- Email: [email protected]
Determination of long-lived radionuclides in concrete matrix by laser ablation inductively coupled plasma mass spectrometry
SPECTROCHIMICA ACTA PART B
ELSEVIER
Spectrochimica Acta Part B 52 (1997) 2051-2059
Determination of long-lived radionuclides in concrete matrix by laser ablation inductively coupled plasma mass spectrometry 1 M. Gastel 2, J.S. Becker*, G. Kiippers, H.-J. Dietze Zentralabteilung 3~urChemische Analysen, Forschungszentrum Jiilich GmbH, 52425 Jiilich, Germany
Received 23 May 1997; revised 30 September 1997; accepted 3 October 1997
Abstract
A laser ablation system using a Nd:YAG laser was coupled both to a quadrupole inductively coupled plasma (ICP) mass spectrometer and to a double-focusing sector field ICP mass spectrometer. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was applied for the determination of long-lived radionuclides in a concrete matrix. The investigated samples were two laboratory standards with a concrete matrix, which we doped with different long-lived radionuclides (e.g. 99Tc, 232Th, 233U, 237Np) from the ng g-l to /~g g-~ concentration range and an undoped concrete material (blank). Detection limits for long-lived radionuclides in the 10 ng g-J range are reached for LA-ICP-MS using the quadrupole mass spectrometer. With double-focusing sector field ICP-MS, the limits of detection are in general one order of magnitude lower and reach the sub ng g- ~range for 233Uand 237Np. A comparison of mass spectrometric results with those of neutron activation analysis on undoped concrete sample indicates that a semiquantitative determination of the concentrations of the minor and trace elements in the concrete matrix is possible with LA-ICP-MS without using a standard reference material. © 1997 Elsevier Science B.V. Keywords: Concrete matrix; Double-focusing sector field ICP-MS; Laboratory standards; Laser ablation ICP-MS; Long-lived radionuclides; Quadrupole ICP-MS
1. I n t r o d u c t i o n
The trace and ultratrace analysis of long-lived radionuclides in environmental or radioactive waste samples is an interesting analytical topic. The determination of long-lived radionuclides is important in radioactive waste from nuclear reactors for recycling and final storage of radioactive waste. In many countries, specifications have been drawn up to limit the Presented at the 1997 Winter Conference on Plasma Spectrochemistry, 12-17 January 1997, Gent, Belgium. 2 Present address: Technische Hochschule Darmstadt, Fachbereich Materialwissenschaft,Fachgebiet Chemische Analytik, 64287 Darmstadt, Germany. * Author to whom correspondence should be addressed.
activity of radioactive waste packages that may be transferred to disposal facilities. The content of around 60 radionuclides has to be specified in the sense that there are different activity limits for the nuclides that must not be exceeded. A fast and reliable analysis - - especially on solid waste material - - is required for the determination o f these nuclides up to the n g g -l concentration range. Radioanalytical methods are well suited for the measurement of 7ray-emitting nuclides but for long-lived ct- and /3ray-emitting nuclides mostly time-consuming radiochemical separation procedures are necessary. Inductively coupled plasma mass spectrometry (ICPMS) as a powerful trace, ultratrace and isotopic analytical method has been used successfully for the
0584-8547/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0584-8547(97)001 I 1-0
2052
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
determination of long-lived radionuclides in aqueous solutions [1-5]. Kim et al. [1] determined the detection limits for the long-lived radionuclides 99Tc, 226Ra, 232Th, 237Np, 238U, 239pu and 24°pu with half-lives of 10 3-1010 years in aqueous standard solutions using a double-focusing sector field ICP-MS ('PlasmaTrace', Fisons) coupled with an ultrasonic nebuliser and obtained detection limits ranging from 2 to 20 pg 1-I. The determination of long-lived radionuclides in radioactive waste materials was mostly performed in aqueous solutions, especially in combination with special sample introduction systems, e.g. for the determination of 129I [5] or 795e [6], using ion chromatography [7], liquid-liquid extraction [8] or electrothermal vaporisation [9]. For a characterisation of solid nuclear waste, a fast direct method for the analysis of solids without any sample preparation is required. The matrix of the radioactive solid waste is often concrete, therefore the analytical method must be capable of analysing even non-conducting samples. The mass spectrometric methods capable of direct analysis of trace elements in non-conducting solids include LIMS ]10], radiofrequency GDMS [11] and laser ablation ICP-MS (LA-ICP-MS). This work focuses on LAICP-MS, which is a well-established method for the analysis of geological materials with detection limits in the 10 ng g-J concentration range [12-14] and has also proven its applicability for the determination of trace elements in ceramic materials [ 15,16]. Until now no investigations have been published on the determination of radionuclides in radioactive waste products with LA-ICP-MS. The main problem in the quantification of analytical results is that no suitable standard reference materials are available. Therefore, synthetic laboratory standards were prepared for the determination of long-lived radionuclides, as described in Ref. [17]. The aim of this work is to demonstrate the capability and the limits of LA-ICP-MS for measurements on long-lived radionuclides (99Tc, 129I, 232Th, 233U, 235U, 237Np, 238U) in concrete matrix, which is a very common matrix in waste packages. Of special interest are the limits of detection of long-lived radionuclides, which are compared for two different types of mass spectrometer (a quadrupole and a doublefocusing sector field ICP-MS) coupled to the laser ablation system.
2. Experimental 2.1. I n s t r u m e n t a t i o n
The laser ablation system, based on a Nd:YAG laser, used for LA-ICP-MS has been developed in our laboratory for the analysis of non-conducting sampies, such as ceramic materials [15,16]. It was coupled either to a quadrupole ICP mass spectrometer (SCIEX-Elan 5000 from Perkin-Elmer) or to a double-focusing sector field ICP mass spectrometer (ELEMENT from Finnigan MAT). Fig. 1 shows the experimental set-up of both LA-ICP-MS configurations. The experimental parameters for the LA-ICPMS, which were optimised in order to yield maximum ion intensities of analytes, are summarised in Table 1. The whole system is computer-controlled and allows different measurement programs to be run automatically. In order to avoid difficulties due to local inhomogeneities of the samples, the laser beam is rastered over the surface by moving the whole laser ablation chamber constructed at our laboratory by using two step motors. Activity measurements of the -y-ray emitters - - for neutron activation analysis - - were performed with a high-resolution -y-ray spectrometer system including a high-purity germanium detector (Ortec), multichannel analyser from Nuclear Data and a Micro-VAX workstation for data processing. For the evaluation of the "r-ray spectra, a peakfit procedure from Nuclear Data was applied. The activities of the c~- and fl-ray emitters were measured with the Tri-Carb 2200CA Liquid Scintillation Analyzer (LSC) from the CanberraPackard Instrument Company. For the fl-ray emitters the 'efficiency tracing mode', which is provided with the instrument, was used to obtain absolute decay rates. The a-ray emitters were also measured with the LSC where the counting efficiency was nearly 100% [18,19]. 2.2. I r r a d i a t i o n
Irradiations for neutron activation analysis to determine the blanks of 232Th and 238U of the concrete matrix were carried out in the FRJ-2 (heavy-watermoderated reactor, Research Center Jiilich, Germany). Samples prepared from about 200 mg were irradiated for 3 h at a thermal neutron flux of
2053
M. Gastel et aL/Spectrochimica Acta Part B 52 (1997) 2051-2059
•, - , ,
J INet ion opU~t,
SEM
/
L
n
.............
