Capillary zone electrophoresis of minocycline

A method is described for analysis of the tetracycline antibiotic minocycline using capillary zone electrophoresis. Potential impurities are 4-epiminocycline, 6-deoxy-6-demethyltetracycline, 7-didemethylminocycline, 7-monodemethylminocycline and 9-minocycline. Method development was undertaken with a mixture consisting of minocycline and its related substances mentioned above. Using a fused silica capillary, the type of buffer and its pH and concentration were investigated. In all cases 1 mM EDTA was added to prevent metal ion complexation. Instrumental parameters such as capillary temperature and applied voltage were optimised. The effects of the sample solvent and of organic modifiers in the buffer were also investigated. The following method is proposed: capillary: fused silica, l = 38 cm, L = 44 cm, 50 microns i.d.; buffer: 25 mM sodium tetraborate, 1 mM EDTA at pH 11.75; voltage, 13 kV; temperature, 15 degrees C; UV detection performed at 254 nm. Relative standard deviations, linearity, LOD and LOQ are reported and compared with those of liquid chromatography.