-14- analyzed in the gamma spectrometer described above. The specific radioisotopes could be identified in a sample when the radioactivity of the isotope was one per cent or more of the total radioactivity of the sample. The fractions which contained more than one radioisotope were further separated by other ion-exchange methods. Anion fractions The anion fractions of plankton samples P-6, P-13 and P-15 + P-17 were dried, redissolved in 0.1 M oxalic acid, and passed through 0. 13 em2 x 26-cm columns containing Dowex-1* (200-400 mesh) for the separation of the anions of Sb, Sn, Te, and Ru. The separation was done by the modified procedure of Smith and Reynolds as described by Palumbo and Lowman The anion fractions of samples P-25 + P-26, P-24 + P-35, and P-13 + P-15, and the fractions from the secondary separation of P-15 + P-17, were dried and redissolved in concentrated HCl to separate the complex radionuclide mixtures, as described by Wisk. In these experiments the samples were passed through 0,03 cm2 x 15 cm-columns containing Dowex-1 (200-400 mesh) and the radionuclides were sequentially removed by using mixtures of hydrofluoric and hydrochloric acids. Because of the corrosive nature of the HF acid, column plugs made of lucite shavings were used and the fractions were collected in * Available from Dow Chemical Co., Midland, Michigan, ¢ por. co CHIE :