TRACE MULTIRESIDUE ANALYSIS OF FENBENDAZOLE AND ITS SULFOXIDE, SULFONE, AND P-HYDROXYLATED METABOLITES IN MILK BY LIQUID-CHROMATOGRAPHY

A liquid chromatographic method for trace analysis of fenbendazole and its sulfoxide, sulfone, and p-hydroxylated metabolites in milk has be en developed. Samples are deproteinized with acetonitrile, and the sup ernatants are extracted with dichloromethane. The separated bottom org anic layers are washed with alkaline buffer and evaporated to dryness, and the residues are partitioned between water and ethyl acetate. The separated top organic layers are evaporated to dryness, reconstituted in mobile phase, and submitted to reversed-phase ion-pair chromatogra phy. Overall recoveries ranged from 82% for the sulfoxide and p-hydrox ylated metabolites to 91% for fenbendazole and its sulfone metabolite. The linearity of the assay was quite satisfactory, whereas the limit of detection was in the range 2-5 ng/mL. Precision studies based on wi thin- and between-days variation suggested overall precision values be tter than 4.3% for all analytes. The method was successfully applied t o quantitate incurred fenbendazole residues in milk from a lactating d airy cow treated with a fenbendazole formulation.