Determination of tetracycline antibiotics in cow's milk by liquid chromatography/tandem mass spectrometry

A liquid chromatographic/tandem mass spectrometric (LC/MS/MS) multiresidue method for the simultaneous quantitative determination of oxytetracycline, 4-epi-oxytetracycline, tetracycline, 4-epi-tetracycline, chlortetracycline, 4-epi-chlortetracycline and doxycycline in milk has been developed. An extraction procedure consisting of a liquid extraction of the milk samples with trichloroacetic acid was performed. The extract was centrifuged and the supernatant was filtered. Solid-phase extraction (SPE) with an OASIS HLB SPE column was used to clean up the sample extracts. The samples were analysed by LC/MS/MS. The LC separation was performed on a reversed-phase C18 column using gradient elution with a mobile phase consisting of water and a mixture of methanol/acetonitrile. The tetracycline analytes were detected with a quadrupole mass spectrometer using positive ion electrospray ionisation. The confirmatory method has acceptable detection limits and the different tetracyclines can be detected at a residue concentration between 5 and 20 microg/L. The method is validated according to the European requirements for veterinary drug residues and all determined parameters were found to conform to the criteria. The recovery values ranged from 90.4 to 101.2% with relative standard deviations (RSDs) no larger than 9.7%. The overall or between-day precision of the analytical assay determined as repeatability at several residue concentrations and expressed as RSD ranged from 3.3 to 10%. This analytical assay is a useful tool within the Belgian monitoring programme for confirmation of samples which have been positively screened for residues of tetracyclines in raw farm cow's milk.