Article pubs.acs.org/ac
Determination of 210Po in Drinking Water and Urine Samples Using Copper Sulfide Microprecipitation Nicolas Guérin*,¶ and Xiongxin Dai*,¶ Radiological Protection Research and Instrumentation Branch, Chalk River Laboratories, Atomic Energy of Canada Limited, Building 513, Chalk River, Ontario K0J 1J0, Canada ABSTRACT: Polonium-210 (210Po) can be rapidly determined in drinking water and urine samples by alpha spectrometry using copper sulfide (CuS) microprecipitation. For drinking water, Po in 10 mL samples was directly coprecipitated onto the filter for alpha counting without any purification. For urine, 10 mL of sample was heated, oxidized with KBrO3 for a short time (∼5 min), and subsequently centrifuged to remove the suspended organic matter. The CuS microprecipitation was then applied to the supernatant. Large batches of samples can be prepared using this technique with high recoveries (∼85%). The figures of merit of the methods were determined, and the developed methods fulfill the requirements for emergency and routine radioassays. The efficiency and reliability of the procedures were confirmed using spiked samples.
P
olonium-210 (210Po) is a naturally occurring radioisotope produced from the decay of uranium-238. This alpha emitter has a high specific activity per unit mass (1.66 × 1014 Bq/g)1 and is considered to be one of the most radiotoxic nuclides.2−5 Due to its acute toxicity, 210Po can cause severe health problems if it is either accidentally or intentionally released in water, food, or the environment.6 The development of rapid methods for its measurement is particularly important for emergency response in the event of a polonium contamination. Polonium-210 is traditionally measured using the spontaneous deposition method, which selectively plates Po onto a metallic disc in an acidic solution (e.g., 0.1 to 1 M HCl).7,8 Silver discs are preferred,7 but nickel, copper, and stainless steel ones have also been used.9−11 The sample solutions are usually heated at high temperature (90−95 °C) under agitation for 3 to 5 h in order to obtain a preferred recovery (∼70− 90%).2,7−10,12−14 The metallic discs are then rinsed with water and counted by alpha spectrometry. Note that 208Po or 209 Po tracers are frequently used to establish the recovery of the procedure.7,15 Polonium is selectively plated from other alpha emitters, but several chemical interfering agents could affect the recovery or the spectral resolution.16 The plating method, even without the sample treatment, requires heating at an elevated temperature for several hours, which is inconvenient and timeconsuming. Recently, microprecipitation techniques for the preparation of thin-layer alpha counting sources of Po have been developed for the improved measurement of 210Po. Maxwell et al.17 have established a bismuth phosphate microprecipitation method to measure Po. This technique shortens the preparation time of the Po counting sources, but it does not provide adequate Published XXXX by the American Chemical Society
selectivity against other alpha emitting radionuclides (e.g., Th) and a purification step is required. Guerin and Dai18 have demonstrated that alpha counting sources of Po can be rapidly prepared using the CuS microprecipitation technique in HCl solutions due to the very low solubility of PoS. The other common alpha emitters (Ra, Th, U, Np, Pu, Am) did not coprecipitate and, thus, would not interfere with the Po counting. Moreover, up to 0.1 mg of the transition metals in the sample, which could produce insoluble sulfides, can be tolerated before degradation of the alpha spectral resolution was observed.18 Compared to the conventional plating method, this CuS microprecipitation method is much faster and easier to use for processing large batches of samples, with no heating step required for a high recovery (80−95%). However, the method has not been applied to determine 210Po in environmental or bioassay samples. Drinking water and urine samples are among the most important media for monitoring 210 Po on a routine basis or for emergency response. Direct application of the CuS microprecipitation method could be challenging for the determination of 210Po in complicated sample matrices (e.g., urine), which may contain a large amount of transition metals or insoluble particles. The objective of the work described in this paper is to apply the CuS microprecipitation method for the rapid measurement of 210Po in drinking water and urine samples. The influence of transition metals in selected drinking water samples was assessed and found to be insignificant. For urine samples, 228
Received: March 31, 2014 Accepted: May 27, 2014
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Table 1. Composition of Drinking Water Samples and Recovery for
210
Po Measurement
sample ID
sample location or name
type of water
Mg2+ (mg/L)
Ca2+ (mg/L)
total transition metala (mg/L)
1 2 3 4 5 6 7 8 9 10
Sainte-Catherine de la Jacques-Cartier, QC Sainte-Catherine de la Jacques-Cartier, QC Sainte-Catherine de la Jacques-Cartier, QC Mont Reine Malouin, Quebec City, QC Quebec City, QC Deep River, ON Real Canadian President Choice Evian Milli-Q
well well well spring municipality municipality bottle bottle bottle UPW
2.78 1.28 0.96 0.98 0.76 1.79 25.9 26.0 24.5 0.13
29.0 8.07 7.71 8.42 6.43 6.74 68.1 68.4 73.4 0.35
0.109 0.039 0.040 0.007 0.036 0.083 0.011 0.058 0.010 0.000
a
recovery (%) 85 88 89 85 89 82 83 85 86 84
± ± ± ± ± ± ± ± ± ±
3 4 4 4 3 3 4 3 4 5
Including Cd2+, Cr3+, Co2+, Fe3+, Pb2+, Mn2+, Ni2+, and Zn2+.
