Electroosmosis with electrophoresis

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. 

Using the SI units, the velocity of the EOF is expressed in meters/second (m s ) and the electric held in volts/meter (V m ). Consequently, the electroosmotic mobility has the dimension of m V s. Since electroosmotic and electrophoretic mobility are converse manifestations of the same underlying phenomena, the Helmholtz-von Smoluchowski equation applies to electroosmosis, as well as to electrophoresis (see below). In fact, it describes the motion of a solution in contact with a charged surface or the motion of ions relative to a solution, both under the action of an electric held, in the case of electroosmosis and electrophoresis, respectively. 

The methods described above all deal with direct current electroosmosis and electrophoresis. If electroosmosis is used with a time-periodic... 

Fig. 4. Controls of the agar electrophoresis of a noimal serum. Arrow 1 shows the control of electroosmosis with o-nitraniline. Arrow 2 gives the position of bromo-phenol blue (free dyestuff) and Arrow 3 the albumin front stained blue. From Wunderly (W8).

Both electroosmosis and streaming potential relate to the motion of electrolyte solutions and are therefore considered in the following section. However, we shall reserve the detailed discussion of streaming potential for the next chapter in connection with the treatment of sedimentation potential, which together with electrophoresis deals with the motion of dissolved or suspended charged particles. 

For electrokinetic injection a low voltage is applied for a brief time with the inlet of the separation column located in the sample vial. Sample enters the column by the combined effect of electroosmosis and electrophoresis. If it is assumed that the conductivity of the sample solution and background electrolyte is equal, then the length of the injected zone, Ljnj, is given by... 

The effect known either as electroosmosis or electroendosmosis is a complement to that of electrophoresis. In the latter case, when a field F is applied, the surface or particle is mobile and moves relative to the solvent, which is fixed (in laboratory coordinates). If, however, the surface is fixed, it is the mobile diffuse layer that moves under an applied field, carrying solution with it. If one has a tube of radius r whose walls possess a certain potential and charge density, then Eqs. V-35 and V-36 again apply, with v now being the velocity of the diffuse layer. For water at 25°C, a field of about 1500 V/cm is needed to produce a velocity of 1 cm/sec if f is 100 mV (see Problem V-14). 

The physical separation of charge represented allows externally apphed electric field forces to act on the solution in the diffuse layer. There are two phenomena associated with the electric double layer that are relevant electrophoresis when a particle is moved by an electric field relative to the bulk and electroosmosis, sometimes called electroendosmosis, when bulk fluid migrates with respect to an immobilized charged surface. 

The electrokinetic processes can actually be observed only when one of the phases is highly disperse (i.e., with electrolyte in the fine capillaries of a porous solid in the cases of electroosmosis and streaming potentials), with finely divided particles in the cases of electrophoresis and sedimentation potentials (we are concerned here with degrees of dispersion where the particles retain the properties of an individual phase, not of particles molecularly dispersed, such as individual molecules or ions). These processes are of great importance in particular for colloidal systems. 

Electrophoresis The physical situation of relative motions of a solution and another (insulating) phase during electrophoresis is exactly the same as in electroosmosis. Hence, the linear velocity of a cylindrical particle (which is the equivalent of a cylindrical pore) is given by the value following from Eq. (31.4). With particles of dilferent shape, this velocity can be written as... 

Theory Cross-flow-electrofiltration can theoretically be treated as if it were cross-flow filtration with superimposed electrical effects. These electrical effects include electroosmosis in the filter medium and cake and electrophoresis of the particles in the slurry. The addition of the applied electric field can, nowever, result in some qualitative differences in permeate-flux-parameter dependences. 

The movement of a charged particle with respect to an adjacent liquid phase is the basic principle underlying four electrokinetic phenomena electrophoresis, electroosmosis, sedimentation potential, and streaming potential. 

With regard to the movement of liquid versus particles under direct current, electrophoresis is the reverse of the effect of electroosmosis.33 If particles move through a liquid that is stationary, this is called electrophoresis conversely, if the liquid moves through particles that are stationary, that is called electroosmosis. 

The so-called paper electrophoresis is described in Ref 7. It consists of applying the sample with. a micropipet at the center of a pencil, line made on strips cut from Wharman No 1 filter paper. About 0.4-0.5ml of appropriate solv saturated with.water is placed in a test tube. After the solv ascends jche strip is dried and sprayed with an indicator (Compare with Electroosmosis)... 

Micellar etectrokinetic chromatography electrophoresis with micelles acting as pseudostationary phase Capillary electrochromatography similar to HPLC, except mobile phase is driven by electroosmosis instead of pressure... 

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. Electroosmosis 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 electroosmosis 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. 

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