ULTRA-TRACE DETERMINATION OF NEPTUNIUM-237 AND PLUTONIUM ISOTOPES IN URINE SAMPLES BY COMPACT ACCELERATOR MASS SPECTROMETRY

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1 AECL Nuclear Review Downloaded from pubs.cnl.ca by on 02/25/18 FULL FULL ARTICLE ARTICLE Ultra-trace analysis of actinides, such as Pu isotopes and 237 Np, in bioassay samples is often needed for radiation protection programs at nuclear facilities. Accelerator mass spectrometry (AMS), particularly the compact ETH Zurich system Tandy, has evolved over the years as one of the most sensitive, selective, and robust techniques for actinide analysis. Employment of the AMS technique can reduce the demands on sample preparation chemistry and increase sample analysis throughput, due to very low instrumental detection limit, high rejection of interferences, and low susceptibility to adverse sample matrices. Initial research and development tests were performed to explore and demonstrate the analytical capability of AMS for Pu and Np urine bioassay. In this study, urine samples spiked with femtogram levels of Np and Pu isotopes were prepared and measured using compact ETH AMS system and the results showed excellent analytical capability for measuring Np and Pu isotopes at femtogram/litre levels in urine. ULTRA-TRACE DETERMINATION OF NEPTUNIUM-237 AND PLUTONIUM ISOTOPES IN URINE SAMPLES BY COMPACT ACCELERATOR MASS SPECTROMETRY Xiongxin Dai a, Marcus Christl a, Sheila Kramer-Tremblay a *, and Hans-Arno Synal b a Canadian Nuclear Laboratories, Chalk River, ON K0J 1J0, Canada b Laboratory of Ion Beam Physics, ETH Zurich, 8093 Zurich, Switzerland Article Info Keywords: neptunium; plutonium; accelerator mass spectrometry; urine bioassay Article History: Received 9 April 2015, Accepted 24 September 2015, Available online 15 December DOI: *Corresponding author: sheila.kramer-tremblay@cnl.ca 1. Introduction Long-lived plutonium (e.g., 239 Pu and 240 Pu) and neptunium (e.g., 237 Np) isotopes are commonly considered to be among the most radiotoxic nuclides to human health due to their highly energetic alpha emission and ability to accumulate onto bone with a long retention time when they get into the body through inhalation or ingestion. For the purpose of radiation protection, urine bioassays of Pu and Np could be required to assess occupational exposure of nuclear workers in nuclear facilities or incidental exposure of the general public after a nuclear accident. Because the Canadian Nuclear Safety Commission (CNSC) regulations require the ability to report all occupational exposures 1 msv committed effective dose, there is a need for extremely sensitive radioanalytical techniques to detect Pu and Np isotopes down to low femtogram levels or even less in urine samples (Table 1). Accelerator mass spectrometry (AMS) is a very sensitive and robust technique for the analysis of intermediate- and long-lived radionuclides. As a result of its high rejection of molecular isobaric interferences and low susceptibility to matrix effects, the AMS technique allows simplification of sample preparation chemistry with a good potential for high sample analysis throughput and reduced cost for the ultra-low level radioassays. This is of particular interest for ultra-trace determination of long-lived actinides in bioassay samples, where other radiometric techniques (e.g., alpha spectrometry) or mass spectrometric methods may not be adequate to meet either the detection limit or throughput requirements. For instance, the measurement of 239 Pu at femtogram (fg) levels by inductively coupled mass spectrometry (ICP-MS) may be compromised by the formation of the 238 UH + isobar and by tailing from 238 U mass [1], as uranium is present in a wide range of concentrations in biological samples. Hence, a highly efficient sample preparation procedure is required to separate Pu and U from the complicated sample matrix, which may be tedious and expensive to operate with only limited reliability for routine measurements. Similarly, thermal ionization mass spectrometry also requires an extremely laborious radiochemical separation procedure to prepare the filament for final analysis [2], which has a significant impact on its analysis throughput and cost. CNL NUCLEAR REVIEW 125

