Survey of stable sulfur isotope ratios (34S/32) of sulfite and sulfate in foods

Abstract

A study was conducted to determine the natural abundance sulfur isotope ratios in foods containing sulphite preservatives. This involved determining (1) the accuracy and repeatability of sulfur isotope ratios measured using an elemental analyser (EA) coupled to an isotope ratio mass spectrometer (IRMS); and (2) the isotope ratios of 21 samples of commercially available S(IV) oxo-anion compounds, nine samples of S(VI) sulfate salts and the isotope ratios of the sulfate obtained by modified Monier-Williams distillation of SO2 from 33 retail foods containing sulfite preservatives. The sulfur isotope ratio data for SO2 recovered from foodstuffs showed a large spread of results, which suggested that the SO2 derived from sulfite preservatives does not have a distinctive sulfur signature ratio. The range of results (3.1–52.1) overlapped with that found for a range of commercially available sulfite and sulfate reagents commonly used to preserve food (sulfites 2.5–13.7, sulfates 10.0–16.9). Whilst the variability in isotope ratios originated from the food samples themselves, evidence from the analysis of SO2 gas suggested that isotope fractionation during dissolution, reaction and recovery was also a confounding factor.