/ [Nd:YAa4uer "
J I I "),., M2, ~4
ICP
quadmpole J
beam divkler focus lens
ion optics .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
i~: ,, ~
laser ablation chamber
.
exit =llt t~lctroliti~c analyzer
Fig. 1. Schematic diagram of the experimental set-up of LA-ICP-MS using a quadrupole ICP-MS and a double-focusing sector field ICP-MS.
1 × 10 13 n cm -2 s -1. Together with the samples a zirconium wire of about 3 mg was irradiated as a monitor for the thermal and epithermal neutron flux.
long-lived radionuclides in all three samples (standard 1, standard 2 and concrete blank sample) as they are known from sample preparation or determined by neutron activation analysis (NAA).
2.3. Investigated samples 2.4. Preparation of the concrete laboratory standards The investigated samples are the two synthetic laboratory standard samples with concrete matrix and known concentrations of some long-lived radionuclides (sample 1 and 2) and a blank sample with the same matrix but without doping of radionuclides. The blank sample also contains 23~Fh, 235U and 238U because these nuclides are present as trace elements in the concrete matrix. The concrete standard 1 was prepared as low level standard in respect to the activity levels to check radiometric methods for activity measurements (c~spectrometry and f3-counting). In concrete laboratory standard 2 higher concentrations of the long-lived nuclides are doped. Both standard samples are suitable to check the performance of LA-ICP-MS. Table 2 s u m m a r i s e s the concentrations of
The preparation of the concrete laboratory standard 1 for some selected long-lived radionuclides, especially intended for the control of radioanalytical separations and activity measurements, is described in Ref. [17]. In concrete laboratory standard 2, further selected nuclides were added. 99Tc, 129I, 232Th, 233U and 237Np were chosen which have very long halflives (see Table 2). Such long-lived radionuclides have low specific activities and their measurements with radioanalytical methods are correspondingly insensitive but they should be particularly suitable for analysis by mass spectrometry. The concrete laboratory standard 2 was prepared by mixing calibrated solutions of the nuclides with cement powder. The solutions of the /3-ray-emitting
2054
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
Table 1 Experimental parameters of the LA-ICP-MS Laser ablation
Nd:YAG laser Wavelength Pulse width Repetition frequency Pulse energy Laser power density Spot diameter Raster width Ion detection
(Surlite 1, Continuum, Santa Clara, USA) 266 nm (4th harmonic) 5 ns 10-20 Hz 15 mJ ~ 10 ~°W c m -2 ~ 100 #m 3 mm × 3 mm SEM
Ablation cell and connection tube to the ICP-MS torch
Tube dimension Ablation cell dimensions
4 mm id, 2 m length 34 mm id, 60 mm high
ICP-MS
Quadrupole ICP-MS SCIEX-Elan 5000, Perkin-Elmer 1060 W 5 ms 50 0.9 1 rain ~Ar 13.5 I min ~ 0.7 1 rain-t Peak hop transient 6-240 u 300
R.f. power Dwell time No. of replicates Carrier gas flow rate Coolant gas flow rate Auxiliary gas flow rate Scanning mode Mass range Mass resolution (m/Am)
n u c l i d e s 99Tc a n d 129I w e r e p r e p a r e d f r o m l a b o r a t o r y stock solutions, o b t a i n e d b y the d i l u t i o n o f s a m p l e s purchased from Amersham-Buchler, Braunschweig, Germany. They were calibrated by liquid scintillation c o u n t i n g (LSC), u s i n g the e f f i c i e n c y t r a c i n g m o d e . T h e s o l u t i o n s o f the a - r a y - e m i t t i n g n u c l i d e s 233U a n d 237Np w e r e c a l i b r a t e d w i t h the L S C w h e r e the c o u n t i n g e f f i c i e n c y w a s n e a r l y 100%. 1 0 0 m g o f e a c h n u c l i d e s o l u t i o n w a s m i x e d w i t h 2 0 m l o f the L S C cocktail ' B a k e r A n a l y z e d ' ( B a k e r ) . T h e p u l s e
Double-focusing sector field ICP-MS ELEMENT, Finnigan MAT 1250 W 5 ms 50 1.0 1 min i Ar 13.5 1 min-i 0.7 1 minPeak hop transient 6-240 u 300
h e i g h t s p e c t r a w e r e e v a l u a t e d as d e s c r i b e d in Ref. [17]. O n e o f t h e a - r a y e m i t t e r s , 232Th, h a s many relatively short-lived a-ray-emitting daughter n u c l i d e s a n d it is t h e r e f o r e difficult or n e a r l y i m p o s s i b l e to d e t e r m i n e the 232Th a c t i v i t y b y L S C . B e c a u s e o f the v e r y l o n g half-life o f 1.41 x 10 l° y e a r s a n d the c o r r e s p o n d i n g l y v e r y l o w specific a c t i v i t y a c a l i b r a t e d s t o c k s o l u t i o n for 232Th c o u l d b e p r e p a r e d b y w e i g h i n g the salt T h ( N O 3 ) a o 5 H 2 0 . T h e activity o f the s o l u t i o n w a s c a l c u l a t e d f r o m t h e
Table 2 Concentrations, specific activities and half-life periods of the radionuclides in the investigated samples Nuclide
99Tc t29I 232Th 233U 235U 237Np
238U
Half-life
Laboratory standard 1
Laboratory standard 2
(years)
Conc. (ng g-t)
Spec. activity (Bq g-~)
Conc. (ng g -I)
Spec. activity (Bq g-i)
2.13 × 10 5 1.57 x 1 0 7 1.4 x 101° 1.6 x 105 7.0 x 108 2.14 x 1 0 6 5.4 X 1 0 9
28.5 _+ 0.9 not added 1800 + 180 51 ± 1 5 ± 0.5 628 _+ 15 730 ± 73
17.9 ± 0.6 not added 0.007 ± 0.0007 18.3 - 0.4 0.0004 ± 0.00004 16.4 ± 0,4 0.009 ___0.0009
1400 _+ 70 1700 - 100 6100 _ 400 1040 ± 70 5 ± 0.5 2270 ± 150 730 ± 73
890 ± 50 11.1 ± 1 0.024 - 0.002 369 ± 25 0.0004 + 0.00004 59 ± 5 0.009 ± 0.0009
Undoped concrete :onc. (ng g-i)
0 0 1800 ~ 180 0 5_+0.5 0 730 -- 73
2055
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
for 232Th+ and 238U+ (with concentrations of 1.8 #g g-J for Th and 0.73 #g g-~ for U - - determined by NAA). The uranium with a natural isotopic composition of 235U/238U (measured isotopic ratio, 7.2 x 10 -3) was found. The precision of isotopic ratio measurements on uranium using a double-focusing sector field LA-ICP-MS with single ion detection is about _+ 5% (relative standard deviation). The use of a multiple collector system in double-focusing LAICP-MS ('Plasma 54' prototype) - - as demonstrated for the determination of 2°6pb/2°4pb in glass by Walder et al. [20] - - allows isotopic ratio measurements at precision levels down to 0.1% (relative standard deviation). Furthermore, the mass spectrum shown in Fig. 2 shows the detection of the long-lived radionuclides (233U+, 234U+, 237Np+ and 232Th+) which were added to the laboratory concrete standard 2. The determination of long-lived radionuclides can be disturbed by isobaric interferences with molecular ions. In Table 3, the measured relative hydride and oxide ion intensities - - which were measured in the 10 -3 range with respect to their atomic ion intensity - - and interferences with different possible actinide ions in waste materials are summarised. (Some of the actinides mentioned in Table 3 are not doped in the concrete matrix, but might as well occur in real radioactive waste samples.) Due to the relatively low molecular formation rate in LA-ICP-MS,
concentration of thorium. The concentration of Th was checked by NAA. Aliquots of the calibrated solutions were poured together and then immediately mixed with cement powder. A ready-to-use mixture of fine-grained cement ('Fugenzement', Ultrament, Essen, Germany) was used. Fine-grained material was used in order to attain good homogeneity of the standard material. The mixture was vigorously stirred and kneaded and cement powder was added until the paste had a consistency that did not exude free solution. The total amount of cement powder used was about 100 g. The paste was filled into small stainless steel dishes (20 mm in diameter and 5 mm in height) to produce easy-to-use samples. The material was exposed to the atmosphere for 4 weeks to harden and dry. When the weight was constant, the exact nuclide concentrations and their specific activities were calculated and the standard was ready for analysis.
