Approaches used in the reduction of buffer electrolysis effects for routine capillary electrophoresis procedures in pharmaceutical analysis.

Electrolysis of the electrolyte in the buffer reservoirs can occur when performing multiple assays in capillary electrophoresis (CE). This effect, which has been termed buffer depletion, results in a pH gradient being formed across the capillary. This gradient can effect both migration time, peak efficiency and selectivity and therefore can cause problems of reproducibility which leads to poor quantitative assays in pharmaceutical analysis. In this study the factors affecting the extent of buffer depletion were investigated. This involved performing a series of multiple assay runs where a number of factors were varied such as the use of dilute or concentrated buffer solutions and single or separate vials for pre-rinse and separation. In addition, the effect of the volume of buffer in the separation vials on the degree of depletion was also examined using standard (4 ml) and small volume (0.3 ml) CE vials. From these studies, it was observed that long multiple assay runs should not be carried out with small volumes of dilute buffer when pre-rinse and separation steps are from the same vial. The rate of electrolysis also increased when using wide-bore capillaries and at elevated temperatures. The use of short-end injections, zwitterionic buffers and reversing the polarity were found to appreciably reduce the effects of buffer depletion. From this work a number of recommendations are provided to maximise the number of acceptable separations that can be obtained in routine CE injection sequences.