Radiation and Nuclear Safety Authority (STUK), P.O. Box 14, FI Helsinki, Finland.

Transcription

1 AN OXIDIZER/LSC METHOD FOR THE DETERMINATION OF 14 C IN FOODSTUFF SAMPLES Ritva Saxén Ulla-Maija Hanste Radiation and Nuclear Safety Authority (STUK), P.O. Box 14, FI Helsinki, Finland. ritva.saxen@stuk.fi. ABSTRACT. This paper presents a method used at the Radiation and Nuclear Safety Authority (STUK) for the determination of radiocarbon in foodstuff samples in Finland. The method was applied retrospectively to milk powder samples taken annually in and to total diet samples taken in In the method, the dried sample is oxidized to convert carbon to CO 2 with an automatic oxidizer, after which the resulting CO 2 is absorbed by a trapping reagent. Following this step, scintillation solution is added and the sample is measured via LSC. The average activity concentration of 14 C in milk powder in the 1960s was found to be 199 ± 27 Bq/kg on a dry weight (d.w.) basis. There was a decreasing trend after 1964 due to the end of atmospheric nuclear weapon tests in The average activity concentration of 14 C in total diet samples from was 125 ± 19 Bq/kg d.w. No significant difference between the sampling locations and sample years was observed. The radiation dose from dietary 14 C was estimated to be 11μSv/yr, which corresponds to what is considered the natural level. INTRODUCTION Radiocarbon 14 C is a pure beta emitter (E max = 156 kev) with a half-life of about 5730 yr. The sources for environmentally occurring 14 C are cosmic radiation, atmospheric nuclear weapons testing, and emissions from nuclear reactors and industry. From the atmosphere, 14 C is distributed into the stratosphere, troposphere, biosphere, and partially absorbed by the oceans. The specific activity of atmospheric carbon dioxide ( 14 CO 2 ) was 227 ± 1 Bq/kg carbon before the beginning of the nuclear weapons testing period (Fritz and Fontes 1980; Kendall and Coldwell 1998; UNSCEAR 2000). The specific activity in the atmosphere was at its highest in the 1960s, with twice as much found in the Northern Hemisphere than in the Southern Hemisphere. In the past years, the specific activity is strongly reduced due to combustion of fossil fuels, i.e. due to release of large quantities (> tons) of dead carbon dioxide into the atmosphere. 14 C as 14 CO 2 then enters the foodchain via photosynthesis in plants. The radiation dose from 14 C is attributed almost entirely (99%) to the 14 C concentration in foodstuffs, with only a minor amount (1%) entering our bodies via inhalation (UNSCEAR 2000). In terms of the collective radiation dose, 14 C is the most significant radionuclide released by nuclear power plants. Thus, the determination of 14 C in foodstuffs is important, especially in areas where levels may be elevated. Fluctuations of 14 C activity concentrations have been studied worldwide with various analytical methods. Moore et al. (1993) determined the 14 C level in foodstuffs with the highly accurate benzene synthesis method. Singleton et al. (2002) compared the benzene synthesis method with a sample combustion method. The combustion method was shown to have many advantages, one of the most important being that it requires less working time. The method used by STUK to determine 14 C in foodstuff samples is presented here, together with the results of the application of the method to milk powder and total diet samples. The annual milk powder samples tracing back to 1963 were analyzed for 14 C retrospectively, as were samples of total diet in the 2000s. The goal was to verify the suitability of the automatic non-catalytic combustion method, as combined with LSC measurement, for the determination of 14 C in foodstuffs. In addition, the level of 14 C in milk powder was determined. These samples were collected in Finland during the years when nuclear weapons testing was most intensive. The activity concentrations were compared, and the decline of the concentrations were weighed against values reported in the literature by the Arizona Board of Regents on behalf of the University of Arizona LSC 2008, Advances in Liquid Scintillation Spectrometry edited by J Eikenberg, M Jäggi, H Beer, H Baehrle, p

