Calculation of the composition of sample zones in capillary zone electrophoresis .4. Weak acids and system characteristics

In this final part, Part IV, of a series of papers about the calculation of parameters in sample zones in capillary zone electrophoresis by a repeated application of a steady-state model, the mathematical model is extended for the calculation of parameters in sample zones of weak anionic analytes, applying background electrolytes (BGEs) with more than one co- and/or counter-ionic species. From the calculations, it appeared to be practically impossible to formulate general rules for the description of the characteristics of anionic and cationic sample peaks in a particular BGE, especially applying more co-ionic species. However, it is possible to describe the most important system characteristics by means of a set of eight figures, which will be called a SystChart. Four figures of such a SystChart are used for the visualization of the relationships between the mobilities at infinite dilution of cationic analytes and the pH, the ratio E(1)m(1)/E(2)m(2), the total concentrations of the cations and of the anions of the BGE in a sample zone with a sample concentration of 5.10(-4) M, respectively, and four figures for the description of the same relationships for anionic analytes. In this way, the most important parameters, from which several peak characteristics can be deduced, such as the fronting/tailing character and peaks/dips for both cationic and anionic sample ionic species, are visualised for a particular BGE. Descriptions are given of SystCharts for BGEs consisting of (1) 0.01 M imidazole and 0.02 M acetic acid at pH 4.7, (2) 0.02 M imidazole and 0.01 M acetic acid at pH 7.0 and (3) 0.02 M histidine, 0.005 M butyric acid and 0.005 M hydrochloric acid at pH 6.1 (two anionic co-ions).