Electroosmotic mobility flow profile

Electroosmotic flow in a capillary also makes it possible to analyze both cations and anions in the same sample. The only requirement is that the electroosmotic flow downstream is of a greater magnitude than electrophoresis of the oppositely charged ions upstream. Electro osmosis is the preferred method of generating flow in the capillary, because the variation in the flow profile occurs within a fraction of Kr from the wall (49). When electro osmosis is used for sample injection, differing amounts of analyte can be found between the sample in the capillary and the uninjected sample, because of different electrophoretic mobilities of analytes (50). Two other methods of generating flow are with gravity or with a pump. 

Depending on the sign of the zeta potential, EOF can be towards either the anode or cathode, and the apparent mobility of an analyte is the sum of its electrophoretic mobility and EOF. Electroosmotic forces act near the capillary wall, which results in a flat flow profile that is less dispersive than the parabolic flow profile associated with pressure-driven flows. EOF can be beneficial since it can enable the analysis of both anions and cations in a single run. Electroosmosis has been investigated extensively as a means for producing fluid flow for various processes in microfluidic devices [227-232]. 

Figure 1.5. Mobile phase flow profile for an open tube and a packed column with pressure-driven and electroosmotic flow.

The primary advantage of such an approach is that it creates a flat flow profile, and thus allows definition of precise flow rates and narrow residence time distributions. Nevertheless, electroosmotic flow (EOF) pumps are severely limited in their widespread application to molecular synthesis due to a need for a conductive solvent and the fact that varying electrophoretic mobilities of reagents and products leads to time-dependent concentration gradients within the reactor that can degrade performance. 

The electroosmotic mobility and bulk EOF velocity are sensitive to the physicochemical properties of the solution and channel walls. Because electroosmosis is a surface-driven phenomenon, EOF fluid transport is characterized by plug flow. Except in the thin EDL, there is no shear in the fluid flow and so the velocity profile is flat. At the same time, the individual species charged permanently or spontaneously in the bulk fluid also undergo electric forces. 

Electroosmotic flow has emerged as a viable alternative transport mechanism to pressure-driven flow in column chromatography. Benefits include a plug-flow profile (reduced transaxial contributions to zone broadening) and a mobile phase velocity that is independent of the column length and particle size. The electroosmotic-driven flow is governed by the dielectric constant of the mobile phase, the zeta potential at the stationary phase/mobile phase interface, and the applied electric field. The efficiency obtainable is limited by double layer overlap or radial dispersion induced by inefficient heat dissipation. 

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. 

True electroosmotic flow has been demonstrated in horizontally mounted layers at modest field strengths (< 1 kV / cm) with mobile phases of high dielectric constant. Still unclear is which solvents can be used, the need for prewetted layers and ions as current carriers, the effect of local heating on zone profiles, and the effect of binder chemistry on flow, mass transfer, and thermal effects. Compared with capillary flow faster separations have been demonstrated, but the influence of flow velocity on efficiency was only treated in a qualitative sense. So far no comprehensive analysis of the kinetic properties of separations under conditions of electroosmotic flow have been performed in thin-... 

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