Pillar-structured microchannell's for on-chip liquid chromatography: Evaluation of the permeability and separation performance

The chromatographic characteristics were determined for a set of microfabricated separation channels structured with cylindrical and diamond-shaped pillars with a characteristic size of 5 µm. Channels with different structures and porosities were etched in a silicon wafer using lithographic techniques. The permeability for flow of the channels was shown to increase strongly with the overall porosity. Diamond-shaped pillars appeared to yield a slightly higher permeability than cylindrical pillars at the same channel porosity. Compared to packed columns, permeabilities were higher by a factor of up to 5. Band dispersion in the channels was measured with an unretained fluorescent probe compound using a fluorescence microscope. A relatively large variation in the observed plate heights between channels was found, which was mainly attributed to the inaccurate geometry of the structure close to the side walls. Reduced plate heights between 0.2 and 1.0 were obtained. The lowest plate heights were found for channels with low porosity. The chromatographic impedances were calculated and compared to the values for the traditional chromatographic systems. For one of the structured microchannels the impedance was found to be more than ten times lower than for a column packed with nonporous spherical particles. With the data collected, predictions are given on the possibilities in terms of efficiency and speed offered by structured microchannels for pressure-driven separations, taking practical constraints into account.