Electrophoresis plate height

C.S. Effenhauser, A. Manz, and M. Widmer, Glass chips for high-speed capillary electrophoresis separations with submicrometer plate heights. Anal. Chem. 65, 2637-2642 (1993). 

Teflon tubes and obtained separations with plate heights less than 10 pm. They also provided a theoretical basis for migration dispersion in free zone electrophoresis. 

In packed columns, all three terms contribute to band broadening. For open tubular columns, the multiple path term, A, is 0, so bandwidth decreases and resolution increases. In capillary electrophoresis (Chapter 26), both A and C go to 0, thereby reducing plate height to submicron values and providing extraordinary separation powers. 

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... 

The main argument for making MIP CEC is to combine the selectivity of the MIPs with the high separation efficiency of CEC. This argument appears to fail, however, if the adsorption isotherm of the MIP is nonlinear, which seems to be the rule. In the case of nonlinear isotherms, the peak shapes depend mainly on the isotherm, particularly so if the separation system is otherwise very efficient (has low theoretical plate height, see Fig. 1). In the case of ionized analytes the situation is more complex. If an ionized analyte is not adsorbed at all on the MIP, then it is separated only due to electrophoresis, and its peak will not be widened due to the nonlinear effect. In this case, however, the MIP is merely behaving like an inert porous material. In intermediate cases an ionized analyte may participate in both separation mechanisms and for this case we do not have exact predictions of the peak shape. 

FIGURE 6.10 Van Deemter plots for capillary electrophoresis ( ) and microchip electrophoresis (o), where plate height (H) of TRITC-labeled bovine serum albumin is plotted vs. migration velocity (v) [174]. Reprinted with permission from the American Chemical Society. 

There is much interest in high-efficiency- and high-speed separation media for liquid chromatography. The plate numbers available in practice have been in the range of 10,000-30,000 in HPLC for 20 years or so, but these are low compared to well over 100,000 theoretical plates in capillary gas chromatography or in capillary electrophoresis. This is caused by the limitation in the use of small-sized particles for HPLC, where a particle-packed column is commonly used under a pressure-drop of up to 40 MPa. An increase in column efficiency by using small particles, which is the approach taken in the past, is accompanied by an increase in the pressure-drop, as expected from Eqns. 5.2 and 5.3, below. Eqns. 5.1-3 describe the efficiency (plate height) and flow resistance of a column packed with particles [1-3], where N stands for the... 

Electroosmotic flow, described in Section 4.9, is another complicating factor in electrophoresis. The electroosmotic flow process is often responsible for nonselective ion transport superimposed on the electrophoretic transport. When electroosmotic displacement is significant, it must be kept in mind that the distance X in the preceding plate height equations is the displacement distance due to electrophoresis alone it does not include electroosmotic displacement. Also the voltage V must be calculated as that applied over the path of electrophoretic displacement only, not including the distance of electroosmotic displacement [41]. 

Davis, J. M., Influence of thermal variations of diffusion coefficient on non-equilibrium plate height in capillary zone electrophoresis, J. Chromatogr., 517, 521,1990. 

A van Deemter plot for capillary electrophoresis is a graph of plate height versus migration velocity, where migration velocity is governed by the net sum of electroosmotic flow and electrophoretic flow. 

Plate height In chromatography and electrophoresis, the measure of the amount of dispersion or spreading of a zone or peak during the separation process. 

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