System peaks and disturbances to the baseline UV signal in capillary zone electrophoresis

Non-steady-state processes in capillary electrophoresis can be estimated by applying a steady-state mathematical model. Calculations with a steady-state model indicate that in capillary electrophoresis, moving boundary zones can originate from discontinuities in the concentration of the co-ions and/or the pH of the background electrolyte. Calculations showed that cationic moving boundaries with high mobilities originate with low system pH values. If the separation capillary and anode compartment are filled with electrolytes, different in concentration or pH, a shift of the baseline UV signal can occur. Block-shaped discontinuities in pH and/or concentrations split up in a migrating part with a mobility determined by the composition of the background electrolyte and a part migrating with the velocity of the electroosmotic flow at the position of the original disturbance. As a result, dips of the electroosmotic flow marker (low background concentration) split up and a negative system peak migrates through the system at low system pH values. Injections of high concentrations of background electrolyte or samples at high ionic strength lead to positive system peaks. These system peaks are, of course, only visible if the background electrolyte shows UV-absorbing properties. Experimentally determined data match the calculated values for these mobilities and baseline shifts.