Electrophoretic flow

Figure 26-31 Separation of natural isotopes of 0.56 mM Cl by capillary electrophoresis with indirect spectrophotometrlc detection at 254 nm. Background electrolyte contains 5 mM CrOJ to provide absorbance at 254 nm and 2 mM borate buffer, pH 9.2. The capillary had a diameter of 75 m, a total length of 47 cm (length to detector = 40 cm), and an applied voltage of 20 kV. The difference in electrophoretic mobility of 36C and 37CI is just 0.12%. Conditions were adjusted so that electroosmotlc flow was nearly equal to and opposite electrophoretic flow. The resulting near-zero net velocity gave the two isotopes maximum time to be separated by their slightly different mobilties. [From C. A Lucy and T. L McDonald, "Separation of Chloride Isotopes by Capillary 35 40 45 Electrophoresis Based on the Isotope Effect on Ion Mobility"Anal.

C. Consider a capillary electrophoresis experiment conducted near pH 9, at which the electroosmotic flow is stronger than the electrophoretic flow. 

Net flow of cations and anions is to the right, because electroosmotic flow is stronger than electrophoretic flow at high pH... 

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. 

Figure 7 Arrays of two-microcoil CE-NMR spectra (LB = 0) of the methyl peak of triethylamine in 1 M borate buffer. Spectra acquired during alternation of electrophoresis flow between two outlet capillaries. (A) All spectra acquired from upper coil (shim settings optimized for upper coil NMR observation switch bypassed). (B) All spectra acquired with lower coil (shim settings optimized for lower coil NMR observation switch bypassed). (C) NMR spectra acquired from microcoil on outlet capillary without electrophoretic flow (shim settings optimized for active coil NMR observation switch in-line). (Reproduced with permission from Ref. 43. Copyright 2001 American Chemical Society.)...

Ghosal has also examined electrophoretic flow in microchannels using a Taylor-Aris dispersion model. In both of the above cases Taylor-Aiis models were reported to work well. On the other hand, Ni, Seebauer and Masel found that at high flow rales over... 

Separations of anions are based on differences in electrophoretic flow. Inorganic ions are generally smaller and therefore more mobile than organic ions. The electrophoretic mobilities of inorganic ions are an inverse function of their hydrated ionic radii. Electrophoretic mobility is also affected by the charge on an ion and by the solvent medium. Tables of limiting ionic conductance are a convenient source for estimating electric mobilities of ions. 

In addition to electrophoretic flow, a second type of migration occurs in the capillary called electroosmotic flow (EOF). A minute charge on the capillary surface (called the zeta potential) results from ionization of silanol groups, which have a pf a of 6 to 7. 

Efficiency and resolution of an electrophoretic separation are influenced by the electrophoretic flow as well as the EOF. The apparent mobility, Papp, of an analyte is determined by the sum of its electrophoretic mobility, pep, and the electroosmotic mobility, peof -... 

Munro et al. showed separation and detection of amino acids on microchips using an indirect fluorescence detection method. Figure 36.10 shows application of this method to urine samples with no pretreatment other than dilution in the appropriate separation buffer. Abnormal amounts of amino acids can easily be detected in the two patient samples compared to the healthy control sample. An absorbance detection based approach was utilized for the clinical analysis of calcium ion in serum, which is important in the regulation of a number of physiological processes. Beads with an immobilized calcium reactive dye were placed into the detection region, and the samples mobilized past the beads using electrophoretic flow. While a true separation was not intended, the interference... 

The second method is to neglect the convection term in Eq. 9 considering that the flow velocity is small for both EOF flow and electrophoretic flow, which decouples the flow field and the EDL potential field. In the second approach, the assumption of a two-species buffer and the application of the Boltzmann distribution are commonly made in order to solve the potential field easily, which yields the well-known Poisson-Boltzmann equation ... 

Electroosmosis Liquid-liquid electroosmotic flow Liquid-solid electrophoretic flow... 

In this chapter, the fundamental aspects of (I) liquid-liquid electroosmotic flow and (If) liquid-particle electrophoretic flow considering the free charge at the interface are studied to investigate the characteristics of such two-phase flow system. 

Key Research Findings Liquid-Solid Electrophoretic Flow... 

E. (a) Capillary electrophoresis was conducted at pH 9, at which electroosmotic velocity is greater than electrophoretic velocity for a particular anion. Draw a picture of the capillary, showing the anode, cathode, injector, and detector. Show the directions of electroosmotic and electrophoretic flow of a cation and an anion. Show the direction of net flow for each ion. 

A van Deemter plot for capillary electrophoresis is a graph of plate height versus migration velocity, where migration velocity is governed by the net sum of electroosmotic flow and electrophoretic flow. 

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