Glow discharge spectrometry for the characterization of nuclear and radioactively contaminated environmental samples

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  • Keywords: Glow discharge spectrometry; Contaminants; Radionuclides

    Spectrochimica Acta Part B 59 (2004) 13591376

    1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1360

    2. General processes and instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1361

    3. Instrumentation requirements for radioactive samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1362

    4. Quantification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1364

    5. Trace and bulk analysis in nuclear samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1364

    5.1. Conducting nuclear samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1364

    5.1.1. Metallic alloy nuclear fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1364

    5.1.2. Zircaloy cladding materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1365

    5.2. Non-conducting nuclear samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1365Review

    Glow discharge spectrometry for the characterization of nuclear and

    radioactively contaminated environmental samples

    Maria Betti*, Laura Aldave de las Heras

    European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany

    Received 28 May 2004; accepted 14 July 2004


    Glow discharge (GD) spectrometry as applied to characterize nuclear samples as well as for the determination of radionuclides in

    environmental samples is reviewed.

    The use of instrumentation for direct current (d.c.) glow discharge mass spectrometry (GDMS) and radio frequency glow discharge optical

    emission spectrometry (rf GDOES), installed inside a glove-box for the handling of radioactive samples as well as the two installations and

    their analytical possibilities, is described in detail.

    The applications of GD techniques for the characterization of samples of nuclear concern both with respect to their major and trace

    elements, as well as to the matrix isotopic composition are presented.

    Procedures for quantitative determination of major, minor, and trace elements in conductive samples are reported. As for non-conductive

    samples three different approaches for their measurement can be followed. Namely, the use of rf sources, the mixing of the sample with a

    binder conducting host matrix, and the use of a secondary cathode. In the case of oxide-based samples, the employment of a tantalum

    secondary cathode, acting as an oxygen getter, reduces the availability of oxygen to form polyatomic species and to produce quenching.

    Considerations on the use of the relative sensitivity factors (RSFs) in different matrices are reported.

    The analytical capabilities of GDMS are compared with ICP-MS in terms of accuracy, precision, and detection limit for the determination

    of trace elements in uranium oxide specimens. As for the determination of isotopic composition, GDMS was found to be competitive with

    thermal ionisation mass spectrometry (TIMS) as well as for bulk determinations of major elements with titration methods. Applications of

    GDMS to the determination of radioisotopes in environmental samples, as well for depth profiling of trace elements in oxide layers, are


    D 2004 Elsevier B.V. All rights reserved.0584-8547/$ - s


    * Correspon

    E-mail addree front matter D 2004 Elsevier B.V. All rights reserved.


    ding author. Tel.: +49 7247 951 363; fax: +49 7247 951 186.

    ess: (M. Betti).

  • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1366

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1367

    f nuclear interest . . . . . . . . . . . . . . . . . . . . . . . 1368

    ronmental samples . . . . . . . . . . . . . . . . . . . . . . . 1371

    fuel cladding material . . . . . . . . . . . . . . . . . . . . . 1373

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1374

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1374

    traces, impurities and depth profiling of solids [13]. Glow composition, is of great importance. These materials can

    rochimica Acta Part B 59 (2004) 13591376Discharge Optical Emission Spectrometry (GDOES) is

    recognised to be a rapid method for depth profiling, capable

    of surface analysis [47], interface and bulk qualitative and

    quantitative analysis of solids [8]. Glow Discharge Mass

    Spectrometry (GDMS) is one of the most powerful solid

    state analytical methods for the direct determination of

    impurities and depth profiling of solids [911]. Glow

    discharge mass spectrometers, which are commercially

    be analysed using several techniques based on nuclear and

    Table 1

    Type of radioactive samples

    Oxide-based nuclear fuels

    Metallic alloy nuclear fuels

    Vitrified wastes

    Zircaloy nuclear cladding materials

    Forensic6. Simfuels . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    7. Nuclear waste glasses . . . . . . . . . . . . . . . . . . . . .

    7.1. Determination of the isotopic composition in samples o

    8. Determination of traces of radioisotopes in contaminated envi

    9. Depth profiling of ZrO2 layers deposited on Zircaloy nuclear

    10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . .

    References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    1. Introduction

    Radionuclides, particularly the long-lived ones along

    with trace elements, represent nowadays, an important

    category of inorganic pollutant that need to be determined

    not only in nuclear samples, like for instance nuclear fuels

    but also in environmental samples, as soils and sediments as

    well as waste materials.

    Radionuclides containing samples can originate through

    different systems and processes. Human activities involving

    nuclear weapons and nuclear fuel cycle (including mining,

    milling, fuel enrichment, fabrication, reactor operation,

    spent fuel stores, reprocessing facilities, medical applica-

    tions and waste storage) are important, and may lead to a

    significant creation of this kind of samples. Human

    technology also releases pre-existing natural radionuclides,

    which would otherwise remain trapped in the earths crust.

    For instance, burning of fossil fuel (oil and coal) dominates

    direct atmospheric release at pre-existing natural radio-


    The ability to develop adequate models for predicting the

    fate of inorganic contaminants, including radionuclides, in

    both surface and subsurface environments, is highly

    dependent on the accurate knowledge of the partitioning

    of these constituents between the solid and solutions phases

    and ultimately on the capability to provide molecular-level

    information on chemical species distributions in both of

    theses phases. Furthermore, the development of environ-

    mentally sound yet cost effective remediation strategies

    requires an understanding of the chemical speciation of the

    contaminants within the sediment, soil and waste material

    matrices in which they are contained.

    Glow discharge (GD) sources that have been widely

    exploited in analytical chemistry for the direct analysis of

    solid samples are, among the solid state analytical methods,

    the most powerful tools for the direct determination of

    M. Betti, L. Aldave de las Heras / Spect1360available with fast and sensitive electrical ion detection,

    allow direct trace elemental determination in solid materialswith good sensitivity and precision in the concentration

    range lower than ng g1 [12,13].Primarily employed for the determination of transition

    elements in steels and metals, GD-based methods have also

    been recently exploited for the characterization of nuclear

    radioactive solid samples [14]. The direct analysis of solid

    samples is very important in the nuclear field since operator

    exposure time to radiation and the quantity of liquid nuclear

    wastes can be strongly reduced. Moreover, a non-destructive

    analysis allows the sample to be reused for further

    investigation, to be reprocessed, or to be kept as an archival

    sample. Nowadays, GDMS can be considered as one of the

    most powerful solid-state analytical methods for the

    determination of traces and depth profiling of solid samples

    of nuclear concern as well as for the monitoring of long-

    lived radioisotopes in radioactively contaminated environ-

    mental samples [9,1417]. Its ability to measure isotope

    ratios in solids has been also evaluated [18,19], and the

    technique has proven to be a good choice for isotopic

    analysis in nuclear and radioactive environmental samples

    [9,2026]. In Tables 1 and 2, the different types of

    radioactive samples and the measurements carried out on

    them via GDMS are summarised, respectively.

    As compared to wet chemistry-based methods, glow

    discharge-based methods using optical emission spectrom-

    etry and mass spectrometry have the advantage of simpler

    sample preparation procedures, as results of carrying out

    measurements directly on solid samples. Therefore, for

    nuclear samples, they meet both requirements of reducing

    time exposure of the operator, as well as the amount of

    liquid wastes.

