Development of a novel method for the

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Development of a novel method for the determination of environmental samples by ICP-MS

99

Tc in

Martin McCartney,*a Kaliaperumal Rajendran,a Valerie Olive,a Richard G. Busbyb and Paul McDonaldb a b

Scottish Universities Research and Reactor Centre, East Kilbride, UK G75 0QF Westlakes Scienti®c Consulting, Moor Row, Cumbria, UK CA24 3LN

Published on 01 January 1999. Downloaded on 08/10/2013 18:36:33.

Received 30th June 1999, Accepted 7th September 1999

A relatively rapid and ef®cient method for the determination of 99Tc in a range of marine samples by ICP-MS was developed. Ruthenium decontamination is achieved by the use of TEVA.Spec resin following the removal of the bulk of the matrix elements by a simple evaporation±recrystallisation step. The resulting solution is devoid of ruthenium and subsequent analysis by ICP-MS is comparatively free from interfering matrix effects. The validity of the method was demonstrated by participation in an international intercomparison exercise. The chemical yield for marine biota averages 80±90% and for sediment is around 50±70%. The limit of detection for a 10 g sample is 1 Bq kg21.

Introduction

Experimental

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Reagents and materials

Tc is a long-lived radionuclide (half-life, 2.136105 years) produced with a relatively high yield (6%) from the ®ssion of 235 U and 239Pu. Releases of 99Tc into the environment result mainly from the reprocessing of spent nuclear fuel. Of particular signi®cance in this respect is the British Nuclear Fuels reprocessing plant at Sella®eld, Cumbria, in the UK, which discharges liquid radioactive waste into the Irish Sea. Annual discharges of 99Tc from this site have increased from 4± 6 TBq in the 1980s and early 1990s to 70±200 TBq from 1994 onwards. Unfortunately, relatively little is known about the behaviour of 99Tc in the environment. There are no stable isotopes of technetium to study and the half-life of the longestlived isotope, 98Tc (4.26106 years), is such that no primordial technetium will remain on earth. Trace amounts of natural 99 Tc, formed by spontaneous ®ssion of 238U and slow neutron induced ®ssion of 235U, have been identi®ed in pitchblende ores,1 but the quantity formed in this way is very small compared with present-day arisings from the nuclear fuel cycle. The comparatively high levels of 99Tc now present in the marine environment provide an ideal opportunity for the more detailed study of the behaviour of technetium. Such work is necessary to determine the radiological signi®cance of past, present and future discharges of 99Tc from the nuclear fuel cycle. To this end, the development of a suitable analytical technique is required. Most published techniques are based on the radiometric counting of the b-particle (Emax~293 keV) emitted in the decay of 99Tc.2±6 These methods are all characterised by extensive sample preparation (to remove interfering radionuclides and convert the 99Tc into a form suitable for radiometric counting) and long counting times. Signi®cant improvements in counting times can be achieved by using ICP-MS. The advantages of ICP-MS for the measurement of long-lived radionuclides have been well documented7,8 and, in recent years, several laboratories have developed this technique for the determination of 99Tc in environmental samples.9±11 Extensive sample preparation is still required, however, in order to (i) extract the 99 Tc from the matrix into a form suitable for analysis by ICPMS and (ii) remove the isobaric interference caused by the presence of 99Ru. The aim of the present study, therefore, was to develop a rapid and ef®cient method for the determination of 99Tc in environmental samples by ICP-MS.

