Effects of organic modifiers on solute retention and electrokinetic migrations in micellar electrokinetic capillary chromatography

Influences of seven organic modifiers, including urea, methanol (MeOH), dio xane (DIO), tetrahydrofuran (THF), acetonitrile (ACN), 1-propanol (1-PrOH) and 2-propanol (2-PrOH), on the solute retention and the electrokinetic mig rations in micellar electrokinetic capillary chromatography (MEKC) are inve stigated with sodium dodecyl sulfate (SDS) micelle as pseudostationary phas e. It is observed that in the limited concentration ranges used in the MEKC systems the effect of organic modifier concentration on the retention can be described by the equation logk'=logk'(w)-SC for most binary aqueous-orga nic buffer, but deviations from this retention equation are observed at ACN and particularly THF as organic modifiers. With parameter S as a measure o f the elutropic strength, the elutropic strength of the organic modifiers i s found to follow a general order urea <MeOH<ACN<2-PrOH<DIO, while THF is f ound to be the most abundant modifier in selectivity among the organic modi fiers studied. The electroosmotic mobility decreases linearly with increasi ng modifier concentrations. The strength of modifiers to suppress the elect roosmotic mobility follows the order urea < MeOH, ACN < DIO, THF, 1-PrOH, 2 -PrOH. The addition of ACN affects the electrophoretic mobility of the SDS micelle in different patterns depending on the buffer composition used, whi le the addition of the other modifiers leads to a rough linear decrease of the electrophoretic mobility of the SDS micelle. The suppressing strength o f THF on the electrophoretic mobility of the SDS micelle is strongly relate d to the buffer composition, whereas the suppressing strength for the other modifiers follows the sequence urea < MeOH < DIO, 1-PrOH, 2-PrOH. The migr ation time window is expanded by the use of organic modifiers. Urea and MeO H have the least expanding strength while ACN, DIO, THF, 1-PrOH and 2-PrOH have the highest expanding strength on the migration time window.