Ultra-trace determination of plutonium in urine samples using a compact accelerator mass spectrometry system operating at 300 kV

Abstract

Accelerator mass spectrometry (AMS) is a very sensitive and robust technique for analysis of long-lived radionuclides. Employment of the AMS technique can reduce the demands on sample preparation chemistry, due to its high rejection of interferences and low susceptibility to sample matrix. This is particularly of interest for ultra-trace determination of ²³⁹Pu in bioassay and environmental samples, as other mass spectrometric methods such as inductively coupled plasma mass spectrometry (ICP-MS) can suffer from isobaric mass interferences by the presence of uranium in the sample. A rapid sample preparation method for analysis of Pu at femtogram levels in large volume urine samples is described. Using the compact ETH AMS Tandy facility operating at ∼300 kV, the method was validated by analysing urine samples spiked with known amounts of ^239/240/241Pu ranging from 1 to 30 fg. The detection limits for the method were estimated to be 0.38 fg for ²³⁹Pu, 0.40 fg for ²⁴⁰Pu and 0.08 fg for ²⁴¹Pu in 1400 mL of urine.

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Introduction

Plutonium is present in varying concentrations in environmental and biological samples as a consequence of activities related to nuclear weapons testing, nuclear waste recycling, and accidental or authorised discharges from nuclear power plants. Plutonium is considered to be the most radiotoxic substance, in particular, when it gets into the human body through inhalation of insoluble dust or aerosol. Occupational exposure to Pu usually comes from inhalation.^1,2 The most commonly used methods to assess internal contamination to Pu are in vitro measurements of its concentration in body fluids, especially in urine samples. As Canadian Nuclear Safety Commission (CNSC) regulations require the ability to report all occupational exposures of 1 mSv or greater committed effective dose, there is an urgent need for ultra-sensitive urine bioassay methods for dose assessment of Pu exposures for nuclear energy workers at the Canadian nuclear utilities. The International Commission on Radiological Protection (ICRP) has also recommended an annual dose limit of 1 mSv for the general public.³ To meet these regulatory requirements, a highly sensitive and accurate method capable of measuring the ^239/240/241Pu isotopes at a concentration of ∼1 fg L⁻¹ in urine samples is required.

Although measurement of Pu isotopes has been routinely undertaken by a number of techniques such as alpha spectrometry^4,5 and ICP-MS,^6,7 it is very difficult to accurately and precisely measure Pu isotopes by these techniques at fg levels in a large volume of urine sample. The minimum detectable activity by alpha spectrometry (AS) is around 0.1 mBq (∼50 fg) for ²³⁹Pu, which does not provide sufficient sensitivity to detect the Pu at the low fg levels required for occupational urine bioassay samples. Another disadvantage is that AS cannot provide simultaneous measurements of the ²³⁹Pu, ²⁴⁰Pu and ²⁴¹Pu isotopes, due to very similar alpha energy emissions between ²³⁹Pu and ²⁴⁰Pu and very low alpha branching ratio (2.45 × 10⁻⁵) for ²⁴¹Pu. In spite of the fact that recent developments of the ICP-MS techniques have pushed the detection limit for analysis of ²³⁹Pu down to sub-fg level,^7–11 employment of the ICP-MS techniques for routine Pu bioassay at low fg L⁻¹ concentration in urine is still very challenging. The presence of high content of uranium in urine could cause significant interferences with ²³⁹Pu by ICP-MS due to uranium hydride (²³⁸UH⁺) formation and peak tailing from neighbouring ²³⁸U⁺ mass.^12,13

Other techniques that have been used in the determination of fg quantities of ²³⁹Pu in urine include fission track analysis (FTA),^14,15 thermal ionization mass spectrometry (TIMS)^16–19 and AMS.^19–23 The main disadvantage of FTA method is its large uncertainty in the results caused by interference from ²³⁵U in the sample which can produce indistinguishable fission tracks for ²³⁹Pu analysis. TIMS has been successfully applied to ultra-sensitive urine bioassay, but the sample analysis throughput is quite limited as preparation of the loading filament requires very tedious and time-consuming chemical separation of the sample matrix elements.

