Multiple paths, chromatography

Capillary electrophoresis provides unprecedented resolution. When we conduct chromatography in a packed column, peaks are broadened by three mechanisms in the van Deemter equation (23-33) multiple flow paths, longitudinal diffusion, and finite rate of mass transfer. An open tubular column eliminates multiple paths and thereby reduces plate height and improves resolution. Capillary electrophoresis reduces plate height further by knocking out the mass transfer term that comes from the finite time needed for solute to equilibrate... 

In 1957 Marcel Golay published a paper entitled Vapor Phase Chromatography and the Telegrapher s Equation [Anal. Chem., 29 (1957) 928]. His equation predicted increased number of plates in a narrow open-tubular column with the stationary phase supported on the inner wall. Band broadening due to multiple paths (eddy diffusion) would be eliminated. And in narrow columns, the rate of mass transfer is increased since molecules have small distances to diffuse. Higher flow... 

Figure 3.6 Illustration of the effect on analyte band dispersion of multiple paths through stationary phase particles, resulting in a path length difference of 8L. Adapted from Scott, http //www.chromatography-online.org/, with permission.

Figure 23-19 Band spreading from multiple flow paths. The smaller the stationary phase particles, the less serious this problem is. This process is absent in an open tubular column. [Adapted from H m. McNair and E. J. Bonelli, Basic Gas Chromatography (Palo Alto. CA Varlan Instrument Division. 1968).]...

Capillary electrophoresis can provide extremely narrow bands. Three mechanisms of band broadening in chromatography are longitudinal diffusion (B in the van Deemter equation 21-7), the finite rate of mass transfer between the stationary and mobile phases (C in the van Deemter equation), and multiple flow paths around particles (A in the van Deemter equation). An open tubular column in chromatography or electrophoresis reduces band broadening (relative to that of a packed column) by eliminating multiple flow paths (the A term). Capillary electrophoresis further... 

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