High purity de-ionised water (15 MV cm) was obtained from a Millipore Milli-U 10 unit (Millipore, Bedford, MA, USA). Aristar grade nitric acid (relative density 1.42), hydrogen peroxide (30% m/v) and AnalaR grade ammonia solution (relative density 0.88) were obtained from BDH (Poole, Dorset, UK). Working standard solutions of Be, Co, Ni, Zn, Mo, In, Pt, Hg, Bi and U were obtained from monoelemental 1 mg ml21 certi®ed stock standard solutions (Johnson Matthey, Royston, Hertfordshire, UK). TEVA.Spec resin (particle size 50±100 mm) was obtained from Eichrom Europe (Paris, France). 99Tc calibration standards were derived from a stock standard solution (34 mg g21) obtained from Amersham International (Amersham, Buckinghamshire, UK). The yield tracer, 95mTc, was obtained from the National Physical Laboratory (NPL, Teddington, Middlesex, UK). Instrumentation 95m

Tc was determined by measuring the intensity of its 204 keV c-ray using a 363 in well-type NaI detector. Samples were measured until over 10 000 counts were observed. 99 Tc was measured using a VG Elemental PQ2 Plus quadrupole-based ICP-MS instrument (VG Elemental, Winsford, Cheshire, UK) ®tted with a Meinhard nebuliser and a water cooled glass Scott double pass spray chamber. Instrument parameters were optimised using an In solution (10 ng g21) and a typical response for 99Tc was 36 104 counts s21 (ppb)21. The acquisition parameters are listed Table 1 ICP-MS acquisition parameters Sample uptake rate Washout time Uptake time Acquisition time

0.8 ml min21 180 s 90 s 30 s

Data acquisition mode Masses monitored Dwell time Points per peak No. of replicates

Peak jumping m/z 99, 101, 115 10.24 ms 3 3

J. Anal. At. Spectrom., 1999, 14, 1849±1852 This journal is # The Royal Society of Chemistry 1999

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in Table 1. Ruthenium contamination was monitored by measuring the 101Ru count rate and 115In was used as an internal standard. Memory effects were eliminated by employing a 180 s washout with 0.8 M HNO3 between samples. Method summary A summary of the analytical method is presented in Fig. 1 and important aspects of the procedure are discussed in the following sections.

Results and discussion


Ashing and leaching Ashing is carried out to remove organic carbon, which would otherwise interfere with the subsequent chemical processing and lead to signi®cant matrix effects in the ICP-MS analysis. At high temperatures, however, technetium may be lost from the sample through the formation of the volatile acid HTcO4, although it has been reported that this mechanism can be suppressed by the addition of ammonia.12 Some studies have suggested that technetium losses can occur at temperatures around 500 ³C,5,6 whereas in other studies signi®cant losses were not observed below 800 ³C.9,12 Therefore, an experiment was ®rst carried out to investigate the effects of (i) different ashing temperatures and (ii) the addition of ammonia to the sample prior to ashing. For each ashing temperature, two 10 g aliquots of dried seaweed (Fucus vesiculosus) powder were spiked with 95mTc. One aliquot was wetted with 10±20 ml of ammonia solution

(relative density 0.88). The samples were gently dried on a hotplate and then placed in a muf¯e furnace. The temperature was ramped at a rate of 100 ³C h21 and left at the speci®ed value for 6 h. The results are presented in Fig. 2(a). No signi®cant losses of technetium were observed at temperatures below 750 ³C and the samples treated with ammonia did not appear to differ markedly from the untreated samples. These samples were all subsequently processed and the 99Tc concentrations determined by ICP-MS. The results are presented in Fig. 2(b) (each data point representing the average of two aliquots ashed at a particular temperature). The data obtained at ashing temperatures above 750 ³C are unsatisfactory as the poor yields result in very large errors associated with the measurement of both the 99Tc concentration and the 95mTc recovery. The 99Tc concentration determined between 450 and 750 ³C gradually increases from 5 to 5.6 Bq g21. This effect was further investigated by repeated analysis of another seaweed sample which had been used in a large intercomparison exercise and thus had a relatively well de®ned 99Tc concentration (17.9¡0.9 Bq g21).13 Six aliquots of this sample were ashed at 550 ³C (treated with ammonia) and six at 750 ³C (treated with ammonia). The 99Tc concentrations derived from the samples ashed at 550 ³C (16.3¡0.6 Bq g21) were consistently lower and in poorer agreement with the consensus value than those ashed at 750 ³C (18.1¡0.8 Bq g21). Wigley et al.6 observed a similar effect although at a lower ashing temperature and concluded that low 99Tc concentrations in samples ashed below 500 ³C resulted from reduced ef®ciency of the subsequent leaching process for the matrix bound 99Tc. Therefore, an ashing temperature of 750 ³C was chosen for routine use and, as a precaution against possible volatilisation, samples were also treated with ammonia. After ashing, sample dissolution (or, in the case of marine sediments, leaching) was achieved by gently heating (v75 ³C) on a hot-plate for 4 h following the addition of 50 ml of 8 M HNO3 and 5 ml of 30% m/v H2O2. The hydrogen peroxide ensures that all the technetium present will be in the z7 state. Any residual particulate matter is removed by ®ltration. Recrystallisation and ruthenium decontamination

