Determination of absolute mobilities, pK values and separation numbers by capillary zone electrophoresis: effective mobility as a parameter for screening

Migration times or apparent mobilities can never be used for the identification of ionic species in capillary zone electrophoresis if an electroosmotic flow (EOF) is present, because the velocity of this flow varies considerably with the “state” of the capillary. From the migration times of the EOF and the ionic species, the effective mobilities can be calculated. These effective mobilities are nearly independent of the concentrations of the sample ionic species. Although a large excess of one of the sample components can cause different values of the calculated effective mobility, they are reproducible if the matrix has a constant composition and in this way effective mobilities can be used for screening purposes. In the determination of effective mobilities the use of a “true” EOF marker is extremely important. If effective mobilities are measured in two different electrolyte systems at different pH values, at which the degrees of dissociation differ sufficiently, the absolute ionic mobilities and pK values of ionic species can be calculated. Values obtained in this way, for mobility and pK were compared with data obtained isotachophoretically, showing good agreement. Theoretically, the separation number in zone electrophoresis, defined as the number of components that can be separated within a unit of mobility, varies widely with the mobilities of the ionic species and the EOF. Experimentally obtained values of the separation number are significantly lower than the calculated values owing to the method of injection, temperature effects during analysis and amount of sample. For low-molecular-weight ionic species separations are possible if the effective mobilities differ by about one unit for cations and 0.24.3 for anions. A negative wall charge (at higher pHs) diminishes the separation number of cations considerably, especially on applying small diameter capillaries, owing to attractive forces between the wall and analytes.