Trace analysis of endectocides in milk by high performance liquid chromatography with fluorescence detection

An analytical method has been developed for the simultaneous determination of the following endectocide drugs in milk: ivermectin, abamectin, doramectin, moxidectin, eprinomectin, emamectin and nemadectin. Samples were extracted with acetonitrile, purified with solid-phase extraction on a reversed phase C₈, derivatised with N-methylimidazole, trifluoroacetic anhydride and acetic acid to a stable fluorescent derivative, and were further analysed by gradient high performance liquid chromatography (HPLC) on an endcapped reversed phase Supelcosil LC-8-DB. The derivatisation step was mathematically optimised and the method was validated according to the requirements of Commission Decision 2002/657/EC, using fortified raw bovine milk. Mean recovery was between 78 and 98%. The repeatability (CV_r) and within-laboratory reproducibility (CV_W) ranged from 4.6 to 13.4% and from 6.6 to 14.5%, respectively. Decision limits (CCα) for analytes with MRL values, namely eprinomectin and moxidectin, were determined to be 24.8 and 50.6 μg kg⁻¹, respectively. CCα values for unauthorised endectocides ranged from 0.1 to 0.2 μg kg⁻¹. Due to high acceptability regarding the required criteria and applicability to ovine and caprine milk, giving similar results, this multi-analyte method has been successfully implemented in pharmacokinetic research studies as well as statutory residue monitoring in Slovenia.