Redetermination of Low-level 99 Tc in Planchet Samples by ICP-MS

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1 Redetermination of Low-level 99 Tc in Planchet Samples by ICP-MS S. Uchida, K. Tagami and M. García-León* Environmental and Toxicological Sciences Research Group, National Institute of Radiological Sciences, Anagawa 4-9-1, Inage, Chiba, , JAPAN *Faculty of Physics, University of Seville, Apdo. 1065, Seville, SPAIN Technetium-99 concentrations in environmental samples are usually very close to or under the detection limit of a radiation counter. In recent years, ICP-MS has been popularized to measure low 99 Tc concentrations. Besides, the method is useful to check the concentration of determined 99 Tc by a radiation counting method. Thus, in this study, ICP-MS has been used to re-measure 99 Tc in samples prepared for a radiation counting technique. Subjected 99 Tc sources were planchet samples on which Tc was electroplated after chemical separation from algae. The 99 Tc activities were previously determined by radiation counting. Tc was continuously removed from each planchet sample with 2M and 2M. During the first Tc dissolving step from the planchet with 2M, a small amount of Tc, which was thought to be fixed as TcO 2, was dissolved. However, additional two extraction steps increased the percentage of extracted Tc to 93 97%. After the solution containing Tc was adjusted to 0.1M, Tc was extracted on a TEVA resin to concentrate Tc and to remove Ru. It was found that Tc was completely separated from Ru in the final solution for ICP-MS by our separation method. Thus, the isotope, whose mass was 99, could be identified as 99 Tc. The results of 99 Tc measurements by both radiation and ICP-MS methods agreed well with each other. The total recoveries for Tc on the planchet samples were almost the same, with an average of 91%. INTRODUCTION Technetium-99 (t 1/2 = 2.1 x 10 5 y) is produced by thermal fissions of 235 U and 239 Pu with relatively high yield of about 6%. At present, most part of 99 Tc discharged into the environment originates from nuclear weapons testing ( ). In the near future, it is predicted that the quantity of 99 Tc in the environment will increase due to releases from nuclear power plants, nuclear facilities or nuclear fuel waste disposal vaults. Its long half-life assures its presence in the environment. Consequently, 99 Tc will be a significant contributor to the future collective dose received by the population, and will play an important role in the problem of future nuclear waste management. All these reasons make it very relevant to study the presence and clarify the behaviour of 99 Tc in nature. Knowledge on the concentration levels of 99 Tc in environmental compartments, such as soil, vegetables, rain and so on, is important to follow the 99 Tc behaviours from discharges through spreading paths to the biosphere (1-3). So far measurement of 99 Tc in environmental samples, is commonly carried out by radiation counting methods, e.g., low background gas-flow proportional and liquid scintillation counters (4-6). In recent years, it has been reported that inductively coupled plasma mass spectrometry (ICP-MS) is applicable to the determination of 99 Tc, with high sensitivity (7-11). This technique has been able to lower the detection limit for 99 Tc beyond the limit for any radiation counting method, which has facilitated the analysis of 99 Tc in various environmental samples. Many interesting natural samples have been already analyzed by radiometric techniques. The results although interesting, are in many cases difficult to interpret due to the limitations of such methods: as the measured values have high uncertainties. Unfortunately, the original environmental samples are nowadays unavailable for radiochemical analysis because they have been already used. Thus, they cannot be analyzed by ICP-MS, though it would provide us better information on the results. However it is certainly possible to determine by ICP-MS, the 99 Tc present in the final source prepared for radiation counting. This way a more precise 99 Tc activity can be obtained which facilitates the interpretation of the results. This is the main objective of the present paper. Usually, after radiochemical separations, 99 Tc was electrodeposited onto a stainless steel planchet. Thus, the main task of the work consists of efficiently recovering Tc from the planchet and measuring its 99 Tc contents by ICP-MS. To achieve this, some conditions have to be accomplished. 1) To remove as much Tc from the planchet sample as possible; 2) To completely separate Ru from the sample solution, since the stable 99 mass Ru isotope could interfere in the determination of 99 Tc. 3) To remove other matrix elements, so as to reduce the concentration of stable isotopes to a value less than 300 ppm; and 4) To adjust the nitric acid strength in the final solution to less than 10%. 1

