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An emergency bioassay method for 210Po in urine
Applied Radiation and Isotopes 103 (2015) 179–184
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
An emergency bioassay method for
210
Po in urine
Nicolas Guérin n, Xiongxin Dai Canadian Nuclear Laboratories (CNL) Radiological Protection Research and Instrumentation Branch, Chalk River, ON, Canada K0J 1J0
H I G H L I G H T S
A new method was developed to rapidly measurement 210Po in urine. The method is easy to perform and does not require highly qualified staffs. Large sample batches can be simultaneously prepared. The method meets the requirements for an emergency bioassay method.
art ic l e i nf o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 8 June 2015 Accepted 9 June 2015 Available online 16 June 2015
A rapid method was developed to efficiently measure 210Po in urine samples in an emergency situation. Polonium-210 in small urine samples (10 mL) was spontaneously deposited on a stainless steel disc in 1 M HCl at room temperature for 4 h in a polyethylene bottle. The metallic disc was then counted for 4 h by alpha spectrometry. The developed method allowed the preparation of large sample batch in a short time. The method meets the requirements for an emergency bioassay procedure. & 2015 Published by Elsevier Ltd.
Keywords: Polonium Urine Bioassay Spontaneous plating Emergency method
1. Introduction Polonium-210 (210Po) is an extremely toxic radionuclide. It occurs naturally as part of uranium-238 decay chain. It has been attributed to the death of the former Russian security service agent Alexander Litvinenko and its use led to the spread of radioactive contamination across a large number of locations in London in 2006 (Harrison et al., 2007, Cornett et al., 2009 and Bailey et al., 2010). During this event, 210Po activity was monitored in hundreds of urine samples by the Health Protection Agency (HPA) to assess potential exposures (Bailey et al., 2010). The monitoring program using existing methods took a long time and demonstrated the need for a new rapid and simple analytical method for the measurement of 210Po in urine samples. This is important to facilitate a quick response to identify any radiological risk to the public following a similar incident. Internal exposure to 210Po is usually determined by urine bioassay (Bailey et al., 2010); however, few methods are available n
Corresponding author. E-mail addresses: [email protected] (N. Guérin), [email protected] (X. Dai). http://dx.doi.org/10.1016/j.apradiso.2015.06.013 0969-8043/& 2015 Published by Elsevier Ltd.
(Meli et al., 2009; Manickam et al., 2010; Fisenne, 1997; Ham, 2009; Figgins, 1961; Azeredo and Lipsztein, 1991; Guérin and Dai, 2014). These methods can be divided in two categories: spontaneous plating and micro-precipitation procedures. In both cases, the 210Po recovery is corrected using either 208Po or 209Po tracer. For the plating methods, the urine sample is often oxidized and heated in a beaker to solubilize 210Po bound to organic matter (Manickam et al., 2010). Polonium-210 in the sample is usually pre-concentrated by evaporation (Meli et al., 2009; Fisenne, 1997; Ham, 2009; Azeredo and Lipsztein, 1991) or co-precipitation (Manickam et al., 2010) to improve the method sensitivity. The residue/precipitate is then dissolved using a small volume of a HCl solution. Finally, 210Po is spontaneously plated either onto a silver (Meli et al., 2009; Manickam et al., 2010; Ham, 2009) or a nickel (Fisenne, 1997; Figgins, 1961) disc immersed in the HCl solution. The spontaneous plating is usually performed at an elevated temperature (50–95 °C) for few hours (1–4 h) to maximize the Po recovery. After rinsing with water, the metallic disc is dried and the alpha particles emitted from the Po are counted by alpha spectrometry. A pre-concentration approach is tedious and time consuming (sometime it takes a day), which may not be acceptable for an
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emergency procedure (Maxwell and Culligan, 2009). Hence, a copper sulphide micro-precipitation method was developed, which can be completed in only few hours (Guérin and Dai, 2014). In this method, a small amount of urine (10 mL) is heated and oxidized by boiling for 5 min. The sample is then centrifuged and Po in the supernatant is precipitated and filtered to prepare a thinlayer source for counting by alpha spectrometry using the CuS micro-precipitation procedure (Guérin and Dai, 2013). The filtration can be conducted in large batches using a vacuum box. Thus, the method has a high sample throughput (1.5–2 h for 12 samples) and is suitable for emergency situations. Even though, the CuS micro-precipitation method for the measurement of 210Po in urine samples has the advantage that it is faster than the spontaneous plating method, plating Po onto a metallic disc has the advantage that it requires fewer laboratory manipulations and qualified staff to perform the procedure in case of an emergency. Since the Po plating time cannot be reduced without substantially affecting the recovery, large batches of samples have to be simultaneously processed to make the spontaneous plating method appropriate for emergency purpose. To realize this objective, a new emergency method for the measurement of 210Po in urine samples was developed and is described in this paper. Polonium-210 is directly plated onto a stainless steel disc in polyethylene bottles without the need of any oxidation, heating or pre-concentration step. The material used in this plating procedure was inexpensive, easily available and disposable. In this study, the plating conditions were first optimized to maximize the performance of the method. The figures of merit were determined and the method was validated using spiked samples.