3. R e s u l t s
and discussion
Part of the mass spectra of the concrete laboratory standard 2 (solid lines) and the concrete blank sample (broken lines) using the double-focusing sector field LA-ICP-MS are compared in Fig. 2. In the undoped concrete matrix (sample 3) ion intensities in the 10000cps range were measured 1000000 --
232Th237Np*
233U+ 100000 D. 0
238U÷
S"I
10000
C
.=
1000
234U*
_o
235
U+
100
10 ' 230
Z-~UH+
A 232
234
236
238
240
m/z
Fig. 2. Part of the mass spectrum of laboratory standard 2 (solid lines) and the concrete blank sample (broken lines) using the double-focusing sector field LA-ICP-MS.
2056
M. Gastel et al./Spectrochimica Aeta Part B 52 (1997) 2051-2059
Table 3 Relative intensities of molecular ions of long-lived radionuclides in LA-ICP-MS Molecular ion
Relative ion intensity (MX÷/M+; X = O, H)
Possible interference with analyte ion
Half-life of actinide nuclide (years)
232ThtH + 234UIH+ 238UIH+ 232Thl60+
< 9.1 9.1 2.0 9.0 1.1
233U+ 235U+ 239pu+ 248Cm+ 249Cf+
1.6 × 7.0 x 2.4 x 3.4 x 360
233U160+ 237Npl60+
1.6 × 10 -4 × 10 4 × 10 ~' x 10 -3 × 10 4 × 10 -3
isobaric mass interferences of analyte and molecular ions in the investigated samples are not important. Occurring interferences of molecular ions of PbCI + and PbAr + in the mass range of actinide ions in a Pb-containing soil sample digested with an acid mixture containing HCI investigated by doublefocusing mass sector field ICP-MS are discussed in Ref. [21]. 3.1. Limits o f detection o f long-lived radionuclides in concrete matrix
The limits of detection o f 99Tc, 129I, 233U and 237Np were determined from the mass spectrum of the blank sample using the 30 criterium (the limit of detection is given by mb + 3ab, where mb is the mean value of the blank measurements and Ob the standard deviation of five independent measurements of the blank sample). The quantification was done using standards 1 and 2. All mass spectra are averaged over 50 single measurements in order to minimise the influence of plasma instabilities, variations of the amount of ablated material and local inhomogeneities of the samples. Concentrations were evaluated from measured ion intensities of background in the blank sample using the known concentrations of both
10 5 10 8 10 4 105
laboratory standard samples with well known concentrations of long-lived radionuclides. In Table 4 the limits of detection for LA-ICP-MS using the quadrupole ICP-MS and using the doublefocusing sector field ICP-MS are compared and the corresponding specific activities (calculated from the limits of detection) are summarised. Limits of detection in the 10 ng g-~ concentration range are reached using the quadrupole ICP-MS (SCIEX-Elan 5000, Perkin Elmer). The limits of detection for LA-ICPMS using the double-focusing sector field ICP-MS (ELEMENT, Finnigan MAT) are about one order of magnitude lower - - due to higher sensitivity and lower noise - - in comparison with the used quadrupole ICP-MS (SCIEX-Elan 5000, Perkin Elmer). Further investigations of ultratrace analysis of longlived radionuclides on concrete matrix by double focusing LA-ICP-MS (with detection limits up to the 20 pg g ~ concentration level) are described in Ref. [22]. The detection limit for 129I is much higher than those for the other radionuclides due to an isobaric mass interference of 129I+ with 129Xe+, where Xe is a contaminant in the argon carrier and plasma gas. For the determination of 1291in aqueous solution a special sample introduction equipment for ICP-MS was
Table 4 Limits of detection (LoD, determined from the undoped blank sample) and the correspondingspecific activities for radionuclides in concrete matrix, measured with the quadrupole and the double-focusingsector field ICP-MS coupled to the laser ablation system. Nuclide
99Tc 129I 233U 237Np
Quadrupole LA-ICP-MS
Double-focusing sector field LA-ICP-MS
LoD (ng g- ~)
Spec. activity (Bq g i)
LoD (ng g- ~)
Spec. activity (Bq g- t)
12 not determined 9 5
8 not determined 4 0.2
4 300 0.6 0.5
3 200 0.2 0.02
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
2057
developed [5]. The problem 129Xe+ interference in both techniques might be overcome by using xenonfree argon (if available).
LA-ICP-MS is about --- 20% (relative standard deviation, based on three independent measurements) for most of the measured elements.
3.2. Comparison of LA-ICP-MS and NAA results on undoped concrete sample
3.3. Relative sensitivity coefficients in LA-ICP-MS
In order to achieve a complete characterisation of the concrete matrix, some minor and trace elements of the undoped concrete sample were measured by LAICP-MS. The LA-ICP-MS and neutron activation analysis (NAA) results of some minor and trace elements in the concrete sample were summarised in Fig. 3. Using the results obtained by NAA for a comparison of minor and trace analysis in this kind of sample, it is seen that LA-ICP-MS can be used as a fast survey analytical method for semiquantitative characterisation of non-conducting material without any sample preparation-- if suitable standard reference materials with similar matrix composition are not available. The reproducibility of the analytical results for
The relative sensitivity coefficients (RSC) in LAICP-MS were estimated under the consideration of the concentrations determined by NAA as a comparison. Ca (as one of the matrix elements) was used as an internal standard element in LA-ICP-MS. Table 5 summarises the relative elemental sensitivity coefficients for the quadrupole ICP-MS which are comparable to those of double-focusing sector field LA-ICPMS. The RSCs (RSC = measured concentration by LA-ICP-MS/measured concentration by NAA) varied in LA-ICP-MS by a factor of 0.12 to 3 in the investigated concrete matrix. That means a semiquantitative analysis of most trace elements in concrete is possible with LA-ICP-MS in the absence of a suitable standard reference material. This result is in agreement with experimental findings on non-conducting ceramic
1000000
100000
,
,
10000
c
LA-ICP-MS NAA
1000
.o e.o fJ
100
to
o
10 I!
II II
II
0.1 Na
K
Ca Sc
Ti
Cr Mn Fe Co Cu Zn As Rb Sr
Y
Zr Nb Mo Cs Ba La Ce TI
Pb Th
U
Fig. 3. Concentrations of minor and trace elements in concrete as measured with LA-ICP-MS (without a standard reference material) and NAA.
