Liquid chromatographic determination of moxidectin residues in cattle tissues and confirmation in cattle fat by liquid chromatography/mass spectrometry.

Moxidectin, a potent new endo- and ectoparasitic agent, is determined in cattle tissues by liquid chromatography (LC) with fluorescence detection. Moxidectin residues in cattle fat are confirmed with thermospray LC/mass spectrometry (MS). Moxidectin is extracted from the tissue with acetonitrile; the extract is partitioned with hexane, concentrated, and reacted with acetic anhydride, 1-methylimidazole, and dimethylformamide to produce a fluorescent product. The validated sensitivity of the LC/fluorescence method was 10 ppb, with a limit of detection typically between 1 and 2 ppb. Average recoveries from cattle fat, muscle, liver, and kidney were 99, 95, 89, and 92%, respectively. LC/MS confirmatory method determined the underivatized parent compound following the acetonitrile-hexane partitioning step, with an average recovery of 108% at the 250 ppb level in cattle fat.