Development of a positive pressure driven micro-fabricated liquid chromatographic analyzer through rapid-prototyping with poly(dimethylsiloxane) - Optimizing chromatographic efficiency with sub-nanoliter injections

A rapid and low-cost means of developing a working prototype for a positive -displacement driven open tubular liquid chromatography (OTLC) analyzer is demonstrated. A novel flow programming and injection strategy was developed and implemented using soft lithography, and evaluated in terms of chromato graphic band broadening and efficiency. A separation of two food dyes serve d as the model sample system. Sample and mobile phase flowed continuously b y positive displacement through the OTLC analyzer. Rectangular channels, of dimensions 10 mu m deep by 100 mu m wide, were micro-fabricated in poly-di methylsiloxane (PDMS), with the separation portion 6.6 cm long. Using a nov el flow programming method, in contrast to electroosmotic flow, sample inje ction volumes from 0.5 to 10 nl were made in real-time. Band broadening inc reased substantially for injection volumes over 1 nl. Although underivatize d PDMS proved to be a sub-optimal stationary phase, plate heights, H, of 12 mu m were experimentally achieved for an unretained analyte with the recta ngular channel resulting in a reduced plate height, h, of 1.2. Chromatograp hic efficiency of the unretained analyte followed the model of an OTLC syst em limited by mass-transfer in the mobile phase. Flow rates from 6 nl min(- 1) up to 200 nl min(-1) were tested, and van Deemter plots confirmed plate heights were optimum at 6 nl min(-1) over the tested flow late range. Thus, the best separation efficiency, N of 5500 for the 6.6 cm length separation channel, was achieved at the minimum flow rate through the column of 6 nl min(-1), or 3 mi year(-1). This analyzer is a low-cost sampling and chemica l analysis tool that is intended to complement micro-fabricated electrophor etic and related separation devices. (C) 2000 Elsevier Science B.V. All rig hts reserved.