Simultaneous separation of anions and cations by capillary electrophoresis with high magnitude, reversed electroosmotic flow

A method for simultaneous separation and indirect detection of anions and cations by capillary electrophoresis (CE) is reported. An anodic electroosmotic flow (EOF) in excess of −100 × 10−9 m2 V−1 s−1 was achieved by the addition of 0.1 mM didodecyldimethylammonium bromide (DDAB) to the background electrolyte (BGE). This high magnitude EOF enabled rapid co-EOF separation of inorganic anions and counter-EOF separation of inorganic cations having high electrophoretic mobilities. Single-end injection of the sample at the capillary inlet and detection near the capillary outlet was used. Indirect photometric detection of cations was accomplished using a cationic probe (imidazole), with counter-ionic (Kohlrausch) indirect detection being used for anions. A mixture consisting of chloride, bromide, nitrate, lithium, magnesium, sodium, calcium, potassium, and ammonium was separated in less than 3.5 min using a background electrolyte comprising 6 mM imidazole, 12 mM hydroxyisobutyric acid, 0.1 mM DDAB, 2 mM 18-crown-6 at pH 4.0, with indirect detection at 210 nm. The system was well suited to the separation and detection of inorganic cations, with limits of detection (LODs) in the range 0.92–4.65 μM and separation efficiencies from 44 400–208 5000 theoretical plates. By contrast, LODs for inorganic anions ranged from 6.66 to 11.43 μM, with separation efficiencies of 2500–26 300. Analytical potential of the method is discussed.