Compound-Specific δ34S Analysis of Volatile Organics by Coupled GC/Multicollector-ICPMS

We have developed a highly sensitive and robust method for the analysis of delta S-34 in individual organic compounds by coupled gas chromatography (GC) and multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The system requires minimal alteration of commercial hardware and is amenable to virtually all sample introduction methods. Isobaric interference from O-2(+) is minimized by employing dry plasma conditions and is cleanly resolved at all masses using medium resolution on the Thermo Neptune MC-ICPMS. Correction for mass bias is accomplished using standard sample bracketing with peaks of SF6 reference gas. The precision of measured delta S-34 values approaches 0.1 parts per thousand for analytes containing >40 pmol S and is better than 0.5 parts per thousand for those containing as little as 6 pmol S. This is within a factor of 2 of theoretical shot-noise limits. External accuracy is better than 0.3 parts per thousand. Integrating only the center of chromatographic peaks, rather than the entire peak, offers significant gain in precision and chromatographic resolution with minimal effect on accuracy but requires further study for verification as a routine method. Coelution of organic compounds that do not contain S can cause degraded analytical precision. Analyses of crude oil samples show wide variability in delta S-34 and demonstrate the robustness and precision of the method in complex environmental samples.