Rapid ion-pair liquid chromatographic method for the determination of fenbendazole marker residue in fermented dairy products

•Unique method for determining fenbendazole marker residue in dairy products.•Isocratic separation of fenbendazoles by ion-pair liquid chromatography.•Rapid, sensitive, specific, precise and accurate LC method.•Fenbendazole residues in cheese and yoghurt from spiked and incurred ovine milk. A simple, rapid and sensitive liquid chromatographic method that allows for the quantitative determination of fenbendazole residues in fermented dairy products is described. Samples were extracted with a mixture of acetonitrile-phosphoric acid and the extracts were defatted with hexane to be further partitioned into ethyl acetate. The organic layer was evaporated to dryness and the residue was reconstituted in mobile phase. Separation of fenbendazole and its sulphoxide, sulphone, and p-hydroxylated metabolites was carried out isocratically with a mobile phase containing both positively and negatively charged pairing ions. Overall recoveries ranged from 79.8 to 88.8%, while precision data, based on within and between days variations, suggested an overall relative standard deviation of 6.3–11.0%. The detection and quantification limits were lower than 9 and 21μg/kg, respectively. The method has been successfully applied to quantitate fenbendazole residues in Feta cheese and yoghurt made from spiked and incurred ovine milk.