Analysis of colistin sulfate by capillary zone electrophoresis with cyclodextrins as additive

A method for the quantitative analysis of colistin sulfate by capillary zon e electrophoresis is described. Since colistin components have five free am ino groups, they tend to adsorb onto the capillary wall and cause peak tail ing. It was found that triethanolamine (TEA)-phosphate buffer at pH 2.5 was useful to reduce such adsorption. Methyl-beta-cyclodextrin (M-beta-CD) and 2-propanol (IPA) were found necessary for selectivity enhancement. In orde r to optimize the separation parameters and predict the method robustness, a central composite design was performed including three variables, namely concentration of M-beta-CD, TEA, and IPA. The effects of capillary length a nd applied voltage on separation were also investigated. The optimal condit ions established were: 140 mM TEA-phosphate buffer containing 5 mM M-beta-C D and 6% v/v IPA, a capillary with 55 cm total length (50 mu m inner diamet er, 47 cm from inlet to detection window) and 24 kV applied voltage. The me thod was found to be robust when the variables were changed in the followin g range: 4-6 mM M-beta-CD, 5-7% v/v IPA, and 130-150 mM TEA. Further, the l inearity, limit of detection (LOD), and limit of quantitation (LOQ), as wel l as repeatability far both colistin A and B were examined and three commer cial samples were quantitatively analyzed.