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An ultra-performance liquid chromatography coupled with tandem mass spectrometry (UPLC/MS/MS) method was developed to screen and confirm multi-class veterinary drug residues in pig tissues including pig kidney, liver and meat. Twenty-one drugs of two different classes including seven tetracyclines and four types of quinolones (quinoline, naphthyridine, pyridopyrimidine and cinoline) were determined simultaneously in a single run. The homogenized sample tissues were extracted with EDTA-McIlvaine buffer solution and further purified using a polymer-based Oasis HLB solid-phase extraction (SPE) cartridge. An ACQUITY UPLC (TM) BEH C-18 column was used to separate the analytes followed by tandem mass spectrometry using an electrospray ionization source. MS data acquisition was performed in the positive ion multiple reaction monitoring mode, selecting two ion transitions for each target compound. Recovery studies were performed at different fortification levels. The overall average recoveries from pig muscle, kidney, and liver fortified with quinolones and tetracyclines at three levels ranged from 80.2 to 117.8% based on the use of matrix-fortified calibration with the coefficients of variation ranging from 2.1 to 17.8% (n=6). The limits of quantitation (LOQs) of quinolones and tetracyclines in different tissues ranged from 0.03-4.50 mu g/kg and 0.16-10.00 mu g/kg, respectively. The effects of the extraction solvent, SPE cartridge, elution solvent and sample matrix on the analyte recovery as well as the effects of the mobile phase composition and column temperature on the chromatographic behavior were also studied. Copyright (C) 2007 John Wiley & Sons, Ltd.