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Determination of plutonium isotopes (238,239,240Pu) and strontium (90Sr) in seafood using alpha spectrometry and liquid scintillation spectrometry
Journal of Environmental Radioactivity 177 (2017) 151e157
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Determination of plutonium isotopes (238,239,240Pu) and strontium (90Sr) in seafood using alpha spectrometry and liquid scintillation spectrometry Choonshik Shin*, Hoon Choi, Hye-Min Kwon, Hye-Jin Jo, Hye-Jeong Kim, Hae-Jung Yoon, Dong-Sul Kim, Gil-Jin Kang Food Contaminants Division, Food Safety Evaluation Department, National Food and Drug Safety Evaluation, Ministry of Food and Drug Safety, Osong, Cheongju 28159, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 April 2017 Received in revised form 19 May 2017 Accepted 26 June 2017 Available online 4 July 2017
The present study was carried out to survey the levels of plutonium isotopes (238,239,240Pu) and strontium (90Sr) in domestic seafood in Korea. In current, regulatory authorities have analyzed radionuclides, such as 134Cs, 137Cs and 131I, in domestic and imported food. However, people are concerned about contamination of other radionuclides, such as plutonium and strontium, in food. Furthermore, people who live in Korea have much concern about safety of seafood. Accordingly, in this study, we have investigated the activity concentrations of plutonium and strontium in seafood. For the analysis of plutonium isotopes and strontium, a rapid and reliable method developed from previous study was used. Applicability of the test method was verified by examining recovery, minimum detectable activity (MDA), analytical time, etc. Total 40 seafood samples were analyzed in 2014e2015. As a result, plutonium isotopes (238,239,240Pu) and strontium (90Sr) were not detected or below detection limits in seafood. The detection limits of plutonium isotopes and strontium-90 were 0.01 and 1 Bq/kg, respectively. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Plutonium Strontium Seafood Alpha spectrometer Liquid scintillation counter
1. Introduction Radionuclides are naturally present in the environment including air, soil and water. People are exposed to radiation from these radionuclides, such as potassium-40 (40K), in real time. Moreover, people can also be exposed to radiation from artificial radionuclides (e.g. 134Cs and 137Cs) which are released from nuclear power plant accident or nuclear weapons testing. The degree of harm to human is influenced by the type of radionuclides and the exposed time to people. When large amounts of radionuclides are released into environment, they fall onto the surface of food such as fruits or deposited on the animal feed through contaminated rainwater or snow. In addition, radionuclides in water can move into the rivers and the sea, depositing on seafood. Furthermore, radionuclides can be taken up by plants, seafood or ingested by animals (WHO and FAO, 2011; FAO, 1989). In relation to food safety due to radioactive contamination, the Codex Alimentarius Commission (CAC) has developed the guideline
* Corresponding author. E-mail address: [email protected] (C. Shin). http://dx.doi.org/10.1016/j.jenvrad.2017.06.025 0265-931X/© 2017 Elsevier Ltd. All rights reserved.
levels for certain radionuclides in food following a nuclear emergency. The guideline levels have been first developed in 1988 and then revised in 2006. If radionuclide levels in food do not exceed the corresponding guidelines levels, the food should be considered as safe (CODEX, 1995). In South Korea, the regulatory limits of radionuclides in food have been established only for gamma-emitting radionuclides. Accordingly, the representative gamma-emitting radionuclides (i.e. 134Cs, 137Cs and 131I) have been analyzed in imported and domestic food. However, in emergency situations, such as nuclear power plant accident, other radionuclides (e.g. strontium and plutonium) can be analyzed according to the general provisions in Korea Food Code. Especially, in the cases of alpha-emitting and beta-emitting radionuclides, the CAC recommends the levels for plutonium isotopes (238,239,240Pu) and strontium (90Sr) as 1 and 100 Bq/kg in infant food, respectively (CODEX, 1995). Plutonium (Pu) is a toxic and radioactive element which has been widely contaminated in the environment. It can be accumulated in the food chain including the marine biota. The main source of plutonium is considered global fallout from nuclear weapons ska-Parulska et al., 2011). In addition, nuclear testing (Strumin reactor accidents also contribute to local contamination of
152
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plutonium (Bu et al., 2013). Plutonium in the environment consists of various radioactive isotopes including 238Pu, 239Pu and 240Pu which emit high-energy alpha particles. Among them, 239Pu and 240 Pu are the most abundant isotopes which have long half-lives of 24,110 and 6560 years, respectively. In contrast, 238Pu exists at lower concentration in the environment and has shorter half-life of 87.7 years. However, 238Pu is also important isotope for tracing plutonium level in the environment. Accordingly, many countries established regulatory limits for the three kinds of plutonium isotopes in food (CODEX, 1995). Strontium (Sr) commonly occurs in nature as an important
mineral for human health. It is a mixture of four stable isotopes 88Sr, Sr, 87Sr, 84Sr and twelve unstable isotopes. One of its radioactive isotopes is 90Sr, which is produced during nuclear fission and emits high-energy beta particles (Tyka and Sabol, 1995). It has a long halflife of 28.8 years. Since the chemical property of 90Sr is similar to calcium, it can be accumulated in bones of the human body and cause a high risk to human health (Stamoulis et al., 2007). For the analysis of plutonium isotopes and strontium-90, a lot of methods have been developed over the world. Although most of the methods are sensitive and accurate, they require timeconsuming and laborious chemical processes. In practice, in 86
Fig. 1. Representative spectra of plutonium isotopes in standard (A) and blank shark sample (B). Each isotope emits alpha particles at distinct energy levels: 4.90 MeV), 239Pu (5.11 and 5.14 MeV), 240Pu (5.12, 5.13 and 5.16 MeV), 238Pu (5.46 and 5.50 MeV).