2058
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
Table 5 Relative sensitivity coefficients (RSC) of chemical elements in concrete matrix for LA-ICP-MS Element
RSC
Element
RSC
Ca ~ Sc Cr Fe Co Zn Rb Sr
1.00 1.89 0.57 0.50 0.93 0.20 2.91 1.31
Zr Mo Ba La Ce Th U
0.21 0.12 2.00 1.28 1.36 2.73 1.53
a Internal standard element. perovskite samples for solid oxide fuel cells as discussed in Ref. [16]. Finally, it should be discussed whether the measured concentrations and limits of detection for the radionuclides reached by L A - I C P - M S in concrete matrix are sufficient and meet the requirements for the quality control of nuclear waste packages. In the Federal Republic of Germany, upper limits for the specific activities for each radionuclide present in radioactive waste packages are specified by law [23]. Agreement with these specifications has to be verified by analytical techniques before transferring the waste package to the disposal facility. In Table 6, the limits of detection determined using doublefocusing LA-ICP-MS are compared with the concentrations corresponding to the upper limits for the specific activities given in Ref. [23]. For 99Tc, 232Th, 237Np and 238U, the measured real concentrations or the limits of detection for L A - I C P - M S are sufficient to verify the specifications. For 129I and 233U a further improvement of the limits of detection
is necessary. Further investigations of ultratrace analysis of other long-lived radionuclides (e.g. l°Tpd, 23°Th, 236U, 241Am, 239pu, 242pu and 244pu) by double-focusing LA-ICP-MS are in progress.
4. Conclusions L A - I C P - M S is a fast and powerful method for the determination of long-lived radionuclides in (nonconducting) concrete samples. The limits of detection obtained with L A - I C P - M S using a sector field mass spectrometer are below 1 ng g-I for 233U and 237Np and meet the requirements for the analysis of waste packages for all nuclides under investigation except 129Iand 233U. Even with the quadrupole ICP-MS used, limits of detection in the 10ng g - l concentration range are measured, so quadrupole L A - I C P - M S can also be useful for this kind of analysis if no sector field instrument is available. Although ICP-MS is becoming increasingly important due to better quantification
Table 6 Comparison between limits of detection (LoD) of long-lived radionuclides in concrete matrix by LA-ICP-MS and the concentrations corresponding to the upper activity limits allowed for waste packages as specified in Ref. [23] Long-lived radionuclide
Limit of detection (ng g-i)
Max. allowed concentration (ngg ~) according to Ref. [23]
99Tc 1291 2:~2Th 233U 235U Z38U 237Np
4 300 < 1800 a 0.6 < 5a < 730 a 0.5
70 100 720000 0.05 900 1000 1.5
a These nuclides are present in the undoped concrete matrix (blank sample) which was used to determine the LoD, so the values given here are the actual concentrations. The LoD of these nuclides are significant lower.
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
p o s s i b i l i t i e s o f a q u e o u s s o l u t i o n s after d i s s o l u t i o n , L A - I C P - M S is i n t e r e s t i n g for a fast s u r v e y a n a l y s i s o f solid n u c l e a r w a s t e s a m p l e s b e c a u s e n o s a m p l e p r e p a r a t i o n is n e c e s s a r y . B e c a u s e the r e l a t i v e e l e m e n t a l s e n s i t i v i t y factors d o not d i f f e r too m u c h , a s e m i q u a n t i t a t i v e d e t e r m i n a tion o f m i n o r a n d trace e l e m e n t s in c o n c r e t e w i t h o u t an e x t e r n a l c a l i b r a t i o n is p o s s i b l e in c a s e s w h e r e a p p r o p r i a t e s t a n d a r d s a m p l e s are not a v a i l a b l e .
Acknowledgements T h i s w o r k is part o f the E C p r o g r a m m e ' E u r o p e a n N e t w o r k o f T e s t i n g Facilities for the Q u a l i t y C h e c k ing o f R a d i o a c t i v e W a s t e P a c k a g e s ( N u c l e a r F i s s i o n Safety, C 3 . 3 ) ' . T h e a u t h o r s w o u l d like to t h a n k J. W e s t h e i d e a n d D. T e n z l e r for assistance,
References [I] C.K. Kim, R. Seki, S. Morita, S. Yamasaki, A. Tsumura, Y. Takaku, Y. lgarashi, M. Yamamoto, J. Anal. At. Spectrom. 6 (1991) 205. [2] R. Chiappini, J.M. Taillade, S. Brebion, J. Anal. At. Spectrom. 11 (1996) 497. 13] R.W. Morrow, J.S. Crain (Eds.), Applications of Inductively Coupled Plasma-Mass Spectrometry to Radionuclide Determinations, ASTM (American Society for Testing and Materials) Special Technical Publications (STP 1291), West Conshohocken, USA 1995. [4] M. Yamamoto, H. Kofuji, A. Tsumura, S. Yamasaki, K. Yuita, M. Komamura, K. Komura, K. Ueno, Radiochimica Acta 64 (1994) 217. [5] W. Kerl, J.S. Becker, H.-J. Dietze, J. Anal. At. Spectrom. 11 (1996) 723.
2059
[6] K. Hoppstock, J.S. Becker, H.-J. Dietze, 1996 Winter Conference on Plasma Spectrochemistry, 3-13 February 1996, Fort Lauderdale, Florida, S 10. [7] J.M.B. Moreno, J.I.G. Alonso, P. Arbore, G. Nicolaou, L. Koch, J. Anal. At. Spectrom. 11 (1996) 929. [8] Ph. Bienvenu, E. Brochard, E. Excoffier, 1997 European Winter Conference on Plasma Spectrochemistry, 12-17 January 1997, Gent, Belgium, P3-22. [9] J.S. Alvarado, M.D. Erickson. J. Anal. At. Spectrom. I1 (1996) 923. [ 10] J.S. Becker, H.-J. Dietze, Fresenius J. Anal. Chem. 344 (1992) 69. [11] A.I. Saprykin, J.S. Becker, H.-J. Dietze, Fresenius J. Anal. Chem. 355 (1996) 831. [12] F. E Lichte, Anal. Chem. 67 (1995) 2479. 1131 S.F. Dunant, Analyst 117 (1992) 1585. [14] X. Guo, F.E. Lichte, Analyst 120 (1995) 2707. [15] J. Westheide, J.S. Becket, R. Jiiger, H.-J. Dietze, J.A.C. Broekaert, J. Anal. At. Spectrom. 11 (1996) 661. [16] J.S. Becker, U. Breuer, J. Westheide, A.I. Saprykin, H. Holzbrecher, H. Nickel, H.-J. Dietze, Fresenius J. Anal. Chem. 355 (1996) 626. [17] G, Kiippers, G. Erdtmann, J. Radioanal. Nucl. Chem. 210 (1996) 65. [18] D. Yang et al., J. Radioanal. Nucl. Chem. 183 (1994) 245. [19] J.W. McKlveen, W.J. McDowell, Nucl. Instr. Meth. 223 (1984) 372. [20] A.J. Walder, I.D. Abell, I. Platzner, P.A. Freeman, Spectrochimica Acta 48B (1993) 397. [21] J.S. Becker, H.-J. Dietze, J. Anal. At. Spectom. 12 (1997) 881. [22] J.S. Becker, M. Gastel, D. Tenzler, H.-J. Dietze, Advances in Mass Spectrometry, Vol. 14, Proc. of 14th International Mass Spectrometry Conference, 25-29 August 1997, Tampere, Finland, 385. [23] Report by the German 'Bundesamt ffir Strahlensehutz' Federal Office for Radiation Protection, "Anforderungen an endzulagernde radioaktive Abfalle und MaBnahmen zur Produktkontrolle radioaktiver Abf~ille: Endlager ffir radioaktive Abf'fille Morsleben (ERAM), Teil I: Endlagerungsbedingungen', ET-IB-85, 1996.
