Determination of 210Po in low-level wild bilberries reference material for quality control assurance in environmental analysis using extraction chromatography and α-particle spectroscopy

Radiochim. Acta 2019; aop Santiago Hurtado-Bermúdez* and José Luis Mas Determination of 210Po in low-level wild bilberries reference material for quality control assurance in environmental analysis using extraction chromatography and α-particle spectroscopy https://doi.org/10.1515/ract-2019-3141 Received March 19, 2019; accepted May 10, 2019 Abstract: Certified reference materials (CRM) are being widely used for quality control assurance in environmental analysis. For certain CRM, the analytes and/or the range of concentrations are not be available or certified at all. The Joint Research Centre – Institute for Reference Materials and Measurements (JRC-IRMM) of the European Commission has issued a CRM of Wild Berries (IRMM-426) in order to validate radionuclide measurement methods for activity concentrations of the natural radionuclide 40K and the anthropogenic nuclides 90Sr and 137Cs, but not for 210Po. The aim of the work was to determine low-level activity concentration of 210Po in these wild berries. The activity concentration of 210Po was assessed by α-particle spectroscopy after dissolution of the sample by wet digestion and chemical isolation of Po by extraction chromatography. According to the time elapsed since sample collection, the results here shown can be useful not only for ultra low-level analysis of 210Po but also for 210Pb in the reference material. Keywords: 210Po, wild berries, extraction chromatography, radioactivity in food, α-particle spectroscopy, quality control assurance. 1 Introduction International regulations provide maximum guidance levels for radioactivity in food after a radiological emergency [1–4]. Following the nuclear accident at the Fukushima Daiichi Nuclear Power Station (NPS) in 2011, there is a need for certified reference materials (CRM) in wild foods subjected to radioactive fallout in order to validate reliable radiochemical methods. For this reason, IRMM has developed a CRM for the activity concentration of 40K and the anthropogenic radionuclides 90Sr and 137Cs in wild bilberries. However, the activity concentrations for the natural decay chains are not certified in this matrix. This lack of information in CRMs is usual because they are chosen according to a certain set of targeted analytes. As a consequence, testing the robustness of the analytical techniques for a relatively wide set of target isotopes means in turns using different certified materials and even, sometimes, different sample matrices. As a consequence, the scientific communities are continuously providing additional information on these materials, in such a way that they are not certified (information or recommended values) but can be extremely helpful (see, for example, [5–7]). Amongst all of the natural radionuclides we focus on 210Po (T1/2: 138.4 days), an α emitter which is mainly produced from the natural decay of 210Pb (T1/2: 22.3years) through the decay of 210Bi (T1/2: 5.01days) [8]. The quantification of 210Po is critical because its application to environmental sciences and dosimetry [9–15]. The purpose of this work is to determine the activity concentration of 210Po in IRMM-426 low-level CRM. A previous radiochemical method was adapted and simplified [16]. The method was based on the extraction chromatography of polonium with Sr-resin, polonium self-deposition, and finally its determination by α-particle spectrometry. 2 Materials and methods 2.1 Instrumentation An α-spectrometry instrument (Alpha Analyst, Canberra) was used for the measurement of polonium planchets. Each measurement chamber contained a Passivated Implanted Planar Silicon (PIPS) detector with an energy resolution of 18 keV (as Full-Width at Half-Maximum FWHM), and an active surface area of 450mm2. The polonium sources were placed at 1.5 mm from the PIPS detector. Alpha Analyst software were used for α spectrum analysis [17]. 2.2 Reagents and analytical solutions Deionised water (DI water, Millipore) with 18.0 MΩ cm−1 resistivity, and suprapure grade chemicals (HCl, H2O2, *Corresponding author: Santiago Hurtado-Bermúdez, Centro de Investigación Tecnología e Innovación, Universidad de Sevilla (CITIUS), Av. Reina Mercedes 4B, 41012 Sevilla, Spain, Phone: +34-954559750, E-mail: [email protected] José Luis Mas: Dpto. Física Aplicada I, Escuela Universitaria Politécnica, Universidad de Sevilla, Sevilla, Spain 2 S. Hurtado-Bermúdez and J. L. Mas, Determination of 210Po in low-level wild bilberries reference material HNO3, ascorbic acid, and oxalic acid from Merck) were utilized where needed. Chromatographic extraction of 210Po was carried out using 2mL Sr-resin cartridges (TRISKEM, France) placed on a vacuum box with a pressure valve. 