Determination of free, bound and total sulphites in foods by indirect photometry-HPLC

Sulphur dioxide and sodium, potassium, calcium salts of hydrogen-sulphite, disulphite and sulphite ions are extensively used as food additives for their technological efficacy and versatility. They are active as antimicrobial agents, enzyme inhibitors, antioxidants, structure modifiers, in the control of enzymatic and nonenzymatic browning reactions with stabilising and conditioning functions. In this study a modified Monier-Williams method has been utilised as a preparative procedure to obtain both the free and bound sulphite fractions. The two fractions have been analysed by HPLC with indirect photometric detection using a 250 × 4.6 mm LC-SAX column eluted with a solution of potassium hydrogen phthalate. Levels of 5–10 ppm of SO2 in foods, corresponding to 30–60 ng injected are reliably detected by this method. The results confirm that the chromatographic method, unlike the Monier-Williams method, is able to avoid the potential interference of volatile substances derived from matrices or utilised chemicals.