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Methyl-3-quinoxaline-2-carboxylic acid (MQCA) is a possible residue marker for three quinoxaline veterinary medicines (olaquindox, mequindox, and quinocetone). The wide application of mequindox/quinocetone or the illegal use of olaquindox leads to MQCA residue in animal's original food, thereby threatening the safety of human food. The indirect competitive enzyme-linked immunosorbent assay (IC-ELISA) with a specific coating antigen and monoclonal antibody (MAB) was established and optimized for detecting MQCA in swine liver. Samples were acidified with 2 mol l hydrochloric acid, extracted with ethyl acetate-hexane-isopropanol (8 + 1 + 1, v/ v/ v) and then detected by IC-ELISA. The logarithm correlation of standards to OD values ranged from 0.2 to 200 μg l, with IC of 6.46 μg l. Negligible cross-reactivity happened to five quinoxaline antibiotics (olaquindox, mequindox, quinocetone, carbadox, and cyadox) and the metabolite of carbadox and cyadox (quinoxaline-2-carboxylic acid). When spiked with 1 to 100 μg kg of MQCA, the recoveries ranged from 85.44 to 100.02 %, with the intra-assay coefficient of variation (CV) of 6.64-10.57 % and inter-assay CV of 7.29-10.88 %. The limit of detection for MQCA was 1.0 μg kg in swine liver. Furthermore, incurred samples were detected by the IC-ELISA and then conformed by a reported LC/MS/MS method, it shown that there was good correlation between the two methods. All these results indicated that the IC-ELISA method is appropriate for surveillance MQCA residue in animal tissues. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: A sensitive and specific enzyme-linked immunosorbent assay (ELISA) for the detection of 1-amino-hydantoin (AHD) is described. AHD, the marker residue for nitrofurantoin, is a tissue bound toxic metabolite. To monitor the illegal use of nitrofurantoin, a monoclonal antibody-based ELISA method was developed to detect AHD residue in animal tissues. The highly specific antibody against AHD was prepared by monoclonal antibody technology. The antibody exhibited negligible cross reactivity with other nitrofurans, their metabolites and derivatives, and 50% inhibitory concentration was 0.68 μg/L. The limit of detection in four kinds of animal tissues were all below 0.2 μg/kg and recoveries ranged from 75% to 116.7% for fortified samples at levels of 0.2–5 μg/kg with coefficient of variation values below 15%. Analysis of natural contaminated samples by the ELISA method gave similar results to those of the liquid chromatography-tandem mass spectrometry method. These results indicate the ELISA method is suitable for the detection of AHD residue in animal tissues. [Copyright &y& Elsevier]