2 CNL NUCLEAR REVIEW TABLE 1. Calculated urine excretion rates of Np and Pu isotopes for 1 msv committed effective dose as a function of days postexposure using GenmodPC dose calculation software. Urine excretion rate a Day 30 Day 90 Day 365 Isotope Dose coefficient (Sv/Bq) mbq/l fg/l mbq/l fg/l mbq/l fg/l AECL Nuclear Review Downloaded from pubs.cnl.ca by on 02/25/18 Type M inhalation Np 1.5E E E E E E E Pu 3.3E E E E E E E Pu 3.3E E E E E E E Pu 5.9E E E E E E E+00 Type S inhalation Np 8.1E E E E E E E Pu 8.4E E E E E E E Pu 8.4E E E E E E E Pu 8.5E E E E E E E-01 a Daily urinary output of 1.6 L for reference man. Despite the fact that AMS has been demonstrated for the precise and accurate determination of Pu at the fg level in urine bioassay samples [3 5], lack of availability and high costs to operate the complicated AMS system have prevented the extensive use of this technique. Recently, we have demonstrated the applicability of a compact AMS system at the Swiss Federal Institute of Technology (ETH) Zurich for Pu urine bioassay at sub-fg/l levels [6]. This compact system, running at 300 kv terminal voltage with a much smaller footprint than conventional (large) AMS systems and low maintenance requirements, provides a very competitive and cost-effective technique compared with conventional mass spectrometers. Furthermore, we have optimized the target preparation methods for actinides using mixed iron/titanium (Fe/Ti) oxides [7]. In this work, initial research and development tests were performed to optimize sample preparation methods for low-level determination of Np and Pu radionuclides in urine using the compact AMS at ETH. The use of a more convenient TEVA pre-packed cartridge was investigated and an improved AMS target preparation method using mixed Fe/Ti oxide was evaluated. Details of the procedure are described and the results are summarized in this paper. 2. Methods 2.1 Reagents and standards All of the chemicals used in this work were analytical grade or higher and purchased from Sigma-Aldrich Canada (Oakville, ON, Canada). A 1000 mg/l titanium standard solution for AAS and a 1000 mg/l iron standard solution for ICP were used for the target preparation step. The ultra-pure water was obtained from a Millipore Direct-Q5 Ultrapure water system. The extraction resin employed in this work was TEVA resin ( µm) pre-packed in 2 ml cartridges, available from Eichrom Technologies, Inc. (Lisle, IL, USA). Radioactive 242 Pu, 243 Cm and 244 Cm standards were supplied by the National Institute of Standards and Technology (NIST, Gaithersburg, MD, USA). The 243 Cm and 244 Cm standard solutions contained the daughter products 239 Pu and 240 Pu, respectively, whose concentrations have been well calibrated in our previous work [7] within ±5% of total analytical error. The 237 Np standard was purchased from Eckert & Ziegler Isotopes Products (Valencia, CA, USA). 2.2 Samples As shown in Table 2, twelve 1.6 L samples (the daily urine output for reference man) in 3 different pooled urine batches (batch 1: samples ETH 6-2 to 6-5; batch 2: samples ETH 6-8 to 6-11; and batch 3: samples ETH 6-15 to 6-18) were prepared by spiking with a known quantity of 237 Np, and 243/ 244 Cm along with their daughter nuclides 239/240 Pu. Three samples of 400 ml deionised water (samples ETH 6-1, 6-7, and 6-14) were also prepared as procedural blanks to check for possible contributions of analytes from the reagents used. After spiking, all of the urine and water samples were acidified with 25 ml of concentrated nitric acid for preservation. In addition, to normalize the different ionization efficiencies in the AMS target between non-isotopic Pu tracer and the analyte Np nuclide, 2 standards (samples ETH 6-19 and 6-20) were prepared by adding a known quantity of the 242 Pu and 237 Np standards to 13 ml of 0.05 M HCl. As there is no suitable commercially available Np isotope that can be used as a tracer for 237 Np, a non-isotopic 242 Pu tracer was used. The standard samples were forwarded directly to the AMS target preparation step, without following through the bioassay sample preparation procedure. 2.3 Urine bioassay procedure A flow diagram of the urine bioassay procedure is shown in Figure 1. A known quantity ( 5 pg) of 242 Pu tracer was 126

