Separation of neutral compounds by microemulsion electrokinetic chromatography: Fundamental studies on selectivity

The selectivity of microemulsion electrokinetic chromatography (MEEKC) was studied utilizing some uncharged model compounds like aromatic amides, ster oids, and esters of nicotinic acid, The cosurfactant of the microemulsion w as found to be the most important factor affecting the selectivity, and alt eration between 6.6% of I-propanol, I-butanol, tetrahydrofuran, and 2-ethox yethanol caused several substantial changes in the migration order. In addi tion, the nature of the surfactant was found to significantly affect the se lectivity In this case, changes in order of migration was observed by repla cement of half the content of sodium dodecyl sulfate (SDS) with either sodi um dioctyl sulfosuccinate (SDOSS), 3-(N,N-dimethylmyristylammonio) propanes ulfonate (MAPS), polyoxyethylene sorbitan monolaurate (Tween 21), and polyo xyethylene 23 lauryl ether (Brij 35). MEEKC was also accomplished with 3.3% of the anionic surfactant sodium cholate and with the cationic surfactant N-cetyl-N,N,N-trimethylammonium bromide (CTMA). Both provided substantial d ifferences in selectivity as compared to the SDS-based systems. With SDS as surfacant, the concentration was varied within 1.0-4.5%. Minor selectivity changes were observed as the concentration of the surfacant was reduced, b ut the major effect was a reduction in the total migration time. The organi c solvent of the microemulsion droplets was found only to have minor impact on the selectivity.