Determination of closantel and rafoxanide in animal tissues by online anionic mixed-mode solid-phase extraction followed by isotope dilution liquid chromatography tandem mass spectrometry.

A rapid and high-throughput isotope dilution LC-MS/MS method with online sample pre-concentration and clean-up using anionic mixed-mode SPE was described for the determination of closantel and rafoxanide in edible bovine and ovine tissues. Tissue samples were extracted with acetonitrile and acetone mixture (60:40, v/v). Sample pre-concentration, clean-up and analysis were completed simultaneously with the online MAX SPE LC-MS/MS system. Closantel-(13) C(6) and rafoxanide-(13) C(6) were used as the internal standards to improve the precision of the method. The method was validated with edible ovine and bovine tissues (muscle, kidney and liver) fortified at three different levels. The accuracy and RSD were 86-106% and ≤14%, respectively. This high-throughput method was suitable for routine quantitative analysis of closantel and rafoxanide in food safety surveillance samples.