2 In what follows, we describe a method developed by the authors to remove Tc from the planchet samples and to separate Tc from other elements. The method comfortably satisfies the requirements described above. The resulting 99 Tc is measured by ICP-MS. The method has been applied to stainless steel planchets containing 99 Tc radiochemically extracted from algae samples. Their 99 Tc activities were previously determined with a gas-flow proportional counter or a GM counter at the University of Seville. EXPERIMENTAL Samples. The algae samples used had been analyzed and electroplated at the University of Seville. Four samples were collected from the Irish Sea and other four samples were collected along the Mediterranean and Atlantic coasts of Andalucia, Spain. Preparation and chemical separation method to analyze 99 Tc by beta counting is described elsewhere (2, 12). After the chemical separation, the planchet was air-dried and bounded with mylar films (PETF), then it was placed on a plastic holder. The activities of 99 Tc on the planchet samples were measured by beta counting. The planchet samples had been stored at room temperature for 5-7 years until the 99 Tc sources were used in this study. Reagents. Nitric acid solutions, 0.1M, 2M and 8M were prepared from ultra-pure nitric acid (Tama Chemicals, AA-100) diluted with deionized water. 2M and H 2 O 2 were JIS (Japanese Industrial Standards) special grade. Deionized water (>16MΩ) was used throughout the work. A 99 Tc-free 95m Tc (t 1/2 = 61 d), which was generated by irradiation of Nb in a cyclotron (13), was used as a yield tracer. Technetium-99 standard solution was commercially available from Japan Isotope Association, as NH 4 TcO 4 form in 0.1% ammonia solution. Dissolving Tc from the planchet samples. From the 5-7 years stored 99 Tc sample (a stainless steel disk and two mylar films), 99 Tc was separated and its concentration was measured by ICP-MS. The chemical separation procedure is described in what follows. To remove Tc from the planchet and films, 5 ml of 2M with a small portion of H 2 O 2 were added and heated C. At the for same 3 h on time, a hot plate below 80 95m TcO - 4 was added as a tracer. Next, leaching with 5 ml of 2M C. was carried out. Then, the films were transferred to a glass beaker and heated for 2 h at 400 The residue was extracted in 5 ml of 2M and filtered. The leachates with 2M and 2M and filtrate were mixed well. Separation and concentration with a Tc-selective chromatographic resin. The solution containing Tc was passed through a Tc-selective chromatographic resin (EIChroM Industries, TEVA resin) to separate Tc from Ru (14). Forty ml of 2M were directly pipetted into each column to remove matrix elements on the resin. The loading solutions and the 2M solution were allowed to drain completely. Finally, Tc in the column was eluted with 5 ml of 8M. The final eluate was evaporated to dryness and dissolved in 5 ml of 2%. After the activity of 95m Tc was determined, the solution was measured for its 99 Tc concentration by ICP-MS. Chemical recovery. In this study, the recovery during the dissolution steps were obtained by measuring the 99 Tc activity of each planchet sample with a low-background beta spectrometer (Fuji Electrical Co., Pico-beta ) before and after each dissolution step. To estimate the recovery during the TEVA resin extraction, 95m TcO 4 - was added as a tracer to obtain the recovery yield during the steps of Tc stripping from the resin and drying-up of the stripped solution. 95m Tc was measured by gamma spectrometry with a NaI (Tl) autowell scintillation counter (ALOKA, ARC-300). 