Sulphur-containing polynuclear aromatic hydrocarbons from petroleum : Examination of their possible statistical formation in sediments

Abstract

Two procedures are described for the determination of sulphur-containing polynuclear aromatic hydrocarbons (SPAHs) in petroleum and in its derived industrial products. As a general clean-up procedure, the total aromatic fractions were obtained after partition of the sample between dimethyl sulphoxide (DMSO) and pentane, followed by liquid—liquid extraction of the DMSO layer. In the first method, SPAHs were oxidized to sulphones, isolated selectively and back-reduced to SPAHs. The second approach was the direct determination of SPAHs in the aromatic fractions from the clean-up, by gas chromatography with selective detection using a flame photometric detector and gas chromatography—mass spectrometry with selected ion monitoring. The determination of SPAHs in four crude oils and a coker gas oil is reported. The various isomers are assumed to have been formed in sediments by random alkylation and degradation mechanisms, although specific substances with an increased stability compared with that of other isomers accumulated preferentially. For dibenzothiophene congeners, the number of GC peaks observed in a capillary column chromatogram is close to the maximum number of peaks that could be observed, as predicted by the statistical theory of Giddings. For benzothiophene congeners, the observed number is smaller. SPAHs are concluded to result from severe transformation mechanisms of the organic precursors occurring in sediments.