Velocity electrophoretic migration

The velocity of migration over the applied electric field is called the electrophoretic mobility (pe) ... 

Because of the dominating role of EOF each analyte (cationic, anionic and neutral) has a tendency to move towards the cathode. The cation migration velocity is composed of the electrophoretic migration and the electroosmotic flow. Neutral analyses are not separated under common CE conditions, they move together with the electroosmotic flow. The migration velocity of anions is the difference between electroosmotic flow and electrophoretic migration, consequently their migration velocity is lower than that of neutral compounds and cationic ones. 

From Eq. (13.3), it is clear that neutral molecules will have a net velocity. In normal electrophoresis, cations will migrate faster than neutrals, and neutrals will migrate faster than anions. Anions are electrophoretically migrating in a direction opposite to EOF. Separations of neutral molecules, such as organic explosives, can only be achieved by using buffer additives, such as micelles, ionic cyclodextrins, and bile salts. The interaction of neutral analytes with these ionic buffer additives results in a modified mobility that enables separation. 

In all electrophoresis experiments, a compound bearing an electrical charge will migrate under the influence of the electric field E. The migration velocity, also called the electrophoretic migration velocity v EP, depends on the electrophoretic mobility of the ion /iEP. 

Fig. 6.4. Diagram illustrating the principle for counterflow (a) and normal-flow (b) gradients (Reprinted with permission from [31]. Copyright 1999 American Chemical Society). The direction of electroosmotic flow is opposite to that of the electrophoretic movement in both methods and is opposite to the net migration velocity, vmigr (= ve0 - ve ph), in (a) and coincides with the net migration direction in (b). veo is not constant along the capillary, and velph is higher in the direction of the electrophoretic migration.

Boundary effects on the electrophoretic migration of a particle with ion cloud of arbitrary thickness were also investigated by Zydney [46] for the case of a spherical particle of radius a in a concentric spherical cavity of radius d. Based on Henry s [19] method, a semi-analytic solution has been developed for the particle mobility, which is valid for all double layer thicknesses and all particle/pore sizes. Two integrals in the mobility expression must be evaluated numerically to obtain the particle velocity except for the case of infinite Ka. The first-order correction to the electrophoretic mobility is 0(A3) for thin double layer, whereas it becomes 0(A) for thick double layer. Here the parameter A is the ratio of the particle-to-cavity radii. The boundary effect becomes more significant because the fluid velocity decays as r l when the double layer spans the entire cavity. The stronger A dependence of the first order correction for thick double layer than that obtained by Ennis and Andersion [45] results from the fact that the double layers overlap in... 

In HPLC and CZE, separation occurs due to differences in the retention volumes at constant flow, which is conveniently represented by retention times and the electrophoretic migration velocities of the separands, respectively. However, the... 

Since the direction of EOF can be the opposite of the electrophoretic migration of the separand, the velocity factor k e can be negative. 

Figure 1 Schematic illustration of the four main operational modes of CZE and the pertinent values of some key migration parameters. The right-hand side of the chart represents the counterdirectional mode B and the left-hand side shows the two counterdirectional modes C and D. Mode A lies at point L at the bottom. At a fixed electrophoretic velocity of the sparand, the EOF decreases as we go clockwise from K to L. At point L a transition occurs from the co-directional to the counterdirectional CZE, and as we move farther clockwise the magnitude of the EOF increases in the opposite direction in mode C. Point M, where both EOF and electrophoretic migration have the same velocity, is the transition point to mode D, the other counterdirectional mode of CZE. Finally, at point N, the relative magnitude of the EOF becomes very large in the direction opposite to that of electrophoretic migration.

The velocity factor, k e, as defined in Eq. (8), serves as a peak locator and can be used for the evaluation of various separation parameters such as selectivity and resolution [1,4], The velocity factor can either be positive or negative when the electrophoretic migration of the analyte is co- or counterdirectional to the EOF, respectively. For this reason, and since k e is void of thermodynamic or any other kind of fundamental significance, the velocity factor finds very limited applications. 

We have shown recently that the different separation mechanisms of HPLC and CZE preclude the definition of any retention or velocity factor for CEC that would have comparable significance to that of k in HPLC [4], However, as shown in the following section, the CEC system was defined by a combination of a retention factor that measures the retention in CEC and a velocity factor that characterizes the electrophoretic migration. Further, expressions were proposed for possible peak locators that, just like k and k, in HPLC and CZE, respectively, can serve as useful indicators of peak position on the electrochromatogram and facilitate estimation of various separation parameters such as selectivity and resolution. 

Biopolymers as well as synthetic polymers, as polyanions or polycations, exist with the same surface/charge ratio and migrate with almost identical velocity and cannot be separated in a normal electric field without an auxiliary aid. Since these polymers differ greatly in size, however, a gel can be used to influence the mobility of a large polymer more than that of a smaller one. In capillary gel electrophoresis (CGE), the electrophoretic migration of macromolecules is hindered by the gel matrix. The transport of the solutes through the capillary is based on the charge of the macromolecules, but the separation is dependent on the molecular size. It is easy to understand that with this technique EOF should be eliminated because otherwise the gel is extruded from the capillary. 

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. 

Capillary zone electrophoresis (CZE), micellar capillary electrokinetic chromatography (MECC), capillary gel electrophoresis (CGE), and affinity capillary electrophoresis (ACE) are CE modes using continuous electrolyte solution systems. In CZE, the velocity of migration is proportional to the electrophoretic mobilities of the analytes, which depends on their effective charge-to-hydrodynamic radius ratios. CZE appears to be the simplest and, probably, the most commonly employed mode of CE for the separation of amino acids, peptides, and proteins. Nevertheless, the molecular complexity of peptides and proteins and the multifunctional character of amino acids require particular attention in selecting the capillary tube and the composition of the electrolyte solution employed for the separations of these analytes by CZE. 

The EOF brings an additional, unspecific velocity vector to the electrophoretic migration of the separands. The total migration velocity of the analyte, i, is then... 

Identification of sample components based solely on migration time in capillary electrophoresis (CE) requires reproducibilities not normally obtained. These are caused, mainly, by two effects temperature effects and electro-osmotic affects. Migration times in CE are determined by the electro-osmotic velocity Ueof and effective electrophoretic migration velocity Ueef the net migration velocity Vt is the vector sum of both velocities ... 

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