    In the field of nuclear research and technology, the

    chemical characterization of different types of nuclear

    fuels, cladding materials, nuclear-waste glasses and

    smuggled nuclear samples, from the point of view of

    trace, major and minor elements as well as their isotopicEnvironmental

  • non-nuclear methods that can be, to a varying degree,

    tedious and time-consuming. In the last year, non-nuclear

    methods based on MS have become predominant for the

    characterization of samples of nuclear concern. The

    application of ICP-MS has been widely investigated [27

    30] for fission products and actinide determination as well

    as Thermal Ionisation Mass Spectrometry (TIMS) for the

    routine analysis of isotopes in liquid samples [31]. GDMS,

    when used for the chemical characterization of nuclear

    samples, provides information on the chemical composi-

    tion much faster than other techniques, making it possible

    to modify fuel production procedures and reactor con-

    ditions or to quickly recognise smuggled materials. The

    2. General processes and instrumentation

    Glow discharge is a low-energy plasma (Fig. 1) sustained

    between two electrodes that are immersed within a reduced

    pressure, inert gas environment. The sample usually serves

    as the more negatively charged electrode [i.e., the cathode in

    a direct (d.c.) glow discharge system]. Common discharge

    support gas pressures are in the range 0.11 Torr. For

    analytical applications, argon is most commonly used

    discharge support gas even though other gases are some-

    times used. The plasma is created by inserting two electro-

    des in a cell filled with the discharge gas at a low pressure

    and is initiated when a high potential typically of the order

    of 1 kV is established between the two electrodes. The

    application of the high potential causes the bdischarge gasbreakdown electricallyQ, namely, the gas is being split upinto positive ions and electrons resulting in the formation of

    plasma. The positive ions are attracted toward the sample

    surface by electric fields within the plasma, and may reach

    substantial kinetic energy. In a plasma of argon, the Ar+ ions

    Table 2

    Type of measurements performed on radioactive samples

    Trace elements

    Major and minor elements

    Isotopic ratios

    Depth profiling

    M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 13591376 1361advantages of GDMS are low limits of detection, uniform

    element sensibility and capability to measure all elements

    and even isotopes. Clearly, for the handling of nuclear

    materials, instruments need to be properly modified for

    installation in glove-boxes for a- and h-radiation protec-tion [32,33].

    In this work, the exploitation of GD sources for the

    characterization of samples of nuclear concern as well as

    radioactively contaminated environmental samples as for

    trace elements and long-lived radioisotopes is reviewed.

    Instrumentation requirements for radioactive samples as

    well as the type of samples and analyses performed are

    considered. Quantification issues and comparisons with

    other solid state techniques are discussed.Fig. 1. Schematic picture of the main procformed in the glow discharge are accelerated toward the

    sample cathode and the sample material is sputtered at the

    cathode surface by ion bombardment. Sputtered atoms and

    molecules are ionised in the glow discharge plasma

    (negative glow) by Penning and/or electron impact ionisa-

    tion and charge exchanges processes. Mathematical models

    of processes in a direct current glow discharge have been

    developed by Bogaerts et al. [3436]. GDOES is based

    upon the measurement of photons emitted by excited state

    species in the plasma. In GDMS, the positively charged ions

    formed in the argon plasma of the glow discharge source are

    extracted and accelerated into the mass spectrometer, where

    the ion beams are separated according to their mass-to-

    charge and energy-to-charge ratio. The separated ions areesses occurring in a glow discharge.

  • tions with such instrumentation have been recently

    reviewed [62].

    rochimthen, electrically detected by ion/electron/photon conversion

    by ion counting and a Faraday cup for analog ion beam


    The glow discharge ion source has been interfaced to

    most of the standard mass spectrometer types. The first

    commercially available GDMS instrument used a double

    focusing magnetic sector mass analysis system, permitting

    the acquisition of high-resolution spectra with high

    sensitivity [37]. Quadrupole mass spectrometers are

    typically more compact and less expensive than magnetic

    sectors. As a consequence, quadrupoles have been

    employed for fundamental and development research of

    GDMS [38], finally resulting in the commercial avail-

    ability of a quadrupole GDMS system [39]. At present,

    quadrupole glow discharge mass spectrometers are mostly

    in-house built instruments. Promising results have also

    been obtained from the coupling of glow discharge ion

    trap mass spectrometric systems [40], double and triple

    quadrupole instruments [41], time-of-flight mass spec-

    trometers [42,43], Fourier transform mass spectrometers

    [44,45] and a MattauchHerzog double focusing sector

    spectrometer [46]. However, the commercially available

    GD mass spectrometers presently only employ inverse

    NierJohnson double-focusing and quadrupole-based mass


    One of the main challenges for glow discharge

    spectroscopy to overcome is the intrinsic requirement of

    the sample to be electrically conducting. Nevertheless,

    non-conducting solid samples can be directly analysed by

    using a radio-frequency powered source [47], and their

    applications have been recently reviewed [14], with direct

    current device plasma using the secondary cathode

    technique [48,49], or by mixing the sample with a

    conducting host [50,51]. When a secondary cathode with

    a circular orifice in its centre is employed, it is placed on

    top of analytical specimen. Upon initiation of the

    discharge, the sputtering process is naturally concentrated

    in the inner edge of the conductive electrode, and a

    metallic layer is eventually produced on the sample

    surface. This electrically conductive layer promotes

    sputtering of the sample, and the continuous deposition

    of the metallic layer allows extended plasma operation.

    This approach is limited by the geometry of the sample.

    It tends to be complex in terms of the optimisation of the

    discharge conditions in order to balance sputtering of the

    sample and of the secondary cathode. The application of

    a conducting binder that is mixed with the non-conduct-

    ing powdered sample before the pressing of the homo-

    genous mixture to an electrode is very common. The

    physical nature of the sample can affect the efficiency

    with which the mixture can be ground to a size fine

    enough to ensure a stable sample disk and plasma. Both

    techniques for the analysis of non-conductors by d.c.

    GDMS may present additional disadvantages. The for-

    M. Betti, L. Aldave de las Heras / Spect1362mation of new disturbing molecular ions (argides, oxides

    from the secondary cathode or binder material) in massAs a surface analytical technique, GDMS can be used for

    the determination of element concentration as a function of

    sputtered depth. However, depth profile of thin layers using

    GDMS has had a subordinated role compared with GD-

    OES. In fact, to date, most depth profile work has been

    carried out with optical emission instruments. The major

    reason for this is probably the fact that commercial GDMS

    devices are slow and cumbersome to operate compared with

    GD-OES instruments [63]. However, a substantial amount

    of depth profile analysis has also been carried out using

    GDMS. With the development of a faster GDMS such as the

    one recently developed by Dorka et al. [64], GDMS should

    find wider and more frequent use for depth profiling in the


    3. Instrumentation requirements for radioactive samples

    For handling of nuclear materials, difficulties arising

    from the radioactive nature of samples have to be

    overcome. The operator has to be protected from the

    radioactive material, which means that the use of glove-

    boxes is necessary. In addition, in order to avoid

    contamination of the working area, the analytical instru-

    ment has to be modified so that containment is assured

    and no radioactive material leaks either into the

    laboratory or into the environment. Complete instruments

    cannot be introduced into a glove-box because electronics

    are very sensitive to radiation, and only the sampling

    stage takes place inside. Only one GD mass spectrometer

    (VG9000, ThermoElemental) has been integrated in a

    glove-box for the characterization of nuclear material

    [32]. The glove-box encloses the ion source chamber, the

    interlock and the associated pumped system. In Fig. 2, aspectra can be observed. The detection limits increase

    due to the dilution of the powdered sample with the

    conductor binder or due to the secondary cathode

    material (contributing to the blank). The preparation of

    the mixed electrode requires a grinding step, which could

    cause contamination. Other problems arise from trapping

    water vapour and atmospheric gases in the sample during

    the compaction process. In order to avoid these difficul-

    ties, radio frequency-powered GD sources were intro-

    duced for the direct analysis of non-conductors [52,53].

