Separator/accumulator thickness

By filtration mechanism. Although the mechanism for separation and accumulation of solids is not clearly understood, hvo models are generally considered and are the basis for the apphcation of theoiy to the filh ation process. When solids are stopped at the surface of a filter medium and pile upon one another to form a cake of increasing thickness, the separation is called cake filtration. When solids are trapped within tne pores or body of the medium, it is termed depth, filter-medium, or clarifying filtration. 

Fig. 2.4.5 Profile of a phantom made of three 2-mm thick rubber layers separated by glass slides of 2- and 1-mm thick. The CPMG sequence was executed with the following parameters repetition time = 50 ms, tE = 0.12 ms, number of echoes = 48 and 64 accumulations. The profile was scanned with a spatial resolution of 100 pm in 5 min.

Because the mucus layer or the underlying cells may serve as either final accumulation sites of toxic gases or layers through which the gases diffuse en route to the blood, we need simplified models of these layers. Altshuler et al. have developed for these layers the only available model that can be used in a comprehensive system for calculating tissue doses of inhaled irritants. It assumes that the basement membrane of the tracheobronchial region is covered with three discrete layers an inner layer of variable thickness that contains the basal, goblet, and ciliated cells a 7-Mm middle layer composed of waterlike or serous fluid and a 7-Mm outer layer of viscous mucus. Recent work by E. S. Boatman and D. Luchtel (personal communication) in rabbits supports the concept of a continuous fluid layer however, airways smaller than 1 mm in diameter do not show separate mucus and serous-fluid layers. 

The ThFFF separation system is made up of a flat ribbon-like channel obtained by placing a trimming-spacer between two flat bars kept at different temperatures (at the upper wall) and (at the lower wall), with AT = Tg- The thickness of the spacer defines the channel thickness w. In the channel cross section, the thermal diffusion process pushes the analyte toward the so-called accumulation wall, usually the cold wall (thermophobic substances) the combination of the flow profile and the thermal diffusion produces the fractionation. 

In the fourth subtechnique, flow FFF (F/FFF), an external field, as such, is not used. Its place is taken by a slow transverse flow of the carrier liquid. In the usual case carrier permeates into the channel through the top wall (a layer of porous frit), moves slowly across the thin channel space, and seeps out of a membrane-frit bilayer constituting the bottom (accumulation) wall. This slow transverse flow is superimposed on the much faster down-channel flow. We emphasized in Section 7.4 that flow provides a transport mechanism much like that of an external field hence the substitution of transverse flow for a transverse (perpendicular) field is feasible. However this transverse flow—crossflow as we call it—is not by itself selective (see Section 7.4) different particle types are all transported toward the accumulation wall at the same rate. Nonetheless the thickness of the steady-state layer of particles formed at the accumulation wall is variable, determined by a combination of the crossflow transport which forms the layer and by diffusion which breaks it down. Since diffusion coefficients vary from species to species, exponential distributions of different thicknesses are formed, leading to normal FFF separation. 

It has already been stated that a simple way to enhance the resolution of an FFF measurement is to reduce the channel thickness. This however can lead to other problems as the effects of surface irregularities are enhanced, due to the increase of the surface-to-volume ratio of the channel, and lead to increasing, unpredictable solute-wall interactions, etc. Furthermore, non-uniformities in the channel planarity will also be a problem with very small channel thicknesses, especially in Fl-FFF where the accumulation wall is a membrane. Another possibility for the reduction of H is to reduce the flow velocity of the carrier liquid which, in turn, leads to longer retention times and slower separation. However, in Fl-FFF, one has the possibility to increase the flow rate with cost to resolution but simultaneously increase also the cross-flow rate. Nevertheless, this enhances sample absorption onto the membrane. The third possibility for the reduction of H is to increase the solute diffusion. This can be done by decreasing the solvent viscosity by increasing the temperature. 

Fl-FFF is the most universally applicable FFF technique as the separation only relies on differences in the diffusion coefficients. Thus, it nicely complements S-FFF or Th-FFF with respect to size distribution analysis [225]. Fl-FFF is capable of separating almost all particles (up to 50 pm [226] or even much larger) and colloids and polymers down to -2 nm [17] or 103 g/mol [227]. The lower limit is determined by the pore size of the membrane material. The need for such membrane covering the accumulation wall is the principle disadvantage of Fl-FFF due to possible interactions with the solute and the danger of a membrane-induced non-uniformity in the channel thickness, especially since thinner channels are highly favored for faster separations. However, the advantages of Fl-FFF usually more than balance the potential pitfalls and sources of error. Consequently, Fl-FFF is the FFF technique which has been developed the most in recent years in instrumentation [48] and has found the most widespread distribution. 

Traces of certain specific elements (such as C, S, Si, Cl and O) and inclusion of foreign particles or gases may connect both sides of the membrane (with a thickness of 10 pm or less) and thus render it unsuitable for separation purposes. Aluminum foils have b made down to a thickness of 10 pm and special fabrication methods can be used to produce palladium (or its alloys) foils with a thickness under 1 pm [Shu et al., 1991]. Commercially available Pd alloy foils, however, are in the 10 to 100 pm range. Cold rolling often generates lattice dislocation and it can enhance hydrogen solubility in palladium and some of its alloys due to the accumulation of excess hydrogen around the dislocation. 

The characteristic feature of flow FFF is the superimposition of a second stream of liquid perpendicular to the axis of separation. This cross-flow drives the injected sample plug toward a semipermeable membrane that acts as the accumulation wall. The cross-flow liquid permeates across the membrane and exits the channel, whereas the sample is retained inside the channel in the vicinity of the membrane surface. Sample displacement by the cross-flow is countered by diffusion away from the membrane wall. At equilibrium, the net flux is zero and sample clouds of various thicknesses are formed for different sample species. As with other FFF techniques, a larger diffusion coefficient D leads to a thicker equilibrium sample cloud that, on average, occupies a faster streamline of the parabolic flow profile and subsequently elutes at a shorter retention time t,. For well-retained samples analyzed by flow FFF, t, can be related to D and the hydrodynamic diameter d by... 

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