of water. After filtration, the precipitate was rinsed using 1−2 mL of 80% ethanol and air-dried for few minutes. The filter was finally mounted on a stainless steel disc and counted for either 4 h for emergency or 24 h for routine measurements. For urine samples, 200 mBq of 209Po tracer was added to 10 mL of urine in a 100 mL glass beaker and weighed. The sample was brought to approximately 1 M HCl by adding 0.83 mL of concentrated HCl. To each sample, 4.00 × 10−4 moles of KBrO3 was added (1 mL of the working solution). The sample was then heated to boiling for 5 min under a watch glass to agglomerate suspended organic matter. After cooling to room temperature for 10 min, the sample was centrifuged at 3500 rpm for 3 min to remove the insoluble particles. To the supernatant, 1.14 × 10−3 moles of ascorbic acid (2 mL of the working solution) were added to reduce oxidizing species still present in the sample (BrO3−, Br2, Cl2). Then, the same CuS microprecipitation procedure used for drinking water was applied to the supernatant solution for the preparation of thinlayer alpha counting sources. Samples were counted by alpha spectrometry for 4 or 48 h for emergency or routine measurements, respectively. Method Development and Interferences. For drinking water, the concentrations of calcium, magnesium, and selected transition metals (Cd2+, Cr3+, Co2+, Fe3+, Pb2+, Mn2+, Ni2+, and Zn2+) in the water samples were determined by ICP-AES. For urine samples, the effects of strong oxidizing agents (H2O2, KBrO3, and (NH4)2S2O8) on decomposition and agglomeration of the organic matter, both at room temperature and at boiling temperature for 5 min on a hot plate, were studied. For each sample, 10 mL of urine was acidified to a final concentration of 1 M HCl and 0.1% (m/v or v/v) of the oxidizing agent. At room temperature, the sample was left to react for 10 min. After oxidation/heating, the sample was filtered using a 0.1 μm Resolve filter. The filter was then rinsed with ethanol (80% v/v). The amount of precipitate on the filter was weighed. At least 2 replicates were performed for each test. To examine the potential for Po vaporization during the heating step, two sets of 4 urine samples were prepared. One set of samples was spiked with 209Po before the heating step, and the other set was spiked after heating. Also, to determine if Po at the trace level is adsorbed/plated on the plastic centrifuge tubes during the sample storage, a sample of pooled urine was divided in 4 batches of 8 tubes (32 tubes). The tests were conducted under the following conditions: (1) urine stored at room temperature (∼22 °C); (2) urine stored in the fridge (∼4 °C); (3) acidified urine to 1% HCl at room temperature; and (4) acidified urine to 1% HCl in the fridge. All samples were spiked with a known quantity of 210Po. Polonium-210 activity
rapid sample pretreatment was necessary to remove the interferences from suspended organics on the CuS microprecipitation. To optimize the sample pretreatment method, the influence of different oxidizers and heating on the removal of suspended organic particles in urine was evaluated. Spiked samples were analyzed for method validation, and the figures of merit of the methods were determined.