    Despite extensive research performed in this field

    [12,13,5256], it must be pointed out that no commercial

    source exists for a complete rf-GDMS system, even if rf

    devices have been sampled by a very wide range of mass

    analyser types. Among these, single quadrupole [53,57],

    ion trap [58], Fourier transform ion cyclotron resonance

    (FT-ICR) [42], time-of-flight [59,43], and double focusing

    instruments [60,61] are noteworthy. Most of the applica-

    ica Acta Part B 59 (2004) 13591376schematic diagram of the installation of the GD source

    housing in the frame of the glove-box is given. The

  • rochimM. Betti, L. Aldave de las Heras / Spectinstallation of the VG 9000 GDMS into the glove-box is

    described in detail by Betti et al. [14,32]. The glove-box

    encloses the ion source chamber, sample interlock and

    the associated pumping system. All supplies to the ion

    sources (argon discharge support gas and liquid nitrogen

    for the cryogenic cooling of the discharge cell) and the

    pumping ports should be fitted with absolute filters to

    eliminate any external contamination.

    The ion source was re-designed to minimise the number

    of operations and to simplify routine maintenance inside the

    glove-box area. This was achieved by utilising a buniversalQcell for the analysis of both pin-shape and flat samples and a

    bplug-inQ focus stack. The source itself has been split intovarious components comprising a measurable plate with

    removable cell and focus stack assemblies. The source

    mounting position remains fixed to the back wall of the

    sources housing chamber. The focus stack then plugs into a

    recess in the mounting plate and is held in place by four

    fixing rods. Electrical contact to the plates of the focus stack

    is made by a series of copperberyllium contacts. This

    eliminates the need to disconnect any wire when removing

    the focus stack, thus simplifying its removal.

    The focus stack assembly consists of a series of tantalum

    plates, separated by PEEK spacers, mounted into a base

    Fig. 2. Schematic diagram of the installation of a GDMSica Acta Part B 59 (2004) 13591376 1363containing the source-defining slit from the mass spectrom-

    eter. The focus stack provides deflection and focussing of

    the ion beam in the y- and z-directions to give the best

    image on the source-defining slit. The plates are shaped so

    that when the focus stack is in position, they make electrical

    contact with the appropriate connector on the contact

    assembly. The focus stack assembly also contains a

    mounting bracket for the location of the cell and sample


    The buniversalQ cell has been designed to accommodate alarge range of pin-shape and flat samples. The cell itself

    consists of a universal body that plugs into the focus stack.

    This cell body, based around the existing flat geometry,

    remains located in position. The buniversal pin holderQ canbe used for the analysis of a wide range of pin samples. The

    analysis of flat samples is performed with the aid of the flat

    sample holder [65]. Changing the sample geometry from pin

    to flat simply requires changing the appropriate sample

    holder when loading the sample. It is no longer necessary to

    break the vacuum to do this, thus reducing the number of

    operations required inside the glove-box.

    The integration of a radio-frequency (rf)-powdered GD-

    OES system in a glove-box has been also described [33].

    The GD lamp used in this system was specially designed

    in a glove-box and of the modified discharge cell.

  • window, through which the radiation of the GD sources

    can be observed. The main beam enters the optical

    total uranium and plutonium and Thermal Ionisation Mass

    Spectrometry (TIMS) and Inductively Coupled Plasma

    rochimdetection system.

    As with the previous case, in order to prevent contam-

    ination, all parts of the instrument that will come in contact

    with the radioactive samples have been enclosed in a glove-

    box divided in three separated parts.

    The first compartment of the glove-box contains a

    polishing machine. Then the GD lamp and the UV

    polychromator were installed separately in the second and

    third compartments, respectively. To avoid contamination of

    the pumps HEPA filter are used for the evacuation.

    The (rf)-powdered GD-OES is set up in a glove-box to

    determine the concentration level of carbon, hydrogen,

    nitrogen and oxygen (in the range of Ag g1) in nuclearmaterials such as uranium, plutonium or mixed uranium and

    plutonium oxides with relative standard deviation less than

    5% at 20 Ag g1. The authors, from the calibration graphsobtained in non-nuclear samples for all carbon, nitrogen and

    oxygen, calculated the limits of detections, at 3 sigma level,

    of: 10 Ag g1; 40 Ag g1 and 20 Ag g1 for carbon, nitrogenand oxygen, respectively [33].

    4. Quantification

    GDMS has found extensive application for multi-

    elemental determination in high-purity metallic, non-

    conducting and semi-conducting bulk samples [65,67

    69]. Direct solid mass spectrometry techniques have

    major advantages with respect to sample preparation:

    few sample preparation steps are required, and the risk of

    contamination is significantly lower. Quantification of the

    analytical signal in solid mass spectrometry may be

    difficult, especially if no suitable standard reference

    material with the same matrix is available. However,

    GDMS is relatively free from matrix effects due to the

    separation of atomisation and ionisation phenomena in

    time and space during the sputtering of the sample.for the rf powering and is based on a concept previously

    developed [66]. The sample needs to be flat enough to

    form a vacuum seal when being pressed against a PTFE O-

    ring by means of a pneumatically controlled piston and the

    rf potential is applied to the back of the sample. The

    discharge is powered with the aid of a rf power supply

    operating at a frequency of 13.56 MHz. Only one pump is

    required to provide a vacuum in the lamp instead of two as

    in a Grimm-type lamp. The window of the lamp through

    which the radiation passes consists of magnesium fluoride

    to assure also a transmission in the wavelength range of

    120180 nm. The optical path passes through a closed

    construction of stainless-steel tubes and bellows. A small

    circular flat mirror deviates a small part of the radiation

    beam perpendicularly to the optical axis towards a

    M. Betti, L. Aldave de las Heras / Spect1364Screening data can be obtained by GDMS even when

    reference materials are not available. A simple compar-Mass Spectrometry (ICP-MS), both combined with isotopic

    dilution analysis, were employed for the determination of

    the zirconium and neodymium contents. The agreement

    between the concentrations determined by GDMS and those

    by the other techniques was always good, the ratios between

    the results of the two methods being always close to one.ison of the element signal intensity of the analyte with the

    element sensitivity of a reference element, defined as the

    ratio between the signal intensity and the elemental

    concentration, results in an accuracy of about 30% [70].

    More details for specific applications will be later


    5. Trace and bulk analysis in nuclear samples

    GDMS has been used for the chemical and isotopic

    characterization of samples of nuclear concern. Plutonium

    and uranium oxide specimens, mixed uranium and pluto-

    nium oxide (MOX) and metallic fuels, simulated high burn-

    up nuclear fuels (simfuel), Zircaloy cladding materials,

    nuclear-waste glasses and smuggled nuclear materials have

    been investigated using GDMS. Some examples of con-

    ducting and non-conducting samples of nuclear concern

    characterized by a d.c. GDMS are discussed below.