242
Pu (4.86 and
C. Shin et al. / Journal of Environmental Radioactivity 177 (2017) 151e157
emergency situations such as nuclear accident, more rapid and reliable method is required. Accordingly, in previous study, we have developed a rapid and accurate method for the determination of plutonium isotopes and strontium-90 (Lee et al., 2013). In the case of strontium (90Sr) analysis, a method using the Eichrom's Sr resin was used instead of the time-consuming classical method using fuming nitric acid (IAEA , 1989). The Sr resin was originally introduced by Horwitz and his co-worker in 1991. They showed that strontium was efficiently extracted from nitric acid by a solution of 40 ,4''(500 )-di-t-butylcyclohexane-18-crown-6 in 1octanol sorbed on an inert substrate (Horwitz et al., 1991). In practice, the resin has been used in various researches including food analysis (Jeter and Grob, 1994; Filss et al., 1998; Ware et al., 1998; Forsberg and Strandmark, 2001; Saxen, 2002; Brun et al., 2002). The objectives of this study were to determine the activity concentrations of plutonium isotopes (238,239,240Pu) and strontium (90Sr) in domestic seafood in South Korea and to provide safety information about radioactive contamination of seafood to consumers. 2. Materials and methods
153
Technology) in the United States. A stable strontium nitrate (Sr(NO3)2) solution was used to elevate the recovery of 90Sr during the purification process. 2.2. Sample preparation and decomposition Domestic seafood samples, including fish, shellfish, crustaceans, cephalopods and seaweeds, were purchased in Korea. They were prepared by removing non-edible organs prior to homogenization except for the Alaska pollock. For the Alaska pollock, all portions including intestine were used for the analysis considering the Korean's distinct eating culture. For the shark, a portion of muscle was used. Shellfish were prepared by removing shells. Samples were homogenized and dried at 100 C for 24e48 h. Then, the dried samples were ashed at programmed temperature conditions up to 600 C for 48 h. A portion of ash (equivalent to 50 g of sample) was taken for the subsequent analysis. The ashed samples were spiked with 242Pu tracer (NIST), 85Sr tracer (Eckert & Ziegler IPL) and Sr carrier. The spiked samples were digested with 25 mL of 8 N HNO3 in hot plate at 90e100 C for 2e3 h. After the digestion step, the sample solutions were cooled at room temperature for 5e10 h. Then, 3 mL of 12 N HCl were added to the solution to remove remaining organic materials.
2.1. Materials All test solutions were prepared using analytical grade reagents and Milli-Q water. The radioactive tracers (242Pu and 85Sr) were purchased from the NIST (National Institute of Standards and
2.3. Purification of Pu and Sr nuclides by anion exchange resin and Sr resin For the analysis of Pu and Sr nuclides, an anion exchange column
Fig. 2. Representative LSC spectra of standard (A) and blank shark sample (B). LSC counts for the 1st and 2nd measurements are indicated by arrow lines.
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and the Sr column were used, respectively. The anion exchange column (Eichrom, 20 mL) was prepared with the AG 1-X8 resin (Bio Rad, 100e200 mesh) and the Sr column (Eichrom, 2 mL) was prepared with the Sr resin (Eichrom, 100e150 mm). The anion exchange resin and the Sr resin were swelled in water and poured to each column up to 5 cm and 4 cm, respectively. In order to shorten the analytical time, the anion exchange column and the Sr column were combined sequentially. After conditioning the resins with 25 mL of 8 N HNO3, the digested sample solution was filtered through a Whatman® 540 filter paper and passed through the two columns. Then, the columns were washed with 20 mL of 8 N HNO3 to remove remaining interference nuclides. Thereafter, the two columns were disconnected and the anion exchange resin column was washed with 30 mL of 3 N HNO3 to remove remaining radionuclides, including americium (Am) and uranium (U) isotopes. Finally, Pu nuclides were extracted using 80 mL of 0.1 N NH4I/9 N HCl after additional washing with 60 mL of 9 N HCl to remove remaining radionuclides, including thorium (Th) isotopes. In this step, NH4I reduces the oxidation state of Pu to the trivalent state from the tetravalent state, so that Pu nuclides can be extracted from the anion resin (Lee et al., 2013). The disconnected Sr column was washed with 5 mL of 8 N HNO3 to remove 90Y. At this time, the exact time was recorded when the 8 N HNO3 was dropped lastly. The yttrium-90 (90Y), daughter nuclide of 90Sr, is started to be created at this time. Then, 90Sr was eluted using 10 mL of H2O and collected to 20 mL of glass vial. 2.4. Measurement of Pu nuclides In order to measure the activity concentrations of Pu isotopes, purified solution was dried in hot plate after addition of 5 mL of nitric acid and 1 mL of 0.3 M Na2SO4. Then, dried sample was redissolved with 1 mL of sulfuric acid and 4 mL of H2O and electroplated onto a stainless steel disk at pH 2.1e2.3 for 90 min. Before finishing the electroplating, 1 mL of ammonia solution (conc.) was added. After the electroplating process, the stainless steel disk was washed using H2O and acetone and dried. Finally, plutonium iso238 topes ( ,239,240Pu) were measured using an Alpha Analyst™ integrated alpha-spectrometer with passivated implanted planar silicon (PIPS) detectors (Canberra, 450 mm2 area, 18 keV resolution). The representative spectra of plutonium isotopes in standard and sample are shown in Fig. 1. 2.5. Measurement of Sr nuclide The activity concentration of 90Sr was determined by measuring its progeny 90Y using a liquid scintillation counter (LSC) (Tinker
MDAðBq=kgÞ ¼
2:71 þ 4:65
