Compound-Specific Sulfur Isotope Analysis of Petroleum Gases

We describe a simple, sensitive, and robust method for sulfur isotope ratio (³⁴S/³²S) analysis of ppm-level organic sulfur compounds (OSCs) in the presence of percent-level H₂S. The method uses a gas chromatograph (GC) coupled with a multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). The GC, equipped with a gas inlet and a valve that transfers the H₂S to a thermal conductivity detector (TCD), enables a precise heart cut and prevents the saturation of the MC-ICPMS. The sensitivity and accuracy of the method are better than 0.3‰ for OSCs at a concentration of 25 pmol or 1.4 ppm, and better than 0.5‰ for concentrations ≥0.7 ppm of OSCs. An order of magnitude increase in sensitivity, with no effect on accuracy, can be achieved if the loop volume (0.5 mL) is changed to 5 mL. High concentrations of methane (95% v/v) and/or H₂S (20% v/v) had no effect (within 0.5‰) on the precision and accuracy of the gas sample containing 2 ppm of OSCs after heart cut. The applicability and robustness of this method are demonstrated on a gas sample (10% v/v H₂S) that was produced by pyrolysis of sulfur-rich kerogen. The results show good precision and reveal sulfur isotope variability between individual OSCs that may represent key processes during formation and degradation of OSCs.