    5.1. Conducting nuclear samples

    5.1.1. Metallic alloy nuclear fuels

    Metallic alloy nuclear fuels consisting of two matrices,

    UNdZr and UPuZr, the major component of which was

    uranium at 81% and 71%, respectively, have been charac-

    terized by GDMS. Semi-quantitative analysis could be

    performed using the signal intensity of the analyte and

    considering the element sensitivity of uranium. Using the

    relative sensitivity factor (RSF) values obtained for the

    analytes of interest in a matrix of uranium metal, the results

    could be improved in terms of accuracy. Since no uranium

    metal sample certified for the elements of interest was

    available, the strategy consisted of analysing uranium metal

    specimens of different origins by others methods, such as

    ICP-MS and ICP-AES. From these results, RSF values were

    obtained for GDMS and UNdZr and UPuZr samples could

    be analysed. The results of GDMS were found to be in good

    agreement with the theoretical ones. Accuracies of better

    than 10% and a precision exceeding 5% were obtained in

    runs consisting of 10 measurements. Owing to the fabrica-

    tion method of the alloys investigated, these figures of merit

    were expected.

    Several specimens of these metallic fuels were analysed

    with the aid of other analytical methods. For instance,

    tritation (TITR) [71] was used for the determination of the

    ica Acta Part B 59 (2004) 13591376GDMS was, therefore, the appropriate technique to be used

    for the determination of the chemical composition of these

  • metal nuclear fuel alloys, using the RSF values obtained for

    pure uranium metal matrix. However, in the case of more

    complex metallic alloy matrices, e.g., those containing

    minor actinides and rare earths, the use of more appropriate

    RSFs is necessary.

    For the determination of certain trace elements in UZrNd

    and UPuZr alloys, RSFs obtained for uranium metal

    matrices were employed. The GDMS results obtained using

    this approach were in good agreement with those obtained

    by measurement with ICP-MS [9]. For the determination of

    some trace elements in UZrNd and UPuZr, RSFs obtained

    advantage. Indeed, it was found that the U /UO and Pu /

    PuO+ ratios obtained with a tantalum secondary cathode are

    of the same order of magnitude as those obtained with

    tantalum and titanium binders (Fig. 3). The main result of

    the investigation is that for several elements, the RSFs were

    found to depend on the oxygen content in the sample (Fig.

    each component [9]

    Plutonium Cerium Neptunium






    35 45

    M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 13591376 1365for uranium metal were employed. The results obtained by

    this way were in good agreement with those obtained by

    analyzing the same specimens by ICP-MS [9].

    5.1.2. Zircaloy cladding materials

    GDMS has often been used for the analysis of zirconium

    alloys [72]. In the nuclear industry, zircaloy is an important

    material constituting the cladding of the nuclear fuel during

    the irradiation in the reactor. RSF values were obtained from

    reference materials and used for the analysis of zircaloy

    cladding materials [9]. Quantitative analysis of zircaloy

    cladding material was performed by applying RSF matrix

    specific, as well as RSF values obtained for a uranium metal

    sample. Using both RSF values good accuracy was obtained.

    This indicates that for metallic samples the matrix effects are

    negligible, and the technique is applicable for quantitative

    analysis without using matrix specific reference samples.

    5.2. Non-conducting nuclear samples

    Two different approaches have been used for the analysis

    of non-conducting samples. For flat samples, a secondary

    cathode is placed directly in the front of the sample. The

    second approach, for powdered samples, consists in mixing

    a pure conductive host matrix, namely pure silver, tantalum

    and titanium in a ratio of 1+3. In Table 3, a list of the oxide-

    based nuclear fuels analysed as for trace elements is given.

    The percentage of oxygen ranges from about 12% to 18%

    m/m. Oxygen, as a major matrix element, causes severe

    problems due to its release from the oxide during discharge

    processes. Once released into the GD plasma, it influences

    the analytical signal by quenching excitation and ionisation

    Table 3

    Non-conducting oxide-based samples. Composition is given in % m/m for

    Sample Oxygen Uranium

    U3O8 15.2 84.8

    UO2 11.8 88.2

    PuO2 11.8

    (U,Pu)O2 11.8 49.3

    (U,Pu)O2 14.9 61.8

    (U,Pu)O2 13.8 73.8

    (U,Pu)O2 13.2 82.3

    (Pu,Ce)O 17.9 2(U,Pu,Np)O2 13.8 46.1agents [73]. Moreover, in GDMS problems also arise from

    the presence of polyatomic oxides that create spectral

    interferences and give lower analytical sensitivity. bGettermetalsQ such as titanium or tantalum bond strongly withoxygen and reduce its availability to form oxides with the

    analytes or to quench metastable argon atoms.

    In Fig. 3, the ratios U+/UO+ and Pu+/PuO+ obtained with

    different binder materials are shown. Titanium and tantalum

    bind the oxygen stronger than silver, fewer UO+ and PuO+

    ions are formed in the case of silver. The RSFs for the three

    binders for some selected trace elements of nuclear interest

    such as boron, lithium, cadmium and gallium are presented

    in Table 4. RSFs depend on the host matrix and in the case

    of silver higher RFSs are obtained. This can be explained by

    the fact that (i) the oxygen in the plasma is not obtained by

    the silver and (ii) a large quantity of the uranium and

    plutonium in the plasma are oxide species. This reduces the

    number of metal ions in the plasma available for ionisation.

    For a secondary cathode, tantalum was chosen, and it

    was found that its property as a getter for oxygen is also an+ + +

    Fig. 3. U+/UO+and Pu+/PUO+ intensity ratios from oxide-based nuclear

    samples obtained with different binder materials and a Ta seconday

    cathode; m/z values measured were, 238, 254, 239, 255. Reproduced with

    permission of The Royal Society of Chemistry.35 4.1

  • 4). Therefore, a specific matrix reference sample for the

    quantitative analysis of oxygen containing samples is

    necessary [9].

    Materials for nuclear reactor fuel preparation need to be

    Table 4

    RSF values for uranium and plutonium dioxide using different metals as hostmat

    Host matrix

    Ag Ti Ta Ta

    UO2 PuO2 UO2 PuO211B 1.25F0.30 1.33F0.25 0.98F0.20 1.04F0.307Li 2.01F0.23 1.98F0.30 1.12F0.13 1.08F0.20114Cd 1.97F0.30 2.00F0.28 0.87F0.20 0.90F0.2569Ga 1.58F0.15 1.63F0.13 0.95F0.10 0.98F0.12

    M. Betti, L. Aldave de las Heras / Spectrochim1366characterized as for the following two aspects: the isotopic

    composition of the major elements and the concentration

    of trace elements. The acceptable levels of impurities in

    fresh nuclear fuels vary according to the characteristics of

    the reactor. In order to monitor contaminations during the

    fabrication process, the determination of the trace elements

    should be performed in the starting material as well as in

    the final pellets of fresh fuels. For these measurements,

    analytical methods with proven reliability, accuracy and

    precision are necessary. Among currently available techni-

    ques, d.c. GDMS and quadrupole ICP-MS have been

    successfully used [15]. In Table 5, the results obtained

    analysing a uranium oxide reference sample (Morille,

    CEA, France) using standard and matched-matrix RSFs

    are reported. To obtain results with the highest possible

    accuracy, matrix-specified RSFs values are required. The

    data presented in Table 5 demonstrate the high stability of

    the discharge when using the secondary cathode technique

    for the analysis of uranium oxide samples. Typical

    precisions in the order of 10% RSD or better can be

    obtained. For elements with concentrations of 510 Agg1, an analytical precision higher than 10% RSD wasobtained. Using matrix specific RSFs, an accuracy of 5%

    was generally obtained.Fig. 4. RSFs values of some trace elements in uranium and plutonium

    dioxide samples as function of oxygen content. Measurements performed

    using a tantalum secondary cathode. Reproduced with permission of The

    Royal Society of Chemistry.In GDMS, detection limits, in absence of any interfer-

    ence, are calculated from signals equalling three times the

    noise of the background signal. For uranium and plutonium

    oxides analysed with the secondary cathode technique,

    detection limits for several trace elements are at the low Agg1 level, as shown in Table 5, when using an integrationtime of 120 ms per isotope. As can be seen from this table,

    the detection limits for some elements like Fe, Cu, Ni, Mo,

    Ti, W and Zr are higher because the blank contribution of

    these elements stemming from the tantalum mask has to be

    taken into account.