The AMS technique has also been demonstrated for the precise and accurate determination of Pu at sub-fg level in urine bioassay samples.^20–23AMS is considered to be the most sensitive and robust technique because of its high rejection of molecular isobaric interferences and low susceptibility to matrix effects, which allow simplification of sample preparation chemistry. As a result of its high sensitivity and easy chemical separation, there is a good potential of high sample analysis throughput for routine Pu bioassay by AMS. However, lack of availability of accelerators and high costs to operate the complicated AMS system have limited extensive use of this technique. Recent developments at ETH Zurich demonstrated the applicability of a new generation compact AMS system for actinide analysis at low fg levels.^24,25 This compact system, working with a terminal voltage of 300 kV, is much simpler and more cost efficient. This makes low energy AMS a very competitive and cost effective technique compared to conventional mass spectrometers.

Based on the previous development of a rapid emergency bioassay method for actinides,²⁶ a new radiochemical separation method followed by AMS analysis was developed for isotopic Pu analysis in 24 hour urine bioassay samples. Using the ETH compact AMS, the method was validated with spiked urine samples.

Experimental

Reagents and standards

The anion exchange resin was AGMP-1M (Cl⁻ form, 100–200 mesh), purchased from Bio-Rad Laboratories Canada Ltd. (Mississauga, Ontario, Canada). The deionized water (>18 MΩ cm⁻¹) was obtained from a Millipore Direct-Q5 ultrapure water system. All the other chemicals were supplied by Sigma-Aldrich Canada. Most of these chemicals were trace metal grade or above, except for titanium oxychloride, which was of analytical grade. Radiochemical isotope standards were purchased from the National Institute of Standards and Technology (NIST, Gaithersburg, MD, USA), including ²³⁹Pu (NIST 4330B), ²⁴¹Pu (NIST 4340B) and ²⁴²Pu (NIST 4334G). Based on the TIMS analysis, both the ²³⁹Pu and ²⁴²Pu standards were found to be sufficiently pure with no measurable interferences from other Pu isotopes. However, significant impurities of ²³⁹Pu and ²⁴⁰Pu were found in the ²⁴¹Pu standard. The ^239/240Pu concentrations in the ²⁴¹Pu standard were successfully calibrated by TIMS after anion exchange purification to remove isobaric interference from the ²⁴¹Am progeny to ²⁴¹Pu. Thus, the ^239/240Pu measurements in the samples spiked with the ²⁴¹Pu standard can also be used for comparison to evaluate the performance of the present procedure.

Samples

To validate the Pu bioassay procedure, the spike samples were prepared by adding a known amount of the ²³⁹Pu or ²⁴¹Pu standard solutions into 1400 mL of synthetic urine or pooled urine. The synthetic urine was prepared according to the recipe published by McCurdy et al.¹⁹ To correct the procedural backgrounds, the reagent blanks were prepared using 500 mL of deionised water as a sample, which was acidified with 50 mL of concentrated HNO₃ and was processed following through the entire procedure. The urine blank samples were also prepared as 1400 mL of synthetic or pooled urine for subtraction of the Pu backgrounds in urine from the spiked samples.

Sample preparation procedure

A flow diagram of the sample preparation procedure is shown in Fig. 1. 1400 mL of urine sample was transferred into a glass beaker and was acidified with 50 mL concentrated nitric acid. A known quantity of ²⁴²Pu tracer (∼5 pg of ²⁴²Pu) was then added to monitor the chemical recovery. The sample was well mixed at low heating temperature (<80 °C) for about one hour and cooled down to room temperature.

Fig. 1 Flow diagram of the Pu urinalysis procedure.

In the hydrous titanium oxide (HTiO) co-precipitation step, a suitable amount of TiOCl₂ solution was added. After stirring, the sample was neutralized to pH ∼7 with concentrated NH₄OH, and plutonium was co-precipitated with the formation of HTiO from the urine matrix. The sample was stirred for >15 minutes and transferred to a centrifuge bottle. After centrifugation at 3500 rpm for 3 minutes, the supernatant solution was decanted. The precipitate was then dissolved and transferred to a Teflon beaker with ∼10 mL concentrated HNO₃. The sample was heated to boiling with the addition of 0.5 mL of H₂O₂ to decompose the residual organics. The sample was then neutralized with NH₄OH, and the supernatant solution was decanted after centrifugation. The HTiO precipitate was rinsed with water and centrifuged to remove residual salts. In the subsequent step, the precipitate was dissolved in 8 M HNO₃ with the addition of 0.5 mL of H₂O₂ for anion exchange purification.

