Capillary electrophoresis in aqueous-organic media. Ionic strength effects and limitations of the Hubbard-Onsager dielectric friction model

The mobilities of three aromatic sulfonates, ranging in charge from -1 to -3, were investigated by capillary electrophoresis using buffers containing 0 to 75% ethanol or 2-propanol. Absolute mobilities were determined by extrapolation of the effective mobilities to zero ionic strength according to the Pitts' equation. For all buffers studied, ions of higher charge experienced larger ionic strength effects. The resulting ionic strength-induced selectivity alterations were more dramatic when organic solvents were present in the media. Furthermore, for different organic modifier types and contents, the magnitude of the ionic strength effect was governed to a large extent by the 1/(eta epsilon 1/2) dependence in the electrophoretic effect of the Pitts' equation. Addition of ethanol or 2-propanol to the electrophoretic media resulted in changes in the absolute mobilities of the ions. These solvent-induced mobility changes are attributed to dielectric friction. As predicted by the Hubbard-Onsager model, dielectric friction increased with increasing organic content and with increasing analyte charge. As a result, dramatic changes in the relative absolute mobilities were observed, such as a reversal in migration order between sulfonates of -1 and -3 charge in 75% 2-propanol. Within the alcohols, the Hubbard-Onsager model was successful at predicting the relative mobility trends upon changing solvent. However, the relative trends observed between acetonitrile-water and alcohol-water media were not consistent with the model. This may be explained by the continuum nature of the model, whereby the different ion-solvent interactions characteristic to each solvent class are not taken into account.