The Sensitivity Factors of the Rare Earths in an rf Spark Source Mass Spectrograph

Daniel W. Oblas GTE Laboratories Incorporated, Bayside, New York 11360

(Received 11 September 1970; revision received 14 January 1971)

Experimentally determined sensitivity factors for the rare earths in Y20~ are presented along with computed values based upon some electrical and physical properties of the elements and com- pounds. The sample electrodes consisted of 2 mol of pelletizing graphite and 1 mol of Y203 powder. The properties considered are (1) the heat of formation of the gaseous metal, (2) the dis- sociation energy of the monoxide, (3) the ionization potential of the metal, and (4) the experi- mentally determined charge distribution of the metal ions in the rf spark ion source. Even for the rare earths, a class of elements with similar chemical and electrical properties, the experimentally determined and computed sensitivity factors differ by as much as a factor of 6 from one another. INDEX HEADINGS: Mass spectroscopy, spark source; Methods, analytical; Analysis for rare earths.

INTRODUCTION

The advantages of the spark source mass spectro- graph as an analytical tool in trace chemical analysis are well documented. 1 However, early investigations showed tha t the composition of the ion beam produced from the electrodes did not accurately represent the bulk composition of the sample? Experimental evi- dence has since shown tha t most elements, exclusive of the alkali metals, have a relatively sensitivity factor (RSF) E(measured concentra t ion) / (true concentra- tion)~ within a factor of 10 of each other.

Investigations in the area of ion source character- istics, 3 photoplate processing, 4 and electrical detection 5 have helped to improve the precision and accuracy of mass spectrographic analysis. However, without the use of standards or appropriate correction factors, it is still difficult to achieve accuracies of bet ter than a factor of 3 at the parts per million level.

Sensit ivity or correction factors are normally determined experimentally using samples of known composition or by utilizing specially prepared stan- dards. The preparat ion of precise, homogeneous stan- dards to cover even the most common analytical problems would be an enormous if not impossible task.

The need for standards, however, could be amelio- rated if one could predict, using first principles, the relative behavior of the element in the rf spark ion source. Reactions occurring within the rf spark are extremely complicated so tha t sensitivity factors derived from a theoretical t rea tment can only lead to approximate values. Nevertheless, several workers have at tempted, with limited success, to correlate certain physical, chemical, and electrical properties of elements and compounds into empirical expressions for relative sensitivity factors. 6-~°

Experimental ly determined sensitivity factors for the rare earths in Y203 are presented here, along with computed values based upon some electrical and physical properties of the elements and compounds. The properties considered are (1) the heat of formation

o f the gaseous metal, (2) the dissociation energy of the monoxide, (3) the ionization potential of the metal, and (4) the experimentally determined charge dis- t r ibut ion of the metal ions in the rf spark source.

I. EXPERIMENTAL TECHNIQUE AND RESULTS

A CEC 21-110 spark source mass spectrograph was used throughout all of this work. The instrumental conditions were as follows: accelerating potential, 15kV; pulse repeti t ion frequency, i kHz; pulse duration, 15 ~sec; and sparking voltage, 40 kV. The source slit was 0.05 mm and the energy aperture was approximately 300 V in width.

Standards were prepared by mixing a rare ear th ni t rate solution into an HN03 solution of Y203. The mixture was evaporated to dryness, then fired to 1000°C to convert the resultant residue to the oxide form.

This oxide material, consisting of Y203 with 10-50 ppm by weight of the various rare ear th oxides, was then mixed as follows: 1 mol of Y203 to 2 tool of gold-doped graphite. After sparking, using the above

Table I. Relative sensitivity factors of the rare earths. ~

Normalized Metal RSFA b to Dy

La 0.40 0.68 Ce 0.23 0.39 Pr 0.48 0.81 Nd 0.59 1.0 Sm 0.83 1.41 Eu 1.43 2.42 Gd O.48 0.81 Tb 0.36 0.61 Dy 0.59 1.0 Ho 0.37 0.63 Er 0.42 0.71 Tm 0.42 0.71 Yb 0.71 1.25 Lu 0.24 0.41

Sample electrodes composed of rare-earth oxide doped into Y~O8 and pellet- ized with graphite.

b With*!espec t to Au.

