Electroosmotic flow

In the presence of electroosmotic flow, Eq. (4.3) is more accurately written as 

Electroosmotic flow (EOF) is the term used to describe the movement of a liquid in contact with a solid surface when a tangential electric field is applied.5 This movement is also known as electroosmosis or electrooen-doosmosis. Electroosmotic flow can be eliminated if necessary. However, EOF is often used, when flowing in the same direction as the analytes, to increase the speed with which the analytes reach the detector or, when flowing in the opposite direction from the analytes, to improve resolution. 

Electroosmosis occurs in fused silica capillaries because acidic silanol groups at the surface of the capillary dissociate when in contact with an electrolyte solution6,7 (buffer), according to 

To calculate the electrophoretic mobility of an analyte, the contribution from the electroosmotic flow to the apparent mobility must be known. The most common way to estimate the electroosmotic flow rate is to record the migration time of an injected uncharged marker solute,8,9 which will be carried to the detector under the influence of only the EOF. The electroosmotic flow rate is obtained from the migration time of the neutral marker using Eq. (4.6)10  

Examples of neutral markers include water (in CIE), methanol, acetone, benzene, and dimethyl sulfoxide. 

A highly effective means of reducing natural convection is to reduce channel thickness w. Not only does u(y) at any relative position y/w depend on the square of w, but the reduction of tv in some situations will lead to the more rapid dissipation of heat, thus reducing AT as well. 

The reduction of the thickness of the flow channel, as discussed earlier, is equivalent to introducing more surface area per unit volume of medium. High surface areas inhibit all flow, including natural convective flow. One can increase relative surface areas by going to thinner tubes or channels, or by using a fine granular or porous support medium. Both approaches are used in electrophoresis as discussed in a subsequent chapter. 

More subtle tricks can sometimes be used to fight convection. For example, water reaches its maximum density at 4°C, at which temperature the thermal expansion coefficient y goes to zero. Thus, an aqueous separation medium at 4°C is particularly stable against thermal convection. A far out approach is separation in space where gravity is zero electrophoretic separations have already been carried out in orbiting satellites. 

Electroosmotic flow is a flow induced by an electrical potential. When a voltage is applied across a packed bed or a capillary tube filled with an aqueous electrolyte solution, the solution begins to flow along the field axis. Such flow is known as electroosmotic flow. 

Electroosmotic flow is a consequence of the way ions are distributed near surfaces. Nearly all surfaces are charged the surface charge attracts a cloud of oppositely charged ions (counterions) into adjacent layers of liquid, forming a double layer. If there is an electrical field component parallel with the surface, that field will pull the counterions along the surface, dragging the solution with it. Thus flow is induced. 

Capillary wall having negative surface charge 

A somewhat different mechanism of water flow due to the motion of cations having a hydration layer (the solvation shell) has been postulated. It employs porous/ion exchange membranes whose pore diameters are in the range of 1-5 nm. When such a membrane is placed between two electrodes containing an aqueous salt solution, and electrolysis takes place on the application of 

Open separators bulk flow parallel to force and CSTSs 

Useful expressions for the EOF velocity can be obtained as follows. By balancing the viscous forces of the fluid and the electrical forces due to the various ions in the solution, we get from the linearized Navier-Stokes equation (Equation 7.2), 

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Electroosmotic Mobility When an electric field is applied to a capillary filled with an aqueous buffer, we expect the buffer s ions to migrate in response to their electrophoretic mobility. Because the solvent, H2O, is neutral, we might reasonably expect it to remain stationary. What is observed under normal conditions, however, is that the buffer solution moves toward the cathode. This phenomenon is called the electroosmotic flow. 

Schematic diagram showing the origin of electroosmotic flow.

Electroosmotic flow velocity, Veof, is a function of the magnitude of the applied electric field and the buffer solution s electroosmotic mobility, )J,eof. 

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. 

Total Mobility A solute s net, or total velocity, Vtot, is the sum of its electrophoretic velocity and the electroosmotic flow velocity thus. 

The velocity with which the solute moves through the capillary due to the electroosmotic flow (Veof)-... 

Examining equation 12.41 shows that we can decrease a solute s migration time (and thus the total analysis time) by applying a higher voltage or by using a shorter capillary tube. Increasing the electroosmotic flow also shortens the analysis time, but, as we will see shortly, at the expense of resolution. 

First, solutes with larger electrophoretic mobilities (in the same direction as the electroosmotic flow) have greater efficiencies thus, smaller, more highly charged solutes are not only the first solutes to elute, but do so with greater efficiency. Second, efficiency in capillary electrophoresis is independent of the capillary s length. Typical theoretical plate counts are approximately 100,000-200,000 for capillary electrophoresis. 

The direction of electroosmotic flow and, therefore, the order of elution in CZE can be reversed. This is accomplished by adding an alkylammonium salt to the buffer solution. As shown in Figure 12.45, the positively charged end of the alkylammonium ion binds to the negatively charged silanate ions on the capillary s walls. The alkylammonium ion s tail is hydrophobic and associates with the tail of another alkylammonium ion. The result is a layer of positive charges to which anions in the buffer solution are attracted. The migration of these solvated anions toward... 

