Effect of Weak Electrolytes on Electromigration Dispersion in Capillary Zone Electrophoresis

In this paper, simple equations are introduced to predict the electromigration dispersion (EMD) in weak background electrolytes (BGEs) at high pH in capillary zone electrophoresis (CZE) separations. Analytical solutions have been developed in the case of a basic BGE constituted of a neutral acid in equilibrium with its anionic conjugate base, which acts as co-ion to the analytes, and the treatment has been extended to other buffering systems. The treatment assumes that the concentration of the neutral form of the buffer species remains constant throughout the separation and that the pair of ionic species, hydroxide ion and co-ion, effectively act as a single coion. Hydroxide ion is shown to have a significant effect on EMD for pH values >10. The change in hydroxide ion concentration in the sample zone can compensate for changes in conductivity due to the difference of mobility between the analyte and the co-ion. Under appropriate conditions, the hydroxide species can correct the distortion due to the difference of mobility, and a good peak shape can be obtained, even with a high analyte concentration. This effect has been modeled using numerical treatments and experimentally validated using benzoate and 4hydroxybenzoate as test analytes in a BGE constituted of a 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS) buffer with sodium as counterion. At constant co-ion concentration, the peak efficiency improved from 5.1 × 104 to 1.3 × 105 by changing the pH from 10.85 to 11.00. At a constant pH of 10.94, the peak distortion reversed in sign on increasing the concentration of CAPS co-ion from 15 to 25 mM. All experimental electropherograms were successfully fitted using the Haarhoff-Van der Linde function and distortion parameters extracted. Experimental and predicted distortion parameters are in good agreement. This work offers the potential to improve peak shapes in CZE by taking into account the effect of an ion involved in the water autoprotolysis equilibrium, and could lead to improvement in formulation of low and high pH buffers for CZE.