210Po activity concentration and radiochemical yield were calculated using a 209Po standard solution (Eckert & Ziegler) of 0.2404 ± 0.0024 Bq g−1. Finally, the counting efficiency calibration of the α-particle spectrometer was performed with a 241Am standard electroplated source from PTB (Germany) containing 93.3 ± 1.9 Bq. The obtained values for the measurement chambers range from 0.245 to 0.263 and the uncertainty was less than 3 % within a confidence probability of 95 %. 2.3 Samples As described in previous papers [18] bilberry samples were collected near Chernobyl reactor site in the summer of 2005. The material was oven-dried, cryo-milled, sieved, homogenised before being bottled [19]. Homogeneity and an isochronous long-term stability studies were carried out. Given the time elapsed since sample collection and isolation, and bearing in mind the comparatively short half-life of 210Po (134.4 d) with respect to that of its father 210Pb, it can be established that both nuclides are in radioactive equilibrium, hence the 210Po radioactive concentration accounts for that of 210Pb. In this work, each sample was dried to constant weight at 60 °C and stored in plastic bags prior to performing the analysis. 3 Experimental 3.1 Radiochemical method The radiochemical method was adapted and simplified from a previous study [16] and it is shown in Figure 1. Firstly, a dry weight of 1–5 g of wild bilberry sample (five aliquots for the sake of reproducibility of results) was put into a Teflon beaker and 209Po internal standard (0.1 Bq) was added in order to evaluate 210Po chemical recovery. At a first stage the sample was digested at low temperature (<25 °C) with concentrated HNO3 and H2O2 during 24h in order to avoid foam formation. Then the sample aliquot was wet digested following EPA 3050B method [20] with several additions of HNO3 and H2O2. Next, the solution was filtered through a 0.45 μm filter (Millipore), and evaporated to dryness keeping the temperature below 100 °C to avoid losses due to polonium volatilisation [21]. Afterwards, the residue was dissolved with 10mL of 8M HNO3. Previously the resin cartridge was preconditioned with 5mL 8M HNO3, and then the sample solution was passed through the resin. The resin cartridge was successively loaded with 5mL of 8M HNO3, 5mL of 3M HNO3 IRMM426 sample Dissolved sample 10 mL 8 M HNO3 Rinse 5 mL 8 M HNO3 +5 mL 3 M HNO3-0.05 oxalic acid + 5 mL of 3 M HNO3 210Po elutes Evaporate to dryness Dissolve in 50 mL 1 M HCl Self-deposition on silver Load sample Add 0.10 Bq 209Po Precondition in 5 mL 8 M HNO3 210Po measurement Figure 1: Radiochemical procedure for 210Po isolation using prepacked Sr resin columns. S. Hurtado-Bermúdez and J. L. Mas, Determination of 210Po in low-level wild bilberries reference material 3 – 0.05 oxalic acid, and 5 mL of 3 M HNO3, in order to extract polonium. The solution containing polonium was evaporated to dryness and then re-dissolved in 1mL of concentrated HCl acid. This evaporation step was repeated three times and finally dissolved in 50mL 1M HCl. In order to avoid iron plating, few milligrams of ascorbic acid were added to the solution. Polonium self-deposition was carried out on silver discs during 6h in 1M HCl at about 80 °C [22, 23], and then measured using the α-particle spectrometer. 3.2 Method validation IUPAC [24] and ISO [25] recommendations for assessment of performance of laboratories include z-score test, u-test, and trueness and precision tests. The proposed radiochemical methods were performed in two IAEA CRM. Five aliquots of 1 g were analysed for each IAEA CRM (n = 5) and results are shown in Table 1. The z-score and u values obtained are within the acceptable range for trueness, so an “Acceptable” status referring to precision and trueness was achieved. The analysis of the IAEA-437sample gave a 210Po activity of 4.3 Bq kg−1, very close to the reference material reported value and within the 95 % confidence interval. The median value obtained for IAEA-414sample was within the reported 95 % confidence interval. The observed deviations are in the order of the precision values associated with environmental materials, and overall, we conclude that the proposed method is adequate to analyse 210Po in environmental samples. 4 Results and discussion The ratio of the net count rates of the 210Po (n) and 209Po αpeaks (nstd), was used to calculate the activity concentration of 210Po (CPo-210) in the wild bilberry samples on the date of radiochemical separation. The activity concentration of the 209Po standard solution (Cstd) and the mass of the standard solution used (mstd), the sample mass (msample), the decay of 209Po (with standard half-life T) between its calibration date and counting (t), and standard gross count rate (Nstd), background count rate (B), and sample count rate (N), were taken into account. std Po-21 0s td t stdsample m n CC f nm =× ×× (1) nNB=− (2) stdstd nNB=− (3) Table 1: Validation results including 210Po(LAB) obtained in five aliquots (n = 5) and 210Po(REF) certified in IAEA reference materials. Sample 210Po(LAB) (Bqkg−1) Uncertainty (Bqkg−1) 210Po(REF) (Bqkg−1) Uncertainty (Bqkg−1) z-Score u Trueness Precision P (%) IAEA-437 (mussel) 4.3 0.4 4.2 0.7 0.14 0.12 Acceptable 19 IAEA-414 (fish) 2.2 0.4 2.1 0.3 0.33 0.20 Acceptable 23 Uncertainties (σ) are expressed at k = 1. Table 2: Activity concentration for 210Po measured in five IRMM426samples and its associated mean. Sample Weight (g) 210Po (Bqkg−1) Uncertainty (Bqkg−1) Chemical recovery (%) MDA (Bqkg−1) IRMM-426-1 2.248 0.631 0.053 51 0.05 IRMM-426-2 3.419 0.643 0.080 84 0.04 IRMM-426-3 1.505 0.641 0.060 77 0.09 IRMM-426-4 2.622 0.641 0.077 54 0.08 IRMM-426-5 5.099 0.582 0.058 53 0.03 Mean 0.628 0.026 The reference date is 2018-07-20 0:00 UTC. The uncertainties are expanded uncertainties (k = 1). The chemical recovery and the Minimum Detectable Activity (MDA) was also shown for each sample. 