Pressure-driven systems

The flow profiles of electrodriven and pressure driven separations are illustrated in Figure 9.2. Electroosmotic flow, since it originates near the capillary walls, is characterized by a flat flow profile. A laminar profile is observed in pressure-driven systems. In pressure-driven flow systems, the highest velocities are reached in the center of the flow channels, while the lowest velocities are attained near the column walls. Since a zone of analyte-distributing events across the flow conduit has different velocities across a laminar profile, band broadening results as the analyte zone is transferred through the conduit. The flat electroosmotic flow profile created in electrodriven separations is a principal advantage of capillary electrophoretic techniques and results in extremely efficient separations. 

Figure 9.2 Pressure-driven (a) and electrodriven (b) flow profiles. Laminar flow in pressure-driven systems results in a bullet-shaped profile, wliile the profile of electroosmotic flow is plug-shaped, wliich reduces band broadening.

Figure 4.41 Flow profiles in a capillary coluan for a pressure driven system (A) and an electroosmotically driven system (B). (Reproduced with permission from ref. 653. Copyright Friedr. Vieweg 6 Sohn).

Another approach to the pulsatile delivery of drugs with an osmotic pressure driven system has been suggested by Amidon et al. [66], This system provides the option of an immediate bolus dose and a second dose that can be timed to be released at some subsequent point following the administration of the dosage form. One or both of the doses can be composed of multiparticulates, for example, that would themselves be sustained release systems. 

With the need to provide PCR-amplifiable DNA, multiple approaches for incorporation of the extraction protocol onto microchips were examined. Recent development includes the implementation of a solid-phase extraction of DNA on a microchip [49]. The extraction procedure utilized was based on adsorption of the DNA onto bare silica. The silica beads were immobilized into the channel using a sol-gel network. This method made possible the extraction and elution of DNA in a pressure-driven system. 

The most obvious advantage of an electroosmotic pumping system is the flow profile. Electrochromatography has the same pluglike profile that is characteristic of all other modes of CE. Although the pluglike flow is less perfect in packed columns than in open tubes, it is still more uniform than the laminar flow profile of a pressure-driven system. Consequently, the same capillary would be expected to provide higher efficiencies when used in CEC than when used in pressure-driven systems.31 35,36... 

The heat release per unit volume is about 1,500 times larger for electro-driven systems than for pressure-driven systems under typical operation conditions Although self-heating can have deleterious effects on performance in both pressure-driven and electro-driven systems, there is an important difference between the overall effects in these two modes. In pressure-driven systems, which use wide... 

First, a peptide synthesizer was modified to allow solid-phase oligosaccharide synthesis. This platform had the basic function of an automated synthesizer and allowed repeating cycles of glycosylation and deprotection to be mostly software controlled (Fig. 7.2a) [38], This robust pressure-driven system relies on an established... 

FIGURE 12 Three platforms for automated solid-phase oligosaccharide synthesis, (a) Pressure-driven system, (b) Syringe pump-driven system, (c) HPLC pump-driven system. 

Membrane installations operated in nuclear industry are pressure-driven systems majority of them are reverse osmosis plants. Uncontrolled growth of operation pressure may result in module damage and valves leaks resulted in contamination hazard. The selection of appropriate pumps and security devices can avoid the danger of pressure overgrowth and its detrimental implications. The security valves outlets have to be connected with existing waste distribution systems to direct the eventual leaks to the waste collecting tanks. 

Because there is no ionizable groups of the coating in the neutral capillary, the interaction between charged molecules with ionic capillary surface is eliminated. Also, the electro-osmotic flow (EOF) of a neutral capillary is eliminated. However, a continuous and adequate flow of the buffer solution toward the CE capillary outlet is an important factor for routine and reproducible CE-ESI-MS analysis in order to maintain a stable ESI operation, some low pressure applied to the CE capillary inlet is usually needed, especially when the sheathless interface is employed. The disadvantage of the pressure-assisted CE-ESI-MS is the loss of some resolution because the flat flow profile of the EOF is partially replaced by the laminar flow profile of the pressure-driven system. A typical neutral capillary is a LPA (linear polyacrylamide)-treated capillary. Karger and co-workers [6] used mixtures of model proteins, a coaxial sheath flow ESI interface. 

The flow of liquid caused by electro-osmosis displays a pluglike profile because the driving force is uniformly distributed along the capillary tube. Consequently, a uniform flow velocity vector occurs across the capillary. The flow velocity approaches zero only in the region of the double layer very close to the capillary surface. Therefore, no peak broadening is caused by sample transport carried out by the electro-osmotic flow. This is in contrast to the laminar or parabolic flow profile generated in a pressure-driven system, where there is a strong pressure drop across the capillary caused by frictional forces at the liquid-solid boundary. A schematic representation of the flow profile due... 

Fig.l Schematic representation of the flow profiles obtained with the same capillary column connected to an electric-driven system (a) and to a pressure-driven system (b). Arrows indicate flow velocity vectors. 

Pressure-driven systems are preferred compared to vacuum-driven systems for two reasons (a) Generation of pressures over 1 atm is important when viscous polymer networks are used for size separations and (b) interface to the mass spectrometer is simpler. 

There are much more variables that can be applied for the optimization of a separation in CE than in a pressure-driven system. These include the type and concentration of a chiral selector, the pH of the background electrolyte, the concentration and type of the buffer, the achiral buffer additives, the capillary dimensions and the nature of the inner surface, the EOF, the temperature, and so forth. 

Because the driving force of the flow is distributed along the wall of the capillary, the flow profile is nearly flat or pluglike, contrasting the laminar or parabolic flow generated by a pressure-driven system caused by shear forces at the wall. A flat flow profile is beneficial because it does not contribute to the dispersion of solute zones. The magnitude and direction of the EOF can be impacted by the type of electrolyte used, the pH, the ionic strength, the use of additives (e.g.. 

In pressure-driven systems a pressure gradient exists between the column inlet and outlet resulting in a change in volume-dependent terms over the length of the column... 

The linear velocity in a pressure-driven system is given by the following equation described by... 

By comparing the two Equations 5.3 and 5.5, it can be seen that the linear velocity u is proportional to dp in a pressure-driven system while it is independent of the particle diameter in an electrically driven system. Since plate height values are generally lowered as a result of using small diameter particles, it is possible in electrically driven systems to use very small diameter packing materials and still maintain high linear velocities to yield rapid and very efficient separations. 

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