INFLUENCE OF DODECYL-SULFATE COUNTERION ON EFFICIENCY, SELECTIVITY, RETENTION, ELUTION RANGE, AND RESOLUTION IN MICELLAR ELECTROKINETIC CHROMATOGRAPHY

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, Li+ and K+, with the dodecyl sulfate micelles in order to ascertain the influence of the counterion on efficiency, 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 Krafft 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+, Li+, and K+ counterions are made with 15 and 20% acetonitrile added, while comparisons between Na+ and Li+ are also made with 0 and 10% acetonitrile added. Operating currents were lowest with the LiDS system. The LiDS system provided efficiencies almost 2.5 times that generated by SDS, while the KDS system generated efficiencies much lower than both the SDS and LiDS micellar phases. The more hydrophobic test analytes exhibited the greatest enhancement in efficiency with the use of the LiDS micelles. Retention factors for the various test analytes dropped appreciably with the use LiDS and acetonitrile in comparison to the acetonitrile-modified SDS system in which retention factors appeared to stabilize with 15 and 20% acetonitrile. The KDS micelles provided the largest elution range followed by SDS and then LiDS.