2 280 R Saxén & U-M Hanste The level of 14 C in the Finnish diet of the 2000s was also measured and radiation dose additionally estimated. MATERIAL AND METHODS Method Principles The dried sample is combusted non-catalytically with an automatic oxidizer (Model 307, Perkin- Elmer) into carbon dioxide ( 14 CO 2 ). The carbon dioxide is absorbed by the trapping reagent Carbo- Sorb E, and the scintillation solution Permafluor E + is added to the sample vial. The instrument automatically facilitates the combustion, the absorption of the CO 2, and the addition of the scintillation solution to the sample, when the dried sample is placed in the instrument and the program is started. The samples are measured then with the LSC spectrometer Quantulus 1220, LKB. The efficiency of each measurement is obtained from a quench calibration curve. Samples The method was applied retrospectively to milk powder samples taken annually at the same dairy in Nastola, Finland, beginning in the 1960s and continuing until the early 1980s. It was early in this time period that the largest concentrations of nuclear weapon test emissions have been recorded. In order to determine the level of 14 C found in foodstuffs today, total diet samples taken in from central hospital kitchens in the 3 Finnish cities of Helsinki, Tampere, and Rovaniemi were also analyzed. Locations of the sampling places are given in Figure 1. Figure 1 Location of the sampling stations for foodstuffs. From , milk powder samples were obtained on an annual basis from a dairy in Nastola, Finland. Samples of total diet were taken from hospital kitchens in the Finnish cities of Helsinki, Tampere, and Rovaniemi from Sample Treatment The samples were first dried at a temperature of 105 C. The dried samples were then weighed and added to combustion cups. The cups are made of potato starch (80%) and natural cellulose fiber (20%) and give off no residue in the combustion process and have no effect on the sample measurements. Sample sizes varied between and g dry weight (d.w.). Two parallel determinations were carried out for each sample. The cups were covered before they were set in the combustion instrument (sample oxidizer Model 307, PerkinElmer) to prevent flushing, as was recommended in the manufacturer guidelines. The combustion itself occurs in an oxygen flame and takes 1 2 min per sample. The CO 2 formed in the process is then directly absorbed into the 7 ml of

3 Oxidizer/LSC Method for the Determination of 14 C in Foodstuff Samples 281 Carbo-Sorb E in the 20-mL LSC glass vial, which is placed in the designated position in the combustion instrument beforehand. Carbo-Sorb E is an amine, accepting up to 4.8 mm CO 2 per ml with a trapping efficiency of 97% at a minimum. The trapping capacity of 7 ml of Carbo-Sorb E is by far not exceeded with the small sample quantities used. After absorption of the CO 2, the instrument automatically adds 10 ml of the scintillation solution Permafluor E + to the sample, making the sample ready for measurement. Sample Measurement The samples were measured with a Quantulus 1220 and the counting time for each sample was 300 min for sample and blank measurements. The optimal counting window, corresponding to the highest figure of merit, was from channel 130 to 450, i.e. comparable to Varlam et al. (2006) who used a window setting between channels 100 and 450. Calibration Because 14 C is a low-energy beta emitter, its measurement is strongly prone to chemical quenching. A quench calibration curve was therefore performed to determine the counting efficiency of each sample measurement. The curve was obtained by preparing 6 calibration samples with different amounts of Carbo-Sorb E (2, 4, 6, 8, 9, and 10 ml) and with the same amount of carbon standard (91,600 dpm ± 3%). The combustion times of the calibration samples were 0.2, 0.2, 0.3, 0.5, 0.7, and 1.0 min, respectively. Each calibration sample was measured for 20 min. The counting efficiencies of the calibration samples were calculated and plotted against the quench parameter SQP(E) of the Quantulus liquid scintillation spectrometer. A linear fitting gave the best correlation coefficient for the calibration points (Figure 2). The efficiencies of the samples were then obtained from this curve on the basis of the SQP(E) for each sample. Counting efficiencies of the sample measurements varied between 59% and 66%. Figure 2 Quench calibration curve for the determination of efficiency of 14 C in the LSC measurement RESULTS AND DISCUSSION Calculation of the Results and Uncertainty of the Measurements The activity concentrations of 14 C were calculated according to the following formula: A = (a b) 1000 / (60 c m E) Bq/kg

4 282 R Saxén & U-M Hanste where A = the activity concentration of 14 C (Bq/kg d.w.); a = the count rate of the sample (cpm); b = the count rate of the background (cpm); c = the trapping efficiency (97%); m = the amount of the sample analyzed (g d.w.); and E = the efficiency of the sample measurement. Duplicate analyses of each sample were made (Figure 3). The difference between the parallel samples was small in most cases (i.e. within the range of %), but in 3 cases it was higher (17, 23, and 42%). The higher difference in these cases may be associated with the subsampling, although the samples were mixed well before the subsamples were taken. The total uncertainty (1 σ) of the result, consisting of the average statistical uncertainties of the sample and background measurements and the uncertainty of the standard, was on average 5.1%. The total relative uncertainty was calculated using the formula: 2 U = u u 2 +u 3 where U = the total relative uncertainty of the sample measurement; u 1 = the statistical uncertainty of the sample measurement (2.6% on average); u 2 = the statistical uncertainty of the background measurement (3.3% on average); and u 3 = the uncertainty of the standard used in the calibration (3%). Figure 3 Activity concentrations of 14 C (Bq/kg d.w.) in milk powder samples, with duplicate determinations as a function of time. 14 C in the Milk Powder Samples Activity concentrations of 14 C in milk powder samples from the 1960s varied between 159 and 239 Bq/kg d.w. Total organic carbon (TOC) content in the 1960s samples was determined to be able to calculate the results of 14 C as becquerels per carbon content of the samples. The TOC values varied between 430 and 510 g/kg d.w., and the activity concentrations of 14 C were consequently valued between 370 and 484 Bq/kg carbon. The average activity concentrations of 14 C in Finnish milk powder in the 1960s were 199 ± 27 Bq/kg d.w., corresponding to 420 ± 40 Bq/kg C.