2. Experimental 2.1. Reagents and standards Stock solutions were prepared using ultra pure water (UPW) produced from a water purification system (Millipore Direct-Q5, Billerica, MA). Trace metal grade hydrochloric acid and 60 mL polyethylene bottles were purchased from Fisher Scientific (Fair Lawn, NJ). Rubber O-rings (2.54 cm) were obtained from McMaster-Carr Supply Co. (Princeton, NJ) and stainless steel (type 304) discs (25.4 mm of diameter) were produced at the Canadian Nuclear Laboratories (Chalk River, ON, Canada). Copper, silver and nickel discs of 25.4 mm of diameter, used for comparison, were bought from A.F. Murphy Die & Machine Co., Inc. (North Quicy, MA). Polonium-210 was obtained from a 210Pb solution in equilibrium with its daughters (National Institute of Standard and Technology (NIST), Gaithersburg, MD) and 209Po tracer was
purchased from Eckert & Ziegler Isotopes Products (Valencia, CA). Urine samples were obtained from un-identified employees of the Canadian Nuclear Laboratories via the Dosimetry Services Laboratory. 2.2. Procedure Prior to use, stainless steel discs were rinsed with ethanol, immersed in 6 M HCl for a few seconds and dried using a cleaning tissue. The tracer (200 mBq of 209Po) and urine sample (10 mL) were weighed into disposable 60 mL polyethylene bottles. The urine sample was acidified to approximately 1 M HCl by adding 0.83 mL of concentrated HCl. The metallic disc was placed on the top of the bottle opening, sealed with a rubber O-ring, and tightened with the cap (see Fig. 1). The sample bottles were first manually shaken once and then shaken for 4 h with a mechanical shaker (VWR, digital 12 L shaking water bath). A maximum of 20 samples could be shaken at the same time. After the plating step, the metallic discs were rinsed with water and heated at 300 °C on the surface of a hot plate for 10 min to oxidize Po and reduce potential contamination of the alpha detector (Eichrom Technologies LLC., 2009). Finally, Po plated onto the metallic disc was counted by alpha spectrometry. 2.3. Method development Different metallic discs (stainless steel, copper, nickel and silver) were tested for Po plating efficiency. The recovery and energy resolution were compared. Plating recovery of Po as a function of time and HCl molarity onto stainless steel discs were evaluated. The influence of urine storage conditions on Po recovery was examined using the procedure previously described. The samples were stored under 4 different conditions for 23 days: (1) at room temperature ( 22 °C); (2) in the refrigerator ( 4 °C); (3) acidified to 1% HCl and stored at room temperature; and (4) acidified to 1% HCl and stored in the refrigerator. In addition, potential losses of Po during the plate heating step prior to the alpha counting were evaluated by calculating the difference in Po activities measured before and after heating for different times on a hot plate. Two replicates were performed for each measurement. Pooled urine was used for method development experiments. 2.4. Figures of merit and method validation The minimum detectable activity (MDA) of the method was estimated from 12 UPW blank samples using the Currie (Currie, 1968) Eq. (1) as follows:
Fig. 1. Schematic diagram of the plating assembly.
N. Guérin, X. Dai / Applied Radiation and Isotopes 103 (2015) 179–184
MDA =
k 2 + 2⋅k⋅ 2⋅B (mBq/L) T⋅ε⋅R⋅V⋅F
181
(1)
where k is a constant (1.645) to reach 95% confidence interval; B is the number of background counts for a defined time in seconds (T); ε is the counting efficiency ( 0.25); R is the chemical recovery; V is the sample volume in litres (i.e., 0.01 L); and F is a unit conversion factor that equals 10 3. The quantification limit (QL) has been calculated as 3 times the MDA. The method was verified using 12 different spiked urine samples. For each sample, a known quantity of the 210Pb/210Po solution and 11 mL of urine sample were weighed into disposable 15 mL conical tubes. The samples were vigorously shaken, refrigerated, and settled for 24 h to allow 210Po species to reach equilibrium with the components comprising the urine matrix. The procedure previously described was then applied to measure 210Po activity in these samples. Relative bias (Bri) was calculated using Eq. (2), where Ai is the measured activity and Aai the expected activity.
A − Aai Bri = i × 100% Aai
(2)
Relative precision was calculated as the standard deviation of the individual calculated relative bias values. Recovery correction due to 210Po decay was applied for recovery calculation. 2.5. Instrumentation s
Polonium samples were counted using an Octete Plus Alpha Spectroscopy Workstation with eight 450 mm2 ULTRA-AS ionimplanted silicon detectors (AMETEK/ORTEC Inc., Oak Ridge, TN).
3. Results and discussion 3.1. Optimization 3.1.1. Metallic discs The plating recovery of 210Po on different metallic discs was determined and the results are shown in Fig. 2A. Similar recoveries were found for all the metallic discs tested. Compared to the Po recovery obtained by Pornepkasemon (Porntepkasemsan et al., 2011) for water samples of Po plated at room temperature, the results obtained for urine samples showed similar recoveries ( 60% to 65% for water and urine) for Cu and Ni discs and lower values for silver discs ( 80% for water and 50% for urine). Good energy resolution was observed in all cases with relatively low full weight half maximum (FWHM) values of 10 to 30 keV (Fig. 2B).
Fig. 3. Po recovery as a function of time onto stainless steel discs at room temperature.