Volume 25, Number 3, 1971 APPLIED SPECTROSCOPY 325

> 6

~ 5

3

. _ ~ o Do° (MO)

%%%%

o C Or Pm Sm u Gd Tb Oy Ho r Tm b Lu ATOMIC NUMBER

FIo. 1. Variation of dissociation energies of gaseous monoxides and heats of vaporization of rare-earth metals as a function of rare earth with increasing atomic number [after L. L. Ames et al. (Ref. 13) ].

parameters , the photographic plates, I l ford Q It, were developed and processed under s tandard conditions. The photoplates were read on a recording micro- densi tometer and Seidel t ransmission values were used to calculate the concentra t ion values. The corrections for peak width and mass photopla te sensitivity, used in compiling the da ta shown in Table I, were those listed by Owens and Giardino}

The sensit ivi ty factors shown in column 2 of Table I represent the factor by which the exper imental ly determined rare-ear th concentra t ion mus t be divided to yield the t rue concentrat ion. These da ta are valid

Table II. Relative sensitivity factors of the rare earths2

Metal (RSF) i b (RSF) ~ ff (RSF) 111 d

La 0.53 0.64 0.80 Ce 0.79 0.42 1.16 Pr 0.81 0.85 0.69 Nd 0.82 1.15 1.07 Sm 0.74 1.89 • • • Eu 0.83 2.50 1.23 Gd 0.68 1.03 1.18 Tb 0.91 0.68 0.69 Dy 1.00 1.00 1.00 Ho 0.93 0.79 0.69 Er 0.68 0.76 0.62 Tm 1.10 1.10 0.76 Yb 1.08 2.56 • • • Lu 0.57 0.50 0.73 Th 0.37 • • • 0.41

Normalized, R S F of D y =1 . b Gold used as the in ternal s tandard ; 400-ppm rare-ear th oxide in graphi te . e Ca lc ium i m p u r i t y as the in ternal s tandard ; Y2Os matr ix . d Rhen ium as the internal s tandard, f rom Nicholls, see Ref . 11. Electrodes

were oxides mixed in low mel t ing glass plus graphi te .

for the sample electrodes composed of 2 mol of graphi te and 1 mol of Y203 with gold as the internal s tandard.

The last column in Table I shows these same sensit ivi ty factors af ter they have been normalized, arbitrari ly, to D y and represent the relative behavior of the rare-ear th elements with respect to each other for the above matrix.

Table I I shows other normalized relative sensi t ivi ty factors, all normalized arbi t rar i ly with respect to Dy. (RSF)i were obtained using a mixture of rare-ear th oxide a t the 400-ppm concentra t ion level in a gold- doped graphi te matrix. The electrodes were essentially all graphite. These normalized factors differ con- siderably f rom those R S F values obtained for electrodes consisting of Y203 and graphite. (RSF)i i are for a sample of doped Y~03 matrix, with Ca, an impur i ty in Y203, used as an internal s tandard. These results are average values based upon four independent measurements with an over-all precision of ~ 3 0 % . The da ta in the last column of Table I I , ( R S F ) m , were t aken f rom the work of Nichols et al. H for a mixture of rare-ear th compounds pelletized with graphi te arLd using an added Re internal s tandard. There is agree- ment for m a n y elements but poor correlat ion for others.

II. C O M P U T E D S E N S I T I V I T Y FACTOR

The conversion of the electrode mater ia l into ions in the rf spark ion source consists of several basic steps ; for simplicity, the prebreakdown or field emission phenomena will not be discussed here. A detailed review of the mechanisms occurring prior to and during the rf spark has been given by Honig2 In brief, a toms of the solid are first conver ted by the rf spark into the gaseous state, t ha t is, into free atoms, molecules, or ions. Neut ra l species are subsequent ly ionized or dissociated by the electrons within the discharge. In addit ion to ionization phenomena, molecular species not present in the solid sample are formed in the gas phase or on electrode surfaces. Al though differences in ion energy distributions have been shown to produce apparen t elemental sensi t ivi ty differences at the ion- sensitive plate, ~2 the magni tude of such a considerat ion is insignificant in this s tudy (especially for the rare- ear th elements which might be expected to have similar energy distributions) and will not be considered here.

