Determination of erythromycin and related substances by capillary electrophoresis.

Current compendial methods of assay for the analysis of erythromycin and its related substances involve the use of microbiological techniques. These techniques are non-selective and tedious, thus there is a need for the development of highly specific, quantitative analytical methods. Erythromycin was analysed in a 50 mM phosphate buffer (pH 7.5) and run at an applied voltage of 20 kV. Detection sensitivity was enhanced by using a wavelength of 200 nm and selecting an injection solvent of lower conductivity than the electrolyte: acetonitrile-water (20:80, v/v). In order to facilitate the separation of erythromycin and its related substances, the organic solvent ethanol (35%, v/v) was incorporated into a modified 150 mM phosphate buffer (pH 7.5) and run at an applied voltage of 30 kV. Resolution of all the compounds was achieved in approximately 45 min. The methods described are accurate and precise and thus suitable for the quantitative determination of erythromycin and the related substances, erythromycin C, anhydroerythromycin and N-demethylerythromycin A.