Transfer across membranes definitions

The definitions of effective diffusivity tensors are key parameters in the solution of the transport equations above. For an isotropic medium, the effective diffusivity is insensitive to the detailed geometric structure, and the volume fraction of the phases A and B influences the effective diffusivity. When the resistance to mass transfer across the cell membrane is negligible, the isotropic effective diffusivity, Ds e = Dg eI may be obtained from Maxwell s equation... 

It should be mentioned at this point that no definitive function has been found to be associated with the carbohydrate moieties of immunoglobulins, most of which are associated with the Fc fragment. Hinrichs and Smyth (1970) have obtained some evidence that a portion (40%) of rabbit IgG molecules bear a Ch2 oligosaccharide, and these are not transferred across the placental membrane. It is interesting that in human IgM, which is also known not to cross the placental membrane, the Ch2 oligosaccharide occupies a similar position, i.e., close to the inter-H-chain disulfide bond. Moreover, carbohydrate has been reported (Swenson and Kem, 1967 Melchers and Knopf, 1%7) to play a role in the... 

The membranologists denote as facilitated transport [12] a definite sort of mass transfer across the membrane which is enhanced by a chemical reaction in the phase towards which this transport is directed. Facihtated ion transfer is also observed among processes taking place at ITIES [13]. 

To obtain a high selectivity, i.e., discrimination between the analytes and various unwanted matrix compounds, membrane extraction has a clear advantage over other sample preparation techniques, as all compounds that reach the analytical instalment must travel through the membrane. There is no direct connection and possibility for transferring compounds into the analytical instmment in other ways. This is not the case with other extraction techniques. With SPE, SPME, etc., there is a definite possibUity that matrix components are absorbed on the sorbing phase and subsequently being eluted into the extract. With LEE, such a transfer is less probable and it is generally considered that extracts after LLE are cleaner. The possible and common problem of the formation of emulsions at the phase interface with LLE, which is avoided with aU types of membrane extraction, is a source of contamination across the phase border. 

Reverse osmosis (and nanofiltration) by contrast operate by diffusion from a solution. Under the high differencial pressure across the membrane, the solvent from the solution actually dissolves in the material of the membrane, diffuses across it and transfers out into the clean solvent on the ocher side. It is not a perfect separation, because the dissolved species from the feed solution have a definite ability to diffuse through it as well, but the diffusion coefficient for the solvent is so much higher than that for the solute that the separation is virtually complete. 

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