A method for the measurement of 99Tc by Quadrupole ICP-MS in dry fallout and sea grass samples

JAERI-Conf 2003-010 JP0350636 4.41 A method for the measurement of "Te by Quadrupole ICP-MS in dry fallout and sea grass samples. Jose Luis Mas"), Manuel Garcia-Leon 2) and Juan Pedro Bolivar') 1) Dept. Fisica Aplicada. Universidad de Huelva. EPS La Rabida, Ctra. Palos de la Frontera, s/n. 21819 Huelva (Spain) 2) Dept. Fisica Atomica, Molecular y Nuclear. Universidad de Sevilla. Facultad de Fisica. Avda. Reina Mercedes, 41080 Sevilla (Spain) Corresponding author pepernas(2anirs-ooJ12. Address while the stay at NIR;. National Institute of Radiological Sciences. 49-1, Anagawa, Inage-ku, 263-8555 Chiba (Japan) ABSTRACT A method for the detection of "Tc by ICP-MS in environmental samples is proposed and explained in detail. This method combines the capabilities of anion exchange chromatographic TEVA-Spec' resin with prior isotope concentration using solvent extraction to Tn'butylphosphate. The problem that the excess of Mo in the solution analyzed by ICP-MS causes in the instrumental response at m/z ratio 99 is described in detail, and the proposed solution takes the form of the referred radlochemical method. The realistic limits of detection of this technique (bearing in mind the particular methodology here applied) are in the range of 20-70 mBq/kg when treating mass amounts in the range of 20-125 g dry weight. A few environmental samples proceeding from a region unaffected by any closer nuclear industry (the Southwest of Spain) are analyzed for checking '9Tc concentration. Keywords: Technetium, ICP-MS, sea grass, fallout. 1. INTRODUCTION. "Tc is a pure beta emitter with a long half-life, T,2= 2 x 1 O' y, which appears during the fission Of 29pU or 2"U at a relatively high fission rate of some 6. Its environmental relevance is well established, though sub-ppt "Tc levels in non-perturbed sites pose serious lmitations to traditional radiometric methods 1,2). In the last years, many authors have shown the use of inductively coupled plasma mass spectrometry (ICP-MS) as a powerful tool for the dtection of '9Tc in environmental samples 3-6), and even some of these methods have been successively improved, expanding thus the range of potential sample matrix that they are able to analyze 7 It is well know that the major counting interferences are due to the natural isotopes Ru 12.6 relative abundance), due to isobaric overlap, and 9Mo 24%) and "Mo 9.6%), due the first to hydride generation and the second one to an excessive abundance to sensitivity. This could be really a major problem when quadrupole ICP-MS devices are involved in the measurements, as the typical concentrations of Mo could be as high as three orders of magnitude greater that those of Ruthenium or Technetium in natural soils. In our Of I OX I -6, typical conditions, the abundance sensitivity could be estimated in the range which means that when Mo is present in the final analyzed solution in a concentration in the order of I ppb or greater, concentrations of '9Tc as high as I ppt could not be detected (8). The magnitude of hydride generation effect is well lower than for abundance sensitivity. In this paper, a method for "Tc measurement by ICP-MS is developed and its abilities to decontaminate from Mo and Ru are analyzed. This method is applied to a few environmental samples - 355 - JAERI-Conf 2003-010 coming from a region out of the direct influence of any nuclear ndustry. Some conclusions on the obtained results could be arisen. 2. MATERIALS AND METHODS. 2. 1. Samples and Materials. The ICP-MS here involved is an Ar-feed commercial quadrupole spectrometer (Hewlett-Packard 4500) that works with a Babyngton-type nebulizer. For a counting time of 0.1 s, in the nstrumental conditions found optimizing by the variable control method at mass 99, and when Tunning detection mode is used, the instrumental limit of detection is in the range of 02 pg/g, that is, two orders of magnitude lower than the found using the autotunning method. These operating conditions have been checked using the deformable five dimensions Spendley's Smplex algorithm as applied to five of te ion lenses. The chemical recovery is calculated using the nternal standard method. The chemical tracer is the 140.5 keV gamma emitter 99mTc, obtained from 9Mo generators from a Nuclear Medicine Service. Even not being the better option 9), this one is easily achievable, and besides its very low half-life (6.02 h) and its decay to '9Tc, it has been shown (10) that this tracer could be added to the sample in amounts as high as 0.5-1 MBq wthout very hard