ELSEVIER
Spectrochimica Acta Part B 52 (1997) 2051-2059
Determination of long-lived radionuclides in concrete matrix by laser ablation inductively coupled plasma mass spectrometry 1 M. Gastel 2, J.S. Becker*, G. Kiippers, H.-J. Dietze Zentralabteilung 3~urChemische Analysen, Forschungszentrum Jiilich GmbH, 52425 Jiilich, Germany
Received 23 May 1997; revised 30 September 1997; accepted 3 October 1997
Abstract
A laser ablation system using a Nd:YAG laser was coupled both to a quadrupole inductively coupled plasma (ICP) mass spectrometer and to a double-focusing sector field ICP mass spectrometer. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was applied for the determination of long-lived radionuclides in a concrete matrix. The investigated samples were two laboratory standards with a concrete matrix, which we doped with different long-lived radionuclides (e.g. 99Tc, 232Th, 233U, 237Np) from the ng g-l to /~g g-~ concentration range and an undoped concrete material (blank). Detection limits for long-lived radionuclides in the 10 ng g-J range are reached for LA-ICP-MS using the quadrupole mass spectrometer. With double-focusing sector field ICP-MS, the limits of detection are in general one order of magnitude lower and reach the sub ng g- ~range for 233Uand 237Np. A comparison of mass spectrometric results with those of neutron activation analysis on undoped concrete sample indicates that a semiquantitative determination of the concentrations of the minor and trace elements in the concrete matrix is possible with LA-ICP-MS without using a standard reference material. © 1997 Elsevier Science B.V. Keywords: Concrete matrix; Double-focusing sector field ICP-MS; Laboratory standards; Laser ablation ICP-MS; Long-lived radionuclides; Quadrupole ICP-MS
1. I n t r o d u c t i o n
The trace and ultratrace analysis of long-lived radionuclides in environmental or radioactive waste samples is an interesting analytical topic. The determination of long-lived radionuclides is important in radioactive waste from nuclear reactors for recycling and final storage of radioactive waste. In many countries, specifications have been drawn up to limit the Presented at the 1997 Winter Conference on Plasma Spectrochemistry, 12-17 January 1997, Gent, Belgium. 2 Present address: Technische Hochschule Darmstadt, Fachbereich Materialwissenschaft,Fachgebiet Chemische Analytik, 64287 Darmstadt, Germany. * Author to whom correspondence should be addressed.
activity of radioactive waste packages that may be transferred to disposal facilities. The content of around 60 radionuclides has to be specified in the sense that there are different activity limits for the nuclides that must not be exceeded. A fast and reliable analysis - - especially on solid waste material - - is required for the determination o f these nuclides up to the n g g -l concentration range. Radioanalytical methods are well suited for the measurement of 7ray-emitting nuclides but for long-lived ct- and /3ray-emitting nuclides mostly time-consuming radiochemical separation procedures are necessary. Inductively coupled plasma mass spectrometry (ICPMS) as a powerful trace, ultratrace and isotopic analytical method has been used successfully for the
0584-8547/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0584-8547(97)001 I 1-0
2052
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
determination of long-lived radionuclides in aqueous solutions [1-5]. Kim et al. [1] determined the detection limits for the long-lived radionuclides 99Tc, 226Ra, 232Th, 237Np, 238U, 239pu and 24°pu with half-lives of 10 3-1010 years in aqueous standard solutions using a double-focusing sector field ICP-MS ('PlasmaTrace', Fisons) coupled with an ultrasonic nebuliser and obtained detection limits ranging from 2 to 20 pg 1-I. The determination of long-lived radionuclides in radioactive waste materials was mostly performed in aqueous solutions, especially in combination with special sample introduction systems, e.g. for the determination of 129I [5] or 795e [6], using ion chromatography [7], liquid-liquid extraction [8] or electrothermal vaporisation [9]. For a characterisation of solid nuclear waste, a fast direct method for the analysis of solids without any sample preparation is required. The matrix of the radioactive solid waste is often concrete, therefore the analytical method must be capable of analysing even non-conducting samples. The mass spectrometric methods capable of direct analysis of trace elements in non-conducting solids include LIMS ]10], radiofrequency GDMS [11] and laser ablation ICP-MS (LA-ICP-MS). This work focuses on LAICP-MS, which is a well-established method for the analysis of geological materials with detection limits in the 10 ng g-J concentration range [12-14] and has also proven its applicability for the determination of trace elements in ceramic materials [ 15,16]. Until now no investigations have been published on the determination of radionuclides in radioactive waste products with LA-ICP-MS. The main problem in the quantification of analytical results is that no suitable standard reference materials are available. Therefore, synthetic laboratory standards were prepared for the determination of long-lived radionuclides, as described in Ref. [17]. The aim of this work is to demonstrate the capability and the limits of LA-ICP-MS for measurements on long-lived radionuclides (99Tc, 129I, 232Th, 233U, 235U, 237Np, 238U) in concrete matrix, which is a very common matrix in waste packages. Of special interest are the limits of detection of long-lived radionuclides, which are compared for two different types of mass spectrometer (a quadrupole and a doublefocusing sector field ICP-MS) coupled to the laser ablation system.
2. Experimental 2.1. I n s t r u m e n t a t i o n
The laser ablation system, based on a Nd:YAG laser, used for LA-ICP-MS has been developed in our laboratory for the analysis of non-conducting sampies, such as ceramic materials [15,16]. It was coupled either to a quadrupole ICP mass spectrometer (SCIEX-Elan 5000 from Perkin-Elmer) or to a double-focusing sector field ICP mass spectrometer (ELEMENT from Finnigan MAT). Fig. 1 shows the experimental set-up of both LA-ICP-MS configurations. The experimental parameters for the LA-ICPMS, which were optimised in order to yield maximum ion intensities of analytes, are summarised in Table 1. The whole system is computer-controlled and allows different measurement programs to be run automatically. In order to avoid difficulties due to local inhomogeneities of the samples, the laser beam is rastered over the surface by moving the whole laser ablation chamber constructed at our laboratory by using two step motors. Activity measurements of the -y-ray emitters - - for neutron activation analysis - - were performed with a high-resolution -y-ray spectrometer system including a high-purity germanium detector (Ortec), multichannel analyser from Nuclear Data and a Micro-VAX workstation for data processing. For the evaluation of the "r-ray spectra, a peakfit procedure from Nuclear Data was applied. The activities of the c~- and fl-ray emitters were measured with the Tri-Carb 2200CA Liquid Scintillation Analyzer (LSC) from the CanberraPackard Instrument Company. For the fl-ray emitters the 'efficiency tracing mode', which is provided with the instrument, was used to obtain absolute decay rates. The a-ray emitters were also measured with the LSC where the counting efficiency was nearly 100% [18,19]. 2.2. I r r a d i a t i o n
Irradiations for neutron activation analysis to determine the blanks of 232Th and 238U of the concrete matrix were carried out in the FRJ-2 (heavy-watermoderated reactor, Research Center Jiilich, Germany). Samples prepared from about 200 mg were irradiated for 3 h at a thermal neutron flux of
2053
M. Gastel et aL/Spectrochimica Acta Part B 52 (1997) 2051-2059
•, - , ,
J INet ion opU~t,
SEM
/
L
n
.............
/ [Nd:YAa4uer "
J I I "),., M2, ~4
ICP
quadmpole J
beam divkler focus lens
ion optics .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
i~: ,, ~
laser ablation chamber
.
exit =llt t~lctroliti~c analyzer
Fig. 1. Schematic diagram of the experimental set-up of LA-ICP-MS using a quadrupole ICP-MS and a double-focusing sector field ICP-MS.
