A cold chromium distillation procedure for radiolabeled sulfide applied to sulfate reduction measurements

The separation and detection of extremely small amounts of radiolabeled reduced sulfur species for the determination of very low sulfate reduction rates (SRR) with the 35SO42− radiotracer was improved by optimization of the entire distillation and detection process. By reducing the amount of background radioactivity, the threshold from which turnover of radiotracer can be detected was lowered considerably. Reduction of the background radioactivity was achieved by (1) reducing cross‐contamination between distillations by modifying the distillation setup, and (2) preventing an unidentified 35S‐containing compound that greatly contributes to the background from reaching the final trap by lowering the distillation temperature. These improvements allow the measurement of low SRR, shorter incubation times, and the use of less radiotracer. Experiments with pure sulfur minerals and a variety of sediments verified that the efficiency of the new method is equal to the hot single‐step chromium reduction method, but with a greatly improved reproducibility.