effect on the limit of detection, if several simple cares are taken account regarding the tracer elution from the clinical generator. In this case, the classical IUPAC definition of Limit of Detection (understood as the equivalent concentration to 3 of the blank counting) must be adapted to achieve a more realistic Minimum Detectable Mass Concentration. The gross counting rates due to interferences and due to te own tracer are given account. Therefore, is assumed that theoretically every sample could have a slightly different MDMC value according to the chemical recovery there obtained. The gamma counting was performed using a NaI(TI) solid scintillation detector EG&G Ortec. The method performances were characterized using the chemical recovery (RQ) for tracer, the concentration factor (CF) for Tc and the decontamination factor (DF) for Ruthenium and Molybdenum, being these two numerical quantities depending on the initial sample mass involved in the analysis. For checking the suitability of the method, two soil samples, previously analyzed for other elements (I 1), were used. Their negligible Tc contents could be 'ustified on base of their origin from layers below the surface of a metal extraction quarry. That hypothesis is supported on their very low organic matter contents and the activities of some other man-made radionuclides such as "'Cs under the limit of detection of the semiconductor Ge(HP) used. Once checked the method, it was applied to the analysis of two samples of sea grass (Zostera Marina) collected in the Southwest of the Atlantic Spanish coast, at Huelva. The other sample here analyzed is a dry fallout sarnple collected in the rooftops of the Faculty of Physics Sciences of the University of Seville (Spain). This one corresponds to a very dry period in this region, from April to July of 2001, and could serve us to support some other works that uses rainwater as environmental monitor for "Tc. 2.2. The chemical method. A detailed scheme of the proposed method appears in Fig. 1. About 30 g of dry weight sea grass or 125 g of atmospheric deposition are used for the analysis. After sample tracing and pretreatment, Tc remains dissolved in ca 250 ml of 8M HNO3, too concentrated to be load directly onto a TEVA-Spec resin. Thus, a matrix adaptation to H2SO1 is carried out through a reduction-precipitation step, and then the Tc is concentrated using a solvent extraction process to TBP. Some Ru decontarnination is perforined 'n the washing step. After back extraction, Tc is concentrated in a volume almost ten times lower than for the previous leaching, in a matrix easily adaptable to the TEVA-Spec resin management. - 356 - JAERI-Conf 2003-010 As not .IM HNO, but 0.5M HN03 is used to load the Technetiurn onto the column, some differences could appear against previously published results of other authors (7,12). Anyway, if great Solid sample amounts of Mo are involved, this I process of sample concentration and Bath in 5% NH40H, add tracer purification using this Evaporate to almost dryness incineration at 450' C for 1.5 h chromatographic extraction could be repeated until three times with te Leaching with 150 nil M HN03 30 rn! 30% HO, under re flux (T<80' C) same microcolumn. In these I conditions, and if little sample Whatman CF/G glass fiber filtering volumes are involved in the I chemical separations 13), the resin Adjust pH to 34 using 525% NH40H rduce Te using FeSO4-7H20 performances are not affected. This Add FeC13-7H20 carrier, adjust pH to 9 with cone. NH40H way, the DF for Mo could be Dissolvc precipitate using M kso4l o2ddize with (NH4S2(8 increased in more than three orders Solvent extraction to prc Iviously equilibrated TBP of magnitude. Wash organic phase with the same volumc of 3M H2SO4+ HF I The results for these two Back xtracion to 25% NH40H i In the presence of Xylcnc twice) samples appear in Table 1. The Evaporate to dryness, scavengc residue with 20 nil 0.5M HN03 chemical recoveries are qualitative, Filter through 3 nil of Dowex 5OW-X8 cation exchang rsin in the range of 30 %. This means Load solution onto TEVA Spec rsin that optimizing this technique in the Wash the colunuu us Iin& 40 ml M HN03 future is necessary. However, the X achieved US for technetium are Elute Te with 8M HN03 relatively high, and high enough DF Evaporate to almost dryness, Irceovcxwith 0.5M HN03 for Mo and Ru are achieved. The different values of DF for Mo are Gamma countLng - p ICP-MS counting not yt explained. Anyway, these values assures than even when so Figure 1: Detailed scheme of the applied method for Tc high abundance sensitivity appears concentration and purification. at the spectrometer, counting overlaps from the adjacent mass will be avoid. Table 1. The obtained results for two test soil samples used to check the method performances. .......................................................................................................................................... .......... I........... I........................................ Sample Recovery (%) CF for Tc MDMC (Bq/kg) DF for Ruthenium DF for .............................................. . .... ..... ... .. M o ly b d e n u m ..... ... ... ......... ... ...... ... ......... .... .. ................... .................. ...... .... .. . ..... I...... ............... .. S63 32.Otl.4 4.55±0.20 0.013 ;--9500 500 S64 29.0:0.4 3.98±0.19 0.024 ;--9500 3500 ....................................................................................................................................................................................... The method could be easily adapted to the analysis of other environmental samples as borehole water or rainwater. The only differences appear, of course, in pretreatment and the aqueous and organic phase volumes involved in the solvent extraction process. An experimental evaluation of the MDMC dependence on the water volume analyzed is shown in Figure 2. 357 - JAERI-Conf 2003-010 3. RESULTS AND DISCUSSION. The obtained results for the dry fallout sample (DFS) and the two samples of sea grass (ZM I and ZM2) are shown in Table 2 The CF values for technetiurn are quite different depending on the type of sample, as the initial masses involved are too. The DFS sample 2 .5 -. ..... .. ........ ....... ..... . ............ ....... ........... .......... ... presents a mass activity that really is, into the uncertainty intervals, equal -2.0 . .... ...... ................. ... ... ..... ... .. ............. ................................... .............. .... than the MDMC. This shows the Cr extraordinarily scarce contents in co 3 .5 - ............... .. ..... ..... .. ............. .................... .................... ..... .... .. . ......... "Tc of samples from regions not directly affected by nuclear .0 . .. ..... .... ...... ..... ................... ... . ........ industries, and supports the hypothesis of an almost complete 0 .5 . ......... ...... ....... ................ .. ................... .................. ........... ........ .. . .............. back to the earth surface of the Tc 0 injected into the stratosphere due to 0 .0 . .............. .. ....... ... .. ... ... ...... ........... . ..... . ................. ....................... atmosphe ic nuclear tests after a mean residence time of about 0 5 1'0 1'5 iO 25 O 18 months 14). Some other data Water sample volume (1) dealing with the analysis of Technetium in rainwater, which support this result, are to be Figure 2 Experimental dependence of Minimum published. There are not too much Detectable Mass Concentration with the water volume other data concerning the Tc; analysed using the method described in the text. Line is contents in atmospheric deposition. depicted to guide the eye. Tagarni and Uchida (15) recently published some of these data calculated fi7om samples taken at Hitachinata (Japan), estimating them as lower than 04-0.9 mBq m-' month-'. Our result shows an activity deposition equal to or lower than 0.06 mBq m` mondf 1. And then, very high agreement between the referred conclusions is observed. Table 2 Obtained results for two sea grass samples (ZM1 and ZM2), and a dry fallout sample (DFS). Sample Chemical Yield (%) CF MDMC (mB g).. .. 9rc mqqA- ............. ........................................................................................................................... qA ... .... ...... .S).... DFS 35.6±0.9 4.4±0.1 16 19±5 zM1 32.0:0.8 1.04-±0.03 70 66±19 ZM2 26.3-,..0.7 0.84-±0.03 77 6Ot28 Regarding the sea grass samples, no detection of Tc was possible, as expected. Their concentrations in samples of biological concentrators such as seaweed, coming from the same region, were in the range of 0 1-0.4 B/k-g dry weight 16). With the results shown in Table 2 it results clear that the concentrations in sea grass are below 007 B/kg d.w. This is according to the CF values given in the literature 17) for these two marine species, being for sea grass more than three orders of magnitude lower than for seaweed. Different results to those previously explained would have suggested some other local sources for 'Tc not yet identified. 