Capillary flow efficiency

For a while now, the problem of flow and heat transfer in heated capillaries has attracted attention from a number of research groups, with several applications to engineering. The knowledge of the thermohydrodynamic characteristics of capillary flow with evaporative meniscus allows one to elucidate the mechanism of heat and mass transfer in porous media, to evaluate the efficiency of cooling system of electronic devices with high power density, as well as to optimize MEMS. 

Qualitatively equation (7.15) is adequate to describe tiM f influence of layer quality, selectivity, and zone position in the 1 chromatogram upon resolution for a single unidimensional development under capillary flow controlled conditions. The variation of R, with Rf is not a simple function as can be seen from Figure 7.6. The resolution increases with the layer efficiency in a manner that depends linearly on the R, value. — Relatively small changes in selectivity have an enormous impact on... 

The transfer protocols are the same for DNA and RNA. In opposition to the methods given in Protocol 2.5.3, the forces driving the biomolecules from the separating gel to the receiving membrane are diffusion and capillary flow. This type of transfer is also applicable for proteins, but because the pores of polyacrylamide gels used for protein separation are mostly smaller, transfer times are longer and transfer efficiencies are lower than by electrotransfer. 

The large transport efficiencies through the capillary for the aerosol gas-jet technique can be explained in terms of the laminar flow profile of the gas inside the capillary [32], According to Bernoulli s law, the gas pressure at the center of the capillary, where the velocity is highest, is lower than near the capillary walls. Thus, when the sub pm-sized aerosol particles drift away from the center of the capillary, they are subject to a restoring force toward the center of the capillary. Transport efficiencies of over 50% have routinely been achieved for transport capillary lengths over 20 meters. 

The melt viscosity of LCPs is sensitive to thermal and mechanical histories. Quite often, instrumental influences are important in the value of viscosity measured. For example, the viscosity of HBA/HNA copolyesters are dependent on the die diameter in capillary flow (59). LCP melts or solutions are very efficiently oriented in extensional flows, and as a result, the influence of the extensional stresses at the entrance to a capillary influence the shear flow in the capillary to a much greater extent than is usually found with non-LC polymers. 

For packed columns the efficiency is given by the quality and particle size of the column packing. For wall coated capillaries the efficiency is a matter of the coating film properties. Furthermore, the efficiency depends on the injection mode, solvent effects, flow rate and column dimensions. 

True electroosmotic flow has been demonstrated in horizontally mounted layers at modest field strengths (< 1 kV / cm) with mobile phases of high dielectric constant. Still unclear is which solvents can be used, the need for prewetted layers and ions as current carriers, the effect of local heating on zone profiles, and the effect of binder chemistry on flow, mass transfer, and thermal effects. Compared with capillary flow faster separations have been demonstrated, but the influence of flow velocity on efficiency was only treated in a qualitative sense. So far no comprehensive analysis of the kinetic properties of separations under conditions of electroosmotic flow have been performed in thin-... 

For forced flow separations a constant plate height independent of the solvent-front migration distance is obtained. Figure 6.3. The minimum plate height for capillary flow is always greater than the minimum for forced flow. This is an indication that the limited range of capillary flow velocities is inadequate to realize the optimum kinetic performance for the layers. At the mobile phase optimum velocity, forced flow affords more compact zones and shorter separation times compared with capillary flow. As expected the intrinsic efficiency increases with a reduction of the average particle size for the layer. 

Flow Rate Versus Capillary Column Efficiency... 

Figure 10.3A shows a common interface in use today. Most GC-MS systems use capillary columns, and fused silica tubing permits an inert, high efficiency direct transfer between the two systems. For capillary flow rates of 5 mL/min or less, a direct interface is possible. Bench-top GC-MS systems can easily handle these low flow rates, and they provide better sensitivity (transfer of total sample) and better preservation of GC results. 

From Eq. 1, it follows that the capillary radius r has a very important effect on capillary flow a smaller radius leads to more efficient flow. The methods used for preparation of commercial stationary phases and supports cannot ensure aU pores are of equal, ideal diameter this results in side effects that contribute to the broadening of chromatographic spots. Other mechanisms of spot broadening are described below. 

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