3 CNL NUCLEAR REVIEW AECL Nuclear Review Downloaded from pubs.cnl.ca by on 02/25/18 TABLE Np, 239 Pu, and 240 Pu results in urine test samples (total measuring time: 45 min). 237 Np a 239 Pu 240 Pu Sample ID Description Expected (fg) Measured (fg) Deviation Expected (fg) Measured (fg) Deviation Repeat (fg) Deviation Expected (fg) Measured (fg) Deviation ETH 6-1 Water sample ns 0.08 ± ± % 4.2 ± % ns 1.4 ± 0.3 ETH 6-7 Water sample ns 0.03 ± ± % 862 ± % ns 0.72 ± 0.16 ETH 6-14 b Water sample ns 0.23 ± 0.28 ns 1.2 ± ± 1.2 ns 0.61 ± 0.35 ETH 6-2 Urine sample ns 0.18 ± ± % 7.6 ± % ns 0.60 ± 0.35 ETH 6-3 Urine sample ± % ± % 7.4 ± % ± % ETH 6-4 Urine sample ± % ± % 7.4 ± % ± % ETH 6-5 Urine sample ± 5 7.6% ± % 9.4 ± % ± % ETH 6-8 Urine sample ns 0.51 ± ± % 944 ± % ns 0.32 ± 0.09 ETH 6-9 Urine sample ± % ± % 971 ± % ± % ETH 6-10 Urine sample ± % ± % 1012 ± % ± % ETH 6-11 b Urine sample ± % ± % 1009 ± % ± 4 5.5% ETH 6-15 Urine sample ns 0.20 ± 0.22 ns 6.6 ± ± 1.3 ns 0.15 ± 0.03 ETH 6-16 Urine sample ± % ns 5.4 ± ± ± % ETH 6-17 Urine sample ± % ns 5.4 ± ± ± % ETH 6-18 Urine sample ± 8 3.4% ns 9.0 ± ± ± % ETH 6-19 Pu/Np standard ± % ns 0.28 ± ± 0.17 ns 0.19 ± 0.08 ETH 6-20 Pu/Np standard ± 3 2.0% ns 0.86 ± ± 0.18 ns 0.27 ± 0.12 a A correction factor of 0.8 (Pu:Np) has been applied to account for the difference in recovery of 237 Np using the non-isotopic 242 Pu tracer for all of the urine and reagent samples. This correction is very likely due to the higher chemical recovery of Np than Pu for all of the samples following through the entire urine bioassay procedure. Such a correction was not needed for the Pu/Np standards (Samples ETH 6-19 and 6-20), indicating that ionization efficiencies are similar for Pu and Np in the target. b Sample ETH 6-11 and 6-14 yielded very low signal intensity on the 242 Pu tracer, due to the loss of sample during the target packing. Note: ns, not spiked. CNL NUCLEAR REVIEW 127

4 AECL Nuclear Review Downloaded from pubs.cnl.ca by on 02/25/18 CNL NUCLEAR REVIEW 1.6 L urine sample Add 242 Pu tracer HTiO co-precipitation Np/Pu purification using TEVA column Sample target preparation AMS analysis FIGURE 1. Flow diagram of the urine bioassay procedure for neptunium/plutonium by AMS. added to 1.6 L of pooled urine or water samples in a glass beaker for monitoring the chemical recovery. Each sample was well mixed at 80 C for about 1 h and then cooled down to room temperature. In the hydrous titanium oxide (HTiO) coprecipitation step, 4 ml of TiOCl 2 solution (7% in Ti) was added. After stirring, the sample was neutralized to ph 7 with concentrated NH 4 OH, and Np/Pu was co-precipitated with the formation of HTiO from the urine matrix. The sample was stirred for >15 min and transferred to a 500 ml centrifuge bottle. After centrifugation (at nominally 2700 g force) of 500 ml aliquots, the supernatant solution was decanted. The total precipitate was then dissolved and transferred to a Teflon beaker with 10 ml of concentrated HNO 3. The sample was heated with the addition of 0.5 ml of 30% H 2 O 2 to boiling to decompose residual organics. The sample was then neutralized with concentrated NH 4 OH to co-precipitate Np/Pu with HTiO, and the supernatant solution was decanted after centrifugation. The HTiO precipitate was rinsed with water and re-centrifuged to remove residual salt. In the subsequent step, the precipitate was dissolved in an equal volume of concentrated HNO 3 to make up to a final acidity of 8 M nitric acid. With the addition of 0.5 ml of H 2 O 2 to adjust the valencies, the sample was ready for chromatographic column separation. A prepacked 2 ml Eichrom TEVA cartridge was used for purification of Pu and Np in the sample. After preconditioning using 10 ml of water followed by 10 ml of 8 M HNO 3,the sample was passed through the column at 1 ml/min. The resin was then rinsed with 8 M HNO 3 followed by 5 ml of concentrated HCl (to remove any thorium contaminants extracted onto the resin). The Pu/Np was then eluted with 13 ml of 0.1 M HCl M HF. An aliquot of 4 ml of the eluate for each sample was taken for final AMS analysis. 2.4 AMS target preparation The AMS target was prepared by mixed titanium and iron hydroxide co-precipitation methods. To do this, 0.4 mg of Ti and 0.1 mg of Fe from standard solutions were added into the eluate, and the sample was neutralized with concentrated NH 4 OH to ph >9 to co-precipitate the Pu/Np. The supernatant solution was decanted after centrifugation and the precipitate was transferred with methanol to a micro-centrifuge tube. It was then centrifuged and rinsed with methanol for drying in a heating block. The dried sample was finally mixed with 4 5 mg of niobium powder and pressed into a Ti target holder for AMS measurement. 