Practical capillary electrophoresis method for the quantitation of the acetate counter-ion in a novel antifungal lipopeptide

This is the first report of the validation of a capillary ion electrophoresis method for the quantitative determination of acetate (CH3COO-) levels in the acetate salt of a basic lipopeptide. The acetate counter-ion was detected using indirect photometric detection, 4.0 mM 4-hydroxybenzoic acid (HOC6H4COOH) as the carrier electrolyte and the chromophore mobile phase, and OFM Anion-BT as the electroosmotic flow (EOF) modifier. The EOF modifier is primarily a 20 mM myristyltrimethylammonium bromide (TTAB) solution. The apparent pH (pHapp) of the electrophoretic buffer was 6.0. The response was linear from 0.9 microgram/ml to 46 micrograms/ml (r2 = 0.9997). The capillary electrophoresis (CE) system is very efficient and stable, giving R.S.D. values of about 1.0% for the injection-to-infection precision without using internal standards. The method is accurate as judged by comparison to an ion-exchange HPLC method and by the 99.7% recovery of spiked acetate ion in the aqueous solution of the lipopeptide. The method is rugged with regard to critical method parameters, such as different operational voltages and effective capillary lengths. The overall precision is acceptable with a R.S.D. of 1.5% between 0.2-12% (w/w) acetate ion present in the drug substance without using an internal standard. Comparison to the ion-exchange chromatography and advantages of the CE method are discussed. This CE method has been submitted to and accepted by the regulatory agencies and is now in routine use within our laboratories.