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 δ³⁴S 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 O₂⁺ 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 δ³⁴S values approaches 0.1‰ for analytes containing <40 pmol S and is better than 0.5‰ 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‰. 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 δ³⁴S and demonstrate the robustness and precision of the method in complex environmental samples.