Evaluation of amperometric detection for the liquid-chromatographic determination of tetracycline antibiotics and their common contaminants in pharmaceutical formulations.

A sensitive high-performance liquid chromatographic technique with amperometric detection has been developed for the determination of seven commercially used tetracyclines in bulk powders and pharmaceutical preparations. The technique is based on the oxidation of these compounds and their contaminants at the glassy carbon electrode. The extraction procedures are simple and the HPLC conditions separate the tetracyclines from their major degradation products. The chromatography was performed using a commercially available octadecylsilane column, with a mobile phase: KH2 PO4 (pH = 2.5; 0.05 M) - acetonitrile (84:16, v/v) and detection at 1.2 V. The technique permits the simultaneous determination of trace amounts of chlortetracycline, demeclocycline, doxycycline, methacycline, minocycline, oxytetracycline and tetracycline as well as the separation of their common impurities (epi, anhydro and epianhydro contaminants) with detection limits of 0.1-1.0 ng microl(-1) and recoveries of 99.1-100.4%. No interference was observed from the commonly present excipients in pharmaceutical formulations.