Exploring the sulfur species in wine by HPLC-ICPMS/MS

The recent development of the inductively coupled plasma tandem mass spectrometry (ICPMS/MS) offers exciting opportunities for non-metal speciation analysis. In this work, we report the first sulfur speciation study in wine by HPLC-ICPMS/MS. The major sulfur compounds were found to be sulfate (50-81 mg S L^-1) followed by sulfite (18-24 mg S L^-1 free sulfite & 41-63 mg S L^-1 after base hydrolysis). We detected small amounts of methionine in wine (0.5-1.0 mg S L^-1); Thiosulfate was below the detection limit of 0.1 mg S L^-1 (injection volume 1.0 μL). A few unknown compounds (collectively 1.0-2.0 mg S L^-1) were observed in the chromatograms. However, the sum of detected species accounted for only 65-77% of total sulfur concentration (105-165 mg S L^-1). To investigate this gap between total sulfur and the sum of detected peaks, we performed several chemical treatments of wine including hydrolysis, oxidation, and enzymatic digestion and then re-examined the distribution of sulfur species. Furthermore, we developed a chromatographic method involving the direct determination of sulfite in white and red wine. The recovery was within the range of 98-106% and repeatability (RSD%) in the range of 6.9-9.4% in regular and "sulfite-free" red and white wine. The pH of the mobile phase drastically influenced the chromatographic behavior of the sulfite ion. At neutral pH a significant tailing and a more than 10-fold loss in peak area was observed. This effect was not previously reported and must be taken into consideration when attempting direct chromatography of sulfite.