OPTIMIZATION OF DETECTION SENSITIVITY IN THE CAPILLARY ELECTROPHORESIS OF INORGANIC ANIONS

Published reports describing applications of various capillary electrophoresis methods to the analysis of low-molecular-weight anionic species are reviewed and evaluated. An attempt is made to summarize the parameters relevant to the optimization of detection sensitivity in capillary electrophoresis. Based on a comparison with the results obtained by other techniques, indirect photometry is shown to have a number of important advantages for the detection of anions. For a given carrier electrolyte, the range of linearity can be extended by increasing the concentration of the anion responsible for background UV absorption. With chromate-containing carrier electrolytes the linearity range could be extended over more than three orders of magnitude. Good correlation between the relative molar absorptivities and sensitivity was found for six aromatic carboxylic acids serving as carrier electrolytes. Using optimized conditions for electromigrative sample introduction, the achievable detection limits are found to be in the low nanomolar range. This represents more than a 100-fold increase in sensitivity over the results obtained by hydrodynamic sample introduction. An isotachophoretic steady state occurring during the electromigration is presented as an explanation for the trace enrichment of low levels of analytes during sample introduction by electromigration.