Continuous-Flow Stable Sulfur Isotope Analysis of Organic and Inorganic Compounds by EA-MC-ICPMS

Elemental analysis (EA) coupled to isotope ratio mass spectrometry (IRMS) is a well-established method to derive stable isotope ratios of sulfur (³⁴S/³²S). Conversion of sulfur to SO₂ by EA and measurement of SO₂ isotopologues by IRMS represents the simplest and most versatile method to accomplish isotope measurement of sulfur even in bulk samples. Yet, interferences by oxygen isotopes in SO₂ often impair the precision and trueness of measured results. In the current study, we coupled EA to multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) to establish a method that avoids such interferences due to direct measurement of S⁺ ions. In addition, measurement of the ³³S/³²S isotope ratios is possible, thus representing the first bulk method that is suitable to study mass-independent isotope fractionation (MIF). Analytical precision (σ) of available Ag₂S and BaSO₄ reference materials (RMs) was, on average, 0.2 mUr for δ³³S and δ³⁴S, never exceeding 0.3 mUr within this study (1 mUr = 1‰ = 0.001). Measured δ³⁴S values of reference materials agreed within ±0.2 mUr of officially reported values. Measurement of wood samples yielded good precision (0.2 mUr) for sulfur amounts as low as 3.5 μg, but precision deteriorated for samples at lower sulfur contents due to poor peak shape. Finally, we explored cross-calibration of organic liquids separated via gas chromatography (GC) against solid RMs combusted via EA that avoids challenging offline conversion of RMs. Results indicate good precision (≤0.08 mUr) and acceptable trueness (≤0.34 mUr) for determination of δ³⁴S, demonstrating the future potential of such an approach.