Forced flow band broadening

The electroosmotic flow profile is very different from that for a phase moving under forced pressure. Figure 12.40 compares the flow profile for electroosmosis with that for hydrodynamic pressure. The uniform, flat profile for electroosmosis helps to minimize band broadening in capillary electrophoresis, thus improving separation efficiency. 

Botz, L., Dallenbach-Toelke, K., Nyiredy, Sz., and Sticher, O. (1992). Characterization of band broadening in forced flow planar chromatography with circular development. J. Planar Chromatogr.—Mod. TLC 5 80-86. 

One of the main advantages of OPLC over TLC is its better specificity and the higher available Rs (e.g. Fig. 22). It is partly a consequence of the reduced band broadening effect. Furthermore, due to the forced flow, HPTLC plates can be used for 15-18 cm or longer development distance in OPLC without decreasing the efficiency of separation (Fig. 16). To obtain optimal specificity (maximal resolution) in OPLC, beside optimalization of the mobile phase, the linear velocity of eluent should also be optimized. 

Compared with HPLC, CEC uses electroosmotic flow (EOF) rather than high pressure to force the mobile phase through the capillary. The advantage of EOF for CEC is that the flow profile is fiat and thus precludes band broadening by trans-channel of radial diffusion. The result is that the number of plates is at least double that of HPLC. Another advantage of CEC is that the packing particles are smaller than those of today s HPLC systems. 

Sample dispersion is another cause of the long lag time in flow injection techniques where an aqueous carrier fluid is used [63,64]. Dispersion is caused by axial mixing of the sample with the carrier stream. This increases the sample volume, resulting in longer residence time in the membrane. Dilution reduces the concentration gradient across the membrane, which is the driving force for diffusion. The overall effects are broadened sample band and slow permeation. 

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