In micellar electrokinetic chromatography (MEKC), the pseudostationary phase used most often consists of sodium dodecyl sulfate (SDS) micelles. We investigated the effects of using different counterions, E+ and K+, with the dodecyl sulfate micelles in order to ascertain the influence of the counterion on aciency, selectivity, retention, elution range, and resolution in MEKC. A typically used concentration of 50 mM surfactant was employed for all of the surfactants studied. The effect of acetonitrile was also investigated. Due to the high Kr& point of the potassium dodecyl sulfate (KDS), a minimum of 15% acetonitrile had to be used in order to solubilize the KDS surfactant monomers. Comparisons between the Na+, E+, and K+ counterions are made with 15 and 20% acetonitrile added, while comparisons between Na+ and E+ are also made with 0 and 10% acetonitrile added.Operating currents were lowest with the IiDS system. The (13) Ong, C. P.; Ng, C. L.; Lee, H. IC; Li, S. F. Y.

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A retention index system has been developed for micellar electrokinetic chromatography (MEKC). Three homologous series: alkyl aryl ketones (phenones), 1-nitroalkanes, and alkylbenzenes were studied for use as retention index standards. Micellar systems consisting of sodium dodecyl sulfate (SDS), SDS/Brij 35 (polyoxyethylene lauryl ether), and SDS/SB-12 (N-dodecyl-N, N-dimethylammonium-3-propane-1-sulfonic acid) were used as pseudostationary phases. In addition, three organic modifiers: acetonitrile, methanol, and 1-propanol were used with SDS to evaluate their effect on the retention indices calculated for a set of neutral compounds. Retention indices for the neutral compounds did not vary significantly over the range of surfactant concentrations employed for each of the micellar systems (RSD < 2.0% for non-extrapolated retention indices). However, in the systems where an organic modifier was employed, the calculated retention indices showed some variation (RSD < 3.0%) at different SDS concentrations. The 1-nitroalkanes were found to be the most suitable for use as retention index standards. Alkyl aryl ketones were found to be effective retention index standards for more hydrophobic solutes, but they were not effective for very hydrophilic solutes even with a large amount of organic modifier added to the operating buffer. The alkylbenzenes were too hydrophobic (highly retained) than the alkyl aryl ketones and, therefore, cannot be recommended for use as retention index standards in MEKC.