Electro-osmotic factor

Bifurcation analysis in the local Teorell model accounting for negative osmosis and concentration dependence of the electro-osmotic factor as prescribed by (6.4.44). The analysis should be essentially identical to that of 6.3 with the generalized Darcy s law (6.3.11) replaced by the expression... 

Variation of ion mobility with pH is only part of the story with regard to separation by capillary electrophoresis - the other major factor is electro-osmotic flow (EOF). 

If one compares Eqs. (6.317) and (6.314) everything is fine, except that this Stokes law approach gives a numerical factor / =, whereas the electro-osmotic approach gives/= f. It turns out that each is right for a particular set of conditions. This conclusion comes out of an accurate mathematical treatment that results in the following expression for the electrophoretic velocity ... 

Equation (6.1.4) asserts that the volumetric flow rate is a superposition of two components. They are the electro-osmotic component proportional to the electric field intensity (voltage) with the proportionality factor u> and the filtrational Darcy s component proportional to —P with the hydraulic permeability factor i>. Teorell assumed both w and t> constant. Finally another equation, crucial for Teorell s model, was postulated for the dynamics of instantaneous electric resistance of the filter R(t). Teorell assumed a relaxation law of the type... 

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. 

Another factor that controls the migration of the solute is the electro-osmotic mobility //EOS, which results in movement of the electrolyte or electro-osmotic flow. This flow is present in gel electrophoresis to a small extent and to a greater extent in capillary electrophoresis because of the internal wall of the capillary. 

Another factor affecting particle velocity is the relative contribution of electro-osmotic fluid flow and electrophoresis on the particle. Since the observed velocity is the sum of the intrinsic electrophoretically induced particle velocity and fluid velocity, there exist situations where the two combine to render the particle velocity unobservable within the range discussed above. For similar reasons. there are situations where particle velocity may be observable outside the range discussed above. Choice of particle is important in being able to extract the electrokinetic information. 

Because there is no ionizable groups of the coating in the neutral capillary, the interaction between charged molecules with ionic capillary surface is eliminated. Also, the electro-osmotic flow (EOF) of a neutral capillary is eliminated. However, a continuous and adequate flow of the buffer solution toward the CE capillary outlet is an important factor for routine and reproducible CE-ESI-MS analysis in order to maintain a stable ESI operation, some low pressure applied to the CE capillary inlet is usually needed, especially when the sheathless interface is employed. The disadvantage of the pressure-assisted CE-ESI-MS is the loss of some resolution because the flat flow profile of the EOF is partially replaced by the laminar flow profile of the pressure-driven system. A typical neutral capillary is a LPA (linear polyacrylamide)-treated capillary. Karger and co-workers [6] used mixtures of model proteins, a coaxial sheath flow ESI interface. 

Here, is the absolute mobility, that of the ion at infinite dilution, / is the correction factor that takes into account the deviation from ideal behavior. It can be seen that an additional parameter occurs in this equation the mobility of the electro-osmotic flow, /Ueof> which occurs in many cases in the separation systems and leads to an additional velocity vector of the solutes. 

The Helmholtz-von Smoluchowski equation indicates that under constant composition of the electrolyte solution, the electro-osmotic flow depends on the magnitude of the zeta potential, which is determined by the different factors influencing the formation of the electric double layer, as discussed earlier. Each of these factors depends on several variables, such as pH, specific adsorption of ionic species in the compact region of the double layer, ionic strength, and temperature. 

Some other elution modes have been described. They are induced by various factors — cyclical field, secondary chemical equilibria, adhesion chromatography, asymmetrical electro-osmotic flow for a review, see Ref. 2. However, the number of their implementations is rather limited, and for this reason, these modes are not discussed here. 

The velocity of a solute in the capillary is determined by its electrophoretic mobility and electro-osmotic flow (EOF), which are affected by temperature, and which is influenced by the diameter and length of the capillary, its contents, concentration, and pH of running buffer, applied voltage, current, viscosity, and zeta-potential. The subtle variation of EOF is also a main factor in maintaining high reproducibility if CE is automated by the constant temperature of the capillary and running buffer with proper buffering capacity. However, it is difficult to keep the temperature constant, as EOF depends on the condition of the fused silica. 

The stationary phase acts also with varying success as a retaining material. Non-aqueous mobile phases (for hydrophilic analytes) and untreated silica packings may also be used. The separation factor is very high but a change of composition of the mobile phase or of the organic modifiers will affect the electro-osmotic flow and then the selectivity (Figure 8.14). 

There are several primary factors that affect the efficiency of separation through CE the buffer system, the pH of the electrophoresis electrolyte, the voltages applied (higher voltages improve separation), and the interface between CE and ICP-MS, which reduces the ratio of forced flow to electro-osmotic flow. The relative standard deviation values (RSD < 9%) obtained through the ICP-MS technique confirm the suitability for arsenic speciation in saline waters. In terms of reliability, one can say that the ICP-MS method is more reliable than HG-AAS for arsenic speciation in water. 

Over the whole range of variation of the Cl- HC03— ratio in the cathode compartment, the transport number of chloride was somewhat higher than the molar ratio of chloride in the solution. In other words, in all the three membranes tested chloride ion transport is favored over bicarbonate ion transport. This fact is not unrelated to the data on water transport. The electro-osmotic water transport with chloride ions is smaller than with bicarbonate ions. It is known that, other factors being equal, low water transport and high mobility of ions in a membrane-as measured, for instance, by electrical conductivity-are correlated (13,16). 

There has been some controversy about electro-osmosis. No evidence of its occurrence has been found in early laboratory tests or during testing of cores in a realkalisation demonstration project [24,84,85]. On the other hand, recent laboratory experiments by Andrade and co-workers [86,87] have confirmed the occurrence of electro-osmosis, supporting previous observations by BanfiU [80,88]. Electro-osmotic flow was observed in carbonated concrete, while it was not observed in uncarbonated concrete. Full understanding of the mechanism and the controlling factors in terms of practical characteristics of structures needs further study. 

According to Eq. 6.11, the DMFC determination of the methanol permeability requires the knowledge of the methanol drag factor, because the electro-osmotic flux could afford for a considerable fraction of the methanol flow, particularly at high methanol concentrations. An important drawback of this method is that the methanol drag coefficient is not well known, so Ren et al. [299] assumed that it was similar to the water drag coefficient ( =2.5). However, some recent NMR [300] and electro-osmosis [301] studies would indicate that this assumption is not valid, leading to considerable uncertainties in the methanol permeability coefficients determined by this method. 

Detailed three-dimensional measurements of ICEO flows are now possible in microfluidic devices. Using particle-image velocimetry applied to thin optical slices, the ICEO flow field around a platinum cylinder has recently been reconstructed experimentally (Fig. 5b) and found to agree well with the theory, up to a scaling factor which could perhaps be attributable to compact-layer effects [6]. There has also been extensive experimental work on AC electro-osmotic flows in microfluidic devices, as discussed in a separate article. 

Free-solution capillary electrophoresis (FSCE) is the major technique used for drug analysis, considering the fact that many drugs have acidic or basic groups that allow them to analyzed as charged molecules. In this technique, the capillary is filled with a buffer solution and the separation is based on the different electrophoretic mobilities of the solutes. Separation of both anionic and cationic solutes is possible, owing to electro-osmotic flow (EOF). The pH of the buffer has a major influence on selectivity, but other factors such as buffer concentration, additives, etc. should also be considered dining method development. ... 

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