DETERMINATION OF CLOXACILLIN IN MILK AND BLOOD OF DAIRY-COWS BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

A rapid and sensitive high-performance liquid chromatographic method i s described for the determination of cloxacillin residues in milk and serum. Only a clean-up step after deproteinization is necessary before the analysis. The chromatographic system involves the use of a C-18 c olumn and ultraviolet absorbance detection at 225 nm. The mobile phase was acetonitrile-0.02 M KH2PO4 (21:79) at pH 5. Recoveries for cloxac illin were 83.5 and 75.7% in serum and milk, respectively. Detection l imits (10 ng/ml in milk and 50 ng/ml in serum) were below the stipulat ed European Union maximum residue limits for cloxacillin. Thus, the de scribed method showed the same accuracy, precision and sensitivity as the microbiological assays but without interferences caused by other d rugs commonly used in therapy. Analysis of different blood and milk sa mples obtained at different times from dairy cows treated with an intr amammary dose of cloxacillin benzatine showed undetectable cloxacillin levels both in milk and blood samples. (C) 1997 Elsevier Science B.V.