Slightly poorer resolution was found for copper discs ( 30 keV). Since stainless steel discs are cheaper than others and easy to obtain, they were preferred in this study. Note that Ni and Ag discs could also be used without compromise with regard to recovery and resolution. 3.1.2. Plating kinetics The Po plating recovery for urine increased at a relatively constant rate ( 15%/h) for the first 4 h to reach a maximum value of 60% (Fig. 3). Similar plating kinetics were also observed for water, where an optimal, but higher, recovery ( 90%) was reached after 3–4 h at 80–90 °C in HCl solutions (Porntepkasemsan et al., 2011). This suggests that the plating time should be at least 4 h to obtain an optimal recovery. Reducing the plating time (at room temperature) could be an option to speed up the sample analysis throughput, although it will decrease the recovery and increase the MDA. In this case, the amount of tracer needs to be adjusted to cope with the recovery loss. 3.1.3. HCl molarity The recovery as a function of HCl molarity was also evaluated and the results are shown in Fig. 4. For urine samples acidified to 0.001–0.1 M HCl, Po was mostly not plated. For sample acidities of 0.5 M HCl and higher, the recovery rapidly increased to 55% and then it remained constant. A comparison with water samples showed similar results (Fig. 4). The Po recovery was low for 0.001– 0.01 M HCl and it reached a maximal value of 50% at HCl
Fig. 2. Comparison of Po recovery on different metallic discs for 4 h plating at room temperature. *SS: Stainless steel.
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Fig. 4. Po recovery of as a function of HCl molarity after 4 h plating onto stainless steel discs at room temperature for urine and water samples.
molarities above 0.2 M. The maximum Po recovery was reached at a higher HCl molarity for urine than for water, which could be due to neutralization by alkaline components in the urine and/or the complexation of Po with organic ligands in urine. For silver discs, the Po recovery reached a maximum plating recovery for HCl concentrations above 0.5 M, which is similar to stainless steel discs (Mahmood et al., 2012). This indicates that a minimal sample acidity is required to allow the spontaneous plating reaction. For all tests, an HCl molarity of 1 M has been chosen to ensure sufficient acidification of different urine samples. Note that the results of higher acidities (above 2 M HCl) are not shown due to the rapid degradation of stainless steel discs and obvious leaking issues. 3.2. Storage conditions The effect of different storage conditions on Po recovery in urine is shown in Fig. 5. For all the different storage conditions studied, no significant tendency as a function of time was observed, indicating that the storage conditions were not affecting the recovery. The same conclusion was obtained using the CuS micro-precipitation method (Guérin and Dai, 2014). However, a lower and less consistent recovery was found for the plating technique compared to the micro-precipitation method (647 8% and 857 3% for the plating and micro-precipitation methods,
Fig. 6. Po recovery on stainless steel discs before and after heating on a hot plate (300 °C) as a function of time.
respectively). 3.3. Plate heating The influence of the plate heating step prior to alpha counting was assessed (Fig. 6). As shown in Fig. 6, there was no observable difference in recovery after heating for 30 min at 300 °C. This result is consistent with the fact that the decomposition temperature of PoO2 to Po is 500 °C (Holden, 2013). Thus, no significant losses of Po are expected in the plate heating step. 3.4. Figures of merit A MDA of 0.3 Bq/L for 4 h of counting time was obtained (Table 1). Based on a committed effective dose of 100 mSv in the event of a radiological/nuclear emergency, a reference level of 110 Bq/L in urine samples collected 3 days post-exposure has been derived (Li et al., 2012). The MDA obtained using the present method is well below the reference level with a comfortable margin for confidence. One advantage of this method is that a small volume of a spot urine sample is sufficient to meet the sensitivity requirement for emergency bioassay, facilitating easier sample collection and preparation. In the event of an emergency, after 4 h of counting for rapid screening, the samples with a low activity could continue to be counted for a longer time to achieve a better sensitivity. For example, a MDA of 0.05 Bq/L was obtained after a counting time of 48 h (Table 1), which is only slightly above the usual background level of Po in urine (0.004–0.020 Bq/day) (Meli et al., 2009). Polonium in urine spike samples was determined and the results are shown in Fig. 6. The overall chemical recovery of the method was 607 8% (Table 1). For the samples above the Table 1 Figures of merit of the method.
Fig. 5. Po recovery in different storage conditions of urine samples over a period of 23 days. *RT: room temperature, and F: fridge.
Counting time
4h
48 h
Chemical recovery (%) MDA (Bq/L) QL (Bq/L) RL (Bq/L) Mean relative bias (%) Relative precision (%)
60 7 8 0.3 70.1 0.8 70.3 110 3.10* 13.4
637 9 0.05 70.01 0.147 0.04 – 1.90 9.80
MDA: minimum detectable activity, QL: quantification level, RL: reference level. *
Only the samples above QL included.
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183
Fig. 7. Emergency radioassay (A) 4 h counting, and (B) 48 h counting.
quantification limit, the measured values were in good agreement with the expected values. Mean relative bias and relative precision of 3.10% and 13.4% were obtained with a counting time of 4 h (Table 1). These results meet the acceptance criteria for an emergency bioassay procedure given by the ANSI/HPS N13.30, which are 25% to þ50% for relative bias and 740% for relative precision (Health Physics Society, 1996). For a counting time of 48 h, the mean relative bias and relative precision were –1.90% and 9.80%, respectively (Table 1).
other intrinsic organic ligands in urine). Thus, the presence of the complexing agent may not pose a significant impact on the method performance. In addition, a longer counting time can be applied when the Po recovery is significantly lower for the followup urine samples.
