Selective cleavage of acyclic sulphide moieties of sulphur-rich geomacromolecules by superheated methyl iodide

Abstract

A novel selective chemical degradation method was developed to discriminate between different sulphur-bound moieties in sediments and oils. The method was tested with several geochemically relevant model compounds and with a polar and an asphaltene fraction of the Rozel Point oil and involves the use of superheated methyl iodide which cleaves mono-, di- and polysulphide bonds in acyclic sulphides and does not affect cyclic monosulphides. Released iodides are derivated to their corresponding methylthioethers using sodium thiomethoxide. In this way the position of the original C-S linkage in the carbon skeleton was established. Moieties linked to sulphur via a tertiary carbon atom are exclusively released as unsaturated compounds. Significant amounts of methylthioethers and alkenes, sometimes with additional thiolane or thiophene moieties, were released from the polar and asphaltene fractions of the oil by this method. Gas chromatography and mass spectrometry showed that the products were dominated by methylthio-steroids. These results are similar to those obtained by selective cleavage of polysulphide linked moieties, indicating that mono- and polysulphide linkages are at identical positions.