequence and were measured with a 10 u O electrometer resistor (seven times). Altogether, for one sample, 23 sequences were taken, (3 + 7 + 3 + 7 + 3). A third sequence was used to measure the 46Ca, 48Ca and 44Ca isotopes. Baseline zeros were measured at ±0.15 amu from each ion signal. A ratio 40Ca/44Ca = 47.153 ±0.003 (corrected for instrumental mass fractionation) was established as shown above for a CaF2 laboratory standard. No samples of terrestrial or extra-terrestrial origin were found in which calcium was fractionated beyond 0.25% for 40Ca/44Ca.

Platzner and Degani [229] analyzed Merck Analar Ca(N03)2 without further purification. A triple rhenium filament ion source at 1500°C (pyrometer controlled), a single Faraday collector and a fully automatic mass spectrometer

CALCIUM 265

(44Ca/40Ca)x 103

_ 1 L _ _ 1 _ _ ^ _ _ _ ' }± n .°1 4 6 %

(48Ca/40Ca)xio3

f_~_~JL _ _r_~~ y_i~ "^~ ~ _} ±o,i76%

_i i i i i i

60 80 100 120 140 160 t, min

Figure 9.11. Ca isotope ratio measurement of Merck Analar Ca(N03)2 versus time. Each point is a mean of ten ratios

were used, and 44Ca/40Ca = 0.02153 ± 0.00001 and 48Ca/40Ca = 0.001933± 0.000004 (2SD) were measured. Both isotopic ratios were stable between 60 and 160 min. Six sets each of 10 ratios, were collected. An example is shown in Figure 9.11. Four criteria were used in accepting a result: flatness of the 40Ca peak plateau, time independent ratios within the measurement, stability of the sample filament current, and absence of potassium interference. These data can be favorably compared with those of Moore and Machlan [225], who reported 0.02152 ± 0.00004 and 0.001932 respectively (for a material from a different source). All the isotopic ratio data described in this chapter are summarized in Table 9.22.

Lee et al. [168] developed a direct loading technique to determine the isotopic composition of calcium (along with magnesium) in chemically unseparated samples. Thus, 4 pg samples were loaded into the bottom of a zone refined rhenium V-shaped filament, fused with phosphoric acid and silica gel, and analyzed in a thermal ionization mass spectrometer at filament temperatures of 1350-1400 °C. The sample was heated slowly, therefore potassium was evaporated to a level where 39K could not be detected and ^ K did not interfere with 40Ca. A clear ion signal was observed at m/z = 47, which was attributed to 3 1 P 1 6 0 + . The 48Ca+ ion intensity was corrected for the 3ipi7Q+ interference, which contributed about 0.8% to the total ion intensity. Calcium was also chemically separated from the samples and analyzed on oxidized tantalum filaments as described by Russell et al. [227]. It was noted that the unseparated samples (loaded on rhenium filaments) contained 0.5% of the Ca used in Ca separated analyses (10 pg), but the Ca+ ion intensity was 2.5%. The ion current stability was better in the pure Ca sample analyses. The

21.54 21.53 21.52


exponential law was applied for instrumental fractionation correction, assuming 40Ca/44Ca = 47.153 [169].

Esat et al. [172] analyzed the magnesium isotopic composition of interplanetary dust particles. The typical particle size was 10 pm, containing only about 1 % calcium. A special filament type was developed, consisting of a hemispherical dimple, 200 pm in diameter, pressed into flat rhenium filaments. Before sample loading, the dimple was filled with « 5 0 pg silica gel and phosphoric acid using a micro syringe attached to a micro manipulator. The adhesion of silica gel to the rhenium filament was improved by adding trace amounts of a glucose and citric acid solution. Large Ca ion beams (10~n to 5 x 10~10 A) were observed during the magnesium analyses. The 40Ca/44Ca and 42Ca/44Ca ratios could have been evaluated. Optimal analytical conditions for this Ca isotopic ratio measurement were not established.

Jiang and Smith [230] analyzed calcium in urine samples spiked with stable calcium tracers using fast atom bombardment mass spectrometry (FABMS). A resolving power of 3500 eliminated interferences while maintaining a flat top ion signal profile. Two modes of scanning the instrument were compared. Although magnetic scanning provided data with smaller instrumental mass fractionation, it was not used because only 0.03% of the scanning time would be effective in ion monitoring. The higher mass discrimination for voltage scanning of the electrostatic analyzer (ESA) is related to the decrease in the ability to focus high mass ions relatively to lower masses, therefore reducing the ion transmission of heavier ions. This factor amplifies the fractionation intrinsic to the fast atom bombardment process, where lighter atoms are preferentially desorbed from the sample surface. Despite the strong instrumental mass fractionation, the relative standard deviations for ten sample averages were highly acceptable for the purpose of the work. Data for SRM 915 CaC03 are shown in Table 9.23.

Hachey et al. [176] prepared a volatile Ca chelate containing 1,1,1,2,2-pentafluoro-6,6-dimethylheptane-3,5-dione (PPM). The dimer Ca(PPM)2 was introduced into a GC-MS instrument and ionized by electron impact with 70 eV electrons. The 40Ca+, 42Ca+ and 4 4Ca+ ion currents were monitored [as Ca(PPM)J at m/z = 530, 532 and 534], with relative standard deviations of

Table 9.23. FABMS isotopic ratios of SRM 915 Isotopic TIMS [7] FABMS scan mode ratio

magnetic ESA 40Ca/42Ca 149.74 158 170.61 ±0.11 40Ca/44Ca 46.48 51.0 59.59 ±0.06 Note: Quoted uncertainty is ISD.

TITANIUM 267

0.07, 0.06 and 0.04% respectively. The isotopic abundances differed from values in ref. [1] by 0.07, —0.31 and —0.09% respectively.

9.21 SCANDIUM

Scandium is the twenty-first element in the Periodic Table. It has only one stable isotope at mass number 45.

Scandium has an ionization potential of 6.54 eV. White et al. [164] thermally ionized scandium oxide from a tungsten filament. Approx. 10~14 A of 4 5Sc+

ions were observed at a temperature of 1200 °C.

9.22 TITANIUM

Titanium is the twenty-second element in the Periodic Table. It has five stable isotopes at mass numbers 46, 47, 48, 49 and 50, with relative abundance of 8.0, 7.3, 73.8, 5.5 and 5.4% respectively [1].

Nier [186] and Belsheim [231] used gaseous TÍF4 in analyzing the isotopic composition of titanium. T1F3" is the most intense ion in the mass spectrum and was used for the ratio evaluations. The tetrafluoride has the advantage that fluorine is monoisotopic. Herndon and Hibbs [232] used TiBr4 and Hogg [233] used TiCU; in each case the trihalide ions were the most intense ions and the measured ion intensities needed to be corrected for the chlorine and bromine isotopic abundance. Only Belsheim [231 ] calibrated his mass spectrometer to correct for instrumental mass discrimination.

The ionization potential of titanium is 6.82 eV, allowing thermal ionization of this element. Turnbull [234] used a triple tungsten filament ion source. An aqueous solution of Ti02 was deposited on the sample filaments and dried. A 10~13 ATi+ ion current could be obtained. Spitzer and Sites [45] used the same titanium compound in a single tantalum filament ion source. Ti+ and TiO+ ions at a ratio of 1 :5 were formed at temperatures between 1300 and 1600°C; the oxide ion was used for calculations. Corrections accounting for the oxygen isotopic abundance were necessary. The filaments were carefully cleaned to avoid calcium and potassium interferences. Niederer et al. [235] also analyzed Ti02 (Johnson & Matthey, spectrographically pure) by thermal ionization. Thus, 2 pg samples loaded with Ta2Os yielded ion beams of 2 x 1 0 " A for the 48JJ16Q+ j o n Tio+ j o n beams were more intense and stable than Ti+ ion beams, with TiO+/Ti+ » 20, therefore TiO+ ion intensities were used for ratio calculations. Ti isotopic composition was calculated assuming 1 60 : n O : 1 8 0 = 1 : 0.0003708: 0.002045 [91]. The 1 8 0 / 1 6 0 isotopic ratio was also determined in one of the samples, measuring the ion intensities at m/z = 66 and 68; 0.00203 was obtained. In calculating the isotopic ratios, the important large


correction for oxygen occurred at m/z = 66 (50Ti16O+, 4 9 T i n O + , 48Ti180+). No evidence for interferences in the TiO+ or Ti+ mass region could be detected under the conditions of data acquisition. The isotopic fractionation was corrected using normalization to (46Ti/48Ti)noral = 0.108548 by the following expression

(46Ti/48Ti)meas/(46Ti/48Ti)norm - (1 + a)2 (22.1)

and correcting the other measured 'Ti/4 Ti measured ratios. Further titanium isotopic analyses were performed by Imamura et al. [236,237], Haydegger et al. [238], Niemeyer and Lugmair [239] and Shima and Torigoye [240]. These authors [240] pointed out that titanium has the least accurately accepted atomic weight with a quoted uncertainty of 630 ppm (47.88 ± 0.03) [241], which is a combination of chemical and mass spectrometric measurements. They redetermined the isotopic composition of this element using TIMS, correcting their measurements for isotopic mass fractionation with artificial mixtures of separated titanium isotopes of accurately known isotopic and chemical composition. A double focusing instrument (electrostatic, followed with a magnetic analyzer) was modified for this work. A triple rhenium filament assembly inserted in a tantalum block, magnetic peak switching and a Faraday cup or secondary electron multiplier were used. The ion currents from both detecting devices were amplified by a Keithley 642 electrometer and a vibrating reed electrometer. Overall, the Keithley electrometer provided data with the better precision. Filaments were degassed overnight at 3 and 4.2 A (side and center respectively) in the source block. The sample filaments were then treated with silica gel prepared from distilled silicon fluoride hydrolyzed in pure water. 20-50 pg of titanium from a solution were loaded dropwise on the sample filaments and dried in a vacuum system by passing 0.7 and 1.2 A currents through the side and center filaments respectively. At an ion source pressure of 10~7 Torr, first the center and then the sample filaments were heated to produce adequate ion intensities. The usual filament currents were just below 3.9 A and between 1.9 and 2.6 A for the center and the side filaments respectively. Before starting data collection, the SEM detection system was used to check for impurities. The data were collected with the Faraday cup at 10~13 A for the most abundant isotope in the following sequence: 48-44-48-46-48-47-48-49-48-50-48-52-48. One block of data consisted of 21 runs of this sequence, and one complete analysis included several data blocks. Data were normalized to 48Ti+, ion currents at m/z = 44 and 52 monitored impurities, and baseline readings were taken at m/z = 45.5 and 50.5. The Ti sample solutions were prepared from three different sources and analyzed over a period of three years. The synthetic isotope solutions were analyzed in the same way as the natural samples. From all the measured isotopic ratios (n = 18) and their calculated ratios, based on gravimetry, a fractionation correction factor K = 1.5149± 0.0933% u~l was derived, assuming linear dependence of the

VANADIUM 269

Table 9.24. Isotopic abundance ratios in titanium <*Ti/48Ti 47Ti/48Ti 49Ti/48Ti 50Ti/48Ti Error Method Ref"

0.1118 0.1009 0.0737 0.0703 TÍF+, EI [231] ±0.0061 ±0.0031 ±0.0022 ±0.0042 (ISD)

0.108548 0.099315 0.074463 0.072422 TiO+, TIMS [235] ±0.000005 ±0.000004 ±0.000004 (2SD)

0.11190 0.10088 0.07337 0.07034 Ti+, TIMS [240] ±0.00029 ±0.00019 ±0.00014 ±0.00018 (2SD)

fractionation on mass. The following expression was used to obtain the true isotopic ratios:

(ATi/48Ti)tr = (ATi/48Ti)meas[l - (A - 48)*] (22.2)

where A is the mass number. The corrected titanium isotopic ratios from Belsheim [231] and Shima [240]

are given in Table 9.24. The calculated atomic weights are 47.87 ± 0.01 (210 ppm) and 47.8669 ± 0.0005 (11 ppm) respectively, demonstrating the very good accuracy of the earlier measurement (1968) and the dramatic improve-ment in precision of the more recent measurement (1993). The data of Niederer [235] are also included in Table 9.24.

9.23 VANADIUM

Vanadium is the twenty-third element in the Periodic Table. It has two stable isotopes at mass numbers 50 and 51 with relative abundance of 0.250 and 99.750% respectively [1].

Hess and Inghram [242] used vanadyl chloride (VOCI3) evaporated from a tungsten oven in an electron impact ion source. The 5 0V/5 ,V isotopic ratio calculated from the V2+, V+ and VC12+ ions was 0.00274 ± 0.00004, 0.00256 ± 0.00004 and 0.00254 ± 0.00005 respectively. Thermal ionization of V20.<s from a platinum filament produced the V+ and VO+ ions with isotopic ratios of 0.00268 ± 0.00026 and 0.00271 ± 0.00027 respectively. Five different ions were used to improve the accuracy of the data because of isobaric interference corrections. Leland [243] obtained V+ ions by electron bombard-ment of vanadium vapor produced by evaporation of the metal from a tungsten filament. Flesch et al. [244] made absolute isotope ratio measurements by calibrating a mass spectrometer with synthetic samples prepared from separated vanadium isotopes. Vanadium oxytrifluoride (VOF3) was ionized by electron impact in a double collector magnetic sector mass spectrometer. The oxytrifluoride was prepared by vacuum distillation of a solid-solid mixture of


lead vanadate and cobalt trifluoride at a temperature of 550 °C. Gaseous fluoride impurities originating from the sample preparation (HF, SÍF4, S02F2 and CF3OF) were removed by fractional distillation. The ion currents at m/z =123 (50V,6OFJ), 124 (5IV16OF+ and 50V17OF+), 125 (5IVl7OFi; and 50VI8OF^) and 126 (51V18OFj) were used to calculate the isotopic ratio. The absolute isotopic ratio of the 'bulk of world mineral vanadium' was derived by correcting the observed mean ratio with (a) the average variation factor of 12 natural vanadium samples, (b) the gas flow factor, and (c) the recording sensitivity factor. A ratio of 50V/51 V = 0.002503 ± 0.000006 was reported.

White et al. [164] used vanadyl chloride loaded onto a tungsten filament. The 50V/51V isotopic ratio, after correction of 50V for slight isobaric contributions from titanium and chromium, was observed to be 0.00251 ± 0.00009. Spitzer and Sites [45] used vanadium pentoxide slurried in distilled water and thermally ionized at a temperature range between 1400 and 1600 °C. The V+ to VO+ ion intensity ratio was « 10 : 1. The observed isotopic ratio was 0.00241. No vanadium memory effect was noted. Balsinger et al. [245] measured the vanadium isotopic variation in terrestrial and meteoritic samples. A double filament ion source was used, a tantalum filament for sample evaporation and a rhenium filament for sample ionization. The filaments were preheated at elevated temperatures for several hours until no titanium, chromium or vanadium could be detected at the temperature used for vanadium analysis. Stable ion currents in the range 10 12-10~13 A were obtained for several hours and were monitored with an electron multiplier. Three terrestrial and five meteorite samples yielded a mass discrimination uncorrected, weighted mean for the 50V/5IV ratio of 0.002451 ± 0.000018. The instrumental mass discrimi-nation correction factor was evaluated from isotope ratio measurements of other elements as (1.0 ± 0.5)%, yielding an 'absolute' ratio of 0.002425 ± 0.000030. Imamura et al. [246] obtained stable vanadium ion beams by thermal ionization from a single rhenium filament when the sample was loaded together with a dispersed carbon suspension. These authors also described a vanadium ion

Table 9.25. Isotopic abundance ratio in vanadium 50y/51y

0.00264 ±0.00009 0.00235 ±0.00010 0.002503 ±0.000006

0.00251 ±0.00009 0.00241 0.002425 ±0.000030

Compound

VOCl3 V vapor VOF3

VC13 V2O5

a

Method

EI+TIMS EI EI,

calibrated TIMS TIMS TIMS,

calibrated

Ref.

[242] [243] [244]

[164] [45]

[245]

° Vanadium extracted from natural samples.

CHROMIUM 271

exchange separation and purification procedure. The isotopic ratios are summarized in Table 9.25.

9.24 CHROMIUM

Chromium is the twenty-fourth element in the Periodic Table. It has four stable isotopes at mass numbers 50, 52, 53 and 54, with relative abundance of 4.345, 83.789, 9.501 and 2.365% respectively [1].

White and Cameron [43] measured the chromium isotope ratios by electron impact on vaporized CrCl3, monitoring the Cr+ ion. Flesch et al. [247] analyzed chromyl fluoride by EI, monitoring the Cr02Ff ions in the m/z = 120-124 mass range. As ions were also observed at m/z = 121, 125 and 126, the data were corrected for the contribution of 1 80 and 1 70 at m/z — 122, 123 and 124. The isotopic composition of oxygen was determined by decomposing lead Chromate, which was used to prepare the chromyl fluoride, and the water used in the laboratory, and corrected with ratios measured for atmospheric oxygen, for which the absolute isotopic ratio was given by Nier [91]. The mass discrimination effect of the capillary inlet system and the differential ion source pumping was corrected by measuring the isotopic ratio of nitrogen samples prepared from separated isotopes. This approach was a compromise because separated isotopic chromium compounds could not be assayed. The following isotopic ratios were reported: 50Cr/52Cr = 0.0520, 53Cr/52Cr = 0.1135, 54Cr/52Cr = 0.0283. Flesch et al. [247] also studied the isotopic composition of 18 chromite samples from various locations which were the more important sources of commercial chromium; no variations were revealed.

Turnbull [234] used thermal ionization. A few micrograms of the element in aqueous solutions of Cr203, CrÛ3 or Cr(S04)3 were deposited on the tantalum ribbon of a single filament ion source. Cr+ ion intensities of 10~14-10 13 A were measured. Spitzer and Sites [45] used Cr203 and tungsten filaments instead of tantalum. The ionization efficiency was increased by heating the deposited oxide in a vacuum of 2 x 10~2 Torr until melting was observed. The ion intensities were measured at 1300-1600 °C. Shields et al. [248] determined absolute isotopic ratios in a chromium reference material, calibrating two magnetic sector mass spectrometers with samples prepared from nearly pure separated chromium isotopes. A single platinum filament ion source was used. The filaments were cleaned from chromium traces by vacuum degassing under a potential field until no Cr+ ion signal could be observed at the temperatures used in the analyses. About 60 pg of the element from a 3 mg ml - 1 Cr (as chromium nitrate) solution in dilute nitric acid (1 part concentrated acid, 99 parts H20) were deposited on the filament and dried under a heat lamp and the filament current was slowly increased to about 2.2 A. After the filament had cooled, a drop of 10% nitric acid or distilled water was placed on the filament to


spread the sample more evenly and the sample was dried as before. In the analysis, a strict filament heating pattern was followed and the data were collected on a stable or growing 52Cr+ ion current of (3-4) x 10~12 A between 37 and 60 min from the beginning of filament heating. As the observed ratios changed with time, samples not following the above described behavior were discarded. The following absolute ratios were reported:

50Cr/52Cr 53Cr/52Cr 54Cr/52Cr 0.051859 0.113386 0.028222

±0.000100 ±0.000145 ±0.000059

The quoted uncertainties are overall limits of error based on 95% confidence limits for the mean and allowances for the effects of known sources of possible systematic error. Goetz and Heumann [249] determined isotope ratios in chromium with a quadrupole and a magnetic sector mass spectrometer. A single rhenium filament ion source was used. Silica gel and boric acid were added to the samples. At temperatures of 1200-1250 °C, 0.1-1 pg chromium produced Cr+ ion currents of (0.2-2) xlO-11 A in the quadrupole instrument, and the 52Cr/53Cr isotopic ratio in natural samples could be measured on a Faraday collector with an external relative standard deviation of 0.1%. With the magnetic sector mass spectrometer under similar ionization conditions, ion currents 2-3 times larger were observed [250].

9.25 MANGANESE

Manganese is the twenty-fifth element in tbe Periodic Table. It has only one stable isotope at mass number 55.

Manganese has an ionization potential of 7.435 eV. White et al. [164] thermally ionized manganese chloride from a tungsten filament. 55Mn+ ions were observed.

9.26 IRON

Iron is the twenty-sixth element in the Periodic Table. It has four stable isotopes at mass numbers 54, 56, 57 and 58. The isotopic composition is 5.8, 91.72, 2.2 and 0.28% respectively [1].

Early isotope ratio determinations of iron were made by electron impact ionization of such iron salts as iron chloride [43] and iron iodide [251]. Special care had to be taken, as these compounds are hygroscopic and the resulting oxides are not volatile. Fe2Û3 may be deposited on a tantalum filament and

IRON 273

converted by heat to iodide with an excess of HI. Fel2 is evaporated at 200 °C and also ionized by electron impact [45]. Fel+ ions are monitored. Because of large memory effects, the ion source had to be cleaned after each analysis. Recently, Taylor et al. [252] used Fe(PF3)5 for the isotope ratio analysis of iron. This compound sublimes at 22 °C; both phosphorus and fluorine are monoisotopic and Fe+ is the most abundant ion in its EI spectrum, thus it has a high potential for isotope abundance measurements. Five samples were analyzed for 96 h each in a magnetic mass spectrometer. The RSDs are « 0.07 and 0.3% for the 54Fe/56Fe ratio and the abundance ratios of the minor isotopes respectively. The average ratio values differ from those reported for TIMS analysis (see [256] below). This is assumed to be a result of isobaric interference.

Iron has a relatively high ionization potential (7.87 eV). Turnbull [234] thermally ionized this element with a triple rhenium or tungsten filament ion source. Special care has to be taken in TIMS analysis of iron to avoid isobaric interference of 54Cr and 58Ni. These are corrected by monitoring 52Cr and 60Ni and assuming natural isotopic abundance for the interfering elements. Garner and Dunstan [253] ionized iron in a triple filament ion source using high purity zone refined rhenium. 10-15 pg of iron as FeCl3 in nitric acid was loaded onto the filament, evaporated to dryness and then reduced in a hydrogen atmosphere to a mixture of iron and iron oxides. Fe+ ion currents of « 2 x 10~12 A were obtained. Ionization enhancing agents such as silica gel and phosphoric acid improve the Fe+ ion emission [254]. Goetz and Heumann [255] used boric acid instead of phosphoric acid and a single rhenium ion source with a magnetic sector analyzer, and also a quadrupole. They reported an increase of the Fe+

thermal ion current by a factor of at least 10. Their results are shown in Table 9.26. Taylor et al. [256] prepared an isotopic reference material from natural iron, CBNM IRM-014. Two magnetic mass spectrometers were calibrated with five synthetic iron isotopic samples, prepared from carefully characterized enriched iron spikes. The silica gel/boric acid ionization enhancement method was used. Their results are also presented in Table 9.26. Recently, Dixon et al. [257] improved the technique of iron isotopic ratio measurements. A special source filament assembly was constructed, using zone refined platinum ribbon and platinum filament posts with an improved shape given to the filament. The silica gel/boric acid enhancement method was applied and 0.1-1 pg of iron in an HCl solution was loaded. The ion detection was made with a pulse counting system, monitoring the masses 52 (Cr), 54 (Fe ± Cr), 56 (Fe), 57 (Fe) and 58 (Fe + Ni) for 15 s each and 60 (Ni) for 20 s. Five ratio measurement sets repeated four times provided the final ratio. The count rate at 1200-1215 °C for the 56Fe+ ion was (1.0-5.0) x 105 cps. Special sample cleaning was carried out to eliminate isobaric interferences as far as possible. Organic impurities appear-ing at m ±0.1 mass units and tailing from alkali metals were reduced by preheating for 20 min at 1100°C and applying sufficient mass resolution. The


Table 9.26. Isotopic abundance ratios in natural iron 54Fe/56Fe ±(1SD)

0.0618 ±0.0002

0.0626 ±0.0002

0.06370 ±0.00027

0.06335

RSD%

0.32

0.32

0.21 0.143

57Fe/56Fe ±(1SD)

0.0243 ±0.0003

0.0233 ±0.0001

0.023096 ±0.000072

0.023149

RSD%

1.23

0.43

0.16 0.093c

5 8 F e / 5 6 F e

±(1SD)

0.00361 ±0.00019

0.00309 ±0.00005

0.003071 ±0.000029

0.003079

RSD%

5.26

1.62

0.49 0.38c

Ref.

[255]"

[255]

[256]6

[257]d

" Quadrupole thermal ionization spectrometer. * In ref. [256] the quoted uncertainties are 2SD. c The higher of the two SD values is reported. d The isotopic ratios in this work were originally normalized to ' 'Fe.

overall instrumental fractionation was determined with calibrated 54Fe and 56Fe spikes over a 90 min measuring period with an ionization temperature variation of ± 30 °C. The value derived from (54Fe/56Fe)true/(54Fe/5feFe)meas was 0.9943 ± 0.0015 u_ 1 . The reported isotopic ratios were corrected for chromium and nickel isobars and normalized to the 5 7Fe+ ion current. For convenient comparison with other data, the ratios have been recalculated and are presented in Table 9.26. Voelkening and Papanastassiou [258] and Callis [259] also reported data on TIMS iron isotopic ratio measurements.

Iron isotopic ratios have been determined with various mass spectrometric methods other than EI and TIMS. In general, their precisions are lower compared to thermal ionization. FAB-MS has been used by Eagles and co-workers [260,261] and Hansen et al. [262]. Iron bioavailability in infants and absorption during pregnancy have been monitored by ICP-MS. This method suffers from interference of 40Ar16O+ on 5 6Fe+ when the solution is nebulized with argon [263]. It is more successful for spiked solutions, with measurement of ratios not including 56Fe. Whittaker et al. [264] used electrothermal vaporization ICP-MS to determine the 54Fe/56Fe ratio with considerable success. Iron chelated with acetylacetonate has been ionized by electron impact [265]. Fassett et al. [266] reported laser resonance ionization, measuring the 57Fe/56Fe ratio.

9.27 COBALT

Cobalt is the twenty-seventh element in the Periodic Table. Leipziger [267] used a spark source mass spectrograph to demonstrate that this element has only one stable isotope at mass number 59.

NICKEL 275

9.28 NICKEL

Nickel is the twenty-eighth dement in the Periodic Table. It has five stable isotopes at mass numbers 58, 60, 61, 62, and 64, with relative abundance of 68.08, 26.22, 1.14, 3.63 and 0.93% respectively [1].

Early isotope ratio measurements of nickel were made by electron impact ionization of vaporized nickel powder [45] and the mass spectrometric determination of its atomic weight using NiCl2 [43]. Turnbull [234] thermally ionized NiO dissolved in nitric acid with a triple tungsten filament ion source. Traces of isobaric 58Fe were removed by strong heating of the filaments before the data acquisition. Barnes et al. [268] also determined the atomic weight by TIMS. Grämlich et al. [269] prepared a standard reference material of natural nickel, SRM-986. Two mass spectrometers were calibrated with high purity solutions prepared from electromagnetically separated 58Ni, 60Ni and 62Ni isotopes. Nickel was thermally ionized from a platinum filament, which was heated for several hours in dilute hydrochloric acid to remove Fe and Zn impurities (MZn is isobaric with 64Ni). A 5 pg sample of Ni as Ni(N03)2 was deposited on the filament and dried for 5 min at 1 A. Then 5 pi of a solution containing 17 mg of Aerosil 300 powder (Degussa, Frankfurt, Germany) per g of solution, 0.34mg_ ,g AICI3 in the solution and 0.1 gg_ 1 of high purity H3PO3 were added to the filament and dried at currents of 1,1.3 and 1,5 A each for a 5 min period. The filament was then slowly heated to evaporate excess phosphoric acid and further heated for a few seconds to K. 700 °C. The data were collected at 1250°C between 30 and 70 min. The resulting absolute isotopic ratios in SRM-986 are:

58Ni/60Ni 6iNi/60Ni 62Ni/60Ni ^ N i / ^ N i

2.596061 0.043469 0.138600 0.035295 ±0.000728 ±0.000015 ±0.000045 ±0.000024

Grämlich et al. [270] analyzed 29 nickel samples from worldwide sources, compared the data with those for SRM-986, and concluded that, within the uncertainty of the SRM, the isotopic composition of nickel is invariant in nature. Voelkening and Heumann [250] used the silica gel technique with boric acid to enhance Ni+ ion production.

9.29 COPPER

Copper is the twenty-ninth element in the Periodic Table. It has two stable isotopes at mass numbers 63 and 65, with relative abundance of 69.17 and 30.83% respectively [1].


White and Cameron [43] evaporated anhydrous CuCl2 from an oven and ionized it by electron impact. Cu+ and CuCl+ ions were monitored, the latter corrected for 35C1 and 37C1 abundance. Spitzer and Sites [45] placed finely divided cupric oxide on a tungsten filament, slurried it with distilled water and dried it with gentle heat. The sample was introduced into a vacuum chamber which was pumped down to 2 x 10~2 Torr, and the filament temperature was increased until the sample was reduced to metallic copper and melted. The sample was heated in the ion source to a temperature in the 1000-1400 °C range and ionized by electron impact. Cu+ ions were monitored. Memory effects from previous samples were not observed. Schutze and Zahn [188] placed a drop of CUSO4 solution on an aluminum oxide support and reacted it with 0.1 ml 2 N KOH to form Cu(OH)2, followed by 0.1 ml 43% HI solution to convert it to Cul. The final product was dried at 130°C in a small oven, inserted into the ion source, evaporated and ionized by electron impact. Cul+ ions were monitored. Samples as small as 3 pg of copper could be analyzed.

Copper has a relatively high ionization potential (IP = 7.726 eV), preventing efficient thermal ionization. Shields et al. [271] used a triple rhenium filament ion source. On each sample filament « 2 0 0 pg of copper was electroplated from a Cu(N03)2 solution. The 63Cu+ ion current was adjusted to 5 x 10~13 A and data were taken only on growing or stable ion signals. Ratios calculated from decreasing signals showed strong variations with time and were discarded. Two magnetic mass spectrometers were used, which were calibrated with synthetic copper samples prepared from highly purified and almost separated copper isotopes. An absolute value for a copper reference material was established as 63Cu/65Cu = 2.2440 ± 0.0021. The given uncertainty is the overall limit of error based on 95% confidence limits for the mean and on allowances for the effects of known sources of possible systematic error. Kanzaki et al. [272] used a double filament ion source. The tungsten sample filament was loaded with Cu(N03)2 and heated at 2.4-2.8 A, and the rhenium ionization filament was heated at 5.2-5.5 A. Cu+ ion currents in the range 10~u-10~1 0 A were obtained. Murozumi and Abe [273] ionized microgram quantities of copper with a single rhenium filament ion source using silica gel and phosphoric acid as enhancement agents. Platzner [274] used a triple rhenium filament ion source, electroplating 20 pg of copper on each sample filament. A drop of the copper solution was placed on the degassed filament, a pure platinum wire serving as anode was dipped into it, and DC was passed through for 30 s. The ionization and sample filaments were heated at 5.2 and 1.0 A respectively, yielding a stable 3 x 10~13 A 63Cu+ ion current. 36 ratios were collected for one sample. The relative standard deviations for six samples measured in the single and the double collector mode of operation were 0.041 and 0.031% respectively. The isotopic ratios are summarized in Table 9.27.

ZINC 277

Table 9.27. Isotopic abundance ratio in copper 63Cu/65Cu

2.220 2.2352 2.2440 ±0.002 I a

2.2503 ±0.0009fc

Compound

CuCl2 CuO Cu Cu

Method

EI EI TIMS TIMS

Ref.

[43] [45]

[271] [273]

a Uncertainty quoted as 95% confidence limit corrected for isotopic fractionation. b One standard deviation.

9.30 ZINC

Zinc is the thirtieth element in the Periodic Table. It has five stable isotopes at mass numbers 64, 66, 67, 68, and 70, with relative abundance of 48.6, 27.9, 4.1, 18.8 and 0.6% respectively [1].

Leland and Nier [275] evaporated zinc iodide from a tantalum oven and ionized it by electron impact. The isotopic abundance was calculated using isotopic ratios obtained from the Znl2 , Znl + , and Zn + ions, the last of which had the highest intensity. Spitzer and Sites [45] placed zinc oxide on a tantalum filament, slurried it with distilled water and dried the sample with gentle heat. The sample was heated to 800-1200 °C and ionized by electron impact. The Zn+ ion intensity decreased soon after its first appearance. Additional temperature increase produced a stable and intense ion current. Hydrocarbon background frequently caused interferences. Memory effects from previous samples posed a very serious problem. Zinc iodide and zinc fluoride were also used. Rosman and Jeffery [276] developed an electron impact ion source capable of producing reasonably large ion currents from small Zn samples. The source also reduced hydrocarbon interferences to a negligible level by ionizing the sample with a low energy electron beam in a liquid nitrogen-cooled environment. The boat-shaped sample filament contained a tantalum sinter, into which zinc chloride solution was loaded and converted to oxide by adding nitric acid and applying heat. The ion source was attached to a six inch radius, 60° deflection, single focusing mass spectrometer equipped with an electron multiplier detector. The zinc isotopic ratios measured by Rosman [277] with this instrument are shown in Table 9.28. The isotopic composition of a laboratory reference standard was established with a ^ Z n - ^ Z n modified double spike procedure. The observed instrumental discrimination was 0.74 ± 0.25 u ~ \ favoring the lighter isotopes. The corrected isotopic abundance ratios for the laboratory standard were calculated as follows

('Zn/64Zn)corr = CZn/64Zn)meas[l + 0.0074(i - 64)] (30.1)


Table 9.28. Isotopic abundance ratios in zinc " Z n / ^ Z n

12.01 ±0.12 11.90 11.86

66Zn/67Zn

6.83 ±0.07 6.77 6.80

68Zn/67Zn

4.57 ±0.05 4.52 4.57

70Zn/67Zn

0.1500 ±0.0015 0.1509 0.15

Ionization method

EI EI EI

Compound used

Znl2 ZnO ZnCl2

REf.

[275] [45]

[277]

where ('Zn/ Zn)corr is the corrected ratio and ('Zn/ Zn)meas is the measured ratio. It was assumed that the only instrumental discrimination present is linear mass fractionation and that all the measured Zn samples (spike, spiked mixtures and laboratory standard) were isotopically fractionated to the same extent.

Zinc has a high ionization potential (IP = 9.394 eV), which makes it difficult to ionize thermally. Crouch [278] developed a 'borax bead' technique which allowed Zn+ ion beams in the region of 10~10 A to be obtained from a single platinum filament ion source. No isotopic ratios for zinc were reported in this work. Goetz and Heumann [279] used a single rhenium filament ion source and a Faraday collector. Zn+ ions were produced by applying the silica gel ionization enhancement procedure, following Barnes et al. [280]. Thus, 15 pi silica gel in 0.25 pi portions were dried on the filament, passing a 1.15 A current, then the sample dissolved in nitric acid was loaded and dried at the same current, and finally 1 pi of 0.25 M phosphoric acid was added and dried at 1.5 A. The data were collected at temperatures between 1400 and 1480 °C. The procedure was applied for zinc trace determination using the isotope dilution method. Only the 66Zn/68Zn isotopic ratio was reported.

9.31 GALLIUM

Gallium is the thirty-first element in the Periodic Table. It has two isotopes at mass numbers 69 and 71, with relative abundance of 60.108 and 39.892% respectively [1].

Inghram et al. [281] thermally ionized Ga(N03)3 (IP = 5.999 eV) prepared from meteoritic and terrestrial samples. The material deposited on the filament was gently heated in air, converting the nitrate to an adhering oxide. The emission of Ga+ ions started at 650 °C. For meteoritic and terrestrial samples the ratios 69Ga/71Ga = 1.509 and 1.510 respectively were reported. The authors estimated the contribution of isotopic fractionation and instrumental discrimination within 0.75% of the above ratios. Inghram and Hayden [199] also analyzed Gal3, vaporizing it from a tungsten or tantalum oven and ionizing it by EI. Ga+ ions were monitored. Spitzer and Sites [45] used thermal ionization. Gallium from a solution of Ga203 was loaded onto a tungsten

GALLIUM 279

filament in a single filament ion source, and Ga+ ions were observed between 700 and 1200 °C. De Laeter and Rosman [282] calibrated their mass spectrometer by preparing synthetic mixtures from separated and highly enriched gallium isotopes. The silica gel technique was used, with loading of « 1 pg spectroscopically pure gallium metal dissolved in 6 N HCl on a previously degassed, zone refined rhenium filament. Ion beams of 5 x 10 12 A were stable for at least 24 h. Data were collected with an electron multiplier and with a Faraday collector. An absolute 69Ga/71Ga isotopic ratio of 1.5049 ±0.0068, based on averaging the results from the two measuring systems, has been reported. De Laeter [283] analyzed six meteoritic gallium samples and reported a maximum deviation of 0.11% relative to his terrestrial standard. Isotopic fractionation of gallium has been reported in physical and chemical processes, e.g. for electrical current flow in a capillary of liquid gallium [284,285] and ion exchange chromatography [286]. A survey carried out on the 6 9Ga/7 ,Ga isotopic ratio in high purity commercial gallium materials revealed a range of « 0.25% for this ratio, which may be a result of isotopic fractionation during the multiple recrystallization used for purification [287,288]. Furthermore the discrepancy between the existing atomic weights of gallium stimulated the certification of an isotopic standard reference material, SRM 994 [287]. Two magnetic single focusing mass spectrometers with single tungsten filament ion sources were used. Highly enriched (> 99%), separated gallium isotopes were purified and assayed, their isotopic composi-tion was determined, and they were mixed to prepare synthetic solutions for calibration of the instruments. The tungsten filaments were degassed for 45 min at a current of 4.5 A. After this treatment no gallium signal or isobaric interferences could be detected; although occasionally small peaks at m/z — 69 and 71 attributed to hydrocarbons were observed, these could be resolved from the gallium ion currents, and their contribution to an error in the natural gallium ratio measurement was estimated to be less than one part in 105. 500 ng of gallium from a nitrate solution were loaded onto the filament in a Class 100 clean air hood and dried with a 1 A current for 10 min, followed by a current of 3 A for 5 min. The sample loading procedure had an effect on the isotopic fractionation during the analysis, therefore the samples were loaded with a programmable sample dryer, which automatically controlled the timing and the currents during the drying. After drying, the filaments were heated in a separate Class 100 clean air hood for 15 s at 900 °C without air flow. The temperature was adjusted with the aid of an optical pyrometer. The heat was needed to convert the sample to the most stable crystalline form of gallium oxide, ß-Ga203. In the ion source the initial filament current was set to 2.15 A, corresponding to 700 °C. After 2-3 min the Ga+ ion current appeared and rapidly increased to about 2 x 10~n A. Then, after 5, 10 and 15 min, the ion current was adjusted to 2 x 10~", 4 x 10~" and 6 x 10~" A respectively. Data collection started at 20 min (after baseline measurements were taken) and


continued for 30 min. On each instrument the reference material was run 28 times. The absolute 69Ga/71Ga isotopic ratio was determined as 1.50676 ± 0.00039, the uncertainty being the overall limits of error based on two standard deviations of the mean and allowances for the effects of known sources of possible systematic error. For the error calculation see also Section 8.2.5.

9.32 GERMANIUM

Germanium is the thirty-second element in the Periodic Table. It has five isotopes at mass numbers 70, 72, 73, 74 and 76, with relative abundance of 21.23, 27.66, 7.73, 35.94, and 7.44% respectively [1].

In the early stages of germanium isotopic analysis, electron impact on its gaseous or volatile compounds was applied. Ge(CH3)4 was used by Aston [289], Bainbridge and Nier [290] and Dibeler [291]; GeF4, and Gel4 were used by Hibbs et al. [292], Graham et al. [293] and Reynolds [179]. The isotopic ratios observed by Reynolds are shown in Table 9.29. Spitzer and Sites [45] prepared BaGeFô in aqueous solution, deposited it on a tantalum filament and heated it in the ion source. Between 750 and 1200 °C, barium fluorogermanate decomposed to BaF2 and GeF4, which on electron impact yielded GeF^ as the most intense ion together with other germanium ions. Analyses in which the Ge+ and GeF+ ions had about the same intensity as GeFj" were discarded. Arschakuni et al. [294] improved this technique by adding tungsten powder. A sample of 0.5 mg barium fluorogermanate yielded a 10- 1 1 A GeF+ ion beam at 600-650 °C.

The ionization potential of germanium is high (7.90 eV), making thermal ionization of this element somewhat unfavorable. Furthermore, germanium has a high vapor pressure and germanium compounds have low sublimation points. Turnbull [234] dissolved Ge02 in water or diluted nitric acid and loaded the solution onto a tungsten filament in a triple filament ion source. Ge+ ion intensities of \0U A were observed. Shima [295] used ionization enhancing

Table 9.29 Isotopic abundance ratios in germanium 70Ge/73Ge

2.644 2.7518

±0.0019 2.71708 2.6418

72Ge/73Ge

3.535 3.5848

±0.0017 3.55769 3.5412

74Ge/73Ge

4.709 4.6580

±0.0011 4.66027 4.7049

76Ge/73Ge

1.000 0.9647

±0.0007 0.96798 0.9987

Error

(2SE)a

(ISD)6

Ref.

[179] [296]

[297] [298]

0 30 individual analyses of Johnson & Matthey Specpure GeÛ2 sample (laboratory standard). 6 Relative standard deviations of the original data are in the range 0.037-0.060%.

GERMANIUM 281

gels to reduce the fast vaporization of the element from the sample filament. Green et al. [296] developed an ionization enhancement gel technique and analyzed microgram size samples of germanium. Various gel compositions were tried; the best results were obtained by using a mixture of boric acid and an aqueous silica suspension, MOX 170 (Degussa, Aerosil MOX 170) which contains « 1 % aluminum oxide. Ge02 was loaded onto rhenium or tantalum filaments, dried and covered with the gel suspension. Ion intensities in the range 10 14-2 x 1014 A which lasted for 2-3 h were obtained at 1400°C for « 20 pg samples. A magnetic sector mass spectrometer with an electron multiplier was used. The instrument linearity was established with uranium isotopic standards in the 235U/238U ratio range 0.005-187. The instrumental mass discrimination was determined with the double spike technique, in which enriched 73Ge and 76Ge were used. The technique was employed to demonstrate that no variations exist in the isotopic composition of terrestrial germanium minerals within experimental error. The isotopic ratios were not corrected for mass fractiona-tion. Johnson & Matthey Specpure Ge02 was used as an laboratory standard and 11 terrestrial samples were analyzed. Within the limits of experimental uncertainty, no variations in the isotopic compositions of these samples were observed. The results for the laboratory standard are shown in Table 9.29.

Nishimura et al. [297] determined the isotopic composition of this element in a 99.999% pure germanium pellet (Furuuchi Chem. Co., Japan) using secondary ion mass spectrometry. A modified ion microprobe mass analyzer with 300 mass resolution produced a primary bombarding 12 keV oxygen ion beam of (2-5) x l O - 7 A ion intensity with (70-100) x 10- 6 m diameter. The Ge+ ion beams were monitored with a secondary electron multiplier. The ultrasonically cleaned sample was mounted in a sample holder and covered with a tantalum foil with a 3 mm diameter aperture to avoid mass discrimination due to imperfections in target geometry. A liquid nitrogen cold finger mounted close to the sample prevented interference from hydride ions originating from residual water and hydrocarbon vapors. Atomic and molecular interferences at m/z = 70, 72, 73, 74 and 76, originating from Zn and Se, doubly charged Ce, Nd, Gd and Sm, oxides of Fe, Ni and Ca and hydrides of Ga, Ge and As, could not be observed, their estimated intensities being negligible. The measured ratios were corrected for the SEM discrimination assuming the multiplier efficiency for an atomic ion of mass m to be inversely proportional to the square root of m:

Rcon = ( m / 7 4 ) 1 / 2 x Rmem (32.1)

where m = 70, 72, 73 and 76. The corrected isotopic ratios, originally normalized to 74Ge, were recalcul-

ated, and are presented in Table 9.29. Both Green et al. [296] and Nishimura et al. [297] pointed out the good

linearity of Reynolds' data [179] when per mil deviations of isotopic ratios


g

LU ^

il oc

«fe-à number

<3 Reynolds

•A Shima

Q Green et. al.

Figure 9.12. Per mil deviations of isotopic ratios reported by Green et al. [296] and Nishimura et al. [297] from Reynolds' data [179]

relative to their data are plotted as a function of mass number. Both also agree that their data were susceptible to mass discrimination effects. Furthermore, regarding the three sets of data, those of Reynolds are the least affected by this effect. Figure 9.12 demonstrates the per mil deviations of Green's and Nishimura's data from Reynolds' data. Nishimura and co-workers pointed out that the multiplier discrimination correction may be insufficient. The 'residual' fractionation difference may be due to mass dependent fractionation related to secondary ion formation in the sputtering process of a solid sample. Also, the deviations of 10-13%o per mass unit in Green and co-workers' data are higher than that expected from the SEM discrimination, œ 7%o per mass unit. The isotopic ratio data summarized in this section clearly demonstrate the need for an absolute isotope ratio determination of germanium.

Deng [298] analyzed germanium with a boat-shaped filament ion source. Silica gel was added to germanium dioxide to enhance the germanium ion current. The methods of correcting the isotopic ratios for mass fractionation and electron multiplier discrimination were described in detail in the original article. The results are included in Table 9.29.

9.33 ARSENIC

Arsenic is the thirty-third element in the Periodic Table. Nier [62] vaporized elemental arsenic and ionized it by electron impact. Leipziger [267] used a spark source mass spectrograph to demonstrate that this element has only one stable isotope at mass number 75. Wachsmann and Heumann [185] applied

SELENIUM 283

negative thermal ionization to produce As02 ions. A single rhenium filament ion source with a BaO filament coating yielded an ion current of 10~12 A.

9.34 SELENIUM

Selenium is the thirty-fourth element in the Periodic Table. It has six stable isotopes at mass numbers 74, 76, 77, 78, 80 and 82, with relative abundance of 0.89, 9.36, 7.63, 23.78, 49.61 and 8.73% respectively [1].

White and Cameron [43] analyzed selenium hexafluoride using electron impact. SeFj and Se+, the most intense ions in the mass spectrum, were used for isotopic ratio determinations. Spitzer and Sites [45] used lead selenate, PbSe04, which was prepared by dissolving the element in ammonia and hydrogen peroxide and precipitation by adding lead acetate and acetic acid. An aqueous solution of the selenate was loaded onto a tantalum ribbon, dried and evaporated in an EI ion source. Between 400 and 700 °C the compound decomposes to Pb02 and Se02, producing Se+, SeO+ and SeOj ions, the last of which is the most intense and is used for ratio determinations, taking in account the oxygen isotopic abundance.

Wachsmann and Heumann [299] applied negative thermal ionization to analyze selenium. A magnetic sector mass spectrometer with a double rhenium filament ion source was used. A BaO coating on the ionization filament was applied to reduce the rhenium work function and a silica gel suspension was used to enhance the Se~ ion current. The filaments were washed with nitric acid and degassed at 5 A for « 0.5 h. 0.5 pg of Se as H2Se03 solution was used. The solution was evaporated to dryness, then 10 pi silica gel suspension added. The mixture was deposited on the sample filament and heated to dryness, and 30 pg of Ba as Ba(OH)2 were deposited on the ionization filament. In the ion source, the ionization filament was heated to « 850-900 °C at an initial rate of 0.15 A min ' and then at a reduced rate of 0.01 A min~' to 930-960 °C, when an intense and stable ion signal in excess of 10~" A appeared. In several cases the sample filament was heated at 0.5-1 A. The whole ionization process was

Table 9.30. Isotopic composition of selenium, taken from ref. [299] Isotope Isotopic abundance (%) 74Se 0.889 ±0.003 (ISD) 76Se 9.366±0.018 (ISD) 77Se 7.635 ±0.010 (ISD 78Se 23.772 ±0.020 (ISD) 80Se 49.607 ±0.017 (ISD) 82Se 8.731 ±0.010 (ISD)


controlled by an optical pyrometer. The results (given by the authors only as isotopic abundances), were accepted by IUPAC as 'best measurements' [300]. They are shown in Table 9.30. The quoted errors are ISD.

9.35 BROMINE

Bromine is the thirth-fifth element in the Periodic Table. It has two stable isotopes at mass numbers 79 and 81, with relative abundance of 50.69 and 49.31% respectively [1].

The early isotope ratio measurements of bromine were performed using electron impact ionization. Blewett [301] ionized gaseous bromine and reported 1.026 ±0.026 for the 79Br/81Br ratio. Williams and Yuster [177] also ionized gaseous bromine, observing Br2+, Br+ and Bi2 ions. The last of these (at 158, 160 and 162 amu) were used for the abundance calculation. These authors reported 1.021 ±0.004. White and Cameron [43] evaporated AgBr, reporting 1.0210 ± 0.0020. Cameron and Lippert [302] evaporated NaBr and observed a ratio of 1.0217 ±0.0002.

Turnbull [234] used CaBr2 for thermal ionization isotope ratio analysis of bromine in a triple rhenium filament ion source. CaBr+ ions were monitored at an ion current of 10- 1 3 A. In the ratio calculation the contribution of 42Ca79Br+

to the 40Ca81Br+ ion current should be accounted for. Catanzaro et al. [303] determined an absolute value for the isotope ratio of bromine in a commercial sodium bromide designated as NBS Isotopic Reference Sample No. 106. Two magnetic sector spectrometers with triple rhenium filament ion sources were used. The instruments were calibrated with samples of known isotopic composition prepared from almost pure separated bromine isotopes. The ratios were measured by magnetic field scan. Samples of 60 pg bromide were loaded on each sample filament in the form of ammoniacal solutions of silver bromide, containing 3mgml _ 1 Br. This solution produced ion beams of much better stability compared with sodium bromide solutions. A strict pattern of filament heating was followed and ten measurements were made on a growing signal of (3-5) x l0~ 1 3 A between 38 and 52 min after filament heating was started. Samples which did not follow the signal growth pattern were discarded. The resulting absolute 79Br/81Br ratio was 1.02784 ± 0.00190. This uncertainty is the overall limit of error based on 95% confidence limits for the mean and allowances for the effects of known sources of possible systematic error plus a component to cover possible natural variations in isotopic composition [303]. The NBS team also analyzed 29 commercial and natural samples to detect variations in the isotopic composition of bromine, but the phenomenon could not be proved. To eliminate memory effects, when these were observed, uranium samples were run before and after standard samples if the values differed from the reference value by more than 1%. The

BROMINE 285

high temperature at which the uranium is analyzed removed any bromine residuals.

In the case of natural samples such as sylvites (KCl), carnallites (KMgCl3 • 6H20), brines and sea water, where the matrix element is chloride and the bromide concentration ranges between 0.01 and 0.1%, a method of separating and purifying the bromine was developed [303]. A measured quantity was dissolved in 500 ml of H 2 0 and 1 ml of nitric acid was added. The solution was transferred to a distillation apparatus and, to remove iodine, 50 ml of the solution was distilled into ammonium hydroxide solution. 75 ml of nitric acid were then added and the bromine produced was distilled into dilute ammonium hydroxide. As chloride was the dominant halide, it was necessary to repeat the oxidation and distillation to free the bromine from chlorine. After the second distillation, the solution was made acidic with nitric acid, Br - was converted to silver bromide, and the bromide concentration was adjusted to 3mgml _ I Br.

Heumann et al. [217] analyzed bromine with a magnetic sector mass spectrometer and a quadrupole using negative thermal ionization. They observed that a more intense and stable bromide ion current could be achieved with a double rhenium filament ion source when the ionization filament was coated with lanthanum. Carbonized or thoriated tungsten filaments may also be used, but the application of lanthanum to a rhenium filament was preferred for practical reasons. The reported relative standard deviations of the 79Br/8IBr ratios are 0.1 and 0.6% for the magnetic and the quadrupole instrument respectively. Recently, Xiao et al. [304] also applied the procedure they developed for measuring chlorine isotope ratios in cesium chloride by positive TIMS [218] to bromine. Cs2Br+ is emitted from CsBr, the ionization being enhanced by the addition of 100 pg graphite slurry of spectroscopic grade graphite in an 80% ethanol/20% water (v/v) solution to the tantalum filament before sample loading. KBr was converted to HBr by cation ion exchange, and the solution was neutralized with CS2CO3, loaded onto the filament and dried with a current of 1.1 A for 2 min. The data were collected when the Cs2Br+ ion current reached (3-4) x 10- 1 2 A (at 1.15-1.25 A filament current). Five blocks of ten ratios were acquired. The optimal measuring conditions were: preparation of fresh HBr solution before the measurement, Cs/Br ratio in neutralized solution about 1 :2, pH value of the solution in the range 3-5, and bromine sample size 8-32 pg. The reported 7 9Br/8 lBr isotopic ratio is 1.02654 ±0.00012 (2SD). The advantages of this method are the high ionic masses, m/z = 345 and 347, which introduce only a small and reproducible fractionation effect, and the fact that cesium is monoisotopic, the use of RbBr, KBr, NaBr and LiBr, which were also studied was thus superseded. The presence of chloride as CsCl in the CsBr solution interferes with the measurements. Five 24 pg bromide samples contaminated with chloride in the range 3.5-28 pg yielded an average 79Br/81Br value of 1.02742 ± 0.00042.


Table 9.31. Interlaboratory comparison of bromine isotopic ratio measurements Ionization Method"

EI EI El EI TIMS

PTIMS

Source Compound

Br2 Br2 AgBr NaBr NaBr, AgBr

(NBS SRM 106) CsBr

79Br/81Br Ratio

1.026 ±0.026 1.021 ±0.004 1.0210 ±0.0020 1.0217 ±0.0002 1.02784 ±0.00190

1.02654 ±0.00012

RSD (%)

2.5 0.4 0.2 0.02 0.18

0.011

Ref.

[301] [177]

[43] [302] [303]

[304] 0 EI, Electron impact; NTIMS, Negative thermal ionization mass spectrometry; PTIMS, Positive thermal ionization mass spectrometry.

It was suggested that a correction factor of 1.02654/1.02742 = 0.9991 can be applied when analyzing solutions with a CI/Br ratio lower than unity. For 35 and 51.5 pg chloride in the sample solutions no interference was observed. A summary of the analytical techniques and the results observed is given in Table 9.31.

9.36 KRYPTON

Krypton is the thirty-sixth element in the Periodic Table. It has six stable isotopes at mass numbers 78, 80, 82, 83, 84 and 86, with relative abundances in air of 0.35, 2.25, 11.6, 11.5, 57.0 and 17.3% respectively [1].

The isotopic composition of atmospheric krypton reported by Nier [158] in 1950 is the accepted value for this element. The instrumental mass discrimination was corrected by calibration with a synthetic mixture of 36Ar and 40Ar isotopes. Melton et al. [161] used two magnetic mass spectrometers to measure the isotopic abundance of argon, neon and krypton. The purpose of the work was to make direct determinations without the need to calibrate the instruments with synthetically prepared isotopic mixtures. Mass discrimination was eliminated by constructing a specially designed high transmission ion source with a wide ion exit slit. Krypton from two commercial sources was used. The given values represent the average from about fifty measurements on each mass spectrometer. One measurement consisted of six scans of each isotope. The background pressure was 2 x 10~8 Torr. Walton et al. [305] redetermined the krypton isotopic abundance, although they did not made an absolute determination. They used a magnetic sector mass spectrometer fitted with a glass inlet system, a molecular leak and a Faraday collector. Compressed krypton (Aireo Tank No. 50548) was used. The data are in good agreement, as shown in Table 9.32.

RUBIDIUM 287

Table 9.32. Isotopic abundance ratios in atmospheric krypton 78Kr/84Kr

0.0062 0.0062 0.0063

*°Kr/MKi

0.040 0.0396 0.0400

82Kr/84Kr

0.203 0.2027 0.2033

»Kr/WKr

0.203 0.2024 0.2022

86Kr/84Kr

0.305 0.3039 0.3037

Ref.

[158] [161] [305]

9.37 RUBIDIUM

Rubidium is the thirty-seventh dement in the Periodic Table. It has two naturally occurring isotopes at mass numbers 85 and 87, with relative abundance of 72.165 and 27.835% respectively [1], 87Rb is a radionuclide decaying by ß emission to 87Sr with a half-life of 4.88 x 1010 years. This decay process is the theoretical basis of the well accepted and widely used Rb/Sr geochronological dating method.

Rubidium has a low ionization potential (IP = 4.177 eV), and therefore it is easily thermally ionized. Spitzer and Sites [45] used a single tantalum ion source for ionization of the element. The filament was degassed to remove rubidium and other impurities at 6.0 A for 1.5 min at a pressure lower than 5 x 10- 5 Torr. A solution of rubidium chloride containing less than 1 pg of Rb was loaded onto the filament and ionized at a temperature of 900-1300 °C. Memory effects from previous Rb samples posed a very serious problem. An isotopic ratio of 85Rb/87Rb = 2.5907 was determined. Shields et al. [306] analyzed rubidium in the form of rubidium sulfate. A triple rhenium filament ion source was used, with loading of 0.4 pg on each sample filament. All the data were obtained under controlled conditions of a constant sample size and strict adherence to a time schedule of sample heating and data acquisition. Rb+ ion currents were pa 2 x 10~n A. A 85Rb/87Rb abundance ratio of 2.5995 ±0.0015 was obtained for a Rb2S04 reference material. The quoted uncertainty is the 95% confidence interval. No isotopic abundance variation larger than the experimental uncertainty could be detected in 27 natural silicate samples. Catanzaro et al. [307] determined an absolute value for the 85Rb/87Rb isotopic abundance ratio of a RbCl NBS standard reference material, SRM 727. Calibration of the mass spectrometers was performed with synthetic samples of known isotopic composition prepared from nearly isotopically pure and highly chemically purified, separated rubidium isotopes. A triple rhenium filament ion source was used, and the filaments were degassed until no rubidium ion signals were observed. One drop of solution (0.2 pg Rb) from 10 pg - 1 ml RbCl solutions was loaded on the sample filaments and dried with a heat lamp and an electric current of 0.5 A for 10 min. The analyses were started at an ion source pressure lower than 6 x 10~7 Torr. No memory or background ion signals were


ever noted. The data collection period was only 10 min, reducing the isotopic fractionation to the order of 0.03%. A set of 10 ratio measurements including baseline readings was monitored. Generally the fractionation was not measurable within this short time interval. Changing sample sizes, data collection duration and heating parameters introduced significant isotopic fractionation. A resulting absolute 85Rb/87Rb ratio of 2.59265 ± 0.00170 was obtained. The quoted uncertainties are overall limits of error based on 95% confidence limits for the mean and allowances for the effects of known sources of possible systematic error. Platzner [274] used a triple rhenium filament ion source, and the filaments were degassed until no rubidium ion signals were observed. On each sample filament 0.1 pg Rb as nitrate was loaded from a slightly acidic solution. When the ion source reached a pressure of 2 x 10~8 Torr, the ionization and the sample filaments were raised to 2 and 0.25 A respectively within 10 min. Then the temperature of the ioni-zation filament was adjusted with a pyrometer to 1180°C and the total Rb+

ion current was raised to 2.5 x 10-11 A by increasing the sample filament current. A set of four ratios was taken. Then the total ion current was increased to 3.5 x 10~u and 4 x 1 0 " A each time a set of four ratios was taken. Finally, at 4 x 10~" A, five sets of 10 ratios were collected and used for the ratio calculation. It was very important to increase the total ion current slowly, otherwise its stability was lost. Analyses were performed with a single or double Faraday cup collector system. The data collection periods were 50-110 and 38-70 min respectively from the beginning of the analysis. Six samples of a laboratory standard in each mode of data collection yielded 85Rb/87Rb isotopic ratios of 2.59968 ± 0.00136 (single collector) and 2.60025 ±0.00114 (double collector); uncertainties are quoted as 2SD. The standard deviation for each sample in the double collector measurement was about one order of magnitude better than for the single collector measurement. No time dependent isotopic fractionation was revealed in either mode of these measurements. Nevertheless, the double collector data, which were taken in an earlier stage of the run than the single collector data, are slightly higher, indicating at first sight a low fractionation. On the other hand, both means are within 2SD of each of them. Hosoe [308] tested isotopic fractionation and stability in the measured values of the isotopic ratios for rubidium nitrate, sulfate, chloride and iodide prepared from the isotopic NBS-SRM 984 Rb standard. The lowest fractionation effect and the highest stability were observed for rubidium iodide. A double filament thermal ionization source was utilized.

Note: Recently Lapijtas et al. [545] prepared a 87Rb isotopic reference material, IRMM-618. The Rb concentration is (0.11443 ± 0.00017) x 10 3

mol kg ' , the 87Rb concentration is (0.11213 ±0.00017) x 10- 3 mol kg"1, and the isotopic composition is 97.9913 ± 0.0023 mol% 87Rb and 2.0087 ± 0.0023 mol% 85Rb. The quoted uncertainties are 2SD.