1 × 10 13 n cm -2 s -1. Together with the samples a zirconium wire of about 3 mg was irradiated as a monitor for the thermal and epithermal neutron flux.
long-lived radionuclides in all three samples (standard 1, standard 2 and concrete blank sample) as they are known from sample preparation or determined by neutron activation analysis (NAA).
2.3. Investigated samples 2.4. Preparation of the concrete laboratory standards The investigated samples are the two synthetic laboratory standard samples with concrete matrix and known concentrations of some long-lived radionuclides (sample 1 and 2) and a blank sample with the same matrix but without doping of radionuclides. The blank sample also contains 23~Fh, 235U and 238U because these nuclides are present as trace elements in the concrete matrix. The concrete standard 1 was prepared as low level standard in respect to the activity levels to check radiometric methods for activity measurements (c~spectrometry and f3-counting). In concrete laboratory standard 2 higher concentrations of the long-lived nuclides are doped. Both standard samples are suitable to check the performance of LA-ICP-MS. Table 2 s u m m a r i s e s the concentrations of
The preparation of the concrete laboratory standard 1 for some selected long-lived radionuclides, especially intended for the control of radioanalytical separations and activity measurements, is described in Ref. [17]. In concrete laboratory standard 2, further selected nuclides were added. 99Tc, 129I, 232Th, 233U and 237Np were chosen which have very long halflives (see Table 2). Such long-lived radionuclides have low specific activities and their measurements with radioanalytical methods are correspondingly insensitive but they should be particularly suitable for analysis by mass spectrometry. The concrete laboratory standard 2 was prepared by mixing calibrated solutions of the nuclides with cement powder. The solutions of the /3-ray-emitting
2054
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
Table 1 Experimental parameters of the LA-ICP-MS Laser ablation
Nd:YAG laser Wavelength Pulse width Repetition frequency Pulse energy Laser power density Spot diameter Raster width Ion detection
(Surlite 1, Continuum, Santa Clara, USA) 266 nm (4th harmonic) 5 ns 10-20 Hz 15 mJ ~ 10 ~°W c m -2 ~ 100 #m 3 mm × 3 mm SEM
Ablation cell and connection tube to the ICP-MS torch
Tube dimension Ablation cell dimensions
4 mm id, 2 m length 34 mm id, 60 mm high
ICP-MS
Quadrupole ICP-MS SCIEX-Elan 5000, Perkin-Elmer 1060 W 5 ms 50 0.9 1 rain ~Ar 13.5 I min ~ 0.7 1 rain-t Peak hop transient 6-240 u 300
R.f. power Dwell time No. of replicates Carrier gas flow rate Coolant gas flow rate Auxiliary gas flow rate Scanning mode Mass range Mass resolution (m/Am)
n u c l i d e s 99Tc a n d 129I w e r e p r e p a r e d f r o m l a b o r a t o r y stock solutions, o b t a i n e d b y the d i l u t i o n o f s a m p l e s purchased from Amersham-Buchler, Braunschweig, Germany. They were calibrated by liquid scintillation c o u n t i n g (LSC), u s i n g the e f f i c i e n c y t r a c i n g m o d e . T h e s o l u t i o n s o f the a - r a y - e m i t t i n g n u c l i d e s 233U a n d 237Np w e r e c a l i b r a t e d w i t h the L S C w h e r e the c o u n t i n g e f f i c i e n c y w a s n e a r l y 100%. 1 0 0 m g o f e a c h n u c l i d e s o l u t i o n w a s m i x e d w i t h 2 0 m l o f the L S C cocktail ' B a k e r A n a l y z e d ' ( B a k e r ) . T h e p u l s e
Double-focusing sector field ICP-MS ELEMENT, Finnigan MAT 1250 W 5 ms 50 1.0 1 min i Ar 13.5 1 min-i 0.7 1 minPeak hop transient 6-240 u 300
h e i g h t s p e c t r a w e r e e v a l u a t e d as d e s c r i b e d in Ref. [17]. O n e o f t h e a - r a y e m i t t e r s , 232Th, h a s many relatively short-lived a-ray-emitting daughter n u c l i d e s a n d it is t h e r e f o r e difficult or n e a r l y i m p o s s i b l e to d e t e r m i n e the 232Th a c t i v i t y b y L S C . B e c a u s e o f the v e r y l o n g half-life o f 1.41 x 10 l° y e a r s a n d the c o r r e s p o n d i n g l y v e r y l o w specific a c t i v i t y a c a l i b r a t e d s t o c k s o l u t i o n for 232Th c o u l d b e p r e p a r e d b y w e i g h i n g the salt T h ( N O 3 ) a o 5 H 2 0 . T h e activity o f the s o l u t i o n w a s c a l c u l a t e d f r o m t h e
Table 2 Concentrations, specific activities and half-life periods of the radionuclides in the investigated samples Nuclide
99Tc t29I 232Th 233U 235U 237Np
238U
Half-life
Laboratory standard 1
Laboratory standard 2
(years)
Conc. (ng g-t)
Spec. activity (Bq g-~)
Conc. (ng g -I)
Spec. activity (Bq g-i)
2.13 × 10 5 1.57 x 1 0 7 1.4 x 101° 1.6 x 105 7.0 x 108 2.14 x 1 0 6 5.4 X 1 0 9
28.5 _+ 0.9 not added 1800 + 180 51 ± 1 5 ± 0.5 628 _+ 15 730 ± 73
17.9 ± 0.6 not added 0.007 ± 0.0007 18.3 - 0.4 0.0004 ± 0.00004 16.4 ± 0,4 0.009 ___0.0009
1400 _+ 70 1700 - 100 6100 _ 400 1040 ± 70 5 ± 0.5 2270 ± 150 730 ± 73
890 ± 50 11.1 ± 1 0.024 - 0.002 369 ± 25 0.0004 + 0.00004 59 ± 5 0.009 ± 0.0009
Undoped concrete :onc. (ng g-i)
0 0 1800 ~ 180 0 5_+0.5 0 730 -- 73
2055
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
for 232Th+ and 238U+ (with concentrations of 1.8 #g g-J for Th and 0.73 #g g-~ for U - - determined by NAA). The uranium with a natural isotopic composition of 235U/238U (measured isotopic ratio, 7.2 x 10 -3) was found. The precision of isotopic ratio measurements on uranium using a double-focusing sector field LA-ICP-MS with single ion detection is about _+ 5% (relative standard deviation). The use of a multiple collector system in double-focusing LAICP-MS ('Plasma 54' prototype) - - as demonstrated for the determination of 2°6pb/2°4pb in glass by Walder et al. [20] - - allows isotopic ratio measurements at precision levels down to 0.1% (relative standard deviation). Furthermore, the mass spectrum shown in Fig. 2 shows the detection of the long-lived radionuclides (233U+, 234U+, 237Np+ and 232Th+) which were added to the laboratory concrete standard 2. The determination of long-lived radionuclides can be disturbed by isobaric interferences with molecular ions. In Table 3, the measured relative hydride and oxide ion intensities - - which were measured in the 10 -3 range with respect to their atomic ion intensity - - and interferences with different possible actinide ions in waste materials are summarised. (Some of the actinides mentioned in Table 3 are not doped in the concrete matrix, but might as well occur in real radioactive waste samples.) Due to the relatively low molecular formation rate in LA-ICP-MS,
concentration of thorium. The concentration of Th was checked by NAA. Aliquots of the calibrated solutions were poured together and then immediately mixed with cement powder. A ready-to-use mixture of fine-grained cement ('Fugenzement', Ultrament, Essen, Germany) was used. Fine-grained material was used in order to attain good homogeneity of the standard material. The mixture was vigorously stirred and kneaded and cement powder was added until the paste had a consistency that did not exude free solution. The total amount of cement powder used was about 100 g. The paste was filled into small stainless steel dishes (20 mm in diameter and 5 mm in height) to produce easy-to-use samples. The material was exposed to the atmosphere for 4 weeks to harden and dry. When the weight was constant, the exact nuclide concentrations and their specific activities were calculated and the standard was ready for analysis.
