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) micell es. We investigated the effects of using different counterions, Li+ an d K+, with the dodecyl sulfate micelles in order to ascertain the infl uence of the counterion on efficiency, selectivity, retention, elution range, and resolution in MEKC. A typically used concentration of 50 m M surfactant was employed for all of the surfactants studied. The effe ct 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. Co mparisons between the Na+, Li+, and K+ counterions are made with 15 an d 20% acetonitrile added, while comparisons between Na+ and Li+ are al so made with 0 and 10% acetonitrile added. Operating currents were low est with the LiDS system. The LiDS system provided efficiencies almost 2.5 times that generated by SDS, while the KDS system generated effic iencies much lower than both the SDS and LiDS micellar phases. The mor e hydrophobic test analytes exhibited the greatest enhancement in effi ciency with the use of the LiDS micelles. Retention factors for the va rious test analytes dropped appreciably with the use LiDS and acetonit rile in comparison to the acetonitrile-modified SDS system in which re tention factors appeared to stabilize with 15 and 20% acetonitrile. Th e KDS micelles provided the largest elution range followed by SDS and then LiDS.