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 012345 210Po activity concentration (Bq kg–1) Sample Figure 2: Plot of the activity concentration results of 210Po in the IRMM-426wild bilberries. All uncertainties are combined uncertainties at k = 1. The solid horizontal line indicates the weighted mean and the dashed lines the expanded uncertainty from the standard deviation (k = 2). 4 S. Hurtado-Bermúdez and J. L. Mas, Determination of 210Po in low-level wild bilberries reference material The decay correction factor (ft) was calculated using the elapsed time between the separation time and the start of measurement (t) as follows: ln(2) t T t fe × =(4) According to the Guide for the expression of Uncertainty in Measurement GUM [26] the combined uncertainty corresponding to the 210Po activity concentration on the separation date (u(CPo-210)) was calculated according to the following equations: Po-210 2 22 22 2 sample std stdstd t Po-210 22 22 22 std sample stdstd t u(C) um un um uC uf un Cnn mmC () () ()() () ( f ) =+ ++ ++ (5) 22 u(n) NB=+ (6) 22 stdstd u( n) NB=+ (7) tt u(f) ftu(T)××= (8) The activity concentration results and the uncertainties obtained for 210Po measured in the IRMM426 CRM are summarized in Table 2. The analytical procedure was applied to five aliquots of the CRM. High chemical yields (>50 %) were obtained in the analysis carried out by α-particle spectrometry. The reference date was 2018-07-20 0:00 UTC. The uncertainties in Table 2 are expanded uncertainties with k = 1. The mean activity concentration of 210Po in IRMM-426 CRM was 0.628 ± 0.026 Bq kg−1. In Figure 2, the results for 210Po are plotted and in Table 3 the uncertainty budget for a typical measurement is given. The final combined uncertainty was calculated with a coverage factor of k = 1. The uncertainty analysis for the obtained results was performed using GUM Workbench software [27] recommended from BIPM [28]. The main contributor to the uncertainty of the activity concentration of the measured sample is the gross count rate (N) of the sample (90.7 %). Minor contributions to the overall uncertainty are the background count rate (B) with 8.0 %, and gross count rate (Nstd) of the 209Po standard (0.2 %). This may be due to the fact that IRMM-426has a low 210Po activity concentration, and the measurement time had to be extended up to 1 · 106 s. 5 Conclusions The IRMM-426wild bilberry CRM can be used for quality assurance and quality control of low-level 210Po analysis of radionuclides in biota for taking decisions on radiological protection, as well as for development and validation of radioanalytical methods, and for training of laboratory analysts. References 1. Health Canada: Canadian Guidelines for the Restriction of Radioactively Contaminated Food and Water Following a Nuclear Emergency (2000). 2. U.S. Food and Drug Administration: Supporting Document for Guidance Levels for Radionuclides in Domestic and Imported Foods (2004). 3. FSCJ: Food Safety Risk Assessment Radioactive Nuclides in Foods (2011). 4. United Nations FAO/WHO: Codex General Standard for Contaminants and Toxins in Food and Feed (1995). 5. Altzitzoglou, T., Bohnstedt, A.: Characterisation of the IAEA-375 Soil Reference Material for radioactivity. Appl. Radiat. Isot. 109, 118 (2016). Table 3: Summary of the uncertainty budget for 210Po using GUM Workbench software. Parameter Value Standard uncertainty Distribution Uncertainty contribution Index Cstd 240.40 Bq kg−1 2.4 Bq kg−1 Normal 6.4 · 10−3 Bq kg−1 1.2 % t 3.9232 a – – – – T 124.00 a 1.73 a Rectangular 200 · 10−6 Bq kg−1 0.0 % mstd 2.099200 · 10−3 kg 100 · 10−9 kg Normal 31 · 10−6 Bq kg−1 0.0 % N 192.2 · 10−6 cps 14.4 · 10−6 cps Normal 0.055 Bq kg−1 90.6 % B 25.30 · 10−6 cps 4.28 · 10−6 cps Normal −0.016 Bq kg−1 8.0 % Nstd 0.057318 cps 248 · 10−6 cps Normal −2.8 · 10−3 Bq kg−1 0.2 % msample 2.248000 · 10−3 kg 1.00 · 10−6 kg Normal −290 · 10−6 Bq kg−1 0.0 % The parameters used in uncertainty budget calculation of the activity concentration are standard activity concentration (Cstd), standard mass (mstd), 209Po half-life (T), standard gross count rate (Nstd), background count rate (B), sample count rate (N), and sample mass (msample). S. Hurtado-Bermúdez and J. L. Mas, Determination of 210Po in low-level wild bilberries reference material 5 6. 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