Determination of benzylpenicillin, oxacillin, cloxacillin, and dicloxacillin in cows' milk by ion-pair high-performance liquid chromatography after precolumn derivatization

Abstract.

A high-performance liquid-chromatographic method has been developed for the determination of five penicillin compounds (benzylpenicillin, phenoxymethylpenicillin, oxacillin, cloxacillin, and dicloxacillin) at trace levels in commercially available milk samples. This method comprises extraction of the lipids with ethyl acetate, clean-up and concentration on a C-18 solid-phase extraction column, and derivatization with 1,2,4-triazole and mercury(II) chloride solution, pH 8, at 65 °C for 10 min. The derivatized compounds are eluted from a C-2 column with a mobile phase containing acetonitrile and phosphate buffer loaded with sodium thiosulfate and tetrabutylammonium hydrogen sulfate as ion-pairing reagent. The limit of determination was found to be 4 µg L^–1 milk for benzylpenicillin and 10 µg L^–1 for the others. This meets EU criteria according to decision No. 93/256/EEC.