Electrical migration held charging

Another separation technique utilizes an electric field. An electric held is an electrically charged region of space, such as between a pair of electrodes connected to a power supply. The technique utilizes the varied rates and direction with which different organic ions (or large molecules with charged sites) migrate while under the influence of the electric held. This technique is called electrophoresis. Zone electrophoresis refers to the common case in which a medium such as cellulose or gel is used to contain the solution. A schematic diagram of the electrophoresis apparatus resembles an electrochemical apparatus in many... 

A particle with a net charge moves under the influence of an electric held toward an oppositely charged electrode. This phenomenon, defined as electrophoresis, is the basis for various electrophoretic methods in biochemistry research. Separation of particles via electrophoresis is achieved through the difference in their migration distances, which is dependent upon their electrophoretic mobility. 

The electrophoretic driving force for a particle to move, depending on the electric held strength (E) and the net charge on the particle (z), is balanced by the frictional resistance if) which the particle must overcome to migrate. 

Proteins are positively charged in solutions at pH values below their pi and negatively charged above their pis. Thus, at pH values below the pi of a particular protein, it will migrate toward the cathode during electrophoresis. At pH values above its pi, a protein will move toward the anode. A protein at its pi will not move in an electric held. 

Figure 9.2 Formation of an electrical double layer responsible for electroendosmotic flow in an uncoated fused-silica capillary. The negative charges on the surface of the capillary are neutralized by positive charges of cations present in the buffer, which form an electrical layer near the surface of the capillary. When the electric held is apphed, the positive charges migrate toward the negative electrode, generating a bulk flow of the solution contained within the column. Electroosmosis exhibits a flat prohle, in contrast to hydraulic flow, which is parabolic.

Light absorption causes formation of an electron/hole (e h ) pair in the interfacial region of the solid and, in the presence of an electric field (e. g. when the solid is held in an electrolyte), the electrons migrate inwards towards the bulk of the solid and the holes move towards the surface and react with the FeOH groups, i.e. the charges separate. The surface reaction is, Fe-OH + hye Fe(OH)s where s = surface and hvB is a hole. A feature of the iron oxides is electron/hole pair recombination - many electrons recombine with the holes and are neutralized - which decreases the photo-activity of the solid. The extent of recombination depends to some extent on the pH of the solution and its effect on the proportion of FeOH groups at the surface (see Chap. 10 and Zhang et al., 1993). 

Figure 3.38. Principle of the photorefractive effect By photoexcitation, charges are generated that have different mobilities, (a) The holographic irradiation intensity proHle. Due to the different diffusion and migration velocity of negative and positive charge carriers, a space-charge modulation is formed, (b) The charge density proHle. The space-charge modulation creates an electric Held that is phase shifted by 7t/2. (c) The electric field profile. The refractive index modulation follows the electric field by electrooptic response, (d) The refractive index profile.

The value of the electrophoretic mobility can be calculated considering the migration of an ion in an electrolyte solution at inhnite dilution where no ionic interactions occur. Under the action of an electric held, the ion is accelerated by a force Pa, directed toward the appositively charged electrode, which is given by... 

In a cc experiment, a thin capillary is lilted with a run bulTcr and a voltage is applied across the capillary. The underlying impetus for separations in ee is. in general, derived front the fact that charged species migrate in response to an applied electric held proportionately to their charge and inversely proportionately to their sire. 

The velocity of migration of a charged analyte in an electric held depends on its electrophoretic mobility and on the magnitude of the applied electric field. 

Microwaves are electromagnetic waves (see p. 329) and there are electric and magnetic held components. Charged particles start to migrate or rotate as the electric held is applied,which leads to further polarization of polar particles. Because the concerted forces applied by the electric and magnetic components of... 

The ions, from the feed water to be desalted, migrate towards the membranes on the application of an electric field (Figure 4.11). Anions pass through the anion-exchange membranes and cations through the cation-exchange membranes, in opposite directions. Both ion flows are then stopped by the next manbrane that they reach, because it is only permeable to ions of the opposite charge. The ions thus held in alternate compartments combine to form a concentrated solution of brine. 

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