    Plutonium oxide standards are not commercially available

    and one approach for the determination of bias is to apply

    different analytical techniques. The analysis of uranium and

    plutonium dioxides, using tantalum as secondary cathode,

    has shown that the RSF values are very similar in both

    matrices [9]. This fact indicates that accurate GDMS results

    can be obtained using RSF values from a matrix of similar

    composition. The results for the Pu samples were obtained

    by using RSF values derived for a uranium oxide matrix.

    This confirms that both U oxide and Pu oxide matrices have

    the same behaviour in a glow discharge source. Quantitative

    analysis by ICP-MS using a multi-standard addition in order

    to obtain the most accurate and precise results as possible

    was performed. In Table 6, the results obtained by GDMS

    and ICP-MS in the analysis of a plutonium oxide specimen

    are shown. As can be seen, good agreement exists between

    the results obtain by both techniques [15].

    Rf-GD-AES has been exploited for the analysis of

    plutonium oxide and nuclear waste glasses [74]. The goals

    of these applications included the parametric evaluation of

    the plasma operating conditions an assessment of the limits

    UO2 PuO2 UO2 PuO2

    0.97F0.20 1.05F0.20 0.98F0.30 0.98F0.281.15F0.28 1.12F0.25 1.13F0.20 1.10F0.200.90F0.28 0.88F0.25 0.90F0.25 0.93F0.280.95F0.12 0.95F0.13 0.98F0.10 1.02F0.10rices and secondary cathode [9]

    Secondary cathode

    ica Acta Part B 59 (2004) 13591376of detection for trace analysis in simulated vitrified waste

    samples and the quantification for this type of samples.

    The final goal was to design a contained rf-GD-AES

    system for remote sampling of radioactive materials. The

    results demonstrate the ability of rf-GD-AES to directly

    analyse non-conductive simulated waste glasses containing

    various inorganic components.

    6. Simfuels

    Simfuels replicate the chemical state and microstructure

    of irradiated fuels. These samples are also oxide-based (see

  • Table 5

    GDMS quantitative analysis of Morille uranium oxide reference sample based on

    Element Certified value

    (Ag g1)Matrix-specific RSF

    (Ag g1)Bias* (%)

    Ag 10.4F1.6 10.2F1.3 1.9Al 99F6 87F5 12.1Ba 3.8F1.6 3.5F1.5 7.9Ba 9.6F0.4 bBea 5.4F0.6 3.8F0.4 29.6Bi 24.4F1.9 20.9F1.7 14.3Caa 93F8 94F9 1.1Cd 4.9F0.7 5F0.4 2Co 9.8F2 11.1F0.8 13.3


    M. Betti, L. Aldave de las Heras / SpectrochimCr 99F2 102F5 3Cu 50.2F1 52.1F3.3 3.8Dy 0.5F0.06 cEu 0.52F0.03 cFe 211.6F6.5 207.2F10.8 2.1Gd 0.56F0.06 cIn 9.4F1 10.4F0.5 10.6Mg 19.3F1.5 19.4F1.6 0.5Mn 24.5F0.5 29.3F1.1 19.6Mo 147F5 144F9 2Ni 147F3 142F4 3.4Pb 101F3 103F9 2Table 7), and are here considered in the group of non-

    conductive samples even though, to some extent, they can

    also have conductive properties as some elements are present

    as metallic precipitates [75]. Two of these materials with a

    different chemical composition have been examined by the

    use of a tantalum secondary cathode. Their quantitative

    analysis has been made on the basis of the RSFs obtained on

    real oxide-based fuels with the same oxygen content. In

    Table 7, the results obtained are compared with those

    Sia 100F8 93F6Sm 0.5F0.12 cSn 18.5F5.6 20.8F3 12.4Th 6.2F0.8 bTi 49.2F2.6 48.6F8 1.2V 48.7F2.8 47F2 3.5W 100F9 106F11 6Zn 98.6F5.5 102F10 3.4Zr 59.9F4.1 64F7 6.8*Bias (%)=(certified valueGDMS value)100/certified value.a: Possible interferences: 9Be: 36Ar4+; 10,11B: 40Ar4+H,40Ar4+; 40,41,42,43,44Ca: 40A12C16OH+, 14N16O+.b: Not determined. c: Below detection limit.

    Table 6

    Mean concentration (mg kg1 PuO2) and intervalsof confidence (%)referring to the 95% confidence level (n=25) [15]

    Element/isotope GDMS ICP-MS

    (mg kg1) IC (%) (mg kg1) IC (%)

    Na 1.32 22.0 0.99 36.2

    Mg 0.70 68.6 1.06 42.4

    Al 3.88 13.4 3.73 6.5

    Fe 0.91 73.6 1.75 54.3234U 19.80 1.0 19.40 44.4235U 638.00 1.1 637.00 1.2236U 170.00 1.8 171.00 4.9237Np 122.00 1.6 111.00 7.64.6 8.1F0.3 14.3 17.9 12.2F1 36.8 0.13.6 30F1 22.4 1.46 175F11 19 0.92.7 143F25 2.7 6.28.3 111F7 9.9 0.4matrix-specific and standard RSF [15]

    RSD (%)


    Standard RSF

    (Ag g1)Bias* (%) Detection limit

    (Ag g1)

    12.1 9.3F2.3 10.8 0.15.5 87F3 11.9 0.540.8 2.5F0.8 35.5 0.2

    11.4F0.3 18.8 0.710 b 0.5

    7.7 41F3 68 0.69.1 95.8F4.2 0.47.6 3.4F1 30.6 0.56.9 9.5F0.3 1.34.7 94F11 4.7 1.96 63F7 25.6 0.6



    5 313F22 47.9 2.4

    ica Acta Part B 59 (2004) 13591376 1367reported by other researchers [75] for the same materials.

    As can be seen when the RSFs obtained for a matrix with the

    same oxygen content are used, the GDMS results are in good

    agreement with the others.

    7. Nuclear waste glasses

    The possibility of analysing glass samples by GDMS

    using a secondary cathode in tantalum has been already

    reported [76]. The same procedure has been applied to

    examine vitrified nuclear wastes. RSFs have been obtained

    on the basis of reference nuclear waste glasses. In Table 8,

    the RSFs are reported and compared with those available in

    literature [76]. In this case a complete disagreement was

    found. In Fig. 5, the RSFs obtained for other matrices have

    been reported and compared with those of the reference of

    Table 8 giving all values respect to iron. As can be observed

    in Fig. 5b and c for fission products, the RSF data available

    for comparison are very few and the spread among those

    reported by the different authors is very large. For the

    quantitative analysis of nuclear waste glasses, it is necessary

    to obtain matrix specific RSFs.

