Use of electrolytes containing multiple co-anions in the analysis of anions by capillary electrophoresis using indirect absorbance detection

Background electrolytes (BGEs) containing more than one UV-absorbing probe co-anion were investigated as possible means to control peak symmetries and improve the sensitivity of indirect detection in the separation of a mixture of inorganic and organic anions having a range of electrophoretic mobilities. In initial experiments, chloride and propanoate, which do not absorb at the detection wavelength, were added individually to a BGE containing phthalate as the UV-absorbing probe co-anion. The response ratios (i.e., the detector response for an analyte obtained with the BGE containing the probe and added co-anion divided by the response of the BGE containing the probe alone) were found to be dependent on the relative mobilities of the analyte, probe, and co-anion. In general, it was found that the analyte mainly displaced the BGE component to which its mobility was closest and exclusively displaced any BGE component having the same mobility. This behavior was utilized to design BGEs containing multiple probes to improve peak shapes by matching the mobilities of the BGE components with those of the analytes. A BGE comprising chromate and phthalate as probes was used to demonstrate the improvement in peak shapes when such an approach was used. This was further extended to a BGE containing three probes, namely, chromate, phthalate, and benzoate. System peaks were observed for each multiple-component BGE and for n BGE co-anions; n - 1 system peaks were induced. A simple linear function relating the mobility of the system peak for a two-co-anion BGE to the mobilities and relative concentrations of each of the co-anions was derived empirically. Finally, a series of probes was investigated to determine the optimum multiple BGE composition giving the best peak shapes and sensitivity in the separation of a mixture of 15 analytes. The best combination was a two-probe BGE consisting of chromate and 4-hydroxybenzenesulfonic acid.