Fig. 1 Flow diagram of analytical method.

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Prior to ICP-MS analysis, it is necessary to remove the isobaric interference 99Ru (natural abundance 12.7%). The removal of ruthenium is also required when using radiometric techniques as the radionuclide 106Ru interferes with the b-counting of 99Tc. Ruthenium decontamination has normally been achieved through the use of solvent extraction2±4,14±16 although more recently the use of TEVA.Spec resin, an extraction chromatographic material, has been proposed.6,10,17 Technetium, in the pertechnetate form, is strongly adsorbed by the resin at low concentrations of nitric acid whereas ruthenium is not effectively retained. The technetium can subsequently be eluted with higher concentrations of nitric acid. When applied to environmental samples with complicated matrices and, in some cases, high ruthenium levels, it has been found that complete Tc±Ru separation cannot be achieved by use of TEVA.Spec resin alone. Most workers have found it necessary to include additional clean-up steps to improve the Tc±Ru separation. Butterworth et al.17 used a combination of iron hydroxide precipitation and anion exchange to clean up the samples prior to the use of the TEVA.Spec resin. Beals10 recommended the use of solvent extraction for samples with high ruthenium levels whereas Wigley et al.6 preceded the TEVA.Spec stage with an iron hydroxide precipitation and succeeded it with a solvent extraction step. Although all these solutions to the problem are effective, they add to both the cost and time necessary to complete the analysis. In this study, we made use of a simple yet effective recrystallisation stage prior to the use of the TEVA.Spec

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Fig. 2 Effect of ashing temperature on (a) technetium recovery and (b) ®nal result. Error bars (1s) are shown where they exceed the symbol size.

