Quantitative Analysis of Minocycline by Liquid Chromatography on Poly(styrene-divinylbenzene)

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Abstract
An isocratic liquid chromatographic method has been developed for assay and purity control of minocycline. All the potential impurities are well separated from the main component and from each other. The stationary phase is poly(styrene-divinylbenzene) (PSDVB), PLRP-S, 250 mm × 4.6 mm, which is heated at 60 °C. The mobile phase is 2-methyl-2-propanol (x g/100 ml) - 0.2 M potassium phosphate buffer pH 10.5 (10.0 ml) - 0.02 M tetrabutylam-monium sulphate pH 10.5 (10.0 ml) - 0.01 M sodium edetate pH 10.5 (10.0 ml) - water (up to 100 ml). The flow rate is 1.0 ml/min and UV detection is performed at 254 nm. The total analysis time does not exceed 30 min. Official standards were compared and a number of commercial bulk samples and specialties were analysed. 4-Epiminocycline, 6-deoxy-6-demethyltetracycline and 7-monodemethyl-minocycline are the main impurities. 7-Didemethylminocy-cline, 9-minocycline and some impurities of unknown identity also can be present.