Determination of organic sulfur markers in crude oils by gas chromatography triple quadrupole mass spectrometry

Abstract

Organic sulfur compounds, such as benzothiophene and dibenzothiophene, are present in crude oils, and specific isomers of these classes are known to be geochemical markers of maturity, lithology and biodegradation. Thus, knowledge of the individual concentrations of these target molecules enables us to characterize petroleum basins for better exploration. In this work, a simple and effective gas chromatography–triple quadrupole mass spectrometry method without preliminary fractionation was developed and validated for the determination of 14 organic sulfur compounds assumed to be geochemical markers for crude oils. The research was focused on the optimization of a highly selective GC-based analytical strategy using triple quadrupole MS in multiple reaction monitoring. Sample preparation was carried out by simple dilution in DCM. The method was validated, and the following parameters were expressed in terms of intraday precision: linearity, limit of detection, accuracy and precision. The limit of detection ranged from 0.005 μg mL−1 (benzothiophene) to 0.159 μg mL−1 (benzonaphthothiophene). Intraday assay precision was lower than 20% in all cases. Recoveries ranged from 78.2% (1,4-dimethyldibenzothiophene) to 117% (2,4,7-trimethyldibenzothiophene). After validation, the procedure was successfully applied to the determination of S-markers in 8 crude oil samples, and the method was determined to be precise, accurate, and suitable. Diagnostic ratios based on benzothiophenes to dibenzothiphenes were applied as tools to evaluate maturity, biodegradation and depositional ambient conditions. A cross-plot of methyldibenzothiophene versus dimethyldibenzophene ratio was utilized to classify oils according to their maturity.