Optimization of cation detection by capillary zone electrophoresis with indirect UV absorbance

The indirect UV detection of cations by capillary zone electrophoresis gives peaks of very different height or area between cations. We show that the height depends on the electrophoretic mobility of the cation compared with the electrophoretic mobility of the chromophore used. Moreover, the limit of detection can be easily improved by about Lt-fold by adjusting the concentration of complexing agent or by using a chromophore with the same velocity as the studied cation. Using a new parameter termed sensitization power we can optimize the limit of detection and have determined the best chromophores for each family of cations. Sensitization power is a maximum for: alkali metals and alkali earths with imidazole and UV Cat 2 (R), for transition metal with pyridine, for light rare earth with ephedrine and UV Cat 1 (R), for heavy lanthanides with 1-naphthylamine and phenyl-trimethyl-ammonium bromide. Corrected areas of all cations studied have normal distributions directly proportional to their ionic charge. Certain behaviour can be predicted for elements such as actinides. Detection can also be enhanced because the relation of proportionality between ionic charge and corrected area allows comparison of the performance of all chromophores.