    6.1 245F11 145 0.10.9

    13.7 15.3F4.6 17.3 0.40.4

    15.7 b 1.4

    4.1 50F1 2.6 0.79.9 95F3 4.8 2.19.3 148F8 50 0.810.4 b 0.9

    r+,40ArH+, 12C14N16O+, 12C16O2+; 28,29,30Si: 56Fe2+, 27AlH+, 14N2+, 12C14N,

  • Table 7

    Simfuel composition: comparison of GDMS results with those by other authors [75]

    Compound Sample A Sample B

    GDMS ICP-AES [76] ORIGENCode [76] GDMS ICP-AES [76] ORIGENCode [76]

    BaCO3 0.13 0.11 0.147 0.30 0.39 0.311

    CeO2 0.26 0.23 0.285 0.53 0.53 0.526

    La2O3 0.10 0.09 0.106 0.20 0.15 0.194

    MoO3 0.33 0.28 0.359 0.70 0.64 0.730

    SrO 0.076 0.06 0.072 0.12 0.12 0.110

    Y2O3 0.04 0.03 0.041 0.06 0.07 0.061

    ZrO2 0.34 0.23 0.339 0.60 0.58 0.601

    Rh2O3 0.03 0.02 0.028 0.03 0.03 0.034

    PdO 0.15 0.14 0.149 0.44 0.37 0.440

    RuO2 0.34 0.30 0.364 0.75 0.69 0.764

    M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359137613687.1. Determination of the isotopic composition in samples of

    nuclear interest

    In nuclear technology and generally in nuclear research,

    the precise and accurate measurement of isotope ratios is of

    great interest. The widely accepted method for their

    determination is TIMS. In this method, the sample must

    be dissolved and chemical separation of the analyte of

    interest is required before the analysis. GDMS has also been

    exploited for the determination of the isotopic composition

    in samples of nuclear concern [21].

    Lithium and boron are light elements having two

    natural isotopes whose abundances are widely diverse:6Li (7.5%), 7Li (92.5%), 10B (19.9%) and 11B (80.1%).

    Their detection in nuclear material is important for the

    determination of its purity. Boron is a neutron capture

    element and lithium is used as a doping additive. The most

    widely used method for their isotopic abundance determi-

    nation is TIMS, after sample dissolution. The use of ICP-

    MS is hindered by its low detection power for light

    elements. By GDMS, detection at the ng g1 levels iseasily achievable.

    Nd2O3 0.44 0.41 0.460

    Composition given in w/w %.As to boron, at mass 10 the interference caused by the

    formation of 40Ar4+ occurs. According to the sample matrix80Kr8+, 40Ca4+ and 30Si3+ are also formed. The necessary

    Table 8

    RSFs obtained for nuclear waste glasses with respect to silicon

    Elements Ref. [9] Refer. values [76] Elements Ref. [9]

    Si 1.000 1.000 Cs 0.517

    B 0.662 0.381 Eu 5.964

    Al 0.192 0.621 Gd 0.027

    Na 0.405 1.051 La 0.038

    Mg 0.777 0.953 Nd 0.043

    Ca 0.236 0.502 Ni 0.195

    Ti 0.144 0.345 Mn 1.240

    Ba 0.229 1.621 Mo 0.193

    Ce 0.080 nr Pd 0.093

    Cr 0.122 nr Pr 0.040

    nr: Not reported.mass resolution to separate 10B from all these interferences

    is equal to 500. As for lithium interferences on 6Li+ and 7Li+

    can arise from 12C2+ and 14N2+, respectively. Also, in these

    cases, the minimum resolution necessary for separation is

    500. Double-focusing GDMS was compared with TIMS

    [21]. Several samples of nuclear origin containing either

    lithium or boron were examined for their isotopic compo-

    sition. The two methods always gave results in good


    Chartier and Tabarant [26] have also performed

    isotopic measurements directly on solid samples of ZrB

    by HR-GDMS. By this way they could resolve the

    interference 40Ar4+ from 10B+, that was not possible by a

    quadrupole GDMS. The authors compared the accuracy

    and the precision of the HR-GDMS based method with

    those figures obtained by TIMS as reported in Table 9.

    The instrumental mass discrimination determined for

    HR-GDMS was 1.4%; the internal precision obtained

    varied from 0.2% to 0.5%. The external precision was

    of 0.3%.

    Silicon isotopic composition in aluminium matrices has

    been also determined by GDMS [21]. Aluminium is a

    0.90 0.87 0.912material used in nuclear technology. After reactor

    operations the isotopic composition of natural silicon

    contained in aluminium changes. The determination of

    Refer. values [76] Elements Ref. [9] Refer. values [76]

    nr Rb 6.590 2.720

    nr Rh 17.681 nr

    nr Ru 28.018 nr

    nr Sm 0.038 nr

    nr Sr 0.201 0.854

    0.914 Te 0.361 nr

    0.878 Y 0.003 nr

    nr Zr 0.017 0.282

    nr Pu 29.461 nr

    nr U 0.275 nr

  • rochimM. Betti, L. Aldave de las Heras / Spectsilicon cannot be accomplished by the worldwide diffused

    quadrupole-based Inductively Coupled Plasma Mass

    Spectrometers (ICP-MS). The resolution required for the

    separation of 28Si+ from 12C16O+ and 14N2+ is 1625 and

    957, respectively. Moreover, in aluminium matrix the tail

    of Al and the formation of 27AlH+ produce also

    interferences on 28Si+ that cannot easily resolved. The

    other two isotopes of silicon have a low abundance:

    Fig. 5. Comparison of RSFs values obtained for diffica Acta Part B 59 (2004) 13591376 13694.67% for 29Si and 3.10% for 30Si, and a high detection

    power must be reached to obtain accurate isotopic

    compositions. The dissolution of those type of sample is

    tedious and sometimes, due to presence of other metals,

    incomplete. Double-focusing GDMS can be used success-

    fully for the measurement of silicon isotopic abundance.

    On 28Si+ the interferences due to formation of 27AlH+,12C16O+ and 14N2

    + can be easily resolved as well as those

    erent matrices: (a) BZn; (b) GeY; (c) ZrU.

  • produces a population of gaseous ions directly amenable

    to isotopic analysis by a mass spectrometer. Neutrals are

    also produced that are amenable for detection in a variety

    Table 910B/11B ratio in different ZrB samples [25]

    Sample TIMS HR GDMS D%

    1 0.2523 0.2509 0.52 0.2520 0.2511 0.33 0.2528 0.2513 0.64 0.2528 0.2525 0.1

    M. Betti, L. Aldave de las Heras / Spectrochim1370interferences due to 28SiH+, 27AlH2+ and 12C16OH+ on

    29Si+ and that due to 14N16O+ on 30Si+. 27AlH+ comes

    together with 12C16O+, as well as the 27AlH2+ with

    12C16OH+. 27AlH3+ should appear at mass 30.005, but

    no evidences for its formation appeared. Based on results,28SiH+ and 14N16O+ appear to be well separated from29Si+ and 30Si+, respectively. Five aluminium samples

    containing traces of silicon were measured using a mass

    resolution of 2000, and for each sample 10 separate runs

    were carried out. In Table 10, the mean values obtained

    for the three isotopes are reported along with the external

    precision (expressed as RSD%) and the accuracy

    (expressed as bias %) calculated on the basis of data

    reported in literature [77]. As seen both the figures of

    merit for the method are good. On the basis of standard

    samples prepared in-house, the relative sensitive factors

    (RSFs) relative to Al were also calculated for the three

    isotopes, and these were found to be 1.235 for 28Si, 1.257

    for 29Si and 1.248 for 30Si. Other authors [78] reported an

    RSF value of 2.0F0.3 for Si in a steel matrix whencalculated with respect to Fe. If this value is converted to

    obtain the RSF with respect to Al, a value of 1.0 results,

    which is in reasonable agreement with those found in this


    Many samples containing uranium with different isotopic

    composition have been measured in our laboratory. An

    example is given in Table 11, where ratios of the results

    obtained by GDMS and TIMS for the four isotopes of

    uranium, including the non-natural 236U, in two samples of

    uranium with different enrichment are reported.

