Analysis of beta-lactam antibiotics in incurred raw milk by rapid test methods and liquid chromatography coupled with electrospray ionization tandem mass spectrometry

A recently developed confirmatory LC-MS method has been applied to the quan tification of five major fi-lactam antibiotics in suspect raw bovine milk s amples that gave a positive response with receptor-based (BetaStar) and rap id microbial inhibitory screen tests (Delvotest SP). In total, 18 presumpti ve positive raw milk samples were reanalyzed; 16 samples showed traces of a ntibiotic residues that could be identified and quantified by the LC-MS met hod, ranging from the limits of confirmation up to 38 mug/kg. Of the positi ve samples, only five (similar to 30%) were found to be violative of EU max imum residue limits. The most frequently detected antibiotic residues were cloxacillin and penicillin G, the former often in combination with amoxicil lin or ampicillin. This study compares the results obtained by the three me thods on identical samples and addresses how these relate to certain criter ia such as sensitivity and selectivity. Furthermore, the limitations of the LC-MS method and the potential impact of the presence of frequently more t han one residue in the same milk sample on the response of the rapid test m ethods are discussed.