Apparent baseline irregularities for neutral markers in capillary zone electrophoresis with electroosmotic flow

An explanation is given for experimentally observed baseline irregularities in the trace obtained from the UV absorbance detector in capillary zone electrophoresis. Such disturbances often occur as positive or negative jumps on both sides of the signal of the neutral marker, especially when the marker substance is injected in a solution with an ionic concentration that is different from that of the background electrolyte (BGE). It is demonstrated that a source of these irregularities is the different migration velocity of the concentration boundary between the neutral marker zone and the BGE (which migrates exactly with the velocity of the electroosmotic flow) on the one hand, and the concentration boundary between the electrolyte zones having different salt concentrations on the other hand. Such different migration velocities, leading to an apparent de-mixing of the components of the sample zone, are caused by divergent dependencies of the transference numbers on the concentrations of the ions involved. This de-mixing leads to a kind of system peak resulting from the recorded sum of the absorbance signals of the BGE and the neutral marker. The approach to explain experimentally found irregularities is supported by the results of a computer simulation.