closed room. After 5 days, the solution was measured secondly at the same conditions of the 1st measurement. The activity concentration of 90Sr was calculated using the difference of LSC counts between 1st and 2nd measurements. Recovery of strontium-90 was determined by measuring the concentration of 85Sr tracer using a gamma spectrometer. The representative LSC spectra of standard and sample are shown in Fig. 2. Also, the representative spectra of 85 Sr in standard and sample are shown in Fig. 3. 3. Results and discussion For the analysis of plutonium isotopes and strontium-90, a method developed by the previous study was used. In order to verify the developed method, we have examined recovery, minimum detectable activity (MDA), analytical time, etc. The analysis results of plutonium isotopes and 90Sr in seafood samples are summarized in Table 1. For the analysis of plutonium isotopes (238,239,240Pu), alpha spectrometer was used. Since the energies of 239Pu and 240Pu cannot be resolved in alpha spectrometer, the two plutonium isotopes were determined as a sum of 239Pu and 240Pu. Prior to analyze plutonium in sample, alpha spectrometer was calibrated and efficiencies of each chamber was determined for accurate measurement. The recovery of plutonium isotopes was obtained by measuring the chemical yield of 242Pu added to each sample. Recoveries of Pu isotopes for different matrices and individual samples are shown in Fig. 4A. The average of plutonium recovery in all samples was sufficiently high (87.6 ± 21%). However, some samples showed low recoveries (hairtail: 44.9e53.0%, Alaska pollock: 41.9%, shark: 44.3%, brown seaweed: 40.9%) or enhanced recoveries (squid: 130.7%, saury: 129.8%). It was considered that variable contents of interference nuclides can affect the recoveries including enhanced recoveries in squid and saury. The activity concentrations of the seafood samples were below the minimum detection limits (0.01 Bq/kg) of the current method for plutonium isotopes. In Korea, the regulatory limits for plutonium isotopes in food have not been established. Meanwhile, the Codex Alimentarius Commission (CAC) recommends the levels for plutonium isotopes (238,239,240Pu) as 1 and 10 Bq/kg in infant food and other food, respectively. The minimum detection limits of the method were 1% levels of the guideline values (i.e. 0.01 Bq/kg). In conclusion, all the seafood samples analyzed here were considered to be safe in relation to radionuclide contamination. Minimum detectable activities (MDAs) of plutonium isotopes for different matrices are shown in Table 2. The MDAs were calculated based on background activity, detector efficiency, recovery and sample mass (Currie, 1968), as follows:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Background counts Sample counting time ðsecÞ Background counting time ðsecÞ
efficiencyð%Þ Recoveryð%Þ Sample weightðgÞ Sample counting ðsecÞ Counting 100 100
et al., 1997). Since the activities of 90Sr and 90Y are equal at equilibrium, the activity of 90Sr was inferred from the equilibrium activity of 90Y (L'Annunziata, 1998; EPA, 1976). Because 90Y emits beta particles with an Emax sufficiently high enough to cause Cerenkov radiation, the activity concentration of 90Y was directly counted without using any scintillation cocktails. The radiochemically separated strontium solution was firstly measured for 180 min (1st measurement) after 1 h of stabilization time under the dark and
(1)
In the experiments, it was considered that the MDAs in plutonium analysis were not affected by other parameters such as the amounts of ash and seafood type or species. For the analysis of 90Sr, the activity concentration of 90Sr was measured by a liquid scintillation counter (LSC) after purification using the Sr resin. 90Sr decays to its daughter nuclide, 90Y, emitting beta particles with maximum energy of 546 keV. Subsequently, 90Y
C. Shin et al. / Journal of Environmental Radioactivity 177 (2017) 151e157
155
(Stamoulis et al., 2007). Instead of the time-consuming classical method using fuming nitric acid (IAEA , 1989), we used the simple purification method using the commercially available Sr resin. The resulting solution containing 90Sr was directly measured by LSC. Recovery of strontium was determined by measuring 85Sr tracer using a gamma spectrometer. Recoveries of 90Sr for different matrices and individual samples are shown in Fig. 4B. The average recovery of 90Sr was relatively low (55.4 ± 27%) in all samples. In addition, several samples showed very low recoveries (crab: 27.2e32.4%, shrimp: 22.8%, brown seaweed: 16.0e23.2%, sea tangle: 18.1e22.3%). In the case of seaweeds and crustaceans, including crab, shrimp, brown seaweed and sea tangle, showed high ash content. Thus, it was considered that high ash content affected the chemical recovery of strontium. The activity concentrations of the seafood samples were below the minimum detection limits (1 Bq/kg) of the current method for 90Sr. In South Korea, the regulatory limits for 90Sr in food have not been established. Meanwhile, the Codex Alimentarius recommended the levels for 90 Sr as 100 Bq/kg in food including infant food. The minimum detection limit of the method was 1% level of the guideline value (i.e. 1 Bq/kg). In conclusion, all the seafood samples analyzed here were considered to be safe in relation to radionuclide contamination. The MDA values of strontium-90 for different matrices are shown in Table 2. The MDAs were calculated based on background activity, detector efficiency, recovery and sample mass as mentioned in plutonium analysis (Currie, 1968). In the cases of seaweeds and crustaceans, the MDAs were relatively high depending on the ash content. It was considered that high levels of mineral content (e.g. calcium) in seaweeds and crustaceans affect the MDA values. 4. Conclusion Fig. 3. Representative spectra of shark sample (B).
85
Sr (internal standard) in standard (A) and fortified
decays to stable zirconium-90 (90Zr) emitting beta particles with maximum energy of 2280 keV and half-life of 64 h. It was known that 90Y exists in secular equilibrium with its parent nuclide, 90Sr. Thus, the 90Sr activity of the sample can be estimated by measuring 90 Y activity in a sample. Fast moving electrons emitted by the 90Y decay, produce Cherenkov radiation in a liquid medium such as water. If electrons move faster than light in the medium, Cherenkov radiation takes place. The energy threshold of the fast moving electrons to produce Cherenkov radiation in water is 256 keV (Rao et al., 2000). Cherenkov radiation lies in the optical and UV parts of the electromagnetic spectrum, so it can be detected by LSC
Table 1 The activity concentration and recovery of Types
Fish Shellfish Crustaceans Cephalopods Seaweeds a b c d
Forty seafood samples were collected from the local markets in Korea. Plutonium isotopes (238,239,240Pu) and 90Sr in the samples were sequentially purified using an anion exchange column and the Sr column. Measurements of activity concentrations of strontium90 and plutonium isotopes were carried out using liquid scintillation counter (LSC) and alpha spectrometer, respectively. As a result, plutonium (238,239,240Pu) and strontium (90Sr) were not detected or below the detection limits. The detection limits of the method were 0.01 and 1 Bq/kg for plutonium isotopes and strontium-90, respectively. Thus, it can be considered that the seafood samples analyzed here were safe in relation to radionuclide contamination of plutonium (238,239,240Pu) and strontium (90Sr). However, in order to ensure the food safety from radionuclide contamination, continuous monitoring should be carried out.
