Development of a single fluorescence-based optosensor for rapid simultaneous determination of fungicides benomyl and thiabendazole in waters and commercial formulations

A novel, sensitive, and straightforward spectrofluorimetric flow injection method is proposed in this work for the resolution of a binary mixture of two widely used fungicides (thiabendazole and benomyl). The continuous flow methodology is based on the implementation of on-line solid phase extraction (SPE), preconcentration, and separation of both analytes on a surface of C(18) silica gel beads placed just in the flow cell, with solid surface fluorescence detection. A 45- and 25-fold sensitivity enhancement was obtained for benomyl and thiabendazole, respectively (in relation to the liquid phase measurements in the absence of solid support). The separation of the pesticides was performed because of the different retention-desorption kinetics in their interaction with the solid support, in the zone where the stream impinges the solid material. No previous separation of the analytes before they reach the flow cell is needed, simplifying extraordinarily both the procedure and the manifold. Using a sample volume of 3200 microL, the system was calibrated in the range of 0.4-20 and 20-400 ng x mL(-)(1) with detection limits of 0.06 and 3.6 ng x mL(-)(1) for thiabendazole and benomyl, respectively, and RSD values (n = 10) smaller than 0.8% for both analytes. The RSD values obtained replacing the solid support in each measurement were lower than 3%, and the day-to-day reproducibility RSD value was also lower than 5%. Sampling frequencies of 10 and 18 h(-)(1) were obtained with 600 and 3200 microL of sample volume. Recovery studies carried out on natural water samples spiked with known amounts of both analytes at concentration levels in the range of 1-10 and 25-200 ng x mL(-)(1) provided mean recovery percentages ranging from 98.8 to 102% and from 98 to 103% for thiabendazole and benomyl, respectively. The proposed methodology was also applied to pesticide formulations.