On capillary ion-exchange preconcentration of inorganic anions in open-tubular capillary electrochromatography with elution using transient-isotachophoretic gradients. 2. Characterization of the isotachophoretic gradient

Diffuse transient-isotachophoretic boundaries can be used as an elution gra dient of increasing eluotropic strength to elute inorganic anions that have been preconcentrated on an open-tubular ion-exchange stationary phase prio r to electrophoretic separation. The generation and characteristics of thes e gradients for elution after preconcentration have been investigated. The gradients are generated by placing a low-mobility, weak ion-exchange compet ing anion in the capillary (weak electrolyte, WE), and a high-mobility, str ong ion-exchange competing anion in the electrolyte vials (strong electroly te, SE), Application of voltage establishes a diffuse boundary with the com position changing from the weak anion to the strong anion, Comparison of el ution gradients generated with different electrolyte systems was accomplish ed by comparing the eluotropic strength (a function of eluent concentration , ion-exchange selectivity coefficient, and charge) and the shape of the pr ofile as it changes from WE to SE. The ion-exchange selectivity coefficient of the SE competing anion is important in establishing a sharp change in e lution strength. A large difference in mobility between the WE and SE compe ting anions gives an SE with a higher final eluotropic strength, but the sl ope of the gradient is shallower. This results in a reduction in the effici ency of analyte focusing. To ensure maximum focusing efficiency, the WE and SE electrolytes should be selected such that the SE has the highest possib le eluotropic strength for a given concentration of WE. The SE competing an ion should also have a sufficiently low electrophoretic mobility to ensure focusing for the maximum number of analytes, and the mobility difference be tween the WE and SE competing anions should be as small as possible.