99 Tc measurement by ICP-MS. 99 Tc concentration in the final solution was measured by ICP-MS (Yokogawa, PMS-2000). The 99 Tc standard solution was used to make a working curve for 99 Tc by ICP-MS measurement. Counting time was approximately 12 min per solution. The instrumental detection limit for ICP-MS was 0.05 ppt (0.03 mbq/ml). During the measurement, mass number 102 was also measured for Ru in the samples. Ru has 7 stable isotopes; at mass 102, Ru has an isotopic abundance of 31.6%. If counts at mass 102 of sample solution are higher than those of deionized water, 99 Ru would interfere with 99 Tc counting. RESULTS AND DISCUSSION Recovery of 99 Tc Before the 99 Tc sources were used in this study, they had been stored for 5-7 y at room temperature. It 2

3 was reasonable to expect that Tc was plated as TcO 2 or metal as reported by Kotegov et al. (15). Therefore, an oxidizer, such as nitric acid, was expected to remove most of the Tc from the planchet and films as the higher oxidized state of TcO - 4. In order to clarify the removed 99 Tc amount from the planchet and the films in each dissolving step, the samples collected from the Irish Sea was used. The concentrations of 99 Tc in the samples were high enough to determine the contribution of each dissolving step by the low-background beta spectroscope. In the first leaching step, 2M solution was used, however, a smaller amount of Tc was dissolved than expected. The percentage of the extracted Tc ranged from < 1 to 86% (Fig. 1). The next leaching was carried out with 2M, because, (a) the 99 - Tc was electroplated in 2M and (b) TcO 4 is stable in an alkaline solution. After the treatment with 2M, the amount of 99 Tc remaining on the sample was still high (9-38%). As can be seen from Fig. 1, most of the 99 Tc in the samples was fixed on the films accounting for 5-30% of the initial 99 Tc amounts on the 99 Tc C for sources. Then the films were put into glass beakers and incinerated at h. Previously, C (16). With we a reported 400 C incineration, that Tc started no to evaporate above 500 loss of Tc was expected and Tc would remain in the beaker. Tc in the beaker was extracted with 2M and filtered. The filtrate was transferred into the same plastic bottle with the solutions obtained by the previous steps. The remaining amount of 99 Tc in each residue on the filter was less than 2% of the total 99 Tc. In each plastic bottle, the mixture obtained was adjusted to about 0.1M. In such an oxidized condition, the chemical form of Tc was TcO - 4. During these dissolving steps, 93-97% of 99 Tc was collected from the 99 Tc sources (Table 1). Percentage of 99 Tc remaining on sample HNO -total 3 -films -planchet S-1 S-2 S-3 S-4 Fig. 1. Percentage of 99 Tc remaining on planchet samples (S-1 to S-4) after extractions with 2M followed with 2M. Table 1. Recoveries of Tc from the planchet samples at dissolving and separation steps. Sample 99 Tc recovery Tc recovery and leachings TEVA extraction and Dry-up Total Tc recovery (%) (%) (%) S / / /- 0.6 S / / /- 0.8 S / / /- 0.7 S / / /- 0.6 Average / / /

4 Next, separation and concentration of 99 Tc from the solution by the TEVA resin was carried out. From this step, 95m TcO - 4 was added as a tracer. The resin was used because Tc can be completely separated from Ru with high recovery (14); as TcO - 4, Tc was extracted on the resin from ca. 0.1M solution with almost 100% recovery and Tc was stripped from the resin with 5 ml 12M. The recoveries from the sample solutions to the 5 ml stripped solutions were more than 98%. The solution could not be introduced directly into the ICP-MS because of its high nitric acid concentration. For the ICP-MS system, 1-2% nitric acid concentration is preferable. Since no remarkable Tc loss from C (17), a nitric the acid solution is expected when the solution is heated at a temperature lower than 80 stripped solution, of almost 12M, was evaporated to dryness. The dried matter was dissolved in 5 ml of 2%. Finally, the recoveries measured with 95m Tc during the separation steps with TEVA resin extraction and heating of the stripped solution were 95-98% as shown in the second column in Table 1. Total recoveries ranged from 89-94% (Table 1). Due to the fixation of 99 Tc on the mylar films, several steps were added to the procedure. At