STRONTIUM 289

9.38 STRONTIUM

Strontium is the thirty-eighth dement in the Periodic Table. It has four stable isotopes at mass numbers 84, 86, 87 and 88, with relative abundance of 0.56, 9.86, 7.00 and 82.58% respectively [1]. 87Rb is a radionuclide decaying to 87Sr, therefore in nature the 87Sr/86Sr ratio in rubidium- and strontium-bearing minerals may show fluctuations. This process is the theoretical basis of the well accepted and widely used Rb/Sr geochronological dating method. The value of 0.1194 determined by Nier [309] for the 86Sr/88Sr ratio has been adopted by the geological community as an interlaboratory standardization value and an internal normalization correction factor for isotopic fractionation in thermal ionization.

The first precise isotope analysis of strontium was performed by Nier [309]. The element was evaporated from a tungsten oven and ionized by electron impact to yield Sr+. This procedure is not applied any more, having been replaced by thermal ionization. The ionization potential of Sr is 5.695 eV, thus ionization is easily achieved. Spitzer and Sites [45] used solutions of Sr(N03)2 or SrSGj, deposited and dried on a tantalum filament in a single filament ion source. SrCl2 was found less suitable. Rubidium and strontium impurities on the filament were removed by degassing for a few minutes at a pressure below 5 x 10"5 Torr. The Sr+ ion was monitored between 1200-1500 °C. If 85Rb+

appears, the 87Sr+ ion current can be corrected using the known 85Rb/87Rb ratio. Interferences from previous strontium samples were observed. Samples as small as 10 ng could be analyzed. Urbach et al. [310] analyzed Sr(N03)2 from sea water and limestone in a triple rhenium filament ion source. They reported a 0.2% relative external precision for the 87Sr/86Sr ratio. Seibt et al. [311] verified these results with a double collector measuring system.

Moore et al. [312] prepared an absolute isotopic standard reference material of ('natural') strontium: SRM 987. A triple filament rhenium ion source with filaments rigorously degassed at T > 2000 °C for 2 h was used. The absolute 87Sr/86Sr and 86Sr/88Sr ratios are 0.71034 and 0.11935 with 1RSD of 0.037 and 0.034% respectively. This standard is used to calibrate mass spectrometers. Platzner [313] calibrated a fully automatic thermal ionization instrument fitted with five Faraday collectors against the SRM 987 standard. Thus, 1 pg p i - 1 of Sr as nitrate in a slight excess of HNO3 was loaded onto a previously degassed (T > 2000 °C for 30 min) tantalum filament. After drying, 1 pi of 10% H3PO4 was added and dried until white fumes ceased. Then the sample was heated to a glowing dull red. Four different modes of operating the mass spectrometer were compared. The best results for the 87Sr/86Sr ratio (0.0045%, 2RSD, n = 16) were obtained applying a triple collector/peak jump dynamic multi-collection. This procedure eliminates the gain factors of the individual Faraday collectors and corrects for rubidium impurities. 50 ng samples were also easily analyzed. 30 samples measured during almost seven years showed excellent accuracy and


long term instrumental stability [314] with a ratio 87Sr/86Sr = 0.71023 ± 0.000023 (2SD, n = 30, measurement period May 1984-Junuary 1991). The normalization factor used was 86Sr/88Sr = 0.1194. Correction to the NIST value 87Sr/86Sr = 0.71034 demands 86Sr/88Sr = 0.11938, which is in very good agreement with Moore et al. [312], 0.11935 ± 0.00004. Birck [315] used Ta205 as an ionization enhancement agent. A 0.5 pi portion of Ta solution containing 1% Ta, 2% H3PO4, 1% HF and 2% HNO3 by weight in water was loaded onto a tungsten filament and dried by heating the filament to a very dark red. The Sr sample dissolved in 0.5 pi concentrated nitric acid was loaded on top of the Ta2Os layer, and the filament was heated to dark red for 10-20 s until the load had a white color and then cooled to a temperature below 100 CC. Finally, the sample was coated with 0.2 pi Ta solution and dried to dark red. The filament was heated to 1200-1300°C within 15 min and the 88Sr+ ion current was adjusted to (0.3-1) x lO - 1 1 A; data could be collected for several hours. The precision obtained for the 87Sr/86Sr isotopic ratio after mass discrimina-tion correction was a few parts in 105. Sample from 1 pg to subnanogram amounts of Sr can be used. Recently, Callis and Abernathey [316] applied the total volatilization method for 10 ng samples of SRM 987. A triple rhenium filament was used. They obtained 0.11934 for 86Sr/88Sr (NIST certified value 0.11935 ±0.00004).

Wälder and Freedman [317] analyzed strontium on a new prototype double focusing multiple collector ICP mass spectrometer. They reported a mean value for 87Sr/86Sr higher by 0.1% relative to the NIST value and a RSD of 0.015% (2SD, 28 samples). This technique is entirely new. So far it has generated very reliable data for a few other elements, such as Nd, Hf, Pb and U [318] and Mo, Te, Sn and W [319], thus prospects for improvement in the near future are very good.

9.39 YTTRIUM

Yttrium is the thirty-ninth element in the Periodic Table. It has only one stable isotope at mass number 89.

Yttrium has an ionization potential of 6.38 eV. Collins et al. [320] thermally ionized a purified yttrium compound to prove that this element has only one isotope.

9.40 ZIRCONIUM

Zirconium is the fortieth element in the Periodic Table. It has five stable isotopes at mass numbers 90, 91, 92, 94, and 96, with relative abundance of 51.45, 11.22, 17.15, 17.38 and 2.80% respectively [1].

ZIRCONIUM 291

White and Cameron [43] measured the isotopic abundance of zirconium by electron impact on Z1CI4 and ZrF4, vaporized from a tungsten oven. Zr+ in Z1CI4 and ZrFj in ZrF4 were monitored. Spitzer and Sites [45] thermally ionized Zr02 with a single filament ion source. Turnbull [234] used the same compound in a triple tungsten filament ion source. In both cases Zr+ was monitored. Minster and Ricard [321] studied the isotopic composition of zirconium and calculated its atomic weight. A triple filament rhenium ion source with a 60 ° magnetic sector mass spectrometer and a Faraday collector were used in this work. Zirconium (1-2 pg) was dissolved in 2 -4 pi of concentrated nitric acid and loaded onto the filaments. Nomura et al. [322] analyzed commercial zirconium oxychloride to establish the atomic weight of zirconium. The oxychloride was dissolved in dilute perchloric acid and 10-15 pg was deposited on a tantalum filament in a triple filament thermal ionization ion source, having an ionizing filament made from rhenium. The measurements were made with a 90° magnetic sector mass spectrometer equipped with a Faraday collector. Data were acquired when the ion current of 90Zr+ was in the range (6-8) x l0~ 1 2 A. The mass spectrometer was calibrated with two enriched 90Zr and 94Zr spikes and the data were corrected for molybdenum traces. The resulting absolute isotopic ratios are:

91Zr/90Zr 92Zr/90Zr ^ Z r / ^ Z r 96Zr/90Zr

0.21814 0.33324 0.33779 0.05440 ±0.00022 ±0.00013 ±0.00021 ±0.00009

Table 9.33 compares the isotopic abundance determinations of zirconium in various laboratories. Rosman [323] pointed out that the data in ref. [321,322]

Table 9.33. Isotopic composition of zirconium" 90Zr

51.46 51.46 51.449

±0.059 51.444

+ 0.085 - 0.025 51.452

0.009* 51.12

±0.11

91Zr

11.23 11.23 11.320

±0.015 11.214

+ 0.008 - 0.004

11.223 0.012*

11.22 ±0.05

92Zr

17.11 17.11 17.189

±0.021 17.150

+ 0.008 -0 .017

17.146 0.007*

17.40 ±0.04

94Zr

17.40 17.40 17.283

±0.021 17.393

+ 0.016 - 0.057

17.380 0.012*

17.57 ±0.04

96Zr

2.80 2.80 2.759

±0.004 2.798

+ 0.008 -0 .019

2.799 0.005* 2.79

±0.10

Ref.

[43] [45]

[324]

[321]

[322]

[325]

(1949) (1963) (1978)

(1981)

(1983)

(1988)

" Data given in at%. * Quoted uncertainties 95% confidence level.


and Shima's values [324], despite their high precision, should not be considered as absolute data, as no valid correction has been made for instrumental discrimination. Koeppe and Heumann [325] thermally ionized zirconium with a quadrupole mass spectrometer. They reported ratio precisions of 0.5%.

9.41 NIOBIUM

Niobium is the forty-first element in the Periodic Table. It has only one stable isotope at mass number 93.

Niobium has an ionization potential of 6.88 eV. White et al. [164] used a niobium filament in a thermal ionization ion source to produce Nb+ ions. Molybdenum impurities contributed to the mass spectrum, but without an isobaric interference at m/z — 93.

9.42 MOLYBDENUM

Molybdenum is the forty-second element in the Periodic Table. It has seven stable isotopes at mass numbers 92, 94, 95, 96, 97, 98 and 100, with relative abundance of 14.84, 9.25, 15.92, 16.68, 9.55, 24.13 and 9.63% respectively [1].

Williams and Yuster [177] analyzed Mo(CO)6 by electron impact. At 400 °C, Mo+ and MoCO+ ions are observed and the atomic ion is monitored. M0O3 evaporated from a tungsten filament at 700-1000 °C and ionized by electron impact yields MoOj as the most intense ion. The abundance calculation has to account for the isotopes of oxygen [45].

The thermal ionization of molybdenum poses a few difficulties. (1) The first ionization potential of Mo is 7.099 eV, which is high to achieve good efficiency in thermal ionization; (2) Mo samples are easily volatilized at temperatures prevailing under the thermal ionization conditions; and (3) interference is caused by Zr and Ru impurities. Turnbull [234] deposited 3 pg Mo as M0CI3 on a single tantalum filament source, and observed about 10~15 A of Mo+. Ion stability was improved by replacing tantalum with rhenium. Crouch and Tuplin [327] deposited molybdenum sulfide (20 pg Mo) on a rhenium filament and reduced it to the metal at elevated temperatures in a hydrogen atmosphere. Stevens [328], using a triple filament source and loading molybdenum from a NH4OH solution, reported ratio precisions of 0.06-0.1% at the 95% confidence limit. Moore et al. [329] applied the double spike technique. They used zone refined rhenium in a triple filament source, reducing ammonium molybdate in a H2 atmosphere at red heat. Forty pg of Mo at 1760 °C yielded a fairly stable ion beam of (5-8) x lO - 1 2 A. The results are shown in Table 9.34. Koeppe and Heumann [325] thermally analyzed molybdenum with a double filament ion source in a quadrupole mass spectrometer, generating MoO^ ions. The reported

MOLYBDENUM 293

Table 9.34. Isotopic abundance ratios in molybdenum 92Mo/98Mo

0.61477 0.607926

±0.000013* 0.6159c

0.61459 ' 0.614764

±0.000025*

94Mo/98Mo

0.38315 0.3802°

0.3842 ±0.00019d

0.38294 0.38315'

95Mo/98Mo 96Mo/98Mo

0.65968 0.69099 0.655964 0.688146

±0.000013 ±0.000011 0.6602'' 0.6910°

0.65950 0.69028 0.659839 0.690918

±0.000021 ±0.000024

97Mo/98Mo

0.39594 0.394947

± 0.000007 0.3966

±0.00032 0.39604 0.395762

±0.000018

100Mo/98Mo Ref.

0.39919 0.400129

±0.000012 0.3988

±0.00024 0.39893 0.398478

±0.000010

[329]

[330]

[332] [333]

[319] * Normalization ratio. * uncertainty quoted as 1 SE. c Calculated from isotopic abundance in ref. [332]. d Uncertainty quoted as ISD. ' Calculated from isotopic ratios of ref. [333], originally normalized to '

[329]. f Normalization ratio, taken from ref. [329]. 8 Uncertainty quoted as 2SD.

"Mo/'5Mo = 0.6049, taken from ref.

ratio precision is 0.5% (ISD). Qi-Lu and Masuda [330] analyzed a 99.999% M0O3, Aldrich Chemicals Co. standard in a triple filament zone refined rhenium ion source at 1750°C. Molybdenum (20 pg) was loaded on the side filaments as ammonium paramolybdate together with 2 pi of a saturated aqueous solution of boric acid and 1 pi of 1 M nitric acid. After 3 h of preheating, a fairly stable 96Mo ion beam of 2 x 10 12 A could be observed for 5 h. Data acquisition was made by peak jumping using a single Faraday collector. The ratios were normalized to 98Mo and mass fractionation was corrected by normalizing to 94Mo/98Mo = 0.3802, applying the exponential correction law. The mean isotopic ratios of seven samples are shown in Table 9.34 (errors given are one standard error of the mean). These data are the most precise isotopic ratios of molybdenum published in the literature. The same authors have also developed the chemical separation of microgram quantities of Mo from gram amounts of iron meteorites and the purification of Mo from Zr and Ru [331]. Recently, Turnlund et al. [332] developed a relatively fast technique to analyze 1-2 pg Mo samples, monitoring simultaneously, with a five Faraday collector thermal ionization mass spectrometer, the 94Mo, 96Mo, 97Mo, 98Mo and I00Mo ion intensities. The calculated ratios were corrected for isotopic fractionation. A double filament ion source was used, and the zone refined rhenium filaments were degassed at a current of 4.5 A for 15 min. The samples were loaded with an automatic sample loader in a laminar flow bench under a heating lamp. A 10 pi silica gel suspension was loaded onto the filament and dried with a 1 A current for 3 min, then the molybdenum sample (5 pi) was applied in dilute hydrochloric acid and dried at the same conditions, and finally 5 pi of ultrapure 0.25 M H3BO3 solution was added and dried first with a 1.5 A


filament current for 1 min and then automatically with a 0.5 A min- 1 increment to 2 A, after which the current was turned off. Data collection started at an ion intensity of 2 x 10~12 A, which was achieved by heating the ionization filament to 4.1 A in a controlled procedure and finally adjusting the evaporation filament current to an appropriate level. Integration for 8 s, followed by 5 s idle time comprised one measurement cycle, and the ratios were calculated from 10 cycles. The results are presented in Table 9.34.

Further progress in the isotope ratio analysis of molybdenum was made by Kawashima et al. [333]. Hexavalent molybdenum is reduced to a less volatile trivalent compound, and 10-20 ng of it in ascorbic acid solution is loaded on a rhenium-platinum pretreated rhenium ribbon in a single filament ion source. The Re-Pt-carbon alloy enhances the ionization efficiency by increasing the work function of rhenium from 5.1 to « 6 . 0 - 6 . 1 eV. This amount of molybdenum yielded 3 x 10 15 to 2 x 10~14 A Mo+ ion current, which was monitored with a Daly electron multiplier detection system. The sample reduction procedure is as follows. A 1 ppm molybdenum solution was prepared by diluting a 1000 ppm atomic absorption standard solution (Aldrich Chemical Co.) with 6 M HCl. 1-10 pi of this solution were evaporated on a hot plate or under an IR lamp, forming Mo02Cl2. Without leaving the sample to stand, a few mg of 6 M HCl containing 40 mg ml - 1 ammonium iodide and 15 mg ml - 1

ascorbic acid were added, loaded on a pre-prepared filament, evaporated and heated at 1.2 A until the ascorbic acid was carburized. The proposed reduction reaction is:

Mo( 6 + )02Cl2 + 4HC1 + 3NH4I -* Mo ( 3 + ) Clf + 3NH^ + 2H20 + 3/2I2

(42.1)

The preparation of the filament included mixing high purity Re and Pt powders in a 1:2 ratio, slurrying with sub-boiled water, loading a few milligrams of the slurry on a rhenium filament and heating the filament at 4.8 A for 30 min. Molybdenum impurities were removed and a Re-Pt-carbon alloy was pro-duced. Zirconium is isobaric with 92Mo, 94Mo and 96Mo. The possible presence of Zr impurities was corrected for by measuring 90Zr/95Mo. Isotopic fractionation was corrected using the power law and 100Mo/95Mo = 0.6049 [329] as the normalization factor. Uncertainties are given as the average of the deviation from the standard ratios normalized to 95Mo (Moore et al. [329]) and as relative standard errors of deviation. The largest values of the average of deviation are -0 .098 and 0.102 for 96Mo/95Mo and 97Mo/95Mo respectively, and ±0.144% for the relative standard error of deviation for 92Mo/95Mo. For comparison purposes, the data in Table 9.34 are normalized to 98Mo. Qi-Lu and Masuda [334] studied the natural variance of Mo isotopes in a shelf reagent standard, terrestrial molybdenites and iron meteorites using TIMS. Twenty pg

TECHNETIUM 295

of molybdenum was loaded as ammonium paramolybdate together with 2 pi of saturated boric acid and 1 pi of nitric acid on a degassed, zone refined Re filament. A triple filament ion source was used at 1750 °C. Isotopic fractionation was corrected using the exponential law and 94Mo/98Mo = 0.3802 as the normalization factor. No especially evident variations in isotopic composition were observed. High precision molybdenum isotope ratio measurements have recently been achieved with a double focusing multiple collector ICP-MS system by Lee and Halliday [319]. The instrument is equipped with seven Faraday collectors. 1-2 ppm molybdenum solutions of Johnson & Matthey pure metal at a flow rate of 0.3 ml - 1 were introduced into an argon ICP ion source. Typical total ion current intensity was 4 x 10~" A for a 1 ppm solution. Each sample analysis consisted of over 100 measurements of each isotope, detected simultaneously with 10 s integration time. The raw data were corrected for the time independent mass discrimina-tion, applying the exponential law, and normalized to 94Mo/98Mo = 0.38315 [329]. Table 9.34 summarizes the isotopic abundance ratios in molybdenum.

9.43 TECHNETIUM

Technetium is the forty-third element in the Periodic Table. It has no naturally occurring isotopes. Nineteen isotopes of technetium with mass numbers from 90 to 108 are known. 99Tc is produced with high yields from the fission of 235U (6.3%) and 239Pu (6.1%). 97Tc, 98Tc and " T c have half-lives of 2.6 xlO6, 4.2 xlO6 and 2.1 xlO5 years respectively.

Spitzer and Sites [45] evaporated a solution of ammonium pertechnetate (NH4Tc04) on an iridium filament and reduced it to the metal by heating it in a hydrogen atmosphere. Alternatively, electroplating of Tc on the filament and heating in H2 forms a well adhering layer of Tc. Tc+ ions are formed by electron impact. Technetium will produce Tc + ions thermally on iridium filaments [335]. Anderson and Walker [336] developed an improved isotope dilution mass spectrometric technique to analyze 99Tc in environmental samples. The sample was spiked with 97Tc and technetium was isolated from the sample by sequential ion exchange chromatography and ion-association solvent extraction. The isolated Tc was adsorbed onto anion exchange beads and each bead was individually analyzed on a V-shaped rhenium filament. Since no primary Tc isotopic standard is available, instrumental bias was calibrated with molybdenum. The sample was loaded into the ion source and, when the vacuum reached 10~7 Torr, it was slowly heated until the pressure had increased to 10~6 Torr. After vacuum recovery the heating continued by the same procedure to a temperature of 1000 °C. The time required for decomposition of the bead was 5 min. Then the sample was slowly heated to 1900 °C and data were collected with a counting system. A 1 ng 97Tc spike


provided reliable data at about 5 x 104 cps. Determination of as little as 1 pg 99Tc has been achieved.

9.44 RUTHENIUM

Ruthenium is the forty-fourth element in the Periodic Table. It has seven stable isotopes at mass numbers 96, 98, 99, 100, 101, 102 and 104, with relative abundance of 5.52, 1.88, 12.7, 12.6, 17.0, 31.6 and 18.7% respectively [1].

Ordzhonikidse and Akirtava [337] mixed ruthenium and tungsten powders (in 1:3 ratio), placed the mixture on a tungsten filament, vaporized the ruthenium and ionized it by electron impact. In view of the scarcity of volatile ruthenium compounds, Friedman and Irsa [338] used gaseous ruthenocene, Ru(CsH5)2. Only the molecular ion was observed when this molecule was ionized with 10 eV electrons. The only corrections needed were those for 13C and deuterium, using their natural abundances of 1.108 and 0.015% respectively. Also, statistical distribution of these isotopes was assumed.

The ionization potential of ruthenium is relatively high (IP = 7.37 eV), therefore thermal ionization is accomplished at high temperatures or by use of ionization enhancement agents. White et al. [164] suspended powdered ruthenium sponge in amyl acetate and dried it on a V-shaped tungsten filament. Extremely high ruthenium ion currents, stable for long periods of time and over a wide range of temperature, were obtained. Molybdenum interference was always observed, therefore the lower mass ruthenium isotopes were corrected by about 2%. Spitzer and Sites [45] used a single tungsten filament ion source. Finely divided ruthenium metal was loaded on the filament and placed in a vacuum chamber, which was pumped down to 2 x 10~2 Torr and heated until the sample melted. Data were taken in the temperature range 1600-2000 °C. The optimal temperature must be reached very slowly after the first appearance of Ru+ ions. At elevated temperatures ruthenium forms alloys with tungsten, causing the filament to melt. The Ru+ ion beam becomes unstable just before the melting occurs. Interference from previous samples was not observed. Devillers et al. [339] used a single rhenium filament ion source. Total Ru+ ion currents of pa 10"13 A were obtained at temperatures corresponding to 5-6.8 A filament current. Using the silica gel technique, ion currents up to 10~12 A were reached at 3.8-4.6 A filament current. Hydrated ruthenium trichloride (RuCb • «H20, Engelhard Industries, USA) and ammonium aquochlororuthe-nite (NH4[Ru(H20)Ci5], spectrographic standard, Johnson & Matthey, UK) were used as laboratory standards without further purification. The compounds were dissolved in 0.1 N HCl to yield solutions of 1 mg ml"1 metal and loaded onto the filament. Intense ion beams could be obtained only with high purity samples. Two magnetic sector mass spectrometers equipped with electron multipliers were used. The linearity of the system, measured with NIST

RUTHENIUM 297

uranium and lead isotopic standards, was better than 1 %o. The ratio values were averages of at least six sets of 10 mass scans, i.e. 60 ratios. Systematic errors associated with isotopic mass fractionation were not corrected. Further limitations related to the precision of the measurements were caused by interference at all masses in the range 94-105 and molybdenum contamination at m/z = 96 and 98. The first was minimized by prolonged heating in vacuum before reaching the ionization temperature. The second, which was apparent at high Ru+ ion intensities, could not be eliminated, but could be monitored by measuring the 94Mo/99Ru ratio. Poths et al. [340] improved the isotopic ratio measurements of ruthenium. The silica gel-boric acid technique was adopted, using a very careful sample loading procedure. RUCI3 solution (Ventron-Alpha products, 1000 ppm) and Ru metal (Koch & Light, 99.9%) were used as laboratory standards, both loaded from 1.5 N hydrochloric acid. In natural samples, Ru was separated by conversion to RUO4 and volatilization at 80-100 °C. Zone refined, V-shaped and degassed rhenium filaments were used, with concentration of the silica gel, sample and boric acid on a minimum sized spot in the filament. Heating of the filament started at an ion source pressure of 10 8

Torr, with increase of the filament current to 4.2 A within 15-20 min. For samples of 25 ng Ru or larger, the Ru+ ion current at this stage showed a tendency to decrease while the heating current was kept constant. Further filament heating to 1500°C (read by an optical pyrometer) increased the ion current to a level at which data collection was carried out. Without any filament current setting changes, the temperature increased to pa 1600 °C during the measurement. For 100 ng samples, a total Ru+ ion current of 5 x 10"12 A was observed for at least 4 h. Compared with the work of Devillers et al. [339], 50 times higher sensitivity was achieved. A single Faraday collector with a 10n 0 feedback resistor was used. 120 spectra in six data blocks were collected and the data were normalized to the 101Ru+ ion current. Corrections for mass fractionation were performed on each spectrum, using 96Ru/10lRu = 0.324851 for normalization and assuming that the fractionation behaved as predicted by Rayleigh's law. As the true isotopic abundance of ruthenium is not known, the normalization factor is an arbitrary choice, resulting from measurements of 12 laboratory standard samples. Table 9.35 summarizes the ruthenium isotopic ratio measurements discussed in this section, except those of Poths et al. [340];

Table 9.35. Isotopic abundance ratios in ruthenium ^Ru/'OORu 9 8RU/1 0 0RU 99Ru/100Ru l01Ru/looRu 102RU/100RU 104Ru/,00Ru Ref.

0.443 0.442 0.4366 0.4380

0.150 0.148 0.1482 0.1476

1.001 1.008 1.0079 1.0111

1.340 1.357 1.3526 1.3532

2.484 2.508 2.5048 2.5055

1.471 [338] 1.468 [164] 1.4722 [45] 1.4809 [339]


Table 9.36. Isotopic abundance ratios in ruthenium 98Ru/101Ru

0.109550 ± 0.000004

0.10954 ±0.00007

"Ru/101Ru

0.747757 ±0.000007

0.74813 ±0.00013

iooRu/.oiRu

0.738608 ±0.000047

0.74089 ±0.00028

102 R u / 101 R u

1.849977 ±0.000031

1.85052 ±0.00059

104 R u / 101 R u

1.092339 ±0.000090

1.09216 ± 0.00056

Ref.

[340]

[339]

Note: All values normalized to 96Ru/101Ru = 0.324851. The uncertainties are ISD of the mean of 12 samples in the upper line data and of 7 samples in the lower line data.

these are shown in Table 9.36. Data from Devillers et al. [339], normalized to 96Ru/i°iRu _ 0.324851, were included in the table for comparison. Poths et al. [340] attributed the larger errors in 100Ru/101Ru and 104Ru/101Ru ratios to samples with the occasional presence of interferences at m/z = 100 and 104, suggested as being 40Ca28Si16O2

h and 8 8 S r l 6 0 + respectively.

9.45 RHODIUM

Rhodium is the forty-fifth element in the Periodic Table. Leipziger [267] used a spark source mass spectrograph to demonstrate that this element has only one stable isotope, at mass number 103.

9.46 PALLADIUM

Palladium is the forty-sixth element in the Periodic Table. It has six stable isotopes at mass numbers 102, 104, 105, 106, 108 and 110, with relative abundance of 1.02, 11.14, 22.33, 27.33, 26.46 and 11.72% respectively [1].

Spitzer and Sites [45] loaded metallic palladium powder onto iridium or rhenium ribbons, melted it in a vacuum better than 10 2 Torr and ionized the vapor produced at sa 1000°C by EI. The 204Hg2+ ion in the ion source back-ground interfered with the minor 102Pd isotope. This interference was corrected by measuring the 202Hg2+ ion intensity and the known 2 0 2Hg2 +/ Hg2+ ratio. Turnbull [234] applied thermal ionization notwithstanding the high ionization potential (8.38 eV) of this element. About 30 pg of palladium as Pd(NÛ3)2 were loaded onto tungsten filaments in a triple filament ion source with a rhenium ionization filament. A Pd+ ion current of 10~14 A was obtained.

Shima et al. [341] measured the isotopic composition of palladium by loading a drop of a solution containing a few micrograms of palladium together with a drop of freshly prepared ammonium sulfide solution onto a rhenium sample filament in a double filament ion source, where a second tungsten

PALLADIUM 299

filament was producing an ionizing electron beam at an energy of 30 eV After drying the sample, the filament was slowly heated with an electrical current until a stable ion beam was produced. The instrumental mass discrimination was determined with accurately known synthetic isotopic mixtures, and the measured ratios of a laboratory standard and two natural samples were corrected assuming a linear mass dependence of the mass discrimination. No differences in isotopic composition were revealed for the three samples. The corrected ratios are (errors quoted 2SD):

102pd/106pd 104pd/106pd 105pd//106pd 108pd/106pd 110pd/106pd

0.03734 0.4077 0.8172 0.9681 0.4290 ±0.00030 ±0.0018 ±0.0013 ±0.0029 ±0.0024

Mermelengas et al. [342] determined the isotopic composition of five ter-restrial and seven meteoritic samples applying thermal ionization. Sub-micro-gram palladium samples were loaded onto previously degassed zone refined rhenium filaments with silica gel and phosphoric acid. Ion beam intensities of « 10~14 A, stable for several hours, were monitored with an electron multiplier. Magnetic peak jumping to the peak centers was used for data collection. No palladium contamination from the filaments or the ion source was observed. Isobaric interference from ruthenium at m/z =102 and 104 was also not observed. Samples which presented interferences at m/z = 106, 108 and 110 due to cadmium were discarded. The double spike technique [343] (spikes of enriched I02Pd — 15% and 108Pd — 81%) were used to correct for the isotopic fractionation. Four terrestrial and the seven meteoritic samples had the same isotopic composition, shown below:

102pd/108pd 104pd//108pd 105pd/108pd 106pd/108pd U0pd/108pd

0.039568 0.42831 0.85648 1.04116 0.43797 ±0.000082 ±0.00030 ±0.00061 ±0.00053 ±0.00029

These data are in good agreement with the work of Shima et al. [341] and with data published by Kelly and Wasserburg [344], who also analyzed the palladium terrestrial and meteoritic isotopic composition. The fifth sample, originating from the Bushveld Igneous Complex of South Africa, exhibited an isotopic fractionation of 0.38% per mass unit, with enrichment occurring in the heavier isotopes. It is believed that this is a true phenomenon independent of chemical processing, although not understood. Rosman et al. [345] redeter-mined the isotopic ratios of this palladium mineral, confirming a fractionation of 0.36 ±0.01% per mass unit. A multicollector thermal ionization mass spectrometer equipped with Faraday collectors was used for this work.


9.47 SILVER

Silver is the forty-seventh element in the Periodic Table. It has two stable isotopes at mass numbers 107 and 109, with relative abundance of 51.839 and 48.161% respectively [1].

Spitzer and Sites [45] vaporized silver iodide from a tantalum ribbon at 200 °C and ionized it by electron impact. The Ag+ ion was used for ratio determination; other ions such as Ag J , AgJ, Agl+, Ag2I+ and Ag3l+ were also observed in the mass spectrum.

The ionization potential of silver is relatively high at 7.58 eV, but still the element may be thermally ionized. Crouch and Turnbull [346] dissolved pure silver in 3 N HNO3, evaporated the solution to dryness, dissolved the nitrate in concentrated NH3 and dissolved the Ag(NH3)4N03 in saturated boric acid. 20 pg silver from this solution was loaded onto a tungsten filament in a single filament ion source. Ag+ ion currents between 10~12 and 10~n A were measured. Spitzer and Sites [45] modified the boric acid method. A pure solution of silver nitrate was loaded onto the tungsten filament followed by a drop of saturated boric acid. The filament was then carefully heated in air for a few seconds to glowing red, which corresponded to »750°C. Ag+ ions were collected between 1100 and 1400 °C. Shields et al. [347] used a triple rhenium filament ion source, loading onto each sample filament « 100 pg silver from a 15 mg ml - 1 silver nitrate solution and drying it carefully. Then the filament was heated under a hydrogen atmosphere until the silver nitrate started to melt. This procedure reduced the nitrate completely to silver. A stable Ag+ ion beam could be maintained for several hours. The mass spectrometer was calibrated against mass discrimination effects with synthetic mixtures of silver isotopes prepared from pure and almost completely separated isotopes. The reported 107Ag/109Ag ratio was 1.07547 ± 0.00206 (total uncertainty at 95% confidence level). Shields et al. [348] measured isotopic ratios of a commercial silver nitrate sample, 13 samples of native silver and 11 silver minerals from various deposits. One sample exhibited a statistically significant variation. The mean ratio of all samples was 1.07597 + 0.00135. Powell et al. [349] redetermined the absolute isotopic abundance of a silver reference material to improve its accuracy and precision, and also to reassess the isotopic ratio in the isotopic variant sample observed by Shields [348]. A single platinum filament ion source and the silica gel technique were used. The sample preparation was carried out in two stages: the low and the high temperature drying stages. A 5 pi drop of silica gel suspension [280] was dried on the filament at 1.0 A for 5 min, then a second drop was applied and dried. Four pg of silver as silver nitrate dissolved in (1 + 9) nitric acid were loaded on the silica gel and dried at 1.0 A for 5 min. A drop of 0.75 N phosphoric acid was then added and dried at 1.5 and 2 A, each for 5 min. This drying procedure was carried out using a programmable sample drier in a Class 100 clean air hood under a heat lamp, which was adjusted to

CADMIUM 301

provide a temperature of 70 °C at the filament surface. The high temperature drying stage was performed in a nitrogen atmosphere under a bell jar. The filament was purged for 5 min, then the nitrogen flow was stopped and the temperature was adjusted with a pyrometer to 1040 °C for 60 s. At an ion source pressure of 10~7 Torr the filament temperature was set to 760 °C, and a 107Ag+

ion beam intensity of (1-2) x l0~ 1 2 A was observed. After 25 min the temperature was increased to 790 CC and the observed ion intensity was in the range 5 x 10~12 to 1 x 10~" A. Samples which did not follow this heating pattern were discarded. Data were collected between 30 and 50 min. The mass spectrometer was calibrated with accurately known mixtures of the separated isotopes. A very careful sample purification and a study of possible interferences were carried out in this procedure. The absolute isotopic ratio 107Ag/109Ag = 1.07638 ± 0.00022 was obtained and the reference material was designated as NBS SRM 978a. After extensive purification, the sample, that had originally exhibited a different isotopic ratio ultimately yielded the same ratio.

9.48 CADMIUM

Cadmium is the forty-eighth element in the Periodic Table. It has eight stable isotopes at mass numbers 106, 108, 110, 111, 112, 113, 114 and 116. The isotopic abundance is 1.25, 0.89, 12.49, 12.80, 24.13, 12.22, 28.73 and 7.49% respectively [1].

Cadmium and several of its compounds are easily vaporized and thus may be ionized by electron impact. Leland and Nier [275] used cadmium iodide and monitored the Cd+ and Cdl+ ions. Spitzer and Sites [45] vaporized cadmium oxide from a tantalum ribbon. Between 800 and 1000 °C the Cd+ ion is observed, its intensity decays at the early stage of the analysis, but is increased and stabilized with slight temperature elevation. Zahn [350] evaporated the metal directly.

The high ionization potential of Cd, 8.993 eV, makes direct thermal ionization quite difficult. Crouch [278] used a single tungsten filament ion source. CdSÛ4 followed by boric acid or a borax solution was loaded onto the filament, heated in air until the mixture dried and melted to a glassy material. In the mass spectrometer, Na2BÛ2 ion followed by NaKB02 ions appeared and, after their decay and, a further temperature increase, Cd+ ions were formed. Rosman and De Laeter [351] used a 90° magnetic sector mass spectrometer equipped with a single rhenium filament ion source and an electron multiplier. The silica gel-phosphoric acid ionization method, following Cameron et al. [352], was applied on degassed filaments. Microgram quantities of cadmium as Cd(N03)2 produced ion currents of « 10"13 A for at least 24 h. The electron multiplier current was amplified by a vibrating reed electrometer with a 109 fi input resistance. A voltage-to-frequency converter followed by an electronic


counter provided digital presentation of the data, which were finally processed with a small digital computer. Contamination of the Cd was not observed. The masses were magnetically scanned in two mass groups: 110-116, which contains the major isotopes, and 106-110 for the minor isotopes. From each set of 10 scans the mean and the standard deviation were calculated. Approxi-mately 120 measurements yielded standard errors of the mean better than 0.1% and <0.05% for the isotopic ratio for the minor and the major isotopes respec-tively. Only when a stable ' 12Cd+ ion beam was achieved was data acquisition commenced. Time dependent isotopic fractionation was not observed. The silica gel method produces interfering hydrocarbon peaks. These could be reduced by heating the sample to a temperature which yielded a 112Cd+ ion beam up to 100 times larger than that used for data collection. A liquid nitrogen cold finger was also installed in the ion source. Increasing the mass spectrometer resolving power could reveal the presence of hydrocarbons. Interference by In at mass 113 can be detected by monitoring the ' l 5In+ ion, which is the abundant isotope of this element. Usually indium was efficiently removed by the ion exchange procedure applied for preparation of the cadmium sample. Palladium isotopes at 106, 108 and 110 mass units, and also tin isotopes at 112, 114 and 116 mass units, may interfere, but were not detected. If present, 104Pd, 105Pd and 120Sn can be monitored and used for isobaric interference correction. The instrumental mass discrimination was assessed by analyzing the NBS 981 common lead isotopic standard and the NBS 978 silver isotopic standard and by the double spike method with cadmium of known isotopic composition. Rosman et al. [169] studied the natural composition of cadmium in the Brownfield chondrite. A 90° magnetic sector mass spectrometer with a thin lens 'z' focusing ion source and a deep Faraday collector was used. The data were collected by peak jumping and acquired by a minicomputer. 500 ng Cd [as Cd(N03)2] were loaded together with silica gel and phosphoric acid onto

Table 9.37. Isotopic abundance ratios in cadmium

1 0 6Cd/U 2Cd l 0 8Cd/U 2Cd 110Cd/112Cd m Cd/ 1 1 2 Cd 113Cd/112Cd ,14Cd/112Cd u 6Cd/1 1 2Cd

Leland and Nier [275]

0.0505 0.0364 0.5147 0.5297 0.5093 1.1990 0.3182

Rosman and De Laeter [351] "

0.0518 0.0370 0.5184 0.5309 0.5064 1.1898 0.3096

Rosman et al [169]*

0.0518 0.0369 0.5176 0.5305 0.5064 1.1906 0.3080

" Matthey-Johnson Pty. Ltd. Spectroscopically pure cadmium metal, ratios corrected for mass fractionation of 0.68% per mass unit. b Originally normalized to 110Cd/"4Cd = 0.438564.

INDIUM 303

a rhenium filament. Within 15 min the temperature was raised to 1120 °C, and data collection started after 30 min. The Cd+ ion current was 3 x 1 0 ' " A and decayed uniformly during the analysis period. No hydrocarbon or other interferences were observed, but as a general precaution a liquid nitrogen cold finger was used. The change in mass fractionation during a 3 h analysis period was less than 0.01% per mass unit. The extent of isotopic fractionation was measured with spiked samples. The results show that cadmium in the Brownfield chondrite is isotopically fractionated, with the heavier isotopes relatively enriched by 0.27% per mass unit. In Table 9.37 results from refs. [275], [351] and [169] are presented.

9.49 INDIUM

Indium is the forty-ninth element in the Periodic Table. It has two stable isotopes at mass numbers 113 and 115, with an isotopic composition of 4.3 and 95.7% respectively [1],

Indium is a metallic element with a low ionization potential (IP = 5.786 eV), thus it is easily thermally ionized. White and Cameron [43] and White et al. [164] performed isotope ratio analyses of this element. The latter group used metallic indium as a source of In+ ions and a two stage magnetic mass analyzer, followed by a twenty stage electron multiplier and high speed scaling circuits. Their result is accepted as the 'Best Measurement' by IUPAC [1] and is shown in Table 9.38, together with other data. Spitzer and Sites [45] used In203 dissolved in aqueous solution, loaded onto a tantalum ribbon of a single filament ion source. In+ ions are monitored between 700 and 1000 °C. The temperature must be increased slowly, otherwise excessive heat produces erratic ion beams. Also, when the ion beam has developed to a sufficient intensity, the temperature must be slightly reduced to avoid losing the sample. Interferences from previous indium samples cause problems. Turnbull [234] also used In203 in a triple tungsten filament ion source. The heat radiated from the ionization filament was sufficient to evaporate the sample, and 10- 1 3 A ion currents were monitored.

Table 9.38. Isotopic abundance ratios in natural indium Ref. [43] [164] [45] ll3In/115In 0.04417 0.04526 0.04471


9.S0 TIN

Tin is the fiftieth element in the Periodic Table. It has ten stable isotopes at mass numbers 112, 114, 115, 116, 117, 118, 119, 120, 122, and 124, with relative abundance of 0.97, 0.65, 0.34, 14.53, 7.68, 24.23, 8.59, 32.59, 4.63, and 5.79% respectively [1].

Early isotope ratio measurements of tin were made by electron impact ionization of Snl2 or SnCl2. The Sn+ ions were monitored [43]. Spitzer and Sites [45] developed a different EI technique: tin powder was deposited on a heated tungsten ribbon, evaporated in the ion source at a vacuum better than 2 x 10"2 Torr, and bombarded with electrons. In this case SnO+ ions were also observed with a Sn+/SnO+ ratio strongly dependent on the vaporization temperature.

The relatively high ionization potential of tin (IP = 7.344 eV) does not favor thermal ionization of this element without ionization enhancement agents. Turnbull [234] deposited tin chloride with a 20 mg ml-1 sodium metaborate solution onto a tungsten ribbon of a triple filament thermal ionization ion source, observing a Sn+ ion current of « 10~13 A.

De Laeter and Jeffery [353] thermally ionized tin, monitoring the ion currents with an electron multiplier, which introduced a mass dependence into the results. These authors also drew attention to possible indium contamination in most of the samples they analyzed, with consequent interference at mass 115. Devillers et al. [354] measured the isotopic composition of tin in order to establish its absolute isotopic abundance and atomic weight. About 500 pg of metallic tin were electrodeposited onto a rhenium side filament of a triple filament ion source. Ion beams larger than 10~13 A were obtained for the most abundant isotopes. Interferences at mass 115 from 115In, and at mass 114, which is believed to be due to an organic residue, were reported, thus accurate abundances for the low abundance 1l4Sn and 115Sn isotopes could not be established. De Laeter et al. [355] applied the silica gel-phosphoric acid ionization enhancement method introduced by Cameron et al. [352] to analyze microgram quantities of tin. Interferences, particularly that at mass 119 known to be due to ^Ca31?16©^, decreased the accuracy of the measurements. Rosman et al. [356] modified the silica gel-phosphoric acid technique by replacing phosphoric acid with boric acid and adding alumina. With this procedure the interference at mass 119 was eliminated and indium, which interfered at mass 115, was not present in the samples. Johnson & Matthey 'Specpure' JMC 530 tin oxide and JM 540 tin metal were analyzed. The samples were loaded on degassed zone refined rhenium single filaments either as a tin oxide/water slurry or as tin chloride in 8 M HCl, the latter salt being produced from the metal. In the case of the slurry, > 100 pg was evaporated to dryness and the gel mixture was added. The chloride samples (10 pg) were either evaporated to dryness by passing a current of 0.5 A through the filament

TIN 305

or electrodeposited on the filament (as cathode) in 5 pi of 2.5 M NH4CI and 2.5 M NH4OH. The ionization enhancing gel was prepared from a mixture of saturated boric acid solution and an aqueous suspension of Degussa Aerosil MOX 170, which contains between 0.3 and 1.3% aluminum oxide and increased the ionization efficiency of the gel over that prepared with pure silicon dioxide. The optimum concentration of boric acid was determined by preparing mixtures with different boric acid contents. Monitoring the 120Sn+

ion profile vs B2Û3 wt% concentration at three temperatures (1320, 1380 and 1450 °C), two maxima at sa 36 and 74% B203 were observed. The 74% mixture was chosen because of the better sensitivity at lower temperatures and the ability to maintain ion beams for longer periods. Most of the measurements were performed with a single Faraday cup collector mass spectrometer. 3 x 10 12 A ion currents were obtained. A second mass spectrometer of the same type, but equipped with an electron multiplier, was used mainly to confirm the absence of "5In by searching for the less abundant 113In isotope. Several ratio measurements were also made with this instrument. Isobaric interference with tin by cadmium and tellurium isotopes was checked at mass 110 and 128 respectively, but was not detectable. The resulting isotopic ratios of Rosman et al. [356] are in good agreement with those of Devillers et al. [354] for all isotopes except the two minor isotopes 1I4Sn and 115Sn. The results are shown in Table 9.39. Rosman and McNaughton [357] studied the natural variation of tin isotopes in ten different high purity tin samples. The fractionation was determined by the double spiking method [358] using a mixture of ll7Sn and 122Sn as the double spike. Each sample was dissolved in 10 M HCl to give a solution of 1 mg g_1 of Sn. An amount of 2 pg Sn was deposited on a degassed, zone refined rhenium filament and evaporated to a small drop. An ionization enhancement agent slightly different from that used by De Laeter et al. [355] was prepared by mixing high purity boric acid and silica gel doped with aluminum chloride. This mixture was applied to the sample filament and evaporated to dryness and the filament was heated to redness in air for 3-5 s. At

Table 9.39. Isotopic composition of Johnson & Matthey tin 112Sn/l20Sn ""Sn/'^Sn 115Sn/l20Sn "7Sn/120Sn ll8Sn/120Sn ,19Sn/12°Sn l22Sn/l20Sn 124Sn/120Sn Ref

0.02986 0.02022 0.01039 0.23538 0.74295 0.26345 0.14211 0.17753 [356] ±0.00005 ±0.00005 ±0.00004 ±0.00008 ±0.00020 ±0.00013 ±0.00007 ±0.00010

0.029812 0.020195 0.010366 0.235313 0.742935 0.263430 0.142086 0.177588 [319] ±0.000004 ±0.000014 ±0.000007 ±0.000048 ±0.000076 ±0.000046 ±0.000013 ±0.000052

Notes: (1) Ref. [356]. The mean given is of four JMC 530 tin oxide and three electrodeposited JMC 540 tin metal samples. (2) Ref. [319] Johnson & Matthey, AAS standard tin solution. (3) The errors given are 95% confidence intervals of the means. (4) The isotopic ratios were normalized to "6Sn/l20Sn = 0.4460, taken from Ref. [354].


a temperature of 1300 °C Sn+ ion intensities of 1 0 _ n A, lasting for at least 1 h, were obtained. Five isotopes, 116Sn, U7Sn, 118Sn, 120Sn and ,22Sn, were monitored in this work with a multicollector magnetic sector mass spectro-meter. Five data blocks of 25 ratios each were collected for every isotopic pair with a 5 s integration period. The errors of the measured ratios, given in 2SD, were in the range 0.013-0.036%, with a few exceptions of 0.06%. No isotopic variations relative to a laboratory standard (Johnson & Matthey 'Spec-Pure' Sn metal rod) in nine out of ten samples were observed, i.e. the variation in isotopic abundance was less than 1 part in 104 per mass unit. Only one sample showed a relative enrichment in the light isotopes of 0.012 ± 0.006% (2SD) per mass unit.

High precision tin isotope ratio measurements have recently been achieved with a double focusing multiple collector ICP-MS instrument by Lee and Halliday [319]. The instrument is equipped with seven Faraday collectors. A 1-2 ppm Johnson & Matthey AAS standard tin solution, at a flow rate of 0.3 ml - 1 , was introduced in an argon ICP ion source. Typical total ion current intensity was 4 x 10~u A for a 1 ppm solution. Each sample analysis consisted of over 100 measurements of each isotope, detected simultaneously with 10 s integration time. The raw data were corrected for the time independent mass discrimination by applying the exponential law and were normalized to I16Sn/120Sn = 0.4460, as given by Devillers et al. [354]. The results are presented in Table 9.39.

9.51 ANTIMONY

Antimony is the fifty-first element in the Periodic Table. It has two stable isotopes at mass numbers 121 and 123, with relative abundance of 57.36 and 42.64% respectively [1].

White and Cameron [43] evaporated SbCl2 from a tungsten oven and ionized it by EI. The Sb+ ion intensity was monitored. Spitzer and Sites [45] melted antimony powder in air on a tantalum ribbon and ionized it by EI at 400-750 °C. Sb+ and SbO+ ions, the latter about ten times more intense, were observed in the spectrum. When the oxide ion appeared, the temperature was increased slowly. The ratio calculation was done using the oxide ions, correcting for the contribution of 121Sb180+ to 123Sb160+. The reported 121Sb/ 123Sb isotopic ratio was 1.3392 ± 0.0017. The ion source was cleaned after each analysis to avoid memory effects.

In spite of the high ionization potential of antimony (8.64 eV), Turnbull succeeded in thermally ionizing this element [234]. Sb02 from a liquid solution was deposited on a tungsten filament in a single filament ion source. SbOj and SbO+ ions were observed and used for ratio determinations. The SbO+ ion intensities were « 10- 1 4 A. 137Ba+ interferes with 1 2 1Sb1 60+, thus the sample

TELLURIUM 307

should be carefully purified and the filament degassed. The presence of barium traces in the sample could be identified by monitoring non interfering barium isotopes such as 135Ba or 138Ba. De Laeter and Hosie [359] also determined the isotopic composition of antimony by TIMS. 99.999% spectroscopically pure antimony metal was dissolved in 6 M HCl in the presence of a small amount of nitric acid and 5 pg of the metal were deposited on a degassed, zone refined rhenium filament. Ionization enhancing gel and a single filament ion source were used. Data were collected on a Faraday collector at a filament temperature of 1340°C, and the ion beam intensity was « 5 x 10"13 A. 1.3453 ±0.0014 (2SD) for the 121Sb/123Sb ratio was reported. In this determination two isotopically enriched antimony tracers were used, but their analysis was achieved by applying the isotope dilution technique with the solution used for the isotopic ratio measurement of the natural sample, thus the instrumental mass discrimination was not defined. Chang et al. [360] determined the chemical purity of their isotopic tracers by independent chemical methods and calibrated the mass spectrometer with eight synthetic mixtures of the isotopes 121 Sb and 123Sb. Sb203 was dissolved in 4.8 M HCl, and lOpg antimony from this solution was loaded onto a V-shaped rhenium filament previously cleaned and degassed at a current of 5.5 A for 30 min. The silica gel-phosphoric acid technique was used to enhance ionization efficiency. At 1450°C the SbO+ ion beams reached 5 x 10~12 A and were simultaneous collected on two Faraday collectors at m/z = 137 (121Sb160+) and m/z = 139 (123Sb160+ + 1 2 l Sb l 8 0 + ) . No interference from barium was observed at this temperature when monitoring the major l38Ba+ isotope. Five loadings were made, taking 60 ratios for each. The data were corrected for the 1 2 ISb1 80+ contribution at m/z = 139 by the known , 8 0 / 1 6 0 ratio of 0.00200. The measured 12lSb/123Sb ratios were com-pared with the calculated ratio and the mass discrimination correction factor was evaluated. Ten samples from various origins were analyzed and yielded a mean ratio of 1.34145 (RSD < 0.02%), which was corrected to 1.33714, providing an absolute isotopic ratio for antimony. The results also show no natural isotopic fractionation in this element. Wachsmann and Heumann [185] measured the natural isotopic composition of antimony by NTIMS. A single rhenium filament ion source was used; and 20 pg of Sb from a K[Sb(OH)ô] solution followed by 30 pg of Ba from a Ba(OH)2 solution were loaded onto the filament. At a temperature of 1000 °C an SbOT ion current of 10~12 A was obtained. Five loadings yielded a mean ' Sb/ , 2 3Sb isotopic ratio of 1.3351 ±0.0018 (ISD).

9.52 TELLURIUM

Tellurium is the fifty-second element in the Periodic Table. It has eight stable isotopes at mass numbers 120, 122, 123, 124, 125, 126, 128 and 130, with


relative abundance of 0.096, 2.603, 0.908, 4.816, 7.139, 18.95, 31.69 and 33.80% respectively [1].

Williams and Yuster [177] used electron impact ionization of TeF6. Six TeF^ ions (n = 0,1..5) were observed in the mass spectrum, TeFjj" being the most intense. White and Cameron [43] also ionized the hexafluoride, using the ion intensities of TeF^ and TeFj for ratio evaluations. These authors also measured the isotope ratios directly by vaporizing elemental tellurium. Spitzer and Sites [45] analyzed silver tdlurate, prepared by dissolving tellurium in NH3 and H202 and adding a neutral solution of silver ions. The solution of AgóTeOo was deposited on a tantalum ribbon in an electron impact ion source, dried and heated in vacuum to 200 °C. At this temperature the tdlurate decomposes to silver and tellurium oxides, which were subsequently ionized. Te+, TeO+ and Te02 ions appear in the mass spectrum, together with silver and silver oxide ions. TeOj is the most intense ion and was used for ratio evaluations. The ion source was cleaned after each analysis to avoid memory effects.

Smith et al. [361] thermally ionized tellurium when searching for natural isotopic variations in tellurium-bearing terrestrial and meteoritic minerals. The results were compared with those for a laboratory standard prepared from spectroscopically pure tellurium metal. Nanogram size samples were loaded on

Table 9.40. Isotopic abundance ratios in tellurium Ref. Williams White and Cameron Smith Wachsmann De Laeter

and Yuster [43] et al. and Heumann [362] [177] [361] [299]

120Te/130Te

i 2 2 T e / i 3 0 T e

123Te/,30Te

124T e/130T e

125Te/130Te

l26Te/l30Te

l28 T e y l30 T e

Error

Method

0.00256

0.0705

0.0247

0.133

0.202

0.542

0.923

EI

(a)

0.0026

0.0722

0.0230

0.134

0.203

0.543

0.920

EI

(b)

0.0027

0.0680

0.0258

0.132

0.205

0.543

0.955

EI

0.00284 ±0.00002

0.07701 ±0.00004

0.02687 ±0.00002

0.14250 ±0.00006

0.21122 ±0.00006

0.56073 ±0.00014

0.93753 ±0.00016

2SD

PTIMS

0.0028"

0.0736 ±0.0004

0.0256 ±0.0002

0.1372 ±0.0005

0.2053 ±0.0006

0.5480 ±0.0020

0.9280 ±0.0010

ISD

NTIMS

0.00268 ±0.00003

0.07368 ±0.00004

0.02583 ±0.00003

0.13767 ±0.00007

0.20551 ±0.00005

0.54828 ±0.00015

0.92688 ±0.00014

2SD

PTIMS

" Value taken from ref. [1].

TELLURIUM 309

degassed, zone refined rhenium filaments. Ion intensities of «a 10~13A were produced, lasting for several hours, which were monitored with an electron multiplier. A magnetic sector mass spectrometer was used in this work, and the source was equipped with a liquid nitrogen cold finger to condense any hydrocarbons present. Contaminations at m/z = 119 and 125 were detected, disappearing upon increasing the filament temperature. The values shown in Table 9.40 are the mean of means of the laboratory standard and six terrestrial samples each measured several times. No natural variations in the isotopic composition were detected. All the ratio data were normalized to 126Te/,30Te = 0.56073 using a fractionation factor D determined as

D = [(126Te/,30Te)meas/0.56073]1/4 (52.1) where the subscript meas denotes the measured mean. Each isotopic ratio can then be corrected as shown, for example:

(128Te/130Te)corr = (,28Te/,30Te)meas/D2 (52.2) No electron multiplier discrimination corrections were made on the measured data.

Good agreement was observed with the data of White and Cameron [43] except for small discrepancies at the 123Te and 124Te isotopes, and reasonable agreement with those of Williams and Yuster [177]. Wachsmann and Heumann [299] applied negative thermal ionization to analyze tellurium. A magnetic sector mass spectrometer with a double rhenium filament ion source was used. A BaO coating on the ionization filament was applied to reduce the rhenium work function, and a silica gel suspension was applied to enhance ionization. The filaments were washed with nitric acid and degassed at 5 A for about 0.5 h. l-2pg of Te as H2Te03 solution was used. The solution was evaporated to dryness, then 10 pi of silica gel suspension was added. The mixture was deposited on the evaporation filament and heated to dryness. 30 pg of Ba as Ba(OH)2 was deposited on the ionization filament. This filament was heated to about 930°C at an initial rate of 0.15 Amin"1 and then at a decreased rate of 0.04 Amin ' controlled by an optical pyrometer. The evaporation filament was heated to 1.0 A at a rate of 0.05-0.1 Amin-1. Te- ion intensities in excess of 10-11 A were obtained. The results are shown in Table 9.40. It is evident that they are not in agreement with the data of Smith et al. [361] but in good agreement with De Laeter [362]. De Laeter measured the tellurium isotopic composition with a Faraday cup detector. The Te samples were dissolved in 8.5 M HCl and the element was electroplated onto degassed rhenium filaments. The ionization efficiency was improved with 25 ng aluminum (as AICI3) added together with a silica gel activator. Data were collected at 1050°C, after checking for Sn, Ba and Sb interferences at 950 °C. De Laeter also pointed out that the need for absolute tellurium isotopic ratio measurements (against calibrated isotopically enriched solutions) still exists.


Table 9.41. Isotopic abundances and atomic weight of tellurium

1 2 0 T e

122Te 123Te 124Te 125Te 126 T e

128 T e

! 3 0 T e

Atomic

Method

Smith et al. [361]

0.0960 + 0.0007 2.603 ±0.001 0.908 ±0.001 4.816 ±0.002 7.139 ±0.002

18.952 ±0.004 31.687 ±0.004 33.799 ±0.003

weight 127.5856 ±0.0003

TIMS

De Laeter [362]

0.0918 ±0.0007 2.523 ±0.001 0.885 ±0.001 4.714 ±0.002 7.037 ±0.002

18.773 ±0.004 31.736 ±0.004 34.240 ±0.003

127.6114

TIMS

Lee and Halliday [319]

0.0927 ±0.0004 2.5277 ±0.0005 0.8860 ±0.0003 4.7165 ±0.0001 7.0509 ±0.0006

18.8066 ±0.0010 31.7540 ±0.0001 34.1656 ±0.0007

127.60834 ±0.00006

ICP-MS

IUPAC [363]

0.096 ±0.002 2.603 ±0.004 0.908 ±0.002 4.816 ±0.006 7.139 ±0.006

18.95 ±0.01 31.69 ±0.01 33.80 ±0.01

127.60 ±0.03

All the quoted uncertainties are 2SD.

High precision tellurium isotope ratio measurements have recently been achieved with a double focusing multiple collector ICP-MS by Lee and Halliday [319]. The instrument is equipped with seven Faraday collectors. 1-2 ppm tellurium solutions of the Johnson & Matthey pure element, at a flow rate of 0.3 m l ' 1 were introduced into an argon ICP ion source. Typical total ion current intensity was 2.5 x 1 0 " A for a 1 ppm solution. Each sample analysis consisted of over 100 measurements of each isotope, detected simultaneously with 10 s integration time. The raw data were corrected for the time independent mass discrimination by applying the exponential law and normalized to 124Te/,28Te = 0.14853, given by De Laeter [362]. Table 9.40 summarizes the isotopic abundance ratios in tellurium.

The quality of the ICP-MS data is demonstrated in Table 9.41, where isotopic abundances and the atomic weight of tellurium are compared.

9.53 IODINE

Iodine is the fifty-third element in the Periodic Table. It has only one stable isotope at mass number 127. 129I is a radioactive, ß emitting isotope of iodine with a half-life of 1.57 x 107 years. The recent interest in the 127l/129l isotopic ratio measurement has arisen from its application to quantitative trace analysis of iodine and iodine-containing compounds of environmental and nuclear origin using isotope dilution mass spectrometry [364, 365]. A NIST SRM 4949A containing 13.90% 127I and 86.10% I29I is available for IDMS.

IODINE 311

Nier [62] ionized the vapor of elemental iodine by electron impact. Spitzer and Sites [45] evaporated 1 pg of sodium iodide at 200 °C from a tantalum filament and ionized it also by EI; Nal+ ions were monitored. Strong memory effects made it necessary to clean the ion source after each admission. Kishi and Kawano [366] developed a method for continuous monitoring of trace amounts (0.1-10ppm) of iodine in air. The instrument was constructed from a heated capillary gas inlet system and a magnetic mass spectrometer with an EI ion source heated to sa 200 °C. The electron energy and the ionizing current were 70 eV and 100 pA respectively. The I" ions were monitored with an electron multiplier. After studying the appearance potential curves, it was suggested that the I - ions were produced by dissociative electron transfer:

0 - + I 2 - + 0 + l + r (53.1)

rather than by dissociative electron capture:

I2 + e - • r + 1 (53.2)

Turnbull [234] thermally ionized barium iodide in a triple rhenium filament ion source; 10 pg of Bal2 in his instrument yielded a Bal+ ion current of 10~14A. Delmore [367] demonstrated the feasibility of measuring 1 2 7 I / ' I ratios at 104-108 levels by applying NTIMS. The ionization efficiency of iodine was increased by several orders of magnitude using LaB6 as a low work function ionizer, rather than high work function pure metals. Three types of LaB0 ionizer were developed: a smooth dense coating, a thin porous coating and a thick porous coating; the last was mechanically unstable. Delmore [368] and also Grämlich and Murphy [364] observed that the porous coatings provide maximum ion intensity, but the smooth coatings produce higher inter- and intra-analysis precision at the expense of ionization efficiency. The process of the cataphoretic deposition of LaBß onto rhenium filaments has been described in detail [364, 367, 368]. The procedure for producing the smooth dense coating following ref. [364] will be given briefly. The LaBô specified by the manu-facturer for use in producing thermionic coatings on rhenium was ground to a mesh size finer than 325, washed with pure ethanol and agitated in an ultrasonic cleaner, the mixture was centrifuged and the solvent was decanted and discarded. The washing procedure was repeated to remove any boric oxide, then the LaBg was vacuum dried and stored in a desiccator over magnesium Perchlorate. The solution used for deposition was a mixture of 100 mg LaB6 in 10 ml of anhydrous spectrograde methanol. This solution, if kept sealed from the atmosphere, was effective for 2-3 weeks. A rhenium filament previously degassed in vacuum and under a potential field at a current of 4.5 A for 1 h was placed in a specially designed borosilicate glass apparatus 1 mm away from a flat (1 cm x 1 cm) platinum foil anode and covered with the solution. The deposition of LaBö was accomplished within 5-10min by applying 60-100 V d.c, sufficient to draw a current of 2.5 mA. The uniformity of thickness of the


coatings was judged visually . The rhenium filament with the LaBö coating is easily poisoned by oxygen, hydrogen, water vapor and carbon dioxide. Reactivation is possible by stepwise heating in a vacuum of 1 x 10 7 Torr at 1200, 1300 and 1400 °C for 5 min at each temperature.