The anion exchange column, containing 2 mL of AGMP-1M resin, was pre-conditioned with 10 mL of water followed by 8 M HNO₃ before use. The sample was passed through the column at ∼1 mL min⁻¹. After being rinsed with 8 M HNO₃, any thorium extracted onto the resin was removed with 5 mL of concentrated HCl. Plutonium was eluted with 10 mL of 0.2 M HNO₃ + 0.05 M HF.

The AMS target was prepared by either iron hydroxide or titanium hydroxide co-precipitation methods. To do this, ∼1 mg of Fe or Ti in the stock standard solutions was added, and the sample was neutralized with NH₄OH to pH > 9 to co-precipitate the Pu isotopes. The supernatant solution was decanted after centrifugation, and the hydroxide precipitate was dried under an infrared lamp. The dried sample was baked in a furnace for 2–3 hours at 650 °C. It was finally mixed with 4–5 mg niobium powder and pressed into a Ti target holder for the AMS measurement.

AMS analysis

The AMS measurements were performed with the compact (0.6 MV) AMS system TANDY at ETH Zurich.²⁷ Since the AMS set-up for actinide measurements is described in detail elsewhere,^24,25,28 only the main points and the most recent developments are summarized here. Negatively charged PuO ions are 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 is used as a stripper gas to break up the injected molecules and to generate positively charged Pu ions. On the high energy side, Pu³⁺ ions are selected. Recently, it has been demonstrated that the transmission for the actinides, Th and U through the accelerator (into the 3+ charge state), increased from about 12% when using Ar-stripping to almost 40% when using He-stripping.²⁹ This implies that the sensitivity for Pu-detection with the compact ETH AMS system Tandy has increased by a factor of three compared to the previous set-up. The final ion identification is made with a dedicated low noise gas ionization detector³⁰ equipped with a 30 nm thick SiN entrance window.³¹ The Pu isotopic ratios of interest (i.e., ²³⁹Pu/²⁴²Pu, ²⁴⁰Pu/²⁴²Pu, and ²⁴¹Pu/²⁴²Pu) were determined by a sequential measurement. Switching between the different Pu-isotopes typically takes two seconds (1 s for switching plus 1 s of idle time to ensure stable operation conditions). To enable fast isotope switching, only electrostatic devices (no magnetic fields) are changed (including the pulsing system on the low energy side, the acceleration voltage, and the electrostatic analyser on the high energy side). The measurement time for each isotope can be adjusted individually for each set of samples according to the expected counting rate. For this study, ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu were measured for 25 s each, while the ²⁴²Pu tracer was counted for 8 s. In addition, for each sample, the background counting rate on amu (atomic mass unit) 239 was monitored for 8 s by injecting ²³⁸U¹⁶O⁻ into the accelerator while leaving the high energy side tuned to amu 239.

To summarize, the measurement of one sequence as described above takes about 100 s. This sequence was repeated four times before measuring the next sample. The isotopic ratios of interest were calculated from the average counting rates acquired during the repeated sequential measurements. The whole measurement procedure as described above was repeated 8–12 times for each set of samples. The final Pu isotopic ratios were calculated from the error weighted mean of all single measurements. In total, each Pu sample was measured between 60 and 90 minutes on average.

Results and discussion

The mass of the analyte Pu isotopes (including ²³⁹Pu, ²⁴⁰Pu and ²⁴¹Pu) present in the samples was calculated from the measured ratio of the respective Pu isotope to the ²⁴²Pu tracer (e.g., ²³⁹Pu/²⁴²Pu, see above) by reference to the known amount of the tracer added. The average counting rate derived from the ²⁴²Pu tracer depends on the actual operating conditions of the ion source as well as the chemical recovery of the sample preparation procedure. The blanks and urine samples presented in this study typically yielded between 40 and 300 counts s⁻¹ pg⁻¹ at amu 242. Background corrections (e.g., for ²³⁸U tailing on amu 239) were negligible for all samples. The cross-talk in the ion source (i.e., the potential interference caused when the sputtered material re-deposits on the sample wheel, and thus, contaminates neighbouring samples) was on the order of 10⁻⁶ for both sets of samples.