3. R e s u l t s
and discussion
Part of the mass spectra of the concrete laboratory standard 2 (solid lines) and the concrete blank sample (broken lines) using the double-focusing sector field LA-ICP-MS are compared in Fig. 2. In the undoped concrete matrix (sample 3) ion intensities in the 10000cps range were measured 1000000 --
232Th237Np*
233U+ 100000 D. 0
238U÷
S"I
10000
C
.=
1000
234U*
_o
235
U+
100
10 ' 230
Z-~UH+
A 232
234
236
238
240
m/z
Fig. 2. Part of the mass spectrum of laboratory standard 2 (solid lines) and the concrete blank sample (broken lines) using the double-focusing sector field LA-ICP-MS.
2056
M. Gastel et al./Spectrochimica Aeta Part B 52 (1997) 2051-2059
Table 3 Relative intensities of molecular ions of long-lived radionuclides in LA-ICP-MS Molecular ion
Relative ion intensity (MX÷/M+; X = O, H)
Possible interference with analyte ion
Half-life of actinide nuclide (years)
232ThtH + 234UIH+ 238UIH+ 232Thl60+
< 9.1 9.1 2.0 9.0 1.1
233U+ 235U+ 239pu+ 248Cm+ 249Cf+
1.6 × 7.0 x 2.4 x 3.4 x 360
233U160+ 237Npl60+
1.6 × 10 -4 × 10 4 × 10 ~' x 10 -3 × 10 4 × 10 -3
isobaric mass interferences of analyte and molecular ions in the investigated samples are not important. Occurring interferences of molecular ions of PbCI + and PbAr + in the mass range of actinide ions in a Pb-containing soil sample digested with an acid mixture containing HCI investigated by doublefocusing mass sector field ICP-MS are discussed in Ref. [21]. 3.1. Limits o f detection o f long-lived radionuclides in concrete matrix
The limits of detection o f 99Tc, 129I, 233U and 237Np were determined from the mass spectrum of the blank sample using the 30 criterium (the limit of detection is given by mb + 3ab, where mb is the mean value of the blank measurements and Ob the standard deviation of five independent measurements of the blank sample). The quantification was done using standards 1 and 2. All mass spectra are averaged over 50 single measurements in order to minimise the influence of plasma instabilities, variations of the amount of ablated material and local inhomogeneities of the samples. Concentrations were evaluated from measured ion intensities of background in the blank sample using the known concentrations of both
10 5 10 8 10 4 105
laboratory standard samples with well known concentrations of long-lived radionuclides. In Table 4 the limits of detection for LA-ICP-MS using the quadrupole ICP-MS and using the doublefocusing sector field ICP-MS are compared and the corresponding specific activities (calculated from the limits of detection) are summarised. Limits of detection in the 10 ng g-~ concentration range are reached using the quadrupole ICP-MS (SCIEX-Elan 5000, Perkin Elmer). The limits of detection for LA-ICPMS using the double-focusing sector field ICP-MS (ELEMENT, Finnigan MAT) are about one order of magnitude lower - - due to higher sensitivity and lower noise - - in comparison with the used quadrupole ICP-MS (SCIEX-Elan 5000, Perkin Elmer). Further investigations of ultratrace analysis of longlived radionuclides on concrete matrix by double focusing LA-ICP-MS (with detection limits up to the 20 pg g ~ concentration level) are described in Ref. [22]. The detection limit for 129I is much higher than those for the other radionuclides due to an isobaric mass interference of 129I+ with 129Xe+, where Xe is a contaminant in the argon carrier and plasma gas. For the determination of 1291in aqueous solution a special sample introduction equipment for ICP-MS was
Table 4 Limits of detection (LoD, determined from the undoped blank sample) and the correspondingspecific activities for radionuclides in concrete matrix, measured with the quadrupole and the double-focusingsector field ICP-MS coupled to the laser ablation system. Nuclide
99Tc 129I 233U 237Np
Quadrupole LA-ICP-MS
Double-focusing sector field LA-ICP-MS
LoD (ng g- ~)
Spec. activity (Bq g i)
LoD (ng g- ~)
Spec. activity (Bq g- t)
12 not determined 9 5
8 not determined 4 0.2
4 300 0.6 0.5
3 200 0.2 0.02
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
2057
developed [5]. The problem 129Xe+ interference in both techniques might be overcome by using xenonfree argon (if available).
LA-ICP-MS is about --- 20% (relative standard deviation, based on three independent measurements) for most of the measured elements.
3.2. Comparison of LA-ICP-MS and NAA results on undoped concrete sample
3.3. Relative sensitivity coefficients in LA-ICP-MS
In order to achieve a complete characterisation of the concrete matrix, some minor and trace elements of the undoped concrete sample were measured by LAICP-MS. The LA-ICP-MS and neutron activation analysis (NAA) results of some minor and trace elements in the concrete sample were summarised in Fig. 3. Using the results obtained by NAA for a comparison of minor and trace analysis in this kind of sample, it is seen that LA-ICP-MS can be used as a fast survey analytical method for semiquantitative characterisation of non-conducting material without any sample preparation-- if suitable standard reference materials with similar matrix composition are not available. The reproducibility of the analytical results for
The relative sensitivity coefficients (RSC) in LAICP-MS were estimated under the consideration of the concentrations determined by NAA as a comparison. Ca (as one of the matrix elements) was used as an internal standard element in LA-ICP-MS. Table 5 summarises the relative elemental sensitivity coefficients for the quadrupole ICP-MS which are comparable to those of double-focusing sector field LA-ICPMS. The RSCs (RSC = measured concentration by LA-ICP-MS/measured concentration by NAA) varied in LA-ICP-MS by a factor of 0.12 to 3 in the investigated concrete matrix. That means a semiquantitative analysis of most trace elements in concrete is possible with LA-ICP-MS in the absence of a suitable standard reference material. This result is in agreement with experimental findings on non-conducting ceramic
1000000
100000
,
,
10000
c
LA-ICP-MS NAA
1000
.o e.o fJ
100
to
o
10 I!
II II
II
0.1 Na
K
Ca Sc
Ti
Cr Mn Fe Co Cu Zn As Rb Sr
Y
Zr Nb Mo Cs Ba La Ce TI
Pb Th
U
Fig. 3. Concentrations of minor and trace elements in concrete as measured with LA-ICP-MS (without a standard reference material) and NAA.
