The high-performance liquid chromatographic method published by Aliu & Odegaard (1983) was found to give poor
peak separation when used to determine plasma diminazene concentrations in cattle. Before bioequivalence studies
could be carried out, the method had to be modified. Solid-phase extraction with acetonitrile/0,025 M Na-octane
sulphonate and 2% acetic acid as eluent, followed by sample concentration, gave recoveries of > 90% for
diminazene and the internal standard. A mobile phase of acetonitrile/0,005 M Na-octane sulphonate, 0,1% triethylamine, pH 3,2 with acetic acid on a Nova Pak C 18 column was used for the analysis. Wavelength switching
was used to determine the internal standard (imidocarb) and diminazene at their respective wavelengths of maximum
absorbance, resulting in a fivefold increase in the limit of detection for diminazene. The modified method attained
a detection limit of 2 ng/ml (peak 4x baseline noise) , limit of quantitation of 10 ng/ml (coefficient of
variation < 15 %) and an accuracy of > 96% over the range from 10-5000 ng/ml.
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