    Moreover, values regarding the external precision of the

    methods are given and are expressed as RSD%. In both

    samples, 236U and 234U were present at ng/g levels. The

    ratio between the highly enriched sample (indicated as

    5 0.2521 0.2540 +0.7

    6 0.2521 0.2531 +0.4sample 1 in Table 11) and the low enriched one (indicated as

    sample 2 in Table 11) had a value of 10. The results

    obtained by GDMS were always in good agreement,

    Table 10

    Isotopic composition of silicon in an aluminium matrix [21]

    Isotope Exp. value RSD% Ref. value [81] Bias %

    28Si 92.22 0.05 92.23 0.0129Si 4.68 0.28 4.67 +0.2130Si 3.096 1.1 3.10 0.13independently of the sample enrichment, with those

    obtained by TIMS. The advantage of the use of GDMS is

    omission of the time-consuming dissolution and dilution


    As for the determination of the isotopic composition

    of Pu, corrections for 238U and 241Am are necessary. The238U isotope interferes with 238Pu and at mass 241, and

    there is interference due to the 241Am isotope, which is

    to some extentalways present in samples containing

    plutonium. Mathematical corrections on 238Pu and 241Am

    can be made when results are available from other

    techniques such as Multi-Group Analysis (MGA) [79],

    which consists of a gamma-ray spectrum analysis code

    for determining plutonium isotopic abundance. When

    corrections can be performed, GDMS results were in

    agreement with those obtained by TIMS for which

    americium and uranium have been previously chemically

    separated from plutonium. For plutonium, the uncorrected

    data from GDMS is complementary to non-destructive

    techniques, such as MGA, when no sample dissolution

    can be performed.

    Glow discharge optogalvanic spectroscopy has been

    investigated for the determination of uranium and actinide

    isotope ratios [4,80,81]. In these investigations, a hollow

    cathode glow discharge has been coupled with a tunable

    laser for isotopically selective excitation of gaseous

    uranium atoms produced by cathodic sputtering. In a

    glow discharge, the cathode bombarded by energetic ions

    Table 11

    Comparison between TIMS and GDMS results for the determination of

    isotopic composition of two uranium samples having different enrichment


    Isotope Sample 1,



    Sample 2,







    234Ua 0.9045 0.9125 1.000 0.500235Ub 0.9999 0.9939 1.200 0.020236Ua 0.8997 0.8896 1.100 2.420238U 1.0004 0.9999 0.050 0.002

    a ng/g levels.b Ratio (sample 1/sample 2)=10.

    ica Acta Part B 59 (2004) 13591376of other techniques, like absorption, emission, and laser-

    induced fluorescence spectroscopy. In the investigation

    performed by Young et al. [80], the sputtered atoms are

    resonantly excited to a higher lying energy level, from

    which they may be ionised by collision. The change in

    the discharge voltage brought about by disruption of the

    ionisation equilibrium serves as an indicator of optical

    absorption and is the basis of optogalvanic signal

    collection. The instrumentation developed had, as a final

    goal, to be applied in the field. For this application,

    accuracy and precision at a comparable level to the

    results obtained with a laboratory mass spectrometer are

  • rochimgenerally not required. However, sensitivity to changes of

    the isotopic composition, free from interferences, is of

    high importance. The authors evaluated the measurement

    precision and accuracy uranium isotopic composition and

    found that for depleted uranium the precision of the

    isotope ratios was F15%, and for enriched uranium itwas F3%. The method can be used for a rapid andinexpensive screening and has been applied for the

    determination of uranium isotope ratios in different

    sample matrices as uranium oxide, fluoride and metal [4].

    The increasing interest in isotopic composition studies

    and ultralow-level (abundance below 1010) radionuclidemetrology for a variety of applications in environmental

    and biomedical applications led to the development of a

    continuous-wave resonance ionisation mass spectrometer

    (cw-RIMS) system at the National Institute of Standards

    and Technology [2325]. Resonance ionisation mass

    spectrometry (RIMS) can determine ultratrace levels of

    a single isotope of a particular element in a sample. The

    laser excitation scheme of RIMS increases the isotopic

    selectivity obtained by standard mass spectrometry and

    eliminates isobaric interferences. The NIST cw-RIMS

    consists of three major parts: a sample loading system

    (glow discharge or graphite furnace) as the atom source,

    a continuous wave laser system to ionise the atoms, and

    a mass spectrometer to analyse and detect the resulting

    ions. The apparatus can simultaneously produce copious

    amounts of atoms for analysis while not interfering with

    the high-vacuum conditions required for accurate mass

    spectrometric analysis. A glow discharge method to

    introduce a sample to the cw-RIMS system (GD-RIMS)

    with a magnetic sector analyser has been successfully

    tested on caesium by Pibida et al. [23]. This method

    involved selective laser ionisation, with one-step laser

    excitation scheme. Careful atomic beam collimator and

    alignment of the lasers with respect to the atomic beam

    reduce the Doppler broadening thereby increasing the

    laser ionisation selectivity and ionisation efficiency. The

    overall efficiencythe number of detected ions divided

    by the total number of atoms emittedwas measured to

    be 108; as a whole, the selectivity (the detectable ratiobetween adjacent isotopes based on mass) was estimated

    to be between 109 and 1010, from which a factor of 103

    was estimated to be the enhanced selectivity due to

    single-step laser excitation followed by ionisation. With

    this set-up of the GD-RIMS, the system could be used to

    determine trace amounts of radionuclides, with half-lives

    in the 105 years range, at normal environmental levels

    and with a minimum of chemical preparation.

    Recently, laser ablation-ICPMS has been applied for the

    determination of long-lived radionuclides and their isotopic

    composition [13,82]. The method revealed to be very

    competitive with the GDMS one and it would be worth-

    while to analyse the samples with both techniques for

    M. Betti, L. Aldave de las Heras / Spectvalidation of results for quality control/quality assurance

    purposes.8. Determination of traces of radioisotopes in

    contaminated environmental samples

    The nuclear accident at Chernobyl provided a point

    source for distribution of radionuclides and a unique

    opportunity to trace the mechanisms by which they are

    distributed and accumulated in the food chain and become

    available for human consumption. GDMS has been

    exploited for the determination of traces of uranium in soil

    samples [83]. Caesium, strontium, plutonium, uranium and

    thorium in soils, sediments and vegetation have been also

    investigated [20]. More recently, d.c. GDMS has been

    exploited for the determination of 237Np in Irish Sea

    sediment [22].

    The method applied for the analysis of non-conducting

    samples, using a secondary cathode, has also been

    employed for environmental samples. According to this

    method, the samples (soils, vegetation, and sediments)

    need to be pressed into disc-shaped electrodes. Discs

    obtained without blending with silver powder were found

    to be too fragile. Therefore, 5% of silver powder was

    added to the samples. This concentration was the minimum

    amount necessary to obtain stable disc electrodes without

    too much dilution. Stable discharge was obtained using a

    tantalum secondary cathode during the analysis. The

    combination of the use of blending material and a

    secondary cathode has been used for all environmental

    types of samples mentioned above.

    As for the determination of elements and radioisotopes,

    at the trace level, sensitivity is an important parameter. The

    procedures based on GDMS are mainly affected by

    interferences arising from the sample matrix, the blending

    material and the discharge gas. Interferences from the matrix

    and discharge gas can be eliminated by using a high mass

    resolution. This normally results in a decrease of sensitivity,

    and hence is not suitable for trace elements determinations.