resin. The ®ltrate from the leaching stage is evaporated gently (v75 ³C) to incipient dryness. As the volume decreases, recrystallisation of dissolved salts takes place. A 30 ml volume of water is then added in 10 ml aliquots, the slurry ®ltered and the ®ltrate retained. Analysis of the redissolved solids by ICP-MS indicates that these mainly consist of salts of Group II elements (Mg, Ca, Sr and Ba), transition metals (Fe, Mn, Cu, Ni and Zn), lead and uranium. On average, approximately 20% of the ruthenium in the original solution is retained within these salts whilst losses of technetium are minimal (v2%). Although the extent of Tc±Ru separation achieved by this step is small, it does provide a relatively matrix free solution from which it is possible to separate these two elements completely. The ®ltrate from the recrystallisation step is then added to a preconditioned TEVA.Spec column (0.3 g of resin retained in a Pasteur pipette with a glass-wool plug, washed with 30 ml of 4 M HNO3 followed by 30 ml of 0.1 M HNO3). At the low nitric acid concentration of the ®ltrate, technetium, unlike ruthenium, is retained by the resin. Ruthenium decontamination is then completed by washing the resin with 30 ml of 0.1 M HNO3 and the technetium eluted with 30 ml of 4 M HNO3. The reliability of the method was checked by the multiple analysis of a seaweed sample containing approximately 140 ng g21 (87.5 Bq g21) of 99Tc and 20 ng g21 of Ru. Analysis of the eluates was carried out by ICP-MS. The results are presented in Table 2. It may be that the importance of the recrystallisation step lies in the removal of a large fraction of the matrix rather than in the amount of ruthenium removed. Certainly, the ef®ciency of the Tc±Ru separation achievable using the TEVA.Spec resin is enhanced by this pretreatment. This is further demonstrated in Fig. 3(a) and (b), which show the mass spectra (m/z 95±103) of the TEVA.Spec eluate with and without the recrystallisation step. The eluate obtained without use of the recrystallisation step shows a small but signi®cant ruthenium peak at m/z 101. The ef®ciency of the ruthenium decontamination is routinely monitored by measuring the count rate at m/z 101 (the natural abundance of 101Ru is 17%). The ruthenium contribution to the count rate at m/z 99 is directly proportional to that at m/z 101, hence it is possible to correct for small amounts of ruthenium remaining in the ®nal solution. In practice, in the analysis of over 200 environmental samples, no signi®cant contribution from ruthenium has been observed. From the repeat analysis of a seaweed sample with a Ru concentration of 20 ng g21, a decontamination factor in excess Table 2 Average recoveries of technetium and ruthenium in the eluate from the three stages of the TEVA.Spec resin decontamination process (as a percentage of the concentration of these elements in the loading solution), based on 10 repeats Process

Technetium (%)

Ruthenium (%)

Loading Washing Elution

0 0 95

80 20 0

Fig. 3 ICP-MS spectra of ®nal solution (a) with and (b) without recrystallisation step.

of 103 was determined. It was only possible to determine the lower limit of the decontamination factor as the ®nal solution contained no detectable levels of ruthenium. Hence the lower limit of the decontamination factor was determined using the instrumental detection limit of 2 pg ml21 for 101Ru. Chemical recovery This method was developed primarily for the determination of Tc in marine biota. Chemical yields for seaweed and a wide variety of other types of marine biota (lobster, mussels, winkles, crab and Nephrops) average between 80 and 90%. The method has also been applied to sediment samples, which display a lower recovery of around 50±70%.

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ICP-MS Interferences. In addition to the major isobaric interference presented by ruthenium, there are a variety of other elements which, through the formation of polyatomic (59Co40Arz, 62Ni 37 z 64 Cl , Ni35Clz, 64Zn35Clz and 98Mo1Hz) and doubly charged (198Hg2z and 198Pt2z) species, may contribute to the count rate at m/z 99. Although the concentration of some of these elements may be relatively high in environmental materials, the chemical procedure used for the Tc±Ru separation has the added bene®t of excluding most of these elements from the ®nal solution. ICP-MS analysis of a variety of sample types (sediment, lobster, mussels and seaweed) suggests that the concentrations of these elements in the ®nal solution are not likely to exceed 10 ng ml21. Solutions containing 100 ng ml21 of these elements were analysed by ICP-MS in order to test the extent of the potential interference. In each case, the contribution to the count rate at m/z 99 was found to be negligible. Internal standard. The suitability of a variety of nuclides (9Be, 59Co, 115In, 209Bi and 238U) was tested for use as an internal standard. The behaviour of 115In was found to be the J. Anal. At. Spectrom., 1999, 14, 1849±1852

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most similar to that of Tc, as would be expected given the proximity of the masses. One potential problem in using 115In as internal standard is caused by the formation of 99Tc16Oz (m/z 115).18 Formation of the oxide, however, was found to be insigni®cant (v0.02%) and it was therefore concluded that 115 In could be used to correct for changes in sensitivity resulting from changing instrumental conditions throughout the run.

