Interlaboratory comparison of environmental relevant nuclides with spinach powder as sample medium

Spinach is cited as a representative medium for leafy vegetables in the Integrated Measurement and Information System for the surveillance of environmental radioactivity (IMIS) in Germany. Fresh spinach, however, is not suitable in interlaboratory comparisons on the determination of spiked radionuclides because of the difficulties in homogeneously distributing the radionuclides and attaining a known specific activity in the samples. In contrast, spinach powder is finely milled, so that homogeneous distribution and known specific activities of the nuclides are more readily achievable. For this interlaboratory comparison spinach powder was mixed with the pure beta emitter Sr-90 and the gamma-emitting nuclides I-131, Cs-134 and Cs-137. After homogenization samples were dispatched to 77 laboratories from Germany and other European countries (59 in Germany, 5 in Switzerland, 4 each in the UK and Austria, and one each in France, Italy, the Netherlands, Ireland and Luxembourg). In addition to the added nuclides participants had to determine the natural radionuclide K-40. The participants were instructed to use a fast method for the determination of dry matter (DM). To check the homogeneity of the nuclide distribution 14 samples of the labeled spinach powder were randomly selected and analyzed in the Coordinating Laboratory for the Surveillance of Radioactivity in the Environment of the Max Rubner-Institute (MRI). According to DIN 13528:2005 the samples showed sufficient homogeneity of the added nuclides. For the evaluation of the interlaboratory comparison the Physikalisch-Technische Bundesanstalt (PTB) determined reference values for the the specific activities (Bq per kg DM) of the gamma emitters. The values with the expanded uncertainties (k = 2) were as follows: I-131: 181 ± 6 Bq/kg; Cs-134: 34.4 ± 1.1 Bq/kg; Cs-137: 11.1 ± 0.4 Bq/kg; K-40: 1240 ± 40 Bq/kg. Since a reference value of the PTB for the specific activity of Sr-90 was not available the general average of the participating laboratories were used as reference [103 ± 19.2 Bq/kg DM; specific activity with the reproducibility standard deviation]. The proportion of outliers was generally small and greatest for the DM and Sr-90 determinations. However, the general average values of the participating laboratories for the specific activities were 5.6%-10.2% less than the reference values determined by the PTB. In the case of DM the general average of all participants was 2% greater than the PTB value. The evaluation of the z-scores revealed three unsatisfactory results each for the Cs-134 and K-40 determinations, respectively, and one for the determination of DM. Although there are slight difficulties in handling this material, finely ground spinach powder proved to be an appropriate medium for proficiency tests in environmental radioactivity surveillance.