90
Sr and plutonium isotopes in seafood samples.
Species (n)
Alaska pollock (4), mackerel (1), saury (2), shark (2), hairtail (2) abalone (2), oyster (3), short necked clam (2), common oriental clam (1) shrimp (3), crab (3) squid (3), small octopus (3) laver (2), brown seaweed (4), sea tangle (2), green laver (1)
ND: Not detected. Minimum detection limit (Pu: 0.01 Bq/kg, Sr: 1 Bq/kg). Recovery of Pu was determined using 242Pu (0.02 Bq). Recovery of Sr was determined using 85Sr (6 Bq).
238
Pu,
239þ240
90
Pu
Sr
Concentration (Bq/kg)
Recovery (%)d
41.9e129.8 62.1e98.9
ND <1b ND <1
22.4e89.6 36.2e88.8
85.7e111.8 91.0e130.7 40.9e99.4
ND <1 ND <1 ND <1
22.8e82.0 29.3e80.1 16.0e67.0
Concentration (Bq/kg)
Recovery (%)
NDa <0.01b ND <0.01 ND <0.01 ND <0.01 ND <0.01
c
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C. Shin et al. / Journal of Environmental Radioactivity 177 (2017) 151e157
Fig. 4. Recoveries of Pu isotopes (A) and strontium-90 (B) for different matrices and individual samples.
Table 2 Minimum detectable activities of plutonium isotopes and strontium based on sample mass, recovery, and detector efficiency. Matrix (n)
Fish (11) Shellfish (8) Crustaceans (6) Cephalopods (6) Seaweeds (9)
Mass (g)
Minimum detectable activity (MDA) (Bq/kg)
Sample
Ash
50 50 50 50 50
1.07 0.82 2.94 0.69 5.17
238
Pu
± ± ± ± ±
0.42 0.10 1.67 0.19 4.67
0.0030 0.0015 0.0025 0.0015 0.0020
± ± ± ± ±
0.0022 0.0007 0.0007 0.0009 0.0018
239þ240
Pu
0.0020 0.0019 0.0014 0.0017 0.0032
± ± ± ± ±
90
Sr
0.0010 0.0010 0.0010 0.0011 0.0018
0.35 0.33 0.64 0.41 0.83
± ± ± ± ±
0.25 0.13 0.23 0.22 0.36
C. Shin et al. / Journal of Environmental Radioactivity 177 (2017) 151e157
Acknowledgements This research was supported by the Grants 14161MFDS078 in 2014 and 15161MFDS666 in 2015 from the Ministry of Food and Drug Safety in Korea. References Brun, S., Bessac, S., Uridat, D., Boursier, B., 2002. Rapid method for the determination of radiostrontium in milk. J. Radioanal. Nucl. Chem. 253 (2), 191e197. Bu, K., Cizdziel, J.V., Dasher, D., 2013. Plutonium concentration and 240Pu/239Pu atom ratio in biota collected from Amchitka Island, Alaska: recent measurements using ICP-SFMS. J. Environ. Radioact. 124, 29e36. CODEX, 1995. General standard for contaminants and toxins in food and feed. CODEX STAN 193e1995. Amended in 2015. www.fao.org/input/download/ standards/17/CXS_193e_2015.pdf. Currie, L.A., 1968. Limits for qualitative detection and quantitative determination. Anal. Chem. 40, 586e593. EPA, 1976. Measurement of strontium-89 and strontium-90 in environmental water : a tentative reference method. United States. Environ. Prot. Agency, Office Res. Dev. 1e10. FAO, Food and Agriculture Organization of the United Nations, 1989. Radioactive fallout in soils, crops and food. www.fao.org/docrep/T0228E/T0228E00.htm. Filss, M., Botsch, W., Handl, J., Michel, R., 1998. A fast method for the determination of strontium-89 and strontium-90 in environmental samples and its application to the analysis of strontium-90 in Ukrainian soils. Radiochim. Acta 83, 81e92. Forsberg, S., Strandmark, M., 2001. Migration and chemical availability of 137Cs and 90 Sr in Swedish long-term experimental pastures. Water Air Soil Pollut. 127, 157e171. Horwitz, E.P., Dietz, M.L., Fischer, D.E., 1991. Separation and preconcentration of Sr
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from biological, environmental, and nuclear waste samples by extraction chromatography using a crown ether. Anal. Chem. 63, 522e525. IAEA (International Atomic Energy Agency), 1989. Measurement of Radionuclides in Food and Environment-a Guidebook. Technical Series Report No. 295. Vienna. Jeter, H.W., Grob, B., 1994. Determination of radiostrontium in milk using an extraction chromatography column. Radioact. Radiochem 5, 8e17. L'Annunziata, M.F., 1998. Cherenkov counting. In: L'Annunziata, M.F. (Ed.), Handbook of Radioactivity Analysis. Academic Press, San Diego, pp. 453e505. Lee, S.H., Oh, J.S., Lee, J.M., Lee, K.B., Park, T.S., Choi, J.K., 2013. Development of radioanalytical methods for plutonium and strontium in foodstuffs. MFDS’s R&D Rep. Republic of Korea. Rao, D.D., Mehendarge, S.T., Chadramouli, S., Hedge, A.G., Mishra, U.C., 2000. Application of Cherenkov radiation counting for determination of 90Sr in environmental samples. J. Environ. Radioact. 48, 49e57. Saxen, R., 2002. Determination of 90Sr by extraction chromatography in the presence of 210Pb in environmental samples. Proceedings of the LSC 2001 Conference, Radiocarbon, Tucson USA. Stamoulis, K.C., Ioannides, K.G., Karamanis, D.T., Patiris, D.C., 2007. Rapid screening of 90Sr activity in water and milk samples using Cherenkov radiation. J. Environ. Radioact. 93, 144e156. ska-Parulska, D.I., Skwarzec, B., Fabisiak, J., 2011. Plutonium bioStrumin accumulation in seabirds. J. Environ. Radioact. 102, 1105e1111. Tinker, R.A., Smith, J.D., Cooper, M.B., 1997. Determination of Strontium-90 in environmental samples containing thorium. Analyst 122, 1313e1318. Tyka, R., Sabol, J., 1995. Low Level Environmental Radioactivity. Technomic Publishing Company, Inc, USA. Ware, A.R., Odell, K.J., Martin, J.P., 1998. Analysis of radioactive metal ions in environmental and liquid effluent samples. J. Radioanal. Nucl. Chem. 228, 5e13. WHO and FAO, World Health Organization and Food and Agriculture Organization of the United Nations, 2011. Nuclear accidents and radioactive contamination of foods. www.who.int/foodsafety/fs_management/radionuclides_and_food_300 311.pdf.
