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 A— 10 -9 m 2 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 M 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. © 2004 Elsevier B.V. All rights reserved.