Confirmation of carbadox and olaquindox metabolites in porcine liver using liquid chromatography–electrospray, tandem mass spectrometry

A method is described for the quantitative determination of quinoxaline-2-carboxylic acid (QCA) and methyl-3-quinoxaline-2-carboxylic acid (MQCA), the metabolites that have been designated as the marker residues for the veterinary drugs, carbadox and olaquindox, respectively, in swine tissue. The method is suitable for use as a confirmatory method under EU National Surveillance Schemes. Porcine liver samples were subjected to protease digestion followed by liquid–liquid extraction. Further clean-up was performed by automated solid phase extraction (SPE) and was followed by a final liquid–liquid extraction step. Analysis was performed using a narrow bore column HPLC coupled to electrospray MS/MS, operated in positive ion mode. MS/MS product ions were monitored at m/z 102 and 75 amu for QCA, m/z 145 and 102 amu for MQCA and at m/z 106 and 152 amu for the d4-QCA and d7-MQCA internal standards, respectively. The method has been validated at 3.0, 10, 50 and 150 μg kg−1 for both metabolites. The method performance characteristics—the decision limit (CCα) and the detection capability (CCβ) have been determined for QCA at 0.4 and 1.2 μg kg−1, respectively, and for MQCA at 0.7 and 3.6 μg kg−1, respectively.