2.5 AMS analysis The AMS measurements were performed using the compact (0.6 MV) AMS system TANDY at ETH. The AMS set-up for actinide measurements was described in detail elsewhere [8]. Negatively charged actinide oxide ions (AnO ) ions were extracted from the Cs-sputter ion source and injected into the accelerator running at a terminal voltage of about 300 kv. At the terminal, helium was used as a stripper gas to break up the injected molecules and to generate positively charged actinide ions. On the high energy side, An 3+ ions were selected by a serial combination of 2 high-energy magnets with an electrostatic analyzer in between them. The final ion identification was made with a dedicated low noise gas ionization detector equipped with a 30 nm thick SiN entrance window. 239 Pu, In this study, the analyte Pu and Np isotopes (i.e., 240 Pu, and 237 Np) were analyzed relative to a 242 Pu tracer. The analyte-to-tracer ratios (i.e., 239 Pu/ 242 Pu, 240 Pu/ 242 Pu, and 237 Np/ 242 Pu) were determined by a sequential measurement. In a cycle, 237 Np was measured for 20 s, 239 Pu for 10 s, 240 Pu for 20 s, and the 242 Pu tracer for 10 s. Switching between different isotopes typically took 2 s (1 s for switching plus 1 s of idle time to ensure stable operation conditions). Each sample was measured for a total time of 45 min. 3. Results and Discussion The results for 237 Np, 239 Pu, and 240 Pu in all of the test samples are given in Table 2. A 242 Pu non-isotopic tracer was used to calculate the 237 Np recovery. As the recoveries of the 2 isotopes are not identical when samples pass through the entire procedure, a correction factor of 0.8 (Pu:Np) has been applied to the 237 Np results using 242 Pu tracer for all of the urine and reagent samples to obtain reasonable agreement between the expected values and the measurements. This correction is required and is very likely due to the higher chemical recovery of Np than Pu for all of the samples following through the entire urine bioassay procedure. Such a correction was not needed for the Pu/Np standards (samples ETH 6-19 and 6-20) prepared without a chemical 128

5 CNL NUCLEAR REVIEW separation, confirming that the difference between the ionization efficiencies of Pu and Np was minimal using the present AMS target preparation method. However, further tests and improvements are required to guarantee a consistent recovery ratio (ideally the same recovery) between 237 Np and non-isotopic 242 Pu tracer during the bioassay procedure to confirm the validity of the recovery correction. and 240 Pu (1.2 fg/l for 239 Pu and 0.9 fg/l for 240 Pu) were found in the present work. This can be explained because only a small fraction of the eluate (4 ml out of 13 ml) was used for the measurement. In addition, a consistent amount of 239 Pu ( 5 9 fg) was found in all of the 4 urine samples comprising batch 3 (samples ETH 6-15 to 6-18), indicating AECL Nuclear Review Downloaded from pubs.cnl.ca by on 02/25/18 As shown in Table 2, after the correction factor was applied to account for the recovery difference between Np and Pu, the measured 237 Np in the spike samples matched well with the expected values in the femtogram range, and with excellent linearity (Figure 2) on the 1:1 reference line. The average 237 Np blanks for the unspiked water and urine samples were found to be 0.10 ± 0.11 fg and 0.30 ± 0.19 fg, respectively (Table 2). The detection limit (DL) for 237 Np was calculated to be fg/l in urine using Currie s equation [9], which easily meets the bioassay DL requirement (Table 1). For this radionuclide with a long half-life of years, ICP-MS analysis should also be adequate with the preference for use owing to its good availability and low cost. In our previous work, we demonstrated the suitability of Pu bioassay using the ETH compact AMS with the DLs of sub-fg/l for 239,240,241 Pu [6]. In this work, good agreements were observed for both of the 239/240 Pu isotopes in the samples with the expected concentration of >2 fg/l (see Table 2, and Figures 3 and 4 along with the 1:1 reference line). Note that, compared with the previous work [6], higher DLs for 239 Pu Measured (fg) 1, Np-237 Measured (fg) 2, Pu-239 Repeat ,000.0 Expected (fg) FIGURE 3. Measured vs. expected 239 Pu in the spiked urine samples. Measured (fg) Pu ,000.0 Expected (fg) FIGURE 2. Measured vs. expected 237 Np in the spiked urine samples Expected (fg) FIGURE 4. Measured vs. expected 240 Pu in the spiked urine samples. CNL NUCLEAR REVIEW 129

