Determination of iodine-131 at low levels in milk: collaborative study.

The official method for the determination of iodine-131 in milk has a lower limit of detection (LLD) of 10 pCi/L (0.37 Bq/L). The Nuclear Regulatory Commission had recommended that a method claiming to have an LLD of <0.3 pCi/L (<1.1E-02 Bq/L) be used. That method, which is capable of measuring iodine-131 below the level of detection of the Official Method, was collaboratively studied. The method uses a palladium iodide precipitate to concentrate the iodine-131 and measures (counts) its 364 keV gamma energy in coincidence with the beta decay or with a low-background beta-counting system. The study was performed by using 3 concentrations of iodine-131 in milk: 2.6, 5.0, and 8.0 pCi/L (9.62E-02, 1.85E-01, and 2.96E-01 Bq/L, respectively). Eleven laboratories agreed to participate in the study. Eight laboratories submitted data for the study. The averages of the results were 2.68, 5.30, and 8.12 pCi/L (9.92E-02, 1.96E-01, and 3.00E-01 Bq/L, respectively), respectively. The intra- and interlaboratory variations were acceptable. The method was adopted First Action by AOAC INTERNATIONAL.