Transfer ratio in indirect UV detection in capillary zone electrophoresis: A mathematical approach

Abstract

Generally, the transfer ratio (TR) is defined as the number of molecules of the background electrolyte (BGE) with UV absorbing properties, displaced by each analyte molecule, and this concept plays an important part in the choice of the BGE in capillary zone electrophoresis (CZE) in the indirect UV mode. Less transparent is this concept and its importance, applying BGEs with more than one co-ions, in the application of UV absorbing counterions and for the use of BGEs at low or high pH values. In this paper, the concept of the TR is discussed for these cases and applying a mathematical model, with which all parameters in the sample zones can be calculated, the TR values are calculated for several BGEs. Hereby, different values are obtained for anionic and cationic TR values for all ionic species of the BGE. Discontinuities in the relationships of the calculated TR values versus the mobilities of the sample components indicate that system peaks are present in the electropherograms applying BGEs with two co-ions and also applying BGEs with a single co-ion at both high and low pH values. This means that in the latter cases OH − and H + ions act as a second co-ion, whereby the mobilities of the system peaks generally increase for higher and lower pH values, respectively. If the mobilities of the analytes approach the mobility of a system peak, the TR values increase considerably. These phenomena are confirmed experimentally.