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Spectrochimica Acta Part B
Review
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
Abstract
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
discussed.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Glow discharge spectrometry; Contaminants; Radionuclides
Contents
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1365
0584-8547/$ - s
doi:10.1016/j.sa
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59 (2004) 1359–1376
ee front matter D 2004 Elsevier B.V. All rights reserved.
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ding author. Tel.: +49 7247 951 363; fax: +49 7247 951 186.
ess: [email protected] (M. Betti).
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–13761360
6. Simfuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1366
7. Nuclear waste glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1367
7.1. Determination of the isotopic composition in samples of nuclear interest . . . . . . . . . . . . . . . . . . . . . . . 1368
8. Determination of traces of radioisotopes in contaminated environmental samples . . . . . . . . . . . . . . . . . . . . . . . 1371
9. Depth profiling of ZrO2 layers deposited on Zircaloy nuclear fuel cladding material . . . . . . . . . . . . . . . . . . . . . 1373
10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1374
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1374
Table 1
Type of radioactive samples
Oxide-based nuclear fuels
Metallic alloy nuclear fuels
Vitrified wastes
Zircaloy nuclear cladding materials
Forensic
Environmental
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 earth’s crust.
For instance, burning of fossil fuel (oil and coal) dominates
direct atmospheric release at pre-existing natural radio-
activity.
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
traces, impurities and depth profiling of solids [1–3]. Glow
Discharge Optical Emission Spectrometry (GDOES) is
recognised to be a rapid method for depth profiling, capable
of surface analysis [4–7], 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 [9–11]. Glow
discharge mass spectrometers, which are commercially
available with fast and sensitive electrical ion detection,
allow direct trace elemental determination in solid materials
with good sensitivity and precision in the concentration
range lower than ng g�1 [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 [9,14–17]. 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
[9,20–26]. 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 isotopic
composition, is of great importance. These materials can
be analysed using several techniques based on nuclear and
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) 1359–1376 1361
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
advantages 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 proc
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 0.1–1 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 gas
breakdown electricallyQ, namely, the gas is being split up
into 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
formed 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. [34–36]. 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 are
esses occurring in a glow discharge.
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–13761362
then, electrically detected by ion/electron/photon conversion
by ion counting and a Faraday cup for analog ion beam
measurement.
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 Mattauch–Herzog double focusing sector
spectrometer [46]. However, the commercially available
GD mass spectrometers presently only employ inverse
Nier–Johnson double-focusing and quadrupole-based mass
spectrometers.
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-
mation of new disturbing molecular ions (argides, oxides
from the secondary cathode or binder material) in mass
spectra 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
[12,13,52–56], 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-
tions with such instrumentation have been recently
reviewed [62].
As 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
future.
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, a
schematic diagram of the installation of the GD source
housing in the frame of the glove-box is given. The
Fig. 2. Schematic diagram of the installation of a GDMS in a glove-box and of the modified discharge cell.
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–1376 1363
installation 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 into
various components comprising a measurable plate with
removable cell and focus stack assemblies. The source
mounting position remains fixed to the back wall of the
source’s 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 copper–beryllium 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
containing 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
holder.
The buniversalQ cell has been designed to accommodate a
large 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
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–13761364
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
120–180 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
window, through which the radiation of the GD sources
can be observed. The main beam enters the optical
detection 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 g�1) in nuclear
materials such as uranium, plutonium or mixed uranium and
plutonium oxides with relative standard deviation less than
5% at 20 Ag g�1. The authors, from the calibration graphs
obtained in non-nuclear samples for all carbon, nitrogen and
oxygen, calculated the limits of detections, at 3 sigma level,
of: 10 Ag g�1; 40 Ag g�1 and 20 Ag g�1 for carbon, nitrogen
and 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.
Screening data can be obtained by GDMS even when
reference materials are not available. A simple compar-
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
discussed.
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
total uranium and plutonium and Thermal Ionisation Mass
Spectrometry (TIMS) and Inductively Coupled Plasma
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.