Procedural blank and detection limit

Two sets of samples, including reagent (procedural) blanks, urine blanks, and spike samples, were prepared to evaluate the performance of the method. The first set of samples were prepared at the Chalk River Laboratories (CRL) in December 2010 and analysed at the ETH in February 2011; while the second set of samples were prepared in April 2011 and analysed in May 2011. The results for the blank samples are listed in Table 1.

Table 1 Results for the reagent and urine blanks (the uncertainties quoted are 1σ analytical errors only)

Sample	Description	^239Pu/fg	^240Pu/fg	^241Pu/fg
The first set of samples
T1-8	Reagent blank	1.62 ± 0.45	0.84 ± 0.30	0.84 ± 0.30
T1-9	Reagent blank	1.95 ± 0.50	0.97 ± 0.43	0.97 ± 0.43
T1-6	Synthetic urine blank	0.92 ± 0.24	0.93 ± 0.30	0.93 ± 0.30
T1-7	Synthetic urine blank	2.18 ± 0.53	1.36 ± 0.43	1.36 ± 0.43
T1-14	Pooled urine blank	1.93 ± 0.28	1.40 ± 0.23	1.40 ± 0.23
The second set of samples
T2-7	Reagent blank	0.57 ± 0.05	0.27 ± 0.04	0.05 ± 0.04
T2-8	Reagent blank	0.30 ± 0.04	0.13 ± 0.03	<0.01
T2-9	Reagent blank	0.44 ± 0.11	0.40 ± 0.10	<0.02
T2-11	Pooled urine blank	0.46 ± 0.07	0.15 ± 0.05	0.07 ± 0.05
T2-12	Pooled urine blank	0.59 ± 0.24	0.36 ± 0.19	<0.04

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Higher Pu backgrounds were found in the first set of samples due to possible cross-contamination introduced during the AMS target preparation step, as several samples containing much higher Pu content (at picogram levels) were dried together with this set of samples under the same infrared lamp. The laboratory cross-contamination significantly decreased for the second set of samples by cleaning the lamp and careful control of the sample drying step; thus, lower Pu backgrounds (especially for ²⁴¹Pu) were achieved. In addition, no apparent difference was seen between the reagent and urine blank samples, suggesting that the Pu backgrounds are dominated by the reagents used in the procedure.

Based on the measurements of all the reagent and urine blanks for the second set of samples, the average procedural backgrounds were found to be 0.47 ± 0.12 fg for ²³⁹Pu, 0.26 ± 0.12 fg for ²⁴⁰Pu and 0.038 ± 0.024 fg for ²⁴¹Pu. Assuming a standardized normal distribution for the procedural blanks, the detection limit (L_d) can be calculated by:³²

L_d = 2 × 1.645 × σ₀

where σ₀ is the standard deviation of the blank measurements (fg). Therefore, the detection limits for the Pu isotopes by the present procedure were estimated to be 0.38 fg (0.88 μBq) for ²³⁹Pu, 0.40 fg (3.3 μBq) for ²⁴⁰Pu and 0.08 fg (0.32 mBq) for ²⁴¹Pu.

Performance tests for spike samples

The results of the urine samples spiked with known amounts of ²⁴¹Pu and ²³⁹Pu standard solutions are summarized in Table 2 and Table 3, respectively. The measured values have been blank corrected with ±1σ uncertainties. Measurements for the ^239/240/241Pu isotopes ranged from ∼1 to 30 fg per 1400 mL of urine sample, agreeing very well with the expected values. As a result of higher blanks found in the first set of samples, elevated detection limits of <1.6 fg ²³⁹Pu and <1.3 fg ²⁴⁰Pu for the samples T1-12, T1-13 and T1-16 are reported in Table 2.

