Electromigration velocity

Even without molecular sieving or charge retardation associated with the support, observed electromigration velocities will generally be affected by electroosmotic flow and by capillary flow through the porous medium. These flow effects make the process unsuitable for mobility measurements. However, by somewhat empirical means, it is today the principal analytical procedure used for protein and amino acid analysis because it is simple, cheap, enables complete separation of all electrophoretically different components, and because small samples can be studied, which is often important for biochemical analyses. 

The problem is a complex one for not only is there convection of the liquid by electroosmosis but if the dissolved contaminants are themselves charged, then an additional electromigration velocity will be imposed on them. Moreover, to the extent that concentration gradients are set up, there will also be transport of dissolved species by diffusion. In addition, there are chemical reactions in the bulk fluid and at the electrodes, together with adsorption or desorption at the soil surface. 

Here, we have defined two velocities that appear in the brackets. The first is the electromigration velocity... 

The extent of electromigration of a given ion depends on the conductivity of the soil, soil porosity, pH gradient, applied electric potential, initial concentration of the specific ion, and the presence of competitive ions. The electromigrative velocity... 

The electromigration velocity in Equation 2.13 is the speed of ion movement in the pore water caused by an electric field in infinitely dilute solutions. In the pore fluids with finite ion concentrations, which more closely resemble the electrochemical systems of contaminated clays, influence of the interionic attraction should be considered. Generalized ion mobility that account for the possibility of interactions... 

Typical approximate values for ion mobilities in a free solution at 25 °C are Na 5, K 8, H 36, OH 21 (l(T m /(V s)) (Atkins, 2001). Note that hydrogen ions move almost twice as fast as OH ions. So, in a field of 1 V/cm, the electromigration velocity of a sodium ion in a free solution is about 1.8cm/h. In soil, tortuosity must be taken into account, so it will be somewhat less than this. Acar and Alshawabkeh (1993) reported that the electromigration velocity in soil is typically about 4.7 times less than that in a free solution, so this will reduce the velocity in the above example to about 0.38 cm/h. 

For the barrier to prevent the movement of heavy metal cations, it follows that the hydraulic flow velocity of groundwater through the soil (vHyd) should not be greater than the sum of the electroosmotic velocity (veof) plus the electromigration velocity (vion), assuming diffusion is neglected ... 

Fig. 18. Arrangement at counter-flow isotadioidioresis. Pomp (I) pushes out the leading electrolyte from reservoir (2) through valve (3) into separation capillary (4) in the direction opposite to the electromigration (rf zones. The zone migrates at the velocity given by the difference of electromigration velocity (v) and counterflow (v ...

Vhyd is the advection velocity due to hydraulic processes, is the diffusion velocity (expressed in terms of the first Pick s law in the above relation, A represents the aquifer section), Vion is the electromigration velocity, and v op is the electroosmotic flow velocity. The last two terms depend on the strength of the applied electric field, AF/AL. 

A variety of microscale separation methods, performed in capillary format, employ a pool of techniqnes based on the differential migration velocities of analytes under the action of an electric field, which is referred to as capillary electromigration techniques. These separation techniques may depend on electrophoresis, the transport of charged species through a medium by an applied electric field, or may rely on electrically driven mobile phases to provide a true chromatographic separation system. Therefore, the electric field may either cause the separation mechanism or just promote the flow of a solution throughout the capillary tube, in which the separation takes place, or both. 

The first section of the book explores emerging novel aspects of HPLC and related separation methods based on the differential velocity of analytes in a liquid medium under the action of either an electric field (capillary electromigration techniques) or a gravitational field (field-flow fractionation). The section focusing on applications highlights four significant areas in which HPLC is successfully employed chiral pharmaceutical, environmental analysis, food analysis, and forensic science. 

The vacancy current is therefore due solely to the cross term arising from the current of conduction electrons (which is proportional to E). The coupling coefficient for the vacancies is the off-diagonal coefficient Lvq which can be evaluated using the same procedure as that which led to Eq. 3.54 for the electromigration of interstitial atoms in a metal. Therefore, if (5V) is the average drift velocity of the vacancies induced by the current and Mv is the vacancy mobility,... 

Recent results by Chen et al. (1999) show, however, the transport velocity of H+ in natural soils during EO is considerably more retarded compared to the migration velocity in pure low CEC clays such as kaolinite. In EO experiments containing only pure kaolinite, very rapid electromigration of H is observed relative to natural soils because kaolinite is essentially a stoichiometric clay mineral and has limited capacity in absorbing cations. The transport ofH+ is therefore rapid. 

Electroosmotic mobility (in capillary electromigration), u or (teo -> Electroosmotic velocity, i/eo, divided by -> electric field (strength), E. 

Electroosmotic velocity, i/eo — See electroosmosis. In -> capillary electromigration, this velocity is... 

Different mechanisms have been devised to achieve chiral resolution by electromigration, but often we have to deal with mixed-mode separations rather than pure processes. In any case, chiral resolution results from stereospecific interactions of a chiral selector, with the enantiomers of the compound giving rise to a difference in migration velocity between the two entities. Chirally selective ligands, such as Cu(II)-L-histidine and Cu(II)-aspartame, have been used for derivatized amino acid mixtures. 

Recycling isoelectric focusing, like its parent method lEF, is an equilibrium process in which each component migrates to a steady-state position and remains there. By contrast, zone electrophoresis is a rate process in which each component moves at a steady-state velocity. In order to convert ZE to a recycle system, it is necessary to provide a counterflow to offset electromigration of the solutes. 

The concepts of ionic mobility ui, m /s-V) and effective ionic mobility ( f, m /s-V) are introduced as representative parameters of electromigration (ionic migration). The effective ionic mobility defines the velocity of the ionic species under the effect of a unit electric field, which can be theoretically estimated using the Nemst-Townsend-Einstein relation (Holmes, 1962). Ionic mobility is related to the ionic valence (z,) and molecular diffusion coefficient (Z) m /s) of species as follows ... 

As pointed out in the previous section of this chapter, attention should be paid to electromigration rather than electroosmosis to establish the removal mechanisms of mixed heavy metals. However, high concentrations of multiple metal contaminants, especially divalent cations, can affect the electroosmotic flow in electrokinetic remediation, which is a factor that should be taken into consideration with regard to the removal mechanisms of mixed heavy metals. The electroosmotic velocity, V, is given by the Helmholtz-Smoluchowski equation (Acar and Alshawabkeh, 1993 Mitchell, 1993) ... 

The movement of ions toward the oppositely charged electrode is called electromigration, which is quantified by the effective ionic mobility. The effective ionic mobility (f/f) is defined as the velocity of ion within the pore space under the influence of a unit electrical potential gradient. The Nernst-Einstein equation is used to relate the ionic mobility to the diffusion coefficient of the ion in a dilute solution (Koryta, 1982) as follows ... 

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