Sensitive and selective flow injection analysis of hydrogen sulfite/sulfurdioxide by fluorescence detection with and without membrane separation by gas diffusion

Highly sensitive and selective FIA flow injection analysis procedures for t he determination of sulfite/hydrogen sultite/sulfur dioxide were developed on the basis of an in situ-generated o-phthalaldehyde (OPA)/ammonium reagen t and fluorescence detection. The highest sensitivity was achieved at an ex citation wavelength of 330 nm, an emission wavelength of 390 nm, and at pH 6.5. Sulfite concentrations between 2.5 nM and 5 muM can be determined with relative standard deviations between 10.5 and 1.0% (n = 5, confidence leve l alpha = 0.05) by utilization of a reagent that contains 0.2 mM OPA and 0. 4 M NH4Cl in 50 mM potassium phosphate buffer. A concentration of 0.1 mM su lfite can be selectively detected in the presence of thiosulfate, thioglyco late, tetrathionate, cysteine, and ascorbate, The fluorometric sulfite dete ction was combined with a membrane gas diffusion step to improve the select ivity with respect to nonvolatile fluorescing substances. The total sulfite content can be quantitatively separated as sulfur dioxide into an acceptor solution before its now detection. Between 40 nM and 0.1 mM sulfite can be determined. After 1000-fold dilution, the total sulfite content can be det ermined in white and red wines.