A NOVEL METHOD TO REDUCE AND QUANTIFY ELEMENTAL SULFUR

The concentration of elemental sulfur in modern and ancient geologic systems can reveal the biogeochemical conditions at the time of deposition. Such measurements can be made from extracting the sulfur from sediment or water samples and quantifying the sulfur at each depth as sulfide. Elemental sulfur concentations can themselves be analyzed using HPLC and other complex methodologies; however, the preparation and analysis times can be long and these instruments often require regular maintenance. Current reduction methods involve the use of costly and specialized glassware in addition to toxins such as chromium chloride or cyanide to reduce the sulfur to sulfide for concentration measurements as well as d³⁴S isotopic analysis. Our new reduction method uses dithiothreitol (DTT) as a less toxic reducing agent to obtain both elemental sulfur concentrations and isotopic signatures. The sample is dissolved in a liquid medium and upon reaction with DTT, the elemental sulfur is volatalized as sulfide and collected in a basic trap using an inexpensive gas extraction apparatus. Our method development shows that the evolved sulfide concentrations can easily be measured using a field absorbance spectrometer or voltammetry techniques. Using the field spectrometer, sulfide quantification has a wide range from 5 µM to 100 µM, with sulfide detection by the instrument even lower at 1 µM. We scaled the method to 10 ml gas-tight reaction vessels that fit a 36-position block heater, and thus many samples can be extracted simultaneously. The procedure is quantitative at >95% recovery, and highly specific to elemental sulfur. Multiple extractions demonstrate that the method does not extract pyrite (FeS₂), acid-volatile sulfides (FeS), or sulfate. We demonstrate the utility of our new extraction procedure on environmental samples with elemental sulfur concentration profiles of modern lacustrine surface sediments collected from a lake in Minnesota.