Electroosmotic flow control

Poison, N.A., Hayes, M.A., Electroosmotic flow control of fluids on a capillary electrophoresis microdevice using an applied external voltage. Anal. Chem. 2000, 72, 1088-1092. 

GlaxoSmithKline Pharmaceuticals in Harlow, UK, performed the Hantzsch synthesis of 2-bromo-4 -methylacetophenone and l-acetyl-2-thiourea in NMP (N-methyl-2-pyrrolidone) using a microchip reador under EOF conditions [10] (for EOF see [11]) [10] This is claimed to be the first example of a heated organic reaction performed on a glass chip reactor under electroosmotic flow control, whereas only room temperature reactions were made earlier. In a wider scope, the Hantzsch synthesis is a further example to evaluate the potential of microfluidic systems for high-throughput... 

Culbertson, C. T. and Jorgenson, J. W., Increasing the resolving power of capillary electrophoresis through electroosmotic flow control using radial fields. Journal of Microcolumn Separations 1999,11, 167-174. 

J.E. Melanson, N.E. Baryla and C.A. Lucy, Dynamic capillary coatings for electroosmotic flow control in capillary electrophoresis. Trends Anal. Chem., 20, 365-374, 2001. 

Combined Pressure-Driven Flow and Electroosmotic Flow - Control of Micro-Fluidics... 

Katayama H, Ishihama Y, Asakawa N (2000) Enantiomeric separation by capillary electrophoresis with an electroosmotic flow-controlled capillary. J Chromatogr A 875 315-322... 

Capillary wall Generally, the most straightforward approach is to use an uncoated fused silica capillary. But sometimes this is not possible because of adsorption problems to the capillary wall, or other wall properties are needed to control the electroosmotic flow. In literature, there are multiple examples. Besides permanently coated capillaries, there are several descriptions of dynamic coatings available, e.g., triethanolamine, Triton X-100, Polybrene, and quaternary ammonium salts. The advantage of these dynamic coatings is that the coating can be renewed between injections, which could improve repeatability and reproducibility of the separation. 

For fused silica the magnitude of the EOF is controlled by the pH value of the electrophoretic buffer used. At high pH where the silanol groups are predominantly deprotonated, the EOF is significantly greater than at low pH (pH < 4) where they become protonated. Depending on the specific conditions, the EOF can vary by more than one order of magnitude between pH 2 and pH 12. In nonionic materials such as Teflon and other polymers, electroosmotic flow is also encountered. The electrical double layer in this case results from adsorption of buffer anions to the polymer surface. 

While the electroosmotic flow is a positive attribute of electrophoresis in most cases, there are instances where EOF needs to be carefully controlled. For example, too high an electroosmotic flow may decrease resolution, especially of cations with similar mobility. In a different case, when analyzing anions of very different mobilities (anorganic and organic) in one run, the electroosmotic flow needs to be reversed (5). Furthermore, alternative elec-... 

The effect of the electroosmotic flow on the resolution is also evident from Eq. (16). A high electroosmotic flow in the direction of the moving ions can significantly diminish resolution. Theoretically, infinite resolution of two peaks could be reached when jlEOF is equal but opposite to the average mobility m.Av. In this case one of the solutes would migrate in the direction of the detector and the other one in the opposite direction. In other words, the separation run would be infinitely long. Thus, for a practical separation the electroosmotic flow should be controlled in a way to achieve baseline resolution (R = 1) at minimal separation time. 

The optimum pH for separating cations is pK + 0.30 K. K.-C. Yeung and C. A. Lucy, Isotopic Separation of [14N]- and fI5N] Aniline by Capillary Electrophoresis Using Surfactant-Controlled Reversed Electroosmotic Flow, Anal. Chem 1998, 70. 3286. 

Santi, P., and R.H. Guy. 1996. Reverse iontophoresis—Parameters determining electroosmotic flow I. pH and ionic strength. J Control Release 38 159. 

The mechanism of separation in NCE is based on the difference in the electrophoretic mobility of the separated species. Under NCE conditions, the migration of the separated species is controlled by the sum of the intrinsic electrophoretic mobility (fxe/)) and the electroosmotic mobility (fxeo), due to the action of electroosmotic flow (EOF). The observed mobility 0bs) of the species is related to xeo and juep by the following equation ... 

As in the case of normal chromatography both stationary and mobile phases are also required in NLC. On the other hand, in NCE hydrophilic channel walls with improved control over electroosmotic flow are required for better separation of biological samples. Briefly, the separation efficiencies and selec-tivities in NLC and NCE depend on the properties of the microchannels, and, therefore, surface modification of the microchannel is usually necessary to achieve good separation of a variety of analytes. Recently, Muck and Svatos... 

