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A method of high-performance liquid chromatography with UV detection has been established for simultaneous quantitative determination of quinoxaline-2-carboxylic acid (QCA) and methyl-3-quinoxaline-2-carboxylic acid (MQCA), the marker residues for carbadox (CBX) and olaquindox (OLA), respectively, in the muscles and livers of porcine and chicken and in the muscle of fish. Tissue samples were subject to acid hydrolysis followed by liquid-liquid extraction and Oasis MAX solid-phase extraction clean-up. The method was validated according to the EU Commission Decision 2002/657/EC. The decision limits (CC alpha) were 0.7-2.6 mu g/kg and the detection capabilities (CC beta) were 1.3-5.6 mu g/kg for QCA and MQCA in tissues. The recoveries of QCA and MQCA, spiked at levels of 2-100 mu g/kg, were from 70 to 110%; the relative standard deviation values were < 20%. This simple, fast and economic method could be applied to the monitoring for the possible misuse of CBX and OLA in animal edible tissue samples. (c) 2006 Elsevier B.V. All rights reserved.