3.5. Analysis throughput and turn-around time
210
Using the present procedure, batches containing a large number of samples could be easily processed. The number of samples that can be simultaneously prepared is largely limited by the capacity of the mechanical shaker used. The samples could also be manually shaken at a constant frequency (e.g., every 20 min) with a lower recovery of 30% to 40% (results not shown). The method does not require any heating or oxidation step, which saves a considerable amount of sample preparation time. In addition, no pre-concentration is necessary to meet the sensitivity requirements. Using a single shaker, 20 samples can be prepared by one person in approximately 1–1.25 h (weighing tracer, weighing urine, adding HCl, discs preparation, closed cap). In the event of an emergency, more than 400 samples could be processed in one day, assuming that 20 samples are prepared each hour and 4 shakers are available. In addition, only minimal training is required to perform this procedure. At this rate, the availability of alpha spectrometers could be the limiting factor. To improve sample analysis throughput, one option is to reduce the counting time since the reference 210Po level in emergency urine sample is high. However, the quantity of tracer added has to be adjusted. This method could be a very interesting alternative to the CuS microprecipitation procedure for emergency urine bioassay. 3.6. Decorporation agent In case that a decorporation agent would be administrated to affected personals who may require immediate medical treatments and be present in the follow-up urine samples, the Po recovery could be potentially reduced. However, even the Po recovery would be reduced, the current method could still be used as any recovery loss would be corrected using the 209Po tracer. As showed in Table 1 and Fig. 7, there is a large margin for the developed method to meet the sensitivity requirements for emergency bioassay needs. In the current method, the Po plating was performed under acidic condition, which may reduce the complexing strength of the ligands (either the decorporating agent or
4. Conclusions A very rapid and convenient method was developed to measure Po in emergency urine samples by alpha spectrometry following spontaneous plating of polonium onto stainless steel discs. Large sample batches were simultaneously prepared without heating using small volume urine samples (10 mL) and an acceptable recovery of 6078% was obtained. The method meets all the sensitivity and precision requirements for an emergency bioassay procedure.
Acknowledgements This research was funded by the Government of Canada (RD-1.4.4.5-5897) under Canadian Nuclear Laboratories Science and Technology program. The authors wish to thank the Dosimetry Services Laboratory staff of Chalk River Laboratories for their assistance with this work.
References Harrison, J., Leggett, R., Lloyd, D., Phipps, A., Scott, B., 2007. Polonium-210 as a poison. J. Radiol. Prot. 27, 17–40. Cornett, J., Tracy, B., Kramer, G., Whyte, J., Moodie, G., Auclair, J.P., Thomson, D., 2009. Polonium-210: lessons learned from the contamination of individual Canadians. Radiat. Prot. Dosim. 134 (3–4), 164–166. Bailey, M.R., Birchall, A., Etherington, G., Fraser, G., Wilkins, B.T., Bessa, Y., Bishop, L., Brown, J., Dorrian, M.D., Ewers, L.W., Fell, T.P., Ham, G.J., Hammond, D.J., Forrester, S., Hodgson, A., Howarth, C., Ibrahimi, Z.F., Maguire, H., Marsh, J., Phipps, A.W., Puncher, M., Ruggles, R., Shutt, A., Smith, J.R.H., Turbitt, D., Youngman, M.J., Wildin, D., 2010. Individual Monitoring Conducted by the Health Protection Agency in the London Polonium-210 Incident. Health Protection Agency, New York. Meli, M.A., Desideri, D., Roselli, C., Feduzi, L., 2009. 210Po determination in urines of people living in Central Italy. J. Environ. Radioact. 100, 84–88. Manickam, E., Sdraulig, S., O’Brian, R., 2010. An improved and rapid radiochemical method for the determination of polonium-210 in urine. Aust. J. Chem. 63, 38–46. Fisenne, I.M., 1997. Polonium in Water and Urine. Environmental Measurements Laboratory, US Department of Energy, New York, NY. Ham, G.J., 2009. The determination of Polonium-210 in urine following the Litvinenko incident. Radioprotection 44 (5), 41–46. Figgins, P.E., 1961. The radiochemistry of polonium. N.A.S.-N.R.C. Azeredo, A.M.G.F., Lipsztein, J.L., 1991. 210Po excretion in urine: a comparison of an occupational exposed group and a control. Radiat. Prot. Dosim. 36 (1), 51–54.
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Guérin, N., Dai, X., 2014. Determination of 210Po in drinking water and urine samples using copper sulfide microprecipitation. Anal. Chem. 86 (12), 6026–6031. Maxwell, S.L., Culligan, B.K., 2009. New column separation method for emergency urine samples. J Radioanal. Nucl. Chem. 279 (1), 105–111. Guérin, N., Dai, X., 2013. Rapid preparation of polonium counting sources for alpha spectrometry using copper sulfide microprecipitation. Anal. Chem. 85 (13), 6524–6529. Eichrom Thecnologies, LLC., 2009. Lead-210 and Polonium-210 in Water. Analytical Procedures. Currie, L.A., 1968. Limits for qualitative detection and quantitative determination. Application to radiochemistry. Anal. Chem. 40 (3), 586–593. Porntepkasemsan, B., Srisuksawad, K., O-Manee, A., 2011. deposition efficiency of polonium-210 on various metals. In: Proceeding of the TIChE International
Conference, Hatyai, Thailand, Paper code: ms006. Mahmood, Z.U.W., Mohamed, N., Ariffin, N.A.N., Ishak, A.K., 2012. Improved Optimum Condition for Recovery and Measurment of 210Po in Environmental Samples. Research and Development Seminar, Bangi, Malaysia. Holden, N.E., 2013. CRC Handbook of Chemistry and Physics. Available from url: http://www.hbcpnetbase.com/ (accessed June, 2014). Li, C., Vlahovich, S., Dai, X., Richardson, B., Daka, J.N., Kramer, G.H., 2012. Requirements for radiation emergency urine bioassay techniques for the public and first responders. Health Phys. 99, 702–707. Health Physics Society, 1996. Performance Criteria for Radiobioassay American National Standard: ANSI N13.30. American National Standard Institute, New York.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
An emergency bioassay method for
210
Po in urine
Nicolas Guérin n, Xiongxin Dai Canadian Nuclear Laboratories (CNL) Radiological Protection Research and Instrumentation Branch, Chalk River, ON, Canada K0J 1J0
H I G H L I G H T S
A new method was developed to rapidly measurement 210Po in urine. The method is easy to perform and does not require highly qualified staffs. Large sample batches can be simultaneously prepared. The method meets the requirements for an emergency bioassay method.
