Capillary Electrophoresis of Inorganic Anions in Nonaqueous Media with Electrochemical and Indirect UV Detection

Abstract

Methanol and dimethylformamide were evaluated for the separation of a series of inorganic anions. As these solvents were added to an aqueous electrolyte, the electroosmotic flow went through a minimum at 70-80% (v/v) organic solvent and then increased as the composition approached that of the pure solvent. Reproducibility of electroosmotic flow was good (%RSD=1.1 for dimethylformamide), and Joule heating was not as important as in water systems. The effect of electrolyte concentration on electroosmotic flow was similar to that expected for aqueous solutions, but significant differences in separation selectivity were observed, with many ions showing reversed separation order compared to aqueous systems. In addition, there was some evidence of ion association effects that led to changes in selectivity as a function of concentration and nature of the electrolyte. The application of indirect UV detection in methanol was briefly evaluated with chromate, phthalate, and benzoate electrolytes. Phthalate electrons were slightly less sensitive than chromate, but chromate was not stable in methanol. Calibration of 11 inorganic anions separated in a phthalate electrolyte gave linear curves in the range of 5×10 -5 -8×10 -4 mol/L; detection limits were in the range of 2.0×10 -5 -3.4×10 -5 mol/L. Amperometric detection at a 25 μm Pt disk electrode gave detection limits in the range of 1×10 -9 -6×10 -8 mol/L for some anions, but calibration curves were linear only over small ranges of concentration. Peak shapes obtained with the electrochemical detection systems were good, and theoretical plate counts were in the range of 140 000-450 000