Development with forced flow

Figure 7.2 Relation between the solvent front position and tine for (1) an enclosed layer with forced-flow development, (2) an exposed layer in a saturated chamber with capillary controlled flow, (3) a covered layer (sandwich chamber) with capillary controlled flow, and (4) an exposed layer in an unsaturated atmosphere with capillary controlled flow. (Reproduced with permission from ref. 30. Copyright Or Alfred Huethlg Publishers).

Kalasz, H. Bathori, M. Ettre, L.S. Polyak, B. Displacement thin-layer chromatography of some plant ecdyste-roids with forced-flow development. J. Planar Chromatogr. 1993, 6, 481-486. 

Organization for Economic Cooperation and Development Overpressnred Layer Chromatography (TEC with forced flow)... 

Equation (6.13) provides values that are about 25% smaller that those calculated by Eq. (6.12). Some typical results from theory or determined by experiment are summarized in Table 6.4. Results from theory are probably too high and represent an upper limit. Experiment indicates a zone capacity of about 12-14 for a single development with capillary flow. This rises to about 30 - 40 for forced flow. Automated multiple development with capillary flow provides a similar zone capacity to forced flow. Two-dimensional thin-layer chromatography employing different retention... 

Mobile-phase velocity is higher with forced-flow development than in capillary-flow TLC. The actual flow rate is influenced by the type of chamber (rectangular or sandwich, saturated or unsaturated), the pressure and solvent viscosity (OPLC), or the rotational speed (RPC) (Nyiredy et al., 1988a). Nyiredy (1992) discussed the relation among resolution, separation distance, and time for forced-flow planar chromatography compared to capillary flow. It was stated that for separation of nonpolar compounds by FFPC on silica gel, a separation time of 1—2.5 min over a separation distance of 18 cm can be used without great loss in resolution. By contrast, longer separation times are needed for separation of polar compounds. 

The aim of breaking up a thin film of liquid into an aerosol by a cross flow of gas has been developed with frits, which are essentially a means of supporting a film of liquid on a porous surface. As the liquid flows onto one surface of the frit (frequently made from glass), argon gas is forced through from the undersurface (Figure 19.16). Where the gas meets the liquid film, the latter is dispersed into an aerosol and is carried as usual toward the plasma flame. There have been several designs of frit nebulizers, but all work in a similar fashion. Mean droplet diameters are approximately 100 nm, and over 90% of the liquid sample can be transported to the flame. 

The treated water contains sufficient concentration of surface film-forming agents if cold water spends about 12 min and warm water at least 20 min in the tank [19]. Sudden temperature variations over 10°C must be prevented because the active form of Al(OH)3 is sensitive to them [20]. If mixing with cold water or subsequent warming cannot be avoided, a short-term electrolytic aftertreatment must be provided in a small reaction tank. The development of undisturbed protective films in the tubing assumes continuous water flow with forced circulation by pumps [20]. 

Figure 8.19 illustrates another example of the versatility of multidimensional OPLC, namely the use of different stationary phases and multiple development ("D) modes in combination with circular and anticircular development and both off-line and on-line detection (37). Two different stationary phases are used in this configuration. The lower plate is square (e.g. 20 cm X 20 cm), while the upper plate (grey in Figure 8.19) is circular with a diameter of, e.g. 10 cm. The sample must be applied on-line to the middle of the upper plate. In the OPLC chamber the plates are covered with a Teflon sheet and pressed together under an overpressure of 5 MPa. As the mobile phase transporting a particular compound reaches the edge of the first plate it must-because of the forced-flow technique-flow over to the second (lower) stationary phase, which is of lower polarity. 

The high-pressure oil flows in piping through a control valve. The control valve changes the direction of oil flow. A relief valve, set at a desired, safe operating pressure, protects the system from an over pressure condition. The oil that enters the cylinder acts on the piston, with the pressure acting over the area of the piston, developing a force on the piston rod. The force on the piston rod enables the movement of a load or device. 

Two-phase flows in micro-channels with an evaporating meniscus, which separates the liquid and vapor regions, have been considered by Khrustalev and Faghri (1996) and Peles et al. (1998, 2000). In the latter a quasi-one-dimensional model was used to analyze the thermohydrodynamic characteristics of the flow in a heated capillary, with a distinct interface. This model takes into account the multi-stage character of the process, as well as the effect of capillary, friction and gravity forces on the flow development. The theoretical and experimental studies of the steady forced flow in a micro-channel with evaporating meniscus were carried out by Peles et al. (2001). These studies revealed the effect of a number of dimensionless parameters such as the Peclet and Jacob numbers, dimensionless heat transfer flux, etc., on the velocity, temperature and pressure distributions in the liquid and vapor regions. The structure of flow in heated micro-channels is determined by a number of factors the physical properties of fluid, its velocity, heat flux on... 

A significant advance in this area was recently made by Li and coworkers [30,31], who developed a laminar flow technique, that allowed the direct contact of two liquids with better-defined mass transport compared to the Lewis cell. Laminar flow of the two phases parallel to the interface was produced through the use of flow deflectors. By forcing flow parallel to, rather than towards, the interface, it was proposed that the interface was less likely to be disrupted. Reactions were followed by sampling changes in bulk solution concentrations. 

Development in TLC is the process by idiich the mobile phase moves through the sorbent layer, thereby inducing differential migration of the suple components. The principal development modes used in TLC are linear, circular and anticircular with the velocity of the mobile phase controlled by capillary forces or forced-flow conditions. In any of these modes the development process can be extended by using continuous development or multiple development. 

For two-dimensional TLC under capillary flow controlled. conditions it should be possible to achieve a spot capacity, in theory, on the order of 100 to 250, but difficult to reach 400 and nearly impossible to exceed 500 [52,140]. Theoretical calculations indicate that by forced-flow development it should be relatively > easy to generate spot capacities well in excess of 500 with an upper bound of several thousand, depending on the choice of operating conditions. -fE... 

Resolution in forced-flow development is not restricted by the same limitations that apply to capillary flow controlled systems. The maximum resolution achieved usually corresponds to the optimum mobile phase velocity and R, increases approximately linearly with the solven)t migration distance (48). Thus there is... 

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