Electroosmotic mobility measurement

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... 

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 ... 

Gusev et al. monitored the conductivity of his modified monolithic polystyrene-based columns for over 3 months and observed no changes [49]. Similarly, the electroosmotic mobility was measured over a number of days and again almost no changes were found [50]. This demonstrates excellent stability of the polystyrene-based monolithic column. 

Electroosmotic hold-up time (in capillary electromigration), teo — Time required for a liquid in a capillary to move due to -> electroosmosis through the effective length of the capillary, Leff. This time is usually measured as the -> migration time of a neutral compound, called an electroosmotic flow marker, which is assumed to have an -> electroosmotic mobility that is negligible compared to that of the analyte. 

It should be emphasized that both k" and k" need to be evaluated under conditions used in the CEC experiments. First, the electrophoretic mobility of the sample component is obtained from separate CZE measurements using the mobile phase used in CEC. Then electroosmotic mobility, which is the interstitial EOF mobility in the packing, is evaluated from the results of measuring the currents and the EOF with CEC columns [9,10]. This allows for calculation of k" according to Eq. (16) followed by calculation of k" from Eq. (11) using the migration times of the different sample components in the CEC column. 

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. 

A knowledge of the electrolyte temperature is important in CE as temperature changes in the electrolyte influence precision, accuracy, separation efficiency, and method robustness [7,14,32], During the past two decades, a considerable amount of research has been conducted toward electrolyte temperature measurements in CE [1,14,19,21,32 2], Early methods have included using the variation of electroosmotic mobility (eieof), electrophoretic mobility (p-ep), and electrical conductivity (k) to measure temperature [38,39], More recently, techniques such as external thermocouples [21], Raman thermometry [19,40], NMR spectroscopy [32,35], thermochromic probes [41], and the variation in fluorescence response [42] have been used to measure temperatures. Most of these methods require the modiflcation of the existing instrument and/or the purchase of additional equipment. 

Two noninvasive methods of temperature measurement based on the electroosmotic mobility and conductance are discussed in further detail below. The linear relationships between both the electroosmotic mobility and the conductance versus PIL are illustrated in Figure 18.7. 

It should be noted that the electroosmotic mobility (peof) is not a measure of the average elee-trolyte temperature (/Mean, see Figure 18.2) over the whole cross section but instead reflects the average temperature of the electrolyte near the inner wall of the capillary (Twau)- This is because the electroosmotic flow is generated at the capillary wall. To determine the average temperature of... 

There are three commonly used methods to measure the zeta potential, which involves pressure-driven flow, electrophoresis, and electroosmotic-driven flow, respectively. For the measurement of the specific surface conductivity, there are primarily two methods involving pressure-driven flow and electroosmotic-driven flow. In this entry, we will focus on the electrical current monitoring methods which can be used to measure the zeta potential, specific surface conductivity, volumetric flow rate, and electroosmotic mobility by employing electroosmotic flow with a solution displacement process. 

The electroosmotic flow rate can then be calculated hy Q = Mave c- Although it is possible to directly measure the electroosmotic mobility using the ultramicroscope technique [1],... 

The above equation is a different form of the Helmholtz-Smoluchowski equation. Equation 14 shows that if the average electroosmotic velocity is determined from the experimentally measured current-time relationship, the zeta potential can be calculated. Introducing the electroosmotic mobility concept, Peo = u -vJEx, Eq. 14 can be further simplified in the following format ... 

Concentration/separation of sample solutes is one of most important functions in micro- and nanofluidic systems. TGF has proved to be a promising technique that can achieve concentration and separation in microfiuidic devices. However, so far very limited experimental and theoretical investigations have been reported. Experimentally, it is highly desirable to develop various microfiuidic structures that can be utilized by the TGF technique to cmicentrate different samples. Furthermore, more experiments should be carried out to characterize the thermoelectrical properties of buffers and samples so as to obtain the temperature-dependent electroosmotic mobility and electrophoretic mobility, as well as buffer conductivity, viscosity, and dielectric permittivity for each individual sample and buffer solution. In addition, the development of reliable, accurate, high-resolution, experimental techniques for measuring fiow, temperature, and sample solute concentration fields in microfiuidic channels is needed. Theoretically, the model development of TGF is still in its infancy. The models presented in this study assume the dilute solute sample and linear mass flux-driving forces correlations. However, when the concentrations of the sample solute and the buffer solution are comparable, the aforementioned assumptions break down. Moreover, the channel wall zeta potential in this situation may become nonconstant. More comprehensive models should be developed to incorporate the solute-buffer and solute-channel wall... 

In electroosmosis, the stationary and mobile phases are exchanged in relation to electrophoresis. As measurement of the rate of movement of a liquid through a capillary is difficult, the force that it exerts is measured, i.e. the electroosmotic pressure, or, alternatively, the volume of liquid transported through a capillary in a given time interval. The electroosmotic velocity, veo, is... 

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