Electrophoresis-convection

Fig. 1. Effect of pH on the corrected sedimentation constant of conalbumin at ionic strength 0.1, protein concentration 1.1—1.3 g./lOO ml. , conalbumin prepared by electrophoresis—convection O. conalbumin prepared by the acid precipitation method Q, conalbumin prepared by electrophoresis—convection, exposed for 1 hr. to pH 2—3 before sedimentation at pH 5.2 A, conalbumin prepared by the acid precipitation method, exposed for 1 hr. to pH 2.5—3.4 before sedimentation at pH 4.5—7.5. (Arch. Biochem. Biophys. 61, 51 [1956]).

Electrodecantation A separation process in which a colloidal dispersion is separated from a noncolloidal solution by an applied electric field together with the force of gravity. Also called electrophoresis convection. 

Electrophoresis convection, where the undesirable thermal convection is eliminated because separation takes place in a relatively narrow channel. 

Zone electrophoresis is defined as the differential migration of a molecule having a net charge through a medium under the influence of an electric field (1). This technique was first used in the 1930s, when it was discovered that moving boundary electrophoresis yielded incomplete separations of analytes (2). The separations were incomplete due to Joule heating within the system, which caused convection which was detrimental to the separation. 

The form of the effective mobility tensor remains unchanged as in Eq. (125), which imphes that the fluid flow does not affect the mobility terms. This is reasonable for an uncharged medium, where there is no interaction between the electric field and the convective flow field. However, the hydrodynamic term, Eq. (128), is affected by the electric field, since electroconvective flux at the boundary between the two phases causes solute to transport from one phase to the other, which can change the mean effective velocity through the system. One can also note that even if no electric field is applied, the mean velocity is affected by the diffusive transport into the stationary phase. Paine et al. [285] developed expressions to show that reversible adsorption and heterogeneous reaction affected the effective dispersion terms for flow in a capillary tube the present problem shows how partitioning, driven both by electrophoresis and diffusion, into the second phase will affect the overall dispersion and mean velocity terms. 

Countercurrent electrophoresis can be nsed to split a mixtnre of mobile species into two fractions by the electrical analog of elntria-tion. In such countercurrent electrophoresis, sometimes termed an ion still, a flow of the suspending flnid is maintained parallel to the direction of the voltage gradient. Species which do not migrate fast enough in the applied electric field will be physically swept out of the apparatus. An apparatus based mainly on this principle bnt nsing also natural convection currents has been developed (Bier, Electrophoresis, vol. II, Academic, New York, 1967). 

In traditional electrophoresis, separation efficiency is limited by thermal diffusion and convection. Owing to long analysis times and low efficiencies, these procedures never enjoyed wide usage. Problems have arisen when trying to differentiate between structurally related drug residues such as streptomycin and dihydrostreptomycin, tetracyclines, lincomycin and clindamycin, and erythromycin and oleandomycin (83, 84). To overcome these problems, anticonvective media, such as polyacrylamide or agarose gels, have also been used. 

FIGURE 3-19 Electrophoresis, (a) Different samples are loaded in wells or depressions at the top of the polyacrylamide gel. The proteins move into the gel when an electric field is applied. The gel minimizes convection currents caused by small temperature gradients, as well as protein movements other than those induced by the electric field, (b) Proteins can be visualized after electrophoresis by treating the gel with a stain such as Coomassie blue, which binds to the proteins blit not to the gel itself. Each band on the gel represents a different pro-... 

This technique represents the transposition of classical polyacrylamide or agarose gel electrophoresis into a capillary. Under these conditions, the electro-osmotic flow is relatively weak. In this approach, the capillary is filled with an electrolyte impregnated into a gel that minimises diffusion and convection phenomena. In contrast to its use for proteins that are fragile and thermally unstable, CGE is ideal for separating the more rugged oligonucleotides. 

Biological Materials. The degree of purity ol biological materials severely limits their usefulness. Electrophoresis is a commonly used mclhod of separation and purilicalion of substances such as cells, enzymes and proteins. This lechniquc relies upon the luei that surface charge distribution, and thus mobility in an electric field, vary from one material lo another The degree of separation, product yield, and purity are limited by convection which is caused by coucenlialion gradients within lire process medium. 

In general terms, capillary electrophoresis is the electrophoretic separation of a substance from (usually) a complex mixture within a narrow tube filled with an electrolyte solution which is normally an aqueous buffer solution. Although one example of separation performed in a totally non-aqueous solution has been reported (50), neutral and slightly basic buffer solutions are generally used. Small tubes dissipate heat efficiently and prevent disruption of separations by thermally driven convection currents. Therefore, capillary electrophoresis can use... 

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