Residue analysis of 15 penicillins and cephalosporins in bovine muscle, kidney and milk by liquid chromatography–tandem mass spectrometry

A sensitive and specific liquid chromatographic–tandem mass spectrometric (LC–MS–MS) method for most of those penicillins and cephalosporins for which EU maximum residue limits (MRL) were set in Council regulation (EEC) 2377/90 was developed and validated in bovine muscle, kidney and milk. The analytes were extracted with acetonitrile/water and cleaned-up by a single reversedphase solid-phase extraction step. Highest sensitivity for the analytes was obtained when amoxicillin, ampicillin, cephalexin, cephapirin, desacetylcephapirin, cephalonium, cefquinome and cefazolin were measured in the positive electrospray ionisation mode (ESI (+)) and cefoperazone, benzylpenicillin, phenoxymethylpenicillin, oxacillin, cloxacillin, dicloxacillin and nafcillin in the negative electrospray ionisation mode (ESI (-)). Chromatography was performed with a formic acid/methanol gradient. Collision-induced dissociation (CID) with argon was used for fragmentation of the pseudomolecular ions to achieve the required specificity. Possible adverse matrix effects on the electrospray ionisation process caused by co-eluting matrix components were investigated. The method was validated closely to the new EU guidelines and applied to positively screened samples from official food control allowing the identification and quantification of the residual (beta)-lactams.