ANALYSIS OF δ34S IN INDIVIDUAL ORGANIC COMPOUNDS BY COUPLED GC/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-ICP-MS). The system requires minimal alteration of hardware, and is amenable to virtually all GC methods and columns. 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 ICP-MS. 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 two 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 degradation of 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 and accuracy. Analyses of crude oil samples indicate a wide range of δ³⁴S values for individual components, and demonstrate the robustness and precision of the method in complex environmental samples. Analyses of dimethylsulfide extracted from ~2L seawater demonstrate the extreme sensitivity of the method. We conclude that the method is suitable for isotopic analysis of a wide range of geological sample types, including atmospheric trace gases, dissolved organics in water, crude oil and natural gas, and extractable components in organic-rich sediments and rocks.