art ic l e i nf o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 8 June 2015 Accepted 9 June 2015 Available online 16 June 2015
A rapid method was developed to efficiently measure 210Po in urine samples in an emergency situation. Polonium-210 in small urine samples (10 mL) was spontaneously deposited on a stainless steel disc in 1 M HCl at room temperature for 4 h in a polyethylene bottle. The metallic disc was then counted for 4 h by alpha spectrometry. The developed method allowed the preparation of large sample batch in a short time. The method meets the requirements for an emergency bioassay procedure. & 2015 Published by Elsevier Ltd.
Keywords: Polonium Urine Bioassay Spontaneous plating Emergency method
1. Introduction Polonium-210 (210Po) is an extremely toxic radionuclide. It occurs naturally as part of uranium-238 decay chain. It has been attributed to the death of the former Russian security service agent Alexander Litvinenko and its use led to the spread of radioactive contamination across a large number of locations in London in 2006 (Harrison et al., 2007, Cornett et al., 2009 and Bailey et al., 2010). During this event, 210Po activity was monitored in hundreds of urine samples by the Health Protection Agency (HPA) to assess potential exposures (Bailey et al., 2010). The monitoring program using existing methods took a long time and demonstrated the need for a new rapid and simple analytical method for the measurement of 210Po in urine samples. This is important to facilitate a quick response to identify any radiological risk to the public following a similar incident. Internal exposure to 210Po is usually determined by urine bioassay (Bailey et al., 2010); however, few methods are available n
Corresponding author. E-mail addresses: [email protected] (N. Guérin), [email protected] (X. Dai). http://dx.doi.org/10.1016/j.apradiso.2015.06.013 0969-8043/& 2015 Published by Elsevier Ltd.
(Meli et al., 2009; Manickam et al., 2010; Fisenne, 1997; Ham, 2009; Figgins, 1961; Azeredo and Lipsztein, 1991; Guérin and Dai, 2014). These methods can be divided in two categories: spontaneous plating and micro-precipitation procedures. In both cases, the 210Po recovery is corrected using either 208Po or 209Po tracer. For the plating methods, the urine sample is often oxidized and heated in a beaker to solubilize 210Po bound to organic matter (Manickam et al., 2010). Polonium-210 in the sample is usually pre-concentrated by evaporation (Meli et al., 2009; Fisenne, 1997; Ham, 2009; Azeredo and Lipsztein, 1991) or co-precipitation (Manickam et al., 2010) to improve the method sensitivity. The residue/precipitate is then dissolved using a small volume of a HCl solution. Finally, 210Po is spontaneously plated either onto a silver (Meli et al., 2009; Manickam et al., 2010; Ham, 2009) or a nickel (Fisenne, 1997; Figgins, 1961) disc immersed in the HCl solution. The spontaneous plating is usually performed at an elevated temperature (50–95 °C) for few hours (1–4 h) to maximize the Po recovery. After rinsing with water, the metallic disc is dried and the alpha particles emitted from the Po are counted by alpha spectrometry. A pre-concentration approach is tedious and time consuming (sometime it takes a day), which may not be acceptable for an
180
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emergency procedure (Maxwell and Culligan, 2009). Hence, a copper sulphide micro-precipitation method was developed, which can be completed in only few hours (Guérin and Dai, 2014). In this method, a small amount of urine (10 mL) is heated and oxidized by boiling for 5 min. The sample is then centrifuged and Po in the supernatant is precipitated and filtered to prepare a thinlayer source for counting by alpha spectrometry using the CuS micro-precipitation procedure (Guérin and Dai, 2013). The filtration can be conducted in large batches using a vacuum box. Thus, the method has a high sample throughput (1.5–2 h for 12 samples) and is suitable for emergency situations. Even though, the CuS micro-precipitation method for the measurement of 210Po in urine samples has the advantage that it is faster than the spontaneous plating method, plating Po onto a metallic disc has the advantage that it requires fewer laboratory manipulations and qualified staff to perform the procedure in case of an emergency. Since the Po plating time cannot be reduced without substantially affecting the recovery, large batches of samples have to be simultaneously processed to make the spontaneous plating method appropriate for emergency purpose. To realize this objective, a new emergency method for the measurement of 210Po in urine samples was developed and is described in this paper. Polonium-210 is directly plated onto a stainless steel disc in polyethylene bottles without the need of any oxidation, heating or pre-concentration step. The material used in this plating procedure was inexpensive, easily available and disposable. In this study, the plating conditions were first optimized to maximize the performance of the method. The figures of merit were determined and the method was validated using spiked samples.
