Determination of closantel residues in milk and animal tissues by HPLC with fluorescence detection and SPE with oasis MAX cartridges.

A liquid chromatographic method for the determination of closantel residues in milk and tissues is developed and validated. An acetonitrile-acetone solution (80:20, v/v) is used for the extraction of closantel residues from milk and animal tissues, and the extract is purified by solid-phase extraction with Oasis MAX cartridges and a mixture of formic acid-acetonitrile (5:95, v/v) as the elution solution. A C(18) bonded silica column is used for chromatographic separation. The mobile phase consists of acetonitrile-water (85:15, v/v) containing 0.05% triethylamine at pH 2.5, adjusted with phosphoric acid with the flow-rate set at 1.0 mL/min. Using the fluorescence emission of closantel at lambda(ex) = 335 nm and lambda(ex) = 510 nm, the calibration curve is linear, with a correlation coefficient of 0.9999 over the concentration range of 10-5000 microg/kg for the tissue sample and 10-5000 microg/L for the milk sample. The detection limit (s/n = 3) is 3 microg/kg for tissue sample and 3 microg/L for milk sample. The intra- and inter-day repeatabilities are between 3.35-7.66% and 4.04-8.67%, respectively. The proposed method enables the quantitative determination of closantel residues at levels as low as 10 microg/kg in animal tissue samples and 10 microg/L in milk samples.