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 microns diameter) were trapped. ODS beads were mobilized into and out of the cavity using electroosmotic flow 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 flow of 1.2 nL/s. The material retained on the ODS phase was then eluted by electroosmotic flow of acetonitrile with a concentration enhancement of up to 500 times. Capillary electrochromatography was conducted using a similar device. BODIPY and fluorescein were loaded onto a 200-micron-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-micron plate height.