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 W-absorbing probe c o-anion were investigated as possible means to control peak symmetries and improve the sensitivity of indirect detection in the separation of a mixtur e of inorganic and organic anions having a range of electrophoretic mobilit ies. In initial experiments, chloride and propanoate, which do not absorb a t the detection wavelength, were added individually to a BGE containing pht halate as the UV-absorbing probe co-anion, The response ratios (i.e., the d etector 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 analyt e, probe, and co-anion, In general, it was found that the analyte mainly di splaced the BGE component to which its mobility was closest and exclusively displaced any BGE component having the same mobility, This behavior was ut ilized 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 th e improvement in peak shapes when such an approach was used. This was furth er extended to a BGE containing three probes, namely, chromate, phthalate, and benzoate, System peaks were observed for each multiple-component BGE an d for n BGE co-anions; n - 1 system peaks were induced. A simple linear fun ction relating the mobility of the system peak for a two-co-anion BGE to th e mobilities and relative concentrations of each of the co-anions was deriv ed empirically. Finally, a series of probes was investigated to determine t he optimum multiple BGE composition giving the best peak shapes and sensiti vity in the separation of a mixture of 15 analytes. The best combination wa s a two-probe BGE consisting of chromate and 4-hydroxybenzenesulfonic acid.