Solid-phase extraction followed by liquid chromatography-mass spectrometry for trace determination of beta-lactam antibiotics in bovine milk.

A confirmatory assay able to unambiguously identify and quantify 10 approved-for-use beta-lactam antibiotics in milk below stipulated U.S. and EU tolerance levels is presented. beta-Lactams are extracted from 10 mL of intact milk by a Carbograph 4 cartridge. After solvent removal, residue reconstitution, and filtration, a completely transparent and uncolored extract is injected into a liquid chromatography -mass spectrometry (LC-MS) instrument equipped with an electrospray (ES) ion source and a single quadrupole. During the chromatographic run, the ES/MS system is operated first in the positive-ion mode (PI) and then in the negative-ion (NI) mode. This is done to circumvent matrix interferences resulting in remarkable signal weakening of the last-eluted analytes, when detecting them as M+H]+ adduct ions. MS data acquisition is performed by a time-scheduled three-ion selected ion monitoring program. At the 5 ng/mL level, recoveries of the beta-lactams are between 70 (nafcillin) and 108% (cephalin), with relative standard deviations ranging between 5 (oxacillin) and 11% (amoxicillin and ceftiofur). The response of the ES/MS detector is linearly related to injected amounts up to 500 ng, irrespective of the chemical characteristics of the beta-lactams and the acquisition mode selected (PI or NI modes). Limits of quantification, based on a minimal value of the signal-to-noise ratio of 10, were estimated to be within 0.4 (cephalin) and 3 ng/mL (dicloxacillin). Analyses of milk samples taken after intramammary application of amoxicillin showed that 1.2 ng/mL of this penicillin was still present 6 days after treatment. At this concentration level, the identification power of the method is not weakened, as signals of the three product ions of amoxicillin are still well distinguishable from the background noise.