DETERMINATION OF BETA-LACTAM RESIDUES IN MILK USING PERFUSIVE-PARTICLE LIQUID-CHROMATOGRAPHY COMBINED WITH ULTRASONIC NEBULIZATION ELECTROSPRAY MASS-SPECTROMETRY

The introduced electrospray (ESP) technique combined with quadrupole m ass spectrometry (MS) was applied for the trace residue detection (10 ppb) of commonly administered beta-lactam antibiotics in bovine milk. Because of the widespread use of penicillin G, ampicillin, amoxicillin , cephapirin, cloxacillin, and ceftiofur in veterinary medicine, these six popular drugs were chosen to develop a fast and reliable microcol umn liquid chromatographic (micro-LC) separation method for residue-co ntaining milk extracts. Furthermore, the analytes were selected to tes t the chromatographic behavior of a novel stationary phase, a perfusiv e-particle column lacked with derivatized porous polystyrene divinylbe nzene. The effects of mobile phase additives on separation and ESP ion ization efficiency were investigated. The ionization of the eluted ana lytes took place either in a conventional ESP interface with a capilla ry shield allowing flow rates up to 40 mu L/min or in the latest model of the ultrasonic nebulization ESP interface. The perfusive-particle column, although not optimized for the separation of small molecules, exhibited sufficient resolution of the antibiotics for its routine usa ge, especially when considering the remarkable time-saving advantage c ompared with ordinary reversed phase micro-LC columns. The application of the novel ultrasonic nebulization interface lowered detection limi ts and improved the stability of the signals over a much wider flow ra te range than possible for ionization with the ESP system equipped wit h the conventional needle.