Electrically electrophoresis

In electrophoresis, the motion of charged colloidal particles under the influence of an electric field is studied. For spherical particles, we can write... 

Electrophoresis is used primarily to analyze mix tures of peptides and proteins rather than individual ammo acids but analogous principles apply Because they incorporate different numbers of ammo acids and because their side chains are different two pep tides will have slightly different acid-base properties and slightly different net charges at a particular pH Thus their mobilities m an electric field will be differ ent and electrophoresis can be used to separate them The medium used to separate peptides and proteins is typically a polyacrylamide gel leading to the term gel electrophoresis for this technique... 

The basic instrumentation for capillary electrophoresis is shown in Figure 12.41 and includes a power supply for applying the electric field, anode and cathode compartments containing reservoirs of the buffer solution, a sample vial containing the sample, the capillary tube, and a detector. Each part of the instrument receives further consideration in this section. 

Applying the Electric Field Migration in electrophoresis occurs in response to the applied electric fleld. The ability to apply a large electric fleld is important because... 

An injection technique in capillary electrophoresis in which an electric field is used to inject sample into the capillary column. 

Electrophoresis (qv), ie, the migration of small particles suspended in a polar Hquid in an electric field toward an electrode, is the best known effect. If a sample of the suspension is placed in a suitably designed ceU, with a d-c potential appHed across the ceU, and the particles are observed through a microscope, they can all be seen to move in one direction, toward one of the two electrodes. AH of the particles, regardless of their size, appear to move at the same velocity, as both the electrostatic force and resistance to particle motion depend on particle surface this velocity can be easily measured. 

Electrophoresis and electro osmosis can be used to enhance conventional cake filtration. Electrodes of suitable polarity are placed on either side of the filter medium so that the incoming particles move toward the upstream electrode, away from the medium. As most particles carry negative charge, the electrode upstream of the medium is usuaHy positive. The electric field can cause the suspended particles to form a more open cake or, in the extreme, to prevent cake formation altogether by keeping aH particles away from the medium. 

Response to Electric and Acoustic Fields. If the stabilization of a suspension is primarily due to electrostatic repulsion, measurement of the zeta potential, can detect whether there is adequate electrostatic repulsion to overcome polarizabiUty attraction. A common guideline is that the dispersion should be stable if > 30 mV. In electrophoresis the appHed electric field is held constant and particle velocity is monitored using a microscope and video camera. In the electrosonic ampHtude technique the electric field is pulsed, and the sudden motion of the charged particles relative to their counterion atmospheres generates an acoustic pulse which can be related to the charge on the particles and the concentration of ions in solution (18). 

Capillary Electrophoresis. Capillary electrophoresis (ce) is an analytical technique that can achieve rapid high resolution separation of water-soluble components present in small sample volumes. The separations are generally based on the principle of electrically driven ions in solution. Selectivity can be varied by the alteration of pH, ionic strength, electrolyte composition, or by incorporation of additives. Typical examples of additives include organic solvents, surfactants (qv), and complexation agents (see Chelating agents). 

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

Behavior. Diffusion, Brownian motion, electrophoresis, osmosis, rheology, mechanics, and optical and electrical properties are among the general physical properties and phenomena that are primarily important in coUoidal systems (21,24—27). Of course, chemical reactivity and adsorption often play important, if not dominant, roles. Any physical and chemical feature may ultimately govern a specific industrial process and determine final product characteristics. 

Electrical Enhancement of Dewatering. Electrophoresis (qv) can be used to prevent a filter cake from forming on a filter medium while allowing water to pass through the medium from the slurry. Electrophoresis is used to move the particles upstream, opposite to the Hquid movement, in order to prevent blinding of the medium. 

The term electrophoresis refers to the movement of a soHd particle through a stationary fluid under the influence of an electric field. The study of electrophoresis has included the movement of large molecules, coUoids (qv), fibers (qv), clay particles (see Clays), latex spheres (see Latex technology), basically anything that can be said to be distinct from the fluid in which the substance is suspended. This diversity in particle size makes electrophoresis theory very general. 

Electrophoresis uses the force of an apphed electric field to move molecules or particles, often through a polymer matrix. The electric field acts on the intrinsic charge of a substance, and the force on each substance is proportional to the substance s charge or surface potential. The resulting force on the substance results in a distinct velocity for the substance that is proportional to the substance s surface potential. If two different substances have two different velocities, an electric field apphed for a fixed amount of time results in different locations on the matrix for these substances. 

Theory of Electrophoretic Motion. The study of the mechanics of electrophoresis focuses on the basis of electric potential on the surface of an object, and the relation of the electric potential to the velocity of the particle. Whereas research has been generally limited to nonmolecular particles of weU-defined geometry and is not strictly apphcable to molecules such as proteins and DNA fragments, this work is useful for understanding the physics of electrophoretic motion. 

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. 

Electrokinetics. The first mathematical description of electrophoresis balanced the electrical body force on the charge in the diffuse layer with the viscous forces in the diffuse layer that work against motion (6). Using this force balance, an equation for the velocity, U, of a particle in an electric field... 

Generation of Heat in Electric Fields. One of the practical problems encountered in electrophoresis is the generation of heat from resistive dissipation of energy in the electrophoretic medium. The generation of heat (foule heating) is given by... 

The heating effect is the limiting factor for all electrophoretic separations. When heat is dissipated rapidly, as in capillary electrophoresis, rapid, high resolution separations are possible. For electrophoretic separations the higher the separating driving force, ie, the electric field strength, the better the resolution. This means that if a way to separate faster can be found, it should also be a more effective separation. This is the opposite of most other separation techniques. 

Differences in mobilities of ions, molecules, or particles in an electric field can be exploited to perform useful separations. Primary emphasis is placed on electrophoresis and dielec trophoresis. Analogous separation processes involving magnetic and centrifugal force fields are widely apphed in the process industiy (see Secs. 18 and 19). 

There are four related electrokinetic phenomena which are generally defined as follows electrophoresis—the movement of a charged surface (i.e., suspended particle) relative to astationaiy hquid induced by an applied ectrical field, sedimentation potential— the electric field which is crested when charged particles move relative to a stationary hquid, electroosmosis—the movement of a liquid relative to a stationaiy charged surface (i.e., capiUaty wall), and streaming potential—the electric field which is created when liquid is made to flow relative to a stationary charged surface. The effects summarized by Eq. (22-26) form the basis of these electrokinetic phenomena. 

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

Process Concept The application of a direct elecdric field of appropriate polarity when filtering should cause a net charged-particle migration relative to the filter medium (electrophoresis). The same direct electric field can also be used to cause a net fluid flow relative to the pores in a fixed filter cake or filter medium (electroosmosis). The exploitation of one or both of these phenomena form the basis of conventional electrofiltration. 

Introduction Dielectrophoresis (DEP) is defined as the motion of neutral, polarizable matter produced by a nonimiform electric (ac or dc) field. DEP should be distinguished from electrophoresis, which is the motion of charged particles in a uniform electric field (Fig. 22-30). 

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