2058
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
Table 5 Relative sensitivity coefficients (RSC) of chemical elements in concrete matrix for LA-ICP-MS Element
RSC
Element
RSC
Ca ~ Sc Cr Fe Co Zn Rb Sr
1.00 1.89 0.57 0.50 0.93 0.20 2.91 1.31
Zr Mo Ba La Ce Th U
0.21 0.12 2.00 1.28 1.36 2.73 1.53
a Internal standard element. perovskite samples for solid oxide fuel cells as discussed in Ref. [16]. Finally, it should be discussed whether the measured concentrations and limits of detection for the radionuclides reached by L A - I C P - M S in concrete matrix are sufficient and meet the requirements for the quality control of nuclear waste packages. In the Federal Republic of Germany, upper limits for the specific activities for each radionuclide present in radioactive waste packages are specified by law [23]. Agreement with these specifications has to be verified by analytical techniques before transferring the waste package to the disposal facility. In Table 6, the limits of detection determined using doublefocusing LA-ICP-MS are compared with the concentrations corresponding to the upper limits for the specific activities given in Ref. [23]. For 99Tc, 232Th, 237Np and 238U, the measured real concentrations or the limits of detection for L A - I C P - M S are sufficient to verify the specifications. For 129I and 233U a further improvement of the limits of detection
is necessary. Further investigations of ultratrace analysis of other long-lived radionuclides (e.g. l°Tpd, 23°Th, 236U, 241Am, 239pu, 242pu and 244pu) by double-focusing LA-ICP-MS are in progress.
4. Conclusions L A - I C P - M S is a fast and powerful method for the determination of long-lived radionuclides in (nonconducting) concrete samples. The limits of detection obtained with L A - I C P - M S using a sector field mass spectrometer are below 1 ng g-I for 233U and 237Np and meet the requirements for the analysis of waste packages for all nuclides under investigation except 129Iand 233U. Even with the quadrupole ICP-MS used, limits of detection in the 10ng g - l concentration range are measured, so quadrupole L A - I C P - M S can also be useful for this kind of analysis if no sector field instrument is available. Although ICP-MS is becoming increasingly important due to better quantification
Table 6 Comparison between limits of detection (LoD) of long-lived radionuclides in concrete matrix by LA-ICP-MS and the concentrations corresponding to the upper activity limits allowed for waste packages as specified in Ref. [23] Long-lived radionuclide
Limit of detection (ng g-i)
Max. allowed concentration (ngg ~) according to Ref. [23]
99Tc 1291 2:~2Th 233U 235U Z38U 237Np
4 300 < 1800 a 0.6 < 5a < 730 a 0.5
70 100 720000 0.05 900 1000 1.5
a These nuclides are present in the undoped concrete matrix (blank sample) which was used to determine the LoD, so the values given here are the actual concentrations. The LoD of these nuclides are significant lower.
M. Gastel et al./Spectrochimica Acta Part B 52 (1997) 2051-2059
p o s s i b i l i t i e s o f a q u e o u s s o l u t i o n s after d i s s o l u t i o n , L A - I C P - M S is i n t e r e s t i n g for a fast s u r v e y a n a l y s i s o f solid n u c l e a r w a s t e s a m p l e s b e c a u s e n o s a m p l e p r e p a r a t i o n is n e c e s s a r y . B e c a u s e the r e l a t i v e e l e m e n t a l s e n s i t i v i t y factors d o not d i f f e r too m u c h , a s e m i q u a n t i t a t i v e d e t e r m i n a tion o f m i n o r a n d trace e l e m e n t s in c o n c r e t e w i t h o u t an e x t e r n a l c a l i b r a t i o n is p o s s i b l e in c a s e s w h e r e a p p r o p r i a t e s t a n d a r d s a m p l e s are not a v a i l a b l e .
Acknowledgements T h i s w o r k is part o f the E C p r o g r a m m e ' E u r o p e a n N e t w o r k o f T e s t i n g Facilities for the Q u a l i t y C h e c k ing o f R a d i o a c t i v e W a s t e P a c k a g e s ( N u c l e a r F i s s i o n Safety, C 3 . 3 ) ' . T h e a u t h o r s w o u l d like to t h a n k J. W e s t h e i d e a n d D. T e n z l e r for assistance,
References [I] C.K. Kim, R. Seki, S. Morita, S. Yamasaki, A. Tsumura, Y. Takaku, Y. lgarashi, M. Yamamoto, J. Anal. At. Spectrom. 6 (1991) 205. [2] R. Chiappini, J.M. Taillade, S. Brebion, J. Anal. At. Spectrom. 11 (1996) 497. 13] R.W. Morrow, J.S. Crain (Eds.), Applications of Inductively Coupled Plasma-Mass Spectrometry to Radionuclide Determinations, ASTM (American Society for Testing and Materials) Special Technical Publications (STP 1291), West Conshohocken, USA 1995. [4] M. Yamamoto, H. Kofuji, A. Tsumura, S. Yamasaki, K. Yuita, M. Komamura, K. Komura, K. Ueno, Radiochimica Acta 64 (1994) 217. [5] W. Kerl, J.S. Becker, H.-J. Dietze, J. Anal. At. Spectrom. 11 (1996) 723.
2059
[6] K. Hoppstock, J.S. Becker, H.-J. Dietze, 1996 Winter Conference on Plasma Spectrochemistry, 3-13 February 1996, Fort Lauderdale, Florida, S 10. [7] J.M.B. Moreno, J.I.G. Alonso, P. Arbore, G. Nicolaou, L. Koch, J. Anal. At. Spectrom. 11 (1996) 929. [8] Ph. Bienvenu, E. Brochard, E. Excoffier, 1997 European Winter Conference on Plasma Spectrochemistry, 12-17 January 1997, Gent, Belgium, P3-22. [9] J.S. Alvarado, M.D. Erickson. J. Anal. At. Spectrom. I1 (1996) 923. [ 10] J.S. Becker, H.-J. Dietze, Fresenius J. Anal. Chem. 344 (1992) 69. [11] A.I. Saprykin, J.S. Becker, H.-J. Dietze, Fresenius J. Anal. Chem. 355 (1996) 831. [12] F. E Lichte, Anal. Chem. 67 (1995) 2479. 1131 S.F. Dunant, Analyst 117 (1992) 1585. [14] X. Guo, F.E. Lichte, Analyst 120 (1995) 2707. [15] J. Westheide, J.S. Becket, R. Jiiger, H.-J. Dietze, J.A.C. Broekaert, J. Anal. At. Spectrom. 11 (1996) 661. [16] J.S. Becker, U. Breuer, J. Westheide, A.I. Saprykin, H. Holzbrecher, H. Nickel, H.-J. Dietze, Fresenius J. Anal. Chem. 355 (1996) 626. [17] G, Kiippers, G. Erdtmann, J. Radioanal. Nucl. Chem. 210 (1996) 65. [18] D. Yang et al., J. Radioanal. Nucl. Chem. 183 (1994) 245. [19] J.W. McKlveen, W.J. McDowell, Nucl. Instr. Meth. 223 (1984) 372. [20] A.J. Walder, I.D. Abell, I. Platzner, P.A. Freeman, Spectrochimica Acta 48B (1993) 397. [21] J.S. Becker, H.-J. Dietze, J. Anal. At. Spectom. 12 (1997) 881. [22] J.S. Becker, M. Gastel, D. Tenzler, H.-J. Dietze, Advances in Mass Spectrometry, Vol. 14, Proc. of 14th International Mass Spectrometry Conference, 25-29 August 1997, Tampere, Finland, 385. [23] Report by the German 'Bundesamt ffir Strahlensehutz' Federal Office for Radiation Protection, "Anforderungen an endzulagernde radioaktive Abfalle und MaBnahmen zur Produktkontrolle radioaktiver Abf~ille: Endlager ffir radioaktive Abf'fille Morsleben (ERAM), Teil I: Endlagerungsbedingungen', ET-IB-85, 1996.