Simultaneous Compound-Specific Analysis of δ33S and δ34S in Organic Compounds by GC-MC-ICPMS Using Medium- and Low-Mass-Resolution Modes.

Compound-specific isotope analysis of sulfur (δ34S-CSIA) in organic compounds was established in the last decade employing gas chromatography connected to multiple-collector inductively coupled plasma mass spectrometry (GC-MC-ICPMS). However, δ33S-CSIA has not yet been reported so far. In this study, we present a method for the simultaneous determination of δ33S and δ34S in organic compounds by GC-MC-ICPMS applying medium- and also low-mass-resolution modes. The method was validated using the international isotope reference materials IAEA-S-1, IAEA-S-2, and IAEA-S-3. Overall analytical uncertainty including normalization and reproducibility for δ33S and δ34S was usually better than ±0.2 mUr (σ) for analytes containing at least 100 pmol of S. Further, it is demonstrated that, despite small isobaric interferences, results obtained at low mass resolution are indistinguishable from medium mass resolution offering the benefit of increased sensitivity and versatility of this method. Additionally, the method was applied for the δ33S and δ34S isotope analysis of industrially produced organic compounds to investigate potential mass-independent fractionation (MIF). The relation between δ34S and δ33S in these compounds followed a mass-dependent fractionation trend (MDF; Δ33S ≤ ±0.2 mUr). Degradation of dimethyl disulfide by direct photolysis caused a small but significant MIF (Δ33S = 0.55 ± 0.04 mUr, n = 3), demonstrating sufficient sensitivity of the method for these types of studies.