Fast stopped-flow enzymatic sensing of fenitrothion in grapes and orange juice

This article describes a fast analytical method for the determination of the organophosphorus pesticide fenitrothion in spiked orange juice and grape samples. Fast data acquisition (2 s) facilitates excellent selectivity because no interference from kinetically slower side reactions in the matrix occurs on such a short time scale. The mechanical mixing of the reagents and short residence time in the reactor after onset of reaction allow a precise study of the mechanistic details of the reaction. The stopped-flow device used to make measurements was coupled to a fluorescence detector to obtain excellent sensitivity. The precision, in terms of repeatability (expressed as relative standard deviation, n = 7), was 0.27% at a 5 μg mL−1 level. The detection limit was 1.34 μg mL−1. Calibration data over the range 5.0-35 μg mL−1 afforded the correlation coefficient R = 0.998. The proposed method was applied to directly determine fenitrothion in orange juice samples as well as in fresh grapes crushed and filtered after solid-phase extraction. Mean recovery values were 99.3 ± 0.28% and 97.2 ± 2.37%, respectively. The enzymatic mechanism was discussed on the basis of the obtained stopped-flow data.