Rapid ion–pair liquid chromatographic method for the determination of fenbendazole in cows' milk

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Abstract
A simple and rapid methodology for the analysis of fenbendazole residues in cows' milk, at levels down to 3 ng ml–1, has been developed. Samples are extracted with acetonitrile and centrifuged. The supernatant is de-fatted with isooctane, and mixed with dichloromethane. The separated aqueous layer is discarded, while the bottom organic layer is washed with a phosphate buffer (pH 10) and evaporated to dryness. The residue is dissolved in the mobile phase and analysed by ion-pair reversed-phase liquid chromatography, using octanesulfonate as the ion-pair reagent. Over-all recovery was found to be 99.0%, linearity was excellent and precision data based on within- and between-day variation suggested an over-all variation of 2.0%.