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Determination of plutonium isotopes (238,239,240Pu) and strontium (90Sr) in seafood using alpha spectrometry and liquid scintillation spectrometry Choonshik Shin*, Hoon Choi, Hye-Min Kwon, Hye-Jin Jo, Hye-Jeong Kim, Hae-Jung Yoon, Dong-Sul Kim, Gil-Jin Kang Food Contaminants Division, Food Safety Evaluation Department, National Food and Drug Safety Evaluation, Ministry of Food and Drug Safety, Osong, Cheongju 28159, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 April 2017 Received in revised form 19 May 2017 Accepted 26 June 2017 Available online 4 July 2017
The present study was carried out to survey the levels of plutonium isotopes (238,239,240Pu) and strontium (90Sr) in domestic seafood in Korea. In current, regulatory authorities have analyzed radionuclides, such as 134Cs, 137Cs and 131I, in domestic and imported food. However, people are concerned about contamination of other radionuclides, such as plutonium and strontium, in food. Furthermore, people who live in Korea have much concern about safety of seafood. Accordingly, in this study, we have investigated the activity concentrations of plutonium and strontium in seafood. For the analysis of plutonium isotopes and strontium, a rapid and reliable method developed from previous study was used. Applicability of the test method was verified by examining recovery, minimum detectable activity (MDA), analytical time, etc. Total 40 seafood samples were analyzed in 2014e2015. As a result, plutonium isotopes (238,239,240Pu) and strontium (90Sr) were not detected or below detection limits in seafood. The detection limits of plutonium isotopes and strontium-90 were 0.01 and 1 Bq/kg, respectively. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Plutonium Strontium Seafood Alpha spectrometer Liquid scintillation counter
1. Introduction Radionuclides are naturally present in the environment including air, soil and water. People are exposed to radiation from these radionuclides, such as potassium-40 (40K), in real time. Moreover, people can also be exposed to radiation from artificial radionuclides (e.g. 134Cs and 137Cs) which are released from nuclear power plant accident or nuclear weapons testing. The degree of harm to human is influenced by the type of radionuclides and the exposed time to people. When large amounts of radionuclides are released into environment, they fall onto the surface of food such as fruits or deposited on the animal feed through contaminated rainwater or snow. In addition, radionuclides in water can move into the rivers and the sea, depositing on seafood. Furthermore, radionuclides can be taken up by plants, seafood or ingested by animals (WHO and FAO, 2011; FAO, 1989). In relation to food safety due to radioactive contamination, the Codex Alimentarius Commission (CAC) has developed the guideline
* Corresponding author. E-mail address: [email protected] (C. Shin). http://dx.doi.org/10.1016/j.jenvrad.2017.06.025 0265-931X/© 2017 Elsevier Ltd. All rights reserved.
levels for certain radionuclides in food following a nuclear emergency. The guideline levels have been first developed in 1988 and then revised in 2006. If radionuclide levels in food do not exceed the corresponding guidelines levels, the food should be considered as safe (CODEX, 1995). In South Korea, the regulatory limits of radionuclides in food have been established only for gamma-emitting radionuclides. Accordingly, the representative gamma-emitting radionuclides (i.e. 134Cs, 137Cs and 131I) have been analyzed in imported and domestic food. However, in emergency situations, such as nuclear power plant accident, other radionuclides (e.g. strontium and plutonium) can be analyzed according to the general provisions in Korea Food Code. Especially, in the cases of alpha-emitting and beta-emitting radionuclides, the CAC recommends the levels for plutonium isotopes (238,239,240Pu) and strontium (90Sr) as 1 and 100 Bq/kg in infant food, respectively (CODEX, 1995). Plutonium (Pu) is a toxic and radioactive element which has been widely contaminated in the environment. It can be accumulated in the food chain including the marine biota. The main source of plutonium is considered global fallout from nuclear weapons ska-Parulska et al., 2011). In addition, nuclear testing (Strumin reactor accidents also contribute to local contamination of
152
C. Shin et al. / Journal of Environmental Radioactivity 177 (2017) 151e157
plutonium (Bu et al., 2013). Plutonium in the environment consists of various radioactive isotopes including 238Pu, 239Pu and 240Pu which emit high-energy alpha particles. Among them, 239Pu and 240 Pu are the most abundant isotopes which have long half-lives of 24,110 and 6560 years, respectively. In contrast, 238Pu exists at lower concentration in the environment and has shorter half-life of 87.7 years. However, 238Pu is also important isotope for tracing plutonium level in the environment. Accordingly, many countries established regulatory limits for the three kinds of plutonium isotopes in food (CODEX, 1995). Strontium (Sr) commonly occurs in nature as an important
mineral for human health. It is a mixture of four stable isotopes 88Sr, Sr, 87Sr, 84Sr and twelve unstable isotopes. One of its radioactive isotopes is 90Sr, which is produced during nuclear fission and emits high-energy beta particles (Tyka and Sabol, 1995). It has a long halflife of 28.8 years. Since the chemical property of 90Sr is similar to calcium, it can be accumulated in bones of the human body and cause a high risk to human health (Stamoulis et al., 2007). For the analysis of plutonium isotopes and strontium-90, a lot of methods have been developed over the world. Although most of the methods are sensitive and accurate, they require timeconsuming and laborious chemical processes. In practice, in 86
Fig. 1. Representative spectra of plutonium isotopes in standard (A) and blank shark sample (B). Each isotope emits alpha particles at distinct energy levels: 4.90 MeV), 239Pu (5.11 and 5.14 MeV), 240Pu (5.12, 5.13 and 5.16 MeV), 238Pu (5.46 and 5.50 MeV).
