Multiresidue liquid chromatographic method for determining residues of mono- and dibasic penicillins in bovine muscle tissues

A liquid chromatographic method with UV detection at 325 nm was developed f or simultaneous determination of amoxicillin, ampicillin, penicillin G, and cloxacillin residues in bovine muscle tissue as their mercaptide derivativ es, The penicillins are extracted from bovine tissues with 0.1M phosphate b uffer (pH 8.5), cleaned up on a t-C-18 Sep-Pak cartridge, and eluted with 2 mt acetonitrile, After the acetonitrile in the eluate is evaporated to dry ness, the residue is dissolved in 200 mu L (40 + 60, v/v) acetonitrile-phos phate buffer (pH 6.5) and derivatized with acetic anhydride and mercuric ch loride in the presence of 1,2,4-triazole at 65 degrees C for 30 min. Gradie nt analysis on a Spherisorb 5 mu m ODS(2) (octadecyl silane) analytical col umn using a binary mobile phase consisting of acetonitrile and 0.10M phosph ate buffer (pH 6.5) in the presence of 0.0157M sodium thiosulfate at 1 mL/m in permits determination of each intact penicillin in bovine muscle tissue at greater than or equal to 10 ppb with recoveries greater than or equal to 72%. This laboratory method provides detection sensitivities equivalent to those of rapid tests used for screening p-lactam drug residues in bovine t issue samples for regulatory enforcement.