Dispersion in Large Aspect Ratio Microchannels for Open-Channel Liquid Chromatography

Solute dispersion in open-channel liquid chromatography is often dominated by transverse diffusion limitations in the mobile phase (Martin, M.; Guiochon, G. Anal. Chem. 1984, 56, 614−620) convecting the solute species. While such dispersion is known to scale with the square of the Peclet number based on the narrower dimension of the conduit, the proportionality constant may significantly vary with the aspect ratio of the channel geometry. In this article, we investigate the effect of channel sidewalls on axial dispersion in electrokinetically and pressure-driven chromatographic systems. The analysis presented here clearly identifies the contribution from flow, wall retention, and the interaction between the two to the overall slug dispersion in the mobile phase for any arbitrary channel geometry. The particular geometries that have been investigated in this work, however, are the rectangular and the isotropically etched profiles often employed in microanalysis systems. Further, the effectiveness of simple double-etched profiles proposed elsewhere (Dutta, D.; Leighton, D. T. Anal. Chem. 2001, 73, 504−513) to diminish the effect of channel sidewalls on Taylor−Aris dispersion has also been examined. Analysis shows that dispersion arising due to shear and wall retention, as well as the interaction between the two, may be significantly reduced in large aspect ratio microchannels for optimized channel geometries.