Separation of positional isomers of organic acids and dyestuff food additives by nonaqueous capillary electrophoresis : Environmental Analysis Using Capillary Electrophoresis and Related Techniques, Including Capillary Electrochromatography

The merits of capillary electrophoresis in pure organic solvents were investigated for the separation of closely related positional isomers of dinitrobenzoic acid and isomeric pairs of dyestuff food additives, amaranth-new coccine and light green SF yellowish-brilliant blue. Instead of ordinary buffer salts, cetyltrimethylammonium bromide (CTAB), in methanol or methanol-acetonitrile, and sodium dodecyl sulfate (SDS), in methanol-ethanol, were the media used for the separation, in which a complete resolution of the tested compounds was achieved in a relatively short time. Both nonaqueous systems showed low levels of generated electric current, especially SDS in methanol-ethanol, which showed submicro ampere levels. As a result of low-joule heating and diminished adsorption of solutes on the capillary wall, a relative standard deviation of <0.4% was obtained for the migration times without use of temperature control and a washing step between runs. Addition of urea to the CTAB/methanol system was used for manipulation of electroosmotic flow, analysis time, and resolution. Although elution of the tested compounds in CTAB/acetonitrile was very fast, methanol-acetonitrile was found to be better for optimizing separation and analysis time. For the CTAB/methanol system, the separation pattern was independent of the sample matrix because of the opposite directions of the electroosmotic flow and the electrophoretic mobility of the anionic organic acid isomers. This caused removal of samples constituents including neutral and positively charged species from the capillary after the start of the electrophoretic run.