Quantitative determination of I-131, Cs-137, and Ba-140 in milk: collaborative studies

The official method for Cs-137 in milk by gamma-ray spectroscopy was extended to include I-131 and Ba-140. A collaborative study was performed on this method applied to I-131 concentration in cow's milk; the original collaborative study of the method including all 3 nuclides was reviewed. In the I-131 study, 1 aliquot of a milk sample containing 82 pCi/L was sent to each of 60 laboratories for triplicate analyses. From 40 responses, the mean of the reported values was 81.6 pCi/L, indicating a method bias below the 5% statistical detectability limit. Within- and between-laboratory coefficients of variation (CVs) were 7 and 8%, respectively. In the 3-nuclide study, 2 samples were sent to 25 laboratories for triplicate analyses; one sample contained 633, 305, and 515 pCi/L, respectively, of I-131, Cs-137, and Ba-140 and the other contained 98, 52, and 72 pCi/L. For the high-activity sample, within-laboratory CVs were 4-5% for the 3 nuclides and between-laboratory CVs were 4-7%. For the low-activity sample, the corresponding results were 6-9% and 8-16%. The method bias was statistically significant at 95% confidence only for Cs-137 in the high-activity sample; reported results were 3% below the known concentration. The extended method was adopted official first action.