A highly selective method for quantitative determination of impurities in tetracycline antibiotics based on non-aqueous capillary electrophoresis has been developed. In tetracycline hydrochloride the degradation products 4-epitetracycline, anhydrotetracycline and 4-epianhydrotetracycline can be determined with limits of detection corresponding to 0.06%, 0.04% and 0.02% of the drug substance, respectively. The relative standard deviations were about 4% at the 0.1% level of impurity in tetracycline hydrochloride. Furthermore, it is possible to detect desmethyltetracycline as well as a number of unknown impurities within 10 min. The separation of tetracycline, oxytetracycline, doxycycline and chlortetracycline by non-aqueous capillary electrophoresis is also demonstrated. The nature of the solvents and electrolytes in the electrophoresis medium exhibit a major influence on the separation selectivity. Solvents such as methanol and acetonitrile are more volatile than water and it is demonstrated that evaporation of the solvent from sample as well as from the electrophoresis medium may cause severe problems in some capillary electrophoresis instruments.
