Electrophoresis and electroosmosis

Four different electrokinetic processes are known. Two of them, electroosmosis and electrophoresis, were described in 1809 by Ferdinand Friedrich Renss, a professor at the University of Moscow. The schematic of a cell appropriate for realizing and studying electroosmosis is shown in Fig. 31.1a. An electrolyte solution in a U-shaped cell is divided in two parts by a porous diaphragm. Auxiliary electrodes are placed in each of the half-cells to set up an electric held in the solution. Under the inhuence of this held, the solution starts to how through the diaphragm in the direction of one of the electrodes. The how continues until a hydrostahc pressure differential (height of liquid column) has been built up between the two cell parts which is such as to compensate the electroosmotic force. 

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... 

The ionic species j represents both the ions migrating into the receiver from the donor including ionic species i and the oppositely charged counter-ions migrating into the donor from the receiver. In dominant electrotransport (negligible passive diffusion contribution) and combining the electroosmosis and electrophoresis terms in Eq. (6), Eq. (9) is commonly seen in the iontophoresis literature ... 

Electroosmosis and electrophoresis are electrokinetic methods that are being considered for contaminated soil remediation." The principle behind these... 

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... 

We consider the velocity field bounded by a cylindrical slip surface, which excludes a thin electric double layer (EDL) [16]. The velocity scale Ut includes the effects of local pressure gradients, electroosmosis, and electrophoresis and can be expressed as follows ... 

Li and Harrison carried out the first cell assay in microchannels [2]. This seminal work made use of electrokinetically driven flow (electroosmosis and electrophoresis) to transport bacteria, yeast, and mammalian cells in channels and to implement low-volume chemical lysis (cell death). This theme of microfluidics-based cell transport, sorting, and lysis has continued to be a popular application, as well as related work in using microfluidics to culture cells and to pattern them into structures. The utility of these methods is acknowledged (and that they are featured in several good reviews [1] and other entries in the encyclopedia) but focuses here on describing microfluidics-based cell assays that fit the definition described above - application of a stimulus and measurement of a response. 

In one approach, the microchip interface was constmcted from modified 1/16-inch high-performance liquid chromatography (HPLC) fittings [30]. It incorporates a freestanding liquid junction formed via continuous delivery of a flow of suitable solvent which carries the separation effluent through a pneumatically assisted electrospray needle located in front of the MS orifice. In some cases, stmctural features of microchips can be utilized as parts of the ion source (e.g., ESI emitter). Thus, the resulting coupled microchip-MS systems are more compact, and the delay time between the on-chip incubation and MS detection can be decreased. For example, a capillary nanoESI emitter was successfully incorporated into a microchip CE channel for on-line CE-MS analysis [31]. Such microchip-MS systems do not require the use of external pumps because analytes can be driven toward the ion source (ESI or nanoESI) by means of electroosmosis and electrophoresis [32]. 

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. 

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 streaming potential (Dorn effect) relates to a movement of liquid that generates electric potential, and electroosmosis occurs when a direct electric potential causes movement of the liquid. The sedimentation potential relates to sedimentation (directed movement) of charged particles that generates electric potential, and electrophoresis occurs when a direct electric potential causes a movement of charged particles. 

Lab-on-a-chip separations are reliant upon the pumping of solutions by electroosmosis and the separation of charged ions in an electric field by electrophoresis. Each ion has an electrophoretic mobility, which is proportional to its charge and inversely proportional to the frictional forces that act upon it [13]. The velocity at which an ion migrates in the electric field is dictated by its size, charge, and the applied potential, as seen in Eq. (13.1), where v is the velocity of the ion, /xg is the electrophoretic mobility, E is the applied potential, q is the charge of the ion, q is the viscosity of the solution, and r is the radius of the ion ... 

Estimates of the isoelectric point of different plasma cholinesterases are shown in Table 10. There is a wide variation within the range pH 3 to 5, which is probably due to artifacts associated with the different methods which were used. Sources of error include evaporation, electroosmosis, and variation in texture of the supporting medium, with consequent variable adsorption of protein. Correction may be made for these factors by the electrophoresis, under identical conditions, of an uncharged reference substance such as dextran (S5, Wl). When subjected to preparative isoelectric focusing, human plasma cholinesterase showed a ma-... 

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