A study on the preparation of 35S-labeled elementary sulfur

A study has been made on the preparation of 35S-labeled elementary sulfur by the reduction of 35S-labeled sulfate to hydrogen sulfide followed by oxidation to elementary sulfur with potassium ferricyanide. The aim was to obtain labeled sulfur with specific activity exceeding 80 mCi/mgS, using labeled sulfate in quantities as small as 1 mg. The direct-oxidation method, in which hydrogen sulfide is absorbed in an oxidizing solution, and the indirect-oxidation method, in which hydrogen sulfide is absorbed in a caustic solution and then oxidized, were both studied. The influences brought on recovery performance by different values of pH of the oxidizing solutions and by the amount of sulfate were examined. Tin(II)-Strong phosphoric acid was found to reduce quantitatively sulfate ranging from 0.1 to 50 mg in sulfur content. In both the direct- and indirect-oxidation methods, the pH of the oxidizing solutions and the amounts of sulfate affected the recovery of sulfur, and the effect was more marked in the latter than in the former method. The direct oxidation method showed relatively a high labeled sulfur recovery of 75% with only 1 mg sulfate, when the pH of the oxidizing solution was adjusted to the range 9–11. Thin-layer chromatography revealed that it was oxidation of sulfide to oxy-acids in the absorbing solutions that was the cause of the low recovery with small sulfate batches and high pH of the absorbing solution.