6 AECL Nuclear Review Downloaded from pubs.cnl.ca by on 02/25/18 CNL NUCLEAR REVIEW the potential 239 Pu contamination of this batch of pooled urine. Nevertheless, the results confirmed that the present AMS method would be adequate to meet the requirements for Pu bioassay (Table 1). Compared with our previous Pu bioassay method [6], a few modifications were made in the present procedure. The main changes include (i) pre-packed Eichrom TEVA resin was examined for its efficacy, whereas the anion exchange AGMP-1 resin was used in the previous procedure; and (ii) improved AMS target preparation method using mixed Fe/Ti oxide was applied in the present procedure, which eliminated the conversion of hydroxide to oxide by baking the samples at a high temperature of 650 C. The results confirmed the suitability of the more convenient pre-packed TEVA resin cartridges for Np/Pu purification from urine interferences, improving sample throughput. Without a high temperature baking step, the new AMS target preparation method is more economical (with no need to use in-house made high-purity quartz crucibles), and it is faster and easier for batch processing, with much less risk of crosscontamination. 4. Conclusions Measurements of ultra-low levels of actinides in urine samples are often required for dose assessment when incidental exposure to these highly toxic radionuclides occurs. AMS allows the simplification of sample preparation chemistry and increased sample analysis throughput compared with other methods. By improving the AMS method using the more convenient TEVA resin cartridges and eliminating the high temperature baking step, the sample throughput and cost effectiveness increased and the risk of cross-contamination decreased. Urine samples, spiked with known amounts of 237 Np, 239 Pu, and 240 Pu, were measured using compact AMS at ETH. The results agreed well with the expected spike values in the fg/l levels. The present AMS method has a sufficiently low detection limit to meet the routine bioassay requirements for regulatory purposes. REFERENCES [1] V.N. Epov, K. Benkhedda, R.J. Cornett, and R.D. Evans, 2005, Rapid Determination of Plutonium in Urine using Flow Injection On-line Preconcentration and Inductively Coupled Plasma Mass Spectrometry, Journal of Analytical Atomic Spectrometry, 20, pp doi: / b501218j. [2] N.L. Elliot, G.A. Bickel, S.H. Linauskas, and L.M. Paterson, 2006, Determination of Femtogram Quantities of 239 Pu and 240 Pu in Bioassay Samples by Thermal Ionization Mass Spectrometry, Journal of Radioanalytical and Nuclear Chemistry, 267, pp doi: /s [3] A.A. Marchetti, T.A. Brown, C.C. Cox, T.F. Hamilton, and R.E. Martinelli, 2005, Accelerator Mass Spectrometry of Actinides, Journal of Radioanalytical and Nuclear Chemistry, 263, pp [4] N.D. Priest, G.M. Pich, L.K. Fifield and R.G. Cresswell, 1999, Accelerator Mass Spectrometry for the Detection of Ultra-low Levels of Plutonium in Urine, Including that Excreted After the Ingestion of Irish Sea Sediments, Radiation Research, 152, pp. S16 S18. PMID: [5] H. Hernández-Mendoza, E. Chamizo, A. Yllera, M. García-León, and A. Delgado, 2010, A Highly Sensitive Method for the Reassessment and Quantification of 239 Pu in Urine Samples Based on a 1 MV Accelerator Mass Spectrometry System, Journal of Analytical Atomic Spectrometry, 25, pp doi: /C002420A. [6] X. Dai, M. Christl, S. Kramer-Tremblay, and H.-A. Synal. Ultra-trace Determination of Plutonium in Urine Samples Using a Compact Accelerator Mass Spectrometry System Operating at 300 kv, Journal of Analytical Atomic Spectrometry, 27, pp doi: /C1JA10264H. [7] M. Christl, X. Dai, J. Lachner, S. Kramer-Tremblay, and H.-A. Synal, 2014, Low Energy AMS of Americium and Curium, Nuclear Instruments and Methods in Physics Research Section B, 331, pp doi: / j.nimb [8] M. Christl, C. Vockenhuber, P.W. Kubik, L. Wacker, J. Lachner, V. Alfimov, and H.-A. Synal, 2013, The ETH Zurich AMS Facilities: Performance Parameters and Reference Materials, Nuclear Instruments and Methods in Physics Research Section B, 294, pp doi: /j.nimb [9] L.A. Currie, 1999, Detection and Quantification Limits: Origins and Historical Overview, Analytica Chimica Acta, 391, pp doi: / S (99)