Trapping of bead-based reagents within microfluidic systems: On-chip solid-phase extraction and electrochromatography

A 330-pL chromatographic bed was fabricated on a glass substrate as part of an electroosmotically pumped microfluidic system. Two weirs within a sample channel formed a cavity in which octadecylsilane (ODS) coated silica beads (1.5-4 mu m diameter) were trapped. ODS beads were mobilized into and out of the cavity using electroosmotic now through a bead-introduction channel which. accessed the cavity. This procedure allowed the beads in the cavity to be repeatedly exchanged. A 1 nM solution of a nonpolar analyte (BODIPY 493/503) in buffer was loaded onto the beads for different lengths of time using an electroosmotic now of 1.2 nL/s. The material retained on the ODS phase was then eluted by electroosmotic now of acetonitrile with a concentration enhancement of up to 500 times. Capillary electro-chromatography was conducted using a similar device. BODIPY and fluorescein were loaded onto a 200-mu m-long chromatographic bed and then separated in an isocratic CEC run with an acetonitrile/buffer mobile phase. Complete separation was achieved in less than 20 s with a 2-mu m plate height.