Capillary zone electrophoresis also can be accomplished without an electroosmotic flow by coating the capillary s walls with a nonionic reagent. In the absence of electroosmotic flow only cations migrate from the anode to the cathode. Anions elute into the source reservoir while neutral species remain stationary. 

Because micelles are negatively charged, they migrate toward the cathode with a velocity less than the electroosmotic flow velocity. Neutral species partition themselves between the micelles and the buffer solution in much the same manner as they do in HPLC. Because there is a partitioning between two phases, the term chromatography is used. Note that in MEKC both phases are mobile. ... 

The capillary used for CGE is usually treated to eliminate electroosmotic flow, thus preventing the gel s extrusion from the capillary tubing. Samples are injected... 

Capillary Electrochromatography Another approach to separating neutral species is capillary electrochromatography (CEC). In this technique the capillary tubing is packed with 1.5-3-pm silica particles coated with a bonded, nonpolar stationary phase. Neutral species separate based on their ability to partition between the stationary phase and the buffer solution (which, due to electroosmotic flow, is the mobile phase). Separations are similar to the analogous HPLC separation, but without the need for high-pressure pumps, furthermore, efficiency in CEC is better than in HPLC, with shorter analysis times. 

Methanol, which elutes at 4.69 min, is included as a neutral species to indicate the electroosmotic flow. When using standard solutions of each vitamin, CZE peaks are found at 3.41 min, 4.69 min, 6.31 min, and 8.31 min. Examine the structures and p/Ca information in Figure 12.47, and determine the order in which the four B vitamins elute. 

McKillop and associates have examined the electrophoretic separation of alkylpyridines by CZE. Separations were carried out using either 50-pm or 75-pm inner diameter capillaries, with a total length of 57 cm and a length of 50 cm from the point of injection to the detector. The run buffer was a pH 2.5 lithium phosphate buffer. Separations were achieved using an applied voltage of 15 kV. The electroosmotic flow velocity, as measured using a neutral marker, was found to be 6.398 X 10 cm s k The diffusion coefficient,... 

Electro osmosis often accompanies electrophoresis. It is the transport of Hquid past a surface or through a porous soHd, which is electricaHy charged but immovable, toward the electrode with the same charge as that of the surface. Electrophoresis reverts to electroosmotic flow when the charged particles are made immovable if the electroosmotic flow is forcibly prevented, pressure builds up and is caHed electroosmotic pressure. 

Electroosmotic flow is also dependent on the zeta potential at the immobilized surface and the strength of the electric field. For electroosmosis, the flow rate generated is... 

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. 

Electroosmotic flow (EOE) is thus the mechanism by which liquids are moved from one end of the sepai ation capillai y to the other, obviating the need for mechanical pumps and valves. This makes this technique very amenable to miniaturization, as it is fai simpler to make an electrical contact to a chip via a wire immersed in a reservoir than to make a robust connection to a pump. More important, however, is that all the basic fluidic manipulations that a chemist requires for microchip electrophoresis, or any other liquid handling for that matter, have been adapted to electrokinetic microfluidic chips. 

The mechanism by which analytes are transported in a non-discriminate manner (i.e. via bulk flow) in an electrophoresis capillary is termed electroosmosis. Eigure 9.1 depicts the inside of a fused silica capillary and illustrates the source that supports electroosmotic flow. Adjacent to the negatively charged capillary wall are specifically adsorbed counterions, which make up the fairly immobile Stern layer. The excess ions just outside the Stern layer form the diffuse layer, which is mobile under the influence of an electric field. The substantial frictional forces between molecules in solution allow for the movement of the diffuse layer to pull the bulk... 

The flow profiles of electrodriven and pressure driven separations are illustrated in Figure 9.2. Electroosmotic flow, since it originates near the capillary walls, is characterized by a flat flow profile. A laminar profile is observed in pressure-driven systems. In pressure-driven flow systems, the highest velocities are reached in the center of the flow channels, while the lowest velocities are attained near the column walls. Since a zone of analyte-distributing events across the flow conduit has different velocities across a laminar profile, band broadening results as the analyte zone is transferred through the conduit. The flat electroosmotic flow profile created in electrodriven separations is a principal advantage of capillary electrophoretic techniques and results in extremely efficient separations. 

Figure 9.2 Pressure-driven (a) and electrodriven (b) flow profiles. Laminar flow in pressure-driven systems results in a bullet-shaped profile, wliile the profile of electroosmotic flow is plug-shaped, wliich reduces band broadening.

Electroosmotic flow, 195 End column detection, 89 Energy barrier, 16 Enzyme electrodes, 172, 174 Enzyme immunoassays, 185 Enzyme inhibition, 181 Enzyme reconstitution, 178 Enzyme wiring, 178 Equilibrium potential, 15 Ethanol electrodes, 87, 178 Exchange current, 14... 

Manz, B Stilbs, P Jonsson, B Soderman, O Callaghan, PT, NMR Imaging of the Time Evolution of Electroosmotic Flow in a Capillary, Journal of Physical Chemistry 99, 11297, 1995. Matthew, JB Hanania, GIH Gurd, FRN, Electrostatic Effects in Hemoglobin Bohr Effect and Ionic Strength Dependence of Individual Groups, Biochemistry 18, 1928, 1979. 

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