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EXPERIMENTAL SECTION Reagents and Standards. All solutions were prepared using ultrapure water (UPW) produced from a Millipore Direct-Q5 water purification system (Billerica, MA). Trace metal grade hydrochloric and nitric acids were employed (Fisher, Fair Lawn, NJ). Copper chloride (CuCl2·2H2O), sodium sulfide (Na2S·9H2O), potassium bromate (KBrO3), ammonium persulfate ((NH4)2S2O8), hydrogen peroxide 30% v/v (H2O2), and ascorbic acid were purchased from Fisher (Fair Lawn, NJ). Solutions of Cu2+ 500 mg/L in 1% v/v HCl, Na2S·9H2O 1% m/v, KBrO3 6.7% m/v, and ascorbic acid 10% m/v were prepared and used for the microprecipitation procedure. A certified standard solution of 210Pb (National Institute of Standards (NIST), Gaithersburg, MD) in secular equilibrium with its daughters was used as a precursor of 210Po. Polonium-209 was obtained from Eckert & Ziegler Isotope Products (Valencia, CA). Sample Collection. Drinking water samples of various types were collected, and main characteristics are noted in Table 1. All water samples were taken in the provinces of Quebec and Ontario in Canada. Drinking water samples were filled in 4 L plastic bottles. A volume of 10 mL of HNO3 (0.25% v/v) was added at their arrival in the laboratory. For commercial bottled water samples, nitric acid was directly added to the samples in the same proportion. Urine samples were provided by Chalk River employees through the Dosimetry Services Laboratory and kept refrigerated until use except where otherwise specified. Procedure. For drinking water, a known amount of 209Po tracer (200 mBq for emergency samples or 50 mBq for routine samples) was weighed to 10 mL of water samples in disposable 50 mL polypropylene conical tubes. The sample was acidified to approximately 1 M HCl by adding 0.83 mL of concentrated HCl. A quantity of 7.87 × 10−7 moles of Cu2+ (0.05 mL of the working solution) followed by 4.17 × 10−5 moles of S2− (1 mL of the working solution) was pipetted to the sample. The solution was vigorously shaken and allowed to react for 10 min. The CuS precipitate was filtered using an Eichrom Resolve filter (Eichrom Technologies Inc., Lisle, IL), which was prewetted with 1−2 mL of 80% ethanol followed by 1−2 mL B
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was then measured at specific times using the method described previously. Figure of Merits. MDA. Minimum detectable activity (MDA) of the method was determined by measuring 210Po counts in 10 water blanks using the procedures previously described. MDA was calculated using the Currie equation19 (1) as follows: MDA =
k2 + 2·k· 2·B (mBq/L) T ·ε·R·V ·F
(1)
where k is a constant (1.645) to reach the 95% confidence interval, B is the number of background counts for a defined time in seconds (T), ε is the counting efficiency, R is the chemical recovery, V is the sample volume in liters (i.e., 0.01 L), and F is a unit conversion factor which equals 10−3. Spiked Samples. To validate the drinking water method, 10 spiked samples were prepared (5 for emergency and 5 for routine tests). In 50 mL plastic tubes, exact known amounts of 210 Po were weighed into 20 mL of drinking water samples. The concentration of 210Po in the sample was determined using the CuS microprecipitation procedure described previously. Twelve different urine samples spiked with known amounts of 210Po were also prepared for validation of the urine method. The relative bias (Bri) was calculated using eq 2 and the relative precision (SB) (standard deviation of the relative bias) by eq 3: Bri =
A i − A ai × 100% A ai
Figure 1. Gravimetric measurement of organic particles in urine after different oxidiation pretreatments. RT: Room temperature; CuS: CuS microprecipitation; Cent.: centrifugation step; Heat: heating step.
(2)
N
SB =
∑i = 1 (Bri − Br )2 N−1
× 100%
(3)
where Ai is the measured activity, Aai is the added activity, Br is the mean relative bias, and N is the number of replicates. Instrumentation. Polonium samples were counted using an Octete Plus Alpha Spectroscopy Workstation with eight 450 mm2 ULTRA-AS ion-implanted silicon detectors (AMETEK/ ORTEC Inc., Oak Ridge, TN).