Grämlich and Murphy [364] performed isotope dilution analyses of iodine at trace levels in two biological reference materials. The materials were spiked with 129I and wet ashed and the iodine was separated as a solution of Agí dissolved in ammonium cyanide containing 10pgml~'1. A triple filament ion source was used for the ratio determinations. 50 ng of iodide was deposited on each of the two (previously degassed) rhenium sample filaments. The filaments were dried using a heat lamp and a current of 1 A for 5 min, and the current was then increased until the sample melted on the filament. When the vacuum in the ion source reached 2 x 10- 7 Torr, the ionizing LaB0-coated filament was reactivated at 1400 °C for 5 min and its temperature was reduced to 1000 °C. The sample filaments were heated in steps to 0.25 A, then after 5 min to 0.50 A, and after an additional 5 min to a final current producing a total I - ion current of 6 x l O ~ u A . Data collection started after 10min of stabilization. When samples of different isotopic composition were analyzed, memory effects from previous samples had to be controlled and cleaning of the ion source was necessary. Iodine concentrations of 2-3 ppm with 0.15-0.44% RSD were determined. The precisions, compared with those of other measuring methods, were better by factors of 40-140. Gaebler and Heumann ([365] and references therein) analyzed iodine-containing samples of diverse origin. The samples were spiked with natural or radioactive iodine, converted to an ammoniacal Agi solution and analyzed in a double rhenium filament ion source. A 10 pg amount of lanthanum as La(NÛ3)3 was deposited on the ionizing filament to reduce the work function of rhenium. For the ratio measurements, the ionization filament was adjusted to 1200-1250 °C and the sample filament was not heated. With a magnetic sector mass spectrometer precisions of 0.1 % and detection limits of 6 ppb were achieved. Carbonized and thoriated tungsten filaments were also used as ionization filaments, producing ion intensities and stabilities comparable with those for the lanthanum-treated filament.

9.54 XENON

Xenon is the fifty-fourth element in the Periodic Table. It has nine stable isotopes at mass numbers 124, 126, 128, 129, 130, 131, 132, 134 and 136, with relative abundance of 0.10, 0.09, 1.91, 26.4, 4.1, 21.2, 26.9, 10.4 and 8.9% respectively [1].

The isotopic composition of xenon in the terrestrial atmosphere was determined by Nier [158] and by Podosek et al. [369]. The abundance of

CESIUM 313

Table 9.42. Isotopic abundance ratios in terrestrial atmospheric xenon l24Xe/130Xe

0.0236 0.02335

±0.00014

,26Xe/l30Xe

0.0220 0.02176

±0.00014

128Xe/l30Xe

0.4709 0.4708

±0.0017 0.4680

l2 'Xe/ ,30Xe

6.488 6.505

±0.015 6.519

13IXe/l30Xe

5.188 5.224

±0.012 5.261

132Xe/l30Xe

6.599 6.614

±0.012 6.613

l34Xe/l30Xe

2.562 2.567

±0.007 2.562

l36Xe/l30Xe

2.176 2.182

±0.006 2.207

Note: The ratios in the upper, middle and lower line were taken from refs. [158], [369] and [370] respectively. The quoted uncertainty is 2SD.

seven of the nine xenon isotopes was determined also by deKoning et al. [370]. Other sources of natural xenon, including lunar and meteoritic samples, were also studied for their isotopic composition [3]. The 131Xe, 132Xe, 134Xe and 136Xe isotopes are produced by fission in a uranium nuclear reactor.

Podosek et al. [369] used a magnetic sector mass spectrometer and com-mercial xenon (Linde, USA) for instrumental discrimination calibrations. The correction factor was calculated from the uncorrected (measured) l29Xe/I32Xe ratio, assuming linear discrimination and the value 129Xe/132Xe = 0.9833 obtained by Nier [158]. The calculated discrimination was 0.39% u_ 1, favoring the lighter isotopes, which is very close to the {m\/m2)x'2 mass dependence of 0.38% expected for electron multiplier mass discrimination. The extraction of xenon from the samples and its purification were described by Podosek et al. [369]. DeKoning et al [370] used a double focusing magnetic sector mass spectrometer with reverse geometry. In Table 9.42 the isotopic ratios of terrestrial atmospheric xenon are shown.

9.55 CESIUM

Cesium is the fifty-fifth element in the Periodic Table. It has one stable isotope at mass number 133.

Cesium is a metallic element with a low ionization potential of 3.894eV. White et al. [164] showed that Cs is monoisotopic. A solution of CSNO3 was evaporated to dryness on a niobium filament and thermally ionized. Cs+ ions were monitored at a temperature of 500 °C. Spitzer and Sites [45] ionized cesium chlorostannate (Cs2Sn00) from a tantalum filament at 600-800 °C. Shields [371] used cesium nitrate in a 3mgl _ 1 Cs and 0.1 N HNO3 solution: 60 ng cesium and a single platinum filament ion source were applied. Inghram et al. [372] analyzed the abundance of unstable l35Cs and ,37Cs isotopes, pro-duced by uranium fission. Kiselev and Glasunow [373] used tungsten filaments for the same purpose.


9.56 BARIUM

Barium is the fifty-sixth element in the Periodic Table. It has seven stable isotopes at mass numbers 130, 132, 134, 135, 136, 137 and 138. The isotopic composition is 0.106, 0.101, 2.417, 6.592, 7.854, 11.23 and 71.70% respectively [1],

Nier [309] evaporated metallic barium and ionized the vapor by electron impact. The author evaluated his precision as 2% for the I34Ba-I37Ba and 4% for the i30Ba-l32Ba isotopic abundance.

Barium has a low ionization potential of 5.21 eV, thus the element is conveniently thermally ionized. Turnbull [234] used a single tantalum filament ion source; 1 pg BaO loaded from a water solution and dried on the filament yielded a Ba+ ion current of about 10~13A. Spitzer and Sites [45] used Ba(N03)2. The loaded tantalum filament was slowly heated in air until the sample melted and then it was slowly heated in the ion source to avoid sample flaking. Stable Ba+ and also BaO+ ion beams were produced between 1300 and 1650 °C, their relative intensity depending on the temperature. Before each analysis, the filament was degassed at 6 A for 2 min at a pressure lower than 5 x 10"5 Torr to eliminate Ba+ background. Umemoto [374] loaded BaS04 onto a tantalum filament in a single filament ion source. The filament was heated to an optimal temperature within 2 -4 h and the measurement was carried out for 12-14 h by magnetic scanning from the lowest to the highest mass and back to the lowest. From this scan one set of ratios, relative to I38Ba+, was calculated, and was accepted only if the signal intensity difference was smaller than 3%. When the ratios were plotted in the order of measurement, no isotope fractionation could be revealed. Eugster et al. [375] used the double spike isotopic dilution technique to correct the mass fractionation: 1 pg of barium as chloride was analyzed with a single tantalum filament ion source and a magnetic sector mass spectrometer. 134Ba and 137Ba were the enriched isotopic spikes used. Detailed equations for the fractionation corrections and error analysis were given. De Laeter and Date [376] also used a magnetic sector mass spectrometer set to a mass resolution of « 400. A triple rhenium filament ion source with previously degassed filaments was used. No barium background from the filaments was observed. One pg of barium, loaded as BaCl2, produced a 138Ba+ ion intensity of R¿ 10~12 A, which lasted for several hours without a marked decrease; however, lOpg of Ba were used in practice. The detection system included an electron multiplier with a gain of 104, and a vibrating reed electrometer with a 109 ÍÍ input resistor. The data were digitally displayed by an electronic counter via a voltage-to-frequency converter and also on a chart recorder. The ion currents were monitored at a constant electron multiplier gain and for each isotope in the same electrometer range. Within an 8 h interval a time dependent mass fractionation of 2% for the 130Ba/138Ba ratio was observed. By measuring the isotopic ratios of NIST rubidium and strontium

LANTHANUM 315

Table 9.43. Isotopic abundance ratios in barium ( x 104) l30Ba/138Ba

14.1 14.215

±0.020 14.76

±0.02 15.47

±0.06 14.71 14.0

l32Ba/138Ba

13.6 13.712

±0.017 14.12

±0.02 14.68

±0.06 14.13 14.0

134Ba/l38Ba

337 331.97 ±0.34 337.1 ±0 .3 344.9 ±0 .3 336.3 338

135Ba/138Ba

920 908.0 ±0 .8 919.4 ±0 .2 937.4 ±0 .9 919.8 920

136Ba/l38Ba

1090 1086.8

±0 .9 1095.3

±0.5 1110.9 ±0 .9

1097.0 1090

137Ba/138Ba

1680 1547.9

±1 .0 1566.5

±0 .5 1579.2

±1 .0 1569.3 1580

Ref.

[309] [374]

[375]

[376]"

[376]* [377]

" Mean measured values of terrestrial and meteoritic Ba samples, uncertainty quoted as 3SD. b Terrestrial Ba, corrected for mass fractionation by 0.625% u"1.

isotopic standards, the authors established a fractionation correction factor of 0.625% u_ 1 which was used to correct the measured ratios for barium. An excellent agreement with Eugster et al. [375] was noted. Neither research team, Eugster et al. [375] or De Laeter and Date [376], could confirm the isotopic variance of Ba in meteorites, compared with terrestrial barium, previously reported by Umemoto [374]. Chow [377] also determined the Ba isotopic composition. Table 9.43 summarizes the isotopic ratio data from refs [309,374, 375,376 and 377].

9.57 LANTHANUM

Lanthanum is the fifty-seventh element in the Periodic Table. It has two naturally occurring isotopes at mass numbers 138 and 139, and their isotopic abundance is 0.0902 and 99.9098% respectively [1]. 138La undergoes a branched radioactive decay by ß emission to the stable 138Ce and by electron capture to the stable 138Ba nuclei.

l38La -^ , 3 8 Ce ± ß~, rI /2 = 2.69 ± 0.24 x 10" y (57.1)

138La ± e- - , . 1 3 8 Ba, r1/2 = 1.51 ±0 .10 x 10u y (57.2)

A rare-earth geochronometer is based on the ß decay reaction (57.2). Lanthanum has an ionization potential of 5.58 eV and therefore is easily

thermally ionized. Isotope ratio determinations of this element were reported by Inghram et al. [378], White et al. [164], Masuda étal. [379], Yanagita [380] and Makishima et al. [381]. The lanthanum isotope ratio is determined from the LaO+ ion currents, which are more stable and have higher intensity by a factor of « 100 than the atomic ion. Makishima et al [381] used a fully automatic,


Table 9.44. Mass spectrum of lanthanum oxide mlz LaO+ ions Isobaric interferences

154 155 156

B 8 L a i 6 0 + 138La170+ + l39La160+ l38La280++ ,39La170+

138 C e 16 0 + ; 154 G d + i 1 5 4 S m + ,55Gd+

157 139L a180+

Table 9.45. Isotopic abundance ratio in lanthanum 138La/139La Error Ref.

0.000891 0.000893 ±0.000016 0.0008873 ±0.0000024 0.000890 ±0.000007 0.0009025 ±0.0000005

2SD 2SD 2SD

[378] [164] [379] [380] [381]

Table 9.46. Isotopic abundance ratios in 1 7 0 / 1 6 0 1 8 0 1 6 0 Error

0.0003749 0.002044 0.000387 0.00211 0.0003916 + 14 x 10~7 0.002129 ±10 x IO"6 2SD

oxygen

Method

EI TIMS TIMS

Ref.

[91] [153] [381]

double focusing thermal ionization mass spectrometer, equipped with a single rhenium filament ion source and a Faraday cup collector. La2Û3 was used and (5-6) x 10""" A LaO+ ion currents were monitored. The oxygen isotopic ratio was also determined, for which the singly isotopic praseodymium oxide was chosen, as it easily forms monoxide PrO+ ions when thermally ionized. Thus, 10 pg Pr were loaded onto a rhenium filament in a triple filament ion source. A total ion beam current of 6 x 10~" A for PrO+ ions was monitored for 15h, accumulating 900 ratios. 1 4 4Nd1 60+ and 140Ce16O+ were also monitored to detect Nd and Ce interferences if any. There were only very small or undetec-table contributions from these elements. In both ratio measurements, lanthanum and oxygen, mass fractionations were not corrected. Tables 9.44, 9.45 and 9.46 show the La+ ions in the mass spectrum and possible interferences, and deter-minations of the lanthanum and the oxygen ratios respectively. The last table is given to compare the oxygen isotopic ratios used by Makishima et al. [381] with other accepted data. It is evident that these are slightly different from the results of other measurements.

CERIUM 317

9.58 CERIUM

Cerium is the fifty-eighth element in the Periodic Table. It has four stable isotopes at mass numbers 136, 138, 140 and 142, with an isotopic composition of 0.19, 0.25, 88.48 and 11.08% respectively [1].

Cerium has a favorable ionization potential (IP = 5.47eV) for thermal ionization. Spitzer and Sites [45] placed finely divided cerium oxide (Ce02) on a tantalum filament and partially digested it with concentrated nitric acid. The sample was dried and the filament was slowly heated to dull red heat ( « 750 °C). One pg of cerium, heated between 1300 and 1600°C, produced a CeO+ ion current of 10~10 A for more than 1 h. The observed CeO + /Ce + ratio was greater than 100/1. No cerium interference from previous samples was observed. Corrections for l 8 0 may be necessary. Umemoto [374] compared the isotopic abundance of meteoritic and terrestrial cerium. Ceric nitrate chemical reagent was further purified by iodate precipitation and ion exchange (Dowex 50W-X8). This procedure provided a laboratory standard completely free from neodymium and samarium. The ceric nitrate was loaded onto a tantalum filament in a single filament ion source. The filament was heated to an optimal temperature within 2-4h , and the measurement was carried out during 12-14h by magnetic scanning from the lowest CeO+ mass to the highest mass and back to the lowest. From this scan one set of ratios, normalized to 140Ce+, was calculated, and was accepted only if the signal intensity difference for the two identical ions was smaller than 3%. No isotope fractionation has been reported.

Tanaka and Masuda [382] proposed the 138La isotope decays by ß~ emission to 138Ce and the electron capture to 138Ba for use as the La/Ce geochronological dating method. The Johnson & Matthey Ce reagent, JMC 304 was used as a laboratory standard. Cerium isotopes were measured as CeO+ with a triple rhenium ion source, « 5 pg Ce were loaded, and ion currents in the (0.3-2) x 10~n A range were monitored [383]. Generally 100 scans were accumulated in one run during 3-4 h, and 2 -4 runs were measured for each sample. The data were corrected for l 8 0 abundance, and 136Ce/142Ce = 0.01720 was used for normalization. 140Ce/142Ce was evaluated as 7.992 and the mean 138Ce/142Ce ratio for 15 runs was 0.0228559 ± 0.0000011 (2SD). The 138Ce/142Ce ratio is usually measured as the variable in the La/Ce dating method, ignoring the 140Ce16O+ ion, which is off the amplifier scale. Dickin [384] used tantalum sample filaments and rhenium as ionization filament and a single Faraday collector for data acquisition. Traces of BaO and BaF, which interfere with 138Ce, disappeared at the early stage of the analysis. La, Pr, Nd and Sm interferences also appeared during the first scans, but then decreased to negligible levels. Makishima et al. [154] measured the cerium isotopic ratios with a fully automatic double focusing thermal ionization mass spectrometer with a Faraday collector. Johnson & Matthey reagent JMC 304 was used as a Ce standard. Thus, « 5 pg Ce were loaded with phosphoric acid onto rhenium


filaments in a triple Re filament ion source. This source provided the most stable CeO+ ion beams, better than those with Re or Ta single filament and Ta-Re-Ta triple filament ion sources. All the cerium isotopes were monitored and the total ion beam intensity was set at 1 x 10~10 A. The measurement duration was 11-12 h and more than 300 ratios were collected. 136Ce/142Ce = 0.01688 was used for normalization. Masses 153, 155, 157, and 160 were monitored for possible BaO+, LaO+, PrO+ and NdO+ interferences. Barium was not detected and the other interferences were very small. Oxygen corrections were made using l 8 0 / 1 6 0 = 0.002129 and 1 7 0 / 1 6 0 = 0.0003916, determined by measur-ing the ion intensities of monoisotopic PrO+. Details of this procedure were described in Section 9.57.

Xiao et al. [385] developed a cerium isotopic ratio determination procedure, measuring Ce+ ions. A single tantalum filament ion source was used with a fully automatic multiple collector thermal ionization mass spectrometer. The filament was degassed at 4.0 A for 1 h, expelling Ba impurities to avoid 136Ba+

and l38Ba+ interferences. A 3 pi portion of a graphite slurry (about 100 pg graphite), prepared by mixing spectroscopic grade graphite with an 80% ethanol-20% water (v/v) solution, was loaded onto the center of the filament, dried and followed with 20 pg of Ce. Johnson & Matthey reagent JMC 304, converted to the nitrate, was used as Ce standard. The 140Ce+ ion intensity was adjusted to (1.5-2) x 10~ n A with a 2.8-3.1 A filament current. Data were collected by magnetic peak switching after about 1 h from the start of filament heating, when the I37Ba+ ion disappeared. The contribution of 142Nd+ to ,42Ce+ was corrected by monitoring 143Nd+ and taking 142Nd/143Nd = 2.2274. The minor 138La isotope contribution was corrected by monitoring 139La+

and taking ,38La/139La = 9.02xl0~4. No isotopic fractionation was observed during the 150 min time interval of data acquisition. The highest Ce+ ion intensity emission (5 x 10-11 A) and Ce + /CeO + ratio (10:1) were observed at a temperature of 1260 °C when a slurry of Ce(N03)3 was used. The chloride was less favorable and in the sulfate the Ce + /CeO + ratio was only « 0 . 0 1 .

Recently Chang et al. [386] described an absolute determination of the isotopic composition of cerium using two samples of enriched Ce isotopes, 140Ce (99.5 at%) and 142Ce (94.2 at%). Only neodymium, 142Nd, was a potential interference in the specified impurities. Purification reduced its level below 0.01%. A total impurity analysis was carried out for both isotopically enriched samples. A double rhenium filament ion source was used, and the filaments were degassed at 5.5 A for 1 h and then at 6.0 A for 10min. Four pg of cerium (as nitrate) were loaded and dried with a current of 1.5 A for RS 6 min. The ionization filament was set to 5.7 A and the sample filament was slowly heated to 1.5 A, and the ion current of the major isotope was adjusted to 10- 1 1 A. Four Faraday collectors were simultaneously used to monitor the atomic ions, and 60 ratios were collected. A mass discrimination factor was experimentally

CERIUM 319

Table 9.47. Isotopic abundance ratios in cerium 136Ce/140Ce

0.002181 0.0021526a

± 0.0000029 0.0021286

±0.000010 0.002097c

±0.000005 l36Ce/142Ce

0.01720'

0 .01688M

138Ce/140Ce

0.002825 0.002866

±0.000004 0.002871

±0.000010 0.002846c

±0.000005 138Ce/l42Ce

0.0228559 ±0.0000011

0.0225762 ±0.0000014

I42Ce/140Ce

0.1251 0.12525

±0.00008 0.12523

±0.00005 0.125237 e

±0.000010 , 4 0Ce/ l 4 2Ce

7.992

7.9471 ±0.0003

Error

2SD

SD

2SD

2SD

2SD

Ref.

[45] [374]

[385]

[386]

[382]

[154]

0 Mean of 30 ratios. b Johnson & Matthey JMC 304 Ce reagent. 0 Fractionation corrected mean of six natural samples. d Normalization factor.

Table 9.48. Interferences on cerium oxide ions from isobaric rare earths and rare earth oxide ions

CeO+ ion

136 C e 16 0 +

138Ce160+

140Ce16O+

142C e160+

'36Ce180+

138 C e 18 0 + 140Ce18O+

Interfering ions , 3 6Ba1 60+, 1 3 8Bal 60+, 135Ba19p + 138BaI80+, 142 N d 16 0 + >

152Sm+ 136Ba180+,

I38La180+, 141pr170+

I 3 8La1 60+ ,

139La,70+

, 5 4Sm+

established /140 142, /140/-^ = ( 1 - C e / - C e ) a b s / ( - C e / ' w C e ) o b s (58.1)

1̂42/140 = 100337 and /Ci3g/i40 = 0.99663, with Ar,36/,4o = Q.99326, were calculated. Six samples, from Mongolia (1), China (3), UK (1) and USA (1), were analyzed without observing any natural fractionation.

The isotopic ratio measurements in cerium are summarized in Table 9.47. The possible interferences are listed in Tables 9.48 and 9.49.

ISOTOPE RATIO MEASUREMENT PROCEDURES

Table 9.49. Isobaric interferences to atomic cerium ions

Ce+ ion Interfering ions 136Ce+ 136Ba+

i38C e+ i 3 8 B a + i i38L a+ 140Ce+ None 142Ce+ 142 N d +

9.59 PRASEODYMIUM

Praseodymium is the fifty-ninth element in the Periodic Table. It has only one stable isotope at mass number 141 [320]. It is easily thermally ionized (IP = 5.42eV). Spitzer and Sites [45] placed finely divided praseodymium oxide (Pr203) on a tantalum filament and partially digested it with concentrated nitric acid. The sample was dried and the filament was slowly heated to a dull red heat («750°C). One pg of praseodymium, heated to between 1400 and 1700 °C, produced a PrO+ ion current of 10~10A for more than l h . The observed PrO + /Pr + ratio was greater than 100:1. No praseodymium interference from previous samples was observed. The 157Gd+ ion is a potential isobaric interference.

9.60 NEODYMIUM

Neodymium is the sixtieth element in the Periodic Table. It has seven stable isotopes at mass numbers 142, 143, 144, 145, 146, 148 and 150, with an isotopic composition of 27.13, 12.18, 23.80, 8.30, 17.19, 5.76 and 5.64% respectively [1]. The 143Nd isotope is also produced by a decay of 147Sm with a decay constant of 6.54 x 10 1 2 y _ 1 ; thus, in samarium/neodymium-bearing minerals, the 143Nd/144Nd isotopic ratio is a variable related to the age of the mineral. The samarium/neodymium method is applicable for dating meteorites and terrestrial and lunar rocks.

9.60.1 Isotopic Ratio Determinations on NdO+ Ions

Neodymium has an ionization potential of 5.49 eV, which is favorable for thermal ionization. White et al. [164] used a single tungsten filament ion source and specially purified rare earth compounds to avoid potential isobaric interferences. Spitzer and Sites [45] loaded finely divided neodymium oxide on a tantalum filament and partially digested it with concentrated nitric acid. The sample was dried and the filament was slowly heated to dull red heat,

NEODYMIUM 321

Table 9.50. Isotopic abundance ratios in neodymium I43Nd/,42Nd

0.4513 0.4489 0.4485

l44Nd/l42Nd

0.8719 0.8797 0.8774

1 4 5 N d / 1 4 2 N d

0.3037 0.3062 0.3056

146Nd/142Nd

0.6260 0.6352 0.6322

148N d / /142N d

0.2077 0.2114 0.2113

150Nd/l4ZNd

0.2037 0.2073 0.2069

Ref.

[164] [45]

[388]

( « 750 °C). One pg of neodymium ionized in the temperature range 1300-1600°C produced an ion current of 10~10A for more than 1 h. The observed NdO+/Nd+ ratio was greater than 100:1. No neodymium interferences from previous samples were observed. Corrections for 1 7 0 and 1 80 were necessary. Richard et al. [387] used 200 ng neodymium on a single, zone refined rhenium filament ion source. NdO+ ions with a 142NdO+ ion intensity of 2 x 10~12 A, which were stable for several hours, were monitored at filament temperatures of 1280-1300 °C. A method involving dissolution, Nd separation and purification for various terrestrial rock samples was also developed. Holliger and Devillers [388] also thermally ionized natural Nd. Isotopic ratios in neodymium are presented in Table 9.50. The data shown were not corrected for isotopic fractionation.

Wasserburg et al. [153] performed very detailed and precise Nd isotope ratio measurements. Ultrapure Nd metal was used. The monitored ion was NdO+, and the calculations were based on their own oxygen isotope ratio determinations of 1 8 0 / 1 6 0 = 0.00211 and 1 7 0 / 1 6 0 = 0.000387. These values differ from Nier's [91] accepted oxygen values of 0.002045 and 0.0003708 respectively. It was suggested that the difference in the isotopic composition may be due to mass fractionation relative to atmospheric oxygen occurring during the oxidation of Nd on the rhenium filament while the sample is loaded, or to very slight air leakage into the ion source while the solid sample (Nd203) is evaporated and ionized to form gaseous NdO+ and 0 2 . Alternatively, the difference may be due to mass fractionation in the mass spectrometer used by Nier. Table 9.51 shows the isotopic composition of normal neodymium as

Table 9.51. Isotopic abundance ratios in normal neodymium, from [153] 142 N d /144 N d

1.138305 1.141827 1.138266

,43Nd/l44Nd

0.511847 0.512638 0.511836

l45Nd/144Nd

0.348956 0.348417 0.348968

146Nd/,44Nd

0.724134 0.7219h

0.724109

148Nd/l44Nd

0.243075 0.241578 0.243079

150Nd/144Nd

0.238619 0.236418 0.238581

Notes

a b c

" Mass fractionation correction factor l,6Nd/142Nd = 0.636151, oxygen correction 1 80/1 60 = 0.00211 and 1 7 0 / , 6 0 = 0.000387. b Mass fractionation correction factor 146Nd/l44Nd = 0.7219, oxygen correction l 80 / 1 60 = 0.00211 and l 7 0 / 1 6 0 = 0.000387. c Mass fractionation correction factor I46Nd/,42Nd = 0.636151, oxygen correction 1 80/1 60 = 0.002045 and , 7 0 / l 6 0 = 0.0003708.


evaluated by Wasserburg et al. [153]. Lines 1 and 3 show a 21 ppm difference in die calculated I43Nd/144Nd ratios based on the two sets of oxygen isotopic composition data. A neodymium isotope fractionation factor, F, per atomic mass unit was defined as:

,1/4 1 (60.1) F = [ r °Nd/^Nd) w / r °Nd/""Nd) r

where the normalization factor is (146Nd/142Nd)w = 0.636151 [153]. Neodymium isotope ratios were calculated from the oxide data and then

corrected for instrumental mass fractionation using:

('Nd/144Nd)corr = ('Nd/144Nd)meas x [1 + F](i-144) (60.2) O'Nions et al [389] proposed a normalization factor 146Nd/144Nd = 0.7219.

Richard et al. [387] developed a correction procedure based on the isobaric interferences of rare earth monoxide ions. The decay of 147Sm and the growth of radiogenic l43Nd are described by the equation:

143Nd/144Nd = (143Nd/l44Nd),. + (147Sm/144Nd) x (eA< - 1) (60.3) thus interferences at mass numbers 159 and 160 as summarized in Table 9.52 may yield erratic 143Nd/ Nd ratios. The presence of 144Sm after the chemical separation may be checked with the ¿163/1162 ratio:

¿163/M62 = (147Sm160+/¿162) ± (145Nd180+//162) + (146NdnO+/¿i62). (60.4) In the absence of Sm, and using Nier's [91] isotopic composition of oxygen,

the sum of the second and third terms is 1.36 x 10-3, which is larger only in the presence of samarium. The interferences of cerium and praseodymium oxides at mass numbers 159 and 160 may be detected and corrected for in a similar way.

Table 9.52. Interferences on NdO ions from isobaric rare earth monoxide ions Mass No.

156 157 158 159 160 161 162

160 161 162 163

Isobaric interfering ions

,40Ce16O+ 140Ce17O+ i4ipr160+ 140Ce,8O+ 14 ,Pr170+ I 4 2Cel 60+

i 4 . P r i 8 0 + i 4 2 C e i 7 0 + 142Ce180+

145N d160+ ! 4 5 N d 1 7 0 + H 5 N d 1 8 0 +

142N d160+ l 4 2 Nd 1 7 0 +

1 4 2 N d l 8 0 +

146N d160+ 146N d170+

, 4 3 Nd 1 6 0 + ,43Nd170+ 143 N d 18 0 +

144Sm160+ l 4 4Sm, 70+ ,44Sm180+

i 4 4 N d t 6 0 +

144N d170+ 144Nd180+

147Sm160+

NEODYMIUM 323

9.60.2 Isotopic Ratio Determinations on Nd+ Ions

In recent years many laboratories involved in geochronological research prefer to use double or triple filament ion sources for Nd isotope ratio analyses. In general, the great advantage of this technique is the direct production of the metallic Nd+ ions, the reduced number of interfering ionic species, no need for , 7 0 and l 8 0 corrections, and a better control of the sample evaporation and ionization parameters. Consequently, fewer and simpler interference corrections are needed and better ratio precisions and accuracy can be achieved. Thirlwall [390] analyzed neodymium with a triple filament ion source. The sample, as a nitrate solution, was loaded onto the previously degassed tantalum side filaments and dried at 1 A in air. Care was taken to avoid introduction of traces of sample onto the center filament, as this can produce interfering oxide ion emission. The center filament current was 4.5-4.9 A and the sample filament current was 2.1-2.3 A. Ion beam intensities of 10~ n A were recorded. No interference from BaO+ could be observed. The interferences of isobaric rare earth and barium monoxide ions on neodymium ions are summarized in Table 9.53.

It is a common practice to calibrate mass spectrometers with the La Jolla or the US Geological Survey standard basalt BCR1 neodymium isotopic standard. La Jolla is a pure Nd standard. The USGS BCR1 standard contains 26 and 6.6 ppm of neodymium and samarium respectively. A 100 mg portion of the basalt is dissolved and the Nd is separated and purified, usually following the method developed by Richard et al. [387]. Double or triple filament ion sources are used with rhenium ionization filaments and tantalum or rhenium sample filaments, loaded with 100 ng Nd. The data collection was made with single, triple or five Faraday collector detection systems, and 143Nd+ ion beam inten-sity of 4 x 10~u A was monitored. A combined procedure, using five collectors switched to three positions to monitor the 142Nd, 143Nd, 144Nd, 145Nd and 146Nd isotopes and correcting for possible ,42Ce and 144Sm traces, has been developed [391]. The data were normalized to 146Nd/144Nd = 0.7219, the power law was used to correct mass fractionation, and I42Ce/140Ce = 0.1250 and 144Sm/l47Sm = 0.2501 was taken to correct isobaric interferences. This

Table 9.53. Interferences on neodymium ions from isobaric rare earth and barium monoxide ions

Nd+ ion Interfering ions 142Nd+ 142Ce+ 144 N d + 144 S m +

i 4 6 N d + ( 1 3 0Ba1 6O+) 148 N d + 1 4 8 S m + ( 1 3 2 B a 1 6 0 + ) 150Nd+ lS0S m+ ( 1 3 4 B a l 6 0 + )


Table 9.54. TIMS 143Nd/144Nd isotopic ratio in La Jolla and USGS BCR1 neodymium isotopic standards

i43Nd/i44Nd Normalization factor Notes Ref.

La Jolla

0.511859 + 0.000013 0.511847 + 0.000020 0.511859 0.511876 + 0.000011 0.51185 + 0.00002 0.511857 + 0.000003 0.511861+0.000003 USGS BCR1

0.512638 0.512655 0.51265 + 0.00002 0.512636 + 0.000012 0.512638 + 0.000025

(1) H6Nd/144Nd = 0.7129. (2) l46Nd/145Nd = 2.07199. Notes: SF, single filament; DF, double filament; TF, triple filament; SC, single collector; TC, triple collector; FC, five collectors; MC, muiti collector; SD, standard deviation.

procedure was used for instrumental calibration with the BCR1 standard by Platzner et al. [392]. Three separate dissolutions of 100 mg each were made, and altogether 10 samples were run. The results, together with data reported for the 143Nd/144Nd isotope ratio by other laboratories for the two isotopic standards, are summarized in Table 9.54. Wasserburg et al. [153] also measured the isotopic ratios of normal samarium; the Sm+ ion was monitored and the data were corrected for mass factionation. The results are presented in Table 9.55 A series of calibrated natural and spike samarium, neodymium and samarium/neodymium solutions was prepared by these authors for geochrono-logical applications, and aliquots of them are available upon request.

Recently, Takashima and Masuda [397] proposed measurement of the 143Nd/145Nd ratio for geochronological studies, instead of 143Nd/144Nd, and use of the 146Nd/145Nd ratio as normalization factor instead of 146Nd/144Nd. This technique does not require the separation of Nd from other rare earth elements, since 143Nd, 145Nd and 146Nd have no isobaric interferences. The data acquisition was made with a triple Faraday collector detection system, switching between four positions. The technique was successfully tested with the La Jolla standard and natural samples. The results for the La Jolla standard are also given in Table 9.54. The authors pointed out that the analytical errors in measuring the 143Nd/145Nd ratio are « 2-3 times the errors for the 143Nd/144Nd

(1) TF, SC, 2SD [393] (1) DF, SC, 2SD [393] (1) TF, FC, [394] (1) ISD [395] (1) TF, MC, 2SD [396] (1) TF, TC, ISD [397] (2) TF, TC, ISD [397]

(1) SF, 2SD, [153] (1) [395] (1) TF, SC, 2SD, [398] (1) TF, SC, 2SD, [399] (1) TF, FC, 2SD, [392]

NEODYMIUM 325

Amperes

1.00E-10

1.00E-11

1.00E-12

1.00E-13

1.00E-14

1.00E-15

1.00E-16 0 10 20 30 40 50

time(min)

Figure 9.13. Neodymium ion profile in the total vaporization isotope ratio measurement procedure. A, adjustment phase; B, acquisition phase; C, shut down phase. (Reproduced by permission of Elsevier Science NL from J.C. Dubois et al, Int. J. Mass. Spectrom. Ion Processes, 120, 163 (1992))

ratio when measured with the five Faraday collector procedure. The reason is claimed to be the lower abundance of 145Nd compared with 144Nd.

Dubois et al. [400] applied the total vaporization method for the isotopic analysis of several rare earth elements. Thus, 20 ng of Johnson & Matthey Nd (Nd203 dissolved in 3 N nitric acid) were loaded onto a rhenium sample filament in a triple filament ion source. The ionization temperature was maintained at 2300 °C, and the total ion current was set to 1 0 _ n A. Under these conditions the observed NdO+/Nd+ ratio was lower than 10~4. The mass spectrometer was equipped with five adjustable Faraday collectors simulta-neously monitoring the ion currents. Special software was developed to run the instrument in this mode of operation. The duration of an analysis is 40-50 min, and a typical ion current vs. time profile is shown in Figure 9.13. Seven samples analyzed by this method yielded the following means: l43Nd/144Nd = 0.51099±0.00002, l 4 5Nd/1 4 4Nd= 0.34870 + 0.00001, and 146Nd/144Nd = 0.72333 ±0.00008. The quoted uncertainties are 2SD. The authors proposed the value 146Nd/144Nd = 0.72333 as a new normalization ratio in neodymium isotope ratio analysis. It was also shown that, when the same material was analyzed by the conventional single collector TIMS procedure, l43Nd/144Nd = 0.51097 was observed, which is in excellent agreement with the above value.

Walder et al. [318] used an inductively coupled plasma source with a double focusing, multiple collector mass spectrometer to measure the isotopic ratios of the La Jolla neodymium standard. The solution contained 1 ppm of Nd, six samples were measured, each with use of » lOOng of Nd, and the duration of measurement was 100 s. The mean 143Nd/144Nd ratio was 0.511825 ±


0.000039 (2SD), compared with the accepted value of 0.511859. Six samples of the same solution contaminated with 0.5 ppm of samarium yielded 143Nd/144Nd = 0.511854±0.000059. In both cases the data were normalized to 146Nd/145Nd = 2.07179, and in the second set of data the 144Sm interference was corrected with the measurement of 147Sm and the assumed ratio 147Sm/l44Sm = 4.8389 [1.] It should be noted that in TIMS the separation and purification of Nd from Sm is imperative.

9.61 PROMETHIUM

Promethium is the sixty-first element in the Periodic Table. It has no naturally occurring isotopes. Fourteen isotopes of promethium with atomic masses from 141 to 154 are known. They are produced by fission of uranium and neutron bombardment of neodymium. 147Pm has the longest half-life at 2.64 years.

9.62 SAMARIUM

Samarium is the sixty-second element in the Periodic Table. It has seven stable isotopes at mass numbers 144, 147, 148, 149, 150, 152 and 154, with an iso-topic composition of 3.1, 15.0, 11.3, 13.8, 7.4, 26.7 and 22.7% respectively [1]. The 147Sm isotope is an a emitter, decaying to 143Nd with a decay constant of 6.54 x 10",2y~'. The geochronological samarium/neodymium dating method is based on this process.

Samarium is easily thermally ionized (IP = 5.63 eV). Spitzer and Sites [45] placed finely divided samarium oxide (Sm203) on a tantalum filament and partially digested it with concentrated nitric acid. The sample was dried and the filament was slowly heated to dull red heat, ( « 750 °C). One pg of samarium, heated between 1400 and 1700°C, produced a Sm+ ion current of 10"10 A for more than 1 h. The observed Sm + /SmO + ratio was greater than 100:1. No samarium interference from previous samples was observed. Lugmair et al. [401] reported data for the isotopic composition of terrestrial samarium. The isotopic ratios were normalized to 147Sm/152Sm = 0.56081. Uncertainties quoted as 2SD were in the range 0.03-0.006%. Holliger and Devillers [388] also measured the isotopic ratios of natural Sm. Wasserburg et al. [153] performed precise measurements of normal terrestrial samarium isotopic ratios. The Sm+ ion was monitored and the data were corrected for mass fractionation using 148Sm/154Sm = 0.49419. Uncertainties quoted as 2SD were better than 0.007%. All the data given in Table 9.55 were recalculated relative to 149Sm. A series of calibrated natural and spike samarium, neodymium and samarium/ neodymium solutions, intended for geochronological studies, were prepared by the authors and aliquots were made available upon request.

SAMARIUM 327

Table 9.55. Isotopic abundance ratios in samarium 144Sm/l4S,Sm 147Sm/149Sm 148Sm/149Sm 130Sm/,49Sm l52Sm/149Sm l54Sm/14«Sm Ref.

0.2234 0.2226 0.2233 0.22249 0.22383

±0.00014 144Sm/l52Sm

0.114956" ±0.000013

0.114962" ±0.000002

1.0824 1.0851 1.0860 1.08507 1.08680

l48Sm/152Sm

0.420447 ±0.000005

0.420456 ± 0.000004

0.8127 0.8135 0.8121 0.81348 0.81419

±0.00006

0.5380 0.5340 0.5332 0.53399 0.53366

±0.00015

1.9320 1.9349 1.9306 1.93477 1.93040

±0.00032 149Sm/,52Sm ,50Sm/152Sm 154

0.516839 0.276005

1.6421 1.6463 1.6402 1.64609 1.64008

±0.00035

Sm/152Sm

0.850813 ±0.000009 ±0.000002 ±0.000035

0.516862 0.276002 0.850824 ±0.000003 ±0.000002 ±0.000006

[451 [4011 [388] [1531 [400]

[4021

[404]

" Normalization factor: 147Sm/,52Sm = 0.56083.

Maas and McCulloch [402] used a double Re filament ion source; 1-2 pg of samarium were loaded at sub-red heat onto a degassed filament, and Sm+ ions were collected with a multiple (seven) Faraday collector mass spectrometer in the static mode. The raw data were corrected for instrumental mass discrimina-tion by normalizing to ,47Sm/152Sm = 0.56083 [403].

Dubois et al. [400] applied the total vaporization method for the isotopic analysis of samarium. Twenty ng of Johnson & Matthey Sm (Sm203 dissolved in 3N nitric acid) were loaded onto a rhenium sample filament in a triple filament ion source. The ionization temperature was maintained at 2300 CC and the total ion current was set to 10 n A. Under these conditions the observed SmO+/Sm+ ratio was lower than 10~4. The mass spectrometer was equipped with five adjustable Faraday collectors simultaneously monitoring the ion currents. Special software was developed to run the instrument in this operation mode. The duration of an analysis is 40-50 min, and a typical ion current vs. time profile is shown in Figure 9.13. Two isotopic ratios, 147Sm/149Sm = 1.08680 ± 0.00016 and 152Sm/149Sm = 1.93009 ± 0.00051, were determined by this method. The mean ratios for six samples, measured with the single collector TIMS procedure and normalized to the above 147Sm/149Sm = 1.08680, are given in Table 9.55. The quoted uncertainties are 2SD. The 152Sm/149Sm ratios obtained by the total consumption method: 1.93009, and by conventional TIMS: 1.93040, are in very good agreement.

Recently Hidaka et al. [404] studied the thermal ionization of samarium. A triple Re filament ion source was used. 500 ng of samarium produced a 152Sm+

ion current above 2 x 1 0 " A using 2.2-2.4 A and 4.1-4.2 A for the side and the center filaments respectively. The pressure in the ion source was kept below 5 x 10~8 mbar. A thermal ionization mass spectrometer equipped with seven


Table 9.56. Interferences on samarium ions from isobaric rare earth and oxide ions

Sm+ ion Interfering ions l44Sm+ l44Nd^ ,4SSm+ l48Nd+ (I32Ba160+) i50 S m + i50 N d + ( 1 3 4 B a l 6 0 + ) 152Sm+ 152Gd+ 136 C e 16 0 + ( 1 3 6 B a 1 6 0 + )

154 S m + 154 G d + 138 C e1 6 0+ ( 1 3 8 B a , 6 0 + )

Faraday collectors was used. The seven Sm isotopes and two possible isobaric interferences were measured using the static mode in two separate cycles of 100 ratios each. The collector configuration was:

Low-3 Low-2 Low-1 Axial High-1 High-2 High-3 144Sm 146Nd "7Sm" 148Sm 149Sm li5Srri 152Sm 146Nd ,47Sm 148Sm 149Sm 150Sm 154Sm ,55Gd

Correction for instrumental mass fractionation was made using the expo-nential low and normalizing to 147Sm/152Sm = 0.56083. The results for a commercial Sm203 reagent, TMU-STD (Shin-etsu Chemical Co. Ltd, 99.9% grade purity) are also shown in Table 9.55. The quoted isotopic ratio precisions are within 0.001% at 95% confidence level.

The potential isobaric interferences on samarium are summarized in Table 9.56.

9.63 EUROPIUM

Europium is the sixty-third element in the Periodic Table. It has two stable isotopes at mass numbers 151 and 153. The isotopic composition is 47.8 and 52.2% respectively [1].

Europium has an ionization potential of 5.67 eV and is easily thermally ionized. Hess [405] used Eu203 deposited on a tungsten filament. Between 1400 and 1700°C Eu+ and EuO+ ions were emitted, with relative intensity 100:1. Holliger and Devillers [388], in a study of the Oklo nuclear reactor phenomenon, reported 0.9160 for the 151Eu/153Eu ratio in the natural element. TIMS was used but no further experimental details were given. Chang et al. [406] redetermined the absolute isotopic abundance of europium. A thermal ionization mass spectrometer was calibrated with synthetic mixtures prepared from chemically pure and highly enriched europium isotopes of known isotopic composition. A double rhenium filament ion source was used. The filaments

GADOLINIUM 329

were degassed under vacuum by a current of 4, 5 and 6 A for 20, 5 and 1 min respectively, and 1.5 pg europium as nitrate was loaded onto the sample filament. At a source pressure of 10~8 mbar, the ionization filament current was increased to 5.2-5.7 A within 20 min and the sample filament was heated very slowly, to avoid sample spattering, to 1.0-1.5 A. 151Eu+ and ,53Eu+ ion currents were collected simultaneously with two Faraday cups. The 15lEu/I53Eu isotopic ratio of six synthetic samples was measured five times each for six blocks of 10 ratios with 2SD lower than 0.08%. An isotopic fractionation correction factor

K = fltr/Kmeas (63 .1 )

where Rtr is the calculated ratio, was established as 0.98533 with a RSD of 0.037%. Twelve minerals and commercial samples from various sources were measured following the procedure for the synthetic mixtures and revealed the invariance of the isotopic abundance. The corrected 151Eu/l53Eu ratio is 0.90673 ±0.00235 (95% confidence limit). The quoted error includes the uncertainty of the mass spectrometric analysis, the chemical analysis and the error in composition of the separated isotopes. Recently the same group [407] redetermined their results. It was observed that considerable amounts of anionic impurities in the form of carbonate were disregarded in the first work. The above described procedure was followed with slight modifications, including thermal decomposition of carbonates and use of a 5 pg europium sample instead of 1.5 pg. A ratio 15IEu/153Eu = 0.91609 ±0.00038 has been reported. This value is in very good agreement with that of Holliger and Devillers [388]. The isotopic invariance of europium in nature was also demonstrated.

9.64 GADOLINIUM

Gadolinium is the sixty-fourth element in the Periodic Table. It has seven stable isotopes at mass numbers 152, 154, 155, 156, 157, 158 and 160, with an isotopic composition of 0.20, 2.18, 14.80, 20.47, 15.65, 24.84 and 21.86% respectively [1].

Gadolinium has an ionization potential of 6.14 eV, favorable for thermal ionization. Leland [408] placed on a clean new tungsten filament a drop of pure water, followed by a small amount of Gd203 powder to form a slurry. After drying, the filament was inserted into a magnetic mass spectrometer and heated at a pressure lower than 10~6 Torr to yield an adequate ion current. GdO+ ions were monitored with an electron multiplier. The maximum isotope abundance errors for the major isotopes was estimated to be 1 %, and that for the minor 152Gd isotope was 2.5%. I 70 and l 8 0 abundance corrections were made. Collins et al. [320] used specially purified gadolinium compounds to eliminate isobaric interferences. Spitzer and Sites [45] loaded finely divided gadolinium oxide (Gd203) onto a tantalum filament and partially digested it with con-


centrated nitric acid. The sample was gently dried and the filament was slowly heated to dull red heat (sa750°C). One pg of gadolinium, ionized in a temperature range of 1300-1600 °C, produced an ion current of 10"10 A for more than 1 h. The observed GdO+/Gd+ ratio was greater than 100:1. No gadolinium interferences from previous samples were observed. Corrections for 170 and 180 were sometimes necessary. Eugster et al. [409] developed an ion exchange procedure to separate gadolinium from terrestrial and meteoritic samples. The final duent was evaporated to dryness and dissolved in a drop of 1.5 N HCl and 0.05-2 pg Gd were loaded onto a zone refined, high purity rhenium filament previously degassed at 2000 °C for 2 h at a vacuum of « 10~4

Torr. The loaded sample was heated on the filament in air to « 800 °C for 1 min to convert the gadolinium to the oxide. A stable and most intense GdO+ ion beam was obtained at 1400 °C, with a GdO+/Gd+ ratio higher than 1000. A triple rhenium filament was also tested. The yield for GdO+ ions with the single filament source was about one order of magnitude better than for Gd+ ions with the triple filament ion source, for which Gd+VGdO+œS was observed. Gadolinium samples larger than 20 ng could be analyzed using a Faraday collector with a 10" ft resistor, and for samples of <20 ng a 17 stage electron multiplier at a gain of 104 and a 1010 CI resistor were used. A set of 10 magnetic scans was recorded and averages of the isotopic ratios were calculated. These were corrected for the 170 and 180 contribution using Nier's [91] values normalized to 156Gd/160Gd = 0.9361 and corrected for mass fractionation. One set of ten spectra, including zero measurements at both sides of each peak, were recorded within « 15 min. Uncertainties of (6-20) x 10~5 at 95% confidence

Table 9.57. Isotopic abundance ratios in gadolinium 152Gd/158Gd 154Gd/158Gd ,55Gd/158Gd l56Gd/158Gd 157Gd/158Gd 160Gd/158Gd Error Ref.

0.00801 0.00837 0.00804 0.008170 0.00805 0.00828

±0.00002 l52Gd/,60Gd

0.009263 ±0.000008

0.0865 0.0910 0.0865 0.08782 0.0878 0.08822

± 0.00004 l54Gd/,60Gd

0.099706 ± 0.000060

0.099722 ±0.000002

0.5881 0.6164 0.5923 0.59593 0.5958 0.59785

±0.00013

0.8157 0.8409 0.8231 0.82408 0.8241 0.82595

±0.00012 1S5Gd/,60Gd 1S7Gd/'

0.6266 0.6409 0.6305 0.63024 0.6300 0.63088

± 0.00005 6°Gd '

0.676909 0.715875 ±0.000016 ±0.000015

0.676840 0.715826 ±0.000008 ±0.000006

0.8818 0.8817 0.8806 0.88033 0.8800 0.878636

58Gd/,60Gd

1.135886 ±0.000020

1.135861 ±0.000008

2SD

2SD

2SD

[408] [320] [45]

[409]" [388] [400]

[402]"

[404]°

" Normalization factor: ,56Gd/160Gd = 0.9361. b Normalization factor.

GADOLINIUM 331

levels were reported for the 158_I54Gd/160Gd isotopic ratios and 2 x 10~3 for the l52Gd/160Gd ratio. The measured isotopic ratios are shown in Table 9.57. Holliger and Devillers [388] also measured the isotopic composition of gadolinium. Barium is efficiently ionized in thermal ionization and in chloride medium produces BaCl+ ions, which interfere seriously with almost all the GdO+ ions except ,52Gd160+ and 160Gd16O+. A correction procedure was investigated by Eugster et al. [409], who also discussed all other possible interferences. Tables 9.58 and 9.59 summarize these interferences.

Maas and McCulloch [402] used a double Re filament ion source. 1-2 pg of gadolinium were loaded at sub red heat onto an degassed filament. Oxide and atomic ions were produced. Data were collected using the atomic ions with a seven Faraday collector mass spectrometer in the static mode. A l58Gd+ ion current of 6 « 10~12 A was observed for « 1 h. The measured data were cor-rected for instrumental mass discrimination by normalizing to 156Gd/160Gd = 0.9361 [409].

Dubois et al. [400] applied the total vaporization method for the isotopic analysis of gadolinium. Fifty ng of Johnson & Matthey Gd (Gd2Û3 dissolved in 3 N nitric acid) were loaded onto a rhenium sample filament in a triple filament

Table 9.58. Interferences on gadolinium ions from isobaric rare earths and rare earth monoxide ions

GdO+

168 170 171 172 173 174 176

ions

152G d160+ l58Gd160+ 155Gd160+ 156 G d 16 0 + 1 5 7 Gd , 6 0 +

1 5 8 G d l 6 0 + 160Gd16O+

Interfering

1 5 2 G d . 8 0 +

154Gd18Q+ 1 5 5 G d . 8 0 + 156Gd,80+ l S 8 G d 1 8 0 +

ions

154Gdl7Q+ 155 G d 17 0 + 156Gdl70+ 157Gd17Q+

168j3r+ 170Er+

I76Lu+

168 y b + 170yb+ 17. Y b + 172Yb+ 173 Y b +

174Yb+ 176 Y b +

I52Sm160+ 154Sm160+

l56j)yl6Qf

1 5 8Dy1 60+

160Dy16Q+

Note: l14Hf+ and n6Hf+ ions do not interfere because of the much higher temperature and the presence of ionization enhancement agents needed to ionize Hf.

Table 9.59. Interferences to gadolinium ions from isobaric BaCl4 ions GdO4 ions Interfering BaCl+ ions

,35Ba35Cl+ 136B a35C 1 + l37Ba35Cl+ l38Ba35Cl+

168 170 171 172 173 174 176

i 5 2 G d i 6 0 +

154Gd16Q+ l 5 5 G d 1 6 0 + 156GdI60+ 157G d16G + 158Gd160+ 160Gd16Q+

,34Ba37Cl+ ,35Ba37Cl+ l36Ba37Cl+ l37Ba37Cl+


ion source. The ionization temperature was maintained at 2300 °C and the total ion current was set to 10~n A. Under these conditions the observed GdO+/Gd+ ratio was lower than 10~4. The mass spectrometer was equipped with five adjustable Faraday collectors simultaneously monitoring the ion currents. Special software was developed to run the instrument in this mode of operation. The duration of an analysis is 40-50 min, and a typical ion current vs. time profile is shown in Figure 9.13. Two isotopic ratios, 160Gd/158Gd = 0.87863 ± 0.00022 and 157Gd/158Gd = 0.63084 ± 0.00013, were determined by this method. The mean ratios of six samples, measured with the single collector TIMS procedure and normalized to the above 160Gd/158Gd = 0.87863, are given in Table 9.55. The quoted uncertainties are 2SD. The ,57Gd/158Gd ratios obtained by the total consumption method, 0.63084, and by conventional TIMS, 0.63088, are in excellent agreement. The purity of gadolinium oxide was evaluated by testing for isobaric interferences. An unidentified interference was observed at m/z = 155. The other possible interferences are: 152Sm, 154Sm, 156Dy, 158Dy and 160Dy.

Recently. Hidaka et al. [404] studied the thermal ionization of gadolinium. They observed that a Ta-Re-Ta triple filament ion source is highly effective in producing Gd+ ions. The filaments were degassed at 4.7 A for 20 min and « 1 pg of extensively purified gadolinium was loaded on the Ta side filament. A stable 158Gd+ ion current above 2 x 10~u A was produced using 2.6-2.7 A and 4.5-4.5 A for the side and the center filaments respectively. The pressure in the ion source was kept below 5 x 10~8 mbar. Under these conditions the intensity of the 1 5 8Gd1 60+ ion current was below 10- 1 5 A, undetectable with a Faraday cup collector. A triple Re filament ion source was also tested. At 2.8-2.9 A and 4.5-4.5 A on the side and the center filaments, respectively, 1 pg of Gd yielded ion current intensities of 2 x l 0 ^ n A for 158Gd+ and 8 x 10~13 A for 1 5 8 Gd l 6 0 + . At lower currents on both the side and the center filaments, the oxide ion intensity increased and the atomic ion intensity decreased to approximately the same value. This observation is consistent with that of Dubois et al [400], who also used a triple Re filament ion source at even higher ionization temperature (2300 °C), observing a GdO+/Gd+ ratio lower than 10~4. A thermal ionization mass spectrometer equipped with seven Faraday collectors was used. The seven Gd isotopes and two possible isobaric interferences were measured using the static mode in two separate cycles of 100 ratios each.

The collector configuration was:

Low-3 Low-2 Low-1 Axial High-1 High-2 High-3

i^irn" 152Gd 154Gd 156Gd 158Gd ^ G d 161Dy I54Gd 155Gd 156Gd 157Gd 158Gd 160Gd 161Dy

DYSPROSIUM 333

Corrections for instrumental mass fractionation were made using the exponential law and normalizing to 156Gd/160Gd = 0.9361. The results for a commercial Gd203 reagent, TMU-STD (Shin-etsu Chemical Co. Ltd, 99.9% purity) are also shown in Table 9.64. The quoted isotopic ratio precisions are within 0.001% at 95% confidence level. It should be noted that, following the work of Dubois et al. [400] and Hidaka et al. [404], the accuracy of Gd isotopic composition measurements improved by at least one order of magnitude. The recent data are obtained by measuring atomic ions and not oxides, therefore isobaric l 8 0 and n O oxide ion contributions do not exist.

9.65 TERBIUM

Terbium is the sixty-fifth element in the Periodic Table. It has only one stable isotope at mass number 159 [320]. It is easily thermally ionized (IP = 5.85 eV). Spitzer and Sites [45] placed finely divided terbium oxide (Tb203) on a tantalum filament and partially digested it with concentrated nitric acid. The sample was dried and the filament was slowly heated to dull red heat, («750°C). One pg of terbium, heated between 1400 and 1700°C, produced a TbO+ ion current of 10~10 A for 1 h. The observed TbO + /Tb + ratio was 100:1. No terbium interference from previous samples was observed. The 175Lu+ ion is a potential isobaric interference.

9.66 DYSPROSIUM

Dysprosium is the sixty-sixth element in the Periodic Table. It has seven stable isotopes at mass numbers 156,158, 160, 161, 162, 163 and 164, with an isotopic composition of 0.06, 0.10, 2.34, 18.9, 25.5, 24.9 and 28.2% respectively [1].

Dysprosium has an ionization potential of 5.93 eV and is easily thermally ionized. Leland [408] placed on a clean new tungsten filament a drop of pure water, followed by a small amount of Dy203 powder (15% in holmium oxide) to form a slurry. After drying, the filament was inserted into a magnetic mass spectrometer and heated at a pressure lower than 10~6 Torr to yield an adequate ion current. Dy+ ions were monitored with an electron multiplier. The isotope abundance errors for the major isotopes were estimated to be 1%; significantly larger errors existed for the minor 156Dy and ,58Dy isotopes. Collins et al. [320] used specially purified dysprosium compounds to eliminate isobaric interfer-ences. Spitzer and Sites [45] loaded finely divided dysprosium oxide (Dy2Û3) on a tantalum filament and partially digested it with a drop of concentrated nitric acid. The sample was gently dried and the filament was slowly heated to dull red heat ( « 750 °C). One pg of dysprosium, ionized in a temperature range of 1400-1500 °C, produced an ion current of 10~10 A for > 1 h. The observed


Table 9.60. Isotopic abundance ratios in dysprosium 156Dy/164Dy 158Dy/164Dy 160Dy/164Dy 161Dy/164Dy 162Dy/164Dy 163Dy/164Dy Ref.

0.00203 0.00356 0.0836 0.6762 0.9075 0.8861 [320] 0.00186 0.00320 0.0814 0.6700 0.9060 0.8861 [45] 0.00199 0.00341 0.0830 0.6708 0.9049 0.8833 [388]

Table 9.61. Interferences to dysprosium ions from isobaric rare earth and oxide ions

Dy+ ion Interfering ions ,56Gd+ 140Ce16O+

i58Gd+ i42ce160+ 142Nd160+

!60Gd+ 144Nd160+ 144Sm160+

,45Nd160+

.62Er+ 146Nd160+

147Sm160+

!64Er+ ,48Nd,60+ 148Sm160+

Dy + /DyO + ratio was 10:1 . No dysprosium interferences from previous samples were observed. Holliger and Devillers [388] also measured the isotopic composition of dysprosium. Table 9.60 summarizes the isotopic abundance ratios in dysprosium and Table 9.61 shows the isobaric interferences.

9.67 HOLMIUM

Holmium is the sixty-seventh element in the Periodic Table. It has only one stable isotope at mass number 165 [409,320]. It is easily thermally ionized (IP = 6.02eV). Spitzer and Sites [45] placed finely divided holmium oxide (Ho203) on a tantalum filament and partially digested it with a drop of concentrated nitric acid. The sample was dried and the filament was slowly heated to dull red heat (w 750 °C). One pg of holmium, heated between 1400 and 1600 °C, produced anion current of 10~10 Afor 1 h. The observed Ho + /HoO +

ratio was 10:1. No holmium interference from previous samples was observed. 1 4 9Sm1 60+ is a potential interference.

9.68 ERBIUM

Erbium is the sixty-eighth element in the Periodic Table. It has six stable isotopes at mass numbers 162, 164, 166, 167, 168 and 170, with an isotopic composition of 0.14, 1.61, 33.6, 22.95, 26.8 and 14.9% respectively [1].

156Dy+ 158Dy+ 160Dy+ 16,Dy+ ,62Dy+ ,63Dy+

<«Dy+

THULIUM 335

Table 9.62. Isotopic abundance ratios in erbium 162Er/166Er

0.00462 0.00407 0.00408

i 6 4 E r / , 6 6 E r

0.04796 0.0467 0.04787

i 6 7 E r / i 6 6 E r

0.6841 0.6866 0.6822

168Er/166Er

0.8100 0.8102

0.7968

i 7 0 E r / i 6 6 E r

0.4508 0.4454 0.4442

Ref.

[408] [410] [388]

Table 9.63. Interferences on erbium ions from isobaric and rare earth monoxide ions

Er+ ion Interfering ions

162Er+ 162D + 1 4 6Nd1 60+

164Er+ 164Dy+ 1 4 8Nd1 60+ , 4 8Sm1 60+

.66Er+ 150Nd16O+ ,50Sm16O+

i67Er+ 1 5 1 Eu , 6 0 +

i68Er+ i6 8 Y b + 1 5 2Sm1 60+ 152Gd160+

170Er+ 170Yb+ l 5 4 Sm l 6 0 + 154Gd160+

Erbium has an ionization potential of 6.10eV. Leland [408], Hayden et al. [410] and Holliger and Devillers [388] measured the isotopic ratios of erbium by thermal ionization. No fractionation corrections or instrumental calibrations were reported. Their results are given in Table 9.62.

