Electroosmotic Flow EOF

For a better flow control using only EOF, secondary hydrodynamic flow (HDF) should be avoided. This could be achieved by ensuring that all solution reservoirs were filled to the same liquid level to avoid HDF [159]. In addition, HDF could be prevented by closing the inlet reservoir to the atmosphere using a valve. In this way, better EOF control and more reproducible CE separation (RSD of migration time decreased by 10-30 times) can be achieved [631]. 

Microfluidic Lab-on-a-chip for Chemical/Biological Analysis and Discovery 

This pumping technique uses no moving parts, however, and does not require sophisticated fabrication techniques. It is relahvely easy to implement and operate in microchannels and other miniaturized systems [287]. The disadvantage of EOF is its dependency on huid physiochemical properties (ionic strength, pH, organic content, etc.) to develop the surface charge, so its effechveness will change for each huid used in the pump [290]. 

EOF has been applied to electrophoresis (Sechon 7.7.3), electrospraying [291, 292], and electrochromatography [293]. Pressures of over 5 bar have been generated using EOF techniques [290]. Utilizing high pressures, mists or aerosols can be generated that are often sent to analyzer equipment such as electrophoresis, mass spectrometry, and electrospray ionization. 

Electrochromatography uses EOF to drive a mobile phase through a packed microchannel or capillary (50-150-pm diameter) [293]. The Helmholtz-Smolu- 

A final application of EO F is the ability to direct and control the stream profile. Besselink et al. have developed a system that is designed to accurately position the middle of three streams in a laminar flow chamber [287]. They proposed that the system can be used for sending the middle fluid to selected parts of a chip, which contains a multi-sample array. The system is designed such that three parallel microchannels feed into the flow chamber (a wider channel). The two outer streams sandwich the middle stream to be analyzed. The guiding action of the two outer streams causes the center stream to flow across the chamber. By increasing the flow of one outer stream and decreasing the flow of the other, the middle stream is pushed to the side, as shown schematically in Fig. 7.25 and pictorially in Fig. 7.26. Control of the middle stream s width is adjusted by the flow rate ratio of the guiding streams and the sample stream [287]. 

The electrophoretic migration of all species within the capillary is called electroosmotic flow or EOF. The phenomenon of EOF is a fused silica capillary may be explained as follows [2]. 

Silanol groups in the fused silica are partially ionized. 

This gives a negative zeta potential (Z) at the capillary wall and introduces an excess of hydronium cations into the solution. 

Water dipoles and ions from the electrolyte solution form a fluid profile next to the solid-liquid interface, a relatively rigid and stagnant inner Helmholtz layer closer to the wall. 

In the CE capillary both Helmholtz layers are situated in an electric field with an orientation parallel to the solid surface. 

At the surface of the capillary, an electric double layer is formed. The negative surface charges are compensated by positive ions from the buffer solution. These 

Si Si Si Si native siiica surface Si Si Si Si Si ionised siiica surface Si 

Upon application of an electric field, the cations in the diffuse layer move towards the cathode and drag the bulk solution with them. This movement of 

the direction of the /j-eof of the bulk solution is against the direction of the /Fgp of the analyte ions. This is because for most biomolecular separations, the analyte ions are negatively charged and will be dragged towards the anode, whereas the EOF is directed towards the cathode. 

The flow profile of the EOF has the form of a plug (Fig. 3.4). The flow velocity is identical over the whole capillary diameter, except for the slower moving diffuse layer close to the capillary wall. This homogeneous velocity distribution minimises band broadening and, thus, increases separation efficiency. A radically different situation occurs with the pressure driven flow used in liquid chromatography. Here, the flow profile is parabolic the flow velocities have a large distribution over the column diameter. Analytes in the middle flow considerably faster than analytes 

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Liquid transport is achieved by hydrostatic action, pumping or electroosmotic flow (EOF). So far, chip reactors have been employed at low to very low flow rates, e.g. from 1 ml min to 1 pi min. Applications consequently were restricted to the laboratory-scale or even solely to analytics. However, this is not intrinsic. By choosing larger internal dimensions, similar throughputs as for the other classes of liquid or liquid/liquid micro reactors are in principle achievable. 

Wilson, N. G., McGreedy, T, Microporous silica structures for the immobilization of catalysts and enhancement of electroosmotic flow (EOF) in micro reactors, in Ehreeld, W. (Ed.), Microreaction Technology 3rd International Conference on Microreaction Technology, Proc. of IMRET 3,... 

In the presence of electroosmotic flow (EOF), the mobility of a given molecule is a combination of its own mobility (which is now called apparent mobility, papp) and the mobility of EOF (pE0F). True mobility is then calculated by subtracting the mobility of EOF from apparent mobility ... 

Separation is performed using free-zone electrophoresis, where the capillary is filled with a separating buffer at a defined pH and molarity. This buffer is also called a BGE. During separation, the polarity is set to cathodic or anodic mode, also called normal and reverse mode, depending on the charge of the molecule cation or anion. For anions, the capillary is usually dynamically coated with an electroosmotic flow (EOF) modifier to reverse the EOF and separate the analytes in the co-electroosmotic mode. 

The low electroosmotic flow (EOF) of the PMMA chip material facilitated the rapid switching between analyses of explosive-related cations and anions using the same microchannel and run buffer (and without an EOF modifier) [29], This led to a rapid (<1 min) measurement of seven explosive-related cations and anions down to the low micromolar level. The presence of an 18-crown-6 ether modifier in the run buffer allowed separation of the peaks of the co-migrating ammonium and potassium ions. 

The electrophoretic separation principle is based on the velocity differences of charged solute species moving in an applied electric field. The direction and velocity of that movement are determined by the sum of two vector components, the migration and the electroosmotic flow (EOF). The solute velocity v is represented as the product of the electric field strength E and the sum of ionic mobility uUm and EOF coefficient /a OF ... 

Typically, buffers in the region of pH 7-9 have been used in MEEKC. At these pH values the buffers generate a high electroosmotic flow (EOF). Extreme values of pH have been used in MEECK specifically to suppress solute ionization. For example, a pH of 1.2 of the buffer has been used to prevent the ionization of acids (30,31). To eliminate the ionization of basic compounds, a buffer at pH 12 has been used. These pH values were used in MEEKC to measure the solubility of ionic compounds (30). High-pH carbonate buffers (31) were applied in place of the standard borate or phosphate buffers. 

The state of the channel surface is very important in order to obtain a good performance of the microchips. This depends on the chip material and pretreatment, mainly. Hence, prior to using the microchip, channels have to be pretreated adequately, which is needed in order to clean the channels and obtain an appropriate electroosmotic flow (EOF) [159]. It can also be useful for performing a chemical cleaning of the working electrode. 

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 ... 

Basically, capillary electrochromatography (CEC) is a hybrid technique of HPLC and CE [1-3], which was developed in 1974 by Pretorius et al. [4]. CEC is expected to combine high peak efficiency, which is a characteristic of electrically driven separations, with high separation selectivity. As is the case for electrophoresis, a voltage is applied across the separation plateform and sample moves via electroosmotic flow (EOF). However, in analogy to liquid chromatography, the separation device contains a solid... 

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