Ultrasensitive, simple and solvent-free micro-assay for determining sulphite preservatives (E220-228) in foods by HS-SDME and UV-vis micro-spectrophotometry

摘要

A new method based on headspace single-drop microextraction in combination with UV-vis microspectrophotometry has been developed for the ultrasensitive determination of banned sulphite preservatives (E220-228) in fruits and vegetables. Sample acidification was used for SO_2 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 SO_2 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 SO_2 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 SO_2 in a vegetal sample, thus showing the importance of a sensitive tool for SO_2 detection at low levels.