Table 9.63 summarizes the interferences on erbium ions by isobaric and rare earth monoxide ions. Extensive chromatographic and ion exchange methods may improve the sample purity. Schumann et al [411] showed that the introduction of propane into an ion source at a steady state level of 10"~4 Torr substantially suppressed various monoxide ion interferences.

9.69 THULIUM

Thulium is the sixty-ninth element in the Periodic Table. It has only one stable isotope at mass number 169 [320]. It is easily thermally ionized (IP = 6.18 eV). Spitzer and Sites [45] placed finely divided thulium oxide (Tm203) on a tantalum filament and partially digested it with concentrated nitric acid. The sample was dried and the filament was slowly heated to dull red heat, (R¿ 750 °C). One pg of thulium, heated between 1400 and 1700°C, produced an ion current of 10~10 A for 1 h. The observed Tm+/TmO+ ratio was > 100:1. No thulium interference from previous samples was observed. 1 5 3Eu1 60+ is a potential interference.


9.70 YTTERBIUM

Ytterbium is the seventieth element in the Periodic Table. It has seven stable isotopes at mass numbers 168, 170, 171,172,173,174 and 176, with an isotopic composition of 0.13, 3.05, 14.3, 21.9, 16.12, 31.8 and 12.7% respectively [1].

Ytterbium has an ionization potential of 6.254 eV, which is favorable for thermal ionization. Hay den et al. [412] were the first group who measured the isotopic abundance of all the ytterbium isotopes. Leland [408] thermally ionized ytterbium oxide in a single filament ion source with a 60° magnetic sector mass spectrometer. An overall error of 1% was assessed for each ion current, except for the minor 168Yb isotope, for which 4% was estimated. Collins et al [320] used a tungsten filament and specially purified rare earth compounds to avoid potential isobaric interferences. Spitzer and Sites [45] placed finely divided ytterbium oxide on a tantalum filament and partially digested it with concentrated nitric acid. The sample was dried and the filament was slowly heated to dull red heat ( « 750 °C). One pg of ytterbium produced an ion current of 10- 1 0 A for more than 1 h. The observed Yb + /YbO + ratio was greater than 100:1. No ytterbium interferences from previous samples were observed. McCulloch et al. [413] analyzed ytterbium with a solid source magnetic sector mass spectrometer equipped with an electron multiplier. The measured ratios were normalized to 171Yb/174Yb = 0.4512 and a fractionation factor F per atomic mass unit was derived from:

F = [{m^/mYb)meJQA5\2}^ (70.1)

where the RmeàS represents the measured mean. Corrected ratios were then obtained as follows, e.g.

(170Yb/174Yb)corT = (17°Yb/174Yb)meas/F4 (70.2)

Holliger and Devillers [388] also thermally analyzed natural Yb. The isotopic ratios in Yb from various reports are shown in Table 9.64.

Table 9.65 summarizes the interferences on ytterbium ions by isobaric and rare earth monoxide ions. Extensive chromatographic and ion exchange methods may improve the sample purity. Schumann et al. [411] showed that the introduction of propane into an ion source at a steady state level of 10- 4 Torr substantially suppressed the GdO+ and DyO+ monoxide ion interferences.

Thirlwall [390] analyzed ytterbium with a triple filament ion source. The sample as a nitrate solution was loaded on the previously degassed tantalum side filaments and dried at 1 A in air. Care was taken to avoid introduction of sample traces onto the center filament, as this could produce interfering oxide ion emission. Use of a high center filament current (4.9 A), eliminates interferences from L a O j , CeOj and P r O j , and at low sample filament current (1.6-1.8 A), GdO+ was not observed. Ion beam intensities of 10""12 A were recorded.

LUTETIUM 337

Table 9.64. Isotopic abundance ratios in ytterbium 58Yb/174Yb 170Yb/174Yb 171Yb/174Yb 172Yb/174Yb 173Yb/174Yb 176Yb/174YbRef.

0.00407 0.0950 0.4472 0.6822 0.5068 0.4009 [409] 0.00424 0.0952 0.00429 0.0964

±0.00005 ±0.0001 0.00399 0.0955

" Normalization factor.

0.4494 0.4512 a

0.4486

0.6853 0.6887

±0.0004 0.6858

0.5066 0.5075

±0.0003 0.5068

0.3998 0.3997

±0.0002 0.4009

[45] [4131

[388]

Table 9.65. Interferences on ytterbium ions from isobaric and rare earth monoxide ions

Yb+ ion Interfering ions ,68Yb+ '68Er+ , 5 2 Sm 1 6 0 + , 5 2 Gd , 6 0 +

i70Yb+ nog,* 1 5 4Sm1 60+ 154Gd160+

171Yb+ l 5 5 Gd , 6 0 +

i72Yb+ 1 5 6Gd1 60+ l 5 6Dy1 60+

!73Y b + >57Gd160+

i74Yb+ 1 5 8Gd , 60+ , 5 8Dy1 60+

m Y b + I76Lu+ 160Gd16O+ , 6 0Dy, 6O+

Notes: ,74Hf+ and 176Hf+ ions do not interfere, because of the much higher temperature and the presence of ionization enhancement agents needed to ionize Hf.

9.71 LUTETIUM

Lutetium is the seventy-first element in the periodic table. It has two stable isotopes at mass numbers 175 and 176, with an isotopic composition of 97.416 and 2.584% respectively [1]. 176Lu is a radionuclide, decaying to 176Hf by ß" emission witJi a half-life of (3.57 ± 0.14) x 1010 years.

Lutetium is a metallic element with a low ionization potential of 5.426 eV, thus it is easily thermally ionized. Patchett [414] analyzed a large natural sample of the element on a triple filament ion source and observed i75Lu/i76Lu = 3 7 7 0 i ± o 026 (2SD). From ref. [414] it can be assumed that the filament material was rhenium. No corrections for mass fractionation were made.

Interferences for 175Lu+ may be 1 5 9Tb1 60+, and those for 176Lu+ are i60Gd!6O+ a n d i60Dyi6O+ Hafnium has an isotope at m/z = 176, but this element has a very high ionization potential. It is ionized as a phosphate in a triple rhenium filament ion source at «2200°C, which is higher by 400 -800 °C than the temperature used for analyzing lutetium without an ionization enhancement agent. Thus interference from this isotope has a rather very low probability. Patchett and Matsumoto [415] developed a reproducible method,


with high recovery, for the chemical separation of Lu and Hf from rock samples for geochronological studies. Mattauch and Lichtblau [416], Collins et al. [320], McCulloch et al. [417] and Holliger and Devillers [388] also reported isotopic ratio values in very good agreement with those of Patchett [414]. Dubois et al. [400] applied the total vaporization method for lutetium isotopic analysis. Twenty ng of Johnson & Matthey Lu (Lu2Û3 dissolved in 3 N nitric acid) were loaded onto a rhenium sample filament in a triple filament ion source. The ionization temperature was maintained at 2300 °C and the total ion current was set to 1 0 " A. Under these conditions the observed LuO+/Lu+

ratio was lower than 10 4. The mass spectrometer was fitted with five adjustable Faraday collectors simultaneously monitoring the ion currents. Special software was developed to run the instrument in this mode of operation. The duration of an analysis is 40-50 min. A typical ion current profile vs time is shown in Figure 9.13. Six samples yielded the result 175Lu/176Lu = 37.6719 ±0.0071, in reasonable agreement with Patchett [414]. The quoted uncertainty is 2SD.

9.72 HAFNIUM

Hafnium is the seventy-second element in the Periodic Table. It has six stable isotopes at mass numbers 174, 176, 177, 178, 179 and 180, with an isotopic composition of 0.162, 5.206, 18.606, 27.297, 13.629 and 35.100% respectively [1]. 176Lu is a radionuclide decaying to l76Hf by ß~ emission with a half-life of (3.57 ±0.14) x 1010 years. Thus in nature the 176Hf/177Hf isotopic ratio in lutetium- and hafnium-bearing minerals may reveal fluctuations, setting up the 176Lu-176Hf geochronometer.

Hibbs [418] analyzed HfCU using electron impact. Hafnium is a metal of highly refractory nature and high ionization potential (7.0 eV). White et al. [164] thermally ionized metallic hafnium melted onto a tungsten filament, correcting for the minute (less than 0.1%) contribution of 180W. Patchett [414, 415,419] loaded hafnium as phosphate onto the side filaments of a triple rhenium filament ion source. On heating the center (ionizing) filament to 2200 °C, Hf+ ions are emitted simultaneously with a 187Re+ ion current of « 6 x 1 0 _ n A. A 15 mm long center filament was used to avoid severe temperature gradients in the filament. The results were normalized to 179Hf/ 177Hf=0.7325. Kawashima et al. [333] used 200 ng of hafnium loaded onto a rhenium ribbon in a single filament ion source. A slurry of rhenium and platinum powders together with a solution of hydrochloric acid, ammonium iodide and ascorbic acid was used as an ionization enhancement agent. The ion currents were monitored with a Daly electron multiplier detection system. Further details of this method are described in Section 9.42. Corfu and Noble [420] also used the single filament technique. An aqueous slurry of very fine

HAFNIUM 339

grain iridium, molybdenum metal powder and carbon was loaded onto a rhenium filament, followed by a 2-5 pg hafnium sample, which was almost dried by increasing the filament current. Adding a drop of dilute HCl and heating to 1.5-01.8 A improved sample adhesion to the filament. A dense, very fine grained and homogeneous Ir layer on the filament is the most important factor in the ionization enhancement process. Coarse grain metal strongly reduced the ionization efficiency. Adding an optimal amount of Mo improved the Hi+ ion beam intensity, but too much Mo inhibited emission. The correct amount was determined by a series of tests. The presence of carbon reduced oxide ion formation from possible interfering elements. Total Hf+ ion currents of (0.6-1.6)x 10~n A were obtained at 5-6 A filament currents, corresponding to 2100-2300 °C. 100-200 ratios were collected within 70-140 min. The measurements were performed with a triple Faraday collector detection system operating in the dynamic peak jumping mode. Good data were obtained only with a flight tube vacuum below (l-2)x 10~8 Torr. The measured ratios within a sample remained essentially constant. Isotopic fractionation was controlled by the sample loading conditions. Salters and Hart [421] analyzed Hf with a double zone refined Re filament ion source. Hafnium dissolved in HF was loaded onto the filament, and H3PO4 and slurried elemental boron in water were added. Precision better than 0.015% for 176Hf/177Hf was achieved using static multicollection.

Walder et al. [318,422] used an ICP ion source coupled to a double focusing magnetic sector mass analyzer equipped with seven Faraday detectors to measure isotopic ratios of hafnium. Two different nebulizing systems were

Table 9.66. Isotopic abundance ratios in hafnium 174Hf/177Hf

0.00879 0.00871

0.008748 ±0.000041

176H f /177H f

0.2806 0.282195

±0.000015 0.282157 0.282184

±0.000042 0.282142

±0.000021 0.282194

±0.000020 0.282198

±0.000010

178H f /177H f

1.4602 1.46710

1.467168 1.468572

±0.001469 1.467140

±0.000070

1.46731 ±0.00007

180H f /177H f

1.8975 1.88651

1.886666 1.88392

±0.00188

1.88634 ±0.00025

Ref.

[164]" [414]*

[419]* [333]c

[420]*

[318]*

[422]*

„ "»Hf/'77Hf= 0.7408. h Johnson & Matthey 475 Hafnium was used. The results were normalized to l79Hf/l77Hf=0.7325. Quoted uncertainties are 2SD. c Normalization factor 179Hf/177Hf=0.7325. Quoted uncertainty is ISD. Sample origin not given.


used. Solutions of 1 pg ml - 1 Hf were introduced via a Meinhard nebulizer [318], and 50ng ml - 1 via an ultrasonic nebulizer [422]. The total Hf consumption per sample was almost the same (about 300 and 290 ng respectively). The data are in very good agreement with the TIMS results of Patchett [414]. Table 9.66 summarizes the isotopic ratios observed for hafnium.

9.73 TANTALUM

Tantalum is the seventy-third element in the Periodic Table. It has two stable isotopes at mass numbers 180 and 181, with an isotopic composition of 0.012 and 99.988% respectively [1].

White et al. [164, 423] showed that tantalum is not monoisotopic. They measured the 180Ta/181Ta ratio, using a tantalum filament as a source for Ta+

ions. Palmer [73] also used a tantalum filament for this measurement. Spitzer and Sites [45] loaded finely divided Ta2Os on a rhenium filament and partially digested it with 1:1 hydrofluoric acid. Then the filament was placed in a vacuum of better than 10~2 Torr and slowly heated until the sample was reduced to tantalum and melted. Above 1500 °C the sample was ionized by electron impact, and Ta+, TaO+ and Ta0 2

+ were observed with an approximate ratio of 1:2:8. At natural or close to natural oxygen and tantalum abundance, correction for 1 7 0 at mass 213 (180Ta16O17O) may be neglected. Interferences from previous samples were not observed. The isotopic ratios from the above mentioned measurements are summarized in Table 9.67.

9.74 TUNGSTEN

Tungsten is the seventy-fourth element in the Periodic Table. It has five isotopes at mass numbers 180, 182, 183, 184 and 186, with relative abundance of 0.13, 26.3, 14.3, 30.67 and 28.6% respectively [1].

Inghram [66] used WOF4 and White and Cameron [43] used W F ô to measure the isotopic composition of tungsten. Both compounds evaporate easily, and they were introduced into the ion source through a capillary inlet system and ionized by EI. The more abundant ions for WOF4 are WOF 2, WF 3" and W +

Table 9.67. Isotopic abundance ratio in tantalum Reference [423] [164] [73] [45] 180Ta/18ITa 0.0001230 0.0001232 0.0001170 0.0001230

±0.0000030

TUNGSTEN 341

and those for W F ô are WF^~, WF^, WF^ and WF J, from which the isotopic ratios were calculated. Both compounds are highly corrosive materials, which attack the inlet system and ion source components. Spitzer and Sites [45] evaporated W 0 3 from a tantalum ribbon at 900-1200 °C. To stabilize the ion beam, the oxide was deposited on the ribbon together with concentrated boric acid solution and heated to glowing red ( « 750 °C) in air. The isotopic ratios were calculated from the W 0 2 ion intensities with correcton for the oxygen isotopic composition.

Although tungsten has a high ionization potential (7.98 eV), Turnbull [234] showed that sodium tungstate could be thermally ionized from a tantalum filament in a single filament ion source. Na2WO^ ions were observed at an intensity of about 10~13 A. Voelkening et al. [424] successfully analyzed this element by applying negative TIMS. Thus, 1 pg or 100 ng of tungsten as Na2W04 was deposited onto a rhenium filament. A double filament ion source was used, and on the ionization filament 5 pg of lanthanum from a La(N03)3 solution was deposited. Both filaments were previously degassed at a current of 5 A for 1 h. At an evaporation filament temperature of 1150-1200°C and an ionization filament temperature of 1250-1300 °C, stable WOf ion currents of (3-1.5)xl0~u A from the above mentioned sample sizes were obtained. Twelve runs of 100-150 ratios, normalized to the ,84W isotope and internally normalized for isotopic fractionation to I 8 3W/ W = 0.46712 using an exponential law [227], were made. The fractionation correction factor used was one of the isotopic ratios established in this work. The only interference from the filament material was l 8 5Re1 701 602 on the , 8 6 W l 6 0 j ion at m/z = 234, which was corrected by monitoring the 1 8 7Re1 801 602 ion intensity. Further corrections for n O and 1 80 were made to calculate the tungsten isotopic ratios. The results together with the data of White and Cameron [43], also used in ref. [1], are given in Table 9.68.

High precision tungsten isotope ratio measurements have recently been achieved with a double focusing multiple collector ICP-MS by Lee and Halliday [319]. The instrument was equipped with seven Faraday collectors. 1-2 ppm solutions prepared from H. Cross standard filament material and NIST SRM-3163 spectrometric tungsten solution were introduced into an argon ICP

Table 9.68. Isotopic abundance ratios in tungsten 1 8 0 w y l 8 4 w 182W^184W

l 8 3 W / 1 8 4 \ V ' 8 6 \ y / ' 8 4 W Ref.

0.003919 0.86478 0.46712 0.92767 [424] ±0.000006 ±0.00015 ±0.00015 ±0.00023 0.003897 0.86494 0.46712 0.92763 [319] 0.00411 0.85865 0.46605 0.93470 [1]


ion source at a flow rate of 0.3 ml-1. Typical total ion current intensity was 4 x 10~n A for a 1 ppm solution. Each sample analysis consisted of over 100 measurements of each isotope, detected simultaneously with 10 s integration time. The raw data were corrected for the time independent mass discrimination applying the exponential law and were normalized to 186W/ W = 1.98594, given by Voelkening et al. [424]. The results presented in Table 9.52.1 were recalculated relative to 184W.

9.75 RHENIUM

Rhenium is the seventy-fifth element in the Periodic Table. It has two isotopes at mass numbers 185 and 187, with relative abundance of 37.40 and 62.60% respectively [1]. 187Re is radioactive with a half-life of (4.56 ±0.11) x 1010

years [425], which decays by ß~ emission to stable 187Os. White and Cameron [43] measured the isotope ratio of rhenium by

evaporating Re207 from a tungsten or tantalum oven, ionizing it with electron impact and collecting the ion beams with a Faraday collector. Only Re+ ions were observed. The reported 185Re/187Re ratio was 0.5891 ±0.0011, yielding an isotopic composition of 37.07 and 62.93% for 185Re and 187Re respectively. Spitzer and Sites [45] used ammonium perrhenate (NH4Re207) dissolved in water, deposited and dried on a tantalum ribbon, evaporated at about 200 °C and ionized with electron impact. Re+,ReO+,Re02 jReO^" and ReOj ions were observed, and Re+ was used for the ratio calculation. The ion source had to be cleaned after each sample to eliminate memory effects.

Elemental rhenium has an ionization potential of 7.88 eV, which prevents high thermal ionization efficiency. Barton et al. [426] used an electron multiplier and pulse counting and corrected their results by a mass dependent correction factor of 1.011, which is close to the ratio of 187/185. They did not mention whether the discrimination was a multiplier or a source effect. The corrected ratio was 0.5938 ± 0.0042. Riley [427] also measured the isotopic composition of rhenium by applying thermal ionization. A modified solid source magnetic sector mass spectrometer was used with various detection systems. The reported 185Re/187Re ratios were as follows: (1) d.c. amplifier, plate collector: 0.5976 ± 0.0004; (2) multiplier, current integration: 0.5975 ± 0.0003 (using a mass dependent correction factor 185/187 = 0.9893); (3) multiplier, pulse counting: 0.5945 ± 0.00027. In experiments (1) and (2) the magnet was scanned, in (3) the accelerating voltage was stepped. Both investigators used rhenium ribbons in their thermal ionization experiments as a source of rhenium ions. Grämlich et al. [428] determined an absolute value for the isotopic ratio of a reference rhenium sample, 185Re/187Re = 0.59738+ 0.00039. The indicated uncertainty is the overall limit of error based on 95% confidence limits for the mean and allowances for the effects of known sources

RHENIUM 343

of possible systematic error. Samples of known isotopic composition with 185Re/187Re ratios ranging from » 10 to 0.1 were used to calibrate two thermal ionization mass spectrometers. The first instrument was a two stage mass spectrometer constructed from two magnetic sectors and an electron multiplier for ion detection; the second instrument was a single stage magnetic sector equipped with a Faraday collector. The instrumental bias was determined using samples with synthetic isotopic composition such as the series of 17 uranium isotopic standards, covering a range of isotopic ratios from 235U/238U = 0.005-187 approximately (see Section 92 in this chapter). Both instruments showed that the established bias factor was independent of the isotopic ratios. A slightly different sample loading procedure and ionization temperature were used for each instrument. The procedure applied for the single stage mass spectrometer is given here. On a single tungsten filament, degassed at 2000 °C under a potential field, 10 pi of a solution containing 5mgml~' rhenium as perrhenic acid in 5% sulfuric acid was loaded. A high purity platinum wire anode, cleaned prior to each use with diluted nitric acid, was dipped into the solution drop and the rhenium was electrolytically deposited onto the tungsten for 1 h at a potential of 2.25 V and a current density of 10mA cm"2. The filament was washed in triply distilled water, dried under a heat lamp and placed in a bell jar purged with extra dry hydrogen. Then the filament was heated to fuming, and when the fuming ceased the temperature was slowly increased until the sample passed through a stage of bubbling and became grey; the heating current was maintained for 1 min and then increased until a faint red glow was observed. The current was then reduced to a temperature at which the glow was not visible and maintained at this stage for 30 s. The sequence of sample plating and hydrogen reduction was performed three times in all, then the filament was loaded into the mass spectrometer and analyzed at 2100 °C. A small amount of filament fractionation (less then 0.1%) was observed during the analyses.

Creaser et al. [429] measured the rhenium isotope ratio using negative thermal ionization. Rhenium powder was dissolved in concentrated nitric acid and the solution was diluted to 0.1 N. Solutions were loaded onto a single platinum filament together with 20 pg Ba as a Ba(N03)2 solution in ultrapure H 2 0, and dried at 0.5 A filament current in air. Intense ion currents of ReO^ were observed at 650-850 °C filament temperatures, followed by a less intense ion current of ReO-¡\ Usually the ReO^/ReO^ ratios exceeded 250. The isotopic ratios were measured at m/z — 249 (185Re1604 ) and m/z = 251 (187Re16Oj). The data were corrected for the 1 8 5 Re l 6 0 j 8 0 - interference at m/z = 251 using , 8 0 / 1 6 0 = 0.002045. [91]. For 1 ng and 350pg rhenium samples, 8 x 10~12 and 4 x l 0 ~ 1 2 A ion currents were observed and l85Re/187Re = 0.5977 ± 0.0002 was determined. The quoted uncertainty is 2SD. Recently, Walczyk et al. [430] developed a negative thermal ionization technique, reducing the rhenium blanks arising from commonly used filament materials. This blank prevents accurate isotopic ratio determination, especially


when very small samples are available. Nickel ribbons were etched in dilute nitric acid, coated with a V2Os suspension and degassed for 2 h under high vacuum at a temperature of « 850 °C. Then 20 pg of Ba as Ba(OH)2 solution were deposited on the filament and dried in air with an electrical current and finally the sample was loaded as perrhenic acid and dried. At a filament temperature of 840- 860 °C, samples as small as 1 ng yielded a ReO^ ion current of 7 x 10 12 A, corresponding to 20% ionization efficiency. At this temperature the calculated filament blank was below 1 pg. An 180 corrected 185Re/187Re isotopic ratio of 0.59818 ±0.00026 (ISD) was reported for a rhenium standard of natural isotopic composition.

Richardson et al. [431 ] used inductively coupled plasma mass spectrometry. Typical precision for the l85Re/187Re isotopic ratio, for samples containing 30 ppb rhenium, was between 0.3 and 1 % (2RSD).

Walker and Fassett [432] used resonance ionization mass spectrometry to measure the isotopic composition of microgram quantities of rhenium. A chloride solution was dried onto anion resin beads. This step purified and concentrated the sample. The beads were placed into a notch formed in a bent, miniaturized tantalum filament, dried with a heat lamp and covered with a 10:1 ethanol - flexible collodion mixture. This layer was then covered with a layer of graphite/ethanol slurry. The sample was heated to 1900-2100 °C and the atomic rhenium was ionized with a relatively broad-band 10 Hz laser system at wavelengths of 280-284 or 296-300 nm. The mass analyzer was a 60° magnetic sector mass spectrometer equipped with a thermal ion source, an ion multiplier detection system and a transient digitizer was used to accumulate the pulsed ion flux. The measured atomic composition of a standard Re sample was 62.39% I87Re and 37.61% 185Re, in good agreement with the accepted values of 62.60 and 37.40% respectively [1], indicating equal ionization efficiency for both isotopes.

9.76 OSMIUM

Osmium is the seventy-sixth element in the Periodic Table. It has seven stable isotopes at mass numbers 184, 186, 187, 188, 189, 190 and 192. The representative isotopic composition of this element given by IUPAC in 1991 is 0.02, 1.58, 1.6, 13.3, 16.1, 26.4 and 41.0% respectively [1]. The l87Os isotope is also produced in rhenium-bearing minerals through ß decay of 187Re (half-life (4.56 ±0.11) x 1010years). The geochronological Re-Os dating method is based on this process, and the variable 187Os/186Os or 187Os/192Os ratios are the measured parameters.

Nier [433] used Os04 for the isotopic analysis of osmium. The vapor of the tetroxide was directly introduced into the ion source through a capillary and ionized by electron impact. Os+ ions and all the oxide ions from OsO+ to

OSMIUM 345

Table 9.69. Isotopic abundance ratios in osmium , 8 4 0 s / , 9 2 0 s

0.00043 0.000485

±0.000011 2.3* 0.000474

±0.000020 4.2*

186Os/192Os

0.0387 0.03889

±0.00011 0.3* 0.03842

±0.00014 0.4*

, 8 7 0 s / , 9 2 0 s a

0.0401 0.04017

±0.00003 0.06* 0.03790

±0.00015 0.4*

I88Os/192Os

0.324 0.32474

± 0.00005 0.02* 0.3244c

189Os/192Os

0.394 0.39593

±0.00004 0.01* 0.3955

1 8 90s/ 1 9 20s

0.644 0.64388 e

0.6443 ± 0.0005 ± 0.0007

0.13* 0.1*

Ref.

T4331 [4341

[4371

" Variable ratio due to 187Re decay in nature. * Relative standard deviations in %. c Normalization factor.

Os04+ were observed in the mass spectrum. Os+ ion was used for the osmium ratio evaluations. The results are shown in Table 9.69. Ratio calculations based on the OsO+ ions should be omitted if isobaric mercury ions are present in the ion source background. Spitzer and Sites [45] evaporated Os02 • H 2 0 from a tantalum ribbon at about 500 °C and ionized it by electron impact. Os + , OsO+, O s O j , OsO^ and OsOj ions were observed at relative intensities of 3:2:4:3:10. The ratios were calculated from the most intense ion currents, taking in account the isotopic distribution of oxygen. The ion source was cleaned after each analysis to eliminate memory effects.

The ionization potential of osmium (8.7 eV) is too high for the positive thermal ionization of this element. Voelkening et al. [434] developed the negative thermal ionization of osmium. A double platinum filament ion source with a single focusing magnetic sector mass spectrometer was used. A well mixed solution containing 1 pg of the element in the form of K20s04 and 7.5 pg Ba as Ba(OH)2 was loaded onto the evaporation filament. The same amount of barium as Ba(OH)2 was loaded on the ionization filament. Both filaments were heated to dryness and introduced into the mass spectrometer. Under optimal conditions: 830 °C for the evaporation and 1.9 A (<750°C) for the ionization filament, only OsO^ and OsO;¡" ions were observed at a ratio of 1000:1, with an ion current of 5 x 10~n A for OsOj . lOng and 1 ng osmium samples yielded ion currents of 1.5 x 10~" and 0.1 x 10~n A respectively. The best precision (RSD = 0.01%) was observed for the l90Os/192Os ratio when eight increments of K20s04 sample (Merck, Darmstadt) were analyzed. The measured value of this ratio, 0.64388, was used as an internal normalization factor to correct for isotopic fractionation. Each run included 10 blocks, and in each block all the osmium isotope intensities were monitored 11 times. The resulting data are given in Table 9.69. Voelkening et al. [434] proposed the use of the 1 8 70s/1 9 20s ratio, which can be measured with a precision of 0.06%, for Re-Os dating. They also emphasized that, to obtain high ionization efficiencies by NTIMS, the samples should be purified to the highest possible degree.


Walczyk et al. [435] improved the NTI determination of the osmium isotope ratio. In their procedure, (a) hexachloroosmic acid was used, because this compound is obtained after separation of osmium from geological samples [436]; (b) the platinum ribbon was boiled in concentrated nitric acid before mounting on the filament holders, then the mounted filaments were heated for 30 min at 3 A in air, followed by degassing at 3 A in vacuum; this treatment removed osmium traces in the Pt ribbon by oxidation to the volatile OSO4; (c) the amount of barium deposited on the filaments was increased to 20 pg; (d) oxygen at a pressure of 7 x 10 7 mbar (in preference to Fréons) was introduced into the ion source, increasing the ionization efficiency by a factor of 10, and (e) the evaporation and ionization temperatures were 800-850 and 760-820°C respectively. A 1 ng osmium sample yielded a 1 9 20s l 603 ion current of 2 x 1 0 " " A. Possible interferences were I 9 8Pt1 6Ol 80~ on I84Os16Oj and i87j^ei6Q- o n i87()sl60^, but their intensity was negligible and therefore neg-lected. The data were corrected for l 7 0 and 1 8 0 and normalized to the IUPAC value of 1 9 0Os/1 9 2Os= 0.6439 [1,433]. In general, using this procedure, the calculated RSDs were lower, i.e. 0.04 compared with 0.3 in ref. [434] for the 186Os/192Os ratio. Creaser et al. [429] also determined the osmium isotopic ratios by applying NTIMS. They used a platinum single filament ion source and Ba(N03)2 as ionization enhancing agent. The data were corrected for 1 7 0 and 1 80 contributions to the various Os^O^ ions and normalized to 188Os/192Os = 0.323394, which was derived from 0.3244 [1,433], to correct isotopic frac-tionation. An exponential correction was used. For two 4 ng and two 70 pg samples the 187Os/l86Os ratios of 1.5137 + 0.0008, 1.5145 ± 0.0013 and 1.514 ± 0.005, 1.509 ± 0.005 respectively were reported.

Price Russ and Bazan [437] applied inductively coupled plasma mass spectrometry (ICP-MS) to osmium isotopic ratio determinations. OSO4 vapor was generated and ionized in the argon plasma. This sample introduction procedure enhanced the ion signal by a factor of « 80 relative to the generally used solution nebulization. The average ratios from 12 analyses of a natural osmium sample are given in Table 9.69. In each analysis 1-5 ng of osmium were used. Bazan [438] and Richardson et al. [439] described Os04 generators. Figure 9.14 shows the generator used by Richardson. Osmium (and rhenium) isotope ratio determinations by ICP-MS were reviewed by Richardson et al. [440].

Luck and Allègre [425] used secondary ion mass spectrometry (SIMS) to analyze commercial (Merck) ammonium hexachloroosmate ( N H ^ I O S C I ô . The value for the l 9 2 0s / 1 8 8 0s ratio, 3.08271, obtained by Nier [433] was used as an internal standard to correct for mass fractionation. The resulting isotope ratios and their relative standard deviations are given in Table 9.70. It should be noted that the osmium isotopic abundances published by Nier in 1937 [433] are still considered by IUPAC as the 'Best Measurement from a Single Natural Source' [1]. (See also Appendix 2).

IRIDIUM 347

— ICP-MS

Sampling tube

Condensor

Cooling bath

Sample chamber

Sample reactants

Injection port

vanac »»»»

Glass frit

Waste

Figure 9.14. Diagram of the OSO4 generator. (Reproduced by permission of The Royal Society of Chemistry from J.M. Richardson et al, J. Anal. At. Spectrom., 4,465 (1989))

Table 9.70. SIMS isotopic abundance ratios in osmium [425] 184r Os/,88Os ,86Os/,!i8Os ,8'Os/,88Os ,8,,Os/l88Os lyuOs/,88Os ^ O s / ^ O s

3.08271* 0.0018 ±0.0002

11"

0.12057 ±0.00007

0.06"

0.11367 ±0.00007

0.06"

1.2232 ±0.0004

0.03"

1.9847 ±0.0008

0.04"

" Relative standard deviations in %. ' Normalization factor [433],

9.77 IRIDIUM

Iridium is the seventy-seventh element in the Periodic Table. It has two stable isotopes at mass numbers 191 and 193, with relative abundance of 37.3 and 62.7% respectively [1]. These values were reported by Baldock [441] in 1954, and in 1989 was still accepted by IUPAC as the representative isotopic composition for this element [1].

Spitzer and Sites [45] evaporated iridium powder from a tantalum filament at 1500-1800 °C and ionized the vapor by electron impact. For one analysis at least 1 mg iridium was needed. The difficulty of analyzing this element by


positive TIMS arises from its high ionization potential of 9.1 eV. Holden et al. [442] reported for the isotopic ratio 19lIr/193Ir = 0.595. Creaser et al. [429] determined this ratio by negative thermal ionization as 0.5948 ± 0.0002 (2SD). Thus, 5 ng iridium as hexachloroiridate salt dissolved in 2.5 N HCl was loaded onto a platinum filament previously coated with a Ba(N03)2 solution (20 pg Ba) and dried in air at 0.5 A filament current. The salt was then heated in the mass spectrometer at a pressure < 10"6 Torr to about 500 °C and analyzed as Ir02 in a single filament ion source magnetic mass spectrometer. Ion currents of 2 x 10~12 A were observed. The data were corrected for the contribution of the , 9 , l r l 6 0 , 8 0 " ion current at m/z = 225 to 1 9 3 l r l 6 0 2 . This method was recently improved by Walczyk and Heumann [443] using a double platinum filament ion source. The filaments were degassed under vacuum at a current of 3 A for about 1 h and coated with a Ba(OH)2 solution (20 pg Ba) in bidistilled water. Then the evaporation filament was loaded with a solution of ammonium hexachloroir-idate (usually 100 ng) and dried in air at a current of 1.4 A. Stable Ir02 ion currents above 10-11 A were obtained at 900-950°C and 850-900°C for the ionization and evaporation filaments respectively. Slow heating rates of the evaporation filament (100 mA min"1 and 10 mA min"1) at the early and the final stage of the analysis are essential. Introduction of oxygen into the ion source at a partial pressure of 7 x 10"7 mbar enhanced the iridium ionization efficiency. A further positive effect of the oxygen was the reduction of the high voltage drift, thus stabilizing mass calibration of the mass spectrometer. This effect, known in NTIMS, is related to the electron emission from the barium ionization enhancement agent. The reported iridium isotopic ratio obtained from 11 independent runs is 0.59418 ±0.00037 (ISD). In the NTIMS of iridium, negative platinum oxide ions are observed, thus, 190Pt16O17O" and , 9 2Pt1 601 70~ are potential interferences. In practice their ion intensities are negligible, below 10~15 A. In pure iridium samples no Os02 ions due to impurities were observed, thus interference from the isobaric 190Os and 192Os ions was excluded. Chang and Xiao [444] succeeded in producing Ir+ ions by PTIMS at filament temperatures of 1900°C. 26 pg of iridium was used and the 2.5 x 10~13 A ion current was monitored with a Daly electron multiplier. Five different samples with a total of 25 runs were analyzed. The mass discrimination factor was established by measuring the l 8 5Re/ , 8 7Re isotope ratio and the observed 19lIr/193Ir ratio of 0.59453 was corrected to 0.59399.

9.78 PLATINUM

Platinum is the seventy-eighth element in the Periodic Table. It has six stable isotopes at mass numbers 190, 192, 194, 195, 196 and 198, with an isotopic composition of 0.01, 0.79, 32.9, 33.8, 25.3, and 7.2% respectively [1].

MERCURY 349

Table 9.71. Isotopic abundance ratios in platinum ,9°pt/195pt

0.000376 0.000376

192Pt/I95Pt

0.02308 0.02308

194Pt/195Pt

0.9734 0.9734

196PJ/195PJ

0.7456 0.7485

198Pt/195Pt

0.2127 0.2133

Ref.

[164] [45]

White et al. [164] melted pure platinum onto a tungsten filament and thermally ionized the element at a temperature below 1800 °C. A twenty stage electron multiplier was used, coupled to an ion counting system. Spitzer and Sites [45] loaded finely divided platinum metal onto a tungsten filament. The filament was placed in a vacuum chamber, which was pumped down to a vacuum better than 10~2 Torr. The sample was slowly heated until it melted and was ionized by electron impact between 900 and 1500 °C. WO+ ions frequently appeared and may interfere. Pt+ ions were observed when the WO+ ions started to decrease. Hg+ ions may also interfere. Interferences from previous samples were not observed. The data are summarized in Table 9.71.

9.79 GOLD

Gold is the seventy-ninth element in the Periodic Table. It has one stable isotope at mass number 197. Leipziger [267], using a spark source mass spectrograph, has shown the existence of only one isotope. Gold may be also ionized by electron impact. Its powder was melted in vacuum onto a tungsten filament, vaporized and then ionized. The ionization potential of this element is too high for thermal ionization (IP = 9.225 eV).

9.80 MERCURY

Mercury is the eightieth element in the Periodic Table. It has seven stable isotopes at mass numbers 196, 198, 199, 200, 201, 202 and 204, with relative abundance of 0.15, 9.97, 16.87, 23.10, 13.18, 29.86 and 6.87% respectively [1].

Mercury has an ionization potential of 10.437 eV, which makes thermal ionization of this element impossible. All the available data about the isotopic composition were obtained by electron impact on elemental mercury or one of its heated salts; Hg+ givens the most intense ion current, followed by Hg2+. Mercury appears in the background of electron impact ion sources pumped by mercury diffusion pumps, thus very efficient pump cooling systems or oil diffusion pumps should be used. Alternatively, the mercury background of the mass spectrometer can be used for ratio determinations, increasing the partial pressure (i.e. vapor concentration) by warming the cooling system. Isotope


ratios of mercury were measured by Aston [445], Nier [62,158], Inghram et al. [446], Hibbs [418], Dibeler [447], Fleming [448], von Ubisch and Salg [448], Palmer [448], Spitzer and Sites [45], Nier and Schlutter [449] and Zadnik et al. [450]. Nier [158] had taken account of the mass discrimination effects by correcting the measured Hg2+ isotopic ratios with discrimination factors obtained from the measured and absolute isotopic abundance ratios of krypton and xenon. However, Nier recognized that the mercury used by him may already have been fractionated by distillation processes. Zadnik et al. [450] redetermined the isotopic composition of terrestrial mercury, especially the less abundant 196Hg isotope. A noble gas mass spectrometer with a mercury diffusion pump on the ion source end was used. The operating conditions were: 70 eV electron energy (40 eV in one set of measurements), 100 pA trap current, 4000 V ion accelerating voltage, detection with an electron multiplier using gains of 1.05 x 105 and 7.40 x 105 and a 108 ft resistor, and a mass resolution of 330 (10% valley). The background pressure was about 6 x 10~10 mbar, which corresponded to 0.02 x 10"12 g of mercury. The amount of mercury in the ion source could be increased to 260 x 10~12 g by warming the liquid nitrogen cooling system. The concentration of mercury was calibrated against xenon, assuming approximately the same sensitivity for both elements. For a nominal amount of 1 x 10 12 g of 202Hg, ion currents of 2.3 x 10 ,4 A and 6.0 x 10~15 A were obtained for 202Hg+ and 202Hg 2+ respectively. Mainly Hg+

ion currents were monitored in the static mode of operation, in which a mercury vapor concentration was established and then the ion source was disconnected by valves and isolated from the mercury input, and the element was analyzed.

Measurements were also performed in the dynamic mode at 70 eV, and at 40 eV and 70 eV (static mode), monitoring doubly charged mercury ions. These measurements were made to check on possible impurities and systematic errors. In one analysis, 25 cycles of 19 integral and half-integer masses between m/z = 194 and m/z = 204.5 were measured by magnetic peak jumping. The dwell time on each mass number was 10 x 1.28 s except for mass 196, which was sampled for 150 x 1.28 s. For the peak jump and magnetic field stabilization a dead time of 201.28 s was used. The peaks were centered at m/z = 198 and 204 after every five cycles. The duration of one analysis was «3.5 h. The measurements at m/z= 195, 197 and 203 did not reveal the presence of any hydrocarbon or mercury hydride and were used for baseline corrections. The ratios were calculated in each cycle by taking the baseline corrected values and the corresponding value of the linearly interpolated (between adjacent cycles) 202Hg isotope. Finally, a linear extrapolation of the ratios to zero time was calculated. Altogether 43 analyses were performed (in four modes). In the absence of a mercury isotopic reference material, the 199Hg/202Hg and the 204Hg/202Hg isotopic ratios determined by Nier [158] were assumed as correct values and used for mass fractionation corrections. Consequently, the following ratios were calculated:

THALLIUM 351

196 H g / 202 H g

0.005138 ± 0.000006

198Hg/202Hg

0.3338 ±0.0004

l 9 9 H g / 2 0 2 H g

0.5650 ±0.0005

20O H g / 202 H g

0.7734 ±0.0008

201 H g / 202 H g

0.4414 ±0.0004

2 0 4 H g / 2 0 2 H g

0.2299 ±0.0002

9.81 THALLIUM

Thallium is the eighty-first dement in the Periodic Table. It has two stable isotopes at mass numbers 203 and 205, with relative abundance of 29.524 and 70.476% respectively [1].

Thallium can be ionized directly by electron impact when the metal [451] or thallium iodide [194] is evaporated from a tungsten oven. Tl+ ions are monitored. The more practical method is thermal ionization of thallium salts. The ionization potential of the element is 6.108 eV Spitzer and Sites [45] used thallium nitrate deposited on a tungsten filament with a saturated solution of boric acid in a single filament ion source. The loaded filament was preheated in air to glowing red (750 °C), then heated in the ion source to 1000-1400 °C until Tl+ ions appeared and finally to a slightly higher temperature, at which sufficiently intense and stable ion beams were produced. The authors reported that lead impurity is an interference.

Dunstan et al. [452] established a very careful measurement procedure for the absolute determination of thallium isotope ratio, studying a wide range of experimental parameters. A single tungsten filament is used in preference to rhenium filaments, which may potentially produce ReO+ ions in the mass range 201-205, interfering with the thallium ions at m/z = 203 and 205. Even though no interference from a rhenium filament at 2200 °C had been observed (at ion current sensitivity of 2 x 10"16 A), tungsten was used. Only square flat top filaments were chosen; concave or convex filaments were avoided. The fila-ments were degassed at 3.0 A for 30 min. The current and duration of degassing were found to be critical; higher currents or longer times could shift the 205T1/203T1 ratio by as much as 0.15%. The thallium was stored in Teflon vessels as TICI3 in HNO3 ( 1 + 9 , v/v) at a concentration of 0.1 gl"1 Tl. One 10 pi drop, containing 1 pg TI, was loaded onto the filament. The sample was dried on a Class 100 clean air bench at an air flow of 30 ms~\ utilizing filament currents of 1 A for 10 min and 3 A for 5 min and an infrared heating lamp, which was controlled to produce a temperature of 50 °C on the filament surface. Then the filament was heated to a pyrometer controlled temperature of 860 °C in a dark room under a linear Class 100 clean air flow of 15 ms"1 for 1 min. The 205T1/203T1 ratio depends strongly on the temperature of this drying stage, which is believed to be the key factor in obtaining high precision ratio measurements. This dependence is shown in Figure 9.15. A very careful


2.389

2.388

2.387

to tr CO o cvi S o CM

2.386

E 2.385 to

XI r-

2.384

2.383

2.382 800 820 840 860 880

Drying Temperature (°C)

Figure 9.15. Dependence of 205T1/203T1 isotope ratio (Reproduced by Permission of NIST from L.P. Dunstan et al. Technol, 85, 1 (1980))

900

3n drying température. J. Res. Natl. Inst. Stand.

filament heating and ion current adjustment was used. The data collection started after 35 min at a 205T1 ion intensity of 2.5 V (10 u ft resistor) and lasted 25 min. Four ratio blocks, each of five ratios and each for 5 min, were collected. During this time interval the fractionation was very small, generally of the order of 2-3 parts in 104. The two mass spectrometers used were calibrated with mixtures of separated and purified TI isotopes. The thallium isotope ratio 2 0 5T1/ 203T1 of the natural sample (known as SRM 997) is 2.38714 + 0.00101, and the overall limit of error is the sum of the 95% confidence limits covering the known sources of possible systematic error. Any silica present interferes, decreasing the measured ratio by 0.2%. Potassium and sodium also interfere; 1 pg of Na shifted the measured isotope ratio to higher values. The work of Dunstan et al. [452] also quotes five publications between 1931 and 1949 in which isotopic ratio measurements were used to determine the atomic weight of thallium.

LEAD 353

9.82 LEAD

Lead is the eighty-second element in the Periodic Table. It has four naturally occurring, stable isotopes at mass numbers 204, 206, 207 and 208. Its composition is variable in nature. The representative isotopic composition of common lead is 1.4, 24.1, 22.1 and 52.4% respectively [1]. Only 204Pb is a non-radiogenic isotope of lead; the other three isotopes are end products of the radioactive decay of uranium and thorium:

238U -*206Pb + 84He + 6ß" (82.1) 235y _ 207 p b + 7 4 H e + 4 ß - (g2¿)

2 3 2 T h - > 2 0 8 Pb + 64He + 4ß" (82.3) The isotopic composition of lead in uranium- and thorium-bearing minerals

and also in lead minerals with low contents of these elements is the basis of various geological dating methods.

9.82.1 Electron Impact Ionization of Lead Aston [453] in 1927 was the first to determine the isotopic composition of common lead. Nier [454] reported the variations of lead isotopic ratios in lead minerals (galenas) from different sources. In the earlier works, isotopic ratios were measured by electron impact on the lead halides PbCl2, PbBr2 and Pbl2 [455]. This method demanded correction for the interferences of 204Hg+, usually appearing in the background of the ion source. Ionization of Pb(CH3)4 was also used. Again various interferences complicated the ratio evaluations [456].

9.82.2 Thermal Ionization of Lead Thermal ionization eliminates the interference and background problems in the isotopic analysis of lead. Another advantage is the small sample size of 15-300 ng needed for an ordinary ratio measurement. The ionization potential of lead is 7.416 eV Turnbull [234], with a triple rhenium filament ion source, ionized PbS04 and achieved a 208Pb ion current of about 10"13 A. Spitzer and Sites [45] used Pb(N03)2 with a drop of boric acid on a single tungsten filament. The sample was heated in air to about 750 °C prior to ratio analysis. They observed a stable lead ion current between 1000 and 1400 °C. Various ionization enhancing agents have been developed to increase the ion currents of lead. Akishin et al. [457] used a mixture of 40% Zr02 and 60% Si02, deposited with 10 pg lead (as nitrate) on a single tungsten filament. They observed ion currents up to 3 x 10~10 A at 1300 °C. Barnes et al. [280] used as ionization enhancer a silica gel prepared from hydrolysis of Na2Si03 • 9H20 and also a


suspension of a commercial silica gel powder. Taylor [458] prepared silica gel by hydrolyzing SÍCI4. Platzner [459] compared these three methods and concluded that the best results in terms of ion beam stability and intensity were obtained with a fresh suspension (prepared every 4-6 weeks) of commercial silica gel, following the procedure given by Barnes et al. [280].

Lead is an element with only one non-radiogenic isotope, thus the internal calibration method to correct for time dependent mass discrimination occurring during the analysis cannot be applied. An adaptation of the double spike technique which theoretically provides rigorous correction of the discrimination effect was introduced for lead by Compston and Oversby [460]. Three lead isotopic standard reference materials are available for instrument calibration: NIST SRM-981 (Common Lead), SRM-982 (Equal Atom Lead) and SRM-983 (Radiogenic Lead). Six samples for mass spectrometric calibrations were prepared by mixing weighed portions of two solutions prepared from nearly pure separated 206Pb and 208Pb isotopes with accurately known concentrations. Triple filament ion sources with rhenium sample filaments and a platinum ionization filament were used. 250 pg Pb as nitrate in 2% HNO3 and one drop of 10% NH4OH solution were loaded onto each sample filament. Data were acquired at a total lead ion current of (3-5) x 10"1 ' A within a period of 24 min. Table 9.72 summarizes the isotopic ratios of these SRMs [461]. The commercial multiple collector mass spectrometers introduced in the early 1980s improved the precision, but the accuracy of the ratio measurement is still constrained by the fractionation process. Hamelin et al. [462] also used the double spike technique for precise lead isotope ratio measurements. In their work a slightly lower value of the 208Pb/206Pb ratio in SRM-981 was measured (2.1650 + 0.0018), as observed also by Gulson et al. [463] (2.1630 ±0.0020) and by Platzner [459] (2.16605 ±0.00063). Todt et al. [464] prepared a lead spike solution with the 202Pb and 205Pb isotopes (half-life « 3 x 105 and 3 x 107 years respectively). The 202Pb/20SPb ratio was adjusted to 4.40 and the 202pb//204pb r a t i o w a s e q u a l t o 23 000. The spike was mixed with SRM-981 and a fractionation corrected 208Pb/206Pb ratio of 2.16715 ±0.00012 (2SD) was determined. Other ratios, where 208Pb was not involved, were in very good agreement with the values of Catanzaro et al. [461]. A multiple collector mass spectrometer with five Faraday cups was used.

Table 9.72. Isotopic abundance ratios in NIST lead standard reference materials. 2 0 4 P b / 2 0 6 P b 207 p b / 206p b 208pb /206p b

SRM-981 0.059042 + 0.000037 0.91464 + 0.00033 2.1681+0.0008 SRM-982 0.027219 + 0.000027 0.46707 + 0.00020 1.00016 + 0.00036 SRM-983 0.000371+0.000020 0.071201+0.000040 0.013619 + 0.000024 Errors given as 95% confidence limits [461].

LEAD 355

2.161 2.160 2.159

1 T

VA

18 x 10"

O «o ° 1 O o g

50 100 t-min

150

Fig. 9.16. 208Pb/206Pb ratio of individual data blocks vs. time, single collector data; O, multiple collector data. The arrows indicate that the spread of all the data is better than 18 x 10-5 (±3<r) [459]. (Reproduced by permission of Elsevier Science NL from I. Platzner, Int. J. Mass Spectrom. Ion Processes, 11, 155 (1987))

As already mentioned, the major source of uncertainty in TIMS is the isotopic fractionation process during the ionization. It was observed [459,463] that, when the silica gel method is applied, the mean ratio values of individual blocks (the mean of 10-15 successive ratios within an analysis) is independent of time. This behavior for the 208Pb/206Pb ratio in SRM-981 is illustrated in Figure 9.16. It is entirely different from the time dependent fractionation observed for many other elements. In this case, the mass discrimination factor e may be calculated assuming a first order linear approximation:

(^meas ~ Rti)/Rtr = cAm (82.4)

or Ä,r = Ämeas(l + c A m ) " (82.5)

where Rtv, Rme¡ÍS and Am are the true ratio, the measured ratio and the corresponding isotopic mass difference respectively. In SRM-981 and SRM-982, e = -0.00139 ± 0.00004 u"1 (except for the 208Pb/206Pb ratio in SRM-981) was calculated [459].

A sample for isotope analysis is prepared as follows [459]. Portions of 1 pi of 0.4 N phosphoric acid followed by 4 pi saturated silica gel suspension and 1 pi of 0.3 g I-1 (or less) lead solution in 0.2 N nitric acid are loaded onto a degassed rhenium filament. The filament is dried by passing d.c. and finally the sample is oxidized by heating to a dull glowing red for 2-3 s. Data are collected when the total ion current reaches 6 x 10"11 A. Small samples (15 ng Pb) may be treated in the same way, except that only 1 pi silica gel suspension is loaded. The total ion current is 1 x 10"11 A. Separation of 10 ng and smaller lead samples by anodic deposition has been demonstrated by Barnes et al. [280]. Tera and Wasserburg [465] used the silica gel/phosphoric acid procedure, with V-shaped


rhenium filaments to determine lead by isotope dilution mass spectrometry. High precision isotopic ratio analysis could be achieved for samples with as few as 8 x 1011 lead atoms. Contamination was reduced to a controllable level of 4 x 1010 lead atoms.

9.82.3 Inductively Coupled Plasma Mass Spectrometry and Laser Ablation ICP-MS of Lead

Measurements of lead isotope ratios using quadrupole ICP-MS have been reported since 1983 [466]. Longerich et al. [467] and Ketterer et al. [468] proposed a thallium correction technique. The addition of TI to the Pb standards and the subsequent simultaneous measurement of TI and Pb isotopes allowed corrections for instrumental bias and mass discrimination. Biases for lead ratios, normalized to 204Pb, could be controlled to less than 0.5% and RSDs of 0.2 were obtained.

High precision in lead isotope ratio measurements has been achieved with a double focusing multiple collector ICP-MS by Wälder and Freedman [317]. A 1 ppm lead solution at a flow rate of 0.2 ml"1 was introduced into an argon ICP ion source. Each sample consisted often 10 s measurements. Time independent mass discrimination bias factors of 0.0081-0.0087 u_1 were calculated by comparing the measured ratios in SRM-981 and SRM-982 with the NBS values. The RSDs of the ratios normalized to 204Pb are in the range 0.022-0.054% (1RSD). Walder et al. [318] with the same system measured the isotopic ratios of lead in the NBS SRMs using the thallium correction technique [467,468]. Data of high accuracy and precision were obtained. The measurement of one sample used about 200 ng Pb and took 100 s. Table 9.73 summarizes the results. It should be noted that the quoted errors do not include the uncertainty of the TI isotopic ratio measurements.

Walder et al. [469] coupled a Nd:YAG laser to the double focusing multiple collector ICP-MS and reported high accuracy and high precision in the isotopic

Table 9.73. Isotopic abundance ratios in NIST lead standard reference materials measured with a double focusing multiple collector ICP-MS

208p b / 2O4 p b ( a )

(b) 207Pb/204Pb(a)

(b) 2 0 6 p b / 2 0 4 p b ( a )

(b)

SRM-981

36.691+0.015 36.721+0.036 15.483 ± 0.008 15.491+0.015 16.937 + 0.008 16.937 + 0.011

SRM-982

36.702 + 0.022 36.744 + 0.050 17.141+0.009 17.159 + 0.025 36.711+0.021 36.738 ± 0.037

SRM-983

37.08 + 0.30 36.71+2.04

193.8 ±1 .6 191.9 ±10.5

2723.0 ±22.4 2695 ± 145

(a) Measured values, mean of six samples, errors given as 2SD. (b) NBS values [461].

LEAD 357

Table 9.74. Comparison of isotopic abundance measurements of lead in NIST-610 Glass standard

Method

LA-ICP-MS" TIMS* SIMS*

206 p b / 204 p b

17.051+0.016 17.049 + 0.012

207pb /206pb

0.9096 + 0.0008 0.9095+0.0006 0.9077 + 0.009

208p b /206pb

2.1670 + 0.0018 2.169 ±0.001 2.160 ±0.016

" Walder et al. [469], mean of six samples, errors given as 2SD. b Belshaw et at. [470], errors given as ISD.

ratio analysis of lead in the NIST SRM-610 Glass by laser ablation. The glass contains 426 ppm Pb, 62 ppm TI, botii certified, and an unquantified amount of Hg. Thallium ratio measurement allowed mass discrimination correction, and the well established ratio 204Hg/202Hg = 0.2293 was used to correct for the 204Hg+ contribution to 204Pb+. An analytical event (or sample) comprised the ablation of 12 sites, each for a 5 s period, with a 2 mJ energy laser pulse of 8 ns width and a repetition rate of 8 Hz. The results are shown in Table 9.74, together with data recently obtained by Belshaw et al. [470] using TIMS and SIMS. Compared with TIMS, the results and precision are in very good agree-ment, and the practically non-existent sample preparation chemistry is a powerful advantage of the method.

The transmission efficiency (atoms ablated/ions detected) of the system has also been calculated. Fairly constant crater dimensions of 40 pm diameter and 60-80 pm depth were observed under the above described ablation conditions. Knowing the glass density, lead concentration and isotopic abundance, the number of ablated 208Pb atoms per second corresponded to 2.8 x 1010. During the 5 s ablation, an average 208Pb+ ion current of 1 x 10~" A was recorded, therefore the ratio of ablated lead atoms to detected lead ions is about 450:1 . The atom losses occur mainly within the ablation cell, whereas ion losses take place within the sampler-skimmer cones interface and the ion beam profile shaping lenses at the entrance to the mass analyzer system. Also, the ablation cell was connected to the plasma torch with 3 m long Teflon tubing. Consi-dering the complexity of the ablation/ionization system, this is a remarkable transmission efficiency.

Feng et al. [471] studied the lead geochronology of zircons by laser ablation ICP-MS. A quadrupole mass analyzer was used. The precision of the 207Pb/ 206Pb ratio was in the range 0.5-6% (1RSD), and was strongly dependent on the lead concentration. The accuracy and the limitations of the method were evaluated by analyzing 21 zircon samples ranging in age from 1.0 x 109 to 2.7 x 109 years which had been also dated by conventional U-Pb thermal ionization mass spectrometry. The LA-ICP-MS 207Pb/206Pb ages for zircon grain sizes above 60 pm and 207Pb concentration above 3 ppm were within 1%


of the TIMS ages. The zircon grains used, at least 75 pm in diameter, were glued to the sample holder with epoxy cement. Walder et al. [472] in their LA-ICP-MS work [469] with the double focusing multiple collector mass spectrometer also investigated zircon laser ablation. It was observed that all the glues had a measurable lead blank. The best procedure for holding the zircon grain was to press it into a high chemical purity indium foil, which did not exhibit a lead blank.

When a precision of any of the values quoted above in Tables 9.73 and 9.74 is compared with the precision of the same ratio of an isotopic SRM, it should be noted that the first represents only the measurement uncertainty, whereas the latter includes also errors from various other sources inherent in the preparation of the SRM. It can be assumed that the modern automated multiple collector mass spectrometers are capable of providing data of a quality at least as good as or probably better than that of the instruments used for the above SRM measurements. On the other hand, this instrumental advantage is adequately compensated for by the skill and experience accumulated in certifying laboratories. Therefore a precision comparison based only on the standard deviation values is of limited value. For uncertainty evaluation in SRMs see Section 8.25.

It was noted in the first paragraph of this section that lead has a variable isotopic composition in nature, depending on its origin. In Table 9.75 examples of environmental Pb, including two extreme cases, the Missouri and Idaho ores, are shown.

Table 9.75. Isotopic abundance ratios in lead of natural and environmental origin Sample

Ores

Coal mines

Gasoline additives

Aerosols

Soils

Origin 206

Missouri, USAfl

Broken Hill, Australia Idaho, USA* Kentucky, USA Niederberg, Germany Montana, USA Houston, USA, 1970 Hong Kong, 1968 San Diego, USA, 1974 Helsinki, Finland, 1979 Chester, SC, USA, 1968 San Francisco, USA, 1968

• 204Pb= 1.24%, 206Pb = 27.10%, "̂Pb = 20.92%,

Pb/204Pb 207Pb/206Pb

21.78 17.04

16.45 19.72 18.25 17.64 19.43 17.00 19.02 17.39 19.43 17.45

^ P b ^ 50.73%.

0.772 0.884

0.951 0.799 0.838 0.888 0.820 0.917 0.826 0.891 0.809 0.888

208pb /206p b

1.872 2.179

2.210 1.964

2.128 2.012 2.168 2.028 2.132 2.008 2.138

Ref.

[473] [473]

[473] [473] [474] [473] [473] [473] [473] [475] [473] [473]

204Pb= 1.44%, 206Pb = 23.69%, ^"Pb = 22.53%, 208Pb = 52.35%.

RADIUM 359

9.83 BISMUTH

Bismuth is the eighty-third element in the Periodic Table. It has only one stable isotope at mass number 209. Nier [309] heated a small Nichrome furnace containing the metal and ionized the emitted vapor by electron impact. Leipziger [267] used a spark source mass spectrograph, searching for very low abundance stable bismuth isotopes other than 209Bi.

9.88 RADIUM

Radium is the eighty-eighth element in the Periodic Table. It has no stable isotopes. 226Ra is a naturally occurring nuclide formed in the decay series of 238U, with a half-life of 1622 years. 228Ra has a half-life of 6.7 years.