I n recent work ~3 it was shown tha t the v a c u u m vapor iza t ion of a solid rare-ear th oxide, M203, f rom a Knudsen cell is stoichiometric. The products consist of the monoxide, MO, the metal, M, and a tomic oxygen, O, where the intensi ty of M O > M . These quanti t ies are of p r imary impor tance in determining the behavior of the rare-ear th oxides in an rf spark.

The heats of format ion of the gaseous metals and the dissociation energies of the monoxides, given in electron volts, are shown in Fig. 1.

Since the relative in tensi ty of the M + species will depend upon the ionization potent ia l of the meta l atoms, ~4 these are also included in the calculation for the relative sensit ivi ty factors.

326 Volume 25, Number 3, 1971

In addition, the experimental ly determined values of mult icharge distr ibution were included to account for any unknown effects occurring within the rf spark.

The relative RSF is given by the following expression

R S F (x) D 0 ° (MO)(s td) ~H0 ° (std)

RSF (std) Do ° (MO)(x) AH0 ° (x)

I. P. (std) I0 (x) (1)

I. P. (x) I0 (std) '

where D0 ° (MO) = dissociation energy of the monoxide ; kH0°=ene rgy of format ion of the gaseous metal ; I P = i o n i z a t i o n potent ial of the metal ; I o = M + ' / (M+I+M+2+M+a) , the fractional amount of singly charged ions produced.

Figure 2 shows the calculated and empirically determined sensit ivity factors as a function of the rare- ear th element or atomic number. The correlation between the experimental and calculated values is satisfactory. In m a n y cases, however, par t icular ly at the high mass end above Dy, the absolute agreement is poor.

I I I . D I S C U S S I O N

Among the rare earths, the heats of formation of the gaseous metal and the dissociation energies of the rare-earth monoxides are quite different. They are the major factors affecting the computed values of the normalized sensitivity factors for the rare earths in Y203. These results can he seen by calculating the sensit ivity factors with and without the ionization potentials of the rare ear th and the experimental ly determined mult icharged ion distribution.

In addit ion to these four factors, surface ionization phenomena m a y also be involved, since the ionization potential of bo th the metal and the monoxide are sufficiently low as to yield considerable intensities of M + and MO + directly f rom the electrode surface? 5 Since the local surface tempera tures of the electrodes are quite high, it would appear t ha t the effects of surface ionization could influence the relative sen- s i t ivi ty factors of the rare earths.

I t has also been observed in the course of this work t ha t the absolute values of the relative sensit ivi ty factors can va ry unpredictably f rom experiment to experiment. However, the relative behavior of the rare-ear th elements is similar for electrodes of similar composition. The data show best agreement for those samples having the same or similar electrode compositions, namely (RSF)A, (RSF)H, and (RSF)H~. I t should be noted tha t the ( R S F ) m values were obtained on another type of mass spectrograph, an A E I MS-702. For electrodes of dissimilar composition, analyzed on the same instrument , the agreement is poor. Factors such as electrode shape, ~6 gap width, and position of the electrode spark with respect to the accelerating aper ture t7 all have some effect on the analytical results but were not accounted for or noted in this work.

As an added comparison between the Knudsen cell

3.0

2.5 U

; 2.o

1.0

E 0.5

oCALCULATED BY EO ( I )

• EXPERIMENTAL RSF A I t

! i

',1 I ', t I ',l I I

I o ii tilt I o ~ ' ' - o - ' - - o SS

0 i , i I I i i i i t i i I Lo Ce Pr Nd Sm Eu Gd Tb Oy Ho Er Tm Yb Lu

ATOMIC NUMBER

Fro. 2. Calculated and empirically determined sensitivity factors as a function of rare earth with increasing atomic number.

source and the spark ion source, Table I I I shows the M O / M ion intensi ty for Knudsen cell da ta and M O / M ion ratios determined f rom spark source work. The spark source values were obtained for electrodes whose composit ion were 2 mol graphite, 1 mol Y203. Since in spark source work the most intense ionic species is generally the M +, the M O / M ratios for the rf spark have been multiplied by a factor of 100 to simplify comparison. The relative intensities show agreement with the exception of Ho203, Er20~, and Lu~O~.

