Sulphite stabilization and high‐performance liquid chromatographic determination: A reference method for free and reversibly bound sulphite in food

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Abstract
A high‐performance liquid chromatographic (HPLC) post‐column reactor combination has been developed for the determination of sulphite (free and reversibly bound) in 15 different foods. The foods were treated with a pH 5:1 buffered aqueous 2% formaldehyde solution to convert the labile sulphite to the relatively stable hydroxymethylsulphonate. Reverse‐phase ion‐pairing HPLC with a post‐column detector consisting of an initial reaction with KOH followed by colorimetric reaction with Ellman's reagent buffered at pH 6.3 were used for separation and quantitation. The photometric detector at 412 nm quantitated the strongly absorbing 3‐carboxy‐4‐nitrothiophenolate anion, the product of the sulphite reaction with Ellman's reagent. The recovery at levels of 5–100 ppm as sulphur dioxide was >90% by reverse isotope dilution assay. Repetitive analysis of a single grape juice extract containing 20 ppm SO2 showed a relative standard deviation of 2.2%.