Anionic—zwitterionic mixed micelles in micellar electrokinetic chromatography: sodium dodecyl sulfate—N-dodecyl-N,N-dimethylammonium-3-propane-1-sulfonic acid

Abstract

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 m M, was chosen as a reference to which comparisons could be drawn. With 20 m M SDS, the optimum concentration range of 10–20 m M 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 m M 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 m M SB-12 and by approximately 60% with addition of 40 and 60 m M SB-12, while operating currents remained constant as SB-12 was added. Geometrical isomers p-nitrotoluene and m-nitrotoluene, that co-eluted with 20 m M SDS, were baseline resolved with the addition of 10 m M 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 (⩽ 1.2% R.S.D.).