Optimisation of indirect UV detection in capillary zone electrophoresis of low-molecular-mass anions

Indirect UV detection, in which a UV absorbing probe is added to the carrier electrolyte, is used frequently in capillary zone electrophoresis (CZE) for detection of UV inactive analytes. Sensitivity is governed by the molar absorptivity of the probe, its charge and the transfer ratio, which is the number of moles of probe ions displaced by one mole of sample ions. Equivalent-to-equivalent displacement occurs only when the mobility of the probe is equal to the mobility of the analyte. Transfer ratios can be calculated from the Kohlrausch theory and have been experimentally verified for several low-molecular-mass anions. Highest values of transfer ratios have occurred when the carrier ion had a higher mobility than the analyte ion. The general trend of the experimental data follows the theory, but still considerable deviations between measured and calculated data have been encountered. Besides detection sensitivity, separation selectivity must be optimised as well. Examples given in this article demonstrate that the composition of the carrier electrolyte has to be a compromise between an optimum in sensitivity and an optimum in resolution.