CHANGE OF PH IN ELECTROPHORETIC ZONES AS A CAUSE OF PEAK DEFORMATION

Electromigration dispersion (EMD) was studied theoretically with compa rison of the results to experimental findings. The EMD behavior of a s ample constituent in a given background electrolyte (BGE) could be des cribed by an EMD constant, which determines uniquely the direction and the degree of a peak deformation into a triangular shape in a strong or a weak ion BGE system in capillary zone electrophoresis (CZE). The EMD constant was found to be proportional to the linear sum of the rel ative change of the electric field strength (conductivity effect) and the relative change of the effective mobility of the sample constituen t (pH effect) across the steep boundary between sample and BGE zones. Based on the moving boundary model or equations, the two effects, as w ell as the EMD constants for different BGE types, can be calculated se parately. Analytical solutions for those effects were also obtained fo r some simple cases. Computational results have shown that the conduct ivity effect and the pH effect for an analyte can be quite different w ith different BGE types. In some cases, the effects of conductivity an d pH on EMD act in the same direction, and reinforce the peak broadeni ng. In other cases they act in opposite directions, and therefore coun teract each other partially or completely, leading to a relatively sym metric and narrower peak. To compare the contribution of each dispersi on source, a variance of EMD, which is proportional to the EMD constan t and time, was defined. It was found that the total variance of a pea k can be approximated as the sum of the variance due to EMD and the va riances due to other dispersion sources.