GDMS was, therefore, the appropriate technique to be used
for the determination of the chemical composition of these
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.
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–1376 1365
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
for 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 each c
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)O2 17.9 –
(U,Pu,Np)O2 13.8 46.1
agents [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 with
oxygen 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
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.
omponent [9]
Plutonium Cerium Neptunium
– – –
– – –
88.2 – –
38.9 – –
23.3 – –
12.4 – –
4.5 – –
35 45 –
35 – 4.1
Table 4
RSF values for uranium and plutonium dioxide using different metals as hostmatrices and secondary cathode [9]
Host matrix Secondary cathode
Ag Ti Ta Ta
UO2 PuO2 UO2 PuO2 UO2 PuO2 UO2 PuO2
11B 1.25F0.30 1.33F0.25 0.98F0.20 1.04F0.30 0.97F0.20 1.05F0.20 0.98F0.30 0.98F0.287Li 2.01F0.23 1.98F0.30 1.12F0.13 1.08F0.20 1.15F0.28 1.12F0.25 1.13F0.20 1.10F0.20114Cd 1.97F0.30 2.00F0.28 0.87F0.20 0.90F0.25 0.90F0.28 0.88F0.25 0.90F0.25 0.93F0.2869Ga 1.58F0.15 1.63F0.13 0.95F0.10 0.98F0.12 0.95F0.12 0.95F0.13 0.98F0.10 1.02F0.10
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–13761366
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
characterized 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 5–10 Agg�1, an analytical precision higher than 10% RSD was
obtained. 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 Agg�1 level, as shown in Table 5, when using an integration
time 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
of 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 matrix-specific and standard RSF [15]
Element Certified value
(Ag g�1)
Matrix-specific RSF
(Ag g�1)
Bias* (%) RSD (%)
n=6
Standard RSF
(Ag g�1)
Bias* (%) Detection limit
(Ag g�1)
Ag 10.4F1.6 10.2F1.3 1.9 12.1 9.3F2.3 10.8 0.1
Al 99F6 87F5 12.1 5.5 87F3 11.9 0.5
Ba 3.8F1.6 3.5F1.5 7.9 40.8 2.5F0.8 35.5 0.2
Ba 9.6F0.4 b 11.4F0.3 �18.8 0.7
Bea 5.4F0.6 3.8F0.4 29.6 10 b 0.5
Bi 24.4F1.9 20.9F1.7 14.3 7.7 41F3 �68 0.6
Caa 93F8 94F9 �1.1 9.1 95.8F4.2 0.4
Cd 4.9F0.7 5F0.4 �2 7.6 3.4F1 30.6 0.5
Co 9.8F2 11.1F0.8 �13.3 6.9 9.5F0.3 1.3
Cr 99F2 102F5 �3 4.7 94F11 4.7 1.9
Cu 50.2F1 52.1F3.3 �3.8 6 63F7 �25.6 0.6
Dy 0.5F0.06 c 0.7
Eu 0.52F0.03 c 0.5
Fe 211.6F6.5 207.2F10.8 2.1 5 313F22 �47.9 2.4
Gd 0.56F0.06 c 0.9
In 9.4F1 10.4F0.5 �10.6 4.6 8.1F0.3 14.3 1
Mg 19.3F1.5 19.4F1.6 �0.5 7.9 12.2F1 36.8 0.1
Mn 24.5F0.5 29.3F1.1 �19.6 3.6 30F1 �22.4 1.4
Mo 147F5 144F9 2 6 175F11 �19 0.9
Ni 147F3 142F4 3.4 2.7 143F25 2.7 6.2
Pb 101F3 103F9 �2 8.3 111F7 �9.9 0.4
Sia 100F8 93F6 6.1 245F11 �145 0.1
Sm 0.5F0.12 c 0.9
Sn 18.5F5.6 20.8F3 �12.4 13.7 15.3F4.6 17.3 0.4
Th 6.2F0.8 b 0.4
Ti 49.2F2.6 48.6F8 1.2 15.7 b 1.4
V 48.7F2.8 47F2 3.5 4.1 50F1 �2.6 0.7
W 100F9 106F11 �6 9.9 95F3 4.8 2.1
Zn 98.6F5.5 102F10 �3.4 9.3 148F8 �50 0.8
Zr 59.9F4.1 64F7 �6.8 10.4 b 0.9
*Bias (%)=(certified value�GDMS value)�100/certified value.
