Gas chromatography-mass spectrometry strategies for the accurate and sensitive speciation of sulfur dioxide in wine

- New strategies for quantifying truly and nominally free SO2, total SO2 and free acetaldehyde in wine with low limits of detection and high accuracy.
- Larges discrepancies between truly free and nominally free SO2 that can be more than 70%.
- Deep implications on wine actual antioxidant protection provided by SO2 in real samples.

Understanding the chemistry of wine oxidation requires the accurate and sensitive quantitative determination of the most important molecular species which SO2 can form. An analytical strategy based in three independent static headspace GC-MS determinations is proposed in order to obtain information about the total, nominally free and truly free levels of SO2. Nominally free forms are directly determined after sample acidulation, total forms require the previous incubation at 100 °C, and truly free forms are determined after preconcentration of the headspace of the undisturbed sample in an alkaline solution. The two first determinations provide results equivalent to those reported by the aeration-oxidation (A-O) method, with lower limits of detection (1 mg L−1) and better repeatabilities (<4.0%). Results from the analysis of different wines revealed that levels of nominally free are systematically in excess than those of truly free SO2, which suggests that free SO2 determined by any method using previous acidulation includes at least two different species of SO2, which may have different antioxidant behavior.