Electrophoretic movement

Modes of Operation There is a close analogy between sedimentation of particles or macromolecules in a gravitational field and their electrophoretic movement in an electric field. Both types of separation have proved valnable not only for analysis of colloids but also for preparative work, at least in the laboratory. Electrophoresis is applicable also for separating mixtures of simple cations or anions in certain cases in which other separating methods are ineffectual. 

FIGURE 6.22 Section from the ITP preconcentration to the electrophoretic separation at the bifurcation block on a PMMA chip, (a) stack of zones at the end of the enrichment (b,c) transition from the isotachophoretic migration to zone electrophoretic movement [637]. Reprinted with permission from Wiley-VCH Verlag. 

Fig. 6.4. Diagram illustrating the principle for counterflow (a) and normal-flow (b) gradients (Reprinted with permission from [31]. Copyright 1999 American Chemical Society). The direction of electroosmotic flow is opposite to that of the electrophoretic movement in both methods and is opposite to the net migration velocity, vmigr (= ve0 - ve ph), in (a) and coincides with the net migration direction in (b). veo is not constant along the capillary, and velph is higher in the direction of the electrophoretic migration.

Orida, N. Poo, M.-M. Electrophoretic movement and localisation of acetylcholine receptors in the embryonic muscle cell membrane. Nature 1978, 275, 31-35. 

When the electric strength is equal to or larger than the critical value the particle deposition can be arrested by the electrophoretic movement or the particles will even move in the opposite direction to the permeate flow. It should be noted that electrofiltration requires charged particles in the feed, and that the zeta potential will depend on the pH and the ionic environment. This means it will not be effective close to the isoelectric point or with raised levels of salts. 

Electroporated cells can be used to transfer DNA in bacterial, plant, and mammalian cells. This method offers rapid and efficient incorporation of plasmid and DNA in cells [49]. The in vivo electroporation has been shown to yield enhanced plasmid delivery to a wide range of tissues including muscle, skin, liver, lung, artery, kidney, retina, cornea, spinal cord, brain, synovium, and tumors. The precise mechanisms involved in electroporation applications in vivo are uncertain and require further studies, but appear to involve both electropore formation and an electrophoretic movement of the plasmid DNA. 

It has been shown that DNA must be administered prior to the application of an electric field, as DNA is unable to diffuse through ceU membranes by itself [16]. DNA electrotransfer (Figure 8.4) works by steps involving membrane adsorption, membrane perturbation (by the electric field), electrophoretic movement of the DNA, and finally cellular processing of the internaUzed molecule, as nicely reviewed in [19]. 

CE is a separation technique which uses empty capillaries to effect separation by the electrophoretic movement of charged compounds. Therefore, CE is not a chromatographic method in the strict sense. Recently CE has been applied for the separation and determination of all three surfactant classes (Table 30.7). 

A phenomenon called dielectrophoresis has been used to pattern a variety of cell types on 2D substratesand more recently in 3D culture constructs. Unlike electrophoresis where charged species move in an applied electric field due to Coulombic forces (F = qE), dielectrophoresis capitalizes on the ability of a cell to become polarized when placed in an electric field. Dielectrophoresis is most often used in conjunction with dtemating current (AC) electric fields since AC fields eliminate electrophoretic movement, and have less physiological impact on cells than direct current (DC) fields. When a cell is placed in an AC field, the magnitude and polarity of the induced dipole depend on the frequency of the applied field and the conductivities of the cell and the surrounding medium, described by the equation... 

It was also proposed to use nommiform electric fields, as the electro-osmotic phenomenon would take place much more slowly than the electrophoretic movement. Thus, mobility measurements could be made without any need to correct them for electro-osmosis [41]. This processing has been incorporated in devices, which are now proposed on the market [42]. 

Here, electrophoresis often employs principles used in chromatography - interaction of the separands with another phase, which is called the pseudo-stationary phase. The pseudostationary phase is a substance that is added to the separation system in the column, and can interact with the species to be separated. The substance can be neutral, then it does not have its own electrophoretic movement or it can be charged, then it can move in the column with certain mobility. In both cases, the analytes with their own electrophoretic movements encounter on their way the molecules of the pseudostationary phase and interact with them by forming a temporary complex or by association. During the time when the separands are bound to the pseudostationary phase its mobility is different however, when it is free it moves with its own mobility. As the rate constants of the interaction are mostly very high, in analogy with weak electrolytes, the analyte then moves with a certain mean mobility that lies somewhere between the bound and free mobilities. The mean mobility is in this way dependent on the interaction (complex-ation) constant. Many substances can used for this purpose, such as 2-hydroxyisobutyric acid, which forms complexes with many ions, especially with lanthanides, and enables their electrophoretic separation when added to the separation systems. 

The micelles are highly charged and move in the electric field. They can interact with molecules of analytes, even when they are neutral, and form transient associations. In this way the analytes obtain an electrophoretic movement and can be separated if their interaction constants are different. 

The EOF is especially important in CZE and MEKC. Here, the columns are often made of fused silica, and have negative charge at the inner wall due to dissociation of the silanol groups, which causes the EOF to flow in the direction of the cathode. Therefore, the electrophoretic movement of cationic species is speeded up, while the negative species move slower as the EOF flows against them. Sometimes the latter cannot reach the detector. 

In this section, we consider the rate of capture of polymer chains arising solely from the second term on the right-hand side of Equation 9.7, due to the electrophoretic movement of the polyelectrolyte molecules. We call this limiting situation as the drift-dominated regime. In this limit, the steady-state flux in the number concentration is exactly the same as Equations 8.16 and 8.18, derived for small electrolyte ions,... 

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