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Figure 2. Comparison between the recovery of Po before and after the heating step for urine samples.
RESULTS AND DISCUSSION Method Development and Interferences. Drinking Water. The concentration of potential interfering agents in various drinking water samples was assessed (Table 1). Magnesium and calcium are the predominant cations in drinking water. The highest concentrations of these elements were found in bottled water (approximately 25 and 70 mg/L for Mg2+ and Ca2+, respectively). The amounts of Cd2+, Cr3+, Co2+, and Pb2+ in these samples were below detection limits, and trace amounts of Cu2+, Fe3+, Mn2+, and Zn2+ were found in some samples (results not shown). Note that only the total concentrations of transition metals are given. The highest total concentration of transition metals was found in a well water sample (0.109 mg/L) (Table 1). Calcium and magnesium should not interfere with the method, since they do not form insoluble sulfide salts. The concentration of transition metals that could form insoluble sulfides in drinking water is too low (≪10 mg/L) to affect the CuS microprecipitation.18 A constant Po recovery around 87% was obtained for 9 different samples of drinking water, which is equivalent to UPW (Table 1). This confirms that the amount of Mg2+, Ca2+, and transition metals in 10 mL of drinking water would not interfere with the CuS microprecipitation. Consequently, no purification step is
Figure 3. Recovery of Po during 23 days in different storage conditions for urine samples. RT: Room temperature; F: Refrigerator.
required, which leads to a rapid and convenient method to measure 210Po in drinking water. C
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The direct application of the CuS microprecipitation method to an untreated urine sample was found not to be practical, as very fine suspended particles could be present in urine and the filtration through a 0.1 μm Resolve filter using a vacuum box system could take a long time (several hours for 10 mL of urine). Thus, different sample pretreatment approaches (including oxidation and heating) were tested to agglomerate or decompose the suspended particles and to remove them either by centrifugation or by filtration before the CuS microprecipitation step. The quantity of suspended particles in urine after various sample pretreatment methods was evaluated gravimetrically, and the results are shown in Figure 1. Some organic compounds could precipitate when HCl is added at room temperature to urine, and the precipitate can be captured by the filter. These organic particles were partially later dissolved during the rinsing step with ethanol (∼1.5 ± 0.2 mg of residue left on the filter). A direct oxidation of urine at room temperature led to more residues (∼3−4 mg) on the filter depending upon the oxidizer used (Figure 1). Although more precipitates were formed after oxidation, filtration was faster possibly because the use of an oxidizer agglomerated the suspended particles, which were less problematic than smaller particles to clog the filter. Compared to hydrogen peroxide or potassium bromate, potassium persulfate was found to be slightly less efficient in producing particles that could be retained by the filter. As shown in Figure 1, after the CuS microprecipitation, ∼0.5−1.5 mg of additional residue was found on the filter. Attempts were also made to centrifuge the particles that were formed with oxidation at room temperature following the CuS microprecipitation of the supernatant, as direct filtration of the urine sample without oxidation was impractically long. With centrifugation, slightly fewer amounts of particles were retained on the filters. The samples after oxidiation by KBrO3 filtered faster and produced fewer particles than the two other oxidizers (∼2.5 mg instead of ∼3.5−4 mg) (Figure 1). Centrifugation was partially effective at removing the particles, especially after the oxidation with H2O2 and KBrO3. For ammonium persulfate, no significant difference by centrifugation was found (4.2 ± 0.2 mg without centrifugation and 4.0 ± 0.2 mg with centrifugation). Fewer particles (∼3−4 mg) were removed from urine by centrifugation than by filtration for the room temperature oxidation. This suggests that some particles retained by the filter were too fine to be removed by centrifugation. Note that direct CuS microprecipitation could be performed when KBrO3 is used for oxidation followed by a
Table 2. Figures of Merit for the Drinking Water and Urine Methods Drinking Water counting time
4h
24 h
chemical recovery (%) MDA (Bq/L) required level (Bq/L) mean relative bias (%) relative precision (%)
86 ± 3 0.12 1 −9.5 4.0
85 ± 3 0.04 0.1 −3.9 18.1
Urine counting time
4h
48 h
chemical recovery (%) MDA (Bq/L) required level (Bq/L) mean relative bias (%) relative precision (%)
86 ± 5 0.20 110 5.8 23.2
84 ± 4 0.05 0.4 −1.4 9.2
Figure 4. Results for spiked drinking water samples after HTiO coprecipitation. RL: Required level; MDA: Minimum detectable activity.
