EFFECT OF THE DODECYL-SULFATE COUNTER ION ON SELECTIVITY AND RESOLUTION IN MICELLAR ELECTROKINETIC CAPILLARY CHROMATOGRAPHY - SDS VS MG(DS)2

In micellar electrokinetic capillary chromatography (MECC) the most wi dely used surfactant for the pseudostationary phase is sodium dodecyl sulfate (SDS), although other types of surfactants have been investiga ted. We have synthesized the magnesium salt of dodecyl sulfate (Mg(DS) 2) in order to investigate how changes in the counter ion identity aff ect resolution via changes in selectivity (alpha) and retention. Buffe r solutions containing Mg(DS)2 exhibited operating currents and electr oosmotic velocities one third to one half that of a comparable SDS buf fer solution, and significantly larger methylene and functional group selectivities for the compounds studied. In addition, capacity factors , k', for neutral solutes were between 1.5 and 2.5 times larger for th e Mg(DS)2. The elution range, t(mc)/t(o), for the Mg(DS)2 was independ ent of the percentage of organic modifier in the buffer solution while the elution range for SDS increased as the percentage of organic modi fier increased. Improvements in alpha and increased retention resulted in increased R(s) for neutral solutes in Mg(DS)2. The relatively cons tant elution range for Mg(DS)2 also aided in the retention-mediated op timization of R(s) and R(s) per unit time.