a: Possible interferences: 9Be: 36Ar4+; 10,11B: 40Ar4+H,40Ar4+; 40,41,42,43,44Ca: 40Ar+,40ArH+, 12C14N16O+, 12C16O2+; 28,29,30Si: 56Fe2+, 27AlH+, 14N2+, 12C14N,
12C16OH+, 14N16O+.b: Not determined. c: Below detection limit.
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–1376 1367
Table 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
Table 6
Mean concentration (mg kg�1 PuO2) and intervalsof confidence (%)
referring to the 95% confidence level (n=25) [15]
Element/isotope GDMS ICP-MS
(mg kg�1) IC (%) (mg kg�1) 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 7.6
reported 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.
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
Nd2O3 0.44 0.41 0.460 0.90 0.87 0.912
Composition given in w/w %.
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–13761368
7.1. 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 g�1 levels is
easily achievable.
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
agreement.
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
material 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
Fig. 5. Comparison of RSFs values obtained for different matrices: (a) B–Zn; (b) Ge–Y; (c) Zr–U.
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–1376 1369
silicon 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:
4.67% 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
Table 910B/11B ratio in different ZrB samples [25]
Sample TIMS HR GDMS D%
1 0.2523 0.2509 �0.5
2 0.2520 0.2511 �0.3
3 0.2528 0.2513 �0.6
4 0.2528 0.2525 �0.1
5 0.2521 0.2540 +0.7
6 0.2521 0.2531 +0.4
Table 11
Comparison between TIMS and GDMS results for the determination of
isotopic composition of two uranium samples having different enrichment
[21]
Isotope Sample 1,
high
enrichment
Sample 2,
low
enrichment
RSD%
GDMS
RSD%
TIMS
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.
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–13761370
interferences 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 when
calculated 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
investigation.
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
sample 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 �0.13
independently 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
steps.
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 extent—always 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
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
of 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
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–1376 1371
generally 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 it
was F3%. The method can be used for a rapid and
inexpensive 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 10�10) radionuclide
metrology 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 [23–25]. 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 efficiency—the number of detected ions divided
by the total number of atoms emitted—was measured to
be 10�8; as a whole, the selectivity (the detectable ratio
between 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
validation 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 the
sample. For 236U, detection limits in the pg g�1 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 ZrO2
deposited 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) 1359–13761372
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–1376 1373
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 g�1. A certified marine sediment
doped 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
spectrometry.
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
trend 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 Ch
One of the major effects encountered is the corrosion of
the Zircaloy cladding used for containment of the fuel
pellets.
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 g�1 depending on the
number 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 g�1 of
boron 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
operation.
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 ZrO2
layers 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
of the plasma arising from the change in the electrical
conductivity of the system—when it shifts from the
lear reactor operating conditions: (a) without and (b) with addition of boron
emistry.
M. Betti, L. Aldave de las Heras / Spectrochimica Acta Part B 59 (2004) 1359–13761374
insulating 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 (0–1100 s), the metal
oxide interface (1100–1700 s) and the metallic support
(1700–3000 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
the sample weight gain has now been corroborated with
the depth profiles obtained by GDMS. Fig. 8a and b
compares the profiles of lithium incorporated in the
oxide layer after 7 days of reaction in autoclave with
1000 mg kg�1 of Li and 1000 mg kg�1 of Li+ 1000 mg
kg�1 of B, respectively. The depth profiles show a
reduction 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 of
traces of radioisotopes in environmental matrices, exploit-
ing the technique of the secondary cathode. Mechanisms of
the diffusion of trace elements can be followed by depth
profiling analysis.
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