UV DETECTION IN CAPILLARY ZONE ELECTROPHORESIS PEAKS OR DIPS - THAT IS THE QUESTION

Applying capillary zone electrophoresis with UV detection, both UV-abs orbing and UV-transparent components can be present in electropherogra ms as negative peaks (dips) or as positive peaks. Starting from Kohlra usch's regulation function, derived for fully ionized monovalent ionic constituents and under the assumption that the molar absorptivities o f the UV-absorbing components are identical, eight different cases can be distinguished and in several cases components can occur both as pe aks 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 mobili ties of the two co-ions and determined by the concentration ratio of t hese two co-ions. In the background electrolytes studied, containing t he co-ions potassium and histidine, UV-transparent sample components w ith a mobility higher than that of the system peak migrate as a positi ve peak, whereas UV-transparent components with lower mobilities migra te as negative peaks. System peaks themselves can also be peaks or dip s 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. S imilar phenomena can be measured for anions in background electrolytes containing the co-ions phenylacetate and acetate, indicating that the se phenomena are probably not associated with adsorption phenomena of cations on the fused-silica surface.