Simultaneous separation of ammonium and alkali, alkaline earth and transition metal ions in aqueous-organic media by capillary ion analysis

Abstract

The simultaneous separation of the mono-, di- and trivalent cations NH + 4, K +, Na +, Li +, Mg 2+, Ca 2+, Sr 2+, Ba 2+, Mn 2+, Ni 2+, Zn 2+, Cu 2+ and Cr 3+ by capillary ion analysis was studied in a background electrolyte system composed of imidazole, 2-hydroxyisobutyric acid, 18-crown-6 and methanol. The effects of this system on the electroosmotic and electrophoretic mobility were studied in detail. A competitive complexation between 2- hydroxyisobutyric acid and sulphuric acid was observed which determined the cation mobility. Applying a central composite design, a quantitative description of the electrophoretic behaviour of the inorganic cations as a function of the system parameters was made and the optimum separation could be predicted. It was found in most instances that a linear model is sufficient for mobility modelling. Using a full factorial design, the main effects on the electroosmotic flow were estimated and methanol was found to be the dominating factor. The applicability of the capillary electrophoresis method was examined by separating ions in Chinese tea infusions. Five metal cations (K +, Na +, Ca 2+, Mg 2+ and Mn 2+) were detected. A limit of detection at the μg l level could be achieved using electromigration injection.