Urine. Urine is a complex matrix containing many organic compounds and some inorganic salts.20 For the inorganic content, none of the main constituents (Na+, K+, Mg2+, and Ca2+) precipitates with sulfide. Hence, they are not expected to interfere with the CuS microprecipitation. Transition metals in urine samples are usually present at trace levels in the range of μg/L or lower,21,22 which is well below the maximal limit of 10 mg/L to form sulfide precipitate in urine. The main concern for urine samples is the possible presence of suspended particles, which could be retained by the filter leading to a poor recovery and/or spectral resolution.
Figure 5. Results for spiked drinking water samples. (A) Emergency radioassay (4 h counting) and (B) routine radioassay (24 h counting). RL: Required level; MDA: Minimum detectable activity. D
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Figure 6. Results for spiked urine samples. (A) Emergency bioassay (4 h counting) and (B) routine bioassay (24 h counting). RL: Required level; MDA: Minimum detectable activity.
to be refrigerated or acidified for storage or transportation for the Po bioassay in the event of an emergency, which will simplify the sample collection protocol. Figures of Merit. Sensitivity Requirements and MDA. A guidance level of 0.1 Bq/L for 210Po in drinking water has been recommended by both WHO23 and Health Canada.24 This guidance level was derived as the activity concentration for an intake of 2 L/day of drinking water for 1 year, which results in a committed effective dose (CED) of 0.1 mSv/year for members of the adult public. Following this guidance, the same required level of 0.1 Bq/L for 210Po has been chosen for a routine water assay in our laboratory. In emergency situations, an intervention level of 1 mSv/year is recommended by Health Canada,25 which sets the required level of 1 Bq/L for 210Po for emergency radioassay in water samples. For an emergency urine bioassay, a required level of 110 Bq/L was derived, on the basis of a dose threshold of 100 mSv CED on urinary excretion of the third day after exposure to radionuclide contamination.26 The required level for 210Po in routine urine bioassay was calculated to be 0.4 Bq/L, on the basis of a recommended annual dose threshold of 1 mSv CED with a monthly bioassay sampling frequency. For drinking water, the MDAs obtained for 4 and 24 h of counting were 0.12 and 0.04 Bq/L, respectively (Table 2). It follows that the requirement for emergency measurement in a drinking water sample can be easily met. In contrast, it will be more difficult to meet the sensitivity requirement for a routine radioassay, suggesting a possible need for a preconcentration of a larger sample volume to reach a higher confidence level. To reduce the MDA for routine radioassay of Po in drinking water samples, a rapid coprecipitation of Po with hydrous titanium(IV) oxide (HTiO) could be performed since Ti(IV) (or TiO2+) does not precipitate with sulfide. Approximately 50 mg of titanium(IV) was first added to 50 mL of the water sample, and the HTiO precipitate was formed when the sample was neutralized with ammonium hydroxide. After centrifugation, the precipitate was dissolved with 10 mL of 1 M HCl. Then, the CuS microprecipitation procedure was followed to prepare an alpha counting source. This led to a lower MDA of 0.007 Bq/L after 48 h of counting by alpha spectrometry. An overall recovery of 75 ± 2% was obtained, which is only slightly lower than the direct CuS microprecipitation procedure (∼85%). The HTiO coprecipitation strategy allows for a more comfortable MDA when a better sensitivity is desired. Water samples spiked with low levels of 210Po were prepared to validate the method including the HTiO coprecipitation step, and the results are shown in Figure 4. Mean relative bias and
centrifugation step. This strategy was tested on several urine samples. Although good 210Po efficiency has been achieved for the majority of the samples, poor recovery and spectral resolution were also observed for a few samples (