2. Experimental 2.1. Reagents and standards Stock solutions were prepared using ultra pure water (UPW) produced from a water purification system (Millipore Direct-Q5, Billerica, MA). Trace metal grade hydrochloric acid and 60 mL polyethylene bottles were purchased from Fisher Scientific (Fair Lawn, NJ). Rubber O-rings (2.54 cm) were obtained from McMaster-Carr Supply Co. (Princeton, NJ) and stainless steel (type 304) discs (25.4 mm of diameter) were produced at the Canadian Nuclear Laboratories (Chalk River, ON, Canada). Copper, silver and nickel discs of 25.4 mm of diameter, used for comparison, were bought from A.F. Murphy Die & Machine Co., Inc. (North Quicy, MA). Polonium-210 was obtained from a 210Pb solution in equilibrium with its daughters (National Institute of Standard and Technology (NIST), Gaithersburg, MD) and 209Po tracer was
purchased from Eckert & Ziegler Isotopes Products (Valencia, CA). Urine samples were obtained from un-identified employees of the Canadian Nuclear Laboratories via the Dosimetry Services Laboratory. 2.2. Procedure Prior to use, stainless steel discs were rinsed with ethanol, immersed in 6 M HCl for a few seconds and dried using a cleaning tissue. The tracer (200 mBq of 209Po) and urine sample (10 mL) were weighed into disposable 60 mL polyethylene bottles. The urine sample was acidified to approximately 1 M HCl by adding 0.83 mL of concentrated HCl. The metallic disc was placed on the top of the bottle opening, sealed with a rubber O-ring, and tightened with the cap (see Fig. 1). The sample bottles were first manually shaken once and then shaken for 4 h with a mechanical shaker (VWR, digital 12 L shaking water bath). A maximum of 20 samples could be shaken at the same time. After the plating step, the metallic discs were rinsed with water and heated at 300 °C on the surface of a hot plate for 10 min to oxidize Po and reduce potential contamination of the alpha detector (Eichrom Technologies LLC., 2009). Finally, Po plated onto the metallic disc was counted by alpha spectrometry. 2.3. Method development Different metallic discs (stainless steel, copper, nickel and silver) were tested for Po plating efficiency. The recovery and energy resolution were compared. Plating recovery of Po as a function of time and HCl molarity onto stainless steel discs were evaluated. The influence of urine storage conditions on Po recovery was examined using the procedure previously described. The samples were stored under 4 different conditions for 23 days: (1) at room temperature ( 22 °C); (2) in the refrigerator ( 4 °C); (3) acidified to 1% HCl and stored at room temperature; and (4) acidified to 1% HCl and stored in the refrigerator. In addition, potential losses of Po during the plate heating step prior to the alpha counting were evaluated by calculating the difference in Po activities measured before and after heating for different times on a hot plate. Two replicates were performed for each measurement. Pooled urine was used for method development experiments. 2.4. Figures of merit and method validation The minimum detectable activity (MDA) of the method was estimated from 12 UPW blank samples using the Currie (Currie, 1968) Eq. (1) as follows:
Fig. 1. Schematic diagram of the plating assembly.
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MDA =
k 2 + 2⋅k⋅ 2⋅B (mBq/L) T⋅ε⋅R⋅V⋅F
181
(1)
where k is a constant (1.645) to reach 95% confidence interval; B is the number of background counts for a defined time in seconds (T); ε is the counting efficiency ( 0.25); R is the chemical recovery; V is the sample volume in litres (i.e., 0.01 L); and F is a unit conversion factor that equals 10 3. The quantification limit (QL) has been calculated as 3 times the MDA. The method was verified using 12 different spiked urine samples. For each sample, a known quantity of the 210Pb/210Po solution and 11 mL of urine sample were weighed into disposable 15 mL conical tubes. The samples were vigorously shaken, refrigerated, and settled for 24 h to allow 210Po species to reach equilibrium with the components comprising the urine matrix. The procedure previously described was then applied to measure 210Po activity in these samples. Relative bias (Bri) was calculated using Eq. (2), where Ai is the measured activity and Aai the expected activity.
A − Aai Bri = i × 100% Aai
(2)
Relative precision was calculated as the standard deviation of the individual calculated relative bias values. Recovery correction due to 210Po decay was applied for recovery calculation. 2.5. Instrumentation s
Polonium samples were counted using an Octete Plus Alpha Spectroscopy Workstation with eight 450 mm2 ULTRA-AS ionimplanted silicon detectors (AMETEK/ORTEC Inc., Oak Ridge, TN).
3. Results and discussion 3.1. Optimization 3.1.1. Metallic discs The plating recovery of 210Po on different metallic discs was determined and the results are shown in Fig. 2A. Similar recoveries were found for all the metallic discs tested. Compared to the Po recovery obtained by Pornepkasemon (Porntepkasemsan et al., 2011) for water samples of Po plated at room temperature, the results obtained for urine samples showed similar recoveries ( 60% to 65% for water and urine) for Cu and Ni discs and lower values for silver discs ( 80% for water and 50% for urine). Good energy resolution was observed in all cases with relatively low full weight half maximum (FWHM) values of 10 to 30 keV (Fig. 2B).
Fig. 3. Po recovery as a function of time onto stainless steel discs at room temperature.
Slightly poorer resolution was found for copper discs ( 30 keV). Since stainless steel discs are cheaper than others and easy to obtain, they were preferred in this study. Note that Ni and Ag discs could also be used without compromise with regard to recovery and resolution. 3.1.2. Plating kinetics The Po plating recovery for urine increased at a relatively constant rate ( 15%/h) for the first 4 h to reach a maximum value of 60% (Fig. 3). Similar plating kinetics were also observed for water, where an optimal, but higher, recovery ( 90%) was reached after 3–4 h at 80–90 °C in HCl solutions (Porntepkasemsan et al., 2011). This suggests that the plating time should be at least 4 h to obtain an optimal recovery. Reducing the plating time (at room temperature) could be an option to speed up the sample analysis throughput, although it will decrease the recovery and increase the MDA. In this case, the amount of tracer needs to be adjusted to cope with the recovery loss. 3.1.3. HCl molarity The recovery as a function of HCl molarity was also evaluated and the results are shown in Fig. 4. For urine samples acidified to 0.001–0.1 M HCl, Po was mostly not plated. For sample acidities of 0.5 M HCl and higher, the recovery rapidly increased to 55% and then it remained constant. A comparison with water samples showed similar results (Fig. 4). The Po recovery was low for 0.001– 0.01 M HCl and it reached a maximal value of 50% at HCl
Fig. 2. Comparison of Po recovery on different metallic discs for 4 h plating at room temperature. *SS: Stainless steel.