this stage, we have not been able to determine the chemical form of the Tc, however, we expect that Tc was fixed tightly in the films. Comparison of a radiation counting method and ICP-MS for the determination of 99 Tc As mentioned before, 99 Tc determinations on the planchet samples were previously carried out by a radiation counting method. The obtained results are given in Table 2. The ICP-MS results are also presented in such Table. It was checked that our extraction method was able to purify the solution from Ru, since no stable Ru-isotope was observed in the mass spectra. The lower detection limit for 99 Tc by the machine was 0.03 mbq/ml, therefore, as the final solution, the detection limit was 0.15 mbq/sample. The third column of Table 2 lists the ratios between the results of both measurement methods. The average of the ratios was 1.12; the results of 99 Tc measurements by both methods agreed well. It is necessary to compare one analytical method quantitatively with another analytical method, because each analytical method has qualitative merits and demerits. Because concentrations of 99 Tc in environmental samples are very low, an instrument, which has a high sensitivity and a low detection limit, is attractive. ICP-MS is well known as one of these methods, however, it is a new method so that its accuracy must be checked. From the results, it was clear that the chemical separation procedure for 99 Tc used in this study was suitable for the ICP-MS measurement. Further, the chemical separation procedure carried out at the University of Seville was also suitable for a radiation counting method, i.e. the GM counting system. Determination of low-level 99 Tc concentration in the samples The concentrations of 99 Tc in typical environmental samples were very close to or under the detection limit of a low-background radiation counter. If we could measure the 99 Tc concentrations in these samples, the data obtained would give good information on the behaviour of 99 Tc in the environment. Since the separation procedure proposed in this study provide us high recoveries of 99 Tc from the planchet samples with films, the procedure can be applied to other low-level samples. We used other four planchets separated from algae samples collected along the Mediterranean and Atlantic coasts of Andalucia, Spain. Table 2. Concentration of 99 Tc in planchet samples Beta-counting ICP-MS Ratio (A) (B) (A/B) Sample mbq/sample mbq/sample S / / / S / / / S / / / S / / / Average /

5 Table 3. Concentrations of 99 Tc in algae samples determined by both ICP-MS and beta-counting methods. Recovery ICP-MS (A) Beta-counting (B) Ratio Sample 95m Tc 99 Tc (mbq/sample) 99 Tc (mbq/sample) (A)/(B) A / / A / / A / / A / / Table 3 lists 99 Tc activities in the algae determined by ICP-MS (this study) and by beta-countings (12). The recoveries were , which were obtained from 95m Tc with the correction as mentioned above. The fourth column shows ratios between determined values with ICP-MS and beta countings. The ratio varied from 0.6 to 2.1, that is, the ICP-MS values were from about one half to twice of those by beta countings. It can be said that even though these low-level samples were analyzed, the determined values of 99 Tc were almost the same by both measurement methods. If the activity ratios were all below 1, it would mean that some interferences could have appeared during the counting. In the opposite case, i.e. if all the ratios were above 1, the interferences could have appeared during the ICP-MS experiment. It is apparent that the activity ratio does not show any marked trend. And the analysis of all the ICP mass spectra of the samples demonstrated that no other elements, which might interfere in the 99 Tc counting, were present. In other words Ru and Mo concentrations, for example, could be sufficiently lowered with our method (14, 18). From this study, we can conclude that the values of 99 Tc measurements by both radiation and nonradiation counting methods agreed well with each other. This result strongly supported that both methods were available to determine low-level 99 Tc in environmental samples. Further, due to the development of ICP-MS during the past 10 years, the detection limit for 99 Tc has been lowered beyond the limit for