4. CONCLUSIONS. - 358 - JAERI-Conf 2003-010 A method for the detection of "Tc by ICP-MS in environmental samples is proposed. The method combines the capabilities of anion exchange chromatographic TEVA-SpeCT' resin with prior isotope concentration using solvent extraction to Tn'butylphosphate. The interferences in the mass counting are analyzed and the combination of the radlochemical method and the optimization of counting conditions allow minimizing them. "Tc is used as a tracer of the method. This causes the need of re-defining the limit of detection so as to take into account the contribution to the counting at the mass of interest. The values are in the range of 20-70 mBq/kg when treating mass amounts in the range of 20-125 g dry weight. Some previsions on the possible range of 99Tc concentrations in environmental samples proceeding from a region unaffected by any closer nuclear industry (the Southwest of Spain) are made, and subsequently checked by applying this method. 5. ACKNOWLEDGEMENTS. The authors would like expressing their gratitude to Mr. F. Carrera (Hospital Juan Ramon Jimenez, Huelva, Spain) for his valuable help with the tracer, and to the personnel of the Research Central Services of the University of Huelva (Spain) for allow the access to the ICP-MS facilities. J.L. Mas gratefully acknowledges the financial support of the Scientific Technology Agency of Japan for a scientific visit to the National Institute of Radiological Sciences in Chiba, and is also very grateful to all the staff at the institute for their help and warrn hospitality. This work has been partially supported by the Spanish Ministry of Science and Technology contract BFM2001-38807. 6. REFERENCES. 1. Ehrhardt, K.C., Attrep Jr., M. Technetium-99 in the Atmosphere, Envir. Sci. Technol. 12, 55 1978). 2. Holm, E., Rioseco, J., and Garcia-Leon, M. Determination of "Tc in Environmental Samples, Nucl. Instr. Meth. Phys. Res. 223, 204 1984). 3. Tagami, K., and Uchida, S, Separation procedure for the determination of Technetium99 in soil by ICP-MS, Radiochim. Acta 63,69 1993). 4. Nicholson, S., Sanders, T.W., and Blaine, L.M. The Determination of Low Levels of "Tc in Environmental Samples by Inductively Coupled Plasma-Mass Spectrometry, Si. Total Environ, 130-131, 275 1993). 5. Ihsanullah, and East, B.W., Method for the determination of Technetium-99 using Inductively Coupled Plasma-Mass Spectrometry, Radioactivity and Radiochemistry 2 20, 1994). 6. McCartney, M., Rajendran, K., Olive, V., Busby, R., and McDonald, P., Development of a novel method for the detennination of 99Tc in Environmental Samples by ICP-MS J Anal. Atom. Spectrom. 14, 1849, 1999). 7. Uchida, S., and Tagami, K., Separation and concentration of Technetlurn using a Tc-selective extraction chromatographic resin., J Radional. Nucl. Chem. 221, 35, 1997). 8. Mas, J.L., Garcia-Leon, M., and Bolivar, J.P., 99Tc atom counting by quadrupole ICP-MS. Optimisation of the instrumental response, Nucl. Instr. andMeth. in Phys. Res. A, 484, 660, (2002). 9. Sekine, T., Konishi, M., Kudo, H., Tagami, K., and Uchida, S., Separation of Carrier Free "Tc form Niobium Targets Irradiated with Alpha-Particles, J Radioanal. Nucl. Chem., 239, 48 1999). 10. Mas, J.L., Garcia-Leon, M., Bolivar, J.P., and Sanchcz-Angulo, C.I., The Use of 9Tc as a tracer in the Deterinination of '9Tc by ICP-Mass Spectrometry J Anal. Atom. Spectrom., 15, 1369, 2000). 11. Mas, J.L., Bolivar, J.P., Garcia-Tenono, R,. Aguado, J.L., San Miguel, EG., and Gonzalez, J., A dosimetric model for determining the effectiveness of soil covers for phosphogypsum waste piles, Health Physics, 80, 34, 200 1). _ 359 - JAERI-Conf 2003-010 12. Tagami, K., and Uchida, S., Comparison of the TEVA-Spec resin and liquid-liquid extraction methods for the separation of Technetiurn in soil samples, J Radioanal. Nucl. Chem., 239, 643 1999). 13. Mas, JL., Detection of Technetium-99 by ICP-MS, PhD. Thesis, University of Seville 2002), Seville, Spain. 14. Garcia-Leon, M., Manjon, G., and Sanchez-Angulo, C.I.,"Tc/"'Cs activity ratios in rainwter samples collected in the South of Spain., J Environ. Radioactivity, 20, 49 1993). 15. Tagami, K., and Uchida, S., Analysis of technctium-99 in soil and deposition samples by inductively coupled plasma mass spectrometry., Appl. Radiat. Isot., 47, 1057, 1996). 16. Manjon, G., Garcia-Leon, M., Ballestra, S., and Lopez, JI, The Presence of Man-Made Radionuclides in the Marine Environment in the South of Spain, J Environ. Radioactivity 28, 171, 1995). 17. Coughtrey, P.J., Jackson, D., and Thorne, M.C., Radionuclide Distribution and Transport in Terrestrial and Aquatic Ecosystems., A.A. Balkema Publishers, Rotterdam, The Netherlands, Vol. 3 p210 1983). - 360 -