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Fig. 4. Po recovery of as a function of HCl molarity after 4 h plating onto stainless steel discs at room temperature for urine and water samples.
molarities above 0.2 M. The maximum Po recovery was reached at a higher HCl molarity for urine than for water, which could be due to neutralization by alkaline components in the urine and/or the complexation of Po with organic ligands in urine. For silver discs, the Po recovery reached a maximum plating recovery for HCl concentrations above 0.5 M, which is similar to stainless steel discs (Mahmood et al., 2012). This indicates that a minimal sample acidity is required to allow the spontaneous plating reaction. For all tests, an HCl molarity of 1 M has been chosen to ensure sufficient acidification of different urine samples. Note that the results of higher acidities (above 2 M HCl) are not shown due to the rapid degradation of stainless steel discs and obvious leaking issues. 3.2. Storage conditions The effect of different storage conditions on Po recovery in urine is shown in Fig. 5. For all the different storage conditions studied, no significant tendency as a function of time was observed, indicating that the storage conditions were not affecting the recovery. The same conclusion was obtained using the CuS micro-precipitation method (Guérin and Dai, 2014). However, a lower and less consistent recovery was found for the plating technique compared to the micro-precipitation method (647 8% and 857 3% for the plating and micro-precipitation methods,
Fig. 6. Po recovery on stainless steel discs before and after heating on a hot plate (300 °C) as a function of time.
respectively). 3.3. Plate heating The influence of the plate heating step prior to alpha counting was assessed (Fig. 6). As shown in Fig. 6, there was no observable difference in recovery after heating for 30 min at 300 °C. This result is consistent with the fact that the decomposition temperature of PoO2 to Po is 500 °C (Holden, 2013). Thus, no significant losses of Po are expected in the plate heating step. 3.4. Figures of merit A MDA of 0.3 Bq/L for 4 h of counting time was obtained (Table 1). Based on a committed effective dose of 100 mSv in the event of a radiological/nuclear emergency, a reference level of 110 Bq/L in urine samples collected 3 days post-exposure has been derived (Li et al., 2012). The MDA obtained using the present method is well below the reference level with a comfortable margin for confidence. One advantage of this method is that a small volume of a spot urine sample is sufficient to meet the sensitivity requirement for emergency bioassay, facilitating easier sample collection and preparation. In the event of an emergency, after 4 h of counting for rapid screening, the samples with a low activity could continue to be counted for a longer time to achieve a better sensitivity. For example, a MDA of 0.05 Bq/L was obtained after a counting time of 48 h (Table 1), which is only slightly above the usual background level of Po in urine (0.004–0.020 Bq/day) (Meli et al., 2009). Polonium in urine spike samples was determined and the results are shown in Fig. 6. The overall chemical recovery of the method was 607 8% (Table 1). For the samples above the Table 1 Figures of merit of the method.
Fig. 5. Po recovery in different storage conditions of urine samples over a period of 23 days. *RT: room temperature, and F: fridge.
Counting time
4h
48 h
Chemical recovery (%) MDA (Bq/L) QL (Bq/L) RL (Bq/L) Mean relative bias (%) Relative precision (%)
60 7 8 0.3 70.1 0.8 70.3 110 3.10* 13.4
637 9 0.05 70.01 0.147 0.04 – 1.90 9.80
MDA: minimum detectable activity, QL: quantification level, RL: reference level. *
Only the samples above QL included.
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Fig. 7. Emergency radioassay (A) 4 h counting, and (B) 48 h counting.
quantification limit, the measured values were in good agreement with the expected values. Mean relative bias and relative precision of 3.10% and 13.4% were obtained with a counting time of 4 h (Table 1). These results meet the acceptance criteria for an emergency bioassay procedure given by the ANSI/HPS N13.30, which are 25% to þ50% for relative bias and 740% for relative precision (Health Physics Society, 1996). For a counting time of 48 h, the mean relative bias and relative precision were –1.90% and 9.80%, respectively (Table 1).
other intrinsic organic ligands in urine). Thus, the presence of the complexing agent may not pose a significant impact on the method performance. In addition, a longer counting time can be applied when the Po recovery is significantly lower for the followup urine samples.
