Elimination of adsorption effects in the analysis of water-soluble vitamins in pharmaceutical formulations by capillary electrophoresis

A tendency to an increase in migration times was observed when different wa ter-soluble vitamins were analysed repeatedly in pharmaceutical preparation s by capillary electrophoresis. In order to better understand the origin of this effect, the influence of the vitamins and the excipients, such as cel lulose derivatives, was investigated. These studies indicated that the incr ease in analyte migration times was most probably due to the adsorption of different kinds of constituents to the capillary wall. Different rinsing pr ocedures were tested in order to eliminate these unfavourable effects. A ri nse of the capillary with a 25 mM sodium dodecyl sulfate (SDS) solution in the running buffer between successive runs was found to be particularly eff ective when the analysis was performed by free solution capillary zone elec trophoresis (CZE). When the vitamins were determined by micellar electrokin etic chromatography (MEKC) using SDS as surfactant, a short capillary rinse with the running buffer was sufficient to obtain reproducible migration ti mes. The CZE and MEKC methods developed were validated and compared. Both m ethods could be applied to the determination of water-soluble vitamins in d ifferent multivitamin formulations. (C) 1999 Elsevier Science B.V. All righ ts reserved.