System peaks in capillary zone electrophoresis II. Experimental study of vacancy peaks

An uncoated capillary with selected background electrolytes (BGEs) was used to study the migrating vacancies of UV-absorbing components of the buffer. Picric and salicylic acids were chosen as the UV-absorbing anionic species, to be used either as samples in normal capillary electrophoresis (CE) mode (direct CE) or as BGE components (reverse CE). To study the effect of BGE composition, 2-morpholinoethanesulfonic acid and monochloroacetate, with lower and higher mobilities, respectively, were selected as two different BGE co-ions of picrate and salicylate. In normal CE mode, anionic species provided tailing or fronting peaks (representing fronting or tailing migrating zones), depending on whether their effective mobilities were lower or higher than the mobility of the BGE co-ion, respectively. In reverse CE, the effect was the opposite. Picrate and salicylate vacancies were represented by fronting or tailing dips (migrating as tailing or fronting zones) when their mobilities were higher or lower than that of the BGE co- ion, respectively. The electroosmotic peak may be negative or positive, according to the original value of the Kohlrausch regulating function of the sample. Finally, the difference between migrating vacancies and indirect detection is demonstrated and explained, showing indirect detection of a mixture of potassium and sodium acetate detected simultaneously with a benzoic acid vacancy.