I n recently published paper TM it was shown tha t the dissociation energy of the rare-ear th monosulfides, D0°(1V[S), and the rare-ear th monoxides, Do°(MO), show a similar dependency oi1 atomic number. One could therefore expect tha t the normalized relative sensit ivi ty factors for the rare-ear th elements in the form of sulfides would be similar to the factors reported for the case of an oxide matrix.

IV . C O N C L U S I O N

The experimental ly determined sensit ivity factors of the rare earths for an electrode consisting of 1 mol of Y203 and 2 tool of graphi te differ by as much as a factor of 6 from one another. The normalized relative sensit ivi ty factors of the rare earths in an rf spark

Table Ill. Comparison of MO/M ion intensity ratios: Knudsen cell vs rf spark.

This work Oxide Ames et al. a (X 100)

Y~03 16 20 Sm203 2.0 1.5 Er203 0.1 0.05 Dy203 2.0 0.4 Ho20~ 2.0 0.08 Er~03 2.0 0.26 Tm20~ 0.1 0.05 Yb203 < 0.02 0.08 Lu~O~ 2.0 0.19 Gd~O~ 3.3 8.3 La~O3 > 100 20 Nd~03 > 100 14.2

lKnudsen cell data , 25-eV electrons.

APPLIED SPECTROSCOPY 3 2 7

sou rce mass s p e c t r o g r a p h show a m a r k e d d e p e n d e n c y u p o n the h e a t s of f o r m a t i o n of t he gaseous m e t a l s a n d the d i s soc i a t i on ene rgy of t he monoxide . Since t h e r e l a t i v e v a r i a t i o n of A H a n d D0°(MO) w i t h a t o m i c n u m b e r of t h e r a r e - e a r t h e l e m e n t s for t he ease of t h e sesqu iox ide a re a l m o s t a l ike, t h e r e l a t i ve b e h a v i o r for t h e r a r e - e a r t h e l emen t s in t he rf s p a r k ion source will be a p p r o x i m a t e l y t he s ame w h e t h e r zXH0 °, D0 ° (MO) or t h e i r p r o d u c t is used to ca l cu la t e t he s e n s i t i v i t y fac tors .

1. A. Cornu, in Advances in Mass Spectroscopy, edited by E. Kendriek (The Institute of Petroleum, London, 1968), Vol. 4, p. 401.

2. J. G. Gorman, E. J. Jones, and J. A. Hipple, Anal. Chem. 23, 438 (1963); N. B. Hannay and A. J. Ahearn, ibid. 26, 1056 (1954) ; E. B. Owens and N. A. Giardino, ibid. 35, 1172 (1963).

3. J. Franzen and H. Hintenberger, Z. Naturforseh. 18a, 397 (1963); R. Honig, Final Rep. AFCRL-65-38 (Dec. 1964); J. Franzen and iK. D. Schuy, Z. Anal. Chem. 2, 295 (1967).

4. A. Cavard, in Ref. 1, p. 419. 5. R. A. Bingham, R. Brown, and P. Powers, Pittsburgh Conf.

Anal. Chem. Appl. Speetrosc. (3-8 March 1968) ; H. J. Svee and R. J. Conzemious, Int. Mass Spec. Conf. Berlin (25-29

Sept. 1967) ; F. D. Leipziger and C. A. Evans, Jr., 134th Nat. Meeting Electrochem. Soc., Montreal (6-11 Oct. 1968).

6. N. W. It. Addink, Z. Anal. Chem. 205, 81 (1964). 7. B. B. Goshgarian and A. V. Jensen, 12th Annu. Conf. Mass

Spectrom. ASTM Committee El4, Montreal, 350 (1964). 8. A. J. Socha and R. K. Willardson, Aerospace Res. Lab. ARL

68-132 (AD 676043-July 1968). 9. G. Vidal, P. C. Galmard, P. Lanusse, Tech. Note No. 125,

NASA N68-25303 (1968). 10. J. M. McCrea, Appl. Spectrosc. 23, 55 (1969). 11. G. D. Nicholls, A. L. Graham, E. Williams, and M. Wood,

Anal. Chem. 39, 584 (1967). 12. J. R. Woolston and R. Honig, l l th , 12th Annu. Conf. Mass

Spectrom. ASTM Committee E14 (1963, 1964). 13. L. L. Ames, P. N. Walsh, and D. White, J. Phys. Chem. 71,

2707 (1967). 14. G. R. Hertel, J. Chem. Phys. 48, 2053 (1968). 15. M. Inghram and R. J. tIayden, Nucl. Sci. Ser., Rep. No. 14,

Nat. Acad. Sci. (1954). 16. J. Franzen and K. D. Schuy, in Ref. 1, p. 449. 17. H. J. Svec, R. J. Conzemious, and G. D. Flesch, 15th Annu.