242
Pu (4.86 and
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emergency situations such as nuclear accident, more rapid and reliable method is required. Accordingly, in previous study, we have developed a rapid and accurate method for the determination of plutonium isotopes and strontium-90 (Lee et al., 2013). In the case of strontium (90Sr) analysis, a method using the Eichrom's Sr resin was used instead of the time-consuming classical method using fuming nitric acid (IAEA , 1989). The Sr resin was originally introduced by Horwitz and his co-worker in 1991. They showed that strontium was efficiently extracted from nitric acid by a solution of 40 ,4''(500 )-di-t-butylcyclohexane-18-crown-6 in 1octanol sorbed on an inert substrate (Horwitz et al., 1991). In practice, the resin has been used in various researches including food analysis (Jeter and Grob, 1994; Filss et al., 1998; Ware et al., 1998; Forsberg and Strandmark, 2001; Saxen, 2002; Brun et al., 2002). The objectives of this study were to determine the activity concentrations of plutonium isotopes (238,239,240Pu) and strontium (90Sr) in domestic seafood in South Korea and to provide safety information about radioactive contamination of seafood to consumers. 2. Materials and methods
153
Technology) in the United States. A stable strontium nitrate (Sr(NO3)2) solution was used to elevate the recovery of 90Sr during the purification process. 2.2. Sample preparation and decomposition Domestic seafood samples, including fish, shellfish, crustaceans, cephalopods and seaweeds, were purchased in Korea. They were prepared by removing non-edible organs prior to homogenization except for the Alaska pollock. For the Alaska pollock, all portions including intestine were used for the analysis considering the Korean's distinct eating culture. For the shark, a portion of muscle was used. Shellfish were prepared by removing shells. Samples were homogenized and dried at 100 C for 24e48 h. Then, the dried samples were ashed at programmed temperature conditions up to 600 C for 48 h. A portion of ash (equivalent to 50 g of sample) was taken for the subsequent analysis. The ashed samples were spiked with 242Pu tracer (NIST), 85Sr tracer (Eckert & Ziegler IPL) and Sr carrier. The spiked samples were digested with 25 mL of 8 N HNO3 in hot plate at 90e100 C for 2e3 h. After the digestion step, the sample solutions were cooled at room temperature for 5e10 h. Then, 3 mL of 12 N HCl were added to the solution to remove remaining organic materials.
2.1. Materials All test solutions were prepared using analytical grade reagents and Milli-Q water. The radioactive tracers (242Pu and 85Sr) were purchased from the NIST (National Institute of Standards and
2.3. Purification of Pu and Sr nuclides by anion exchange resin and Sr resin For the analysis of Pu and Sr nuclides, an anion exchange column
Fig. 2. Representative LSC spectra of standard (A) and blank shark sample (B). LSC counts for the 1st and 2nd measurements are indicated by arrow lines.
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and the Sr column were used, respectively. The anion exchange column (Eichrom, 20 mL) was prepared with the AG 1-X8 resin (Bio Rad, 100e200 mesh) and the Sr column (Eichrom, 2 mL) was prepared with the Sr resin (Eichrom, 100e150 mm). The anion exchange resin and the Sr resin were swelled in water and poured to each column up to 5 cm and 4 cm, respectively. In order to shorten the analytical time, the anion exchange column and the Sr column were combined sequentially. After conditioning the resins with 25 mL of 8 N HNO3, the digested sample solution was filtered through a Whatman® 540 filter paper and passed through the two columns. Then, the columns were washed with 20 mL of 8 N HNO3 to remove remaining interference nuclides. Thereafter, the two columns were disconnected and the anion exchange resin column was washed with 30 mL of 3 N HNO3 to remove remaining radionuclides, including americium (Am) and uranium (U) isotopes. Finally, Pu nuclides were extracted using 80 mL of 0.1 N NH4I/9 N HCl after additional washing with 60 mL of 9 N HCl to remove remaining radionuclides, including thorium (Th) isotopes. In this step, NH4I reduces the oxidation state of Pu to the trivalent state from the tetravalent state, so that Pu nuclides can be extracted from the anion resin (Lee et al., 2013). The disconnected Sr column was washed with 5 mL of 8 N HNO3 to remove 90Y. At this time, the exact time was recorded when the 8 N HNO3 was dropped lastly. The yttrium-90 (90Y), daughter nuclide of 90Sr, is started to be created at this time. Then, 90Sr was eluted using 10 mL of H2O and collected to 20 mL of glass vial. 2.4. Measurement of Pu nuclides In order to measure the activity concentrations of Pu isotopes, purified solution was dried in hot plate after addition of 5 mL of nitric acid and 1 mL of 0.3 M Na2SO4. Then, dried sample was redissolved with 1 mL of sulfuric acid and 4 mL of H2O and electroplated onto a stainless steel disk at pH 2.1e2.3 for 90 min. Before finishing the electroplating, 1 mL of ammonia solution (conc.) was added. After the electroplating process, the stainless steel disk was washed using H2O and acetone and dried. Finally, plutonium iso238 topes ( ,239,240Pu) were measured using an Alpha Analyst™ integrated alpha-spectrometer with passivated implanted planar silicon (PIPS) detectors (Canberra, 450 mm2 area, 18 keV resolution). The representative spectra of plutonium isotopes in standard and sample are shown in Fig. 1. 2.5. Measurement of Sr nuclide The activity concentration of 90Sr was determined by measuring its progeny 90Y using a liquid scintillation counter (LSC) (Tinker
MDAðBq=kgÞ ¼
2:71 þ 4:65
