Mobility, electro-osmotic electrophoretic

CE CGE CHOL CIP CPG CTAB CZE dA, dG, dC DBU DEAE DMF DMT DNP DOPE DOTMA EDTA EM EOF ESI-MS Fmoc FPE ICAM-1 Capillary electrophoresis Capillary gel electrophoresis Cholesterol Cahn-Ingold-Prelog nomenclature system for absolute configuration Controlled pore glass Hexadecyltrimethylammonium bromide Capillary zone electrophoresis Deoxyadenosine, deoxyguanosine, deoxycytosine l,8-Diazabicyc o[5.4.0]undec-7-en Diethylaminoethyl- Dimethylformamide Bis(4-methoxyphenyl)phenylmethyl-, (syn. Dimethoxytrityl-) 2,4-DinitrophenyI- Dioleylphosphatidylethanolamine N-[l-(2,3-dioleyloxy)propyl]-N, N, N-trimethylammonium chloride Ethylenediamine tetra-acetic acid Electrophoretic mobility Electro-osmotic flow Electrospray ionization mass spectrometry 9-Fluorenylmethoxycarbonyl-Fluid phase endocytosis Intracellular Adhesion Molecule-1... 

In MEKC, mainly anionic surface-active compounds, in particular SDS, are used. SDS and all other anionic surfactants have a net negative charge over a wide range of pH values, and therefore the micelles have a corresponding electrophoretic mobility toward the anode (opposite the direction of electro-osmotic flow). Anionic species do not interact with the negatively charged surface of the capillary, which is favorable in common CZE but especially in ACE. Therefore, SDS is the best-studied tenside in MEKC. Long-chain cationic ammonium species have also been employed for mainly anionic and neutral solutes (16). Bile salts as representatives of anionic surfactants have been used for the analysis of ionic and nonionic compounds and also for the separation of optical isomers (17-19). 

In capillary electrophoresis, components of a mixture are separated according to two main factors electrophoretic mobility and electro-osmotic flow. These terms apply to ions, molecules or micelles. 

By combining the apparent mobility and the electro-osmotic flow, which is responsible for the migration of the bulk electrolyte, it is possible to calculate the migration velocity or the electrophoretic mobility of charged species. Using equation (8.3), equation (8.5) can be written as ... 

Electric sector, 295 Electrochromatography, 119 Electrode potential, 348 Electromagnetic separator, 294 Electromigration, 114, 117 Electron capture detector, 36 Electron ionisation, 307 Electro-osmosis, 115 Electro-osmotic flow, 114 Electrophoregram, 113 Electrophoretic mobility, 114 Electrospray, 312 ELISA, 336... 

Electrodriven separations, such as capillary electrophoresis (CE) and capillary electrochromatography (CEC), are based on the different electrophoretic mobilities in an electric field of the molecules to be separated. They provide a higher separation efficiency then conventional HPLC since the electrophoretic flow (EOF) has a plug-flow profile. Whereas the mobile phase in CE is driven only by the electro-osmotic flow, it is generated in CEC by a combination of EOF and pressure. CEC has a high sample capacity which favours its hyphenation with NMR. 

Capillary electrophoresis separations are dependent on the relative mobilities of analytes under the influence of an electric field and do not depend on mobile phase/stationary phase interactions. A fused silica capillary is filled with a buffer and both ends submerged into two reservoirs of the buffer. A platinum electrode is immersed in each reservoir and a potential difference (5-30 kV) is applied across the electrode. An aliquot of sample of a few nanoliters is injected onto the capillary by either hydrostatic or electrokinetic injection, and the components migrate to the negative electrode. Separations of analytes arise from differences in the electrophoretic mobilities, which are dependent on the mass-to-charge ratio of the components, physical size of the analyte, and buffer/analyte interactions. An electro-osmotic flow (EOF) of the buffer occurs in the capillary and arises as a result of interactions of the buffer with dissociated functional groups on the surface of the capillary. Positive ions from the buffer solution are attracted to negative ions... 

Note that Makino et al. [57] found that Eq. (21.108) holds between the electro-osmotic velocity U o on a poly(N-isopropylacrylamide) hydrogel-coated solid surface and the electrophoretic mobility of a poly(N-isopropylacrylamide) hydrogel-coated latex particle. 

The results of measurements by the microscopic method show that the electrophoretic mobility of the particles varies with the distance from the wall of the cell particles close to the wall move in a direction opposite to that in which those in the center migrate. In any event, the results show an increase in velocity from the walls to the center of the cell. The explanation of this fact lies in the electro-osmotic movement of the liquid a double layer is set up between the liquid and the walls of the cell and under the influence of the applied field the former exhibits electro-osmotic flow. For the purpose of obtaining the true electrophoretic velocity of the suspended particles it is neceasary to observe particles at about one-fifth the distance from one wall to the other. A more accurate procedure is to make a series of measurements at different distances from the side of the cell and to apply a correction for the electro-osmotic flow. The algebraic difference of the corrected electrophoretic velocity and the speed of the particles near the walls gives the electro-osmotic mobility of the liquid in the particular cell. If the solution contains a protein which is adsorbed on the surface of the walls of the vessel and on the particles, it is possible to compare the electrophoretic and electro-osmotic mobilities in one experiment reference to the significance of such a comparison was made on page 532. 

To distinguish between, and establish, the real electrophoretic and the electro-osmotic velocity, the following analysts may be helpful. Just near the cylinder wall (positions at the dashed lines in fig. 4.15) the mobility of the particle is other positions the liquid velocity according to... 

As the mobile phase moves through the capillary containing the sorbent under the effect of this electro-osmotic flow (EOF), sample components partition between the two phases in sorption and diffusive mechanisms characteristic of liquid chromatography. Ions in the sample move both under the influence of EOF and by their added attraction toward the oppositely charged electrode (electrophoresis). Uncharged components, on the other hand, move only under the influence of EOF. Thus, sample components, in general, separate by chromatographic and, sometimes, electrophoretic processes. 

Using SI units, the velocity of the electro-osmotic flow is expressed in meters per second (m/s) and the electric field in volts per meter (V/m). Consequently, in analogy to the electrophoretic mobility, the electro-osmotic mobility has the dimension square meters per volt per second. Because electro-osmotic and electrophoretic mobilities are converse manifestations of the same underlying phenomenon, the Hehnholtz-von Smoluchowski equation applies to electro-osmosis as well as to electrophoresis. In fact, when an electric field is applied to an ion, this moves relative to the electrolyte solution, whereas in the case of electro-osmosis, it is the mobile diffuse layer that moves under an appUed electric field, carrying the electrolyte solution with it. 

As previously mentioned, electrophoretic separations using open-tube capillaries are based on solute differential mobility, which is a function of charge and molecular size. A different approach is required for separating neutral or uncharged compounds. Because charge is absent, electrophoretic mobility is zero. Electro-osmotic flow would allow them to migrate, but their velocities would be equal. Separation would not be possible with the above method. 

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