Droplet microfluidics for postcolumn reactions in capillary electrophoresis

A postcolumn reaction system based on droplet microfluidics was developed for capillary electrophoresis (CE). Analytes were separated using capillary zone electrophoresis (CZE) and electrophoretically transferred into droplets. The use of a micro cross for positioning a salt bridge-electrode opposite the separation capillary outlet is the key element for maintaining the electrical connection during electrophoretic separation. As the first of its kind, positioning the droplets in the electric field eliminated the need for electroosmotic flow (EOF) or hydrodynamic flow for droplet compartmentalization. Depending on the total flow rate of both aqueous and oil phases, droplets of water-in-oil could be formed having frequencies between 0.7 and 3.7 Hz with a size of approximately 14 nL per droplet. Compartmentalized in the droplets, analytes reacted with reagents already present in the droplets to facilitate detection. The periodate oxidation of paralytic shellfish toxins (PSTs) was demonstrated, overcoming the limitation of precolumn oxidation, which results in multiple and sometimes identical oxidation products formed from the different PSTs. Compartmentalization allows the oxidation products for each peak to be contained and to contribute to a single fluorescence signal, preserving the selectivity of CZE separation while gaining the sensitivity of fluorescence detection.