Recently Volpe et al. [476] and Cohen and O'Nions [477] reported ratio measurements of radium isotopes. Volpe used a 30.5 cm radius, 90 ° deflection magnetic sector thermal ionization mass spectrometer equipped with an ion counting detection system. Purified radium samples typically contained 1-2 fg [(1-2) x l O 1 5 g] of 228Ra and 500-600 fg of 226Ra. About 2-3 pg silica gel (in 1 pi of 0.05 M HCl) was loaded in the center of a platinum filament and almost dried at 1.0 A, followed by the sample dissolved in 1 pi dilute HCl, which was dried, again, at 1.0 A. The filament temperature was increased to 1200 °C within 30 min. Counting periods were between 30 and 60 s for each isotope and for the background at m/z = 227.5, and 10-15 data blocks, each of five ratio measurements were collected at filament temperature between 1250 and 1325 °C at an ion source pressure of 4 x 10~8 Torr or lower. Baseline counts were less than 1 ion s"1, 226Ra+ and 228Ra+ ion intensities were 4000-7000 and 10-225 ions s"1 and respectively. Measured precision in an individual sample was 1% (2RSD). No mass fractionation was revealed during an analysis. Cohen and O'Nions [477] used radium sample sizes of about 1 and 5 fg for 226Ra and 228Ra respectively. The Cambridge radium standard 'Ra-A' solution contained at the date of measurement 7.982 x IO"14 g g""1 radium, with a 228Ra/226Ra ratio of 4.708. A Ta-HF-H3P04 ionization enhancement solution, prepared following Birck [315], was loaded onto a cleaned (in boiling ultrapure water) and degassed (at 10"6 mbar, for 30 min at 5 A) tungsten center filament in a triple filament ion source with tantalum side filaments. This was followed by the sample, dissolved in « 0.5 pi of 1 M HCl, which was gently dried, and finally the filament was glowed at dull redness for 20-30 s. The isotopic ratios were measured with an ion counting system. Hydrocarbon interferences were often observed in the mass range of radium isotopes at an intensity of 1-100 cps. Heating the filaments at temperatures below significant sample evaporation and using a liquid N2 cryotrap located in the ion source reduced the background levels to « 0 . 1 cps. Data were collected when the center (sample) filament


temperature yielded a stable ion beam intensity of 200-300 cps for the 226Ra+

ion. Four determinations of the 228Ra/226Ra ratio in the standard yielded 4.7082 ±0.0212 (2SD). The uncertainty in a single determination was better than 1% (2RSD). Typical ionization efficiencies (ions collected/atoms loaded) were in the range 10-15% for the standard solution and a factor of « 2 lower for other samples, indicating the presence of impurities. No mass fractionation was detected during an analysis. The abundance of radium in the purified standard was determined by isotope dilution using the 226Ra NIST 4953D standard [478].

9.90 THORIUM

Thorium is the ninetieth element in the Periodic Table. It has no stable isotopes. 232Th is a naturally occurring isotope with a half-life of 1.39 x 1010 years. 230Th (half-life 8 x 104 years) is a radioactive daughter in the 238U decay chain.

Chen and Wasserburg [479] developed a procedure for the isotopic deter-mination of uranium in picomole and subpicomole quantities. This procedure, with a minor modification in the ionization temperature, was applied also for thorium ratio measurements [480]. A V-shaped or dimple type zone-refined rhenium filament degassed for 2 h at 2200-2400 °C was used. A suspension of colloidal graphite in water was loaded onto the filament, which was dried with gentle heat to form a thin uniform layer of graphite. The thorium sample in dilute nitric acid was loaded and dried, and finally another drop of the graphite suspension was deposited on the sample and dried. Samples containing 6 x 108

to 6 x 1010 atoms (0.2-20 pg) of 230Th or 232Th and « 2 x 10 u atoms of 229Th spike were analyzed at filament temperatures of 1810-1870 °C. The data were recorded in the sequence 229Th-230Th-230Th-232Th, using an electron multiplier with a gain of 4 x 103 in the analogue mode and an electrometer with a 109 ft feedback resistor. Forty to sixty ratios (20-30 cycles, with two 230Th measurements per cycle) were measured within 40-60 min. In a single analysis, the 230Th/229Th ratio was measured with uncertainties between 0.2 and 3%, depending on the 230Th isotopic content. Mass fractionation of 0.15% u"1 was determined for an analysis of duration 140 min, and about a half of this value for a 60 min period. Electron multiplier mass bias was corrected by multiplying the measured "Th/mTh ratio by (n/m)1/2. Reflection of 185Re+ and 187Re+ ion beams in the flight tube caused a linear background increase in the vicinity of the 230Th+ ion signal. Goldstein et al. [481] pointed out the difficulties in the mass spectrometric determination of the 230Th/232Th isotopic ratios in mid-ocean ridge basalts. The Th concentration in the basalts is of the order of 0.3 ppm and the 230Th/232Th ratios are « (5 -7 ) xlO"6. A thermal ionization mass spectrometer with two magnetic sectors in an S configuration, equipped with an ion counting detection system, was used. The abundance sensitivity was « 2 x 10~7 for Am/m = 2/232 when a triple rhenium filament

THORIUM 361

ion source was used. The dark current of the system was below 0.1 cps. The instrument was calibrated with NIST certified uranium standards. Sample sizes ranged from 50 to 400 ng Th. Samples of 200 ng (typical size) produced a stable 232Th+ ion current of > 4 x 10~12 A for at least 2 h. Ionization efficiency improvement by a factor of at least 10 compared with the single filament graphite loading technique [480] of an equivalent sample was observed. A 229Th tracer was added to the sample before measuring the 230Th/229Th ratio for « 2.5 h, followed by the measurement of the 229Th/232Th ratio for 1 h at reduced center and side filament temperatures. The first ratio was corrected for the 230Th content in the 229Th tracer ( « 1 %). The 230Th/232Th ratio was calculated by multiplying the measured 229Th/232Th and 230Th/229Th ratios. The precision for the 230Th/232Th ratio in a single determination was better than 1% (2RSD) for samples larger than 100 ng. No mass fractionation was revealed in the measurements of the two ratios. Cohen et al. [478] prepared a Cambridge thorium standard 'Th-A containing « 4 0 0 ppm Th in 1 M HNO3. The 232Th/ 230Th ratio was measured in the static mode, using about 400 ng thorium loaded directly on the side filaments of a triple rhenium filament assembly. The 232Th+

and 230Th+ ion currents were monitored with cross calibrated Faraday and Daly collectors respectively. The calibration was performed by switching an ion beam of « 105 cps ( « 1.6 x 10~14 A) between the Daly and the Faraday collector immediately after data collection using the sample ion beam. The mass spectrometer was equipped with a 30 cm diameter electrostatic filter, which enhanced the abundance sensitivity. Nine samples measured over a period of 3 months yielded 2 3 2Th/2 3 0Th= 9.126 x 104 with a precision of 0.35% (1RSD). Palacz et al. [482] measured the 230Th/232Th isotopic ratio in three thorium standards with a high abundance sensitivity, energy filtered thermal ionization mass spectrometer. The instrument was equipped with a 30 cm diameter electrostatic filter and the data were collected in the static mode with Faraday cup and electron multiplier ion counter detectors. 230Th/232Th ratios below 1 x 10~6 were measured with an abundance sensitivity lower than 5 x 10"6 at m/z = 230 with respect to 232. This was considered as an improvement of one order of magnitude over conventional magnetic sector mass spectrometers. The isotopic ratios could be measured with RSDs of ±0 .3% and one standard sample measured over a period of 18 months showed an external precision of ± 0.85%.

Recently, Esat [483] applied charge collection thermal ionization mass spectrometry for isotopic ratio measurements of thorium. For each Faraday cup in a multiple collector mass spectrometer, the original high resistor in the feedback loop of the electrometer was replaced by a capacitor. In this measurement mode a steady ion beam current / results in a monotonie increase in voltage V across the capacitor C (20 pF) during time t, according to

/ = C(SV/St) (90.1)


Table 9.76. Thorium RIMS isotopic ratio determinations at 384.0784 nm wavelength

Technique Enhancement 230Th/232Th laser power (W) ( x 10~4)

RIMS 1.9 3.825 ±0.057 RIMS 2.9 3.919 ±0.027 RIMS 5.2 4.080 ±0.035 TIMS - 4.096 ±0.009

i.e. the slope of V vs t is proportional to /. The technique is applied for Th+ ion currents in the range 10"16-3 x 10"12 A. 15 pg of 230Th yielded ion intensities up to 6 x \QH A over 1 h, with a 4% ionization efficiency. The analytical errors for a single run were better than 0.06%.

Fearey et al. [484] used resonance ionization mass spectrometry to measure the 230Th/232Th isotopic ratio. The resonant excitation was provided by either a frequency doubled Ti-sapphire laser or a standing-wave dye laser. The ionization process was enhanced with an additional Ar+ laser operating in the UV region (333-363 nm). Thus, 10 ng of thorium was loaded onto a rhenium filament, coated with graphite slurry and inserted into the ion source of a magnetic sector mass spectrometer. Ion detection was made with an electron multiplier and a pulse counter. Results for a thorium standard ionized with the 384.0784 nm laser wavelength are given in Table 9.76. The data clearly demonstrate the reduced thorium mass bias with the increase of enhancement laser power. The TIMS value is also included in the table.

9.91 PROTACTINIUM

Protactinium is the ninety-first element in the Periodic Table. It has 20 known radioactive isotopes. The most common isotope is 231Pa, with a half-life of 3.25 x 104 years; the others have half-life of 27.0 days (233Pa) and less.

Kukavadze et al. [485] showed that protactinium is readily thermally ionized from tungsten filaments. The element was loaded onto the filament from 0.1-1 N nitric acid. Differences in sample acidity affected the ionization process. An electron multiplier was used for ion detection. To avoid instrumental contamination, it was necessary to isolate the ion source region from the analyzer tube and detection system by a suitable valve. The latest models of USSR domestic production mass spectrometers (such as MI 1201 and MI 1320 of the UKMP type) provided satisfactory sensitivity and resolving power to analyze nanogram and smaller protactinium samples. Recently Pickett et al. [486] used isotope dilution thermal ionization mass spectrometry to measure

URANIUM 363

231Pa samples spiked with 233Pa. A zone refined, degassed rhenium filament was used, and the ionization was enhanced by the graphite technique used by Edwards etal. [480]. Samples as low as a few tens of femtograms of 23lPa were loaded onto the filament between two graphite layers. A 30.5 cm radius, 90° deflection magnetic sector instrument equipped with an electron multiplier and an ion counting system was used. The uncertainty of the procedure was ± 1 % at 95% confidence level.

9.92 URANIUM

Uranium is the ninety-second element in the Periodic Table. It has 14 radio-active isotopes, three of them naturally occurring at mass numbers 234, 235 and 238 with nominal (slightly variable in nature) abundance of 0.005, 0.720 and 99.275% respectively [1]. Among the eleven artificial isotopes, 233U and 236U have long half-lives of 1.59 x 105 and 2.34 x 107 years respectively. 233U is a fissionable nuclide; it may be used as nuclear fuel and also as a spike in isotope dilution mass spectrometry. It is produced by neutron irradiation of 232Th:

2 3 2Th(n7)2 3 3Pa+ß" -> 233 U + ß" (92.1) 236U is produced in the nuclear reactor burn up of uranium:

235U(n7)236U (92.2)

The unusual importance of this element, mainly the 235U isotope, in the Manhattan Project and later in the post World War II years in nuclear reactor, isotopic enrichment and nuclear weapon technologies and quality, safety and proliferation control, produced a rich crop of documentation on uranium isotope ratio measurements. A large proportion of it exists as laboratory reports, meeting abstracts and various technical notes. The uranium disintegration processes are used in geochemistry for dating uranium-bearing minerals. These include the Pb-U,Th; U, Th-Pb and U-He methods, each with its advantages and drawbacks. The half-lives of the naturally occurring isotopes and their final decay products are given in Table 9.77.

Following the various applications, two essentially different analytical ratio determination methods have been developed. These methods require different chemical form of the samples, different sample sizes and different construction of the mass spectrometer sample inlet and ion source. In nuclear fuel control and geochemical studies, uranium is usually analyzed as uranyl nitrate by the thermal ionization method. In enrichment processes, uranium hexafluoride (ultracentrifuge plants and thermal diffusion plants) or uranium vapor (laser enrichment) are analyzed by electron impact. High precision isotope ratio measurement of uranium using thermal ionization is accomplished with sub-microgram samples at relative standard deviations of «0.05%. The analysis of


Table 9.77. Abundances, half-lives and final decay products of naturally occurring uranium isotopes

Isotope

238U 235TJ 234U

Abundance (%)

99.275 0.720 0.005

Half-life (years)

4.46 x 109

7.04 x 108

2.45 x 105

Final decay product

2 0 6 p b

207p b

2 0 6 p b

uranium hexafluoride requires about 100 mg sample sizes, but precision better by at least one order of magnitude was demonstrated. The first accurate isotope ratio analysis was performed by Nier [487] in 1939. UCI4 and UBr4 were evaporated and ionized by electron impact. The low resolving power of the mass spectrometer allowed ratio calculations only on the less abundant U + ions in the mass spectrum of these halides. The existence of the 234U isotope was also discovered in these experiments. A third, relatively new isotope ratio determination method, applying ICP ionization, is also described.

9.92.1 Electron Impact Ionization of Uranium Hexafluoride

Electron bombardment of U F ö introduces several problems in a conventional electron impact ion source. When stray electrons hit the inner ion source surface, an insulating layer is formed from UF6 molecules adsorbed there. A further reason for solid deposits may be the background H20, which reacts with UFó to form oxyfluoro compounds. The formation of solid deposits made frequent ion source cleaning necessary, since source lifetimes of only a few hundred hours could be attained. Another very important phenomenon is the strong memory effect due to exchange reactions between gaseous UFô and the solid deposits, not only in the ion source but also along the inlet line, especially from the leak to the source. An additional memory problem affecting the measurement results may be due to charge exchange reactions between the solids and fluorouranium ions. Solid deposits may be expected also in many parts of the gas inlet system due to the highly corrosive nature of UFö, various impurities and background water. Brunnee [488] described a double collector mass spectrometer with a special inlet system and a dedicated ion source to minimize solid insulation layer formation and reduce the memory effects. After the sample container is connected to the inlet system, the gaseous UFô flows through a variable leak and a jet system, forming a molecular beam, into an open ion source, where 2 3 5UF^ and 238UF^" ions at m/z = 330 and 333 are produced by an intense, stable and sharply focused electron beam. Liquid nitrogen cold traps were located in the close vicinity, along and at the end of the jet system and the end part of the ion source to condense U Fô and avoid source

URANIUM 365

MOLECULAR BEAM

FILAMENT

COLD TRAP

ION BEAM

COLD TRAP

THREE WAY VALVE

TO-PUMP

LEAK VALVE

SAMPLE CONTAINER

Figure 9.17. Diagram of molecular beam system and ion source of a mass spectrometer for uranium isotopic ratio measurements in UF6. (Reproduced by permission on of Elsevier Science NL from C. Brunee, Adv. Mass Spectrom., 1, 234 (1963))

contamination before and after ionization. The schematics of the molecular beam system and the ion source are shown in Figure 9.17. The isotopic ratio measurement was performed by alternately admitting a sample and a standard gas. The peak tops were simultaneously scanned magnetically for about 3 min. Then the gas was pumped out and the second one was admitted. Each gas was monitored several times with a double collector detection system and the 2 3 5UF^/2 3 8UFj ratio was directly recorded. Ratio measurements of a sample and a slightly enriched standard are shown in Figure 9.18. The ratio of consecutive isotopic ratios was calculated and the RSD was within the limit of 0.03%.

a = (235UF5/238UF5)S , samp/(235UF5/238UF5)s t (92.3)

An example of the calculations after Brunnee [488] is shown in Table 9.78. The memory factor M is defined by

M = (1 - o , ) / ( l - am) (92.4)

where a, is the true ratio of isotopic ratios and am is the corresponding measured ratio. Brunnee [488] showed that, with the modifications in the inlet system and ion source, M < 1.003 was measured. Using the modified ion source but with a direct inlet, a value of M « 1.1 was observed. Smith and Jackson [489] discussed in detail the memory factor calculation in ultra-precise UF6 ratio measurements. The tail contribution from 238U+ ion current to 2 3 5U+


0,7124

Al/W [III [STANDAR!

i. M T\ 1 )

0,7124 i/"t/*i i * ^ n r hui

SAMPLE

[235U]

[239U1 =

L_

-—1 MIN-»

°'I1i2Jà

STANDARD

ZERC m OUTPUT

I Figre 9.18. Uranium isotopic ratio measurement. (Reproduced by permission of Elsevier Science NL from C. Brunnee, Adv. Mass Spectrom., 1, 239 (1963))

Table 9.78. Results of an isotopic ratio measurement

Sample 235U/238U a x 100

1 ° 7 1 9 5 3 0.98756 2 0.71058

0.98777 1 0.71938

0.98761 2 0.71047

0.98726 1 0.71964

0.98738 2 0.71056

0.98764 1 0.71945

0.98755 2 0.71049

0.98764 1 0.71938

Mean 0.9875 ±0.0003

URANIUM 367

(or vice versa in enriched samples) also introduced an error in the ratio measurement and had to be taken in account.

Cowan and Adler [490] made a compilation of 235U/238U isotopic ratio measurements of a variety of natural ores available from the Oak Ridge National Laboratory and the National Bureau of Standards (now the National Institute of Science and Technology), both in the USA. Only data obtained with high precision UFö gas mass spectrometers, which were stated to have relative errors at the 95% confidence level of 3 x 10"4 or better and for which there existed a reasonable basis for normalizing results to a common standard, were considered. Analytical results for 90 samples demonstrated the variability of the natural abundance of 235U. Two modal values of the isotopic ratio with a statistically significant relative difference of 0.03% were revealed.

The Oklo phenomenon, the existence of a naturally occurring fission reactor about 1.8 x 109 years ago in the West African Republic of Gabon, was also discovered through high precision isotopic ratio measurements of UFô. A small but persistent difference in the 235U isotopic abundance from the Oklo mines, relative to the well established and worldwide accepted value for natural uranium, was revealed in the UF(, mass spectrometry laboratory in Pierrelatte, France in 1972. The natural uranium atom abundance was routinely determined to be 0.7202% with a relative precision of 2 x 10"4 or ±0.00014%, compared with 0.7171% from the Oklo mines. Subsequent detailed study of the site revealed 235U depletion down to 0.29% [491]. The Oklo phenomenon will be discussed in more detail in Chapter 10.6.

Nagatoro et al. [492] developed an ion source and gas inlet system for U Fó isotopic ratio measurements with a quadrupole mass spectrometer. Under normal operational conditions, the ion source had a long life, exceeding 500 h (2000 ratio determinations), the memory factor was below 1.01 and both precision and accuracy were within ± 0.40% for UFß samples whose enrichment was beyond 1 % 235U. Depaus et al. [493] measured uranium isotopic ratios in UFö for nuclear material safeguards with transportable quadrupoles.

9.92.2 Thermal Ionization of Uranyl Nitrate

The thermal ionization method is used when small uranium samples are analyzed. The relatively low uranium ionization potential of 6.08 eV makes it possible to generate stable total U + ion currents of « 5 x 1 0 " n A, which are mass analyzed with magnetic sector spectrometers and monitored with Faraday collectors. To measure low isotopic abundance, currents of 3 x 10"10A may also be achieved. A very detailed study of this method was performed by Garner et al. [494] within the framework of preparation and certification of uranium isotopic standard reference materials. This group established and determined the variables affecting a uranium isotope ratio measurement. A set of 18 reference materials: SRM U-0002 to SRM U-970, with nominal 235U content


from 0.02 to 97 at%, was prepared by mixing weighed fractions of separated isotopes in the highly reproducible stoichiometric uranium compound U3O8. The 235U/238U ratios were certified to ±0.1%. Since then, uranium isotopic SRMs have also become available from other sources [495]. Their uses and further details will be discussed elsewhere in this section.

The enormous international expansion of the nuclear industry and increased safety requirements have introduced new demands on isotopic ratio measure-ments of nuclear materials, especially uranium and plutonium. The trend has been to make the sample sizes as small as possible and simultaneously to improve the measurement precision. Two approaches have been combined to solve these problems. Various techniques were developed to enhance the ionization efficiency of these elements, and more advanced mass spectrometers were designed and constructed. Studier [496] showed that heating the loaded filament before the analysis in a reducing atmosphere such as benzene vapor, or adding carbon to the sample, enhanced U + ion emission. Fenner [497] preferred to use hydrogen (mixed with 5-22% oxygen) as reducing atmosphere, avoiding increase of the hydrocarbon background in the mass spectrometer. V-shaped rhenium filaments were loaded with IO"7, 10 - 8 or 10_ 9g of uranium. The hydrogen treatment enhanced the ionization efficiency by almost 3 orders of magnitude relative to untreated samples. Samples treated with benzene showed about the same ionization enhancement. Walker et al. [498] developed the anion resin bead sample loading technique, using V-shaped, zone refined rhenium filaments. Smith and Carter [499] applied the same technique, adding Re powder slurried with a solution of sucrose in water. The ion emission was enhanced and the mass fractionation was significantly reduced. Rokop et al. [500] analyzed lOng uranium samples using electrodeposition on Re filaments. Covering the uranium sample already loaded onto the rhenium filament with a rhenium layer also enhanced the ion emission. Rec [501] used vacuum deposition to obtain the Re layer, and Perrin and Rokop [502] electro-deposited Re over U samples already electrodeposited on Re filaments. Chen and Wasserburg [479] developed a procedure for isotopic determination of uranium in picomole and sub-picomole quantities. A V-shaped or 'dimple' type zone refined rhenium filament degassed at a pressure of 10~7-10~8 Torr for 2 h at 2200-2400 °C was used. A suspension of colloidal graphite in water was loaded onto the filament and dried with gentle heat, forming a thin and uniform layer of graphite. The uranium sample in 0.1 M nitric acid was then loaded and dried. Finally, another drop of graphite suspension was deposited on the sample and dried. The best results were obtained when the Re filament was coated with graphite immediately after degassing. Samples containing 8.4pmol (2ng) of uranium yielded 2 3 8U+ ion currents of 5 x 10~13A for 2-3 h at filament temperatures of 1700-1750 °C. The 'dimple' type filament allowed loading of the sample on a smaller area and seemed to give better results for samples smaller than 40 pg.

URANIUM 369

The measurement precision was significantly improved by advanced instrumentation. In the late 1970s and early 1980s a new generation of thermal ionization mass spectrometers was simultaneously developed by two manu-facturers [503]. The main features of these instruments are: fully automatic, computer controlled operation and data acquisition; improved electronic stability; barrels accommodating up to 20 different samples; special magnet shape [504], which doubles the effective ion dispersion, including z-direction focusing properties; multiple, variable collector detection system equipped with up to nine Faraday collectors and also a secondary electron multiplier; and efficient ion source and flight tube pumping systems to achieve a vacuum of 2 x 10~8 and 2 x 10"9 Torr respectively. High ion current transmission and ion current stability are achieved with these instruments. Consequently, 1 pg or less of the element can be analyzed. Uranyl nitrate in solution of 1 gl"1 U, acidified with 0.1-0.2N HNO3, is loaded onto rhenium [494] filaments. Tungsten [164] and tantalum [505] filaments may also be used. The sample filament is then heated in air with a low electrical current to dryness and for a further short period (1-2 s) to dull redness, transforming the nitrate to oxide. Single, double and triple filament beads or other filament support arrangements are used. In the triple filament technique, 1 pi of the sample, containing 1 pg or less of uranium, is equally deposited on each of the two sample filaments and subsequently ionized on the ionizing filament. Typical heating currents for the generally used rhenium filaments are 1.8-2.5 A for the evaporating and 5 .2 -6A for the ionizing filament (depending on their dimensions). This corresponds to 1400-1600 °C and 2100-2150 °C respectively. Analytical procedures can be written which prescribe various heating conditions, measuring parameters and ratio calculations, including bias corrections for a particular analysis. They are also published as Technical Notes by the manufacturers.

Although uranium is one of the heaviest elements, isotopic fractionation is clearly observed in its thermal ionization. As the isotopes are radioactive, no isotopic pair with a constant ratio value is available, therefore the internal normalization correction technique is not applicable and an experimental instrumental bias factor must thus be established. Figure 9.19 represents a fractionation curve of 16 NIST U-500 samples taken with a fully automatic single Faraday collector instrument [506]. After 70 min from the start of the analysis, the RSDs of each sample (internal precision, RISD) were in the range 0.001-0.002% and the RSD for 16 samples (external precision, RESD) was in the range 0.03-0.04%. The time dependent bias factor /(70), defined as /(TO) = («me» - «tr)/Ätr (where R =2 3 5U/2 3 8U), equals 0.00178, or 0.00059 u . Another simultaneous multi collector ratio measurement of 15 NIST U-500 samples using four collectors is given in Table 9.79. The isotopic ratios were calculated after 60 min from the start of the analysis.

For accurate ratio determinations, the bias factors should be established with a SRM of isotopic composition as close as possible to the sample composition,


1.006

1.005 H

1.004

ce i 1.003 e

1.002

1.00H

1

RESD=0.04% RISD=0.002%

4\

0 —r-10

—r— 20

— i — 30

— i — 40 50

l 60

i 70

— I — 80 9C

t-min

Figure 9.19. Fractionation curve of 16 NIST U-500 samples, taken with a fully automatic single Faraday collector instrument [506]. Reproduced by permission of VG Elemental

Table 9.79. Isotope abundance ratio determination in NIST U-500 with a multicollector detection system

Mean SD RSD (%) Certified value Bias factor (u_1)

234U/238U

0.010449 ±0.000018

0.17 0.010422 0.00065

2 3 5 u / 2 3 8 u 1.001902

±0.000351 0.035 0.99970 0.00073

236IJ /238jj

0.001541 ±0.000011

0.73 0.001519

and each measured ratio should then be corrected accordingly. A constant bias factor (within the experimental error) for a wide range of isotopic ratios is expected if the only or the main reason for the deviation of a measured ratio from the true ratio is isotopic fractionation. One of the critical parameters in the reproducibility of isotopic analysis is to ensure that data in different runs for the same sample are taken on the same portion of an isotopic fractionation curve. A computer programmed procedure controls and tunes the evaporation and ionization filament temperatures or the total ion current in an almost constant sequence of steps, consequently the data are collected at approximately constant time intervals. Other parameters such as reproducibility of filament size, alignment in the source assembly, cleaning and conditioning, chemical uniformity of the sample, sample purity, size, acidity and loading procedure can improve to a great extent the external precision of an isotopic ratio analysis.

URANIUM 371

De Bievre [507] discussed in great detail several of these and also other parameters having general effects on a thermal ionization analysis, and particularly on uranium analyses. Although the article was published before the introduction of the fully automatic thermal ionization mass spectrometers, in practice the content is still of utmost importance. A parameter K, which accounts for the overall bias or the total systematic errors in isotopic ratio measurements, was introduced by Shields and co-workers in the U.S. National Bureau of Standards:

K = Rlb/Robs (92.4) where Rtb and Robs are the theoretical (true) and observed (measured) isotopic ratios. When, for a particular isotopic standard, Rohs equals Rtb within the experimental error, and this error, if quoted as relative external standard deviation, is in the order of 0.05% or better, the measurement is considered of high accuracy. When a suite of isotopic standards with a wide range of ratios is used to determine the K values for a particular mass spectrometer, a calibration or performance test of that instrument was carried out. This procedure is called a 'system calibration'. It measures the inherent inaccuracies caused by isotopic fractionations in the ion source and the non-linearities of the detection and measurement systems. Figure 9.20 shows a calibration result using NIST uranium SRMs [494] where K = 1.0000 ± 0.0005 [507]. The isotopic ratio values of these SRMs vary from /?th = 235U/238U = 0.0001755 to 186.78. They are shown in Table 9.80. Another set of uranium isotopic standards (uranium

i 1 r

I 8 a 8—e S Î-K= 1.0000 ±0.00005(1 s)

J 1 i i

" 2 3 8 10"3 10"2 10"1 1 101 102

U 0.1% 1% 10% 50% 90% 99%

Figure 9.20. Calibration of a thermal ionization mass spectrometer with NIST uranium isotopic SRMs. (Reproduced by permission of Heyden & Sons Ltd. from P. De Bievre, Adv. Mass Spectrom., 7A, 375 (1978))

1.0040 -

1.0020 -

1.0000 -

0.9980 -

0.9960 •

0.9940 -


Table 9.80. XS5\J/3X\J isotopic ratios in NIST uranium SRMs

SRM

U-0002 U-005* U-010 U-015 U-020* U-0306

U-050 U-100 U-150

Certified 235U/238U

ratio"

0.0001755 0.004918 0.010140 0.015566 0.02081 0.03143 0.05278 0.11360 0.18108

Nominal 235TJ

at %

0.02 0.5 1 1.5 2 3 5

10 15

SRM

U-200 U-350 U-500 U-750 U-800 U-850 U-900 U-930 U-970

Certified 235JJ/238TJ

ratio"

0.25119 0.5464 0.9997 3.1646 4.268 6.148

10.375 17.356

186.78

Nominal 235JJ

at %

20 35 50 75 80 85 90 93 97

• Uncertainties ±0.1% (2RSD). * In later NIST SRM price lists (and till 1987), these SRMs were replaced by U-005(A), U-020(A) and U-030(A).

Table 9.81. M3U/235U and 23SV/23aV ratios in CBNM uranium isotopic reference material standards 072/1-15

CBNM-IRM Certified molar ratios 233IT/235 U/235U< 235 u/2 3 8u'

072/1 072/2 072/3 072/4 072/5 072/6 072/7 072/8 072/9 072/10 072/11 072/12 072/13 072/14 072/15

1.00033 0.69967 0.49985 0.29987 0.10001 5.0091 xlO"2

1.9994 xlO"2

1.0165x10 2

5.0000 x l O 3

2.0012 xlO"3

9.6892 xlO"4

5.0088 x 10"4

1.0182 xlO"4

1.9996 xlO"5

1.9995 xlO"6

0.99103 0.99168 0.99212 0.99256 0.99299 0.99310 0.99317 0.99319 0.99320 0.99321 0.99321 0.99321 0.99321 0.99321 0.99321

" Uncertainties ±0.03% (2RSD). b Uncertainties ±0.02% (2RSD).

isotopic reference materials, IRMs) was prepared by Rosman et al. [508] and Lycke et al. [509] at the Central Bureau for Nuclear Measurements (now the Institute for Reference Materials and Measurements—IRMM), Geel, Belgium. It comprises 15 individual samples with 2 3 5u/2 3 8U ratios close to unity and

URANIUM 373 233U/235U ratios ranging from 1.00033 to 1.9995 x 10"6. The isotopic fractionation is determined from the 235u/238U ratio measurements. Assuming linear mass factionation, the measured 233U/235U ratios are corrected for isotopic fractionation. A comparison of the corrected ratios with their absolute values provides an assessment of the mass spectrometer linearity of response over the ratio ranges of interest. The isotopic composition of the IRM solutions, containing 1 mg of uranium (as uranyl nitrate) in 1 g of 2 M nitric acid solution is given in Table 9.81. Recently. Cohen et al. [510] performed high precision isotopic ratio measurements on radium and thorium and of the 235U/234U ratio in natural uranium. About 400 ng of purified uranium was loaded onto a triple rhenium filament assembly. Eight data blocks were collected, each for 40-50min, at an ion beam intensity of w (l-0.5)x 105 cps. The mean value of the 235U/234U ratio for the eight blocks was 132.40 ±0.22 (2RSD). Each data block was within the 2RSD uncertainty of the overall mean, demonstrating repeatability and the absence of significant mass fractionation during the course of a 6 h analysis. The CBNM-IRM standards were used to correct for mass fractionation and to test the overall mass spectrometer performance.

Chevalier et al. [511] prepared a solution of 233U and 236U with a well defined 236U/233U isotopic ratio, calibrated against the 235U/238U ratios of uranium isotopic standards. This spike solution, when mixed with a sample of unknown isotopic ratio, was used as an internal standard to correct for mass fractiona-tion and instrumental bias. Each measured 235U/238U ratio was corrected as follows

(235U/238U)COIT = (235U/238U)meas x Ç t y ^ ~ (92-4)

Ratio data with an accuracy better than 0.05% were obtained. The procedure was intended to be used for determining uranium concentrations by isotope dilution. Smith et al. [512] extensively studied the use of a uranium double spike internal standard for precise isotopic ratio measurements. Two double spike mixtures (233U + 236U) with nominal 233U/236U ratios of 0.89 and 1.0 were prepared with a high isotopic purity (99.674%) 236U spike and a low isotopic purity (89.27%) 236U spike respectively. Isotopic ratio measurements in a mixture of one of the internal standards and NIST U-010 demonstrated 3 to 8-fold improvement, increasing the precision to 0.06%, as compared with data for the same samples when bias corrections were determined externally with NIST U-500. Using a new state-of-the-art isotope ratio multi-collector mass spectrometer, the 233U/236U ratio in the low purity spike was determined with a precision of 0.005%.

Callis and Abernathey [316] showed that the total evaporation method introduced as a technique for isotopic fractionation correction by Wagner et al. [513] yields 235U/238U values for NIST U-010, U-020, U-100, U-500 and U-


900 which deviate from the certified values by +0.010 to +0.040% (the uncertainty of the certified NIST values is ± 0.1%). The important feature of the method is the evaporation of the entire sample and simultaneous integration of the ion signal from each isotope, thus essentially eliminating the effects of isotope fractionation in the evaporation process. A triple rhenium filament source was used and the smallest uranium sample was 20 ng. The total ion current was set between 7 x 10"" and 9 x 10"" A. The mass spectrometer was equipped with five adjustable Faraday collectors simultaneously monitor-ing the ion currents. Special software was developed to run the instrument in this mode of operation. The duration of an analysis was up to 25 min, to sample exhaustion, i.e. when the ion current decreased below 10"13 A. The results for 10 analyses of NIST U-500 (100-200 ng loadings) were as follows:

Mean SD RSD (%) Certfied value

234TJ/238JJ

0.010424 ± 0.000009

0.086 0.010422

235TJ / 2 3 8 I J

0.999768 ±0.000068

0.007 0.99970

236jj /238TJ

0.001523 ± 0.000007

0.46 0.001519

A further interesting advantage of the total evaporization method is the independence of the observed ratio on the sample size. Thus, 20, 50, 100 and 200 ng amounts of uranium from the NIST U-500 were loaded onto the filaments and the 2 3 5u/ 2 3 8U ratio was measured twice for each sample size. The means of two determinations for each loading were 0.99982, 0.99985, 0.99975 and 0.99962 respectively, in very good agreement with the certified value of 0.9997. This behavior is in contrast to the conventional technique, in which only a portion of the sample is consumed and the fractionation behavior, i.e. the curve of observed ratio vs time, is dependent on sample size. Generally, when a constant sample heating procedure is applied, the ratio change (slope of the curve) is more moderate for larger samples.

Fiedler et al [514] also studied the total evaporation method by comparing it with conventional TIMS, confirming the experimental observations of Callis and Abernathey [316]. Using 0.5, 1.0 and 2.0 pg of the Safeguard Analytical Laboratory (SAL, IAEA, Seibersdorf, Austria) 9484 uranium mixture sample, it was observed that in conventional TIMS measurements the 235U/238U isotopic ratio was affected by the sample size loaded onto the filament. This effect was not revealed in the total evaporation measurements. Furthermore, using the total evaporation method, better precision and accuracy were achieved. It was also concluded that the method is less dependent on the quality of the sample preparation chemistry. The results are summarized in Table 9.82. The isotopic abundance ratios in the SAL 9484 uranium isotopic reference material are the

URANIUM 375

Table 9.82. 23SU/Í38U isotopic ratio measurements in the SAL 9484 isotopic reference material

Sample

2

1

0.5

Average ISD RSD (%)

Reference ISD Relative diff. to reference

Total evaporation

235U/238JJ

1.00097 1.00045 1.00051 1.00025 0.99983 1.00020 1.00018 1.00034

1.00034 0.00033 0.033 1.00015 0.00023

+ 0.02%

Average

1.00064

1.00009

1.00027

Conventional TIMS

235JJ/238TJ

1.00415 1.00265 1.00115 1.00022 0.99151 0.99815 0.9991 0.9962

0.99914 0.00397 0.4

1.00015 0.00023

- 0 . 1 %

Average

1.00265

0.99663

0.9977

Diff. to reference

+ 0.00250

- 0.00362

- 0.00245

following:

233IT/238 U/238U 234TJ/-238TJ 235 TJ/238IJ 236T r/238 U/238U

1.05499 ±0.00040

0.03622 ±0.00021

1.00015 ±0.00023

1.0461 ±0.00023

Recently Fiedler [515] extended his total evaporation method studies to uranium and plutonium. It was observed that rhenium and tungsten filaments yield very good results for uranium using a multiple collector mass spectrometer and the NBS U-020 standard, whereas tantalum filaments yielded high results, differing from the standard mean value by almost twice the certified standard deviation. The observations for plutonium will be discussed in Section 9.94.

9.92.3 Inductively Coupled Plasma Mass Spectrometry of Uranium

Price Russ and Bazan [437] performed extensive uranium isotope ratio measurements. Uranium solution concentrations of NIST SRMs up to 1 pg ml"1

were used. The sensitivity was ss 1 x 106 CPS per ugml""1 when the resolution


Table 9.83. Isotope abundance ratios in six samples of NIST U-500, measured with a double focusing ICP-MS with four collectors

234TJ/238TJ 235JJ/238TJ 236^/238^

Mean SD RSD (%) Certified value Bias factor (u_1)

0.010183 ±0.000002

0.023 0.010422 0.0058

0.98174 ±0.00012

0.012 0.99970 0.0061

0.001523 ±0.000003

0.21 0.001519

Bias factor C calculated from Rü = J?meas x (1 + C) m

was set to < 10"4 overlap between adjacent ion signals. The interference of hydride ions relative to U+ was « 1 x 10~4. After correcting for dead time, background and mass bias, the 238U/235U ratios for < l p g m l - 1 solutions agreed with the standard values within 0.5% for ratios ranging from unity to 200. For the minor 234U and 236U isotopes, agreement was within « 10% for abundances as low as 2 x 10~5.

Walder and Freedman [317] and Walder et al. [516] analyzed uranium with a new prototype double focusing multiple collector ICP mass spectrometer; 1 ppm uranium solutions were nebulized with a Meinhard nebulizer. Sample analysis time was significantly reduced and sample preparation was considerably simplified. An external precision of 0.012% for the 235U/238U ratio was calculated on the mean of six samples of NIST U-500 (Table 9.83). The measurement time for all six samples, including between-sample washing, was less than 30 min. NIST U-010 and U-970 yielded for the same ratio 0.010155 ± 0.000004 and 186.27 ± 0.10 respectively, compared with certified values of 0.010140 ±0.000005 and 186.78 ±0.15 (all uncertainties ISD). The ICP ion source transmits heavier ions more efficiently than lighter ones, therefore the measured isotopic ratios show mass discrimination effects; the ratio of heavy to light isotope is larger than the true value. This effect is mass dependent and time independent. A mass bias factor per mass unit, Cp, is calculated using a power correction equation

Ä t r = Ä m e a s X ( l ± C p ) A m (92.5)

where Ru and Rmeas are the true and the measured isotopic ratios of a standard reference material and Am is the mass difference. The calculated bias factors in NIST U-500 are given in Table 9.83. The known bias factor is then used in normal samples to calculate the true ratio. Recently Taylor et al. [517] used the set of uranium isotopic standards IRMM 072 [509] to assess the linearity and mass discrimination of the new instrument. The power correction equation

PLUTONIUM 377

(92.5) and the exponential correction equation (92.6) provided the best results. Ktr = flmeas x exp (AmCe) (92.6)

Resonance ionization mass spectrometry has been applied for uranium isotopic ratio determinations by Donohue et al. [518]. The capability of the technique was demonstrated for U/Pu mixtures, therefore more details are given in Section 9.94.

9.93 NEPTUNIUM

Neptunium is the ninety-third element in the Periodic Table. The 237Np isotope has a half-life of 2.2 x 106 years, and is generated as a by-product in nuclear reactors in the production of plutonium. 236Np has a half-life > 5000 years.

Efurd et al. [519] thermally analyzed minute quantities of neptunium. A surface ionization-diffusion (SID) type ionization source with a single rhenium filament was used. The sample is electrodeposited onto the filament and overplated with platinum. The source is capable of detecting 105 atoms of neptunium and was used also for ruthenium, uranium, plutonium and americium. Further details of the technique are described in Section 9.94. The detection of trace amounts of neptunium was also demonstrated by Trautman [520], using resonance ionization mass spectrometry.

9.94 PLUTONIUM

Plutonium is the ninety-fourth element in the Periodic Table. It has 15 isotopes of which only 239Pu occurs in nature, in uranium ores, in extremely minute quantities as a result of neutron capture from spontaneous fission. The observed concentrations are of the order of one part in 10" parts of uranium. 239Pu is also the most important plutonium isotope; it is produced in extensive quantities in nuclear reactors from natural uranium

238U(n7)239U ->239 Np ^ 2 3 9 Pu (94.1 ) The 240Pu, 24lPu and 242Pu isotopes are formed from successive neutron

capture. Calculated isotopic distribution of irradiated plutonium in a thermal flux of 3 x l O ^ n c m - V is shown in Figure 9.21 [521]. The 238Pu and 244Pu isotopes were produced by deuteron bombardment of uranium in a cyclotron and by prolonged irradiation of plutonium, respectively. Table 9.84 shows the half-lives of the longer lived Pu isotopes.

Plutonium is a highly toxic radioactive element. It must be handled in specially designated laboratories providing adequate protection to the analyst. Plutonium has an ionization potential of 5.7 eV, which is favorable for thermal


100

242 Pu 239 Pu 80

LU O 60 LU

? 40

241 Pu 20 240 Pu

1 1 1

0.5 1.0 YEARS

Figure 9.21. The isotopic distribution of plutonium as a function of time of 239Pu exposure to a thermal neutron flux of 3 x 1014 n/cm"2s"1. (Reproduced by permission of the American Nuclear Society from O.J. Wide (ed.), Plutonium handbook. A Guide to the Technology, Vol. 1, 1980, p. 5)

Table 9.84. Half-lives of long lived plutonium isotopes Isotope

236Pu 2 3 8 p u 239Pu 240Pu

Half-life (years)

2.85 86 2.44 xlO4

6.58 xlO3

Isotope

241 Pu 242p u

244p u

Half-life (years)

14.3 3.79 x 105

8xl0 7

ionization. The upper limit of recommended plutonium sample sizes for thermal ionization mass spectrometric analysis is 10~7-10"8g. Multiple filament ion sources and magnetic sector instruments equipped with electron multipliers were used [522]. Several techniques have been developed to reduce the sample sizes and simultaneously to improve analytical precision. IO"9 g samples were analyzed with V-shaped rhenium filaments [522]. In both cases, multiple or V-shaped filament assemblies, plutonium nitrate in dilute nitric acid is directly loaded onto the filaments, dried and analyzed.

Walker et al [498] developed the resin bead sample loading technique, using V-shaped, zone refined rhenium filaments. The procedure was modified for extraction of uranium and plutonium from highly radioactive solutions for isotopic analysis and for quantitative determination of the two elements,

PLUTONIUM 379

applying the isotopic dilution technique [523]. Instruments with two 30 cm radius, 90° sector magnets and pulse counting detection systems were used, providing high abundance sensitivity and allowing the analysis of nanogram quantities of U and Pu. Dowex-1, 2% cross-linked anion resin beads of 150-250 pm diameter in the nitrate form were used. The sample solution in 8 M nitric acid was agitated with the resin for 10 min to allow sorption of sufficient quantities of uranium and plutonium. Only thorium and neptunium were co-adsorbed, but these elements do not interfere with the mass spectrometric analysis. A single bead was loaded into the V-shaped filament. The filament was slightly crimped to hold the bead and inserted into the mass spectrometer. Plutonium was analyzed at about 1450°C, any excess was burnt off, and uranium was analyzed at about 1750°C. The adsorption capacity of the beads was determined by choosing at random several beads from a larger batch, oxidizing each bead with concentrated nitric acid, redissolving the uranium and plutonium and spiking the solution with 233U and 242Pu. Care had to be taken to avoid sample contamination. The concentration of uranium and plutonium in an original input solution was also determined by isotope dilution. The aliquot of the solution taken for processing was adjusted by considering the following factors: (1) the permissible level of radioactivity; (2) the cost and availability of isotopic spikes; (3) the desired accuracy and precision of the isotopic dilution measurements; and (4) the reduction of contamination. The uranium concentration in the original solution was « 230 mgg"1 and the Pu:U ratio was up to 0.01. A dilution of 400 was the maximum that still yielded isotopic ratios with the desired precision. In isotope dilution analysis, the optimal ratio of the spike isotope to the most abundant sample isotope is 1. Therefore the final amount of the 233U spike should be about the same as the amount of uranium in the sample, and the amount of the 242Pu spike should be about 1% of the uranium spike multiplied by the atom fraction of the most abundant plutonium isotope in the sample. It was observed that an aliquot of the diluted solution, containing 1-2 mg uranium and consequently 10-20 g plutonium, was satisfactory to ensure adsorption of appropriate amounts on 1000 beads and to allow isotopic analyses of both elements from a single bead. Smith and Carter [499] further improved the resin bead technique to enhance ionization efficiency of Pu and U by adding a small amount of rhenium powder slurried with a sucrose solution to the V-shaped filament. Thus, 1 ng of Pu or U analyzed at a filament temperature of 1820°C yielded for several hours (3-4)xl05 cps without decaying. The 240Pu/239Pu isotopic ratio in NIST SRM-947 was measured with a relative standard deviation of 0.14% (n — 4). The isotopic fractionation was also significantly reduced. Recently, the resin bead-rhenium/ carbon slurry technique was utilized for samples as small as 5 fg Pu [524]. Starch or colloidon was used instead of sucrose. Reasonable ratios for 16 fg Pu samples of SRM 947 were obtained for isotopes free of interferences i.e. 240Pu/239Pu and 242Pu/239Pu.


Fassett and Kelly [525] evaluated the resin bead technique by carrying out an interlaboratory analysis program. Nanogram amounts of isotopic SRMs and unknown uranium, plutonium and U-plus-Pu samples were loaded onto anion exchange resin beads and distributed among participating laboratories. Accuracies and precisions achieved in measuring ratios of the major isotopes were better than 0.30%. The major source of imprecision was isotopic fractionation. No significant loading blanks or spectral interferences for U and Pu were observed. It has been shown that the resin bead technique is a convenient means of sample transport, distribution and analysis for safeguards accountability. Smith et al. [526] used the resin bead technique together with a mobile thermal ionization quadrupole mass spectrometer for on-site isotopic ratio measurements of uranium and plutonium; 1-3 ng of each element could be determined with a precision of 1-2%.

Accurate isotope dilution measurements require the establishment of equilibrium between the sample and the spike. This is easily achieved for uranium, because the +VI oxidation state is very stable in nitric acid. Plutonium, however, can exist in nitric acid in at least three oxidation states, IV, V and VI. The affinities of these for anion resin beads in 8 M nitric acid are greatly different, with Pu(IV) > Pu(VI) > Pu(V). Therefore, conversion of all plutonium to the Pu(IV) oxidation state is essential prior to any purification or adsorption treatment using anion resin exchangers. Walker et al. [523] adopted the equilibration method developed by Marsh et al. [527] Fresh and spiked aliquots of solutions of dissolved nuclear fuel were reduced with Fe(II) and sulfamic acid to the Pu(III) oxidation state, followed by oxidation with sodium nitrite to Pu(IV). This procedure, one of ten studied, was found to be > 99.9% effective. Plutonium tends to polymerize in solution, therefore aged dissolver solutions or solutions containing complex molecular species are spiked and evaporated nearly to dryness with perchloric and hydrofluoric acids and then treated by the above described reduction-oxidation procedure. Aggarwal et al.[528] treated the spiked solution with concentrated nitric acid at least twice for depolymerization and adjusted all the plutonium to the Pu(IV) state with H 2 0 2 in 3 M HNO3.

Strebin and Robertson [529] measured the isotopic composition of sub-program (A; 0.4 pg) environmental plutonium samples with a three stage thermal ionization mass spectrometer equipped with an ion electron scintilla-tion converter for ion detection and a 27r alpha proportional counter. V-shaped rhenium filaments carburized in benzene vapor were used, reducing the filament spectral background and improving the Pu+ ion beam intensity. The sample purification and concentration introduced a few percent positive bias at the minor 241Pu and 242Pu isotopic abundance.

Bergey et al. [530] proposed a plutonium electrodeposition procedure. The electrolytes were dimethyl sulfoxide and dimethylformamide. Reproducible, filament centered deposits of 2 mm2 area were produced. The deposited amount

PLUTONIUM 381

ranged between 0.5 and 200 ng with a yield ranging from 40 to 70%. An improvement in precision and sensitivity was demonstrated. Perrin et al. [531] modified a single filament, surface ionization diffusion (SID) type ion source for plutonium isotopic ratio measurements. Nanogram quantities of highly purified plutonium were electrodeposited onto a zone refined rhenium filament and overplated with platinum, again by electrodeposition. The filament was degassed before sample loading in vacuum for 30 min at 2100°C, and a - 9 0 V d.c. potential was applied to it during degassing. Any material in the sample that may co-plate with plutonium or platinum must be eliminated before the plating procedure. The platinum overplating solution was dinitrosulfatoplati-nous acid dissolved in hydrochloric acid. The optimum overplate thickness for a 1 ng sample was 110 ± 10 Â, measured by Rutherford backscattering using 3 MeV alpha particles. The plating procedure for this thickness was described in detail. Rhodium may also be used, but rhenium overplating caused several problems. Seventeen 1 ng samples of a 1:1 239Pu: 242Pu mixture were analyzed to determine the mass fractionation pattern of the platinum overplating technique. The sample was heated to 1450 °C within 20 min, baseline measurements were made for 10 min, and the data were collected for the next 25 min. A fractionation factor of 0.27% u"1 was determined. The same fractionation factor was calculated from nine determinations of the 240Pu/239Pu ratio in the NIST SRM-947. Table 9.85 summarizes the fractionation corrected plutonium isotopic ratio measurements in this SRM.

Callis and Abernathey [316] used the total sample volatilization technique for plutonium isotopic ratio analyses. The important feature of this procedure is the evaporation of the entire sample and simultaneous integration of the ion signal from each isotope, thus essentially eliminating the effects of isotopic fractionation in the evaporation process. A triple rhenium filament source was used. Sample sizes were 10-20ng Pu. The total ion current was set between 7 x 10~" and 9 x 10~u A. The mass spectrometer was equipped with five adjustable Faraday collectors simultaneously monitoring the ion currents. Special software was developed to run the instrument in this mode of operation. The duration of an analysis was up to 12 min, until sample exhaustion, i.e. until

Table 9.85. Plutonium isotopic ratios in NIST SRM 947

(a) (b) (c)

238Pu/239Pu

0.00367 ±0.00003

0.00363

240pu/239pu

0.24142 ±0.00010

0.24142

241Pu/239Pu

0.01580 ±0.00007

0.01580

241Pu/239Pu

0.01559 ±0.00011

0.01559 (a) Means of nine 1 (ig samples. (b) Measured uncertainties quoted as 2SD. (c) Certified values, decay corrected to date of measurement. Normalization ratio: 240Pu/ Pu = 0.24142.


Table 9.86. Isotopic composition of NIST SRM-947 plutonium standard measured by the total volatilization technique

Mean of eight samples SD RSD (%) Certified value

Atomic weight percent 238Pu 239Pu

0.2656 77.6597 0.0007 0.0014

±0.26 ±0.0018 0.264 77.66

2 4 0 p u 241 p u

18.8171 2.0334 0.0014 0.0008

±0.0074 ±0.039 18.816 2.034

242Pu

1.2242 0.0003

±0.025 1.226

Table 9.87. Total volatilization measurements of plutonium isotopic ratios in SRMs SRM Ratio

NIST-946 240Pu/239Pu NIST-947 240Pu/239Pu NlST-948 240Pu/239Pu CRM-128 240Pu/239Pu

Measured value

0.144963 0.241278 0.086353 1.00096

Deviation from SRM

(%)

- 0.019 + 0.003 + 0.049 + 0.022

Uncertainty of SRM

(%)

±0.14 ±0.14 ±0.14 ±0.026

the ion current decreased below 10"1 A. The results for eight analyses of a plutonium standard, NIST SRM-947 (10-20 ng loadings), are shown in Table 9.86. The Table demonstrates the high precision obtainable with this technique. Table 9.87 shows the measured isotopic ratios in various plutonium SRMs, corrected only for amplifier gains. The data are within the uncertainty limits of the 'true', unfractionated ratios, indicating the validity of the total volatilization technique.

Fiedler et al [514] also studied the applicability of total sample volatilization for plutonium isotopic ratio analyses. A double filament ion source was used, and typical sample loadings were 25 ng. Rhenium, tungsten and tantalum were tested with NIST SRM-947 for their performance as evaporation filaments. Tungsten exhibited reproducibility better by a factor of « 2 than that of rhenium. Tantalum filaments were not recommended because it was difficult to achieve a total evaporation measurement. A rhenium filament was used for ionization. It has been shown that the observed isotopic ratios are not affected by the sample size for 5, 10, 20, 50 and 100 ng Pu loadings. The technique provided accuracies at least as good as those of conventional thermal ionization, but the reproducibility of repeated measurements improved by a factor of 2- 4. It was necessary to perform amplifier gain calibrations. In recent work, Fiedler [515] has shown that the total sample volatilization method can be successfully applied for 1 ng Pu samples using the resin bead technique and multiple

PLUTONIUM 383

collector mass spectrometers. In the same work, it has also been shown that 50 ng Pu samples could be analyzed with quadrupole thermal ionization instruments.

One of the more important tasks of plutonium thermal ionization mass spectrometry is the simultaneous determination of Pu and U in irradiated fuel dissolver solutions. The important factors are the amount of plutonium produced in the uranium fuel and the isotopic composition of each of the elements. Maeck et al. [532] developed an isotope dilution technique in which uranium and plutonium, previously spiked with known quantities of 233U and 242Pu, were simultaneously and quantitatively extracted from a sample solution into methyl isobutyl ketone as tetrapropylammonium trinitrate complexes. This provided a separated uranium-plutonium fraction free from cladding compo-nents, fission products, and americium and curium. Uranium and plutonium were then stripped from the organic phase into hydrochloric acid, plutonium was oxidized with HNO3/H2O2, and finally both elements were determined from a single loading on a triple rhenium filament assembly. The ionization filament was held at 5.5-6 A. U+ ions appeared at lower sample filament temperature. The 2 3 5u/2 3 8U ratio was monitored, followed by 233U/235U and, at slightly higher temperatures, the ratios of the minor 234U and 236U isotopes to 235U. Pu+ ions appeared at higher sample filament temperature. The ratios were monitored in the following order: 242Pu/239Pu, 240Pu/239Pu and 241Pu/240Pu. The separation flow diagram is shown in Figure 9.22.

The process of stripping into hydrochloric acid is of particular interest. It is dependent on acid strength and is more efficient for plutonium than for uranium at higher acid concentration. Figure 9.23 demonstrates this phenomenon. Consequently the Pu/U ratio in the aqueous phase can be adjusted to a selected value. In Figure 9.24 the stripped Pu/U ratio is shown as a function of hydrochloric acid concentration based on an initial 1:1 ratio. Assuming a uranium : plutonium ratio of 250 in the dissolver sample, a stripping medium of 4 N HCl will yield a mixture with a 12.6: 1 ratio, which is favorable for the mass spectrometric measurements. A 0.5 ml aliquot of a 500-fold diluted dissolver sample, containing about 1 pg Pu, is an optimal sample on which to carry out the above described determinations. The bias due to isotopic fractionation was corrected by measuring the 235U/238U ratio in NIST SRM U-500, assuming linear mass discrimination. Details of Maeck's procedure [532] are given in ref. [522], Section 2.506.

Hagemann et al. [533] prepared and calibrated a double internal standard for precise uranium and plutonium mass spectrometric determination in irradiated nuclear fuel. The standard contained a mixture of 233U, 236U, 242Pu and 244Pu. Uranium and plutonium isotopic ratios were calibrated against SRMs U-200, U-500, U-850 and Pu-947. The concentration was determined by isotope dilu-tion using French reference materials MU-1 (U) and MP-1 (Pu) of known concentration to an accuracy of 0.1%. The certification results of the quadruple


233 242 Sample mixed with U and Pu spikes, KMn04 oxidant and aluminum nitrate salting solution

U and Pu extraction into methyl isobutyl ketone

—». Aqueous solution discarded

Organic phase

L Addition of tetrahexylammonium iodide reductant

Stripping of U and Pu into HCl

Organic solution discarded

Aqueous phase

I Evaporation with H202 and HN03

Mass spectrometric analysis

Figure 9.22. Uranium-plutonium separation flow diagram

spike solution were: 233U/236U = 2.36985 ± 0.00025 244Pu/242Pu = 0.44970 ± 0.00010 Concentration U = (1426.79 ± 0.09) pgg"1

Concentration Pu = (10.158 ± 0.004) pgg-"1

The spike solution allowed precise correction of mass fractionation. An improvement by a factor of 3 was obtained in the precision of the U and Pu concentrations compared with the procedure utilizing only two spike isotopes.

Dubois et al. [534] applied dynamic multi-collection for simultaneous isotope ratio and isotope dilution analysis of uranium and plutonium mixtures, and compared it with static data acquisition. A five-faraday collector detection system with a single mass unit dispersion was used, initially setting the 235U

PLUTONIUM 385

100

c- 8 0 c <u E a ® 60 o

40

20

• Plutonium

Uranium

2 4 6 8 HCl Concentration (normality)

2 4 6 HCl Concentration (normality)

Figure 9.23 Figure 9.24 Figure 9.23. Stripping of U and Pu as a function of HCl concentration. (Reproduced by permission of McGraw-Hill, New York, from W.J. Maeck et al. Nucleonics, 20, 80 (1962)) Figure 9.24. Pu to U ratio as a function of HCl concentration based on an initial 1:1 ratio. (Reproduced by permission of McGraw-Hill, New York, from W.J. Maeck et al, Nucleonics, 20, 80 (1962))

and 241Pu isotopes into the axial collector. A synthetic sample contained the NIST U-500 and Pu-947 and the certified quadruple spike solution prepared by Hagemann et al. [533] (see above). Amounts of 2pg uranium and 500 ng plutonium were loaded onto the sample filaments of a triple rhenium filament assembly. The total ion current was adjusted to 3 x 10~u A. U + ions appeared before Pu+ ions. The advantages of this procedure are the internal bias correction due to isotopic fractionation and cancellation of the (not accurately known) Faraday collector gains during the course of the actual measurement. The average ratios for seven samples are summarized in Table 9.88.

Table 9.88. Dynamic and static multi-collection isotopic analysis of a double spiked NIST U-500 and Pu-947 mixture

Isotopic ratio 235TT/238 U/238TJ 239 Pu/240Pu

NIST certified value Dynamic multi-collection Deviation from NIST Static multi-collection Deviation from NIST

0.9997 0.99977 + 0.00015

±0.0076% 0.99934 + 0.00015

- 0.036%

0.24127 0.24118 + 0.00003

0.24115 + 0.00006


Ramakumar et al. [535] used a nine variable collector detection system for uranium-plutonium isotopic analysis in dissolver solutions of irradiated nuclear fuel.

Donohue et al. [518] applied the resonance ionization mass spectrometry technique to measure U and Pu isotopic ratios in mixtures of the two elements. Samples were prepared using the NIST U-500 and Pu-947 SRMs. 5 ng of each element was absorbed onto an anion exchange resin bead (Dowex 1-X2) and two beads were loaded into a V-shaped Re side filament of a triple thermal ionization filament assembly and coated with a colloidal graphite suspension. The mass spectrometer was tuned with ions produced by thermal ionization, then Pu analysis was performed by atomizing the sample at a temperature of 1300-1400 CC and irradiating with 588.04 nm photons. Uranium was analyzed at 1500-1600°C with 591.54nm photons. At these wavelengths the photo-ionization was reported to be a three photon, two intermediate level process [536]. In both cases the laser was operated at a repetition rate of 30 Hz and 1 ps pulse duration. A defocused laser beam was used, the laser spot size varying from 2 to 4 mm2 over the 1 cm length of the ion source extraction slit. The ions were collected on a multiplier using the peak jumping mode. The sensitivity was increased by using a pulsed thermal atomization technique synchronized with the ionizing laser. This resulted in a 10-fold improvement of the sample consumption compared with continuous atomization. 238Pu and 241Am are potential isobaric interferences in 235U/238U and 241Pu/239Pu ratio determina-tions, therefore various selectivity ratios for U, Pu and Am were measured at the wavelengths used. The obtained values were sufficient not to obscure the ratios of interest. Table 9.89 lists the averages of the measured ratios from five separate loadings. No mass bias corrections were made. Regarding the obtained sensitivity, selectivity, precision and accuracy, Donohue et al [518] concluded that the RIMS technique is potentially useful in safeguards and nuclear materials accountancy.

Resonance ionization mass spectrometry has also been applied for the detection of plutonium [537-540].

Plutonium isotopic reference materials with certified molar isotopic abund-ance ratios suitable for calibration of isotopic abundance ratios measurements,

Table 9.89. RIMS U and Pu isotopic ratios in a mixture of NIST U-500 and Pu-947 SRMs

Mean SD RSD (%) Certified values

240pu/239Pu

0.2464 0.0006 0.24

0.2414

241pu/239pu

0.0356 0.0005 1.4

0.0341a

235U/238U

1.0280 0.0067 0.65

0.9997 • Corrected to date of analysis for ^'Pu decay.

