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 determination of amoxicillin, cephapirin, procaine penicillin G, ampicillin, cloxacillin, and ceftiofur in bovine milk. The method involved ultrafiltration of milk diluted with an equal volume of 50% acetonitrile through a 10,000 dalton molecular mass cutoff filter. Separation of these beta-lactam antibiotics from other milk components was performed by ion-paired (octane- and dodecanesulfonate) liquid chromatography using a phenyl column eluted with acetonitrile-water solution. Ultraviolet absorbance of the column effluent was monitored in the 200-350 nm range of a photodiode-array detector. For quantitation, the chromatograms were acquired at lambda 210 nm for penicillin G, ampicillin, and cloxacillin; lambda 230 nm for amoxicillin; and lambda 290 for cephapirin, procaine, and ceftiofur. The limit of detection for the simultaneous determination of these antibiotics was estimated to be 100 ppb. Liquid chromatography/electrospray mass spectrometry could be used to confirm these antibiotics for quantities down to 100 pg entering the mass spectrometer.