Table 2 Results for the urine samples spiked with ²⁴¹Pu standard

Sample	Description	^241Pu			^239Pu			^240Pu
		Added/fg	Measured/fg	Deviation	Added/fg	Measured/fg	Deviation	Added/fg	Measured/fg	Deviation
The first set of samples
T1-12	Synthetic urine spike	5.78	6.1 ± 1.2	6.0%	0.3	<1.6	—	1.2	<1.3	—
T1-13	Synthetic urine spike	27.3	27.5 ± 1.3	1.0%	1.5	<1.6	—	5.6	5.2 ± 0.6	−5.7%
T1-16	Pooled urine spike	11.4	12.4 ± 1.2	9.5%	0.6	<1.6	—	2.3	2.8 ± 0.8	19.1%
The second set of samples
T2-15	Pooled urine spike	20.6	20.0 ± 0.3	−2.9%	1.1	1.2 ± 0.1	6.4%	4.2	4.7 ± 0.2	11.9%
T2-18	Pooled urine spike	16.9	16.2 ± 0.4	−3.8%	0.9	0.7 ± 0.3	−27.9%	3.4	3.5 ± 0.3	1.6%

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Table 3 Results for the urine samples spiked with ²³⁹Pu standard

Sample	Description	^239Pu
		Added/fg	Measured/fg	Deviation
The first set of samples
T1-10	Synthetic urine spike	3.59	2.9 ± 0.5	−19.7%
T1-11	Synthetic urine spike	30.1	31.4 ± 1.1	4.4%
T1-15	Pooled urine spike	13.7	13.8 ± 0.9	0.9%
The second set of samples
T2-13	Pooled urine spike	18.6	19.6 ± 1.3	5.1%
T2-14	Pooled urine spike	18.4	20.2 ± 0.7	9.8%
T2-16	Pooled urine spike	16.0	16.7 ± 0.8	4.5%
T2-17	Pooled urine spike	15.4	14.3 ± 0.5	−7.3%

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As shown in Table 4, relative bias (B_r) and relative precision (S_B) are calculated as defined by the guidance provided in ANSI/HPS N13.30.³³ The validation tests for all the analyte Pu isotopes passed the ANSI/HPS N13.30 performance criteria for both relative bias (−25% to +50%) and precision (±40%) by a very substantial margin, demonstrating the excellent accuracy and precision of Pu isotopic analysis with the present method.

Table 4 Relative bias and relative precision for the spiked urine samples

	^239Pu	^240Pu	^241Pu
Relative bias (Br)	−0.3%	6.7%	2.0%
Relative precision (SB)	10.0%	10.6%	5.7%

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Conclusions

AMS is an extremely sensitive and very reliable technique for isotopic analysis of plutonium at low femtogram ranges. In this study, a new ultra-trace bioassay method for the determination of fg amounts of ²³⁹Pu, ²⁴⁰Pu and ²⁴¹Pu in a large volume urine sample has been developed. The performance tests of the method were carried out, and the Pu isotopes were analysed using the compact ETH AMS system operating at 300 kV. The method is relatively simple, fast and robust, with a sensitivity of <0.5 fg for Pu. The method validation tests using spiked urine samples showed excellent agreement between the measured and the expected values for Pu at 1–30 fg in 1400 mL of urine. The results demonstrate the feasibility of the AMS technique using a compact, low energy AMS system for routine isotopic Pu analysis at ultra-trace levels in urine samples, to meet the regulatory requirement for a reportable dose of 1 mSv for occupational Pu exposures in the Canadian nuclear facilities.

Acknowledgements

The authors wish to thank Dr Nicholas Priest (the AECL Chalk River Laboratories), Dr Xiaolin Hou (Risø National Laboratory for Sustainable Energy, Technical University of Denmark) and Dr Vasily Alfimov (ETH Zurich) for helpful discussions and suggestions. Further acknowledgements go to Johannes Lachner (ETH Zurich) for supporting the AMS measurements and for discussions and to Michael Rüttimann for carefully preparing the Pu-AMS targets at ETH Zurich. This study was partially funded by Atomic Energy of Canada Limited.

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