REFERENCES 387

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[515] R. Fiedler, Int. J. Mass Spectrom. ¡on Processes, 146/147, 91 (1995). [516] A.J. Wälder, P.A. Freedman, I.D. Abell, P. Walkden and I. Platzner, Proc. 40th

ASMS Conf. on Mass Spectrometry and Allied Topics, Washington, DC, 1992, p. 766.

[517] PDP. Taylor, P. DeBievre, A.L Wälder and A. Entwistle, J. Anal. At. Spectrom., 10, 395 (1995).

[518] D.L. Donohue, D.H. Smith, J.P. Young, H.S. McKown and C.A. Pritchard, Anal. Chem., 56, 379 (1984).

[519] D.W. Efurd, J. Drake, F.R. Roensch, J.H. Cappis and R.E. Perrin, ¡nt. J. Mass Spectrom. ¡on Processes, 74, 309 (1986).

[520] N. Trautmann, Detection and speciation of trace amounts of neptunium and plutonium, in Transuranium Elements, A Half Century, L.R. Morss and J. Fuger (eds.), American Chemical Society, Washington, DC, 1992, pp. 159-167.

[521] Plutonium Handbook. A Guide to the Technology, Vol. I, O.J. Wick (ed.), American Nuclear Soc, 1980, p. 5.

[522] Selected Measurement Methods for Plutonium and Uranium in the Nuclear Fuel Cycle, R.L Jones (ed.), US Atomic Energy Commission, Washington DC, 1963; and 1983 Annual Book of ASTM Standards, Volume 12.01, Method C-697, ASTM, 1916 Race Street, Philadelphia, PA 19103.

[523] R.L. Walker, LA. Carter and D.H. Smith, Anal. Lett., 14, 1063 (1981). [524] D.H. Smith, D.C. Duckworth, D.T. Bostick, R.M. Coleman, R.L. McPherson and

H.S. McKown, Proc. 42nd ASMS Conf. on Mass Spectrometry and Allied Topics, Chicago, IL, 1994, p. 839.

[525] J.D. Fassett and W.R. Kelly, Anal. Chem., 56, 550 (1984). [526] D.H. Smith, J.R. Walton, H.S. McKown, R.L. Walker and J.A. Carter, Anal. Chim.

Acta, 142, 355 (1982). [527] S.F. Marsh, R.M. Abernathey and J.E. Rein, Anal. Lett., 13, 1487 (1980).

4 0 2 ISOTOPE RATIO MEASUREMENT PROCEDURES

[528] S.K. Aggarwal, G. Chourasiya, R.K. Duggal, R. Rao and H.C. Jain, ¡nt. J. Mass Spectrom. ¡on Processes, 69, 137 (1986).

[529] R.S. Strebin, JR. and D.M. Robertson, Anal. Chim. Acta, 91, 267 (1977). [530] C. Bergey, J. Cesario and S. Deniaud, Analusis, 8, 490 (1980). [531] R.E. Perrin, G.W. Knobeloch, V.M. Armijo and D.W. Efurd, ¡nt. J. Mass Spectrom.

¡on Processes, 64, 17 (1985). [532] W.J. Maeck, M.E. Kussy, TO. Morgan, J.E. Rein and MT. Laug, Nucleonics, 20,

80 (1962). [533] R. Hagemann, J. Cesario, M. Lucas and G. Retali, CEA-CONF-8885, Interna-

tional Conference on Nuclear and Radiochemistry, Beijing, China, 1986, p. 1. [534] J.C. Dubois, E. Eliot and G. Retali, CEA-CONF-9901, 11 Symposium on Safe-

guards, Esarda, Luxembourg, 1989. [535] K.L. Ramakumar, R.M. Rao, L. Gnanayyan and H.C. Jain, Int. J. Mass Spectrom.

Ion Processes, 134, 183 (1994). [536] H. Chen and C. Borzilieri, J. Chem. Phys., 74, 6063 (1981). [537] U. Kroenert, J. Bonn, H.-J. Kluge, W. Rüster, K. Wallmeroth, P. Peuser and N.

Trautmann, Appl. Phys., 38B, 65 (1985). [538] P. Peuser, G. Herrmann, H. Rimke, P. Sattelberger, N. Trautmann, W. Rüster, F.

Ames, J. Bonn, H.-J. Kluge, U. Kroenert, E.-W. Otten, Appl. Phys., 38B, 249 (1985).

[539] N. Trautmann, G. Herrmann, M. Mang, C Muhleck, H. Rimke, P. Sattelberger, F. Ames, H.-J. Kluge, E.-W. Otten, D. Rehklau and W. Rüster, Radiochim. Acta, 44/45, 107 (1988).

[540] N. Trautmann, Detection and speciation of trace amounts of neptunium and plutonium, in Transuranium Elements, A Half Century, L.R. Morss and J. Fuger (eds.), American Chemical Society, Washington, DC, 1992, pp. 159-167.

[541] (a) Institute for Reference Materials and Measurements-IRMM (former Central Bureau for Nuclear Measurements), European Commission-JRC, B-2440 Geel, Belgium. (b) National Institute of Science and Technology, Washington, DC 20234, USA. (c) US DOE, New Brunswick Laboratory, 9800 S.Cass Ave. Argonne, IL 60439.

[542] J.K. Aggarwal and M.R. Palmer, Analyst, 120, 1301 (1995). [543] Reference and intercomparison materials for stable isotopes of light elements,

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[544] K.G. Heumann, S. Eisenhut, S. Gallus, E.H. Hebeda, R. Nusko, A. Vengosh and T Walczyk, Analyst, 120, 1291 (1995).

[545] G. Lapijtas, F. Hendrickx, A. Verbruggen and A. Lamberty, Int. J. Mass Spectrom. Ion Processes, 152, 69 (1996).

CHAPTER

10

APPLICATIONS OF ISOTOPE RATIO MASS SPECTROMETRY

10.1 ATOMIC WEIGHTS 403 10.2 ISOTOPE DILUTION 405 10.3 PHYSICAL AND NUCLEAR CONSTANTS 409 10.4 AGE DETERMINATIONS IN GEO AND COSMOCHEMISTRY 412 10.5 NUCLEAR SCIENCES AND TECHNOLOGY 418 10.6 THE OKLO NATURAL NUCLEAR REACTORS 419 10.7 PLANETARY MASS SPECTROMETRY 421 10.8 STABLE ISOTOPES IN NUTRITIONAL, MEDICAL AND

BIOLOGICAL STUDIES 424 10.9 FOOD AUTHENTICATION AND ADULTERATION CONTROL 428 10.10 ENVIRONMENTAL STUDIES 431 10.11 ISOTOPE EFFECTS AND ISOTOPE ENRICHMENT

PROCESSES 436 10.12 ELUCIDATION OF CHEMICAL REACTION MECHANISMS 438 10.13 ISOTOPE ARCHAEOMETRY 439

REFERENCES 441

10.1 ATOMIC WEIGHTS

The atomic weight (A.W.) of an element can be calculated using the known isotopic abundances and the corresponding atomic masses of the nuclides of that element

A . W . - f ^ M , - x A , - ) / f > (1) i= i i= i

where M, and A¡ are the atomic mass and the abundance of isotope i respectively, and n is the number of isotopes in the element.

Atomic masses have been determined with mass spectrometers since the early days of mass spectrometry [1]. Recently, Wapstra [2] described the contributions of A.O. Nier and his co-workers to accurate atomic mass measurements using double focusing mass spectrographs and mass spectro-meters, published in the 1950's, and pointed out the very good agreement with modern values. Kluge et al. [3] discussed the Penning trap mass spectrometer as

404 APPLICATIONS OF ISOTOPE RATIO MASS SPECTROMETRY

Table 10.1. The most precise known atomic masses Atomic mass Error (ISD) Relative

uua pu error x 1010

'n 'H 2D 3 T

3He 4He 13C 14C

14 N

1 6 0

20Ne 40 Ar

1008664.9236 1007825.03190 2014101.77795 3016049.2677 3016029.3094 4002603.2497 13003354.8383 14003241.9906 14003074.0074 15994914.6223 19992440.1764 39962383.1235

0.0023 0.00057 0.00062 0.0014 0.0012 0.0015 0.0049 0.0042 0.0018 0.0025 0.0030 0.0050

2.3 5.7 3.1 4.6 4.0 3.7 3.8 3.0 1.3 1.6 1.5 1.3

' The atomic mass unit is defined as u = (l/12)m( C).

the most accurate instrument for mass comparisons. Accuracy of 1 part in 1010

can be achieved for masses of stable isotopes, and 1 part in 107 for short lived isotopes. An extensive table of atomic mass values is given by Audi and Wapstra [4]. In Table 10.1 a list of the most precisely known atomic masses is presented [4]. As may be seen from this table, the atomic masses are referred to the I2C nuclide of carbon, defining the relative nuclidic mass of this isotope as 12, therefore the mass unit 'u' is 12C = 12 u. The evolution of this atomic mass scale beginning in 1960, when the chemical atomic weight standard and the physical atomic mass standard were unified, was described in detail by Duckworth and Nier [5]. Duckworth et al. [6] described the evolution of mass spectrometric instrumentation for precise atomic mass determinations. It was shown that, from the 1910s up to the mid 1970s the precision has improved from 1 part in 100 to 1 part in 107. Atomic mass measurements have been reviewed by De Laeter et al. [7].

The most reliable and by far the most frequently used method for die determination of isotopic abundances may be described as calibrated mass spectrometry, which combines the techniques of high precision chemical assay with high precision isotope ratio mass spectrometry.

The limiting factor in atomic weight determinations is the accuracy of the isotopic abundance measurement. A fundamental limitation is imposed by the fact that some elements display natural variations in isotopic composition, thus precluding the unique estimation of their atomic weights. The cases of palladium and lead demonstrate this point.

Mermelengas et al. [8] determined the isotopic composition of five terrestrial and seven meteoritic palladium samples by applying thermal ionization mass

ISOTOPE DILUTION 405

spectrometry. Special care was taken to avoid contamination and isobaric interference effects. No palladium cross-contamination and no isobaric interferences from 102Ru and 104Ru were observed. Samples exhibiting 106Cd, 108Cd and noCd interferences were discarded. The double spike technique [9] with enriched 102Pd and 108Pd was used to correct for the isotopic fractionation. Four out of the five terrestrial samples and the seven meteoritic samples had the same isotopic composition. The fifth sample, an important mineral source of palladium originating from the Bushveld Igneous Complex of South Africa, exhibited an isotopic fractionation of 0.38% per mass unit, with enrichment occurring in the heavier isotopes. Rosman et al. [10] confirmed this observation. This fractionation introduces an uncertainty affecting the atomic weight of palladium by 0.012, thus the accuracy of the atomic weight can be stated only to 1 part in 104.

Lead has four naturally occurring isotopes, 204Pb, 206Pb, 207Pb and 208Pb. Only 204Pb is a non-radiogenic isotope; the other three are final products of the disintegration chains of uranium isotopes. Therefore the lead isotopic abundance depends on the relative Pb/U concentrations in the lead ore, and the accuracy of the lead atomic weight can be stated only to 5 parts in 104. The atomic weights of the elements are given in Appendix 2.

10.2 ISOTOPE DILUTION

Isotope dilution mass spectrometry, IDMS, is a microanalytical quantitative technique to determine the number of atoms of a trace element or elements in a diverse range of samples. To accomplish this, an accurately known number of atoms of the same element but of a different isotope, called a spike, is added to the sample. The mixture is then isotopically equilibrated and the ratio of the unknown number of atoms to the known number of spike atoms is measured. Chemical processing and separation may be needed before the ratio measurement to transform the sample into a favorable form and to remove interfering elements present in the sample. IDMS is extensively applied in the nuclear sciences and technology, geochronology, clinical chemistry, nutrition, food and metabolic studies, water, environmental and agricultural studies, organic chemistry, and the certification of trace elements in reference materials.

IDMS can be applied to all elements with two or more isotopes, and also to monoisotopic elements provided that a long-lived artificial isotope of the element exists. Over 70 elements can be determined using this technique [11]. De Bievre [12] discussed the advantages and limitations of IDMS. Recently the technique was extensively reviewed by Heumann [13] and also by Fassett and Paulsen [14]. The principles of IDMS, the optimization of spike addition, the preparation of spike solutions, sources of possible error and the advantages and disadvantages are discussed. One or two elements in a sample can be


determined by electron impact ionization, thermal ionization (positive and negative) and field desorption ionization, the last-named being applied mainly for alkali and alkaline earth elements. Spark source ionization, inductively coupled plasma, glow discharge, laser and secondary ion mass spectrometry are also used for multielement determinations.

10.2.1 Principles of Isotope Dilution Mass Spectrometry Let us assume a sample which contains an element of at least two natural and stable isotopes. A well defined quantity of a spike isotope, which usually is an enriched (or if available a completely separated) less abundant isotope of the same element, is added to the sample. Depending on the nature of the sample, the sample/spike mixture must be homogenized and equilibrated and precautions must be taken to ensure complete isotopic exchange in the final spiked sample mixture. Following this stage, eventual material losses in the chemical separation procedure do not affect the analytical result. Usually the sample isotope is the most abundant isotope of the sample element.

Figure 10.1 shows a mass spectrum of a sample with two isotopes to which a known quantity of spike isotope was added. Here the enriched lighter isotope (1) was used for this purpose. After chemical processing, including the

¡sotopf (1)

1 Nsp- \ p

Nsa- \ a

isotope 1 sample (sa) (2)

[ ] spike (sp)

1 1 Nsa. 2Asa

1 I Nsp. 2Asp

mass number

Figure 10.1. Schematic mass spectrum of a two isotope element with added spike. Int. J. Mass Spectrom. Ion Process., 66, 55 (1985)

ISOTOPE DILUTION 407

demanded purification and sample separation to avoid isobaric or other interferences, the measured isotopic ratio will be

R = (/Vsa x 2Asa + Nsp x 2Asp)/(Nsa x lAsa + Nsp x lAsp) (2)

where TV is the number of atoms and A is the isotope abundance in (%), which must be known for both the sample and the spike. The highest precision in the ratio measurement is obtained when R is close to one.

Solving eq. (2) for Nsa

Nsa = ATsp(2Asp - R % ) / ( / ? Asa - 2Asa) (3)

The quantity of the sample Gs in p g g - 1 units is then given by

Gs = 1.66 x 10-18(M/Ws)iVsp(2Asp - R %P)/(R Asa - 2Asa) (4)

where M is the atomic weight of the element to be determined and Ws is the sample weight in grams.

Equation (4) can be easily modified for cases in which the sample element is monoisotopic and a pure artificial long lived radioactive isotope is used as a spike. In this instance 'Asa = 0, ]Asp = 100, 2Asa — 100 and 2Asp = 0, and the following equation is obtained

Gs = 1.66 x 10~1&(M/W,)R x Nsp (5)

Probably the best known example very close to this category is the determination of uranium with the 233U spike, where 2Asa — 99.275 for natural uranium.

10.2.2 Advantages and Disadvantages of IDMS

A brief summary of IDMS properties is given below.

Advantages (1) High analytical precision. (2) High accuracy, as the uncertainty of the measured sample/spike ratio

(^sa/sp) c a n D e determined with synthetic isotope mixtures. (3) No quantitative separation or sample handling is necessary after sample/

spike equilibration. (4) High sensitivity and low detection limits may be achieved, as very low

sample sizes may be spiked with a 102—103 times larger spike amount, and /?sa/sp of 0.01 or even 0.001 may be measured with predetermined accuracy.

(5) Multielement, two element and single element analyses are possible, depending on the ionization method.


Disadvantages (1) IDMS is a sample destructive technique. (2) Chemical treatment of the sample is in most cases necessary and may be

time consuming. (3) The technique is expensive relative to other techniques regarding the

instrumentation, skill and spike cost.

10.2.3 Possible Sources of Errors in IDMS

10.2.3.1 Natural Isotopic Variations

For most of the elements, a natural isotopic composition, tabulated in Appendix 1, may be assumed. Small abundance variations do not affect the results of IDMS. If the sample isotopic composition is not known, it must be determined prior to spike addition. Isotopic variations in nature are well known for such light elements as H, B, C, N, O and S and for isotopes of interest in geochronology, especially Pb, as shown in Section 9.82, Table 9.75. Sample irradiation in nuclear reactors also changes the isotopic composition.

10.2.3.2 Isotopic Fractionation During Sample Treatment

This effect is relatively small, but it must be kept in mind that several procedures, such as ion exchange chromatography, and some complex-forming compounds have the capability to introduce isotopic fractionation, which can affect the analytical results if only a fraction of the element is recovered in the sample treatment.

10.2.3.3 Isotopic Fractionation in the Ion Source and Instrumental Mass Discrimination

The best procedure to take care of these effects is calibration of the mass spectrometer with standard isotopic reference materials, and carrying out the sample measurement under exactly (or as hearby as possible) the same conditions as for the measurement of the standards. In Chapter 9 many calibration procedures for electron impact and thermal ionization processes and also an example of mass bias correction in inductively coupled plasma mass spectrometry (Section 9.92) are discussed.

10.2.3.4 Uncompleted Isotopic Equilibration Between Sample and Spike

This is a common reason for errors in IDMS. Spiked solid samples must be completely dissolved using appropriate dissolution techniques. Loss of material during the equilibration phase must also be avoided.

PHYSICAL AND NUCLEAR CONSTANTS 409

10.2.3.5 Erratic Blank Corrections

The blank should be determined by exactly the same analytical procedure as the sample, but without the sample material. Memory effects, such as from ion exchange resins, may introduce blank variations. Replacement of the resin after each run can solve this problem.

IDMS is a well established analytical technique. It is widely applied with electron impact, thermal and, to a lesser extent, with spark source ionization. Until recently, inductively coupled plasma ionization with a quadrupole mass analyzer suffered from low isotopic ratio precisions, no better than 0.1-0.2%, and drifting (fluctuating) ratio values. In the latest instruments an improvement in stability and precision is observed, which will have a strong impact on multielement trace analysis in solutions. Glow discharge mass spectrometry is also expected to become of greater importance to the multielement IDMS of solids.

Note: Recently, Kingston [171] patented a new IDMS technique, speciated isotope dilution mass spectrometry. In this approach the samples are spiked with enriched isotopes in the same speciated form as the species to be measured. After equilibration, the species of interest are separated by chromatography and their concentrations are determined by employing the isotopic ratios. The advantage of the technique is that several different species can be measured. The technique has been adapted to an ion exchange chromatography/ICP-MS system.

10.3 PHYSICAL AND NUCLEAR CONSTANTS

10.3.1 Half-life Determination of Radioactive Nudides

High accuracy isotope ratio mass spectrometry can be applied for the half-life determinations of radioactive nuclides. The decay of a parent isotope or the growth of a daughter isotope is measured relative to a stable or long lived isotope of the same element. The parent decay technique is usually used for short half-life, and daughter growth is applied for long half-life determinations. The reason is that in the latter case even a small number of disintegrations causes a relatively large change in the product abundance.

10.3.1.1 The Parent Decay Method

Clarke and Thode [15] measured the half-lives of isotopes produced by fission of 233U, 235U or 241Pu in a nuclear reactor. The gaseous radioactive products 87Kr, 88Kr, l35Xe and 138Xe were mixed with stable isotopes of Kr and Xe, and their ratios relative to one of the stable isotopes of the same element were monitored with a static gas source mass spectrometer. The half- lives t\/2 were


calculated using the following equation i I / 2 = 0.693r/ln (N/N0) (6)

where N/Nç, is the atom fraction after time t. The resulting half-lives were 76.4 ± 1.0 min, 2.805 ± 0.025 h, 9.15 ± 0.04 h,

and 14.0 ± 0.2 min for 87Kr, 88Kr, 135Xe and 138Xe respectively. Dietz and Pachucki [16] determined the half-life of 137Cs by measuring the

137Cs/135Cs ratio for over 11 years. t\/2 = 30.174 ± 0.034 years was obtained. They measured simultaneously the ratio l35Cs/133Cs, so that the ratio of ratios

R = (137Cs/135Cs)/(135Cs/133Cs) (7)

was essentially free of systematic errors. 135Cs has a half-life of 2 x 106 years, therefore the , 3 5Cs/ , 3 3Cs ratio is practically time independent and it was used as an internal standard, normalizing the 137Cs/135Cs ratio for instrumental mass discrimination. It was assumed that the effect would be the same over the close mass range and the small and equal mass difference, therefore the biases of the two ratios cancel and the measured R is the true value. The half-life of 134Cs was also determined by the same procedure to be 2.062 ± 0.006 years.

The half-life value of the 241Pu isotope is an important factor for nuclear data calculations and nuclear materials safeguards accountability. The values determined during the ten year period 1973-1982 by 15 laboratories showed a variation from 14.24 to 15.06 years, indicating the possibility of large sources of systematic error in some of the data. Before 1973 the spread was even larger. De Bievre et al. [17] measured the variation of the 241Pu/240Pu isotopic ratio and the ratio of the isotope ratios

( 241p u / 240p u ) / ( 240p u / 239p u ) ( 8 )

and obtained t\¡2 — 14.33 ±0 .02 years Kelly [18] described procedures to eliminate two sources of systematic error in mass spectrometric determination of the half-life of 241Pu, namely isotopic fractionation and isobaric interference from the ß decay product241 Am. It was suggested that adoption of the proposed procedures will minimize the errors, increasing the accuracy to about 0.001 year (ISD) after 3 years of 241Pu decay. The use of 242Pu and 244Pu isotopes for this determination was also discussed.

10.3.1.2 The Daughter Growth Method

The accumulation of the daughter atoms with time is given in the following equation

Nd = N0[l - exp(-0.693i/«1/2)] (9)

where Ni is the number of daughter atoms after time t. For a pure sample, at t = 0, also N¿ = 0, therefore the fractional change in A^ can be quite large for

PHYSICAL AND NUCLEAR CONSTANTS 411

values of t that are small compared with the tx/2 of a long half-life isotope, and the latter can be determined in a short period of time.

Lindner et al. [19] reported r1/2 = (4.35 ±0.13) x 1010 years for 187Re, monitoring the ß decay to l87Os in osmium-free rhenium for a period of four years. The rhenium samples were spiked with enriched 190Os and 1920s and the l87Os/l90Os and l87Os/l92Os ratios were measured in the separated osmium. The half-life determination of 87Rb ß decay to 87Sr by Davis et al. [20], íh/2 = (4.89 ± 0.04) x IO10 years], is a further illustration of this technique.

10.3.1.3 The Specific Activity Method

Counting techniques combined with IDMS also provide a very powerful method for the determination of half-lives, especially of long lived isotopes. In this method the half-life is given by the following equation

t1/2 = -0.693r/(cuV/dr) (10)

where dN/dt is the disintegration rate of the parent atom measured by counting, and the number of atoms N is measured by IDMS.

De Bievre et al. [21] and Lounsbury and Durham [22] used this method to determine the half-life of 234U. r1/2 = (2.446 ± 0.007) x 105 years was reported for this isotope. Vaninbrouckx et al. [23] obtained for 233U ( 1.5925 ± 0.0040) x 105 years.

10.3.1.4 The Double Decay

Double ß decay is a process by which a long lived isotope decays spontaneously to an isotope with the same mass but with two additional protons. A few parent-daughter pairs were studied, as the decay of I30Te to 130Xe and 82Se to 82Kr. Very long half-lives, in the order of 10,8-1021 years, were theoretically predicted and also experimentally observed: 1 x 1020 and 8 x 1020 for 82Se and l30Te respectively [24]. Recently Kawashima et al. [25] studied the 96Mo excess in a 1.7 x 109 years zircon sample, determining a half-life of (3.9 ± 0.9) x 1019 years for the double ß decay of 96Zr to 96Mo. The role of isotope ratio mass spectrometry in these investigations is the search for the daughter products using high sensitivity instrumentation. Half-life determina-tion methods for radioactive isotopes have been reviewed by De Laeter [26].

10.3.2 Determination of the Avogadro Constant

The Avogadro constant NA is a fundamental physical constant related to other constants. Two examples are the relationships between the Faraday constant F and the electron charge e

F = NAe (11)


Table 10.2. Avogadro constant values

Avogadro constant Uncertainty Ref. mol-1 (1RSD)

6.0220943xlO23 1.05 x 10~6 [28] 6.022098 xlO23 (1 x 10-6)" [29] 6.0221367xl023 6 x 10~7 [30] 6.0221415xl023 5.8 x 10~7 [31] 6.022137X1023 1.2x10-' [32] 6.0221363X1023 1.1 x 10~6 [33] " Estimated

and between the molar gas constant R and the Boltzmann constant k

R = NAk (12)

Absolute, high accuracy isotope abundance ratio measurements of silicon and consequent accurate atomic weight determinations, combined with accurate determination of density and almost perfect determinations of unit crystal cell dimensions, enabled accurate evaluation of the Avogadro constant. Using the equation of Bragg [27]

NA = nA/pal (13)

where n atoms of average atomic weight A occupy a unit cell volume a\ and p is the macroscopic density.

The work of Deslattes et al. [28] reduced the uncertainty of NA from 30 to 1 ppm. The published A/A values are summarized in Table 10.2.

10.4 AGE DETERMINATIONS IN GEO AND COSMOCHEMISTRY

10.4.1 The Age of the Earth The age of the earth has been a cardinal question through the history of the human race. In the first half of the nineteenth century, eminent geologists such as Lyell and Darwin were convinced that the earth was very old, estimating the age as several tens of billions years. Their hypothesis was of course contra-dictory to the Biblical age, which had dominated many centuries. In the second half of the nineteenth century Lord Kelvin [34] estimated the possible age of the earth as not much more than 100 million years. His calculations were based on the luminosity of the sun, the cooling rate of the earth and the effect of lunar tides on the rotation of the earth. Later, in 1897, he reduced his estimates to 20-40 million years. The discovery of radioactivity by Becquerel at the end

AGE DETERMINATIONS IN GEO AND COSMOCHEMISTRY 4 1 3

of the nineteenth century led to the recognition that radioactive disintegration of an atom is an exothermic process. Therefore the assumption that the earth is undergoing only a cooling process was not correct. Soon after the discovery of radioactivity it became evident that the disintegration processes can provide information about the ages of rocks and minerals. Rutherford, who suggested this possibility, also studied several uranium minerals, obtaining ages of about 500 million years. Holmes [35], in 1913, in his book The Age of the Earth, proposed the first geological time scale, based on the thickness of accumulated sedimentary rocks and on the formation of helium and lead in uranium-bearing minerals. Rutherford [36], in 1929, again used radioactivity measurements on rocks and the U-He method and obtained an age of a billion years, solving a long lasting controversy by shifting the timescale to the > 109 years range.

The application of isotope ratio mass spectrometry to geochronological problems and to the study of the age of the earth was pioneered by Nier [37-40] who measured the isotopic composition of natural lead and uranium. Nier's work stimulated other researchers, among them Holmes [41] and Houtermans [42], who independently formulated a general model for terrestrial lead evolution known as the Holmes-Houtermans model. A brief description of the model follows. A more detailed discussion can be found elsewhere [43]. The basic assumptions of the single stage model are:

(1) originally the Earth was a homogeneous fluid, with an uniform distribution of uranium, thorium and lead;

(2) the isotopic composition of the primeval lead was everywhere the same; (3) small regional differences arose in the U/Pb ratio when the earth solidified. (4) in any given region, the U/Pb ratio changed only as a result of radioactive

decay of U to Pb; (5) at the time of formation of a common Pb mineral (such as galena), the lead

was separated from uranium and thorium and its isotopic composition has remained constant since then.

The 206Pb/204Pb ratio in a U-bearing closed system of age T is:

206pb/204pb = (206pb/204pb)¡ + (*3*U fOipQ^W _ j } ( H )

If lead was withdrawn from such a system / years ago:

(206pb/204pb)( = (206pb/204pb)( + ( 2 3 8 ^ 2 0 4 ^ ^ ( M r _ ] }

- ( 2 3 8 U / 2 0 4 P b ) ( e ( A , ) ' - l ) (15)

or

(206Pb/204Pb), = (206Pb/204Pb),. + (238U/204Pb)(e(A,)7' - e<A'>') (16)


The following equation can be given for the 207Pb/204Pb ratio:

( 207 p b / 204 p b ) i = (207pb /204pb ) j + (l/137.88)(238TJ/204pb)(e(A2)T _ e(A2)()

(17)

where (206Pb/204Pb), and (207Pb/204Pb)t are the isotopic ratios of common lead of age t; (206Pb/204Pb), and (207Pb/204Pb), are the isotopic ratios of primeval lead on the Earth T years ago; Ai = 1.55125 x 10~10 and A2 = 9.8485 x l f J - 'V 1

are the decay constants of 238U and 235U respectively [44]; 238U/204Pb is the ratio of these isotopes in a particular source region of common lead in the interior of the Earth at the present time; T is the age of the Earth; and t is the time passed since removal of a common lead sample from its source.

Also: (206pb /204pb ) ; = ao

(207pb /204pb ) ; = bo

Combining eqs. (16) and (17), the U/Pb ratio is eliminated and eq. (18) is obtained:

r^r»).-j>_(1/,37JB) »(A2)r _ P(A2)n

e(A,)r _ e(A,)t (18) (206pb/204Pb)f _ fl0

The Canyon Diablo meteorite contains iron sulfide (FeS), known as triolite, which is rich in lead but practically free from uranium and thorium. This is the least radiogenic lead available and therefore closest to representing the isotopic composition of primeval lead in the earth. This comparison is based on the assumption that the earth and meteorites were formed at the same time from an isotopically homogeneous nebula. Chen and Wasserburg [45] obtained for Canyon Diablo lead (206Pb/204Pb)¿ = 9.3066, (207Pb/204Pb), = 10.293 and (208pb /204p b ) . = 29.475.

Patterson [46] was the first to determine the age of meteorites, analyzing three stone and two iron meteorites. Equation (18) for t = 0 is reduced to

[ ( 207 p b / 204 p b ) ( _ ¿ , o ] / [ ( 206 p b / 204 p b ) i _ ^

- (l/137.88)[(e(A2)r - l)/(e(A');r - 1)] (19) Plotting the isotopic ratios, and for constant T, a straight line with a slope A should be obtained

A = (l/137.88)[(e<^r - l)/(e(Al):r - 1)] (20) The corresponding calculated age was T = 4.5 x 109 years, which is the

oldest observed age and therefore also assumed to be the age of the earth. The subject of the age of the Earth was recently addressed in detail by

AUegre et al. [47] on the occasion of the retirement of C.C. Patterson and the 40

AGE DETERMINATIONS IN GEO AND COSMOCHEMISTRY 415

year anniversary of his pioneering work on precise determination of the ages of the earth and of meteorites.

10.4.2 Geochronology

The existence of a long lived radionuclide in the isotopic composition of an element and its decay to an isotope of a different element is the principle of various geological dating methods. The rubidium-strontium geochronometer is chosen here to demonstrate the basic concepts. The feasibility of dating Rb-bearing minerals by 87Rb decay to 87Sr was discussed in 1938 and used for the first time in 1943. The development of Nier's isotope ratio mass spectrometer for metallic elements in 1950, the development of cation exchange chromato-graphic separation techniques and the application of the isotope dilution technique to quantitative elemental analysis have contributed to the under-standing and wide use of this geochronometer.

Rubidium has two naturally occurring isotopes, 85Rb and 87Rb. 87Rb is radioactive, decaying to stable isotope 87Sr by ß particle emission

8 7 R b ^ 8 7 S r ± / ? (21)

The growth of the radiogenic 87Sr in a Rb mineral is described by the following equation

87Sr = 8 7 S r , ± 8 7 R b ( e A ' - l ) (22)

where 87Sr is the total number of atoms of this isotope in a unit weight of the mineral at the present time, 87Sr, is the initial number of atoms of this isotope incorporated into the same unit weight of the mineral at the time of its formation, and 87Rb represents the number of atoms of this isotope in the same unit weight of the mineral at the present time. À is the decay constant of 87Rb; its accepted value by the Subcommission on Geochronology of the International Union of Geological Sciences is 1.42 x 10~uy ', and t is the time elapsed in years since the formation of the mineral, i.e. its age. Each term in eq. (22) can be divided by 86Sr, which represents the constant number of these atoms in the same unit weight of the mineral, therefore

87Sr/86Sr = (87Sr/86Sr),. + (87Rb/86Sr)(eA/ - 1) (23)

Equation (23) is the basic expression for the Rb-Sr age determination method. It is valid only for closed systems in which no changes in the concentration of these elements have occurred during the time t. The solution of this equation requires determination of the 87Sr/86Sr ratio, determination of the 87Rb/86Sr concentration ratio, and knowledge of the (87Sr/86Sr), value. The concentration ratio is derived using an appropriate quantitative analytical technique such as isotope dilution and isotopic ratio measurements of both


elements. In principle, the (87Sr/86Sr), value can be obtained from minerals with a high Sr/Rb ratio. If all the parameters are established, t will be given by

t n / A MJ(8 7Sr/8 6Sr)-(8 7Sr/8^Sr) 1 t = (1/A)ln ^ 87Rb/86Sr + 1 j (24>

Systems which were formed in periods short relative to t, such as rocks from cooling of magma, may separate with different chemical composition. If the strontium in the magma was isotopically homogeneous, it may be assumed tbat all the rocks formed from the magma had the same (87Sr/86Sr), ratio and nearly the same age. Under these conditions the data derived from a suite of different samples form a straight line, when in eq. (23) the measured 87Sr/86Sr ratio is plotted against the concentration ratio 87Rb/86Sr. The intercept of the line will be (87Sr/86Sr)¡ and the slope will be (eA' — 1), from which the age t is calculated.

Inspection of eq. (24) clearly demonstrates the important contribution of high accuracy and high precision isotope ratio mass spectrometry to the development of geological dating. Several other geochronological methods exist: K-Ar, 40Ar-39Ar, Sm-Nd, Lu-Hf, Re-Os, and U-Th-Pb are among the more important. It is beyond the scope of this chapter to enter in details of each method. The excellent book by Faure [43], Principles of Isotope Geology, is recommended to obtain a good understanding of this interesting scientific discipline. About 15 years ago, Tanaka and Masuda [48] and Shimizu et al. [49] proposed the ß decay of 138La to 138Ce and the electron capture to 138Ba as the La-Ce geochronological method.

10.4.3 Cosmochemistry Cosmochemistry is a scientific discipline for studying the nuclear and chemical processes in matter related to the formation and evolution of the solar system. In particular, the behavior of chemical elements within and after the formation of the solar system is investigated by studying the distribution of the elements and tiieir isotopic composition. In view of this definition, geochemistry may be considered as a part of cosmochemistry treating the behavior of terrestrial elements. The source of extra-terrestrial materials for cosmochemical research is meteorites and samples from lunar missions. Further information was obtained from unmanned space missions into the atmospheres of Mars and Venus, interplanetary dust, and space encounters with the atmospheres of passing comets, mainly Halley's Comet. There is a hope that deciphering the information transmitted from the Galileo spacecraft mission, which approached Jupiter in December 1995, will provide additional data, (see this Chapter, Section 7). Mass spectrometry plays a dominant role in cosmochemical studies. The most accurate, precise and sensitive technique for elemental distribution

AGE DETERMINATIONS IN GEO AND COSMOCHEMISTRY 417

evaluation is isotope dilution mass spectrometry. Isotopic composition is determined with gas or thermal isotope mass spectrometers or secondary ion mass spectrometers, and to a lesser extent also with spark source and inductively coupled plasma mass spectrometry. In many cases the scarcity of samples dictates the use of static vacuum gas mass spectrometers with high ion beam transmission, high sensitivity and pulse counting systems. Hohenberg [50] described such an instrument for noble gas analysis. All the space missions and the planetary space probes have been equipped with specially designed mass spectrometers which provided much of the elemental and isotopic abundance data about the atmospheres that they penetrated.

Isotopic abundance studies in meteorites have revealed isotopic anomalies in a wide range of elements when compared with the terrestrial composition. Two important discoveries, that of Reynolds [51] in 1960, reporting anomalies in xenon, mainly a large excess of 129Xe, and that of Clayton and coworkers [52] in 1973, reporting anomalous oxygen, contributed to the recognition of isotopic anomalies as a reliable cosmochemical concept. The observation of a whole range of anomalies has affected the understanding of the formation of the solar system, and various hypotheses have been put forward to explain the nucleosynthetic processes. It is believed that the solar system consists of material evolved from compositionally different and imperfectly mixed reservoirs.

Isotopic anomalies in meteorites have been observed in noble gases: Ne, Kr and Xe; in light elements: H, C, N, O; and recently also in Si. The anomalies observed in the iron group elements, measured with conventional thermal ionization mass spectrometry, were very small. This technique requires element separation and purification, and therefore for small samples it is susceptible to contamination. Ion microprobe mass spectrometry (secondary ion mass spectrometry) is capable of analyzing selected sites a few pm in diameter on a solid sample surface, avoiding the need for elaborate sample preparation and the contamination risks involved. Ion probe mass spectrometry revealed anomalies in 48Ca, 50Ti, 49Ti, 47Ti, 54Cr, 64Ni, 68Zn and ^Zn. Heavy element anomalies were observed in Te, Sn, Ba, Sm and Hg. An excellent review on cosmochemistry has been given by De Laeter [53], in which also the references for the anomalies listed here may be found. In a more recent review, the same author discussed the role of mass spectrometry, mainly IDMS, in cosmic element abundance studies [54],

Regarding isotope ratio studies related to cosmochemistry, an important point should be kept in mind. Isotopic variance in nature may originate from physicochemical and nuclear effects. Multiple, or at least three isotope elements provide in principle the possibility to distinguish between the two effects. Physicochemical fractionation is a mass dependent process linearly propor-tional to A(m)/m, therefore in a particular element, for a mass difference of 2, the effect will be approximately twice than for a mass difference of unity. In


nucleosynthesis processes the fractionation is of nuclear origin and independent of the original isotopic composition.

10.5 NUCLEAR SCIENCES AND TECHNOLOGY

Nuclear reactors may be classified into three types: research reactors, power generation nuclear stations, and reactors for production of nuclear weapon material. Isotope mass spectrometry plays a key role in research and in nuclear weapon material production reactors. Hydrogen and its isotopes, lithium, uranium and plutonium are considered the four most important elements in nuclear weapons technology. Not all the isotopes of these elements are equally important in the weapons industry, therefore monitoring of deuterium, lithium-6 and uranium-235 in various enrichment processes, the overall tritium and plutonium production in nuclear reactors and the distribution of their isotopes are extremely important tasks for isotope ratio mass spectrometry. Sections 1, 3, 92 and 94 in Chapter 9 describe in detail the analytical procedures for ratio determinations. Furthermore, the principle of quantitative mass spectrometric determinations, the isotope dilution technique, is described in Section 2 of the current chapter and in detail for uranium and plutonium in Chapter 9, Sections 92 and 94; the effect of isotopic fractionation on the precision and accuracy of these measurements is discussed in Chapter 8; half-life determinations of several of these isotopes in Section 3 of the current chapter; and finally the use of isotopic ratio determinations for reactor characteristics is described in Section 6 of this chapter.

Another important application of isotope mass spectrometry is the monitoring of nuclear activities and occupational risks in nuclear power industry. Reviewing the subject of nuclear activity has two aspects; the first is that most of this work is done by radiometric monitoring of radioisotopes, which is not particularly relevant to this text. Furthermore, it can be assumed that techniques for monitoring energy yields of nuclear devices are considered as classified material. Nevertheless, recently Aregbe et al. [55] have shown that stable radiogenic krypton and, preferentially, xenon isotopes, when released to the atmosphere following fuel reprocessing, may be indicative of nuclear power generation or weapons material production (i.e. plutonium) activity. The isotopic composition of anthropogenic xenon differs significantly from the natural composition. The most sensitive ratio is 136Xe/132Xe. Irradiation of 3.5% 235U-enriched fuel for 3 years in a light water moderated pressurized water reactor (PWR), followed by 3 years of cooling, yielded 240 g 136Xe per 100 kg of processed uranium with t36Xe/132Xe = 2.11755. The same amount of fuel irradiated for one month at the same neutron flux and cooled for one month, yielded 9 g 136Xe, with 136Xe/132Xe = 2.49569. The ratio for natural xenon was determined as 0.3298021 ± 0.0000108 (2SD). It was calculated that,

THE OKLO NATURAL NUCLEAR REACTORS 419

at a blending ratio of 1:10s, which corresponds to diluting 1 g of fission xenon in 2 x 108m3 of air (assuming a natural atmospheric Xe concentration of 4.34 x IO-4 g m~3) is above the detection limit. This dilution yields 136Xe/132Xe = 0.329816, which is above +2SD of the natural value. The isotopic ratio measurements were performed with a high precision magnetic mass spectrometer.

It is common practice at nuclear facilities to perform uranium tests in urine from employees who may be exposed to this element. Gladney et al. [56] compared the routinely used fluorometric analysis, a counting and the delayed neutron assay techniques with ICP-MS, concluding that the last has several advantages: a simple mass spectrum, provision of isotopic information, lower detection limits for either enriched or depleted uranium by at least two orders of magnitude, shorter duration of analysis, low time consumption for sample preparation, and good precision. Allain et al. [57] also studied the determination of uranium in urine and blood by ICP-MS. The limit of quantification of their measurements was 35 ng l-1. They observed that the uranium concentration in urine and blood for an age- and weight-selected group of 20 healthy men was in all cases lower than the above value. It should be noted that recent ICP quadrupole mass spectrometers, which are routinely used for this screening work, may achieve a sensitivity of 500 x 106cps for 1 ppm of uranium, with blank counts in the region of 1-5 cps. Therefore a reading of 50 cps, which is far beyond 3 times the blank SD, will correspond to 10~13g mP1 ofU,or0.1 ng r1.

Regarding plutonium, Taylor [58] recently reviewed the content of this element in humans. Based on measurements made between 1970 and 1980, the human body contains about 40-300 fmol 239,240Pu originating from fall-out from nuclear weapons testing and the Nagasaki atomic bomb. In addition, calculation suggests that the human body may always have contained up to 2 amol plutonium of natural origin, mainly 239Pu and minute quantities of primordial 244Pu. These levels of plutonium are considered to be too low to engender chemical or radiation-imposed risks. Most body plutonium is concentrated in the skeleton and liver. The main concern with plutonium is to prevent significant future release from its global stock, estimated as 1200 tonnes, to the atmosphere or into the human food chain. For further studies about environmental plutonium see 'Plutonium in the environment' in Section 10.10.4

10.6 THE OKLO NATURAL NUCLEAR REACTORS

High accuracy isotopic ratio measurements led to the discovery of one of the more curious phenomena in nature. In 1972, a French team mass analyzing uranium observed a slight but persistent depletion of 235U in uranium supplied

4 2 0 APPLICATIONS OF ISOTOPE RATIO MASS SPECTROMETRY

from the Oklo mines in Gabon, West Africa. The measured 235U abundance was 0.7171%, compared with tbe accepted value of 0.7202% for natural uranium. Later, samples with 235U depletion down to 0.296% were also observed (see Chapter 9, Section 92). Neuilly et al. [59] studied the isotopic composition of rare earth elements extracted from the Oklo uranium ores. The existence of fission products was observed, suggesting the possibility of a natural nuclear reactor activity. Further intensive work on a wide range of related topics was initiated. Hagemann et al. [60] and Lancelot et al. [61] carried out age determinations of the ore deposits. Such reactor characteristics as fluence, neutron spectrum, duration and age were studied by Baudin et al. [62], Neuilly and Dozol [63], Hagemann et al. [60,64,65], Drozd et al. [66], Cowan et al. [67] and Maeck et al. [68]. In most cases the characteristics were determined by studying the isotopic abundance variations of rare earth elements, especially neodymium. It was assumed that this element did not migrate within the period of the nuclear activity or after. On the other hand, migration of other elements, such as alkaline earth elements has been demonstrated by Lancelot et al. [61], and others [69-71].

The possible existence of natural reactors was predicted by Wetherill and Ingham [72] and in more detail by Kuroda [73], provided that certain conditions were fulfilled. Based on the difference in the half-lives of 235U and 238U , the abundance of 235U in natural uranium about two billion years ago was above 3%, which is similar that in to the fuel used in pressurized water reactors. A sufficient amount of uranium, the presence of a moderator and the absence of elements with high neutron capture cross section may create criticality condi-tions and initiate nuclear fission chain reactions. These conditions were met at Oklo, where condensed layers of uranium-rich deposits low in rare earth elements and cadmium are located in a periodically water flooded clay basin. Naudet [74] estimated that the reactors operated for a period of 8 x 105 years, about 2 x 109 years ago. About 800 t of natural uranium were involved, depleting the « 24 t of 235U content by about 600 kg and producing about 5000 kg fission products. Ruffenach et al. [75] also determined the age of a reactor zone. Twelve fission product samples of Nd, Sm, Gd, and Ru were isotopically analyzed and yielded an average age of 1.93 x 109 years.

Equally important to the occurrence of a natural nuclear reactor is the remarkable preservation of the Oklo site for a period of 2 x 109 years, which made feasible the exploration of this unique natural phenomenon. Studies of fission product mobility (or retention) in the peripheral rocks, using IDMS techniques, were carried out by Loss et al. [76, 77] and Curtis [78]. These studies were stimulated also by the possibility of geological burial of man-made radioactive wastes.

Various aspects of the Oklo Phenomenon were intensively studied in different laboratories and documented by the IAEA, Vienna [79],

PLANETARY MASS SPECTROMETRY 421

10.7 PLANETARY MASS SPECTROMETRY

Mass spectrometry has played an important role in the investigation of the upper atmosphere of the earth and the atmospheres of Mars, Venus and, very recently, of Jupiter. In situ studies of the gaseous composition were made possible in the early 1970s by space missions. These provide further important information about the nature of extra-terrestrial material gained from the Moon samples returned to earth by the Apollo lunar landings and from the analyses of meteorites. Variations in isotopic composition were observed in almost every element of extra-terrestrial origin. A summary of the extremes of the variations in these materials was compiled by Shima [80]. In this section the research related to the isotopic composition of elements in the atmospheres of the above mentioned planets will be briefly discussed.

Special air-borne mass spectrometers were designed and constructed to cope with the demands of space missions. Low weight, low power consumption, the ability to withstand vibrations and mechanical shocks and remote controlled operation are essential features of these instruments. In an excellent review, Nier [81] discussed the Atmosphere Explorer program, the Pioneer Venus and the Viking Mars missions. Figure 10.2 shows a schematic drawing of an 'open source' double focusing Mattauch-Herzog [1] mass spectrometer built by Nier et al. [82] for an Atmosphere Explorer satellite. The ion curvature radius of the magnetic field to the high mass collector (one of two available collectors) was 3.81 cm, the magnetic field strength was 4500 G and the total weight, including all the electronics, was 7.3 kg. A typical laboratory mass spectrum of air

Molecular beam

Electron multipliers LOW MASS HIGH MASS

Electric analyzer

SH J1 G

^ 7 123 J2

Magnetic analyzer

5 cm

Figure 10.2. The open source Mattauch-Herzog double focusing Explorer mass spectrometer. (Reproduced by permission of Elsevier Science NL from A. O. Nier, Int. J. Mass Spectrom. Ion Processes, 66, 55 (1985))


10

10

10"

10 r

10"

10"

AIR p = 2x10"5TORR

HpO

Ui

no,

"| Ar

o1 6o , a

i l l I I " 1 6 20 MASS 2 8 3 2

" l^i^^irtii f̂t CO, -

I 40

I 44

INCREASING ION ACCELERATING POTENTIAL

Figure 10.3. Typical laboratory mass spectrum of air in the open source Mattauch-Herzog mass spectrometer. (Reproduced by permission of Elsevier Science NL from A. O. Nier, Int. J. Mass Spectrom. Ion Processes, 66, 55 (1985))

admitted into this instrument is shown in Figure 10.3. From the data provided by this mass spectrometer, valuable information about the gas composition of the upper atmosphere to altitudes of 400 km and above was derived by Nier et al. [82] and information on its temperature was obtained by Kayser et al. [83, 84].

In the Viking mission, two spacecraft equipped with a wide range of scientific instrumentation were launched in 1975 toward Mars [85]. A neutral mass spectrometer was targeted to study atmospheric composition, a retarding potential analyzer to study electron and ionic distribution and a gas Chromatograph mass spectrometer to search for organic compounds in Martian soil and to measure atmospheric composition. Figure 10.4 shows the mass spectrum observed upon the descent of the Viking 1 lander towards the planet at an altitude of 140 km [86]. The presence of CO2, Ar, N2 and 02 was clearly demonstrated. Analysis of the results indicated also the presence of NO. It was concluded that the 1 8 0 / 1 6 0 and 13C/12C isotopic abundance ratios were similar to the terrestrial values and, consequently, so also was the 1 7 0 / 1 6 0 ratio. The l 5N/1 4N ratio on Mars was found to be « 60% higher than on earth. The mass spectrometer mounted on Viking 1 also provided information about the gas density numbers of C0 2 , N2, CO, 02, NO and O at an approximate altitude

PLANETARY MASS SPECTROMETRY 423

Decreasing Mass

Figure 10.4. Mass spectrum of Martian atmosphere recorded with the upper atmosphere mass spectrometer on Viking 1 at 140 km altitude. (Reproduced by permission of Elsevier Science NL from A. O. Nier, Int. J. Mass Spectrom. Ion Processes, 66, 55 (1985))

range of 110-200 km. At altitudes above 200 km, the sensitivity of the instrument was insufficient, and below 110 km the atmospheric pressure was high, preventing proper instrumental function. Ar, Ne, Kr, and Xe at very low abundances and traces of ozone were detected. The 36Ar/40Ar ratio was only about 10% ofthat on earth, 1/3000 compared with the terrestrial value of 1/300. In xenon, the 129Xe abundance was in excess of terrestrial values. In the krypton spectrum, an ion signal at m/z = 80 was attributed to high argon pressure in the instrument. The abundances of Kr and Xe in the Martian atmosphere at the planet surface were estimated to be 0.3 and 0.08 ppm respectively. No organic compounds were detected in the GC-MS experiments, despite gas extraction from soil samples at temperatures as high as 500 °C and passage through the GC column, and despite the ppb sensitivity range of the instrument [87].

Mass spectrometers were also used in the Pioneer missions to explore the atmosphere of Venus. Niemann et al. [88] derived quantitative data on the main gaseous constituents C0 2 , CO, N2, O, N, and He at an altitude of 150-250 km. Another single focusing magnetic instrument, equipped with a pumping system, was designed to operate at higher pressure in the lower Venusian atmosphere, providing information from an altitude of 62 km above the planet surface [89,90]. Small traces of H2S and C2H6 were suggested and upper limits for 3He, Kr, 0 2 , S0 2 , H 2 0 , CI and Hg were derived. The 22Ne/20Ne ratio was observed to be 0.07± 0.02, which is closer to that of solar wind than to the terrestrial value of 0.102. Argon isotopic ratio measurements revealed an 36Ar/40Ar ratio close to unity, which differs immensely from the values on Mars (1/3000) and on earth (1/300). The 36Ar/38Ar ratio was « 5, comparable with the values on Mars and earth. The atmosphere of Venus was also studied by the Russian Venera missions [91].


The Galileo spacecraft mission to study the temperatures, pressures and chemical composition of the atmosphere of Jupiter was planned for the second half of the 1980s. The spacecraft was launched in 1989 following several delays, and after six years, on December 7th 1995, a 350 kg probe equipped with various scientific instrumentation entered the planet's atmosphere, surviving there for 75 min. A quadrupole mass spectrometer with an enrichment and a gas purification system was planned to be included in the probe [92]. Unfortunately, when these lines were been written no scientific data were so far available, except that the probe had successfully transmitted signals to the mothership Galileo orbiter.

Further information about the chemical and isotopic characteristics of extra-terrestrial objects came from the passage of Halley's Comet in the vicinity of the earth in 1986. Several space probes were launched close to the comet: Soviet probes Vega-1 and Vega-2 approached the comet to within 9000 km and the European probe Giotto to within 600 km. They carried time-of-flight mass spectrometers for dust particle studies. An American space probe, ICE, also collected data from the comet. Kissel et al. [93] elucidated the mass spectra from Vega. The data confirm that the dust of Halley's Comet is of essentially solar elemental and isotopic composition. A neutral double focusing mass spectrometer was used to determine the composition, density and velocity of neutral gases and low energy comet ions [94]. Krankowsky et al. [95] reported that water vapor and other light element species are the major neutral gas constituents. The H 2 0 density at 1000 km from the comet was 4.7 x 107

molecules cm"3, and the water vapor production rate (escape rate from the comet) was 1.5 x IO7 g s - 1 . The observed ions were: H 3 0 + , H20+, OH+, 1 2C+, 1 2 C ^ 1 2 C H ^ , 1 6 0 + , N a + , 3 2 S + , 3 4 S + and 56Fe+. The 34S/32S ratio was within ± 25% of the terrestrial value. An ion mass spectrometer was also used to examine the composition, density, energy, and angular distribution of ions in the solar wind and the plasma of the comet [96]. Kissel [97] described the variety of mass spectrometers used in Halley's Comet explorations. The summary of the isotopic ratio observations show that oxygen and sulfur ratios are as on earth, and in meteorites, and D/H in water vapor is significantly higher than in the interstellar medium; the isotopic ratios in carbon, magnesium, silicon and iron are typical of those of materials in the solar system. Recently, Rudenauer [98] surveyed the in situ mass spectrometric space research.

10.8 STABLE ISOTOPES IN NUTRITIONAL, MEDICAL AND BIOLOGICAL STUDIES

Mineral and trace elements are essential to support and maintain the normal metabolic functions of the human body. The dietary requirements depend on the fraction of the element absorbed by the body, which in itself depends on

STABLE ISOTOPES IN NUTRITIONAL, MEDICAL AND BIOLOGICAL STUDIES 425

bioavailability and metabolism. Early studies in these fields involved the use of radioisotopes, which are now restricted to animals and in some cases to humans with terminal diseases.

Mineral nutrition research using stable isotopes is a relatively new but already well established discipline. Its advantage is that it can be applied in humans of all ages and also to sensitive populations such as infants and pregnant and nursing women, provided that the administered dose does not exceed the allowed or recommended daily intake of the element. The main requirements of a properly designed experiment are that the tracer isotope should be in excess of its natural abundance, the isotopic change in the excreted element should be sufficient for isotopic ratio determinations at significant precision levels, and that the extrinsically administered tracer should completely exchange with the endogenous mineral. A priori assumption of complete exchange may lead to erroneous conclusions [99]. Intrinsic labelling is achieved by growing edible flora on, or by feeding or injecting fauna with, labelled nutritional minerals. In general, extrinsic labelling is preferred, because intrinsic labelling is more expensive and time consuming. Mineral elements of interest in nutrition studies are magnesium, calcium, chromium, iron, copper, nickel, zinc, selenium and molybdenum. The biological samples are biological fluids (blood, plasma, urine, saliva and sweat), tissues and feces.

The first experiments were reported in 1963, when neutron activation analysis (NAA) of 58Fe was utilized in plasma studies [100]. This analytical method requires a nuclear reactor, therefore it is restricted to quite a small number of users. A further limitation is that nuclear activation does not transform all the stable isotopes to suitably measurable radionuclides. Rabinowitz et al. [101] in 1973 were the first to apply mass spectrometry for Pb metabolism studies in humans. In 1979, Schwartz and Giesecke [102] had shown the feasibility of measuring the stable 26Mg isotope as a nutrient mineral. A volatile magnesium chelate was ionized by electron impact mass spectrometry (EI-MS). The mass spectrometric isotopic ratio determination methods most frequently used in stable isotope nutrition studies are (a) thermal ionization mass spectrometry (TIMS) for Mg, Ca, Fe, Cu, Zn and Mo; (b) electron impact ionization of chelates, combined with gas chromatography mass spectrometry (EI-GC-MS), for Mg, Ca, Cr, Fe, Cu, Ni, Zn and Se; (c) fast atom bombardment mass spectrometry (FAB-MS) for Zn; and recently (d) inductively coupled plasma mass spectrometry (ICP-MS) for Mg, Ca, Cr, Fe, Cu, Zn, Se, Br and Mo.

It is beyond the scope of this section to describe the various studies in detail. Recently, Crews et al. [103] published a very good review on mass spectro-metric methods for stable isotope determinations of nutrient minerals and trace elements in human metabolic studies. The advantages and disadvantages of EI-MS, GC-MS, FAB-MS, TIMS, and ICP-MS were evaluated in terms of their precision, sensitivity and convenience in use. Studies on selected applications of stable isotopes in nutrition have been edited by Turnlund and Johnson [104].

4 2 6 APPLICATIONS OF ISOTOPE RATIO MASS SPECTROMETRY

Non-mineral isotopes of those elements, such as hydrogen, carbon, nitrogen, oxygen and sulfur, which play a highly important role in food, diet, agricultural and nutritional studies are analyzed by (EI-MS) and (EI-GC-MS). An extensive review on isotope ratio measurements in nutrition and biomedical research has been presented by Hachey et al. [105]. The use of stable isotopes in biomedical research lies in the possibility of labelling and administering in humans a wide range of molecules as food—proteins, fatty acids and carbohydrates, and also drugs, hormones and other substrates. The labelling allows identification of an element, a molecule or a molecular fragment along its biological pathway, measurement of metabolic reaction rates, and quantification of biological pools by isotope dilution. The deviation of isotopic composition in a labelled substrate from the well defined value may be used for clinical diagnostic interpretations. One of the disadvantages of stable isotopes is that they can be analyzed only in situ. The most frequent samples, as mentioned above, are blood, plasma, urine, saliva, sweat, feces, and also expired air. Mass spectro-metric determinations always require sample preparation. Even the 13C/12C analysis of C 0 2 in expired air demands a dry sample, as 1 2C1 602H+ interferes with 1 3C1 60£.

Matwiyoff and Walker [106] reviewed the trends in the use of 13C, 15N and l 8 0 in biochemistry and pharmacology, emphasizing metabolic pathways in humans. 1 3C02 breath tests for the detection of metabolic diseases and disorders, the metabolism of 15N-labelled amino acids and proteins in man and animals, and direct metabolic studies of labelled drugs and other substrates were discussed. Klein et al, in ref. [106], p. 313, discussed the clinical applications of stable isotopes, mainly the principal advantage: the lack of radiation hazard to humans, and the disadvantages: the low oxidation rate of several interesting substrates in the 1 3C02 breath tests. Measurements of protein turnover in man with l5N was reviewed by Garlick and Waterlow, in ref. [106], p. 323, and Hartig et al, in ref. [106] p. 335, studied 15N-labelled amino acids as a quantitative tool for the description of nitrogen metabolism in man.

Klein and Klein [107] discussed the impact of stable isotope substrate enrichment on the saftey of the human body. The margins of safety in the application of the four important stable isotope tracers, D, 13C, 15N and 1 80 , are quite wide; the enrichment of total body water with deuterium is limited to 1-2%, whereas for the three other isotopes it is limited in practice only by their cost. The effect of heavy water on living organism had already been studied by the early 1930s. Taylor et al [108] compared the toxicity of 92% and 30% D 2 0 in tadpoles, aquarium fish, flat worms and paramecia. In the higher concentration all the species succumbed within 1-3 hours, but all survived the 30% D 2 0. In the first mammalian study [109] a mouse fed with 0.66 g of pure D 2 0 , showed symptoms of thirst and intoxication, but survived. The harmful effect of heavy water on the human body is mainly a kinetic isotopic effect, originating from the reduced reaction rates in D 2 0 relative to H 2 0 at

STABLE ISOTOPES IN NUTRITIONAL, MEDICAL AND BIOLOGICAL STUDIES 427

constant body temperature, and consequently the distortion of existing chemical equilibria. Klein and Klein [107] provide values for the natural content of stable isotopes in the human body, their daily consumption, and the quantities used in conventional tracer studies:

Human body Intake as

food water air

Tracer dose

content

D (mg kg"1)

15

0.23 6.7

5

, 3C (mg kg"1)

1980

99.9 -

15

1 5 N

(mg kg"1)

111

0.15 -

10

1 8 0

(mg kg- 1)

130

20.8 40.0 66.4 60

The biological and chemical effects of deuterium were extensively reviewed in articles by Katz [110] and Thomson [111].

Sugino et al. [112] used !5N-labelled urea as a marker in nitrogen metabolism studies in clinical nephrology, particularly in studying chronic renal failure or dialysis. Klein and Klein [113] reviewed the use of deuterium, 15N, l 8 0 and, especially, 13C as tracers in pediatric nutrition and gastroenterology. Techniques using these isotopes have been developed for measuring the digestion, absorption, utilization and excretion of nutrients in premature and full-term infants, and also in young children. Pacy et al. [114] reviewed the advantages of stable isotopes in clinical research.

