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Capillary Electrophoresis and Capillary Electrochromatography

Capillary Electrophoresis and Capillary Electrochromatography. By Naaimat Muhammed. Naaimat’s website   for Capillary Electrophoresis and Capillary Electrochromatography. Electro Osmotic Flow.

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Capillary Electrophoresis and Capillary Electrochromatography

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  1. Capillary Electrophoresis and Capillary Electrochromatography By Naaimat Muhammed

  2. Naaimat’s website   for Capillary Electrophoresis and Capillary Electrochromatography

  3. Electro Osmotic Flow • You can't approach the subject of capillary electrophoresis without immediately running into something called EOF or Electro Osmotic Flow . • EOF is a potent force in CE, and an explanation of exactly how it happens is a bit complex. • Fortunately, it is quite possible to be successful with capillary electrophoresis without understanding the theory behind it .

  4. Here are the basic concepts • EOF is a process that moves fluid from one end of a capillary tube to another. That is, a capillary tube can act as a pump (under the right conditions). • The right conditions include an electrical field, charged particles that are free to move in solution, and fixed charges on the wall of the capillary. • The pumping action is driven from the wall of the capillary. The velocity of flow drops dramatically as distance from the wall increases. This is why the phenomenon is mostly exploited in capillary tubes, where the distance from the wall to the middle of the capillary is small.

  5. The fluid we are most commonly pumping through the capillary is water. • The water must contained charged particles (ions). • They are surrounded by what is called a "hydration shell" of water molecules. • Any movement in one direction would be more-or-less balanced out by movement in the opposite direction, as the charged particles each sought a region of opposite charge.

  6. Effects of various system changes on EOF • Fluid driven by EOF will move toward the electrode that has the same charge as the wall of the capillary • EOF depends on the capillary wall charge; anything that decreases that charge will decrease EOF. High concentration buffers will both decrease wall charge and EOF. The flow can be shown to decrease as the square root of the buffer concentration • EOF decreases as viscosity increases. This is simply because it is harder to pump a thick liquid than it is to pump a thinner one. The increase in EOF than accompanies an increase in temperature is largely because of viscosity effects. • The addition of organics such as methanol or acetonitrile to a CE buffer will decrease the surface charge and should decrease EOF

  7. EOF never goes to zero (although it can become so slow that it is negligible) because the charge on the wall of the capillary can never be fully eliminated. EOF can even be demonstrated in a buffer filled TeflonÔ tube, because ions will adsorb to the inner surface of the tube. • The surface area of the particles may be so much greater than the inner surface area of the capillary that the effect of the inner surface becomes negligible.

  8. EOF and theoretical titration curve

  9. EOF differs from pressure driven flow • When pressure is applied at one end of a fluid filled tube, fluid will begin to move through the tube • Because of this friction, the velocity of pressure driven flow is greatest at the center of the tube, and slowest at the walls. This is known as Parabolic or Laminar flow

  10. The velocity of the electroosmotic flow through a capillary is given by the Smoluchowski equation

  11. Capillary electrochromatography • Capillary electrochromatography (CEC) is an emerging technique that can be applied to the separation of neutral compound mixtures.

  12. References: • 1. Jamie E. Elsila, Nathalie P. de Leon, and Richard N. Zare* • Department of Chemistry, Stanford University, Stanford, California 94305-5080 • 2. http://www.ceandcec.com/presentation.htm • 3. http://www.hbc.ukans.edu/CBAR/Electrochrom.htm • 4. Lukacs, K. D. and Jorgensen, J. W. (1985) Capillary zone electrophoresis: effect of physical parameters on separation efficiency and quantitation J. High Resolut. Chromatogr. Chromatogr. Commun., 8, 407. • 5. Smoluchowski, M. V. (1905) Elektrosche kataphorese. Physik. Z. 6, 529 • 6. Tsuda, T. (1994) Control of electroosmotic flow in capillary electrophoresis. Chapter 22 in Handbook of Capillary Electrophoresis, (J. Landers, ed.)

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