Unlimited-volume electrokinetic stacking injection in sweeping capillary electrophoresis using a cationic surfactant

Sweeping is an effective and convenient way for online sample preconcentration in micellar electrokinetic chromatography. The usual procedure includes a hydrodynamic injection step carried out by applying pressure to the sample vial followed by the subsequent sweeping and separation processes. The injected sample volume is limited by the dimensions of the capillary because a part of the capillary has to be left free of sample solution for the subsequent sweeping and separation steps. In addition, when a short capillary, such as 4-10 cm, is used for sweeping, the injected sample volume is small even if the entire capillary is filled with sample solution. To solve this problem, an electrokinetic stacking injection (EKSI) scheme was developed by using a cationic surfactant, dodecyltrimethylammonium bromide, for sweeping in capillary electrophoresis. An experimental model was proposed, and the entire process was theoretically analyzed. According to the theoretical discussion, the optimal conditions for two model analytes, 5-carboxyfluorescein (5-FAM) and sodium fluorescein (FL), were experimentally determined. The injected sample plug lengths for 5-FAM and FL under 20.1 kV for 60 min were experimentally estimated as 836 and 729 cm, corresponding to 28- and 24-fold the effective capillary length, respectively. The EKSI scheme resulted in increased detection factors for 5-FAM and FL of 4.5 x 10(3) and 4.0 x 10(3) using 60-min injection relative to a traditional pressure injection.