SIMULTANEOUS MULTIRESIDUE ANALYSIS OF BETA-LACTAM ANTIBIOTICS IN BOVINE-MILK BY LIQUID-CHROMATOGRAPHY WITH ULTRAVIOLET DETECTION AND CONFIRMATION BY ELECTROSPRAY MASS-SPECTROMETRY

A multiresidue analytical method was developed for the simultaneous de termination of amoxicillin, cephapirin, procaine penicillin G, ampicil lin, cloxacillin, and ceftiofur in bovine milk. The method involved ul trafiltration of milk diluted with an equal volume of 50% acetonitrile through a 10 000 dalton molecular mass cutoff filter. Separation of t hese beta-lactam antibiotics from other milk components was performed by ion-paired (octane- and dodecanesulfonate) liquid chromatography us ing a phenyl column eluted with acetonitrile-water solution. Ultraviol et absorbance of the column effluent was monitored in the 200-350 nm r ange of a photodiode-array detector. For quantitation, the chromatogra ms were acquired at lambda 210 nm for penicillin G, ampicillin, and cl oxacillin; lambda 230 nm for amoxicillin; and lambda 290 for cephapiri n, procaine, and ceftiofur. The limit of detection for the simultaneou s determination of these antibiotics was estimated to be 100 ppb. Liqu id chromatography/electrospray mass spectrometry could be used to conf irm these antibiotics for quantities down to 100 pg entering the mass spectrometer.