Anionic-zwitterionic mixed micelles in micellar electrokinetic chromatography: sodium dodecyl...

A zwitterionic surfactant, N-dodecyl-N,N-dimethylammonium-3-propane-1- sulfonic acid (SB-12), was used in combination with an anionic surfactant, sodium dodecyl sulfate (SDS), to form a novel pseudostationary phase for use in micellar electrokinetic chromatography. This mixed micellar system was characterized in terms of analyte retention, selectivity, efficiency, elution range, and resolution; and compared to results obtained using only SDS. A typically used SDS concentration, 20 mM, was chosen as a reference to which comparisons could be drawn. With 20 mM SDS, the optimum concentration range of 10-20 mM SB-12 provided efficiencies that were 2-4 times greater than with SDS alone, with minimal (< 15%) changes in the elution range and electroosmotic flow. The addition of 40 and 60 mM SB-12 also resulted in efficiencies on average of 600,000-800,000 theoretical plates/m, but at a significant reduction in the elution range and peak capacity. Retention factors (k') for the various neutral analytes increased by 20% with addition of 10 mM SB-12 and by approximately 60% with addition of 40 and 60 mM SB-12, while operating currents remained constant as SB-12 was added. Geometrical isomers p-nitrotoluene and m-nitrotoluene, that co-eluted with 20 mM SDS, were baseline resolved with the addition of 10 mM SB-12; in addition, methylene selectivity was greatest at this composition. No capillary wall interactions or coating effects were observed with the SDS-SB-12 mixed micellar system, in contrast to previously studied anionic-non-ionic mixed micellar system, SDS-Brij 35. Consequently, migration times were very repeatable (< or = 1.2% R.S.D.).