Affinity Membranes

Ultrafiltration membranes are commonly employed as a 'polishing' stage of multistage separation processes for several commercially important proteins. Consequently attaching standard affinity ligands to create affinity membranes has become an actively researched area. The most obvious advantage of a membrane structure is the high rate of transport of the medium through the porous structure by filtration, thus minimizing the normally encountered diffusion limitations of mass transfer. High adsorption rates are achieved, especially if long distance electrostatic interactions are involved in the binding mechanism. However, in contrast to ion exchange membranes, similar high transport effects are not observed when used in the affinity mode, eliminating much of the initial attraction of this form of device. Other theoretically attractive features included: an inherent ability to control pore size across a very wide range, offering an opportunity to increase capacity of a given system; and ability to operate in either batch mode or filtration mode. In both cases experimental data have not confirmed these assumed advantages; a 10-fold change in the pore size resulted in only a two-fold capacity increase and when in filtration mode, although adsorption is fast, severe peak broadening on elution is experienced.

The chemistry relevant to particulate media is identical to that required for membranes, in effect making the systems compatible and allowing an easy technology interchange. The covalent bonding of affinity ligands to the surface of a membrane follows exactly the same chemistry as that applied to partic-ulate media, and the same adsorption/desorption principles apply to both. Consequently the only difference between membrane systems and those of conventional chromatography is the exploitation of the characteristics of the membrane matrix compared with a particulate bed. Although the high mechanical strength of membranes is one major advantage, plus the scale-up is claimed to be very easy by stacking membranes (although scaling affinity columns is also very straightforward), it has been discovered that if the pressure drop across the membrane is too high sealing problems occur; the mobile phase then flows beyond the edges and past the membranes. Furthermore affinity membranes should be capable of use with unclarified extracts, but is has been generally observed that membrane capacity and lifetime are progressively reduced with time of use. Even with clarified broths, membrane fouling regularly occurs. This is almost certainly the reason why affinity membranes have not found favour in large scale processing.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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