Conf. Mass Speetrom., ASTM Committee El4, Denver, Colo. (May 1968).

18. S. Smoes, P. Coppens, C. Bergman, and J. Drowart, Trans. Faraday Soc. 65, 682 (1969).

Isotopic Analysis of Lithium by Atomic Absorption Spectrophotometry*

J. A. Whea t

Savannah River Laboratory, E. I. du Pont de Nemours and Company, Aiken, South Carolina 29801 (Received 18 September 1970; revision received 14 December 1970)

An atomic absorption method was developed to determine the isotopic composition of lithium. The method utilizes neon-filled 6Li and ~Li hollow cathode emission lamps and a flame atomizer. The absolute precision of the method is =t=0.5 at.% eLi. INDEX HEADINGS: Analysis for lithium isotopes; Atomic absorption spectroscopy; Methods,

analytical; Techniques, spectroscopic.

I N T R O D U C T I O N

Because of large demands in the nuclear industry for 6Li-enriehed lithium, lithium from commercial sources exhibits one of the largest variations of iso- topic compositions and the assay is not ahvays known by the supplier. The 6Li content of samples en- countered in this laboratory has ranged from 4 to 98 at.%. Because many analytical procedures are based on the determination of molar concentrations of lithium, the isotopic composition must be known for correct formulations on a weight basis. In the nuclear industry, the determination of lithium isotopic composition of reactor materials, such as lithium- aluminum, is essential.

The isotopic composition of lithium is normally determined by mass spectrometry; but equipment is

* The information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U. S. Atomic Energy Commission. This work is sponsored by the Division of Peaceful Nuclear Explosives.

expens ive , ex tens ive s a m p l e p r e p a r a t i o n is r equ i r ed , a n d t h e c a p a c i t y of a s ingle s p e c t r o m e t e r is a b o u t fou r s amp le s p e r 8 h. A m o r e r a p i d a n d less expens ive ana lys i s is h i g h l y des i rab le .

Za ide l a n d I ( o r r e n o i 1 h a v e m e a s u r e d t h e 6Li con- c e n t r a t i o n of so lu t ions b y a t o m i c a b s o r p t i o n us ing a 6Li-enr iehed hol low c a t h o d e emiss ion source. Before t h e i r a t o m i c a b s o r p t i o n m e a s u r e m e n t s , t h e t o t a l l i t h i u m c o n c e n t r a t i o n h a d been a d j u s t e d to a c o n s t a n t v a l u e based on a d e t e r m i n a t i o n b y f lame emiss ion s p e e t r o p h o t o m e t r y . T h e a b s o r b a n c e m e a s u r e d w i t h t h e 6Li l a m p was c a l i b r a t e d to t h e i so top ic r a t i o a t c o n s t a n t t o t a l l i t h i u m c o n c e n t r a t i o n .

M a n n i n g a n d S l a v i n 2 used f lame sources of °Li a n d 7Li r a d i a t i o n for a t o m i c a b s o r p t i o n m e a s u r e m e n t s . T h e r a t i o of t h e a b s o r b a n c e s d e t e r m i n e d for a s a m p l e w i t h t h e two sources was a f u n c t i o n of t h e l i t h i u m i so top ic compos i t i on . T h e r e l a t i o n s h i p was c o n s t a n t a t t o t a l l i t h i u m c o n c e n t r a t i o n s of 1 .5-3 p p m . H o w e v e r , t h e r e l a t i v e l y la rge b r o a d e n i n g of t h e l i t h i u m r e s o n a n t

328 Volume 25, Number 3, 1971 APPLIED SPECTROSCOPY