Postcolumn excitation of aflatoxins using cyclodextrins in liquid chromatography for food analysis

Measurement of fluorescence increase was used for the comparative quantification of the effect that several cyclodextrins (alpha-, beta-, heptakis-2,6-beta-o-dimethyl- and gamma-) produce on the fluorescent response of aflatoxins B-1 and G(1). This constitutes a new chromatographic method with stability of the mobile phase, and shows general improvements in the chromatographic conditions with respect to other methods (especially those using an iodine reservoir as a postcolumn reactor). A C-18-type column was used, with methanol-water (60:40, v/v) as the mobile phase. The excitation phase of the natural fluorescence of aflatoxins, a 10(-2) M solution of each cyclodextrin, was introduced postcolumn. The determination of the elution order aflatoxin G(2) > G(1) > B-2 > B-1 was performed for each phase in less than 15 min. As expected using an aqueous-alcoholic medium, an increase in the fluorescence response of aflatoxins with an unsaturated furanic ring was found to occur with all the cyclodextrins studied, except gamma-cyclodextrin. The observed increase was larger for heptakis-2,6-beta-o-dimethyl- than for beta-cyclodextrin (to our knowledge, the only cyclodextrin previously described in the literature to serve for the determination of aflatoxins). The difference is of the order of 70.1-fold in the case of aflatoxin G(1) and 45.2-fold in the case of aflatoxin B-1. The detection limit in the mobile phase used was determined (for aflatoxin B-1) for beta-cyclodextrin and 2,6-beta-o-dimethylcyclodextrin (signal-to-noise ratio 1:3) to be 4 and 9 mg l(-1), respectively.