The versatility of stable isotopes in nutritional and biomedical studies will be demonstrated by a few diverse applications. Samuel [115] studied brain chemistry by applying mainly 1 8 0 . Samuel et al. in réf., [115], p. 203, and Wolf et al. [116] showed that raising three generations of mice in an atmosphere of 90% 1 8 0 2 and giving them 90% H2

1 80 water to drink had no physiological or biochemical effects on reproduction or infant mortality. Baillie and Rettenmaier [117] discussed the use of stable isotopes in drug metabolism and drug biotransformation research, VandenHeuvd [118] discussed the identification of drug metabolites by isotopic labelling, and Wolen [119] discussed applications to drug bioavailability and bioequivalence studies. Van der Merwe et al. [120] reported that a trivariate isotopic analysis of 13C/12C, l 5 N/ l 4 N and 87Sr/86Sr in African elephant bone and ivory is indicative for the habitat area of the animal, and thus may be used as a tool in controlling the illegal ivory trade. Vogel et al. [121], in addition to the isotopes of the above elements, also studied the lead isotopic composition in elephant bone and ivory, concluding that their isotopic ratios provide a clear distinction between several different populations of this animal in Africa.


10.9 FOOD AUTHENTICATION AND ADULTERATION CONTROL

Economical and seasonal incentives exist for producers of pure fruit juices, honey and other food to adulterate these products and mislabel them as containing no added sugars or synthetic constituents. Low cost sweeteners, artificial or natural flavorings, citric acid and water can be added to fruit concentrates, simulating natural juices. Pure honey is also a relatively expensive food which can be adulterated by mixing it with the much cheaper high fructose corn syrup. Adulteration practices constitute a serious consumer fraud and a major threat to the apicultural industry. Isotope ratio mass spectrometry of carbon and, to a lesser extent of hydrogen and oxygen is an accepted technique for the detection of adulteration or establishment of authenticity in food products.

Variations in the 13C/12C isotopic ratio arise from different photosynthetic pathways [122]. In the Calvin cycle, fixation of atmospheric C0 2 starts by reaction with ribulose-l,5-diphosphate, yielding a six-carbon intermediate that breaks down into two molecules of 3-phosphoglycerate, forming carbohydrate. The participating ribulose-l,5-diphosphate carboxylase enzyme discriminates against 1 3C02. Plants using this fixation mechanism are called C3 plants. About 80-90% of agriculturally cultivated plants: wheat, rice, potato, sweet potato, barley, manioc (cassava), soybean, grape, oats, sugar beet and rye belong to the C3 group.

In the Hatch-Slack cycle, fixation of atmospheric C0 2 starts by reaction with phosphoenolpyruvate, yielding oxaloacetic acid, which is converted to malic and aspartic acids. These are decarboxylated and the released C 0 2 is fixed again by ribulose-l,5-diphosphate using the C3 mechanism. The enzyme phosphoe-nolpyruvate carboxylase also exhibits isotopic discrimination. Plants that use this fixation mechanism, the C4 plants, are less frequent. Maize (sweet corn), sugar cane, sorghum, millet and certain pasture grasses are the important species of the C4 group.

A third small group of plants uses the CAM (Crassulacean acid metabolism) fixation pathway. They use both the C3 and the C4 cycles. At night, C0 2 is fixed by the C4 pathway, and is then released in the leaf for subsequent C3 fixation during the next day. CAM plants include the pineapple and cacti.

Carbon isotopic ratios are usually expressed in parts per thousand (or per mil), relative to a laboratory or working standard, using the common ¿-notation

S13C = [(äSA/KLS) - 1] x 1000 (25)

where ÄSA and ALS are the 13C/12C ratios in the sample and a laboratory standard, respectively. Ris may be a carbon isotopic reference material. It is a common practice to quote r513C values relative to Peedee Belemnitella americana, or PDB, a calcium carbonate, in which 1 3 C/ 1 2 C= 0.0112372. This standard is no longer available, thus other carbon isotopic reference materials

FOOD AUTHENTICATION AND ADULTERATION CONTROL 429

Table 10.3. Range of 6UCPDB(%«) in selected carbon pools Source

Marine carbonates Carbonates Terrestrial atmosphere C02 C4 plants CAM plants C3 plants Coal and oil Natural gas

0 CpOB (%o)

0 -15 to +2 -10 to - 9 -20 to - 8 -34 to -10 -35 to -22 -34 to -22 -50 to -28

13C/12C Isotopic ratio

0.0112372 0.01107 to 0.01126 0.01113 to 0.01114 0.01102 to 0.01115 0.01086 to 0.01113 0.01085 to 0.01099 0.01086 to 0.01099 0.01068 to 0.01093

are used and the r5I3C values are recalculated relative to PDB. Further details are given in Chapter 9, Section 6. More negative S values correspond to larger 13C depletion. Table 10.3 shows the ranges of 613CPDB isotope variation and the corresponding isotopic ratios in selected carbon pools.

Doner and Phillips [123] reported the detection of the addition of high fructose corn syrups (HFCSs) to apple juice using 13C/12C ratio determinations. Fructose is a sweeter sugar than glucose. HFCS is a mixture of glucose and fructose prepared by enzymatic isomerization of glucose in corn syrup. Forty one pure apple juices, representing 18 apple varieties, exhibited <5I3C variations between —28 and —22.5%o with a mean of —25.3%o, whereas those for four HFCSs had values of —10 to —9.5%o. One pure apple juice sample and four apple juice HFCS mixtures containing 25 to 70% apple juice were distributed between six laboratories. Five laboratories showed a linear correlation between 20-100% juice content and —23 to — 13.5%o t513C values respectively. Samples with ¿13C values more positive than — 20.2%o, four standard deviations from the mean for pure juice, can be classified as adulterated with a high degree of confidence. Variation of <513C by the addition of HFCSs to orange juice has also been observed. The 13C/12C mass spectrometric method for corn syrup products in fruit juice has been adopted as Official Method 981.09 and 982.21 of the AOAC Official Methods of Analysis (1995, Chapter 37, pp. 18-19) for detecting HFCS in apple juice and orange juice respectively.

Doner and Bills [124] also addressed the issue of authenticity of orange juice. It is a widely used practice to prepare orange juice by the dilution of concentrates, therefore it is important to have tools to verify the purity of the concentrate and to control the declared concentration of natural juice in the reconstituted orange drink. As for apple juice, the r)l3C value of sugar from orange juice is more negative than the value in cane sugar or corn sweetener, but it can still be adulterated with beet sugar, which is a C3 plant, as is the orange tree. Bricout [125] and Bricout and Koziet [126] showed that the 18Q/16Q ancj jyj-i isotopic ratios of water allow a confident differentiation


between natural juices, particularly orange, and reconstituted juices. The enrichment of plant water deuterium and 1 8 0 as compared with rain and ground water occurs through isotopic fractionation by the évapotranspiration process. This enrichment is higher in dry and warm climates, therefore orange juice shows a higher deuterium and 1 8 0 abundance and dilution with rain or ground water is easily detected. The D/H ratio analysis is performed on nitrate esters of sugars precipitated from the juice and burnt in pure oxygen to yield H 2 0 . The water is purified and hydrogen is liberated by reduction on hot uranium. It was concluded [126] that SD values together with <5I3C values allow the detection of the adulteration of orange juice with beet, cane or corn sugar.

Croft [127] discussed the detection of the adulteration of genuine honey with HFCS. Much like fruit juice, the ¿>13C values of honey are between —28 and —23%o, whereas those for HFCS range around — 10%o. Consequently, the American Association of Analytical Chemists considers any honey having r513C less negative than —21%o as having been adulterated with HFCS at a probability of 1:25 000.

The 13C/12C isotopic ratio technique for food authenticity control was utilized also by Bricout et al. [128] for vanilla, by Parker [129] for juice concentrates, by Simpkins and Rigby [130] for whisky, and by Dunbar [131,132] for wine and grape juice. Danho et al. [133] distinguished between natural and synthetic caffeines using mass spectrometric ¿>13C and <515N determinations and SD NMR measurements. Also, natural caffeine could be classified according to its American or African origin. The applications of stable isotope ratio mass spectrometry of carbon in food control were reviewed by Winkler and Schmidt [134] and by Winkler [135] for H, C, N, O and S. Koziet and co-workers, in ref. [136], p. 75, reported intercomparison tests on fruit sugars carried out by 15 European laboratories. The results are summarized in Table 10.4. Statistical treatment of the results for each sugar in compliance with ISO 5725 yielded an average repeatability of 0.03% and an average reproducibility of 0.07%. It was concluded that the sample preparation

Table 10.4. Intercomparison test in sugars Sample origin Mean <513C±1SD value

from 15 laboratories

Orange juice -24.62 ± 0.25 Pineapple juice —12.17 ± 0.23 Beet -25.62 ±0.34 Cane -11.23 ±0.20 Reference: NBS-22" -29.8 ±0.02 ° Normalized against 6>3CPDB.

ENVIRONMENTAL STUDIES 431

errors had no great influence. The results were considered as acceptable, and therefore the technique was recommended as a standard procedure for determination of 13C content in fruit sugars within the European Committee for Standardization.

Recently, Widmer et al. [137] reviewed the isotope ratio determination and other techniques such as NMR, HPLC, tracer studies, UV and visible spectrophotometry, ICP-AES, electrochemical detection and pattern recognition for the detection of the adulteration of fruit juices.

10.10 ENVIRONMENTAL STUDIES

Because of the global importance of research and development related to water resources, atmospheric studies, air pollution, climatic changes and other environmental studies, and the applications of stable isotopes and several natural and anthropogenic radionuclides in this research, an enormous amount of literature is currently available. It is beyond the scope of this chapter to present extensive and detailed reviews of these subjects. It should be pointed out that the International Atomic Energy Agency in Vienna is extremely active in supporting and coordinating environmental research on a global scale. Frequent discussion panels and symposia are organized and their proceedings are widely distributed. Usually the articles and abstracts contain sufficient numbers of updated references. Therefore in this section only a brief account will be presented.

Large geographical areas such as deserts, arid zones, glaciers, polar ice terrains, oceans, river beds, and tropical rain forests are only a few examples of huge environmental 'field laboratories' where isotopes are widely used in various branches of environmental research. Probably the most interesting territory is the Amazon Basin in Brazil, an area of 6 x 106 square kilometers, containing about half of the earth's tropical forests, about 80 000 plant species, and around 30 x 106 animal species, most of them insects. The Amazon river also contributes 20% of the world's river water discharge to the oceans. Furthermore, this part of the world is under permanent ecological change due to massive deforestation. Between 1980 to 1990, 180000 square kilometers of rain forest were lost. Bowen et al. [138] discussed the application of isotope studies in this part of the world.

10.10.1 Hydrology and Water Resource Studies Several stable isotopes, together with a few radionuclides, play an important role in hydrological, hydrogeological and water resource research. More generally, isotopic studies provide information about ground water and surface water sources, sedimentation, residence times (dating), environmental and


water pollution, geochemical evolution processes, paleohydrology and paleoclimatology. Recently, Gascoyne and Kötzer [139] reviewed isotopic methods in hydrogeology. Applications of D, Tand 180,3He and 4He, 13C, 14C, 34S and 180 in S04, Ne isotopes, Ar isotopes, Kr isotopes, 6Li/7Li, UB/10B, 87Sr/86Sr, 36C1, 129I and U isotopes were discussed. The review includes 182 references. 222Rn is also a widely used tracer in dating, ref. [140], p. 249, and chemical kinetics studies [141] of groundwater. Various applications of noble gas isotopes to hydrological and water resources studies were discussed by Rozanski in the Proceedings on 'Isotopes of Noble Gases as Tracers in Environmental Studies', ref. [140], p. 1.

The Proceedings of the 1983 IAEA Symposium on Isotope Hydrology and the Proceedings of the 1991 IAEA Symposium on Isotope Techniques in Water Resources Development [142], both including 78 lectures, 77 poster presentations and more than one thousand references, firmly describe the various activities and the state of the art. The advantages and limitations of existing (up to 1984) mathematical models for the interpretation of isotopic tracer data in groundwater hydrology were reviewed by Zuber [143]. Recent mathematical models in isotope hydrology were discussed at an IAEA meeting in 1993 [144]. The reader is also referred to a book by Mazor [145] on the roles of D, T, 180, 13C, 14C and noble gases in hydrology and to the Proceedings of a recent IAEA Symposium on 'Isotopes in Water Resources Management' [146], which includes 43 papers and over 100 poster presentations.

The importance, continued growth and global spread of isotope applications in hydrological and related geochemical and environmental research have resulted in a demand for high quality isotopic standard reference materials and intercomparison samples, with well determined isotopic composition, for the intercalibration of analytical procedures and results among the various active laboratories. The Section of Isotope Hydrology within the International Atomic Energy Agency in Vienna is active in the preparation, certification and distribution of SRMs for the determination of isotopic composition in natural compounds. A recent IAEA publication [136] presents a comprehensive summary of the available hydrogen, carbon, nitrogen, oxygen and sulfur isotopic SRMs and laboratory intercomparison materials.

10.10.2 The Atmosphere and Air Pollution Besides water, carbon dioxide is the most essential nutrient for plant growth and therefore for the existence of life on earth. C02 is removed from the atmosphere by the photosynthetic processes (see Section 10.9) and by ocean uptake. It is supplied to the atmosphere by decomposition-oxidation of organic material, by human activities such as fuel consumption and respiration, and by deomposition of carbonates. Peng and Takahashi discussed the ocean uptake of C02 [147]. The three major controlling processes are C02 transfer across the sea-air


interface, biological pumping, and ocean water circulation and mixing. Isotopic studies, based mainly on anthropogenic and natural radionuclide tracers and 3He, have been used to understand the rate of mixing of surface ocean water into deeper layers. Roeloffzenn et al. [148] discussed the trends and variations in the 13C isotope in atmospheric C02 .

Kelly [149] discussed the application of enriched stable isotope tracers for studying the transport of pollutants related to acid rain, their chemical transformations occurring during transport, and the location and mechanism of pollutant depositions. For the tracing of sulfur compounds and different aerosols, the two minor isotopes 33S and 36S could be used. Tagging pollution sources with multi-isotopic elements (such as Nd) enables the identification of the source at a particular sampling site. Methods developed at NIST using high precision mass spectrometry have permitted the detection of small isotopic changes introduced by source tagging.

Newman et al. in ref. [150], p. 133, discussed sulfur isotope variations in the atmosphere; Newman and Forrest, in ref. [150], p. 331, performed sulfur isotope measurements relevant to power plant emissions, and Nielsen et al, ref. [150], p. 65, discussed lithospheric sources of sulfur using the <534S concept.

10.10.3 Climatic Changes

Recent environmental changes, such as the concentration increase in air of trace constituents, e.g. C0 2 , CH4, CO, O3, Fréons, nitrogen and sulfur oxides, induced by increased anthropogenic emission, are believed to have significantly harmful global effects. Global warming, known as the 'greenhouse' effect, caused by increases in atmospheric C 0 2 and CH4 is a result of this process. It is expected that the rate of warming of the earth's atmosphere, which over the last 100 years has been about 0.5 °C, will increase dramatically, giving rise to polar ice melting, rising sea levels, flooding of coastal lands, changs in river systems, affecting rainfall distribution and converting agricultural soils into arid land. Environmental isotopes are a powerful tool for investigating climatic variations and the response of nature to fluctuating and changing climate. Rozanski and Gonflantini [151] summarized the isotopes used in climatological research and the climatic information retrieved. The stable isotopes D, l 8 0 and 13C are used in studies of ocean temperature, ocean circulation, ice sheet volumes, air temperature, relative humidity and dynamics of the water cycle on the regional and global scale. The natural radioisotopes 14C, 39Ar, 40K, 210Pb, 222Rn, 230Th, 23'Pa, 234U and 238U are used in studies of oceanic and atmosphereic dynamics and sedimentation rate. Radioisotopes from nuclear resources, such as T, 85Kr, 137Cs and l4C serve for dating of deep-sea, lacustrine and cave deposits and of ocean and ground water and for studies of atmospheric dynamics. Further information on the application of isotopes to climatic studies can be found in


the IAEA Proceedings on Isotopes of Noble Gases as Tracers in Environmental Studies [140], in the IAEA Proceedings on Isotopic Techniques in the Study of Past and Current Environmental Changes in the Hydrosphere and the Atmosphere [152], and in a CEC report [153].

Stievenard et al. [154] measured the D/H and 1 8 0 / 1 6 0 isotopic ratios in Antarctic ice cores in an attempt to reconstruct past climates. Investigations based on the same isotopes plus tritium in snow and ice were reviewed by Moser and Stichler [155]. The environmental record in glaciers and ice sheets, including dating with the use of stable isotopes and radionuclides, was reviewed by Delwiche [156].

10.10.4 Other Environmental Applications

10.10.4.1 Plutonium in the Environment

Plutonium is a newcomer to the earth's environment, an anthropogenic element produced only since the 1940s. Before this period it was not detectable in the environment, despite its production in nature by neutron capture of 238U in extremely small quantities (see also Chapter 9, Section 94). During the past 50 years, more than 1000 tonnes of plutonium have been produced in nuclear reactors around the world. Through nuclear tests, nuclear accidents, nuclear reprocessing plants, usage in space exploration equipment and as fuel in nuclear submarines and other applications, the element has become widespread over land, oceans and in the atmosphere. As all the Pu isotopes are highly radioactive a emitters, and because plutonium is absorbed by bone marrow, it is considered as an extremely hazardous radiological poison. The recommended total accumulated amount in an adult body without producing significant damage is 5 x 10~10 g. Therefore, if only on the ground of this argument, the enormous interest in the environmental behavior of plutonium is self evident.

Plutonium has the unique characteristic that its short existence on the earth has not been sufficient as yet to reach an equilibrium with the surroundings. This is in contrast with natural elements, which have existed in equilibrated environmental systems for at least 4 billion years. Six of the 15 Pu isotopes have relatively long life-times, so their relative isotopic abundance or a particular Pu isotopic ratio may provide a typical fingerprint to indicate the origin (e.g., type of nuclear reactor or type of nuclear weapons used) of plutonium in the environment. Besides the short terrestrial history and easy source identification, studies of plutonium in the environment have the advantages of low background and high sensitivity of mass spectrometric and counting measurements.

In 1994 the first international symposium on 'Plutonium in the Environment' was held in Ottawa, Canada [157]. The topics discussed were: Case Studies; Analysis; Geosphere and Atmosphere; Hydrosphere; and Biosphere. Papers


related directly to isotopic studies are briefly mentioned here. Kudo et al., on p. 1096, discussed the Nagasaki atomic-bomb from the view of an experimental event in global pollution. Plutonium contamination and isotopic distribution at British atomic weapon test sites and locations of nuclear operations and waste disposal in the former Soviet Union were reviewed by Burns et al., p. 1099, and by Baskaran et al., p. 1109, respectively. The Pu mobility in soil, sediment layers and tree rings at Nagasaki was described by Mahara and Kudo, p. 1191. Further discussions included the behavior of the element in oceans (Baxter et al, p. 1213), in the Baltic Sea, where fast scavenging by sediments was observed (Holm, p. 1225), in Lake Ontario (Joshi, p. 1231), and in the Ottawa River Basin (Cornett et al., p. 1239). The incidence of environmental plutonium in humans was discussed by Taylor, p. 1245 (see also Sect. 10.5), Franke et al., p. 1253, and Sun et al., p. 1259. Pentreath, on p. 1279, delivered a brief historical perspective on Pu analysis in environmental samples. All the articles are extensively supported with references.

10.10.4.2 Lead in the Atmosphere

Lead is a useful and widespread element, but is toxic to humans. According to the US OSHA standard, a lead level above 50 pg dl"1 in the blood of adults is considered hazardous, and individuals having this level of contamination must be removed to surroundings having a lower lead content. For children, the normal blood level is below 9 pdl~' [158]. Anemia and enzymatic disturbance effects in adults and a reduction in learning ability in children have been attributed to lead levels above normal.

In an attempt to establish the absorption characteristics of airborne lead into human blood, Facchetti and Geiss [159] replaced the local lead (206Pb/ 2 0 7 Pb« 1.19) in the tetraalkyl-lead added to petrol distributed in the Turin area in Italy with Australian lead from the Broken Hill mine (206Pb/207Pb = 1.04). In a four phase experiment carried out between 1974 and 1985, the lead originally used was gradually replaced by the Australian lead, which was then used almost solely (about 90%) for two and half years and finally again changed back to the European lead. Over these periods the lead concentration and isotopic composition was measured in the atmosphere and in the blood of adult humans. Even though the 206Pb/207Pb ratio in petrol dropped below 1.06, the same ratio in blood collected in the area of greatest car exhaust pollution, namely downtown Turin, never decreased below 1.13-1.14, indicating that airborne lead is not an important factor in human blood lead, or that possibly, within the period during which Australian lead was used, the human body system did not reach equilibrium because of possible lead transfer to other tissues. The experiment was summarized by Facchetti [160,161]. Other experi-ments of this type using lead isotopes with humans were described by Mantón [162] and by Rabinowitz et al. [163,164].


10.10.4.3 Earthquakes and Volcanic Eruptions

Gudzenko, in ref. [140], p. 249, discussed the 222Rn isotope as an earthquake precursor, and Rozanski, ref. [140], p. 1, summarized the use of 4He, 222Rn and 40Ar as indicators for investigating the mechanism and dynamics of processes occurring in seismo-tectonic and volcanic zones. During monitoring of the concentration of 222Rn (f^ = 3.8 days) in thermal mineralized water in Tashkent from 1956, a pronounced concentration increase was observed in 1964-1965. Within the next period of 6 months, from October 1965 to April 1966, the change was small, and immediately after the April 1966 earthquake a threefold decrease in radon concentration was observed. Subsequently, again, a smooth accumulation was monitored, which was interrupted by a sharp reduction in concentration resulting from aftershocks. Similar Rn behavior has been observed in gases released by the Hakone Volcano earthquake in Japan. As this Rn anomaly is not universal, and other behavior patterns have also been recorded, such as decreases in Rn concentration 1 month before a strong earthquake, it may be concluded that 222Rn fluctuations are related to stress processes in the earth's crust, but are not effective on their own as reliable warning signals.

10.11 ISOTOPE EFFECTS AND ISOTOPE ENRICHMENT PROCESSES

The occurrence of isotopes in an element, or isotopes of a particular element in a molecule, may affect the mechanical and the physical or chemical behavior of a substance. A few of the examples are: preferential distillation of H 2 0 over HDO; different electrolytic reaction rates of H 2 0 (fci) and HDO (k2), where k\ > k2; and other kinetic effects, mainly of light isotopes, in various organic and inorganic reactions; the Graham gas diffusion rate law; the time-of-flight isotopic separation in a TOF-MS; and space separation in a magnetic MS. In principle, isotope effects are mass effects, which may be divided into two groups:

1. Effects in which the isotopic mass difference is correlated with such physical properties as gas diffusion, thermal diffusion, or motion in gravitational, electric, magnetic, centrifugal and other force fields.

2. Isotopic substitution in a chemical bond affects vibrational frequencies, therefore spectral shifts in the IR region are observed. Bond strengths are affected, introducing slight differences in chemical reactivity.

Isotopes also exhibit differences in properties related to their nucleus, such as nuclear spin isotope effects, particle emission, energies of radiation, half-lives, cross sections of neutron absorption etc. None of these is included in the above two categories, and they will not be discussed in this section.

ISOTOPE EFFECTS AND ISOTOPE ENRICHMENT PROCESSES 437

Isotope effects have been studied in chemical kinetics and equilibria of a wide range of chemical processes, providing important kinetic and thermo-dynamic data. The understanding of isotope effects, both physical and chemical, laid down the foundations of research and development of isotope enrichment and separation processes, their laboratory and large scale pro-duction and their consequent applications. Different enrichment and separation methods have been developed. Thermal diffusion, gaseous diffusion, electro-magnetic separation, the Calutron, centrifugal enrichment, the Becker nozzle separation and resonant laser enrichment belong to the physical processes. Low temperature adsorption, liquid and gas chromatographic enrichment, chemical exchange processes, electrolytic enrichment, dectromigration enrichment and photochemical separation are among the chemical processes.

Clusius was one of the pioneers of isotopic separation, applying thermal diffusion on a laboratory scale [165]. His achievements between 1939 and 1962 are shown in Table 10.5.

In the 1950s the Oak Ridge National Laboratory (ORNL) started to separate isotopes on a laboratory scale using electromagnetic separation. Numerous metallic isotopes were enriched. Many isotopic ratio measurement procedures were developed to support this operation [166]. Since then, many projects have been undertaken to separate or highly enrich various isotopes, on both the small and the large industrial scale. The production and distribution of about 225 enriched, non-gaseous stable isotopes from 50 multi-isotopic elements at ORNL has been reviewed by Tracy [167]. An interesting separation case shown in Table 10.5 is the enrichment of 38Ar, a low abundance middle isotope, from 0.064% to a purity of 99.984%. The 1 70 isotope, also a middle isotope, of

Table 10.5. Isotopes separated by thermal diffusion [165]

Isotope Natural Enrichment Year Abundance (%)

(%)

75.7 24.3 57.1 17.5 90.5 0.37 1.09 8.9 0.275 0.204 0.064 9.21 0.37

99.4 99.6 98.3 95.5 99.95 99.8 99.8 99.0 99.6 99.75 99.984 99.92 99.991

1939 1939 1942 1942 1950 1950 1953 1955 1956 1959 1959 1960 1962

35C1 37C1 84Kr 86Kr 20Ne u N 13C 136Xe 21Ne 18Q 38 Ar 22Ne 36 Ar


0.038% abundance, has been enriched beyond 90% in a large scale thermal diffusion plant at the Weizmann Institute of Science in Israel.

Vertes and Kiss published good reviews on isotope effects ref. [168], p. 381, and on isotope enrichment processes ref. [168], p. 462. Further literature may be found in the references and monographs listed therein. An interesting symposium report was published by the American Chemical Society [169]. Readers interested in early theoretical studies on isotope effects and experimental work on isotopically substituted model compounds are encour-aged to read the publications of J. Biegeleisen, M. Wolfsberg, W. Spindell, M.J. Stern, W.A. Van Hook and other since the late 1950s.

10.12 ELUCIDATION OF CHEMICAL REACTION MECHANISMS

Chemical reaction interface/mass spectrometry, or CRIMS, is a relatively new technique by which targeted isotopes or elements can be monitored in metabolic studies. In CRIMS, analyte molecules are decomposed in a high

Table 10.6. Reactions and mass analysis applied in CRIMS Element/ isotope

H,D H D2 12,13C

1 4 C

12,13C

c

14,15N

N N

O 1 6 , 1 8 Q

P S

Cl

Se Br

Reactant

NF3 S0 2 H2

so2 H2 NF3

so2 H2

H2

NF3 H 3 5 , 3 7 C ,

NF3

so2 NF3 HCl so2

Product

HF, DF H 2 0 HD co2 14CH4 CF4 CO CH4 C2H2 HCN NO HCN N2 N0 2 H 2 0 C 1 6 , 1 8 0

PF5 S 35 ,37 C 1

SF6 HCl FC1 80Se35-37Cl HBr

Mass number a

20,21 18 3.022 44,45 18.034 69,70 (CF+) 28 16 26 27 30,31 27,28 28,29 46,47 18 28,30 107 (PF+) 67,69 127,129 (SF; 36,38 54,56 115,117 80,82

" Exact mass indicates high resolution analysis.

ISOTOPE ARCHAEOMETRY 439

temperature plasma into their elemental species and reacted with atoms of another gas to form small and simple species that are detected by conventional mass spectrometry. The presence of these species provides qualitative information about the element or isotope, which can be quantified, and the resulting isotopic signature can differentiate it from endogenous materials. By subtracting the known natural abundance of the stable isotope used as a tracer, a spectrum showing only the enriched species can be produced. In this sense CRIMS is parallel to radioisotopic tracing. CRIMS has been coupled with gas and liquid chromatography. Recently, Abramson [170] reviewed this technique and its application to metabolic studies. Table 10.6 summarizes the applications of CRIMS, giving several of the elements or isotopes studied (but not the molecular compound), the reactant gas, the product and the mass numbers at which the product was analyzed.

10.13 ISOTOPE ARCHAEOMETRY

10.13.1 Lead

A. O. Nier and coworkers were the first to study the isotopic composition of lead. When lead in galena (Pbs) from different ore deposits had been compared, variable isotopic distributions were observed. It was suggested that these variations represent mixing of radiogenic lead (see Chapter 9, Section 82) with 'primeval' lead in various proportions, before the deposition of the galena [171].

Based on a survey of several hundred isotopic lead determinations, made in naturally occurring galena ores over almost a period of 30 years, Brill [172] suggested that differences between isotopic compositions should be sufficient to distinguish between leads mined in Greece, England and Spain. Furthermore, it was suggested that the lead isotopic composition in archaeological objects may be helpful in tracing possible locations of mines from which the leads could have been taken. 230 leald samples, consisting of 70 ore samples from ancient mining areas and 160 samples of archaeological origin were collected, 60 of these were analyzed and reported in the first phase of the work. The objectives were: (a) to make an isotopic survey of ancient leads from different places, periods and materials, and (b) to confirm experimentally that the isotopic compositions of archaeological objects, known to have been made from lead mined in specific areas, could be matched to ores from the same areas. The results had shown that the isotopic compositons of the archaeological objects vary over a considerable range, but many of them fall into three definite groups. The same groups also include galena ores from the three best known ancient lead mining areas, namely Laurion, Southern Spain and Roman Britain.


Table 10.7. Lead isotope ratios in archaeological objects Source

Laurion, Greece Roman Britain Southern Spain Sardinia Egypt, various

Finland grave/habitation site, North America

Taurus, Turkey Chinese glass, Western

Han Dynasty Cyprus Nigeria

207pb /206pb

0.828-0.836 0.843-0.853 0.856-0.862 0.872-0.874 0.791-0794 0.802-0.806

0.817 0.837

0.699-0.725

0.820-0.843 0.875-0.895

0.836-0.843 0.834-0.854

208pb/206pb

2.046-2.084 2.079-2.102 2.103-2.109 2.124-2.125 1.987-1.994 2.010-2.014

2.020 2.063

1.873-1.900

2.039-2.088 2.142-2.178

2.072-2.082 2.066-2.095

Ref.

[172] [172] [172] [173] [173] [173] [173] [173] [176]

[179] [185]

[186] [187]

Following this study, it has been accepted by the archaeological community that isotopic distribution of lead is a powerful tool for provenance and authenticity studies of archaeological objects. Explorations of the origin of ancient glasses, pottery and glass glazes, paint pigments, bronze coins, traces of lead in silver and gold, burial memorials and a variety of metallic artifacts had been initiated [173-176] (and references therein), and have been continuously performed since then. Lead isotope data banks have also been established [177,178]. The lead isotope archaeometry has been extensively discussed and rigorously reviewed in a recent series of articles [179,180]. In Table 10.7 a few examples of lead isotopic composition variations are shown.

Gale and Stos-Gale [181] studied Bronze Age archaeometallurgy of the Mediterranean. The lead isotope technique, applied to traces of lead in silver, copper and bronze was proven to be sufficient to provenance metallic artifacts.

The analytical technique for lead isotopic ratio measurements used in archaeological studies is almost always Thermal Ionization Mass Spectrometry. The technique has been described in details in Chapter 9, Section 82. It is well established that for a series of 206,207,208pb/204pb r a t i o determinations (n > 6), uncertainties (given as 2SD), better than 0.03 may be obtained. Experimental results are usually presented as plots of 208Pb/206Pb and/or 204Pb/206Pb ratios vs the 207Pb/206Pb ratios [172,173]. Results for the same geographical area create in the plot a field of isotope ratios. Multivariate analysis [181] was found to be helpful in cases where isotopic fields of different areas are overlapping.

Dolnikowski et al. [182] applied fast atom bombardment mass spectrometry (FAB-MS) to the study of lead isotopic ratios in nineteenth century American glazed ceramic shards. The observed RSDs ranged from 0.3 to 1% in the

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10.13.2 Carbon-14

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[181] N.H. Gale and Z.A. Stos-Gale, Archaeol. Chem. ¡V, Adv. Chem. Ser., 220, (Ed. R.O. Allen), p. 159 1988.

[182] G.G. Dolnikowski, J.T. Watson and J. Allison, Anal. Chem., 56, 197 (1984). [183] M. Suter, Nucl. ¡nstrum. Methods Phys. Res., B64, 321 (1992). [184] R.E. Taylor, D.J. Donahue, T.H. Zabel, P.E. Damon and A.J.T. Jull, Archaeol.

Chem. ¡U, Adv. Chem. Ser., 205 (Ed. J.B. Lambert), 1982, p. 333 [185] J. Wang, P. Ling and X. Zhang, Nucl. Techn., 17, 499 (1994). [186] N.H. Gale and Z.A. Stos-Gale, Science, 216, 11 (1982). [187] K.R. Wilson and T.J. Chow, Nature, 262, 130 (1976). [188] H.M. Kingston, U.S. Patent 5414250, May 9, (1995).

CHAPTER

11

CALCULATIONS

10.0 INTRODUCTION

In this chapter, the calculations of isotopic abundances are described. Inductively coupled plasma ionization has the advantage that only atomic ions are monitored. In thermal ionization, atomic ions are preferentially monitored; several elements form more intense oxide ions, some of them with three or four oxygen atoms, thereby complicating the calculations and increasing the abundance uncertainties owing to the amplification of oxygen uncertainties. For several light elements, mainly lithium and boron, attempts are made to analyze heavier ions to minimize the isotopic fractionation effects. Lithium, apart from atomic ions, is also monitored as Li2F+ and L^BOj , and boron is monitored as Na2BOj or C s ^ O j . In electron impact ionization, except for noble gases, it is unusual to monitor atomic ions. Several elements, metals and non-metals produce gas phase fluorides, most of which are stable and therefore easily ionized and, as fluorine is mono-isotopic, the abundance calculations are simple. The calculations presented in the following discussion are based only on the currently monitored ions.

Margrave and Polansky [1] showed that the relative isotopic mass spectrum can be calculated using probability theory and isotopic abundances. A simple example is the Cl 2 ion, which has ions at m/z = 70, 72 and 74. The number of ways of picking any x atoms of the same kind from a large number N of isotopically different atoms is

Cx = N\/x$N - x)\ (1)

thus for two atoms C2 = N\/2\{N - 2)! (la)

and therefore 35C12/35C137C1 = [N\/2\{N - 2)!]/[An/l!(7V - l)!]2

= [(/V-l)!]2/[2!(rV-2)uV!]

= (l/2)[(N-$/N] (2)

As N—>oo 35Cl2/35C37Cl = l / 2 (3)

A similar calculation gives 35C12/37C12 = 1 (4)

450 CALCULATIONS

and therefore the expected ratios 35C135C1:35C137C1:37C137C1, given equal numbers of 35C1 and 37C1 atoms, are 1 :2 :1 . However, the abundance of the chlorine isotopes is 75.77 and 24.23% respectively, therefore the above ratios must be corrected by considering the partial abundance of each CI isotope

35C12 x 3 5 / 2 = 1 x 0.75772 = 0.5741 (5)

35C137C1 x 3 5 / x 37f = 2 x 0.7577 x 0.2423 = 0.3672 (6)

37C12 x 3 7 / 2 = 1 x 0.24232 = 0.0587 (7) The normalized relative abundances of 35ClJ, 35C137C1+ and 37ClJ in the spectrum will be 100, 64.0 and 10.2 respectively.

Margrave and Polansky [1] also calculated the spectrum of a binary molecule A„Bm, where each atom has three isotopes and their abundance is known. The calculation was applied for the BClJ ion, using 10B = 19.6%, n B = 80.4%, 35C1 = 75.5% and 37C1 = 24.5%. The results are given in Table 11.1, which also includes data calculated with presently accepted IUPAC boron and chlorine isotopic abundances. A calculation for the various WOj ions is given in Section 11.74.6.

The following symbols have been used:

A(mX) abundance of isotope mX in atom per cent; mi measured ion beam intensity of isotope mX; Rmjn ratio of the measured mi ion current over the measured "i ion current;

Table 11.1. Calculated and observed BCI3 mass spectrum Ion mass (m/z)

115 116 117 118 119 120 121 122

Ionic species

10B35C13 "B3 5C13 10B35C12

37C1 "B35C12

37C1 10B35C137C12 UB35C137C12 10B37C13 l lB3 7Cl3

Calculated

25.1 100 23.9 96.0

7.8 30.8 0.9 3.5

Relative abundance

Observed (1)

25.1 100 24.6 98.2

7.9 32.2 0.9 3.5

Observed (2)

24.9 100 23.9 95.9

7.6 30.7 0.8 3.3

(1) I0B = 19.6%, "B = 80.4%, 35C1 = 75.5% and 37CI = 24.5%. (2) 10B = 19.9%. "B = 80.1%, 3SC1 = 75.77% and 37C1 = 24.23%, see Appendix


Ei summation of all the recorded ion currents of the element under consideration.

It is assumed that isotopic mass fractionation has been corrected. The abundance is expressed in terms of isotopic ratios Rm/n rather than in ion

currents. In many applications the isotopic ratio is the variable of interest, therefore Rm/n is the output of the computer operated data acquisition systems. This is also the parameter corrected for mass fractionation and the parameter on which the outlier rejection procedures are applied. The following definitions will be helpful in deriving simplified isotope abundance equations.

a = K33/32 = 1 7 0 1 6 0 / 1 6 0 2 = 0.00075 for molecular oxygen in air; a' = /?33/32/2 = An/16 is the ratio of atomic oxygen 1 7 0 / 1 6 0 in the sample, i.e. 0.000375 for oxygen in air; a" is the 1 7 0 1 6 0 2 / 1 6 0 3 ratio, 0.001125 for oxygen in air; ß is the isotopic ratio of 33S32S/32S2, R65/64 of sulfur in the sample (CS2); r is the carbon isotopic ratio /? l 3 / 1 2 = 1 3C/I 2C in C0 2 , i.e. 0.011237; 6 = K34/32 = 1 8 0 1 6 0 / l 6 0 2 = 0.004088 in molecular atmospheric oxygen; S'= #34/32/2 = /?jg/i6 is the ratio of atomic oxygen 1 8 0 / 1 6 0 in the

sample, i.e. 0.002044 for oxygen in air; S" is the 1 8 0 1 6 0 2 / 1 6 0 3 ratio, 0.006132 for oxygen in air; e is the isotopic ratio R^/w of atomic chlorine in the sample.


m/z = 2; H+(D+) m/z = 3 ; HD+(H+) m/z = 4; D +

It is assumed that the contribution of the ion-molecule reaction product H J to 3t is negligible, or has been corrected. Also, the D4 contribution to H2 has been corrected.

For samples in equilibrium A(D) = 3 i x 100/(3i + 2 x 2 t )

= 100/(1 +Ä2/3), or A(D) = 2 x 4i x 100/(2 x U + 3t)

= 100/[1 + (Ä3/4/2)] For non-equilibrium samples

A(D) = 100(3i + 2 x 4i)/[2(2¿ + 3i + 4i)] = 100(/?3/4/2 + l)/(Ä2/4 + Ä3/4 + 1)

452 CALCULATIONS

Abundance calculations for samples containing tritium and 3He are shown in Chapter 9, Section 1.

11.2 HELIUM

m/z = 3; 3He+

m/z = 4; 4He+

A( 3 He)= 3 ix 100/(3i+4i) = 100/(1 +#4/3)

11.3 LITHIUM

11.3.1 Li+

A(6Li) = 6 / x 100/(6r+ 70 = 100/(1 +R1/6)

11.3.2 Li2F+

31; 6Li2F+ 32; 6Li7LiF+

33; 7Li2F+ 32j x 100/(32¿ + 2 x 330 100/(1 + 2 X«33/32)

100/[l+(#33/3l)1/2]

11.3.3 Li2BO+

m/z = 56; 7Li2 l0B16O+,6Li7Li n B 1602

+

(6Li2 10B 180160+,6Li2

n B n O 1 6 0 + , 6Li7Li10B17O16O+)

m/z = m/z = m/z =

A(6Li) =

A(6Li) =

BORON 453

m/z = 57; 7U2uBl60^

( 7 L i 21 0 B 1 7 O 1 6 O + , 6 L i 2

1 1 B 1 8 O l 6 O + , 6 L i 7 L i 1 0 B 1 8 O 1 6 O + , 6 L i 7 L i u B , 7 O 1 6 O + )

Ignoring the contribution of the ions in parentheses to the ion current at m/z = 56 and 57, the following approximation can be used [2]:

#6/7 = {hb/hi — # io/n) /2

where R6p and #]o/u are the isotopic ratios 6Li/7Li and 1 0 B / n B respectively. Then

A(7Li) = 100/(1 + # 6 / 7 )

The explicit calculations are also given in ref. [2].

11.4 BERYLLIUM

Beryllium is mono-isotopic.

11.5 BORON

11.5.1 BF+

m/z = 48; 1 0B, 9F+

m/z = 49; "B19F2+

A ( 1 0 B ) = 4 8I x l 0 0 / ( 4 8 i + 49¿)

= 100/(1 +# 4 9 / 4 8 )

11.5.2 Na2BO+

m/z = 88; Na2 10B 1 6Oj

m/z = 89; Na2 n B l 6 0+ , Na2

10B 1 7 0 1 6 0 +

89i = 8 9 / „ + 8 9 l , 7 8 9i„ = 89t - a x 88i

where 89<'n and 8 9i ,7 are the Na 2n B 1 6 0+ and Na2

l0B17O16O+ ion current contributions to the measured current 89¿ at m/z = 89, and a is the #33/32 ratio

454 CALCULATIONS

as defined Section 11.1. Therefore

A(10B) = 88ixlOO/(88i + 8 9 i -<*x8 8 i ) = 100/[#89/88 + ( l - a ) ]

11.5.3 Cs2BO¿

m/z = 308; Cs2 10B16O2

m/z = 309; Cs2 n B 160+, Cs2

10B 1 7 0 1 6 0 +

A(,oB) = 100/[/?3O9/308 + ( l - « ) ]

11.5.4 BO2

m/z = 42; 10B 1 60¡ m/z = 43; " B 1 ^

A(l0B) = 100/[#43/42 + ( l - a ) ] m/z = 43; n B l602 ,10B I 701 60~

11.6 CARBON

11.6.1 CO+

m / z = 44; 12C 1602

m/Z = 45; 13C ,602+, 12C l 7 0 , 6 0 +

A(13C) = 100/K44»/45!^) + 1]

where 45ii3 is the 13C1602 ion current contribution to the measured current 45t at m/z = 45.

45 ; = 45/13 + 45/17 and therefore

i (•13,

45 • 45 • 44 •

A(,JC) = 100(/?45/44 - a)/[#45/44 + (1 - a)}

CARBON 455

11.6.2 CD+

m/z = 20; 12CD+ m/z = 21; 13CD+

A(13C) = 100/ (1+# 2 0 / 2 1 )

11.6.3 CO+

m/z = 28; 12C 1 6 0 +

m/z = 29; 1 3 C 1 6 0 + , 12C l 7 0 +

A(13C) = 2 9 ¿ 1 3 x l00 / ( 2 8 t + 29t13) 29; _ 29; 29 ;

»13 — I — '17

where 29i'i7 is the , 2C 1 7 0 + ion current contribution to the measured current 29t at m/z = 29. Therefore

»,-13 = » / - a ' x Mi\

For the definition of a! see Section 11.1.

A(UC) = 100(/?29/28 - a')/[#29/28 + (1 - a')}

M

m/z -

m/z -

m/z -

m/z -

11.6.4

m/z = m/z = ,3C) =

:69; = 70;

CF+

12CF3+ 1 3CF +

: 100/ (1+# 6 9 / 7 0 )

11.6.5 CCIJ

= 117

= 118 = 119

= 120

12C

13c l2C 13c

35C13+

35C13+ 35C12

37C1+ 3 5 C1 2

3 7 C1 +

A(13C) = 100/(1 +#ii7/ii8), and also

A(13C) = 100/(1 +#„9/120)

456 CALCULATIONS

11.6.6 CS J

12r.32ç + m/z = 76 12C32S+ m/Z = 77 ,3C32S+,12C33S32S+

77; _ 77; 77 ; í 13 — I — '33

77; 76; i-ßx lbi

where 7 7 í 3 3 is the 12C33S32S+ ion current contribution to the measured current 77 2 at m/z = 77, and ß is the #65/64 ratio of sulfur in the sample, as defined in Section 11.1. Therefore

A(13C) = 100(/?77/76 - /3)/[#77/76 + (1 - ß)\

11.7 NITROGEN

m/z m/z m/z

11.7.1

= 28; = 29; = 30;

N2+

,4N2+ •5N14N +

1 5 N 2+

(•15» A(,DN) = 100/(2#28/29 + 1)

The following expression may be used when A(15N) > 10%:

A(15N) = 100x1 5N/(1 4N+1 5N) = 100/[(14N/15N) + 1] = 100/[(14N2/I5N2)J + 1] = 100/[(R28/3o)l + l]

Also

A(15N) = 100(29i/2 + 30/)/(28/ + 29i + 3°0 = 100(#2 9/30/2 + l)/(#28/30 +#29/30 + 1)

OXYGEN

11.7.2 NO+

m/z = 44; 14N2160+

m/z = 45; l5N14N160+, 14N2 1704

45; _ 45; 45; '15 — ' — '17

A(15N) = 100 x (45i - 45i17)/(244/ + 45/ - 45i,7) = 100(l-a')/[2R44/45 + ( l - a ' ) ]

11.8 OXYGEN

11.8.1 COJ 12c 16o+ 13^16^ + \2n\lr.\6r.+ C, 00J, , ZC"0'°04

1 2 c 1 8 0 1 6 0 + ) l 2 C l 7 ^ + ) 1 3 C 1 7 0 1 6 0 +

m/z = 44; m/z = 45; m/z = 46;

180 abundance:

A(180) = 46i x 100/(2 x **i + 45t,7 + 46i)

where 45t'i7 is the l2C 170 I 6 0 + ion current contribution to the measured current 45 i at m/z = 45. Therefore

45; _ 45; _ . , 44; l\1 = I — T X I

and T is the carbon ratio # B / I 2 in the sample, as defined in the Section 11.1.

A( l 80) = 100/[(2 - T)/?44/46 + #45/46 + 1]

The above derived expression is valid for samples with natural abundance of carbon isotopes and close to natural abundance of oxygen isotopes. Under these conditions the contribution of I2C17Oj and 13C170160+ to 46i can be neglected. At higher n O and 180 abundances the ion currents at m/z = 47 and 48 should also be accounted for.

170 abundance : A(170) = 45i,7 x 100/(2 x 44t + 45t,7 + 46t)

A( ' 70) = 100(#45/44 - T)/[#45/44 + #46/44 + (2 - T)]

458 CALCULATIONS

11.8.2 O í

m/z = 32 m/z = 33 m/z = 34 m/z = 35 m/z = 36

1 6 0 1 6 0 +

1 7 0 1 6 0 +

18o16o+, 17ol7o+ 1 8 0 1 7 0 +

1 8 0 1 8 0 +

In case of low A( n O)

1 70 abundance : A(170) = 100/(2#32/33 + #34/33 + #36/33 + 1)

#36/33 c a n D e neglected.

1 8 0 abundance : A(180) = 100(34i + 2 x 3 6t)/(2 x 32i + 33¿ + 34/ + 2 x 36i)

A(180) = 100/[(2 x 32/ + 33;)/(34i + 2 x 36i) + 1]

For natural or close to natural isotopic oxygen abundances, 2 x 36i/ui « 0.002, therefore 36i can be neglected, and

A( l 80) = 100/(2#32/34 + #33/34 + 1)

If A ( ' s O ) > 10%, also:

A(180) = 100/[(Ä32/36 +#33/36^ + 1] In the case of non-statistical isotopic distribution:

n O abundance : A(170) = 100(33t72 + 35i/2)/T,i

34 . , O abundance : A(lsO) = 100( ¡/2 + j5¿-/2 + 36¿)/£i

The following equations were also derived for samples enriched with O and 18, O atoms [3]:

A( , 0 0) = 100(^//E/)5

A(180) = 100(36i/Si)2

A(170) = 10000 x 33t-/2A(160) x Si, or

A(170) = 10000 x 35i/2A(180) x S/

m/z = 28 m/z = 29 m/z = 30

18, O abundance: ,18,

OXYGEN

11.8.3 CO+

12 C 1 6 0 +

l2c17o+, , 3 c 1 6 o +

12cI8o+, , 3 c 1 7 o +

28; , 29;

459

A(18O) = 3 ü t 1 8 x l 0 0 / r / + 29M7 + JÜ'',8) 29; _ 29; 29;

'17 — ' — '13

= 29i - T X 2 8 i

Assuming the 13C170+ ion current contribution is negligible A(180) = 100/1(1 - r)#28/30 + #29/30 + 1]

, 7 0 abundance: A(nO) = 100(1 - rÄ2g/29)/[(l - r)/?28/29 + R30/29 + 1]

11.8.4 N20 +

14N2 , 6 0 +

, 5N1 4N1 60+ 14

14NT_ 18r«+ 15 xi 14M 17,-.+ 15 M . 16,-,+ N2

1 7 0 + m/z = 44; m/z = 45; m/z = 46;

A(180) = 100(46/ - «in)l{"i + A5i + 46i = 100(# 4 6 /44 - # 3 0 / 2 8 ) / ( I + #45/44 + #46/44)

'N21 80+, , 3 N l , N " 0 + , ° N 2

, 0 0 /46.. 46; _\//44.. , 45; , 4 6

where 46¿i6/44' = 15N2/14N2 = #30/28» and the ion current contribution of 15NI4N170+ to the measured current 46i at m/z = 46 is neglected.

11.8.5 150NdO+

m/z = 166 m/z = 167 m/z = 168

i s o N d i 6 0 +

1 5 0 N d 1 7 0 +

. 5 0 N d 1 8 0 +

We shall now expand the simple calculation used in Section 11.2 for helium and shall establish a general expression for the atom per cent abundance of an atomic ion aX with mass numbers from a to n

A(aX) = 100 x ai/{ai + bi + ei + di + • • • + "/)

460

therefore

CALCULATIONS

A(aX) = 100/(1 + Rb/a + Rc/a + Rd/a + •••+ Rn/a)

or for CX

A(CX) = 100/(1 + Ra/C + Rb/C + Rd/C + •••+ Rn/C).

Applying to , 8 0 : A(180) = 100/(Ä166/168 +#i67/i68 + 1)

11.8.6 ""PrO

m/z = 147 m/z = 148 m/z = 149

141pr.60+

141pr170+

>41prl80+

The isotopic abundances A(1 60), A(170) and A( l 8 0) are calculated following the expressions in Section 11.8.5.

11.9 FLUORINE

Fluorine is mono-isotopic.

11.10 NEON

m/z = 20 m/z = 21 m/z = 22

20Ne+ 21Ne+

Ne+ 22

Following the expressions in Section 11.8.5 ,20 A(zuNe) = 100 x Mi/(zui + ni + 22i)

= 100/(1 + #21/20+ #22/20)

11.11 SODIUM

Sodium is mono-isotopic.

SULFUR 461

11.12 MAGNESIUM

m/z = 24; 24Mg+

m/z = 25; 25Mg+

m/z = 26; 26Mg+ The isotopic abundances A( Mg), A( Mg) and A(26Mg) are calculated

following the expressions in Section 11.8.5.

11.13 ALUMINUM

Aluminum is mono-isotopic.

11.14 SILICON

m/z = 85; 28SiF^

m/z = 86; 29SiF+ m/z = 87; 30SiF+

The isotopic abundances A( Si), A( Si) and A( Si) are calculated following the expressions in Section 11.8.5.

11.15 PHOSPHORUS

Phosphorus is mono-isotopic.

m/z = 64; m/z = 65;

11.16 SULFUR 11.16.01 SO +

32s 16o+ 3 3 s 1 6 o + , 3 2 s , 7 o 1 6 c

m/z = 66; 34S 1 60+, 33S , 7 0 , 6 0 + , 32S 1 8 0 1 6 0 +

m/z = 67; 34S 1 7 0 , 6 0 + , 3 3 S 1 80 l 60+

m/z = 68; 36S 1 60+, 34S 1 8 0 1 6 0 + , 32S 1 8 0 1 8 0 +

6 5 / 3 3 = 6 5 I . _ 6 5 ¿ i 7 = 6 5 / _ a x 6 4 /

66/34 = 6 6 / - 6 6 / l 8 = 6 6 ' - - ¿ X 6 4 i

A(32S) = 100(64/ + 65t32 + 66t32)/Si

= 100(1 -a-S)/(zZi/Mi) A(32S) = 100(1 - a - S)/{\ + #65/64 + #66/64 + #68/64)

462 CALCULATIONS

where S is the ratio #34/32 as defined in Section 11.1. Ei is the sum of the ion currents at m/z = 64, 65, 66 and 68; the contributions of the 32S ]102 ion and the ions containing 33S, and 36S with combinations of 1 7 0 and 1 80 can be neglected.

For A(33S) and A(34S) the following calculations are used

A(33S) = 100 X 65/33/(64i + 65¿33 + 66'34) = 100(/?65/64 - a)/[#65/64 + #66/64 + I - OL - 6}

A(34S) = 100 X «WC"« + 65'33 + 66'34) = 100(#66/64 - <5)/[#65/64 + #66/64 + 1 - O - Í ]

11.16.2 SO 32s I6o+ 3 3 S 1 6 0 + ) 3 2 S 1 7 0 +

3 4 s 1 6 o + , 3 3 s 1 7 o + , 3 2 s 1 8 o +

3 4 s 1 7 o + , 3 3 s 1 8 o+ 36S160+ 3 4 S 1 8 Q +

m/z = 48

m/z = 49

m/z = 50

m/z = 51

m/z = 52

A(32S) = l O O r »32 + 4y'32 + 5U'32)/S/

A(32S) = 100(1 + a' + S')/{\ + #4 9 / 4 8 + #50/48 + #51/48 + #52/48)

,48

,34 50. A r s ) = ioor¿34+52í34)/Ei Neglecting the S 0 + ion intensity

7n+

50 '34

50 i-S'x 48;

and also

52; _ 52; '36 — '

52 '18 = 52i-S'x 50 '34

A(J4S) = 100{#52/48 + #5o/48(l " « ' )

- [6' + (S')2]}/(1 + #49/48 + #50/48 + #51/48 + #52/4s)

It should be noted that #5i/48 is negligible. At natural abundances 3 4 S 1 7 0 + ss 33S 180+ « 15 x 10"6. Also, {S'f = 4 x 10"6 and therefore may be neglected.

Also, #34/32 may be derived in the following way:

#34/32 =A(3 4S)/A(3 2S) = [#52/48 +#50/48(1 - 6') -S']/(l + a'+ S')

m/z = m/z =

127; 128;

CHLORINE

11.16.3 SF +

32SF+ m/z = 33SF+ m/z =

129; 131;

34SF5+ 36SF+

463

11.16.4 AsS+

m/z =107 ; 75As32S+ m/z = 109; 75As34S+

m/z = 1 8 8 ; 7 5As3 3S+ m/z =111; 75As36S+

In both cases the isotopic abundances A(32S), A(33S), A(34S) and A(36S) are calculated following the expressions in Section 11.8.5.

11.17 CHLORINE

11.17.1 HC1+

m/z = 35; 35C1+

m/z = 36; H3 5C1+

m/z = 37; 37C1+, D3 5C1+, (H235C1+)

m/z = 38; H3 7C1+

A(37C1) = 100/(1+# 3 8 / 3 6 )

11.17.2 NaCl +

m/z = 58; 23 Na35Cl+

m/z = 60; 23 Na37Cl+

A(37C1) = 100/(1 +#58/60)

11.17.3 Cs2Cl+

m/z = 301; 133Cs235Cl+

m/z = 303; 133Cs237Cl+

A(37C1) = 100/(1 +#301/303)

464 CALCULATIONS

11.18 ARGON

m/z = 36 m/z = 38 m/z = 40

36Ar+ 38 A r +

40 A r +

The isotopic abundances A(36Ar), A(38Ar), and A(40Ar) are calculated following the expressions in Section 11.8.5.

11.19 POTASSIUM

m/z = 39; 39K+

m/z = 40; *°K+

m/z = 41; 41K+

The isotopic abundances A(39K), A(40K), and A(41K) are calculated following the expressions in Section 11.8.5.

11.20 CALCIUM

m/z = 40

m/z = 42

m/z = 43

40Ca+ 42Ca+

43Ca+ ,40,

m/z = 44

m/z = 46

m/z = 48 /•43,

44Ca+ 46Ca+ 48Ca+

,44, ,46, The isotopic abundances A(wCa), A(42Ca), A(43Ca), A(44Ca), A p C a ) and ,48 A( Ca) are calculated following the expressions in Section 11.8.5.

11.21 SCANDIUM

Scandium is mono-isotopic.

m/z = 46; m/z = 47; m/z = 48;

11.22 TITANIUM

11.22.1

4 6 T i +

4 7 T i +

4 8 T i +

Ti+

m/z = 49; m/z = 50;

4 9 T i +

5 0 T i +

IRON 465

The isotopic abundances A(46Ti), A(47Ti), A(48Ti), A(49Ti) and A(50Ti) are calculated following the expressions in Section 11.8.5.

11.22.2 TiF +

46 T i F +

4 7 T i F +

4 8 T i F +

m/z = m/z =

106; 107;

49 T i F 4

50 T i F + m/z = 103; m/z= 104; m/z = 105;

The isotopic abundances A(46Ti), A(47Ti), A(48Ti), A(49Ti) and A(50Ti) are calculated following the expressions in Section 11.8.5.

11.23 VANADIUM

m/z = 50; 50V+

m/z = 51; 51V+

A(5°V) = 100/(1+#50/51 )

11.24 CHROMIUM

m/z = 50; 50Cr+ m/z = 53; 53Cr+ m/z = 52; 52 Cr+ m/z = 54; 54 Cr+

The isotopic abundances A(50Cr), A(52Cr), A(53Cr) and A(54Cr) are calculated following the expressions in Section 11.8.5.

11.25 MANGANESE

Manganese is mono-isotopic.

m/z = 54; m/z = 56;

11.26 IRON

11.26.1 Fe+

54Fe+ m/z = 57; 56Fe+ m/z = 58;

57Fe+

58 F e +

466 CALCULATIONS

11.26.2 Fel+

m/z = 1 8 1 ; 54FeI+ m/z = 184; 57Fel m/z = 1 8 3 ; 56FeI+ m/z = 1 8 5 ; 58FeI

In both cases the isotopic abundances A(54Fe), A(56Fe), A(57Fe) and A(58Fe), are calculated following the expressions in Section 11.8.5.

Cobalt is mono-isotopic.

11.27 COBALT

m/z = m/z -m/z -

= 58 = 60 = 61

11.28 NICKEL 58Ni+ 6 0 N i +

6 1 N i +

m/z = 62; m/z = 64;

62Ni+ 6 4 N i +

The isotopic abundances A(58Ni), ¿ ( " N i ) , A(61Ni), A(62Ni) and A(64Ni) are calculated following the expressions in Section 11.8.5.

11.29 COPPER

11.29.1 Cu+

m/z = 63; 63Cu+

m/z = 65; 65Cu+

A(°3Cu) = 100/(l+R6 3/65)

11.29.2 Cul

m/z = 190; 63CuI t/z = 192; 65 CuP

,65 A(03Cu) = 100/ (1+# 1 9 0 / , 9 2 )

also

GERMANIUM

11.29.3 CuCI

467

,35r>i+ m/z = 98; " C u ^ C l m/z = 1 0 0 ; 65Cu35Cl+,63Cu37Cl+ m/z =102 ; 65Cu37Cl+

98'A) 100; 100; '65 = '

,63 9 8 ; / , 9 8 . A(0JCu) = 100 x ™i/Ci + ,uo/ - 98//e) = 1 0 0 / [ ( l - l / e ) + # , o o / 9 8 ]

, 6 3 , 9 8 ; / , 9 8 . A(OJCu) = 100 x y V ( i + £ x 0 = 100/[(1 + e x #,02/98]

For the definition of e see Section 11.1.

m/z = 64

m/z = 66

m/z = 67

11.30 ZINC

64

66

67

ZnH

Zn4

Zn4

m/z = 68;

m/z = 70;

68

70.

Zn4

Zn4

,64 66 r- ,67r ,68,-The isotopic abundances A(MZn), A(°°Zn), A(°'Zn), A(08Zn) and A('uZn) are calculated following the expressions in Section 11.8.5.

11.31 GALLIUM

69(

71Ga+ m/z = 69; ö9Ga+

m/z = 71;

A("Ga) = 100/(1 +#6 9/7i)

m/z = 70; m/z = 72; m/z = 73;

11.32 GERMANIUM

11.32.1

70Ge+ 72Ge+

73Ge+

Ge+

m/z = 74; m/z = 76;

74Ge+

76Ge+

468 CALCULATIONS

m/z = 89; m/z = 91; m/z = 92;

m/z = 127

m/z = 129 m/z = 130

11.32.2 GeF+

70GeF+ 72GeF+ 73GeF+

m/z = 93; m/z = 95;

11.32.3 GeF+

70Ge+ 72Ge+ 73Ge3

+

m/z= 131;

m/z = 133;

74GeF+ 76GeF+

74Ge+ 76Ge+

In all cases the isotopic abundances A(70Ge), A(72Ge), A(73Ge), A(74Ge) and ,76 A( Ge) are calculated following the expressions in Section 11.8.5.

