Electrospray ionization and tandem ion trap mass spectrometry for the confirmation of seven beta-lactam antibiotics in bovine milk

The feasibility of a technique to confirm the presence of residues from seven beta-lactam antibiotics in bovine milk has been demonstrated. The technique makes use of electrospray ionization and tandem ion trap mass spectrometry. Residues are first extracted from milk by reversed-phase solid phase extraction. Target analytes are separated by on-line reversed-phase liquid chromatography and ionized in the electrospray interface. The product ion mass spectra are acquired following collision-induced dissociation of protonated molecules. Confirmation is based on comparison of full scan spectra between unknowns and bona fide standards. The feasibility of this technique has been demonstrated for the six beta-lactams currently approved for use in lactating dairy cattle (penicillin G, ampicillin, amoxicillin, cloxacillin, cephapirin and ceftiofur) and a drug not approved for animal use, cefazolin. The technique has been applied to control milk fortified at 5 ng/mL of penicillin G and 10 ng/mL of the other six drugs.