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 mu L). 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. (C) 2019 Elsevier B.V. All rights reserved.