Development and validation of an improved method for confirmation of the carbadox metabolite, quinoxaline-2-carboxylic acid, in porcine liver using LC-electrospray MS-MS according to revised EU criteria for veterinary drug residue analysis
A method is described for the quantitative determination of quinoxaline-2-carboxylic acid (QCA), the marker residue for the veterinary drug carbadox, in swine liver. Tissue is subjected to alkaline hydrolysis followed by liquid–liquid extraction. QCA residues are cleaned up using automated solid phase extraction (SPE), before a final liquid–liquid extraction step. Analysis is based on LC coupled to positive ion electrospray MS-MS, monitoring product ions at m/z 129, 102 and 75 for QCA and at m/z 106 for the internal standard (d4-QCA). The method has been validated according to draft revised EU criteria for analysis of veterinary drug residues, and is suitable for monitoring tissues taken under national surveillance schemes. The method has been validated at 3, 10, 30, 100 and 300 μg kg−1. The method performance characteristics, CCα (decision limit) and CCβ (detection limit) were determined to be 0.16 and 0.27 μg kg−1, respectively. The described method, which is relatively rapid and applicable to large sample numbers, correlates well (r2 = 0.9799) with a widely used GC-MS assay for QCA.