Table 4 Performance of method

Matrix effects. The concentrations of 99Tc in the sample solutions are obtained by reference to a calibration curve produced by the analysis of standard solutions containing known concentrations of 99Tc. It is necessary, however, to check that the slope of the calibration curve derived from the use of the standard solutions does not vary signi®cantly from that derived from the use of `real' solutions. Seaweed samples, containing low levels of 99Tc, were processed and the ®nal solution spiked with 99Tc to produce concentrations of 0.1, 1, 2, 10 and 20 ng ml21. The slope of the calibration curve obtained [35 100¡1200 cps (ppb)21] was then compared with that obtained for the standard solution [34 500¡1000 cps (ppb)21]. The results show that there is no signi®cant difference in the slopes of the two calibration curves and suggest that there are no unforeseen matrix effects which cannot be corrected for by the use of the 115In internal standard.

Detection limit of method

Instrumental limit of detection. The instrumental limit of detection, based on three times the standard deviation of 11 repeated analyses of a reagent blank, is 2.0 pg ml21 (1.25 mBq ml21). Accuracy and precision There are no readily available standard reference materials for Tc with which to validate the method. The validity, in this case, was assessed through participation in a relatively large international intercomparison exercise involving 14 participating laboratories from eight countries.13 The exercise involved the analysis of ®ve seaweed samples and the initial results obtained by this laboratory are presented in Table 3. The results for samples C, D and E were satisfactory but those for samples A and B appeared to be too high. Subsequent investigations revealed that the cause of the problem was crosscontamination at the ashing stage. It had been our practice to re-use a set of silica crucibles. Tests revealed, however, that despite a rigorous cleaning procedure (soaking overnight in Decon, followed by a 6 h re¯ux in 8 M HNO3), a small amount of carry-over (approximately 0.1%) could still occur. This problem was exacerbated by the large variations in 99Tc levels present in the samples used in the intercalibration study (over four orders of magnitude). The method was subsequently revised and disposable porcelain crucibles are now used. Repeated analysis of sample B, using the revised procedure, produced satisfactory results (61.5 Bq kg21). The 1s standard deviation on 20 repeated analyses of two large seaweed samples averaged 8%. This value for the precision of the method can be taken to be the upper limit of the uncertainty since it is likely that a signi®cant fraction of the error is attributable to sample heterogeneity.

Parameter

Performance

Chemical recovery for marine biota Chemical recovery for sediment Ru decontamination factor Detection limit of method

80±90% 50±70% w1023 1 Bq kg21

The detection limit of the method, based on the average yield for a 10 g biota sample taken up in 5 ml of nitric acid, is 1 Bq kg21.

Conclusions A rapid and ef®cient method for the determination of 99Tc in a variety of marine samples by ICP-MS has been described. Important parameters, relating to the performance of the technique, are summarised in Table 4. Other important ®ndings resulting from the study are detailed below. The choice of ashing temperature appears to be critical to the performance of the technique. At too low a temperature, the subsequent leaching process does not liberate 99Tc quantitatively from the matrix. At too high a temperature, losses of Tc become unacceptably high. For this study, an ashing temperature of 750 ³C was found to be ideal. Despite a rigorous cleaning protocol, cross-contamination from the re-use of ashing crucibles was observed. The use of disposable crucibles is recommended. The incorporation of a simple evaporation±recrystallisation step greatly improves the ef®ciency of the Tc±Ru separation achieved by the subsequent use of the TEVA.Spec resin.

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Table 3

99

Tc intercalibration results (Bq g21 dry¡1s)

Sample

This laboratory

Consensus value

A B B C D E

0.0140¡0.0008 0.0885¡0.0053 (0.0615¡0.0048)a 3.15¡0.20 18.10¡0.92 138¡10

0.0059¡0.0011 0.0583¡0.0046 0.0583¡0.0046 3.91¡0.13 17.91¡0.78 133.1¡5.4

a

Repeat analysis using disposable crucible (see text).

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Paper 9/05274G