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

Abstract

Measurement of fluorescence increase was used for the comparative quantification of the effect that several cyclodextrins (α-, β-, heptakis-2,6-β-o-dimethyl- and γ-) produce on the fluorescent response of aflatoxins B₁ and G₁. 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 C₁₈-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 G₂>GG₁>B₂>B₁ 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 using ring was found to occur with all the cyclodextrins studied, except γ-cyclodextrin. The observed increase was larger for heptakis-2,6-β-o-dimethyl- than for β-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 G₁ and 45.2-fold in the case of aflatoxin B₁. The detection limit in the mobile phase used was determined (for aflatoxin B₁) for β-cyclodextrin and 2,6-β-o-dimethylcyclodextrin (signal-to-noise ratio 1:3) to be 4 and 9 mg l⁻¹, respectively.