Electroosmotic mobility

Electroosmotic Mobility When an electric field is applied to a capillary filled with an aqueous buffer, we expect the buffer s ions to migrate in response to their electrophoretic mobility. Because the solvent, H2O, is neutral, we might reasonably expect it to remain stationary. What is observed under normal conditions, however, is that the buffer solution moves toward the cathode. This phenomenon is called the electroosmotic flow. 

Electroosmotic flow velocity, Veof, is a function of the magnitude of the applied electric field and the buffer solution s electroosmotic mobility, )J,eof. 

The electroosmotic pumping is executed when an electric field is applied across the channel. The moving force comes from the ion moves in the double layer at the wall towards the electrode of opposite polarity, which creates motion of the fluid near the walls and transfer of the bulk fluid in convection motion via viscous forces. The potential at the shear plane between the fixed Stem layer and Gouy-Champmon layer is called zeta potential, which is strongly dependent on the chemistry of the two phase system, i.e. the chemical composition of both solution and wall surface. The electroosmotic mobility, xeo, can be defined as follow,... 

The electroosmotic velocity is also characterized by a mobility, namely, the electroosmotic mobility ( eofX... 

The migration in CE is obviously influenced by both the effective and the electroosmotic mobility. Therefore, the proportionality factor in the relationship of the migration velocity and the electric field strength in such a case is called the apparent electrophoretic mobility (/iapp) and the migration velocity the apparent migration velocity (vapp). The /iapp is equal to the sum of /migration velocity is expressed as... 

The selectivity of a separation is determined by the effective mobility because the effect of the electroosmotic mobility is equal for all the sample constituents. In order to obtain /(err from gapp, knowledge of the magnitude of /(,.<>( is required. Therefore, it is necessary to measure the velocity of the EOF. This can be done by several methods [2] however, the procedure of applying a neutral marker is commonly used. The neutral marker is a neutral compound and thus migrates only because of the EOF. Its migration velocity represents the velocity of the... 

Another kind of bias is related to the electrical resistance (conductivity) of the medium in which the sample constituents are dissolved. Owing to differences in conductivity, both the electrophoretic as well as the electroosmotic mobility is affected. Therefore, injections from samples with different conductivity are bound to show this bias [39]. 

The migration in CE is determined by both the effective and the electroosmotic mobility. 

The selectivity of a separation is determined by the effective mobility because the effect of the electroosmotic mobility is equal for all the sample constituents. 

Fig. 25. Effect of percentage of acetonitrile (A) and methanol (B) on electroosmotic mobility in a packed column. (Reprinted with permission from [56]. Copyright 1997 Elsevier). Conditions capillary column 100 pm i. d., total length 33.5 cm, active length 25 cm packed with 3 pm CEC Hypersil C18, mobile phase organic modifier-water+4% 25 mmol/1 TRIS pH = 8, voltage 30 kV, temperature 20 °C, marker thiourea... 

The pH is an important factor that can influence the ionization of the surface silica groups. As a result, C is directly dependent on the pH. Therefore, the relationship of /teof as a function of pH is governed by the behavior of the dissociation of the silanol groups. Different capillary materials result in different profiles of the electroosmotic mobility as a function of the pH (due to differences in Q. Typically a sigmoid curve behavior resembling the titration curve of the surface active groups is observed. ... 

The overall migration in CE is determined by the combined effect of the effective and the electroosmotic mobility. The apparent electrophoretic mobility is therefore used as the... 

In the section General Principles, a comprehensive description is given of the basic principles of the capillary electrophoretic separation process. The concepts of electrophoretic mobility and electroosmotic mobility as well as band dispersion phenomena and resolution are described, using the equations listed in Table 3. A remarkable difference exists between the equations in both chapters in which the electroosmotic velocity and/or the electroosmotic mobility is used. In the Ph.Eur., the terms 4-feo and 4-/teo are used, whereas in the USP the terms feo and Pco are used in the corresponding equations, with the sentence added The sum or the difference between the two velocities (v p and v o) is used depending on whether the mobilities act in the same or opposite directions. ... 

The prerequisites for the development of a robust method are simple robust equipment, robust staff, and a robust development are needed. Even though the number of CE equipment manufacturers is low, the available instruments can be considered to be robust. In contrast, qualified staff for CE analysis is not often available. Since CE is based on absolutely different physics, i.e., electrophoretic and electroosmotic mobility,than HPLC,... 

The ratio of the velocity of the EOF to the applied electric held, which expresses the velocity per unit held, is dehned as electroosmotic coefficient or more properly, electroosmotic mobility (Heo) [13] ... 

Using the SI units, the velocity of the EOF is expressed in meters/second (m s ) and the electric held in volts/meter (V m ). Consequently, the electroosmotic mobility has the dimension of m V s. Since electroosmotic and electrophoretic mobility are converse manifestations of the same underlying phenomena, the Helmholtz-von Smoluchowski equation applies to electroosmosis, as well as to electrophoresis (see below). In fact, it describes the motion of a solution in contact with a charged surface or the motion of ions relative to a solution, both under the action of an electric held, in the case of electroosmosis and electrophoresis, respectively. 

The effective mobility, expressed by Equation 6.16, can be directly calculated from the observed mobility by measuring the electroosmotic mobility using a neutral marker, not interacting with the capillary wall, which moves at the velocity of the EOF. Accordingly, the effective mobility p of cations in the presence of cathodic EOF is calculated from p ts by subtracting p gf ... 

The virtual migration distance, /q, arising from EOF is expressed as the product of the migration time of the analyte (O and the electroosmotic mobility (fieof) ... 

The equations reported above are related to MEKC and all other EKC separation modes performed with pseudostationary phase, analytes, and EOE moving in the same direction at different velocities. Such condition applies when the electroosmotic mobility is higher than the electrophoretic mobility of the pseudostationary phase migrating to the direction opposite to that of EOF. 

The constant of proportionality between electroosmotic velocity, ueo, and applied field is called electroosmotic mobility, p.eo. 

Electroosmotic mobility is proportional to the surface charge density on the silica and inversely proportional to the square root of ionic strength. Electroosmosis decreases at low pH (Si—O —> Si—OH decreases surface charge density) and high ionic strength. At pH 9 in 20 mM borate buffer, electroosmotic flow is 2 mm/s. At pH 3, flow is reduced by an order of magnitude. 

The apparent (or observed) mobility, xapp, of an ion is the sum of the electrophoretic mobility of the ion plus the electroosmotic mobility of the solution. 

Electroosmotic mobility is the speed of the neutral species, Mneutral, divided by the electric field ... 

The mobility of the neutral marker, which we just calculated, is the electroosmotic mobility for the entire solution. 

Electrophoretic mobility describes the response of the ion to the electric field. We subtract the electroosmotic mobility from the apparent mobility to find electrophoretic mobility ... 

Resolution. Suppose that the electroosmotic mobility of a solution is +1.61 X 10 7 m2/(V s). How many plates are required to separate sulfate from bromide with a resolution of 2.0 Use Table 15-1 for mobilities and Equation 23-30 for resolution. 

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