Purity control of oxytetracycline by capillary electrophoresis

The applicability of capillary electrophoresis for the purity control of oxytetracycline (OTC) was investigated. OTC is a broad-spectrum antibiotic belonging to the group of the tetracyclines. Several related substances can be present due to fermentation or degradation, such as 4-epioxytetracycline, -apooxytetracycline, β-apooxytetracycline, anhydrooxytet racycline, 2-acetyl-2-decarboxamidooxytetracycline, tetracycline and 4-epitetracycline. Using fused-silica capillaries, the influence of buffer type, buffer pH and buffer concentration were investigated. In all cases 1 mM EDTA was added to prevent metal-ion complexation. The influence of the buffer counter-ion type was examined. Consequently, some instrumental parameters were changed such as capillary length and diameter as well as capillary temperature and applied voltage. The following method is finally proposed: fused-silica capillary, l (effective length) = 38 cm, L (total length) = 44 cm, 50 μm I.D.; buffer, sodium carbonate 20 mM-EDTA 1 mM, pH 11.25; voltage, 10 kV; temperature, 10°C. Linearity, limit of detection and limit of quantitation were determined as well as the relative standard deviations for all the analytes involved. This method is less selective then existing liquid chromatographic methods but it may be used as a complementary tool in purity control and stability studies.