ROLES OF ORGANIC MODIFIERS IN MICELLAR ELECTROKINETIC CAPILLARY CHROMATOGRAPHY

In the present work, four organic modifiers, Pit. urea, dioxane, metha nol, and tetrahydrofuran, were comparatively and systematically studie d in terms of their effects on electrokinetic migration behavior and t he retention mechanism of homologous solutes in MECC. The results show ed that the electroosmotic mobility, mu(eo), and the electrophoretic m obility of a micelle, mu(cp,mc), decrease linearly with increasing org anic modifier concentration. The ability of organic modifiers to lower mu(eo) is greater than their ability to lower mu(ep,mc). The negative values of the slopes of these linear relationships, D-eo and D-cp,D-m c, increase along the series the order urea < methanol < dioxane < tet rahydrofuran. The logarithm of the capacity factor (In k') of uncharge d homologous solute, which is mainly determined by the hydrophobic int eraction, decreases linearly with increasing organic modifier concentr ation, due not only to the decrease in the partition coefficient but a lso to the decrease in the phase ratio. A linear relationship was obse rved between the slope of the plot of ln k' vs. organic modifier conce ntration and the carbon number of homologous compound. The slope of su ch a relationship can characterize the hydrophobicity of the organic m odifier. The hydrophobicity of such organic modifiers is also found to increase along the series urea < methanol < dioxane < tetrahydrofuran .