Electroosmotic pumps lack mechanical parts and specific localization in the manifold, producing an even electroosmotic flow. Besides, the flow in interconnected and branched channels can be controlled by switching voltages only. Just two decades ago electroosmotic pumps were attractive and feasible ways for mobile phase flow into microfluidic devices [13] but in the 1990s the conventional pumps available showed a major problem with the high pressures... 

Sample injection in NCE is very important for reproducible results with low limits of detection. In spite of some development in NCE very little effort has been made to develop sample injection devices in this technique. Of course sample injection in NCE is a challenging job due to small volume requirement [87], The controlled injection of small amounts of sample is a prerequisite for successful analysis in NCE. Electrokinetic injection (based on electroosmotic flow) is the preferred method and Jacobson et al. [88] optimized sample injection using this approach. Pinched injection allowing injection in minute quantities [89,90] and double-T shaped fluidic channels [91] have also been used for this purpose. Furthermore, Jacobson et al. [92] used a single high voltage source to simplify instrumentation. Similarly Zhang and Manz [93] developed a narrow sample channel injector to improve... 

A micro channel of height 2 H is equipped with electrodes at the upper (L/,) and lower (L walls [28], These electrodes are used to control the C, potential at the solid-liquid interface. In this way, the direction of the electroosmotic flow near the interface can be changed locally. The external electric field is given as Ex. 

Liu, Y., Fanguy, J.C., Bledsoe, J.M., Henry, C.S., Dynamic coating using polyelectrolyte multilayers for chemical control of electroosmotic flow in capillary electrophoresis microchips. Anal. Chem. 2000, 72(24), 5939-5944. 

Henry, A.C., Waddell, E.A., Shreiner, R., Locascio, L.E., Control of electroosmotic flow in laser-ablated and chemically modified hot imprinted poly(ethylene tereph-thalate glycol) microchannels. Electrophoresis 2002, 23, 791-798. 

Capillary zone electrophoresis (CZE) is the most simple and widely used mode in CE. Separations take place in an open-tube, fused silica capillary under the influence of an electric field. The velocity of the analytes is modified by controlling the pH, viscosity, or concentration of the buffer, or by changing the separation voltage. The electroosmotic flow is often used in this mode to improve resolution or to shorten analysis times. 

For many chemical processing applications, microchemical systems should include solvent extraction and interfacial reaction components utilizing both aqueous and organic (or gas and liquid) solutions. Both solutions must be controlled to realize general chemistry in a microchip. In 1990s, electroosmotic flow was used in microchip electrophoresis (Auroux et al., 2002 Reyes et al., 2002) however, the electroosmotic flow is restricted to the flow control of only one type solution (aqueous buffer). Therefore, electroosmotic flow is not suitable for a flow-control method to... 

Electroosmotic flow is generally reported to be independent of the size of the packing, and consequently the size of the interstitial voids between the particles, unless this size is so small that the electrical double layers overlap [74]. The ability to independently control both the pore size and level of charged functionalities of the methacrylate ester monolithic capillaries enables the direct investigation of the net effect of transport channel size on flow velocity. Recent results clearly demonstrates a... 

Capillary zone electrophoresis is another technique which has been used to separate products such as organic acids.26 Separation is based on differences in the mobility of analytes exposed to an electric field. Resolution and separation time in such systems depends on factors including electroosmotic flow (EOF), and a number of approaches for adjusting the EOF have been examined. While some of the approaches (pretreatment of capillaries) are not useful as means of process control, adjusting buffer pH and the electric field27 seem to be possible handles for true feedback control of the separation, although closed-loop operation does not seem to have been attempted. 

Since electroosmotic flow can exist in both the interparticle and intraparticle spaces, numerous studies have focused on the existence of intraparticle flow in CEC. Several groups have investigated the existence of electroosmotic flow in wide-pore materials [41-44], A model was developed to estimate the extent of perfusive flow in CEC packed with macroporous particles [41] by employing the Rice and Whitehead relationship. Results showed the presence of intraparticle EOF in large-pore packings (> 1000 A) at buffer concentrations as low as 1.0 mM. Additional parameters had been investigated [43,44] to control intraparticle flow by the application of pressure to electro-driven flow. Enhancement in mass transfer processes was obtained at low pore flow velocities under the application of pressure. The authors pointed out that macroporous particles could be used as an alternative to very small particles, as smaller particles were difficult to pack uniformly into capillary columns. 

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