Analysis of erythromycin estolate by liquid chromatography

A method is described for the determination of erythromycin estolate by liquid chromatography. A C18 reversed-phase column (25 x 0.46 cm i.d.) was used with acetonitrile-tetrabutylammonium sulphate (pH 6.5, 0.2 M)-phosphate buffer (pH 6.5, 0.2 M)-water [x:5:5:(90-x), v/v/v/v] as mobile phase. The proportion of acetonitrile (x) has to be adapted to the type of stationary phase used. For RSil C18 LL 42.5% (v/v) was used. The column was heated at 35 degrees C, the flow rate was 1.5 ml min-1 and UV detection was performed at 215 nm. The main component, erythromycin A propionate, was separated from all other components which were present in commercial samples. The impurities most frequently observed were the propionate ester of erythromycin C and the amide N-propionyl-N-demethyl-erythromycin A. Erythromycin A was shown to be present in specialties.