UV detection in capillary zone electrophoresis peaks or dips - that is the question

Abstract

Applying capillary zone electrophoresis with UV detection, both UV-absorbing and UV-transparent components can be present in electropherograms as negative peaks (dips) or as positive peaks. Starting from Kohlrausch's regulation function, derived for fully ionized monovalent ionic constituents and under the assumption that the molar absorptivities of the UV-absorbing components are identical, eight different cases can be distinguished and in several cases components can occur both as peaks or as dips depending on their mobilities and those of the co- ions of the system. Applying background electrolytes containing two co-ions, system peaks are present, with a mobility that is between the mobilities of the two co-ions and determined by the concentration ratio of these two co-ions. In the background electrolytes studied, containing the co-ions potassium and histidine, UV-transparent sample components with a mobility higher than that of the system peak migrate as a positive peak, whereas UV-transparent components with lower mobilities migrate as negative peaks. System peaks themselves can also be peaks or dips depending on the sample composition. Sample peaks in the vicinity of system peaks interact with the system peaks through which both sample and system peaks are enlarged and quantitative properties are lost. Similar phenomena can be measured for anions in background electrolytes containing the co-ions phenylacetate and acetate, indicating that these phenomena are probably not associated with adsorption phenomena of cations on the fused-silica surface.