Determination of anthelmintic drug residues in milk using ultra high performance liquid chromatography–tandem mass spectrometry with rapid polarity switching

A new UHPLC–MS/MS (ultra high performance liquid chromatography coupled to tandem mass spectrometry) method was developed and validated to detect 38 anthelmintic drug residues, consisting of benzimidazoles, avermectins and flukicides. A modified QuEChERS-type extraction method was developed with an added concentration step to detect most of the analytes at <1 μg kg−1 levels in milk. Anthelmintic residues were extracted into acetonitrile using magnesium sulphate and sodium chloride to induce liquid–liquid partitioning followed by dispersive solid phase extraction for cleanup. The extract was concentrated into dimethyl sulphoxide, which was used as a keeper to ensure analytes remain in solution. Using rapid polarity switching in electrospray ionisation, a single injection was capable of detecting both positively and negatively charged ions in a 13 min run time. The method was validated at two levels: the unapproved use level and at the maximum residue level (MRL) according to Commission Decision (CD) 2002/657/EC criteria. The decision limit (CCα) of the method was in the range of 0.14–1.9 and 11–123 μg kg−1 for drugs validated at unapproved and MRL levels, respectively. The performance of the method was successfully verified for benzimidazoles and levamisole by participating in a proficiency study.