Predicting peak symmetry in capillary zone electrophoresis: The concept of the peak shape diagram

Electrophoretic peaks generally deviate from the symmetrical Gaussian shape that is expected when diffusion is the dominant dispersion factor. A basic understanding of the electromigrational dispersion processes which give rise to peak asymmetry shows that these effects are related to the difference in mobilities between the sample ions and the buffer electrolyte ions. There is a currently adopted rule saying that if the mobility of the buffer co-ion is higher than that of the analyte, a tailing peak is obtained, while a lower mobility of the buffer co-ion results in a fronting peak. Unfortunately, the phenomena observed in practice are frequently contradictory to these rules. This paper reveals the principles of these effects and establishes a way to proceed in explaining and predicting the peak shape in electrophoresis. Based on a mathematical model of zone migration in capillary electrophoresis, the key parameter controlling the migration dynamics is found to be the velocity slope, defined as the change in the analyte migration velocity with the analyte molar fraction at infinite analyte dilution. It is shown that the sign of the velocity slope determines the peak asymmetry (fronting or tailing) and that its magnitude relates to peak width due to electromigration dispersion. In experiments where the conversion of a fronting peak of an analyte into a tailing one is induced by changing the pH of the background electrolyte, the proposed theory successfully predicts the conversion point where the peak is symmetric. For a given background electrolyte, the model allows assessment of the symmetry and width of any analyte peak. The concept of the peak shape diagram is introduced where the velocity slope contours are plotted in a ionic mobility-pK coordinate system and its utility for fast and easy predicting of the symmetry and asymmetry of peaks as well as of their migration (detection) order is demonstrated.