Analysis of bacitracin by micellar electrokinetic capillary chromatographywith mixed micelle in acidic solution

A method used for quantitative analysis of bacitracin with micellar electro kinetic capillary chromatography (MEKC) is described. As capillary zone ele ctrophoresis gave poor separation selectivity, MEKC was preferable. It was found that a zwitterionic surfactant, 3-(N,N-dimethylhexadecylammonium)-pro panesulfonate (PAPS) gave the best selectivity among the several surfactant s studied. As the analytes tend to adsorb onto the capillary wail due to th eir positive charge, an acidic solution composed of Tris-phosphate buffer a t pH 2.5 was necessary to diminish such adsorption. The peak tailing caused by relatively strong ion pair interaction between the analyte and PAPS mic elle could be reduced by adding nonionic surfactant Brij 35 to the PAPS sol ution. This phenomenon is possibly explained by a mixed micelle mechanism, in order to obtain the optimal conditions and to test the method robustness , a central composite experimental design was performed. The optimal condit ions are as follows: 44 cm length of fused-silica capillary with 50 mum inn er diameter, 90 mM Tris-phosphate buffer (pH 2.5) containing 17 mM PAPS and 0.3% w/v-Brij 35, 18 kV applied voltage, UV detection at 192 nm and 25 deg reesC column temperature. Under the optimal conditions, more than 50 peaks could be obtained in 30 min. The method had a linearity range from 1 to 0.0 5 mg/mL (concentration of bacitracin A). The limit of quantitation (LOQ) an d limit of detection (LOD) were 0.005 and 0.0012 mg/mL, respectively.