Electroosmotic mobility control

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 ... 

Microfluidic and nanofluidic chips have a wide range of applications in the chemical, biomedical, environmental, and biology areas, where a variety of chemical solutions are used. With the development of microfabrication technology, many new materials such as PDMS and poly (methyl methacrylate) (PMMA) are also employed for chip fabrication. Since each pair of sohd-liquid interface has its unique zeta potential and electroosmotic mobility, which have significant influences on flow control in such small-scale devices, it is very important to experimentally determine these two parameters using the current monitoring technique in order to develop microfluidic and nanofluidic devices for various applications. 

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. 

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... 

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. 

Because the amount of analytes injected depends on both the ion mobility and the electroosmotic flow, variables that are difficult to control, electrokinetic injection is adopted only when hydrodynamic injection is not applicable even though it is theoretically superior in terms of selectivity. A potentially... 

Capillary Zone Electrophoresis (CZE) is a widely used technique that is capable of separating anions as well as cations in the same run. The capillary as well as the buffer reservoirs are filled with the same electrolyte. A voltage is applied and the analyte ions move independent of each other with a different velocity according to their apparent mobility, [ic p. In contrast to gel electrophoresis, where EOF is generally minimised or avoided, a controlled electroosmotic flow is often used during a CZE separation. 

CZE separations are based solely on the differences in the electrophoretic mobilities of charged species, either in aqueous or nonaqueous media (this latter often referred to as nonaqueous capillary electrophoresis, NACE). In CZE, the migration of a species within the capillary column is the net result of mass transport phenomena and chemical equilibria. Two modes of migration are possible, that is, under suppressed electroosmotic flow (EOF), achieved at low pH buffers or by the use of surface modified capillaries, and in the presence of EOF in the latter, two possibilities arise separations under co- and counter-EOF, depending on the relative mobility of the analyte and EOF itself. With the proper control of electrolyte composition (buffer type regarding both co- and counterions, buffer pH and concentration, as well as additives), the analyte mobility can be altered. Flow characteristics are also dependable on the electrolyte composition as well as on the capillary surface condition. 

Microchip capillary electrophoresis (MCE), like conventional capillary electrophoresis (CE) and most other chemical separation techniques, is heavily dependent on surface chemistry. This connection is obvious for techniques that use a stationary phase but is also apparent for separation modes like zone electrophoresis where the separation is based on the mobility of the ions. An important component of zone electrophoresis is the electroosmotic flow (EOF), which is a surface-derived phenomenon. Furthermore, the degree of surface hydrophobicity can cause adsorption resulting in band broadening. As a result of the significance of the surface chemistry, it is important to develop an understanding of methods that provide control of surface chemistry in MCE. This problem is more... 

---