Enhanced radial dispersion in open tubular column chromatography

Abstract

Abstract. Radial diffusion of solutes in the mobile phase limits the practical linear velocities that can be used to increase speed of analysis in conventional open tubular column chromatography. Several methods have been devised to enhance radial dispersion that do not depend only on molecular radial diffusion. In this review, the methods of enhancing dispersion found in the chromatographic literature are discussed. These methods include the use of deformed columns, and spinning band and static mixer columns. These techniques have been found to be of little value in analytical chromatography. Electroosmotic flow has been used to generate a plug-like flow profile in LC and generates one to two orders of magnitude lower plate height than produced at laminar flow conditions. Turbulent and secondary flow have been used to enhance radial diffusion in open tubular columns for gas, liquid, and supercritical fluid chromatography. These techniques are described from their theoretical and practical aspects. Secondary flow has also been studied by several investigators for the enhancement of radial dispersion. A circular, radial flow profile can be generated by centrifugal forces in tightly coiled columns, and after a critical velocity is achieved, columns coiled to appropriate aspect ratios yield lower plate heights than straight columns for unretained solutes as the velocity is increased. Although retained solutes yield higher plate heights in coiled rather than conventional straight columns in some cases due to the resistance to mass transfer at the interphase region, higher speeds of analysis are obtained. Secondary flow chromatography has been successfully applied in gas and supercritical fluid chromatography for the rapid separation of alkanes having low retention. Nevertheless, applications using secondary flow in chromatographic separations are limited.

Concentration Polarization-based Nonlinear Electrokinetics in Porous Media: Induced-Charge Electroosmosis

“…Particularly, the nonlinear macroscopic EOF dynamics (Figure ), relatively extended induced space charge regions (Figure ), flow instability (Figure ), and the resulting pore-scale dispersion (Figure ) constitute key differences. Thus, induced-charge electroosmosis can become a promising tool for process intensification, e.g., in electrodialysis (membrane transport) which is severely limited by stable CP, , or in miniaturized devices concerning the nonlinear pumping of bulk fluid and efficient mixing at low Re, i.e., Re < 1. , In separation science, the realization and control of chaotic instability and turbulent flow over defined temporal and spatial domains has been a challenging task for a long time. − In this respect, nonlinear electrokinetics analyzed in this letter is a unique mechanism for reducing axial dispersion by faster lateral exchange of analytes beween flow velocity extremes during the long-distance transport through porous media employed, e.g., in electrochromatography. Future work needs to address the influence of the applied field and mobile phase ionic strengths, local hydrodynamics, morphology of the pore space, and surface physicochemical properties on the development and consequences of induced-charge electroosmosis in porous media.…”

MEMS Devices for Miniaturized Gas Chromatography

“…In conventional gas chromatography, used columns are tubes functionalized by a stationary phase having length ranging from 10 to 100 m. To obtain an excellent column, few parameters can be optimized: length, inner diameter, film thickness, and the coiling radius [37]. Theory of chromatography predicts an increase of efficiency, while the diameter of a capillary column decreases.…”

Axial mixing with spiral-shaped channel micromixer

“…Such flow distribution is through electroosmotic flows (Ghosal, 2004 andZholkovskij, et al, 2004,), redox-magnetohydrodynamics (Anderson, et al, 2010, Weston, et al, 2012, and Sahore, et al, 2013, or the optimized geometry of the channel cross section under pressure driven flows (Dutta, et al, 2001, Ajdari, et al, 2006, and Parks, et al, 2007. Increasing apparent radial dispersion (Sumpter, et al, 1991) is another way for reducing axial dispersion. Utilizing turbulent flows is one of the methods for increasing radial dispersion.…”