Zone sharpening of neutral solutes in micellar electrokinetic chromatography with electrokinetic injection

Abstract

Zone sharpening has been used extensively to enhance both efficiency and detectability of charged solutes with either electrokinetic or hydrodynamic injection. In the present study, we report the zone sharpening of neutral solutes by sharpening the zones of charged micelles which serve as carriers for neutral (and charged) molecules. Zone sharpening with electrokinetic injection can be accomplished only when the effective electrophoretic velocity, v eff, of the solute and the electroosmotic velocity are in the same direction during the injection process. In order to maximize the amount of solute loaded into the capillary under zone sharpening conditions, it was necessary to use a cationic mixed micelle in the sample buffer. The zone sharpening of a homologous series of alkylphenones was accomplished using electrokinetic injection with cationic mixed micelles of cetyltrimethylammonium bromide and dodecyldimethyl-(3-sulfopropyl)ammonium hydroxide inner salt. The running buffer contained only sodium dodecyl sulfate and phosphate buffer. As a result of the increased amount of solute loaded into the capillary without significant loss in efficiency, the limits of detection (LOD's) for the solutes studied were in some cases lowered by a factor of ten using zone sharpening. Efficiencies for heptanophenone exceeding 1 000 000 theoretical plates were generated in under 10 min on a 50-cm capillary. Such high efficiencies can be predicted from band broadening models developed for micellar electrokinetic chromatography (MEKC). If properly utilized, zone sharpening can eliminate most if not all of the band broadening associated with sample introduction, and can thus be an important tool in the fundamental study of band broadening in MEKC.