Moving-boundary techniques

Moving-boundary electrophoretic techniques, originally demonstrated by Tiselius in 1937, employ a U-tube with the sample occupying the lower part of the U and the two limbs being carefully filled with a buffered electrolyte so as to maintain sharp boundaries with the sample. Electrodes are immersed in the electrolyte and direct current passed between them. The rate of migration of the sample in the electric field is measured by observing the movement of the boundary as a function of time. For colourless samples, differences in refractive index may be used to detect the boundary. Such moving-boundary techniques are used mainly in either studies of the physical characteristics of molecules or bulk preparative processes. 

The conclusion is that the moving boundary technique is simple but not rigorous. For rapid semi-quantitative purposes it may be useful. 

The results obtained by Macinnes and Dole over a range of concentration agree with those obtained by the moving boundary technique (see problem 100). To obtain results of this precision by the Hittorf method extreme care, particularly in the analyses, is necessary. 

A basic feature which differentiates isotachophoresis from the other electrophoretical techniques is that zones migrate at the same velocity from the moment the equilibrium has established. By this the name iso-tachos is explained. Its simplest version is the moving boundary technique applied to the measurement of transference numbers (known since Nemst ° Whetham and Kohlrausch ). Theoretical, experimental and instrumental elaboration of this technique was described in detail by Macinnes and Longsworth Based on the example of this simple method, basic features and characteristics of isotachophoretical migration and its terminology can be described and explained. 

Capillary Isotachophoresis CIT is a moving-boundary technique. The sample is sandwiched between a leading electrolyte with mobility higher than any of the sample components, and a terminating electrolyte with mobility lower than any of the sample components. Upon application of the electrical field, the sample ions are separated into bands according to their electrophoretic mobility. Once the band is formed, all ions that are separated migrate at the same velocity. 

Other methods related to the moving boundary method include the indirect moving boundary method (where the concentration in the trailing edge behind the boundary is monitored) and the analytical boundary method. The latter approach involves analysis of the compositional change within the moving boundary zone and is a hybrid of the Hittorf and standard moving boundary techniques. 

Such methods represent direct applications of Equations (4.31) and (4.32) whereby transport numbers are related to the speeds with which ions move. Moving boundary techniques are based upon the observed rate of movement, under the influence of an applied e.m.f., of a sharp boundary between solutions of two different electrolytes having an anion or cation in common. Measurement of the rate of movement of a sharp boundary presents few problems, since, even if the solutions do not differ in colour, the difference in their refractive indices makes the boundary between them easily distinguishable. A schematic diagram of the relation between two such solutions is shown... 

The determination of electrophoretic velocities may be carried out experimentally by the use of methods suitable for transport number measurements. Moving boundary techniques have proved useful despite the problem of a difficulty in selecting suitable indicator ions. Reliable estimates of electrophoretic velocities make possible the determination of zeta-potentials. Since colloids migrate at characteristic rates under the influence of an electric field, electrophoresis provides an important means of separation. Coatings, such as rubber or graphite, may be deposited on metal electrodes by this means and additives to these may be co-deposited. 

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. 

With the study of the migration of hydrogenium ions (H ) in a phenolphthalein gel by Lodge in 1886 and the description of the migration of ions in saline solutions by Kohlraush in 1897, a basis was set for the development of a new separation technique that we know today as electrophoresis. Indeed, several authors applied the concepts introduced by Lodge and Kohlraush in their methods and when Arne Tiselius reported the separation of different serum proteins in 1937, the approach called electrophoresis was recognized as a potential analytical technique. Tiselius received the Nobel Prize in Chemistry for the introduction of the method called moving boundary electrophoresis. ... 

To solve the preceding set of equations, Equation 5.62 is plugged into Equation 5.60. By separately determining the compaction properties of the fiber bed [32] an evolution equation for the pressure can be obtained. Because this is a moving boundary problem the derivative in the thickness direction can be rewritten [32] in terms of an instantaneous thickness. The pressure field can then be solved for by finite difference or finite element techniques. Once the pressure is obtained and the velocity computed, the energy and cured species conservation equations can be solved using the methodology outlined in Section 5.4.1. 

Of the electrokinetic phenomena we have considered, electrophoresis is by far the most important. Until now our discussion of experimental techniques of electrophoresis has been limited to a brief description of microelectrophoresis, which is easily visualized and has provided sufficient background for our considerations to this point. Microelectrophoresis itself is subject to some complications that can be discussed now that we have some background in the general area of electrical transport phenomena. In addition, the methods of moving-boundary electrophoresis and zone electrophoresis are sufficiently important to warrant at least brief summaries. 

Zone electrophoresis is influenced by adsorption and capillarity, as well as by electroosmosis. Therefore evaluation of mobility (and f) from this type of measurement is considerably more complex than from either microelectrophoresis or moving-boundary electrophoresis. Nevertheless, zone electrophoresis is an important technique that is widely used in biochemistry and clinical chemistry. One particularly important area of application is the field of immunoelectrophoresis, which is described briefly in Section 12.11. Additional information on zone electrophoresis may be obtained from Probstein (1994) and Hunter (1981) and the references given there. Variants of zone electrophoresis also exist see, for example, Gordon et al. (1988) for information on a variant known as capillary zone electrophoresis and Righetti (1983) for information on what is known as isoelectric focusing. 

Dewey et al. (D3) present a numerical scheme for the ablation of an annulus with specified heat fluxes at the outer (ablating) surface and at the inner surface. An implicit finite difference technique is used which permits arbitrary variation of the surface conditions with time, and which allows iterative matching of either heat flux or temperature with external chemical kinetics. The initial temperature may also be an arbitrary function of radial distance. The moving boundary is eliminated by a transformation similar to Eq. (80). In addition a new dependent variable is introduced to... 

Rempel (1994) provided a model for the formation and accumulation of hydrates, using a moving boundary mathematical technique similar to the... 

The development of electrophoretic techniques afforded possibilities for fractionations based on charge density differences. Duxbury (1989) has reviewed applications of different electrophoretic separation methods, including zone electrophoresis, moving boundary electrophoresis, isotachophoresis, and isoelectric focusing (IEF). Preparative column electrophoresis (Clapp, 1957) and continuous flow paper electrophoresis (Hayes, 1960 summarized by Hayes et al., 1985) methods have been used to separate components isolated from sapric histosol soils. These techniques allowed separation of polysaccharides from the colored components the electrophoretograms of the colored components were diffuse, showing a continuum of components of different charge densities. 

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