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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

* Correspon

E-mail addr

59 (2004) 1359–1376

ee front matter D 2004 Elsevier B.V. All rights reserved.

b.2004.07.006

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

ess: betti@itu.fzk.de (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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