Flow manipulation for sweeping with a cationic surfactant in microchip capillary electrophoresis

Abstract

Flow manipulation in sweeping microchip capillary electrophoresis (CE) is complicated by the free liquid communication between channels at the intersection, especially when the electroosmotic flows are mismatched in the main channel. Sweeping in traditional CE with cationic micelles is an effective way to concentrate anionic analytes. However, it is a challenge to transfer this method onto microchip CE because the dynamic coating process on capillary walls by cationic surfactants is interrupted when the sample solution free of surfactants is introduced into the microchip channels. This situation presents a difficulty in the sample loading, injection and dispensing processes. By adding surfactant at a concentration around the critical micelle concentration and by properly designing the voltage configuration, the flows in a microchip were effectively manipulated and this sweeping method was successfully moved to microchip CE using tetradecyltrimethylammonium bromide (TTAB). The sweeping effect of cationic surfactant in the sample solution was discussed theoretically and studied experimentally in traditional CE. The flows in a microchip were monitored with fluorescence imaging, and the injection and sweeping processes were studied by locating the detection point along the separation channel. A detection enhancement of up to 500-fold was achieved for 5-carboxyfluorescein.

Separation of natural antioxidants using PDMS electrophoresis microchips coupled with amperometric detection and reverse polarity

“…Since all the analytes are anions in this pH range, the injection and separation steps were performed with reverse polarity. A cationic surfactant (TTAB) was added to the running buffer in order to reverse the direction of the EOF .…”

Sample preconcentration in microfluidic devices

“…The partition coefficient, k, defined as the ratio of analyte numbers in the micellar phase to that in the water phase, determines the preconcentration extents of sweeping. For neutral analytes, both anionic and cationic surfactants exhibit strong interaction with analytes; for charged analytes, the oppositely charged surfactants have stronger interaction with analytes to obtain high preconcentration effect (Gong et al 2007;Kim et al 2001;Liu et al 2005).…”

“…Sweeping with cationic surfactants is an effective way to concentrate anionic analytes. For example, a cationic surfactant, tetradecyltrimethylammonium bromide (TTAB) was used to concentrate 5-carboxyfluorescein by obtaining a CF of up to 500 (Gong et al 2007) in a simple cross microchip. Although sweeping is an effective concentration method, a large volume of samples is typically injected for obtaining the desired concentration effect, leading to the requirement for a longer separation channel to obtain reasonable separation resolution.…”

Electrophoretic separations in poly(dimethylsiloxane) microchips using a mixture of ionic and zwitterionic surfactants

“…The improved electrochemical response for some species in the presence of surfactants can be attributed to increased solubility of the oxidation product of the electrochemical reaction [39]. In addition, cationic surfactants such as cetyltrimethylammonium bromide (CTAB) [40, 41], tetradecyltrimethylammonium bromide (TTAB) [42], and didodecyldimethylammonium bromide (DDAB) [43] have been added to reverse the EOF direction in microchip CE. Zwitterionic surfactants have also been used in both traditional and microchip CE.…”