11.33 ARSENIC

Arsenic is mono-isotopic.

11.34 SELENIUM

11.34.1 Se+, Se"

m/z = 74; 74Se+ (orSe~) m/z = 78 78 Se4

m/z = 76; m/z = 77;

m/z = 169

m/z= 171 m/z = 172

76Se+

77Se+

m/z = m/z =

11.34.3 SeF +

74SeF + 76SeF + 77SeF +

m/z = 173; m/z = 175;

m/z= 177;

80; 80Se+

82; 82Se+

78SeF + 80SeF5

+

82SeF5+

In both cases the isotopic abundances A(74Se), A(76Se), A(77Se), A(78Se), ,80 ,82 A( Se) and A( Se) are calculated following the expressions in Section 11.8.5.

BROMINE

11.35 BROMINE

11.35.1 Br+, Br-

m/z = 79; 79Br4 (orBr -)

i/z = 81; 81Br+

A(81Br) = 100/(1 + # 7 9 / 8 , ;

469

m/z = 158

m/z= 160

m/z = 162

11.35.2 Br+

79Br+ 7 9 B r 8 1 B r +

81 Br^

A(8,Br) = 100(2 + #,6o/i62)/2(l +#i6o/i62+ #158/162)

11.35.3 NaBr4

m/z = 102; 23Na79Br+ m/z = 104; 23Na81Br+

A(8,Br) = 100/(1 +#,02/104)

11.35.4 AgBr+

m/z =186 ; 107Ag79Br+

m/z = 188;

m/z = 190;

107, Ag8 1Br+, I U 9Ag7 9Br+

109 lRr+

,79 Ag8 ,Br

186;//186; A(/vBr) = 100 x 186;/(I80i + ,88i - 188/79)

A('yBr) = 100/(1 + # , 8 8 / , 8 6 - #109/107)

where >88/79/,86i = 109Ag/107Ag = #109/107.

470 CALCULATIONS

11.35.5 CaBr+

m/z =119; 40Ca79Br+ m/z= 121; 40Ca8,Br+,42Ca79Br+

,79 119; / , 119; A('yBr) = 100x" yí / ( 1 , i í + 121í 121 í'79)

A ( ' y B r ) = 1 0 0 / ( 1 +#121/119 - # 4 2 / 4 0 )

where 121i79/,19i = 42Ca/40Ca = #42/4o-

11.35.6 Cs2Br+

m/z = 345; 133Cs279Br+

m/z = 347; 133Cs281Br+

A ( 8 l B r ) = 1 0 0 / ( 1 + ^ 3 4 7 / 3 4 5 )

11.36 KRYPTON

m/z = 78 m/z = 80 m/z = 82

7 8 K r 4

80Kr-f

82 Kr4

m/z = 83; 83Kr+

m/z = 84; 84Kr+

m/z = 86; 86Kr+ The isotopic abundances A(78Kr), A(80Kr), A(82Kr), A(83Kr), A^Ki) and

A( Kr) are calculated following the expressions in Section 11.8.5.

11.37 RUBIDIUM

m/z = 85; 85Rb+ m/z = 87; 87Rb+

A ( 8 ' R b ) = 1 0 0 / ( 1 + # 8 5 / 8 7 )

11.38 STRONTIUM

m/z = 84; 84Sr+ m/z = 87; 87Sr+

m/z = 86; 86Sr+ m/z = 88; 88Sr+

MOLYBDENUM 471

The isotopic abundances A(84Sr), A(86Sr), A(87Sr) and A(88Sr) are calculated following the expressions in Section 11.8.5.

Yttrium is mono-isotopic.

11.39 YTTRIUM

11.40 ZIRCONIUM

11.40.1 Zr+

m/z = 90 m/z = 91 m/z = 92

90Zr+

91 Zr4

92Zr4

m/z = 94; m/z = 96;

94Zr4

96 Zr4

m/z = 147 m/z = 148 m/z = 149

11.40.2 Z r F j

90

91

92

'ZrF4

ZrF3+

ZrF34

m/z = 151; m/z = 153;

94ZrF3+ 96ZrF3+

In both cases the isotopic abundances A( Zr), A(91Zr), A(92Zr), A(94Zr) and A( Zr) are calculated following the expressions in Section 11.8.5.

11.41 NIOBIUM

Niobium is mono-isotopic.

11.42 MOLYBDENUM

m/z = 92 m/z = 94 m/z = 95 m/z = 96

92

94

95

96

Mo4

Mo4

Mo4

Mo4

m/z = 97; 97Mo+

m/z = 98; 98Mo4

m/z = 100; 100Mo4

472 CALCULATIONS

The isotopic abundances A(92Mo), A(94Mo), A(95Mo), A(96Mo), A(97Mo), A( Mo) and A( Mo) are calculated following the expressions in Section 11.8.5.

11.44 RUTHENIUM

m/z = 96 m/z = 98 m/z = 99 m/z = 100;

96

99

100

Ru+

Ru4

Ru+

Ru4

m/z= 101 m/z = 102 m/z = 104

101

102

104

Ru4

Ru4

Ru4

The isotopic abundances A(96Ru), A(98Ru), A(99Ru), A(100Ru), A(101Ru), A(10 Ru) and A(104Ru) are calculated following the expressions in Section 11.8.5.

11.45 RHODIUM

Rhodium is mono-isotopic.

11.46 PALLADIUM

m/z = 102 m/z = 104 m/z = 105

102 p d +

104pd+

105TJJ+ 3Pd4

m/z = 106 m/z= 108 m/z= 110

106 p d +

108pd+

i i o p d +

,102 104T ,105, ,106T The isotopic abundances A(,uzPd), A(,U4Pd), A(1U3Pd), A(1UDPd), A(1U8Pd) ,110 and A( Pd) are calculated following the expressions in Section 11.8.5.

11.47 SILVER

m/z = 107; 107Ag+

m/z = 109; 109Ag4

,109 A(luyAg) = 100/(1+#,07/109)

m/z = 106 m/z = 108 m/z= 110 m/z = 111

ANTIMONY

11.48 CADMIUM

473

106,

108

no,

m

Cd4

Cd4

Cd4

Cd4

m/z= 112 m / z = 113 m/z = 114 m/z= 116

112

113

114

116.

Cd4

Cd4

Cd4

Cd4

The isotopiç4abundances A(,06Cd), A(,08Cd), A("°Cd), A( l u Cd) , A(II2Cd), A( Cd), A( Cd) and A( Cd) are calculated following the expressions in Section 11.8.5.

11.49 INDIUM

113 In* m/z= 113; m/z =115 ; U 5In4

A ( , l á I n ) = 100/(1 + # „ 5 / „ 3 )

m/z = m/z = m/z = m/z = m/z =

113 114 115 116 117

11.50 TIN U3Sn4

],4Sn+ U5Sn+ 116Sn4

117Sn4

m/z = m/z = m/z = m/z = m/z =

118 119 120 122 124

,18Sn4

"9Sn+

,20Sn4

,22Sn4

124Sn4

The isotopic abundances A(U3Sn), A(, , 4Sn), A(, l5Sn), A(,16Sn), A(117Sn), A(118Sn), A(119Sn), A(120Sn), A(122Sn) and A(124Sn) are calculated following the expressions in Section 11.8.5.

139

11.51 ANTIMONY

m/z =137 ; , 2 1Sb1 60+

m/z = 139; 1 2 3Sb1 604, 1 2 1Sb1 80+

' ; _ 139; ct „ '123 = ' — 0 X 137-

A(I2,Sb) = 100/[#,39/137 + ( l - ( 5 / ) ] The definition of S' is given in Section 11.1.

474 CALCULATIONS

11.52 TELLURIUM

m/z = 120; m/z = 122; m/z = 123; m/z = 124;

11.52.1 Te4, Te 120 Te+ (or Te") m/z-

m/z •• m/z-

122 :Te+

123 T e +

124 T e +

125 126 128

m/z = 130

125

126

128

130

Te4

Te4

Te4

Te4

m/z m/z m/z

158 160 161

m/z =162

11.52.2 TeF4

120 TeF4

122 TeF4

123 TeF + 124 TeF4

m/z =163 m/z = 164 m/z =166 m/z = 168

125

126

128

TeF4

TeF4

TeFj 130 TeF+

11.52.3 TeF4^

m/z : m/z • m/z m/z

215 217 218 219

120

122 TeF4-TeFf

123 TeF 5+

124 TeF4

m/z = 220 m/z = 221 m/z = 223 m/z = 225

,120n

125 TeF t 126 TeF4" 128 TeF4

130 TeF + In all these cases the isotopic abundances A(líVTc), A( Te), A( Te),

A(124Te), A(,25Te), A(126Te), A(I28Te) and A(130Te) are calculated following the expressions in Section 11.8.5.

Iodine is mono-isotopic. 11.53 IODINE

11.54 XENON m/z = 124. m/z = 126: m/z = 128 m/z = 129 m/z= 130

124Xe+

126Xe+

128Xe4

,29Xe4

130Xe4

m/z= 131 m/z = 132 m/z = 134 m/z = 136

131

132

134

136

Xe4

Xe4

Xe4

Xe4

CERIUM 475 ,124 126, ,128 , The isotopic abundances A(,¿4Xe), A(,Z0Xe), A(,¿8Xe), A(,2yXe), A(uuXe),

A(131Xe), A(132Xe), A(l34Xe) and A(I36Xe) are calculated following the expressions in Section 11.8.5.

Cesium is mono-isotopic.

11.55 CESIUM

m/z = m/z = m/z = m/z =

130 132 134 135

11.56 BARIUM 130Ba4

132Ba4

134Ba+ 135Ba+

m/z = m/z = m/z =

136 137 138

136Ba4

,37Ba4

,38Ba4

The isotopic abundances A(130Ba), A(132Ba), A(134Ba), A(135Ba), A(136Ba), A(137Ba) and A(I38Ba) are calculated following the expressions in Section 11.8.5.

11.57 LANTHANUM

m/z = 154; 138LaI604

m/z =155; l 3 9La1 604, , 3 8Lal 70+

At natural oxygen and lanthanum abundances the ion current contribution of l38LaI70+ can be neglected.

,138 A(,J8La) = 100/(1 +#155/154)

11.58 CERIUM

m/z = 136; m/z = 138;

136 Ce4 m/z = 140; ,40Ce+

138 Ce+ m/z = 142; 142 Ce4

The isotopic abundances A(l36Ce), A(I38Ce), A(140Ce) and A(142Ce) are calculated following the expressions in Section 11.8.5.

476 CALCULATIONS

11.59 PRASEODYMIUM

Praseodymium is mono-isotopic.

11.60 NEODYMIUM

m/z = 142 m/z = 143 m/z = 144 m/z = 145

142Nd+ i 4 3 N d +

i 4 4 N d +

145XTJ+ 5Nd4

m/z = 146 m/z= 148 m/z = 150

146

148

150

Nd4

Nd4

Nd4

, 1 4 3 , The isotopic abundances A('*zNd), A(14JNd), A(l44Nd), A( l wNd), A(140Nd), (148Nc

11.8.5. A(148Nd) and A(150Nd) are calculated following the expressions in Section

11.61 PROMETHIUM

Promethium is mono-isotopic.

m/z = 1 4 4 m/z = 147 m/z = 148 m/z = 149

11.62 SAMARIUM

144

147

148

149

Sm4

Sm4

Sm4

Sm4

m/z= 150 m/z= 152 m/z = 154

150

152

154

Sm4

Sm4

Sm4

The isotopic abundances A('44Sm), A(14/Sm), A(148Sm), A(14ySm), A('50Sm), A( 52Sm) and A(l54Sm) are calculated following the expressions in Section 11.8.5.

11.63 EUROPIUM

m/z= 151; m/z = 153;

151

153

Eu4

Eu4

A ( m E u ) = 100/(1 +#,53/15.)

ERBIUM 477

11.64 GADOLINIUM

m/z = 152 m/z = 154 m/z = 155 m/z = 1 5 6

152Gd+

154Gd4

155 G d +

156Gd+

,152,

m/z= 157

m/z = 158

m/z = 160

,154, ,155,

l57Gd4

158Gd4

160 G d +

,156, The isotopic abundances A(,:,zGd), A(,Di,Gd), A(, MGd), A(,;,0Gd), A(157Gd), A(158Gd) and A(' Gd) are calculated following the expressions in Section 11.8.5.

11.65 TERBIUM

Terbium is mono-isotopic.

11.66 DYSPROSIUM

m/z = 1 5 6

m/z = 158 m/z = 160 m/z = 161

156Dy4

158Dy4

160 D y +

161 Dy +

m/z = 162 m/z = 163 m/z = 1 6 4

162 D y +

163Dy4

164Dy4

The isotopic abundances A(IM,Dy), A(158Dy), A( l wDy), A(l61Dy), A(lb2Dy), (163D3

11.8.5. A( Dy) and A( Dy) are calculated following the expressions in Section

Holmium is mono-isotopic.

11.67 HOLMIUM

m/z =162 m/z = 164 m/z = 166

11.68 ERBIUM 162E r+

164E r+

1 6 6 E r +

m/z = 167 m/z= 168 m/z= 170

i 6 7 E r +

1 6 8 E r +

170E r+

478 CALCULATIONS

The isotopic abundances A(162Er), A(164Er), A(166Er), A(167Er), A(168Er), and A( Er) are calculated following the expressions in Section 11.8.5.

Thulium is mono-isotopic.

11.69 THULIUM

11.70 YTTERBIUM

m/z = 168;

m/z = 170;

m/z= 111;

m/z = 172;

168Y| ,+

170 Y b +

171 Y b +

172 VU+ 2Yb4

m/z= 173

m/z = 174 m/z = 1 7 6

173 Yb4

174 Y b +

176 Yb +

The isotopic abundances A(168Yb), A(170Yb), A(171 Yb), A(172Yb), A(173Yb), A(174Yb) and A(176Yb) are calculated following the expressions in Section 11.8.5.

11.71 LUTETIUM

m/z = 1 7 5 ; 1/;,Lu

m/z = 176; 176Lu ,176 A("°Lu) = 100/(1 +# 1 7 5 / , 7 6 )

11.72 HAFNIUM

m/z = 174 m/z = 176

m/z = 111

174 ?+ Hf 176H f+

177H f+

m/z = 178 m/z = 179 m / z = 180

,176 177,

178H f+

179 H f +

180H f+

,178, ,179, The isotopic abundances A("4Hf), AC^Hf), A( '"Hf) , A("8Hf), A("yHf) and ,180 A( Hf) are calculated following the expressions in Section 11.8.5.

11.73 TANTALUM

m/z = 212; 180Ta16Oj m/z = 213; ,81Ta,6O+,180Ta 1 7 0 1 6 0 +

TUNGSTEN 479

At natural oxygen and tantalum abundances the ion current contribution of i8o T a i7 0 i6 0 + c a n b e n e g l e c t e d

A(l80Ta) = 100/(1 +#2,3/2 1 2)

11.74 TUNGSTEN

11.74.1 W+

m/z= 180 m/z= 182 m/z= 183

180W+

182W+

183W+

m/z= 184;

m/z = 186;

184

186 w4

w4

m/z = 218 m/z = 220 m/z = 221

11.74.2 WF 4

180 'WF4

182WF4

183 W F +

m/z = 222; 184

m/z = 224; 186

WF4

WF 4

m/z = 237 m/z = 239 m/z = 240:

11.74.3 WF 4

180

182

183

'WF4

:WF4

WF4

m/z = 241; 184WF+

m/z = 243; '^WF4"

m/z = 256 m/z = 258 m/z = 259

11.74.4 WF 4

180-yyp+ „,/„ _ m 184

182W F +

183 WF 4

m/z = 260; 184WF4

m/z = 262; WF 4

11.74.5 WF4

m/z = 275 m/z = 277 m/z = 278

180

182

WF4

WF54

183W F +

m/z = 279; m/z = 281;

184

186

WF.! WF^

480 CALCULATIONS

In all these cases the isotopic abundances A(I80W), A(182W), A(183W), A( W) and A( W) are calculated following the expressions in Section 11.8.5.

11.74.6 WO3

m/z -

m/z -

m/z -

m/z -

m/z =

m/z -

m/z =

m/z-

m/z -

= 228;

= 230;

= 231;

= 232;

= 233;

= 234;

= 235;

= 236;

= 238;

1 8 0 W 1 6 O -

0.001291 182 W 1 6 Q -

0.261127 1 8 3 W 1 6 0 -

0.141981 l 8 4 w 1 6 o 3 0.304515

1 8 6 W 1 6 0 -

0.283963

1 8 2w1 6o21 7cr

0.000298 1 8 3 W 1 6 0 2 1 7 Q -

0.000162 l84w 1 6 o 2

, 7 o -0.000348

186 W 1 6 Q 2 1 7 0 -

0.000324

180 W 1 6 Q 2 I 8 0 -

0.000008

1 8 2 W 1 6 Q 2 1 8 Q

0.001570 , 8 3w 1 6 o 2

, 8 o -0.000854 184 W 1 6 Q 2 1 8 0 -

0.001831

1 8 6 W 1 6 Q 2 1 8 0 -

0.001707

182 W 1 6 0 1 8 0 -

0.000003 1 8 3 W 1 6 0 1 8 0 -

0.000002 1 8 4 W 1 6 0 1 8 0 -

0.000004 186 W 1 6 Q 1 8 0 -

0.000003

The statistical probability of each ion has been calculated from the accepted abundances of tungsten and oxygen isotopes. In the above table the ions with a probability higher than 1 x 10 6 are listed; they sum to 99.9991% of all the ions provided that the abundances given below are the accurate values.

1 8 0 ^ 182 w > 183W > 184 W i 1 8 6 w

0.13, 26.3,' 14.3,' 30.67 28.6% I60, 17o, 18o, 99.762, 0.038, 0.200

The isotopic abundance expressions will appear to some extent complicated, therefore it is recalled that they were calculated by adding all the intensities of the same tungsten isotope, multiplying by 100 and dividing by the total intensity. The " W 1 6 0 i 8 0 2 ions were omitted from the calculations.

A(180W) = 100228/(l+<5")/£¿ — 100 /? 2 2 g / 2 3 2 ( l + S ) / ( l + # 2 2 8 / 2 3 2 + #230/232 + #231/232

+ #233/232 + #234/232 + #235/232 + #236/232 )

RHENIUM 481

using D for the denominator:

A(l80W) = 100#228/232(l+¿")/D. The definition of S " is given in Section 11.1.

A(182W) = 100 [(230i - 6" x228 i) + a" x (230j - S" x 228i) + é " x ( 2 3 O i - 6 " x 2 2 8 0 ] / S i

= 100[(tf230/232 - 6" x /?228/232) x (1 + a" + 0 ] / D or

A(182W) = 100 [C, x (/?230/232 - 6" x #228/232)]/D where a" is the I 70 l 6 0 2 / l 6 0 3 isotopic ratio; for air oxygen a" = 0.001125, and

Ci = (1 + a" + S") = 1.007257 A(183W) = 100 [C, x #231/232 - (#230/232 - ¿" x #228/232) x (a' + a"6")]/D A('84W) = 100C,[1 - a"#23l/232 - 6" X (Ä230/232 - 6" x Ä22g/232)]/D A(1 8 6W) = 100{/r234/232 + #235/232 + #236/232 ~ S" X 10" 6

x [A(160)3] x 10~2 x [A(184W)]/2320/£>

11.75 RHENIUM

11.75.1 Re4

m/z = 185; 185Re+

m/z=187; 187Re+ ,185 , A(l83Re) = 100/(1 +#,g7/185)

11.75.2 Re04

m/z = 249; 185Re I604 m/z = 251; 187Re,604,185Re1603

180' 25iim = 25ii-S'x249i

A(185Re) = 100/[#25,/249 + (1 - #)]

482 CALCULATIONS

11.76 OSMIUM

11.76.1 Os4

m/z= 184

m/z = 186

m/z= 187

m/z=188

184

186

187

Os4

Os4

Os+

188 Os4

m/z = 189 m/z = 190 m/z = 192

189

190,

192

Os4

Os4

Os4

,187, ,188, ,189, The isotopic abundances A('84Os), A(,8öOs), A(18/Os), A(188Os), A(18yOs), ,192 A( Os), and A( Os) are calculated following the expressions in Section 11.8.5.

m/z = 232 m/z = 233 m/z = 234 m/z = 235 m/z = 236 m/z = 237 m/z = 238 m/z = 239 m/z = 240: m/z = 241 m/z = 242:

184 Os 160

186 187

1 6 o -Os l 6 0 Os 1 60

188Os 1603-, 189rv. I 6 n - f

' 3 .

*Os 1 60 190Os , 6 0

1 9 2 0 s 1 6 0 1 6 o -3 '

11.76.2 OsO;

( 1 8 4 0s 1 6 0 2 17 o-) 186Os I 6 0 2

1 70^ l 8 7 0 s 1 6 0 2

1 7 0 -188Os 1 6 0 2

1 7 0 -1890s 1 6 0 2

1 7 0 -190Os , 6 0 2

1 7 0 -

1 9 2 0 s 1 6 0 21 7 0 -

( 1 8 4 0 s 1 6 0 21 8 0 -

186Os 1 6 0 2 , 8 0 "

l87Os 1 6 0 2 1 8 0 -

l88Os 1 6 0 2 1 8 0 "

189Os 1 6 0 2 1 8 0 -

1 9 0Os1 6O21 8O"

1 9 2 0 s 1 6 0 21 8 0 -

The isotopic abundances of Os may be calculated in a similar way to the calculations for W 0 3 in Section 11.74.6.

11.77 IRIDIUM

m/z = 1 9 1 ; 19lIr+

m/z = 193; I93Ir4

,191 A(iyiIr) = 100/(1 +#i93/i9i)

11.78 PLATINUM

m/z = 190; iyuPt 190 D f +

m/z = 192; 192Pt+

m/z = 194; 194Pt+

m/z = 195 m/z = 196 m/z = 198

195pt+

196pt+

198pt+

LEAD 483

192, ,194T , 1 9 5 , , 1 9 6 r The isotopic abundances A(,yuPt), A(iyzPt), A(iy4Pt), A(,y5Pt), A(,y6Pt) and ,198 A( Pt) are calculated following the expressions in Section 11.8.5.

Gold is mono-isotopic.

11.79 GOLD

11.80 MERCURY

200

201

202

204

Hg+

Hg+

Hg+

m/z = 196; 196Hg+ m/z = 201 m/z =198; 198Hg+ m/z = 202 m/z = 199; 199Hg4 m/z = 204 m/z = 200

The isotopic abundances A(196Hg), A(198Hg), A(,99Hg), A(200Hg), A(201Hg), A(202Hg), and A(204Hg) are calculated following the expressions in Section 11.8.5.

11.81 THALLIUM

m/z = 203; 203T14

m/z = 205; 205T14

,203 A(ZUJT1) = 100/(1 +#205/203)

m/z = 204; m/z = 206;

11.82 LEAD

11.82.1 Pb4

204Pb+ m/z = 207; 206Pb+ m/z = 208;

11.82.2 Pbl+

2 0 7 p b +

208pb+

m/z = 331; 204PbI+ m/z = 334; 207PbI4

m/z = 333; 206PbI4 m/z = 335; 208PbI4

484 CALCULATIONS

In both cases the isotopic abundances A(204Pb), A(206Pb), A(207Pb) and A(208Pb) are calculated following the expressions in Section 11.8.5.

11.83 BISMUTH

Bismuth is mono-isotopic.

11.88 RADIUM

The atomic radium ions 226Ra+ and 228Ra+ are monitored.

11.90 THORIUM

The atomic thorium ions 229Th+, 230Th+ and 232Th+ are monitored.

11.92 URANIUM

11.92.1 U 4

m/z = 234; 234U+ m/z = 236; 2 3 6U4

m/z = 235; 2 3 5U4 m/z = 238; 2 3 8U+

The isotopic abundances A(234U), A(235U), A(236U), and A(238U) are calculated following the expressions in Section 11.8.5.

11.92.2 UF 4

m/z = 329; 234UF+ m/z = 331; 2 3 6 UF 4

m/z = 330; 2 3 5 UF 4 m/z = 238; 333UF;j-The isotopic abundances A(234U), A(235U), A(236U), and A(238U) are

calculated from the measured ratios following the expressions in Section 11.8.5.

11.94 PLUTONIUM

Plutonium is monitored as atomic ions, therefore the abundance of its isotopes can be calculated from the measured ratios according to the expressions in Section 11.8.5.

REFERENCES 485

REFERENCES

[1] J.L. Margrave and R.B. Polansky, J. Chem. Educ, 39, 335 (1962). [2] L.H. Chan, Anal. Chem., 59, 2662 (1987). [3] J. Dattner and J. Fischler, Br. J. Appl. Phys., 14, 728 (1963).

APPENDIX 1

Table of naturally occurring isotopes" Atomic no.

1

2

3

4 5

6

7

8

9 10

11 12

13 14

15 16

17

Element

Hydrogen Deuterium Helium

Lithium

Beryllium Boron

Carbon

Nitrogen

Oxygen

Fluorine Neon

Sodium Magnesium

Aluminum Silicon

Phosphorus Sulfur

Chlorine

H D He

Li

Be B

C

N

O

F Ne

Na Mg

Al Si

P S

Cl

Mass no.

1 2 3 4 6 7 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 36 35 37

Relative abundance (atom%)

99.985 0.015 0.000137

99.999863 7.5

92.5 100

19.9 80.1 98.90

1.10 99.634

0.366 99.762

0.038 0.200

100 90.48

0.27 9.25

100 78.99 10.00 11.01

100 92.23

4.67 3.10

100 95.02

0.75 4.21 0.02

75.77 24.23

Atomic mass

1.007825 2.014102 3.016029 4.002603 6.015123 7.016005 9.012183

10.012938 11.009305 12.000000 13.003355 14.003074 15.000109 15.994915 16.999131 17.999159 18.998403 19.992439 20.993845 21.991384 22.989770 23.985045 24.985839 25.982595 26.981154 27.976928 28.976496 29.973772 30.973763 31.972072 32.971459 33.967868 35.967079 34.968853 36.965903

Ionization energy,

[1]

13.598

24.587

5.392

9.322 8.298

11.260

14.534

13.618

17.422 21.564

5.139 7.646

5.986 8.151

10.486 10.360

12.967

eV [2]

54.416

75.638

18.211 25.154

24.383

29.601

35.116

34.970 40.962

47.286 15.035

18.828 16.345

19.725 23.33

23.81

(continued)

488 APPENDIX 1

Table of naturally occurring isotopes" Atomic Element no.

Mass no.


Atomic mass

Ionization energy, eV

[1] [2] 18

19

20

21 22

23

24

25 26 Iron

27 28

29

30

31

Argon

Potassium

Calcium

Scandium Titanium

Vanadium

Chromium

Manganese Fe

Cobalt Nickel

Copper

Zinc

Gallium

Ar

K

Ca

Sc Ti

V

Cr

Mn 54

Co Ni

Cu

Zn

Ga

36 38 40 39 40 41 40 42 43 44 46 48 45 46 47 48 49 50 50 51 50 52 53 54 55 5.8 56 57 58 59 58 60 61 62 64 63 65 64 66 67 68 70 69 71

0.337 0.063

99.600 93.2581 0.0117 6.7302

96.941 0.647 0.135 2.086 0.004 0.187

100 8.0 7.3

73.8 5.5 5.4 0.250

99.750 4.345

83.789 9.501 2.365

100 53.939612 91.72

2.2 0.28

100 68.077 26.223

1.140 3.634 0.926

69.17 30.83 48.6 27.9 4.1

18.8 0.6

60.108 39.892

35.967456 37.962732 39.962383 38.963708 39.963999 40.961825 39.962591 41.958622 42.958770 43.955489 45.953689 47.952532 44.955914 45.952633 46.951765 47.947947 48.947871 49.944786 49.947161 50.943963 49.946046 51.940510 52.940065 53.938882 54.938046 7.870

55.934939 56.935396 57.933278 58.933198 57.935347 59.930789 60.931059 61.928346 63.927968 62.929599 64.927792 63.929145 65.926035 66.927129 67.924846 69.925325 68.925581 70.924701

15.759

4.341

6.113

6.54 6.82

6.74

6.766

7.435 16.18

7.86 7.653

7.726

9.394

5.999

27.629

31.625

11.871

12.80 13.58

14.65

16.60

15.640

17.06 18.168

20.292

17.964

20.51

Os 00

M S "C

K 3

S °

eu

ï

n > o u « >̂ N 00 2 *-< S u O c —l o

,—1

d

r—1

•* en ON vi ^̂

ON ON 00

II

u o o 3 M e .S 12 S •s g S

cd

es O

•«s n

o c

00

cs

00

Os m en

cs

Os OS Os

en en Os vo. « od —« —i cs

cs — m oo r-Os 0N

o s O r f O N < n v o r ^ r ^ o o ^ ^ ^ O N v o O r - - i n e n - * v o T i - Q ' « t o o e n o < n v o o o T f O s o s c s o o o s v o o o v o o o i n r n • ^ • o o v o r - o o N r - o o o c s o e n o s o s r - o o r n o — ' O o o c s r - o s c s m o - * e n ^ < r ^ r ~ o o o e n t ~ - H O t ~ c s O T » - ™ - > í i n ' < a - c S O N f n i n t ~ - - e n c s e n e n T f - - « n v o o O i - i r t - c s o o v o o o r ~ v o O e n c s e n o o O o o v o O T f T j -Ttcsen'—* • — i i - < c s o s O N t - - v o v o o o v o o v o e n ' < 3 - « o « o s c n O N o o > n i n ^ ' m i n s o o o v o v o > n i n * * v o > n r — N N N N N N N H - H H H - . f . N - n r t H H r t O ' - O O O O O O O O O O O Û O O O O O O S O N O N O N O S O S O S O > O N O N O N O N O N O N O S O S O S O S O S O N O N O N O N O N O S O N O S O S O S O N O S O S C ^ O S O ^ O s ^ r ^ e n v ñ T j : e ñ i n v o r ^ O s - H c ¿ o r - ; O s ' - H C S e n i n ^ v o e n > n v o r ~ o o c ^ v o r ~ - r - - r ~ r - c ~ t ~ r - - r - r - ~ r ~ o o r ~ - o o t - r ~ o o o o o o o o o o o o o o o o o o o o o o o o o s O s o s o s o s O N O N O s o s O s O s o s

00

cs cs

•*

o en p -

m Os vq v-i

•* en CS « cs en

oo -<tf en oo vd vo

cs >n en « •*" vo

OS 00 Os oo O s¿ t~:

vo vo cs r-in vo

oo r» cs en r^ r^

907596

.905287

.905937

« i r -» Os Os Os

3 •S 1

en vo en •* -«t m m . . _ . . „ . , O s s o e n o o — e n o r - i m m v o e n v o v o o o o m c s m o o o •* m es oo in en en c s o o c s v o r - ; O s T t o o e n v q r ^ v q r ^ v o e n e n c s v o i n o e n i - j o o i n o o p i n •* es « en oo oo cs os vo m « vo > n o o r ~

i-«'Kr^inr~ooosc^enoÑodooÑocN--¡i-^Kr-;cst~oc^KcNQ--« >ñ « es e s e s en o e s - * i n - * — —< in « r— cs M O V í « « « O « « « es «

O e N e n ' * v o i n , * v o r ~ o o O C N ) O N « o o o c s e n * v o i n r ~ ' < t v o r - - o o O s O « c s - * v o e n c s - < j - i n v o r ~ o o 2 vo oo os r ^ r ~ r - - r ~ r ^ t ^ r ~ r ^ r ^ t - ~ o o o o r ~ o o t - ~ o o o o o o o o o o o o o o o o o o o o o o o o o s O s O s O s O s O s O N O s O s O s o s O N g Os Os Os

O

O < C/3

en •*!• en en

i -

m

in en

á

§

vo en

s e

I E

c s

t/5

>H N

£ 'S o p N

Os O en TT

, o ,2

S 3 C

Z 2

— (S •* -<*

en •* •* •*

490 APPENDIX 1


45 46

47

48

49

50

51

52

53

Element

Rhodium Palladium

Silver

Cadmium

Indium

Tin

Antimony

Tellurium

Iodine

Rh Pd

Ag

Cd

In

Sn

Sb

Te

I

Mass no.

100 101 102 104 103 102 104 105 106 108 110 107 109 106 108 110 111 112 113 114 116 113 115 112 114 115 116 117 118 119 120 122 124 121 123 120 122 123 124 125 126 128 130 127


12.6 17.0 31.6 18.7

100 1.02

11.14 22.33 27.33 26.46 11.72 51.839 48.161

1.25 0.89

12.49 12.80 24.13 12.22 28.73

7.49 4.3

95.7 0.97 0.65 0.34

14.53 7.68

24.23 8.59

32.59 4.63 5.79

57.36 42.64

0.096 2.603 0.908 4.816 7.139

18.95 31.69 33.80

100

Atomic mass

99.904218 100.905581 101.904348 103.905422 102.905503 101.905609 103.904026 104.905075 105.903475 107.903894 109.905169 106.905095 108.904754 105.906461 107.904186 109.903007 110.904182 111.902761 112.904401 113.903361 115.904758 112.904056 114.903875 111.904823 113.902781 114.903344 115.901744 116.902954 117.901607 118.903310 119.902199 121.903440 123.905271 120.903824 122.904222 119.904021 121.903055 122.904278 123.902825 124.904435 125.903310 127.904464 129.906229 126.904477

Ionization energy,

[1]

7.46 8.38

7.576

8.993

5.786

7.344

8.641

9.009

10.451

eV [2]

18.08 19.43

21.49

16.908

18.869

14.632

16.53

18.6

19.131

ON Ti"

O U

c _

X S Z S3

& <

01

a o sn -•« "C

¡ o

B 'S •S

u s: 5 v£

.°i 3 -a es

es O

I u

a o

<

es es

o en es

- 8 in d es «

rt- cs Os « 00 es en in

vo m O 00 «' d

r-t-->n

r-•* in

in cs m f~ o' d

es os •* •* in in

o r-Os o

in en m vo

m cs «

so «

c

8

« « o o v o o o m o s e n r ^ c s o o o v o v o v o ^ l - i n v o e s o s r ~ - « e n s o c s v o « 0 • i s r s r r \ t s r s - ^ r ^ . v 4 > r ^ . _ . Y \ r ~ ^ r + / - K s t ~ t i r . _ _ . ^ _ _ . i ^ s v * . ^ t . « ^ v * . i ^ . * * . * * , . « . M ^ . * ^ I / — % / — ,

« « O O v o o o m o s c n r ^ e s o o o v o v o v O T r i n v o e S O s r ~ « e n s o c s v o « 0 Os r^csenm«oooenenvoo s c S o o c n o o « r ~ ' * O s « e n r ^ ^ O N V o i n « e n « i n ' j - O s T t - ' * - i n e n e s o s o o c s o o OOenosoO'<i-«vOTtOt~cs « e s i n r - - o s o « e n c s - * e s o > * v o m o o c s « e n « o s - ^ - e s v o r ~ o o o i n « o s o s oosoo«cs r^csoocsoooovo v o ^ e n ^ e n ^ ^ v ) i n v i v o v ) 3 i n ^ i n i n r ^ v o r ^ i n i n o s r ~ - r ~ o s o c s e n v o o c s - * " * r ~ P - o s e s o N « o s o e s o o o o o o o o o o o o o o o o o o o o o o o o o o « « « « e s « « « « « « e s « e s « e s e s Os Os Os Os Os Os Os Os Os Os Os Os Ö\ Os Os Os Os Os Os Os Os ÍTV f>. rty <T\ fT\ ("N Os Os (T\ r*. (T. o. Os Os Os rï\ Os Os rtv <"N ("Í, n \ o o o o o o o o o o o o o o o o o o o o o o o o o o « « « « e s O S O N O N O S O S O S O S O S O N O S O > O S O N O S O S O S O N O S O S O N O N O N O S O S O N O S O N O N O N O N O N en inc~o¿oÑd« 'en ' i nes 'os«eñT j : i nvo r~ r~od in r - -oÑ« 'd«"cSenT) : i ^ c s e s e s e s e S e n e n e n e n e n e s e n e n e n e n e n e n e n e n e n e n e n ' í ' r í - T t ' r t - T i - ' S ' T f ' í T f

i r- es Ti- es oo O Os vo —i r— es Tr o ON

O O N « o o « O s m e n o o s O O N i n o o o o en oo © o Os vo r t « p o s ^ - « e s o s T i ; O s « « r ) ; i n o o c s r - ; p o s « e S T i ; p « « oo en « r-- vq d d « N O i t ' i - i v 6 d o o ' o d d N v o V - < r t d e A d d o ¿ - < o i ^ N m o ó ^ i n i n

v b o o o o o o e s v o r -es es « o O Os O « r- os oo « o cs « es «

« « « « ^ . « t ^ | « Ç^| « « t ^ ^ O S O N O S O S O N O S O S O N O S O S O S O S en \ d r ~ o ¿ o s « e n d c s « e ñ T i :

T t T T T t - T t T t m i n m i n i n m i n

O oo O « p e n o O T f r - ; t - ; O O c s e s « o o e n i n — ' e ñ r - ^ v d e s o o c s d e s r r « « « e s e s r f i n «

T l - v o o o o N O « e s T i - v o e n o e S T r < n v o r ~ o o o o o N v o o o o c s « e s e n T t i n v o o o o T i - r ^ o o o s o c s T t - « e n e S T l - i n e s e s e s c S e n e n e n e n e n e n e n e n e n e n e n e n e n e n e n e n m T i - T i - T t T r T r r h n - T r T t i n T T T j - T t T t i n m i n i n m i n m i n

X

§ e X

m

in cd UPQ

3 3 '¡n "C t j CS UCQ

m vo in in

ca

« u U

1 B •E = § "g « U

r-•n oo i n

« z

•3 6 B 2

13 E o >, tú T3 « »

Os O in «

E S CL, «5

E 3

J3 fl> Ë o t .

Ë 3 •c CS

E C3 0. orj

3 w

Ë 3 O. S -1 w

•o O

E 3 C O

T5 es O

« es vo vo

en vo S

CS t->n vo

O en oo ON m r-

o « es es

os en

os TT

•n en oo ON

es o O « vo vo

oo m « es vô vd

vo cs i* o

Os oo

oo Os

00 00

© r - « « o t ~ e s e n o s i n r ~ e n e n v o « v o i n o o « O e n O e n o o « O s « O e n c S T r c s o s o o p ~ « c s e n ' i - r ^ i n ' t ' i - i n v o v o o o o s e s e s e s c s e s c s e s c s e s c s c s e s ONONOSOSONONONONONONOSOS i n v d t > o s o ( 3 i n t - ~ o Ñ d « c s e ñ i n i n i n i n m i n m i n v o v o v o v o

c s r ~ « m « e n v o i n o O T r o o e n c S e n o o « o v o o o t - ~ e s o r ~ e n O N C S e n t - ~ e s c n o e n T r c S O N r ~ e n e n c S o c e O N O N t N i n ^ m i t v o v o o o eneSCSenenenenenenenenenen ONOSONONONOSONONOSONONONOS T Í « e ñ i n v o r - : o \ o o r ^ o Ñ d « e S v o v o v o v o v o v o v o v o v o v o r ^ r ^ r ^

en vo in TJ- m o en t> r~ oo ON vo cs en oo m r- vo p Tí- es oo es o es O — en en "* T(- Tí- Tf Tí- Tí-ON Os Os Os Os Os Os en in Tí in en in vd r- t> c— r-» r- r- r~

o r-« es r- oo en m TT TT ON Os r~ oo

« Os VO 00 m TT vo t-Tf Tt Os Os ON OS

« CN o r~ oo so Tf Tí-OS Os O Os 00 r»

m m cs TI-CS es 00 O TI- in ON OS « es oo oo

en r~ in r~ Os en O Tf m m Os ON en in oo oo

r-~ m r~ s© Os l— es m in m Os Os Tf vd oo oo

e s v o v o r ^ O N O e s oo r^inTt-vo V O O T T Tf-« in enin cs « a N v o o o o s c s o « o o e n r— o o Ti-vooooo © « en os in ON es « vq vo ON oo ON « p e n o s « o o r ^ T ( ; i n « c s v q c s v o « p o N « e n e n v o v q ' i v q o u ^ ^ ' « o d d e s o o ' i n T i : o o o d « e ñ c s s d T i : o d e ñ T i : « s o « c s r - - e s d i n o e 3 r ~ e ñ es « es es © « e s e s e s o e n e s e s « © « e s « e n « O s « c s « e n Os e s « e n e S e n s o

N O r ~ o o o O N V o o o o « - c s e n T f i n c S T + s o r ^ o o o o N o o o « « c s e n T r v o i n v o T f v o r - - o o o N © o « « o c s e n T r v o i n r ~ i n i n i n v o i n i n i n v o v o v o N O v o v o v o v o v o v o N O t ~ v o v o r - ^ r ~ t > t ~ r ~ r ^ r - - r ~ r ~ r ^ r ~ r ~ t ^ o o o o o o o o o o o o o o o o o o o o

« & HO

« H « W 6 « « £ u «

Ë 3 3 £ Q

Ë •P B S.2 SC (S

« B 6.2 3 S — ? H *

I B •2 .2 S « 3 « J «

E P

H

c u bû B 3 H

E 3 C U X «

m vo vo vo r- oo vo vo Os © vo r- es t - en Tl- m

APPENDIX 1 493


76

77

78

79 80

81

82

83 84 86 88 89 90 92

Element

Osmium

Iridium

Platinum

Gold Mercury

Thallium

Lead

Bismuth Polonium Radon Radium Actinium Thorium Uranium

Os

Ir

Pt

Au Hg

TI

Pb

Bi Po Rn Ra Ac Th U

Mass no.

184 186 187 188 189 190 192 191 193 190 192 194 195 196 198 197 196 198 199 200 201 202 204 203 205 204 206 207 208 209

232 234 235 238


0.02 1.58 1.6

13.3 16.1 26.4 41.0 37.3 62.7

0.01 0.79

32.9 33.8 25.3

7.2 100

0.15 9.97

16.87 23.10 13.18 29.86

6.87 29.524 70.476

1.4 24.1 22.1 52.4

100

100 0.0055 0.7200

99.2745

Atomic mass

183.952514 185.953852 186.955762 187.955850 188.958156 189.958455 191.961487 190.960603 192.962942 189.959937 191.961049 193.962679 194.964785 195.964947 197.967879 196.966560 195.965812 197.966760 198.968269 199.968316 200.970293 201.970632 203.973481 202.972336 204.974410 203.973037 205.974455 206.975885 207.976641 208.980388

232.038054 234.040947 235.043253 238.050786

Ionization energy,

m 8.7

9.1

9.0

9.225 10.437

6.108

7.416

7.289 8.42

10.748 5.279 6.9 6.08 6.194

eV [2]

18.563

20.5 18.756

20.428

15.032

16.69

10.147 12.1 11.2

" Notes: (1) The relative isotopic abundances are from the table of Isotopic Compositions of the Elements 1989, J.R. De Laeter et at., Pure Appl. Chem., 63, 991 (1991). They are the 'Representative Isotopic Composition (Atomic %)'. For the uncertainties quoted for these values, the reader should refer to the original text. (2) The ionization potentials are from the Handbook of Chemistry and Physics, 70th edn., 1989-90, CRC Press Inc., Boca Raton, FL, Tables E 80-81. Columns [1] and [2] are the first and the second ionization potentials respectively.

APPENDIX 2

Table of standard, IUPAC confirmed, atomic weights of elements Atomic Element Atomic weight number

[1] [2] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine

H He Li Be B C N 0 F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br

1.00794(7) 4.002602(2) 6.941(2) 9.012183(3)

10.811(5) 12.011(1) 14.00674(7) 15.9994(3) 18.9984032(9) 20.1797(6) 22.989768(6) 24.3050(6) 26.981539(5) 28.0855(3) 30.973762(4) 35.066(6) 35.4527(9) 39.948(1) 39.0983(1) 40.078(4) 44.955910(9) 47.88(3) 50.9415(1) 51.9961(6) 54.93805(1) 55.847(3) 58.93320(1) 58.69340(2) 63.546(3) 65.39(2) 69.723(1) 72.61(2) 74.92159(2) 78.96(3) 79.904(1)

(continued)

496 APPENDIX 2

Table of standard, IUPAC confirmed, atomic weights of elements

Atomic number

36 37 38 39 40 41 42 45 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

Element

Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Cesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury

Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Hg

Atomic weight

[1] [2]

83.80(1) 85.4678(3) 87.62(1) 88.90585(2) 91.224(2) 92.90638(2) 95.94(1)

-101.07(2) 102.90550(3) 106.42(1) 107.8682(2) 112.411(8) 114.82(1) 114.818(3) 118.710(7) 121.757(3) 127.60(3) 126.90447(3) 131.29(2) 132.90543(5) 137.327(7) 138.9055(2) 140.115(4) 140.90765(3) 144.24(3)

-150.36(3) 151.965(9) 157.23(3) 158.92534(3) 162.50(3) 164.93032(3) 167.26(3) 168.93421(3) 173.04(3) 174.967(1) 178.49(2) 180.9479(1) 183.85(3) 183.84(1) 186.207(1) 190.2(1) 190.23(3) 192.22(3) 195.08(3) 196.96654(3) 200.59(2)

APPENDIX 2 497

T&ble of standard, IUPAC confirmed, atomic weights of elements Atomic number

81 82 83 90 91 92

Element

Thallium Lead Bismuth Thorium Protactinium Uranium

TI Pb Bi Th Pa U

Atomic weight

[1] 12]

204.3833(2) 207.2(1) 208.98037(3) 232.0381(1) 231.03588(2) 238.0289(1)

(1). Atomic Weights of the Elements 1989. J.R. De Laeter and K.G. Heumann, J. Phys. Chem. Ref. Data, 20, 1313 (1991). (2). Atomic Weights of the Elements 1991, IUPAC Commission on Atomic Weights and Isotopic Abundances, J. Phys. Chem. Ref. Data, 22, 1571 (1993). Notes: (1). Atomic weights are reviewed biennially by the IUPAC Commission on Atomic Weights and Isotopic Abundances. The last column in the table shows the changes approved by IUPAC in 1991. (2). The figure in parentheses is the uncertainty of the atomic weight, i.e. 1.00794(7) = 1.00794 ±0.00007.

SUBJECT INDEX

The following abbreviation. are used in this index: EI: electron impact GC-IRMS: gas chromatography isotope ratio mass spectrometry CD: glow discharge GD -IRMS glow discharge isotope ratio mass spectrometry ICP: inductively coupled plasma IRMS: isotope ratio mass spectrometry LC: liquid chromatography RI: resonance ionization RIMS: resonance ionization mass spectrometry SID: surface ionization-diffusion (ion source) SIMS: secondury ion mass spectrometry SRM: standard reference material TI: thermal ionization TIMS: thermal ionization mass spectrometry

The bold page numbers indicate pages where the significant isotope ratio determinations of an element are described.

abundance, isotopic. 487 calculations, 449

abundance sensitivity, 21. 87-89, 111, 135. 139. 361

accuracy, see data evaluation analyzer

double focusing. 46. 86. 135. 137. 138. 139.421

electrostatic sector, 46, 86, 89, 135, 137

magnetic sector. 36, 7, 8, 14. 135, 137

multistage, 144 quadrupole mass filter. 109 three stage

BBE configuration. 139 EBE configuration. 136. 140

appearance potential, 151 applications of IRMS. 403 atomic mass, 487 atomic weight. 3. 103. 403. 495 Avogadro constant, 250. 411

biological studies with stable isotopes. 424

calculation of isotopic abundance, 449 calibration of

helium mass spectrometer, 215 TIMS. 172. 371

chemical reaction mechanism and IRMS 438

cosmochemistry, 416

data evaluation, 69-71. IK3 accuracy. 74. 94.171. 186 error estimation in SRMs, 189 error propagation analysis. 209 precision. 94. 101. 171.184. 196. 206.

229. 235. 241. 358 random and systematic errors, 183 rejection uf measurements, 187

6 notation, 36 £D. 205. 430 fiC, 226. 428 rTN. 231. 430 ¿O. 236 6S, 232

double spike technique, 299. 302, 305, 314. 354. 373. 383

double internal standard. 384

510 SUBJECT INDEX

drawing out field. 26 drawout/repeller plate, 152 dynamic range of

quadrupole mass spectrometer, 112, 114

GD-IRMS. 139

electron affinity. 113. 155. 156 electron beam focusing. 25 electron multipliers

Daly multiplier. 45. 67. 86, 102 secondary. 40. 67

conversion coefficient, 41, 43. 44 conversion dynode, 41 counter threshold. 43, 44 dead lime. 45, 75 linearity. 45, 74

electron work function. 113, 155 environmental studies, 431

atmosphere and air pollution. 432 climatic changes, 433 earthquakes and volcanic eruptions.

436 hazardous elements in humans. 419.

435 hydrology and water resource, 431 lead in

atmosphere. 96. 358. 435 gasoline additives. 358. 435 human body. 435 natural origin. 96, 358

plutonium in environment, 380, 434

Faraday collector cup efficiency, 35. 38. 73

ratio measurement of. 39, 67 detection systems

dual. 7 multiple. 8. 13. 34. 36. 40. 84. 86.

136. 138.369 fixed. 39 variable, 39. 67

food and stable isotopes (caffeine, fruit juices, honey, vine.

whisky) adulteration control, 428 authentication. 428

gas conductance. 49 gas conductivity, 24

GC-IRMS. 7. 227. 233. 249. 254. 266 geological dating (geochronology)

age of earth. 412 methods. 413, 415. 416

geological samples, 98. 99, 104, 106, 357

I I j factor. 26. 76 half-life of radioactive nuclides

daughter growth method, 410 double ß decay, 4M parent decay method. 409 specific activity method. 411

interference in electron impact ion source. 25 isobaric, 21. 95. 104. 105. 199. 322.

335 and pumping systems. 21

ion beam collection. 33 multiple. 35, 38

dynamic, 35, 38 static. 38

ion beam focusing axial, 18 direction ,4 radial. 15 stigmatic, 17

ion beam shaping. 86 ion counting. 41, 102 ionization coefficient, 29 ionization degree. 28 ionization efficiency enhancement agents,

30. 156. 368 amyl acetate. 296 anion exchange (resin) bead. 295. 368,

378. 380 barium hydroxide/oxygen. 346 barium nitrate. 346 benzene. 368 boric acid. 293. 351 boron/phosphoric acid, 339 Freon. 157 graphite. 245. 256. 285, 318. 360. 363,

368 iridium/molybdcnum/carbon. 339 oxygen. 348 porous platinum. 261 rhenium/lanthanum hcxaboridc. 311 rhcnium/plalinum/ascorbic acid, 294

SUBJECT INDEX 511

rhenium/platinum/ascorbic acid/ ammonium iodide/hydrochloric acid. 338

rhenium powder. 368, 379 silica gel. 293. 354. 359 silica gel/aluminum trichloride. 309 silica gel/barium hydroxide, 283 silica gel/boric acid, 113. 272. 273. 297 silica gel/boric acid/aluminum oxide.

281.304 silica gel/phosphoric acid. 113. 247,

254. 265. 266. 273. 276. 278. 279, 281. 296. 300. 301. 302. 307. 355

tantalum/phosphoric acid/hydrofluoric acid. 290. 359

tungsten. 280 vanadium pentoxide/barium hydroxide.

344 ¡onization potential/ionization energy.

113. 151. 153.487 ionization processes

EI. 23. 122, 150 energetics of. 151

GD. 162 Penning ionization. 162

ICP. 158 argon plasma torch. 159 ionization yield, 160 nebulization. 159 plasma temperature. 159 sample transmission, 89

RI, 163 ionization schemes. 163

SIMS. 135. 137. 168 spark. 161 TI. 28. 153 negative ions, 114, 155, 222. 252. 256.

283. 293. 307. 309. 311. 341. 343. 345.348

¡on molecule reactions, 190. 198. 203 ion optics, 19 ion sources

duoplasmatron. 137 EI. 22. 152

cross-beam, 112, 122 linearity, 26 sensitivity. 24

ICP, 32. 84. 158 ¡on extraction interface, 161

RI. 33. 138 SID. 377. 381

SIMS. 135. 138 spark

low voltage d.c. arc, 161 radio frequency. 161

TI. 26. 28. 84. 135 filament/ribbon

dimple. 155. 157, 247. 266, 361, 368

double (also multiple). 27, 153, 154

evaporation. 27. 30. 154 ionization. 27. 30, 154 material. 155 melting points. 155 single, 27, 153 triple (also multiple), 27, 153 and isotopic fractionation. 172 Vor boat shaped, 140. 154. 247.

264. 265. 378 sample magazine/turret. 27, 67 sensitivity. 32. 72

ion statistics, 33 isotope analysis

bulk sample (BSIA). 50. 55 compound specific (CSIA), 50. 57

¡sotope archaeometry lead in. 439 carbon-14 in, 441

¡sotope dilution analysis, 114. 379. 380. 405

isotope effects. 436 isotope enrichment, 43d isotope ratio, absolute, 36

corrections with internal standard, 36, 384 external standard. 36

see also double spike technique isotope ratio accuracy, see data evaluation isotope ratio determinations

in space, 421 of multielement samples. 157 of noble gases. 125 with RI. 167. 344. 362, 385 with SIMS, 219. 249. 281. 346. 357

¡sotope ratio in elements aluminum. 144. 249 antimony, 306 argon. 126. 257 arsenic. 282 barium, 314 beryllium. 141. 144, 219

512 SUBJECT INDEX

isotope ratio in elements (contd) bismuth, 359 boron. 220 bromine, 284 cadmium. 113. 115-117. 119.34)1 calcium. 144, 260 carbon. 12. 129. 130. 144. 223 cerium. 317 cesium, 313 chlorine. 144. 255 cobalt. 274 copper. 113. 115-117.275 chromium. 113. 115-117. 271 dysprosium. 333 erbium. 334 europium. 328 fluorine. 242 gadolinium. 329 gallium, 278 germanium. 280 gold. 349 hafnium. 98. 104, 136. 338 helium. 112,214 holmium, 334 H/D/T. 12. 197

abundance calculation. 213. 451 analyses at

high mass resolution, 211 low mass resolution, 207

indium. 303 indine. 144. 310 iridium, 347 iron. 113. 115. 117.272 krypton. 128. 286 lanthanum. 315 lead. 94. 96. 104. 113. 115-118. 138.

353. 440 lithium. 216 lutetium, 337 magnesium, 245 manganese. 272 mercury. 349 molybdenum. 103. 292 neodymium, 91, 320 neon, 3, 126, 243 neptunium, 377 nickel. 275 niobium, 292 nitrogen. 12. 129.229 osmium, 344 oxygen, 12. 137. 235

palladium. 139, 298 phosphorus, 252 platinum. 348 plutonium, 377 potassium, 258 praseodymium. 320 promelhium. 326 protactinium. 362 radium, 359 rhenium. 342 rhodium, 298 rubidium, 287 ruthenium. 296 samarium. 326 scandium, 267 selenium, 283 silicon. 249 silver, 300 sodium. 244 strontium. 106. 289 sulfur. 12. 138. 168.252 tantalum. 340 technetium. 295 tellurium, 307 terbium. 333 thallium. 94. 104. 113. 117.351 thorium. 136, 360 thulium, 335 tin. 103. 34)4 titanium, 267 tungsten. 103. 106. 340 uranium. 363, 419. 420

electron impact ionization. 126. 364

thermal ionization, 367 inductively coupled plasma

ionization, 99, 375 vanadium. 269 xenon, 127. 312 ytterbium. 336 yttrium. 290 zinc, 113. 115117.277 zirconium. 290

isotope ratio mass spectrometers, 7. 12, 64

accelerator mass spectrometry, 143 basic systems, 65 CAMECA IMS-1270. 138 data systems, 53. 68 glow discharge. 138 helium. 215

SUBJECT INDEX 513

high isotopic abundance sensitivity, 139 H/D/T. 211 ICP multiple collector. 85. 158. 181.

290. 295. 306. 310. 325. 339. 341. 356. 375

ISOLAB-54, 135 ISOLAB-120, 140 RI, 165 SIfRIMP (sensitive high resolution ¡on

micro probe). 137, 249 specifications of

TI mass spectrometer accuracy, 74 baseline stability, 73 cup efficiency, 73 peak flatness, 72 recording system, 73 reproducibility

external, 74 internal, 74

sensitivity, 72 EI mass spectrometer

baseline stability, 76 H3 factor, 76 method specific tests, 76 ratio linearity. 75 sensitivity, 74 system stability, 76

TI quadrupole. 113 TI magnetic sector. 369

Isotope ratio precision, see data evaluation

isotopic exchange reactions, 207 isotopic fractionation

inTl. 30. 119. 171.217,369 correction factors. 31.172, 178. 246.

264. 268. 277, 309. 322. 329. 336. 369. 371

exponential law. 37, 180 linear law, 179, 355 Rayleigh law. 180. 297 power law, 180 see also mass bias correction

experimental parameters, 172 in ICP ¡onization. 181 in gas inlet systems. 182, 198

isotopic fractionation models, 175 isotopic labelling, 425-7 isotopic SRMs, 100. 101. 104. 106. 189.

206. 217, 226. 227. 230. 233. 242, 250. 252. 253. 255. 266, 275. 279.

288. 301. 324, 342. 352, 354. 372, 381. 386

Langmuir- Saha equation, 28, 155 Liouville's theorem. 25 Manhattan Project. 9 mass bias in ICP, 90

correction external. 94, 96. 100 internal normalization, 91 see also isotopic fractionation

correction correction laws

exponential law, 91, 100 linear law, 91 power law. 91. 96. 100. 181

mass discrimination, 26, 171, 216. 231, 257,281.286.318.343.355

see also mass bias mass dispersion. 16 mass spectrometer (spectrograph), 3

air borne, 421 ion transmission. 32 commercial. 64, 85. 107, 113, 125.131,

135. 138. 140.211.233.369 single focusing. 7 static, 7, 214 see also isotope ratio mass

spectrometers mass resolution. 16. 207. 211 medical studies with stable isotopes. 424

metabolic studies, 120. 424-7 memory effect, 237, 365

nuclear industry and technology, 101,418 irradiated fuel. 383. 385

nutrition, stable isotopes in, 424

Oklo natural nuclear reactor. 367.419

photosynthesis. 428 physical constants, determination with

IRMS. 409 planetary IRMS. 421 Poisson distribution, 41, 43 precision, see data evaluation

repeller/arawout plate. 152 resistor noise. 40 resolving power. 17 resolution-mass, 16

high. 21. 130. 137. 138

Copyrighted Material

514 SUBJECT INDEX

sample inlet/iniroduction systems gas inlet systems, 46, 66, 123

batch, 123 continuous flow. 47. 50. 65, 125 dual. 7. 48. 124 standard gas introduction. 48. 51. 56,

58 viscous flow, 47, 48, 65, 182

ICP nebulization systems conventional pneumatic (Meinhard).

90. 159 desolvaling. 90 electrothermal vaporization, 159 flow injection. 159 laser ablation. 159 micro concentric. 90 ultrasonic, 159

isotopic fractionation in, 182 sample preparation devices, 52 sample preparation methods of light gases

carbon as COj from atmosphere, 223 carbonate, 63, 223 organic samples, 224

carbon as CD4, 227 other compounds 228

combustion. 54. 55. 57. 202, 224. 229 elemental analyzer. 56, 60, 126-130,

230 flash combustion, 56 gas Chromatograph. 58, 66 hydrogen from

organic samples. 202 water and hydrides

reduction with carbon. 201

reduction with metals, 200 water electrolysis. 201 water equilibration with hydrogen, 62.

201 laser ablation. 64,103,104.106 LC combustion. 58. 60 nitrogen as N2 from

ammonium sulfate. 229 nitrogen oxides. 239 organic samples. 229. 230

oxygen as CO¿ from water. 238 carbonates, 239 equilibration with water. 62. 236 sulfates. 239 Oj. 240 organic samples, 240

as water, 237 as Oj from

water, 237 phosphates, 238 nitrogen oxides. 239

post-column concentration. 62. 66 pre-column concentration. 61, 66 pyrolysis, 54, 63 summary (table) from, 54

sample size of gases. 206. 229. 235. 241 selectivity in RIMS

elemental, 166 isotopic, 166

sensitivity, elemental in RIMS. 166 space missions. 421

total evaporation TIMS. 121. 179. 290, 325, 327. 331. 338. 373. 381. 382

modern isotope ratio mass spectrometry

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