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 widely used surfactant for the pseudostationary phase is sodium dodecyl sulfate (SDS), although other types of surfactants have been investigated. We have synthesized the magnesium salt of dodecyl sulfate (Mg(DS)2) in order to investigate how changes in the counter ion identity affect resolution via changes in selectivity (alpha) and retention. Buffer solutions containing Mg(DS)2 exhibited operating currents and electroosmotic velocities one third to one half that of a comparable SDS buffer 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 the Mg(DS)2. The elution range, t(mc)/t(o), for the Mg(DS)2 was independent of the percentage of organic modifier in the buffer solution while the elution range for SDS increased as the percentage of organic modifier increased. Improvements in alpha and increased retention resulted in increased R(s) for neutral solutes in Mg(DS)2. The relatively constant elution range for Mg(DS)2 also aided in the retention-mediated optimization of R(s) and R(s) per unit time.