    The blending material (the so-called conductive host matrix)

    can be chosen based on the specific sample requirements, or

    the latter can be neglected.

    For example, in the analysis of traces of Th, the

    addition of silver can be a necessity. Silver produces a

    spectral interference due to the formation of 107Ag109Ag16O+,

    and a mass resolution of at least 1000 is necessary to

    separate the peak of this species from the Th peak. With

    the uranium isotopes, silver does not produce any

    interference. Tantalum, however, gives rise to the formation

    of 180Ta40Ar16O+ and 181Ta40Ar16O+. These two polya-

    tomic ions interfere with 235U+ and 236U+. When working

    at a mass resolution of 300, 180Ta40Ar16O+ can be resolved

    from those of 235U+ as well as 181Ta40Ar16O+ from 236U+.

    For the separation of signal from 180Ta40Ar16O+ and 236U+,

    respectively, a mass resolution of 1700 is necessary. The

    determination of 236U is of great importance because this

    isotope indicates the presence of irradiated uranium in the236 1

    ica Acta Part B 59 (2004) 13591376 1371sample. For U, detection limits in the pg g range or

    even less are often requested. They can be obtained by the

  • Fig. 6. GD mass spectrum for 237Np in a reference soil sample. Reproduced with permission of The Royal Society of Chemistry.

    Fig. 7. (a) GDMS depth profile of a thin layer of ZrO2 deposited on Al. (b) GDMS depth profile for mayor elements and lithium of a thin layer of ZrO2deposited on silicon and doped with lithium. Reproduced with permission of The Royal Society of Chemistry.

    M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 135913761372

  • use of a secondary cathode consisting of gold. In the case

    of 237Np determination, the most important interference is

    due to the formation of 181Ta40Ar16O+ at mass 236.9069,

    and a resolution of at least 1700 is necessary to separate

    this from the 237Np peak at mass 237.0482. In Fig. 6, the

    GD mass spectrum for 237Np and 181Ta40Ar16O+ in a NIST

    Peruvian 237Np-doped soil at a working mass resolution of

    5500 is shown. Neptunium concentration could be deter-

    mined by d.c. GDMS in Irish Sea sediments with a

    detection limit of 80 pg g1. A certified marine sedimentdoped with 237Np was used for the calibration for

    quantitative determination. The 237Np concentration in the

    sediment core layers determined in the low nanogram

    per gram range was in good agreement with the results

    obtained by the measurement of 233Pa by gamma


    9. Depth profiling of ZrO2 layers deposited on Zircaloy

    nuclear fuel cladding material

    Thirty-six percent of the electrical energy produced in

    Europe comes from nuclear power plants [84]. The major

    issues are the competitiveness of nuclear energy com-

    pared with other energy sources, the safety of nuclear

    reactors and the management of nuclear waste. The actual

    One of the major effects encountered is the corrosion of

    the Zircaloy cladding used for containment of the fuel


    In pressurised water reactors (PWR), the lithium hydrox-

    ide added to the primary coolant as alkaliser, in concen-

    trations varying between 2 and 4 Ag g1 depending on thenumber of the reactor cycles [85], may increase the

    oxidation rate of the Zircaloy [86]. In addition, boric acid

    is introduced at a concentration level of 1000 Ag g1 ofboron in the cooling water of western PWR to control the

    core reactivity at the beginning of each reactor cycle. It has

    been found that this additive has a retarding effect on the

    formation of the ZrO2 corrosion layer [87]. In order to

    evaluate the influence of different additives on the corrosion

    induced by the lithium, it is necessary to study their

    incorporation in the layer of ZrO2 formed during reactor


    GDMS has also been used for the study of the

    mechanisms of corrosion of Zircaloy cladding of nuclear

    fuels, measuring the diffusion of the impurities in ZrO2layers by depth profiling [16]. It has been found that the

    use of a secondary cathode is essential. In fact, for a

    depth profile for zirconium oxide layer deposited on

    aluminium, an anomalous peak of zirconium appears

    centred at about 600 s in the metal/oxide interface. This

    behaviour was found to be attributable to the instability

    M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 13591376 1373trend is to increase the burn-up of the nuclear fuel for

    economic reasons and to reduce the production of waste.Fig. 8. Lithium depth profile for a ZrO2 layer obtained in autoclave simulating nuc

    to the corrosive media. Reproduced with permission of The Royal Society of Chof the plasma arising from the change in the electrical

    conductivity of the systemwhen it shifts from thelear reactor operating conditions: (a) without and (b) with addition of boron


  • the sample weight gain has now been corroborated with

    the depth profiles obtained by GDMS. Fig. 8a and b

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    of trace radioisotopes in soil, sediment and vegetation by glow

    rochimcompares the profiles of lithium incorporated in the

    oxide layer after 7 days of reaction in autoclave with

    1000 mg kg1 of Li and 1000 mg kg1 of Li+ 1000 mgkg1 of B, respectively. The depth profiles show areduction of lithium uptake of one order of magnitude

    during corrosion in presence of boron. An interesting

    property noted for boron is that this element does not

    penetrate in the oxide layer, as is evident from Fig. 8b.

    This indicates an external protective effect, e.g., poison-

    ing of adsorption sites or formation of a thin protective

    over layer on the surface. The possible buffering effect

    of the boric acid controlling the pH of the solution

    seems less probable for the simple fact that any other pH

    buffer could also produce the same beneficial effect on

    the oxide growth. Studies on the comparison of the

    GDMS depth profile and cross-longitudinal section

    scanning by SIMS of the samples are currently underway

    (in the laboratory) to obtain independent results on the

    impurities migration mechanisms inside the zirconium

    oxide layer.

    10. Conclusions

    GD-based techniques have been shown to be of great

    use for bulk and depth profiling measurements in the case

    of conductive as well as non-conductive samples, with an

    unrivalled flexibility of applications. In particular, GDMS

    has been successfully applied to the characterization of

    samples of nuclear concern. Its non-destructive nature and

    the fast sample preparation make the technique very

    attractive for the characterization of radioactive samples.

    Moreover, major, minor and trace elements can be

    determined in the same analysis. GDMS has been

    demonstrated to be competitive with TIMS for determi-

    nation of the isotopic compositions of matrix elements.

    The method can also be used for the determination ofinsulating nature of the oxide layer to the conductive

    oxide/metal interface. This problem was overcome by

    applying a secondary cathode. In these experimental

    conditions, three well-defined zones are shown in Fig.

    7a were observed: the oxide layer (01100 s), the metal

    oxide interface (11001700 s) and the metallic support

    (17003000 s). The profile for lithium was also

    registered. Fig. 7b shows that a fairly constant concen-

    tration of Li is found in the oxide layer, then it decreases

    in the interface zone and goes down to zero in the

    metallic phase.

    By GDMS depth profiling, the effect of B on Li

    diffusion has been also studied. The influence of boron

    solution additions considerably reduces the oxidation of

    Zircaloy induced by lithium [16]. The large reduction of

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    Glow discharge spectrometry for the characterization of nuclear and radioactively contaminated environmental samplesIntroductionGeneral processes and instrumentationInstrumentation requirements for radioactive samplesQuantificationTrace and bulk analysis in nuclear samplesConducting nuclear samplesMetallic alloy nuclear fuelsZircaloy cladding materials

    Non-conducting nuclear samples

    SimfuelsNuclear waste glassesDetermination of the isotopic composition in samples of nuclear interest

    Determination of traces of radioisotopes in contaminated environmental samplesDepth profiling of ZrO2 layers deposited on Zircaloy nuclear fuel cladding materialConclusionsReferences


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