Extremely large volume electrokinetic stacking of cationic molecules in MEKC by EOF modulation with strong acids in...

In this report, a novel facile way of online preconcentration of trace levels of analytes in capillary electrophoresis is presented. The proposed strategy is based on the combination of strong acidic phosphate as sample buffers with borate separation buffer containing sodium dodecyl sulfate. When injection voltage is applied, the continuous introduction of low pH sample causes the apparent bulk flow inside the capillary gradually slows down. Finally at a certain point, it reaches the same magnitude as that of the oppositely migrating anionic micelles, thus the frontier of the micelle zone becomes stagnant. This steady state can be maintained for a very long time so that essentially extremely large volume of sample solutions can be injected into the capillary, and the cationic analytes may be efficiently stacked at the neutralized micelle zone. A theoretical model was proposed and preconcentration conditions of two model analytes, matrine and oxymatrine, were optimized with the aid of the model. Under optimized conditions, more than 1000-fold increase in sensitivity was obtained as compared with the normal hydrodynamic injection without sample stacking. The limits of detection for oxymatrine and matrine were 0.81 and 0.18 ng/mL, respectively, using photodiode array UV detection at wavelength 211 nm.