3.5. Analysis throughput and turn-around time
210
Using the present procedure, batches containing a large number of samples could be easily processed. The number of samples that can be simultaneously prepared is largely limited by the capacity of the mechanical shaker used. The samples could also be manually shaken at a constant frequency (e.g., every 20 min) with a lower recovery of 30% to 40% (results not shown). The method does not require any heating or oxidation step, which saves a considerable amount of sample preparation time. In addition, no pre-concentration is necessary to meet the sensitivity requirements. Using a single shaker, 20 samples can be prepared by one person in approximately 1–1.25 h (weighing tracer, weighing urine, adding HCl, discs preparation, closed cap). In the event of an emergency, more than 400 samples could be processed in one day, assuming that 20 samples are prepared each hour and 4 shakers are available. In addition, only minimal training is required to perform this procedure. At this rate, the availability of alpha spectrometers could be the limiting factor. To improve sample analysis throughput, one option is to reduce the counting time since the reference 210Po level in emergency urine sample is high. However, the quantity of tracer added has to be adjusted. This method could be a very interesting alternative to the CuS microprecipitation procedure for emergency urine bioassay. 3.6. Decorporation agent In case that a decorporation agent would be administrated to affected personals who may require immediate medical treatments and be present in the follow-up urine samples, the Po recovery could be potentially reduced. However, even the Po recovery would be reduced, the current method could still be used as any recovery loss would be corrected using the 209Po tracer. As showed in Table 1 and Fig. 7, there is a large margin for the developed method to meet the sensitivity requirements for emergency bioassay needs. In the current method, the Po plating was performed under acidic condition, which may reduce the complexing strength of the ligands (either the decorporating agent or
4. Conclusions A very rapid and convenient method was developed to measure Po in emergency urine samples by alpha spectrometry following spontaneous plating of polonium onto stainless steel discs. Large sample batches were simultaneously prepared without heating using small volume urine samples (10 mL) and an acceptable recovery of 6078% was obtained. The method meets all the sensitivity and precision requirements for an emergency bioassay procedure.
Acknowledgements This research was funded by the Government of Canada (RD-1.4.4.5-5897) under Canadian Nuclear Laboratories Science and Technology program. The authors wish to thank the Dosimetry Services Laboratory staff of Chalk River Laboratories for their assistance with this work.
References Harrison, J., Leggett, R., Lloyd, D., Phipps, A., Scott, B., 2007. Polonium-210 as a poison. J. Radiol. Prot. 27, 17–40. Cornett, J., Tracy, B., Kramer, G., Whyte, J., Moodie, G., Auclair, J.P., Thomson, D., 2009. Polonium-210: lessons learned from the contamination of individual Canadians. Radiat. Prot. Dosim. 134 (3–4), 164–166. Bailey, M.R., Birchall, A., Etherington, G., Fraser, G., Wilkins, B.T., Bessa, Y., Bishop, L., Brown, J., Dorrian, M.D., Ewers, L.W., Fell, T.P., Ham, G.J., Hammond, D.J., Forrester, S., Hodgson, A., Howarth, C., Ibrahimi, Z.F., Maguire, H., Marsh, J., Phipps, A.W., Puncher, M., Ruggles, R., Shutt, A., Smith, J.R.H., Turbitt, D., Youngman, M.J., Wildin, D., 2010. Individual Monitoring Conducted by the Health Protection Agency in the London Polonium-210 Incident. Health Protection Agency, New York. Meli, M.A., Desideri, D., Roselli, C., Feduzi, L., 2009. 210Po determination in urines of people living in Central Italy. J. Environ. Radioact. 100, 84–88. Manickam, E., Sdraulig, S., O’Brian, R., 2010. An improved and rapid radiochemical method for the determination of polonium-210 in urine. Aust. J. Chem. 63, 38–46. Fisenne, I.M., 1997. Polonium in Water and Urine. Environmental Measurements Laboratory, US Department of Energy, New York, NY. Ham, G.J., 2009. The determination of Polonium-210 in urine following the Litvinenko incident. Radioprotection 44 (5), 41–46. Figgins, P.E., 1961. The radiochemistry of polonium. N.A.S.-N.R.C. Azeredo, A.M.G.F., Lipsztein, J.L., 1991. 210Po excretion in urine: a comparison of an occupational exposed group and a control. Radiat. Prot. Dosim. 36 (1), 51–54.
184
N. Guérin, X. Dai / Applied Radiation and Isotopes 103 (2015) 179–184
Guérin, N., Dai, X., 2014. Determination of 210Po in drinking water and urine samples using copper sulfide microprecipitation. Anal. Chem. 86 (12), 6026–6031. Maxwell, S.L., Culligan, B.K., 2009. New column separation method for emergency urine samples. J Radioanal. Nucl. Chem. 279 (1), 105–111. Guérin, N., Dai, X., 2013. Rapid preparation of polonium counting sources for alpha spectrometry using copper sulfide microprecipitation. Anal. Chem. 85 (13), 6524–6529. Eichrom Thecnologies, LLC., 2009. Lead-210 and Polonium-210 in Water. Analytical Procedures. Currie, L.A., 1968. Limits for qualitative detection and quantitative determination. Application to radiochemistry. Anal. Chem. 40 (3), 586–593. Porntepkasemsan, B., Srisuksawad, K., O-Manee, A., 2011. deposition efficiency of polonium-210 on various metals. In: Proceeding of the TIChE International
Conference, Hatyai, Thailand, Paper code: ms006. Mahmood, Z.U.W., Mohamed, N., Ariffin, N.A.N., Ishak, A.K., 2012. Improved Optimum Condition for Recovery and Measurment of 210Po in Environmental Samples. Research and Development Seminar, Bangi, Malaysia. Holden, N.E., 2013. CRC Handbook of Chemistry and Physics. Available from url: http://www.hbcpnetbase.com/ (accessed June, 2014). Li, C., Vlahovich, S., Dai, X., Richardson, B., Daka, J.N., Kramer, G.H., 2012. Requirements for radiation emergency urine bioassay techniques for the public and first responders. Health Phys. 99, 702–707. Health Physics Society, 1996. Performance Criteria for Radiobioassay American National Standard: ANSI N13.30. American National Standard Institute, New York.