Authors: E. Gómez-Otero, M. Costas, Isela Lavilla, Carlos Bendicho
Journal: Analytical and Bionalytical Chemistry , 406: 2133-2140, 03/2014
A new method based on headspace single-drop microextraction in combination with UV–vis micro-spectrophotometry has been developed for the ultrasensitive determination of banned sulphite preservatives (E220–228) in fruits and vegetables. Sample acidification was used for SO2 generation, which is collected onto a 5,5′-dithiobis-(2-nitrobenzoic acid) microdrop for spectrophotometric measurement. A careful study of this reaction was necessary, including conditions for SO2 generation from different sulphating salts, drop pH, 5,5′-dithiobis-(2-nitrobenzoic acid) concentration and potential interference effects. Variables influencing mass transfer (stirring, sample volume and addition of salt) and microextraction time were also studied. A simple sulphite extraction was carried out, and problems caused by oxidation during the extraction process were addressed. A high enrichment factor (380) allows the determination of low levels of free SO2 in fruits and vegetables (limit of detection 0.06 μg g−1, limit of quantification 0.2 μg g−1) with an adequate precision (repeatability, relative standard deviation 5 %). In addition, the sulphiting process was studied through the monitoring of residual SO2 in a vegetal sample, thus showing the importance of a sensitive tool for SO2 detection at low levels.