5 Oxidizer/LSC Method for the Determination of 14 C in Foodstuff Samples 283 In order to estimate the temporal change that took place between 1963 and 1980 in the 14 C levels of Finnish milk powder, annual average values were used (Figure 4). The values of 14 C were at their highest in A slight downward trend was observed in the 14 C activity concentrations of the milk powder samples from The decline in the activity concentrations expressed as the observed half-time in the same period was 23.4 yr. The observed half-time estimated for the time period in Finland (18.8 yr) is somewhat higher than the value reported by Otlet et al. (1989, 1997) for the period of in southern England, determined as 15.5 yr. The major reason for the decrease is the nuclear test ban in 1963, which stopped the atmospheric nuclear weapon tests. The continuous decrease is due to absorption of 14 C in the oceans and the increased combustion of 14 C-free fossil fuels. Finnish nuclear power plants started operation in the 1970s, producing 14 C emissions. Figure 4 Annual averages of 14 C (Bq/kg d.w.) in milk powder samples during C in the Samples of Total Diet To determine the level of 14 C in foodstuffs in the 2000s, we analyzed annual samples of mixed diet from the hospital kitchens of 3 cities in Finland, collected in The cities are located in southern, central, and northern Finland (see Figure 1). The average activity concentrations of 14 C in the total diet samples of were found to be 121, 128, and 126 Bq/kg d.w. in the cities of Helsinki, Tampere, and Rovaniemi, respectively. No trends with respect to time and no significant difference in the levels of 14 C between the cities were noticed (Figure 5). This was in line with the UK study of Otlet et al. (1997), which reported no difference between various types of food (milk, grain, leaf and root vegetables) or any significant regional differences in UK 14 C concentrations (in Bq/kg C). In the UK in 1992, the 14 C level of foodstuffs was found to be 259 ± 0.5 Bq/kg C (Otlet et al. 1997). Our results are very close to the average value of 14 C for vegetation samples from 1993 (123 Bq/kg d.w.), as reported by Singleton et al. (2002). Radiation Doses via Dietary 14 C According to UNSCEAR (2000), the annual consumption of food in Finland is on average 580 kg per person. The samples of daily meals analyzed by STUK weighed kg, corresponding to an annual food consumption of kg/person. The average dry matter percentage of the diet samples was 26.3%, which means that the ratio of fresh weight to dry weight in our samples was 3.8. The dose conversion coefficient of 14 C via ingestion for an adult person is Sv/Bq. Under these assumptions, the annual dose from 14 C in the Finnish diet was estimated to be about

6 284 R Saxén & U-M Hanste 10 μsv/yr. This result is consistent with the 12 μsv/yr dose resulting from the natural level, as presented by UNSCEAR (2000). Our results thus indicate no increments due to discharges from nuclear power plants. A recent study of the area surrounding French nuclear power sites (Roussel- Debet et al. 2006) shows that discharges from the power plants have a very slight impact on the 14 C level in the terrestrial environment. The relative increase in the specific activity was estimated to be somewhere on the order of 3 Bq/kg C in the influenced area, with a near-negligible increase, on average <0.1 μsv in the annual dose. CONCLUSIONS Figure 5 Activity concentrations of 14 C (averages with variation, Bq/kg d.w.) in total diet samples from three Finnish cities: Helsinki, Tampere and Rovaniemi. The study showed that the automatic non-catalyst oxidizing method, combined with the LSC measurements, is well suited for determining 14 C in foodstuff samples. The steps of the combustion-lsc method are as follows: Drying of the samples; Weighing of the samples; Combustion of the samples into 14 CO 2, using the sample oxidizer Model 307 by PerkinElmer; Trapping of the 14 CO 2 into a trapping agent (Carbo-Sorb E); Addition of scintillation solution (Permafluor E + ); Measurement with the LSC (Quantulus 1220); Quench calibration; Calculation of the results. The total time for combustion of the samples into 14 CO 2, trapping of the CO 2, and addition of the scintillation solution is only a few minutes per sample. Very little working time is required of the laboratory personnel for analysis. The most time-consuming stages in the method are the sample drying and the measurement of the formed 14 CO 2. The only costs, after the initial instrument investments, arise from the scintillation vials, the trapping and scintillation solutions, and the 14 C standard. Nevertheless, the measurement depends on chemical quenching and therefore requires quench calibration. With this method, results are obtained in Bq/kg of dried sample. Determination of the carbon content of the sample with some other method is necessary if a Bq/kg C result is desired. This study shows that the 14 C concentration in foodstuffs in Finland is of a normal or background level that can be found worldwide. No increase in 14 C levels due to nuclear power plant emissions was observed.

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