Peaks in capillary zone electrophoresis: fact or fiction.

Non-steady-state electrophoretic processes can be estimated by a repeated application of a steady-state model based on the electroneutrality equation, the modified version of Ohm's law, and the mass balances of the co- and counterions. With such a mathematical model, all parameters in sample zones in capillary zone electrophoresis (CZE) can be calculated. The relationships between the calculated parameters for sample zones in CZE, such as the pH, concentrations of co- and counterions, and the ratio E1m1/E2m2 versus the mobilities of both anionic and cationic analytes can be visualized in a SystChart, a set of eight panels, for a given background electrolyte (BGE). All properties of a zone, such as the fronting/tailing character and the question of peaks/dips can be read from such a SystChart. Applying n coions, n-1 discontinuities are present in such a SystChart, indicating the presence of system peaks applying that BGE. For BGEs with one coion, no system peaks (discontinuities) exist at moderate pH values. SystCharts calculated for BGEs with a low pH do show discontinuities, however, which indicates that system peaks are present in electropherograms applying BGEs at low pH. Experimentally, it is shown that system peaks are indeed present in electropherograms applying BGEs with one coion at low pH and the mobilities of the system peaks generally increase with decreasing pH. Hydrogen ions seem to act as a second coionic species. Of course, these system peaks are only visible in the UV signal if the BGE has UV-absorbing properties.