Selectivity optimization after prediction of the migration behaviour of anions in capillary ion analysis in the presence of micelles

Above the critical micellar concentration of the modifier (cationic surfactant) anion separation in capillary ion analysis is mainly influenced by two chemical equilibria, the ionic dissociation which depends on the pH of the buffer electrolyte and the micellar partitioning in function of the modifier concentration. Experimentally obtained data were fitted to empirical and theoretical (physical) models in order to predict the migration behaviour. Both models resulted in acceptable predictions. The empirical models were second order polynomials whereas the theoretical models described the mobility of the anions in terms of physical and chemical constants of each ion (acid dissociation constant: Ka, ion association constants: km, ion partitioning constants: k′) the pH of the electrolyte, and the concentration of the modifier ([M]) available as micelles in the buffer electrolyte. The experimental domain was examined with the aid of a central composite design. After prediction of the mobility of each solute over a two-dimensional pH[M] space, a selectivity optimization was performed in order to obtain the optimum separation conditions for 10 anions.