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

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.