any radiation counting method. Therefore, ICP-MS is a most powerful tool for determining low-level 99 Tc, which provides decreasing sample mass and counting time. Although the planchet samples used in this study were all separated from algae, the method might be applied to planchet samples from other environmental samples such as water and soil as well. However, the matrix elements and their concentrations should depend on the samples. There is a possibility that their factors would affect the chemical separation procedure. For this reason, other samples may be checked by ICP-MS. Acknowledgments All the authors would like to thank Mr. T. Fujikawa, Kansai Environmental Engineering Center Co., Ltd., for assistance in the chemical separation from samples. The authors are grateful to Dr. T. Sekine, Tohoku University, for providing us with 95m Tc tracer solution. REFERENCES 1. M. Attrep, J.A. Enochs and L.D. Broz, Atmospheric technetium-99. Environ. Sci. Technol., 5, (1971). 2. M. García-León, G. Manjón and C.I. Sánchez-Angulo, 99 Tc/ 137 Cs Activity ratios in rainwater samples collected in the south of Spain. J. Environ. Radioact., 20, (1993). 3. K. Tagami and S. Uchida, Analysis of technetium-99 in soil and deposition samples by inductively coupled plasma mass spectrometry. Appl. Radiat. Isot., 47, (1996). 4. E. Holm, J. Rioseco and M. García-León, Determination of 99 Tc in environmental samples. Nucl. Instr. Meth. Phys. Res., 223, (1984). 5. D. Jordan, R. Schupfner and H.A. Schuttelkopf, New very sensitive LSC procedure for determination of Tc- 99 in environmental samples. J. Radioanal. Nucl. Chem., Articles, 193, (1995) 6. A.E. Nevissi, M. Silverston, R.S. Strebin and J.H. Kaye, Radiochemical determination of technetium-99. J. Radioanal. Nucl. Chem., Articles, 177, (1994) 7. C.K. Kim, M. Otsuji, Y. Takaku, H. Kawamura, K. Shiraishi, Y. Igarashi, S. Igarashi and N. Ikeda, Application of inductively coupled plasma mass spectrometry to the determination of 99 Tc in soil samples. Radioisotopes, 38, (1989). 5

6 8. S. Morita, C.K. Kim, Y. Takaku, R. Seki, R. and N. Ikeda, Determination of technetium-99 in environmental samples by inductively coupled plasma mass spectrometry. Appl. Radiat. Isot., 42, (1991). 9. S. Nicholson, T.W. Sanders and L.M. Blaine, The determination of low levels of 99 Tc in environmental samples by inductively coupled plasma mass spectrometry. Sci. Total Environ., 130/131, (1993). 10. M. Hollenbach, J. Grohs, S. Mamich, M. Kroft and E.R. Denoyer, Determination of technetium-99, thorium-230, and uranium-234 in soils by inductively coupled plasma mass spectrometry using flow injection preconcentration. J. Anal. Atom. Spect., 9, (1994). 11. K. Tagami and S. Uchida, Determination of 99 Tc in rain and dry fallout by ICP-MS. J. Radioanal. Nucl. Chem., Articles, 197, (1995). 12. G. Manjón, M. García-León, S. Ballestra and J.J. Lopez, The presence of man-made radionuclides in the marine environment in the south of Spain. J. Environ. Radioact., 28, (1995). 13. T. Sekine, M. Konishi, H. Kudo, K. Tagami and S. Uchida, Separation of carrier free 95m Tc from niobium targets irradiated with alpha particles. J. Radioanal. Nucl. Chem., 239, , S. Uchida and K. Tagami, Separation and concentration of technetium using a Tc-selective extraction chromatographic resin. J. Radioanal. Nucl. Chem., 221, (1997). 15. K.V. Kotegov, O.N. Pavlov and V.P. Shvedov, Advances in Inorganic chemistry and radiochemistry, 2, Academic Press, New York, pp (1968). 16. K. Tagami and S. Uchida,, Separation procedure for the determination of technetium-99 in soil by ICP-MS. Radiochim. Acta, 63, (1993). 17. K. Tagami and S. Uchida, Influence of concentrations of elements upon technetium preconcentration in deposition samples during heating. J. Radioanal. Nucl. Chem., Articles, 198, (1995). 18. K. Tagami and S. Uchida, Comparison of the TEVA Spec resin and liquid-liquid extraction methods for the separation of technetium in soil samples. J. Radioanal. Nucl. Chem., 239, (1999). 6

Analysis of Technetium-99 in Marshall Islands Soil Samples by ICP-MS

CYRIC Annual Report 2003 VI. 4. Analysis of Technetium-99 in Marshall Islands Soil Samples by ICP-MS Tagami K., Uchida S., and Sekine T. * Environmental and Toxicological Sciences Research Group, National