SEPARATION AND CHARACTERIZATION OF TETRACYCLINE ANTIBIOTICS BY CAPILLARY ELECTROPHORESIS

The electrophoretic behavior of the antibiotics tetracycline, chlortetracycline, demeclocycline, oxytetracycline, doxycycline, methacycline and minocycline has been characterized by capillary electrophoresis in phosphate buffer solutions in the pH range 4-11. A complete set of acid-base equilibrium constants and electrophoretic mobilities was determined and, subsequently, was used in a computer-assisted optimization program to assess the separation of the tetracyclines. Under the predicted optimum conditions (pH 7.5, 18.2 mM ionic strength, 4.3 mM buffer concentration, and constant current of 20 mu A), the separation was performed satisfactorily and all tetracyclines were readily identified. Moreover, the common impurities in tetracycline resulting from dehydration and epimerization reactions were discriminated under the same conditions. The determination of tetracycline, doxycycline and minocycline was performed in commercially available pharmaceutical formulations. During the dissolution of the contents of hard-filled capsules, a minimum recovery of 95% of the active ingredient was obtained. A calibration curve of peak height versus concentration gave a slope of 6.15.10(-4) cm M(-1), intercept of -1.18.10(-5) cm, and coefficient of determination equal to 0.9989. The analysis using UV-absorbance detection at 260 nm provided a detection limit of 1.10(-5) M at a signal-to-noise ratio of approximately 3, with a linear range of two orders of magnitude.