closed room. After 5 days, the solution was measured secondly at the same conditions of the 1st measurement. The activity concentration of 90Sr was calculated using the difference of LSC counts between 1st and 2nd measurements. Recovery of strontium-90 was determined by measuring the concentration of 85Sr tracer using a gamma spectrometer. The representative LSC spectra of standard and sample are shown in Fig. 2. Also, the representative spectra of 85 Sr in standard and sample are shown in Fig. 3. 3. Results and discussion For the analysis of plutonium isotopes and strontium-90, a method developed by the previous study was used. In order to verify the developed method, we have examined recovery, minimum detectable activity (MDA), analytical time, etc. The analysis results of plutonium isotopes and 90Sr in seafood samples are summarized in Table 1. For the analysis of plutonium isotopes (238,239,240Pu), alpha spectrometer was used. Since the energies of 239Pu and 240Pu cannot be resolved in alpha spectrometer, the two plutonium isotopes were determined as a sum of 239Pu and 240Pu. Prior to analyze plutonium in sample, alpha spectrometer was calibrated and efficiencies of each chamber was determined for accurate measurement. The recovery of plutonium isotopes was obtained by measuring the chemical yield of 242Pu added to each sample. Recoveries of Pu isotopes for different matrices and individual samples are shown in Fig. 4A. The average of plutonium recovery in all samples was sufficiently high (87.6 ± 21%). However, some samples showed low recoveries (hairtail: 44.9e53.0%, Alaska pollock: 41.9%, shark: 44.3%, brown seaweed: 40.9%) or enhanced recoveries (squid: 130.7%, saury: 129.8%). It was considered that variable contents of interference nuclides can affect the recoveries including enhanced recoveries in squid and saury. The activity concentrations of the seafood samples were below the minimum detection limits (0.01 Bq/kg) of the current method for plutonium isotopes. In Korea, the regulatory limits for plutonium isotopes in food have not been established. Meanwhile, the Codex Alimentarius Commission (CAC) recommends the levels for plutonium isotopes (238,239,240Pu) as 1 and 10 Bq/kg in infant food and other food, respectively. The minimum detection limits of the method were 1% levels of the guideline values (i.e. 0.01 Bq/kg). In conclusion, all the seafood samples analyzed here were considered to be safe in relation to radionuclide contamination. Minimum detectable activities (MDAs) of plutonium isotopes for different matrices are shown in Table 2. The MDAs were calculated based on background activity, detector efficiency, recovery and sample mass (Currie, 1968), as follows:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Background counts Sample counting time ðsecÞ Background counting time ðsecÞ
efficiencyð%Þ Recoveryð%Þ Sample weightðgÞ Sample counting ðsecÞ Counting 100 100
et al., 1997). Since the activities of 90Sr and 90Y are equal at equilibrium, the activity of 90Sr was inferred from the equilibrium activity of 90Y (L'Annunziata, 1998; EPA, 1976). Because 90Y emits beta particles with an Emax sufficiently high enough to cause Cerenkov radiation, the activity concentration of 90Y was directly counted without using any scintillation cocktails. The radiochemically separated strontium solution was firstly measured for 180 min (1st measurement) after 1 h of stabilization time under the dark and
(1)
In the experiments, it was considered that the MDAs in plutonium analysis were not affected by other parameters such as the amounts of ash and seafood type or species. For the analysis of 90Sr, the activity concentration of 90Sr was measured by a liquid scintillation counter (LSC) after purification using the Sr resin. 90Sr decays to its daughter nuclide, 90Y, emitting beta particles with maximum energy of 546 keV. Subsequently, 90Y
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(Stamoulis et al., 2007). Instead of the time-consuming classical method using fuming nitric acid (IAEA , 1989), we used the simple purification method using the commercially available Sr resin. The resulting solution containing 90Sr was directly measured by LSC. Recovery of strontium was determined by measuring 85Sr tracer using a gamma spectrometer. Recoveries of 90Sr for different matrices and individual samples are shown in Fig. 4B. The average recovery of 90Sr was relatively low (55.4 ± 27%) in all samples. In addition, several samples showed very low recoveries (crab: 27.2e32.4%, shrimp: 22.8%, brown seaweed: 16.0e23.2%, sea tangle: 18.1e22.3%). In the case of seaweeds and crustaceans, including crab, shrimp, brown seaweed and sea tangle, showed high ash content. Thus, it was considered that high ash content affected the chemical recovery of strontium. The activity concentrations of the seafood samples were below the minimum detection limits (1 Bq/kg) of the current method for 90Sr. In South Korea, the regulatory limits for 90Sr in food have not been established. Meanwhile, the Codex Alimentarius recommended the levels for 90 Sr as 100 Bq/kg in food including infant food. The minimum detection limit of the method was 1% level of the guideline value (i.e. 1 Bq/kg). In conclusion, all the seafood samples analyzed here were considered to be safe in relation to radionuclide contamination. The MDA values of strontium-90 for different matrices are shown in Table 2. The MDAs were calculated based on background activity, detector efficiency, recovery and sample mass as mentioned in plutonium analysis (Currie, 1968). In the cases of seaweeds and crustaceans, the MDAs were relatively high depending on the ash content. It was considered that high levels of mineral content (e.g. calcium) in seaweeds and crustaceans affect the MDA values. 4. Conclusion Fig. 3. Representative spectra of shark sample (B).
