Sample preconcentration by field amplification stacking for microchip-based capillary electrophoresis

A microchip structure for field amplification stacking (FAS) was developed, which allowed the formation of comparatively long, volumetrically defined sample plugs with a minimal electrophoretic bias. Up to 20-fold signal gain s were achieved by injection and separation of 400 mum long plugs in a 7.5 cm long channel. We studied fluidic effects arising when solutions with mis matched ionic strengths are electrokinetically handled on microchips. In pa rticular, the generation of pressure-driven Poiseuille flow effects in the capillary system due to different electroosmotic flow velocities in adjacen t solution zones could clearly be observed by video imaging. The formation of a sample plug, stacking of the analyte and subsequent release into the s eparation column showed that careful control of electric fields in the side channels of the injection element is essential. To further improve the sig nal gain, a new chip layout was developed for full-column stacking with sub sequent sample matrix removal by polarity switching. The design features a coupled-column structure with separate stacking and capillary electrophores is (CE) channels, showing signal enhancements of up to 65-fold for a 69 mm long stacking channel.