Asymmetric channel

Figure 30 lists schematics of three asymmetric channel geometries (A, and C) inspired by the designs presented in Ogyu et al. (2004), Young et al. (2004) and Bardon et al. (2004) studied (at the same value of open area fraction). 

FFF techniques were pioneered by Giddings in 1966 [1]. Starting from this point, a remarkable development has taken place resulting in a diversity of different FFF methods. Figure 1 gives an overview of the different techniques with their time of invention. The number of different methods is directly related to the variety of force fields which can be applied for the separation of the samples. Practically, only three of those FFF methods are commonly used and commercially available at the present time namely sedimentation-FFF (S-FFF), flow-FFF (Fl-FFF) and thermal-FFF (Th-FFF). The range of possible techniques was established in the early years whereas the main development of the last years is seen in a continuous optimization of the methodology and the instrumentation. This becomes most evident for the case of flow-FFF, where an asymmetrical channel with better separation characteristics has been developed. 

To calculate the retention ratio R, one needs to know the void time t0 (See Eq. (7)) which for an asymmetrical channel is given by [249] ... 

In 1996, a paper was published which was dedicated to selecting suitable membranes for separations in organic solvents [466]. Membranes tested in an asymmetrical channel included polysulfone MWCO 20,000 g/mol, regenerated cellulose MWCO 20,000 g/mol, PTFE pore size 0.02 mm, polyaramide MWCO 50,000 g/mol, poly(vinylidene fluoride) MWCO 50,000 g/mol, poly(phenylene oxide) MWCO 20,000 g/mol and a DDS fluoro polymer MWCO 30,000 g/mol. The first membrane was tested with water, the others with THF or a THF/ace-tonitrile mixture. Numerous problems occurred with the different membranes. The best membrane for THF was found to be the DDS fluoro polymer membrane. 

J. J. Kirkland, C. H. Dilks, S. W. Rementer, and W. W. Yau. Asymmetric-channel flow fleld-flow fractionation with exponential force-field programming. J. Chromatogr. 593 339 (1992). 

The separation efficiency of an asymmetrical flow FFF system has been known to be higher than that of a conventional symmetrical channel. Because an asymmetrical channel utilizes only one frit, nonuniformity of flow that could arise from the imperfection of frits can be reduced. In addition, the initial sample band can be kept narrower in an asymmetrical channel, due to the focusing-relaxation procedure, which is an essential process in an asymmetrical channel. The relaxation processes, which provide an equilibrium status for sample components, are necessary in both symmetrical... 

In addition to the rectangular channels in FIFFF described thus far, a cylindrical channel system has been developed with the use of hollow fibers in which the fiber wall is made of a porous membrane, as shown in Fig. Ic. It also requires a focusing-relaxation process, as does an asymmetrical channel. Retention in hollow-fiber flow FFF (HF-FIFFF) is controlled by the radial flow, which effectively acts as the cross-flow of a conventional flow FFF system, and the retention in a hollow fiber resembles that of an asymmetrical channel system. 

Frit-inlet asymmetrical flow field-flow fractionation (FIA-FIFFF) [1-3] utilizes the frit-inlet injection technique, with an asymmetrical flow FFF channel which has one porous wall at the bottom and an upper wall that is replaced by a glass plate. In an asymmetrical flow FFF channel, channel flow is divided into two parts axial flow for driving sample components toward a detector, and the cross-flow, which penetrates through the bottom of the channel wall [4,5], Thus, the field (driving force of separation) is created by the movement of cross-flow, which is constantly lost through the porous wall of the channel bottom. FlA-FlFFF has been developed to utilize the stopless sample injection technique with the conventional asymmetrical channel by implementing an inlet frit nearby the channel inlet end and to reduce possible flow imperfections caused by the porous walls. 

Sobey, I.J. 1982. Oscillatory flows at intermediate Strouhal number in asymmetric channels. J. Fluid Mech. 125 359-373. 

Figure 5. Asymmetrical channel for FlFFF. (Reproduced with permission from reference 20. Copyright 1989.)...

A specific advantage of the asymmetrical channel design in FlFFF is that the injected sample can be prefocused into a very sharp band before... 

Asymmetric channel structure in (Er(-)-6.62) from four erbium ions and four L-tartrate ligands (reproduced with permission from reference, copyright 2005, The Royal Society of Chemistry). 

Retention in the separation segment of the FIA-HFFF channel is expected to be equivalent to that observed in a conventional asymmetrical channel system, if complete hydrodynamic relaxation can be obtained. It will follow basic principles, as shown by the retention ratio, R, given by... 

There are two main categories of flow FFT channel systems, depending on the use of frit wall. The above-described flow FFF system has a frit on both walls this is classified as a symmetrical channel, as shown in Fig. la. An asymmetrical channel system is being widely studied in which only one permeable frit wall is used, at the accumulation wall, and the depletion wall is replaced with a glass plate (Fig. lb). In an asymmetrical channel, part of the flow entering the channel is lost by the accumulation waU and this acts as a field force to retain the sample components in the channel, as does the cross-flow in a symmetrical channel. 

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