85
Sr (internal standard) in standard (A) and fortified
decays to stable zirconium-90 (90Zr) emitting beta particles with maximum energy of 2280 keV and half-life of 64 h. It was known that 90Y exists in secular equilibrium with its parent nuclide, 90Sr. Thus, the 90Sr activity of the sample can be estimated by measuring 90 Y activity in a sample. Fast moving electrons emitted by the 90Y decay, produce Cherenkov radiation in a liquid medium such as water. If electrons move faster than light in the medium, Cherenkov radiation takes place. The energy threshold of the fast moving electrons to produce Cherenkov radiation in water is 256 keV (Rao et al., 2000). Cherenkov radiation lies in the optical and UV parts of the electromagnetic spectrum, so it can be detected by LSC
Table 1 The activity concentration and recovery of Types
Fish Shellfish Crustaceans Cephalopods Seaweeds a b c d
Forty seafood samples were collected from the local markets in Korea. Plutonium isotopes (238,239,240Pu) and 90Sr in the samples were sequentially purified using an anion exchange column and the Sr column. Measurements of activity concentrations of strontium90 and plutonium isotopes were carried out using liquid scintillation counter (LSC) and alpha spectrometer, respectively. As a result, plutonium (238,239,240Pu) and strontium (90Sr) were not detected or below the detection limits. The detection limits of the method were 0.01 and 1 Bq/kg for plutonium isotopes and strontium-90, respectively. Thus, it can be considered that the seafood samples analyzed here were safe in relation to radionuclide contamination of plutonium (238,239,240Pu) and strontium (90Sr). However, in order to ensure the food safety from radionuclide contamination, continuous monitoring should be carried out.
90
Sr and plutonium isotopes in seafood samples.
Species (n)
Alaska pollock (4), mackerel (1), saury (2), shark (2), hairtail (2) abalone (2), oyster (3), short necked clam (2), common oriental clam (1) shrimp (3), crab (3) squid (3), small octopus (3) laver (2), brown seaweed (4), sea tangle (2), green laver (1)
ND: Not detected. Minimum detection limit (Pu: 0.01 Bq/kg, Sr: 1 Bq/kg). Recovery of Pu was determined using 242Pu (0.02 Bq). Recovery of Sr was determined using 85Sr (6 Bq).
238
Pu,
239þ240
90
Pu
Sr
Concentration (Bq/kg)
Recovery (%)d
41.9e129.8 62.1e98.9
ND <1b ND <1
22.4e89.6 36.2e88.8
85.7e111.8 91.0e130.7 40.9e99.4
ND <1 ND <1 ND <1
22.8e82.0 29.3e80.1 16.0e67.0
Concentration (Bq/kg)
Recovery (%)
NDa <0.01b ND <0.01 ND <0.01 ND <0.01 ND <0.01
c
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Fig. 4. Recoveries of Pu isotopes (A) and strontium-90 (B) for different matrices and individual samples.
Table 2 Minimum detectable activities of plutonium isotopes and strontium based on sample mass, recovery, and detector efficiency. Matrix (n)
Fish (11) Shellfish (8) Crustaceans (6) Cephalopods (6) Seaweeds (9)
Mass (g)
Minimum detectable activity (MDA) (Bq/kg)
Sample
Ash
50 50 50 50 50
1.07 0.82 2.94 0.69 5.17
238
Pu
± ± ± ± ±
0.42 0.10 1.67 0.19 4.67
0.0030 0.0015 0.0025 0.0015 0.0020
± ± ± ± ±
0.0022 0.0007 0.0007 0.0009 0.0018
239þ240
Pu
0.0020 0.0019 0.0014 0.0017 0.0032
± ± ± ± ±
90
Sr
0.0010 0.0010 0.0010 0.0011 0.0018
0.35 0.33 0.64 0.41 0.83
± ± ± ± ±
0.25 0.13 0.23 0.22 0.36
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Acknowledgements This research was supported by the Grants 14161MFDS078 in 2014 and 15161MFDS666 in 2015 from the Ministry of Food and Drug Safety in Korea. References Brun, S., Bessac, S., Uridat, D., Boursier, B., 2002. Rapid method for the determination of radiostrontium in milk. J. Radioanal. Nucl. Chem. 253 (2), 191e197. Bu, K., Cizdziel, J.V., Dasher, D., 2013. Plutonium concentration and 240Pu/239Pu atom ratio in biota collected from Amchitka Island, Alaska: recent measurements using ICP-SFMS. J. Environ. Radioact. 124, 29e36. CODEX, 1995. General standard for contaminants and toxins in food and feed. CODEX STAN 193e1995. Amended in 2015. www.fao.org/input/download/ standards/17/CXS_193e_2015.pdf. Currie, L.A., 1968. Limits for qualitative detection and quantitative determination. Anal. Chem. 40, 586e593. EPA, 1976. Measurement of strontium-89 and strontium-90 in environmental water : a tentative reference method. United States. Environ. Prot. Agency, Office Res. Dev. 1e10. FAO, Food and Agriculture Organization of the United Nations, 1989. Radioactive fallout in soils, crops and food. www.fao.org/docrep/T0228E/T0228E00.htm. Filss, M., Botsch, W., Handl, J., Michel, R., 1998. A fast method for the determination of strontium-89 and strontium-90 in environmental samples and its application to the analysis of strontium-90 in Ukrainian soils. Radiochim. Acta 83, 81e92. Forsberg, S., Strandmark, M., 2001. Migration and chemical availability of 137Cs and 90 Sr in Swedish long-term experimental pastures. Water Air Soil Pollut. 127, 157e171. Horwitz, E.P., Dietz, M.L., Fischer, D.E., 1991. Separation and preconcentration of Sr
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