On-line Dialysis Membranes

The dialysis membrane is a critical factor contributing to the efficiency of the dialysis separation system. In addition to their specificity on allowing the passage of a certain species of analyte while obstructing interfering components, which is a common requirement for all dialysis systems, on-line systems demand better mechanical and kinetic properties. Dialysis membranes of FI systems are expected to allow the achievement of high transfer factors within very short contact times of usually less than 30 seconds, while the membranes should be able to withstand hundreds of analytical cycles without significant deterioration in their performance. 

Dialysis membranes usually belong to one of the following types microporous, homogeneous and ion-exchange, of which microporous membranes with pore sizes of 1-10 /im are most f equentl used. 

Homogeneous membranes effect transfer of a species from donor to acceptor by molecular diffusion. Therefore the dialysis factor depends on the solubility and diffusivity of the species in the membrane and not on their particle sizes. 

Van Staden and Rensburg [6] compared the performance of eight different commercially available membrane types possibly produced from cellophane, cellulose acetate or natural cellulose with pore sizes ranging from 1.5 to 6 /xm. The best dialysis performance was obtained using Technicon type C membrane which is probably made of cellophane. This was attributed to its smaller thickness of 0.015 mm and the relatively large pore size of 4-6 /xm. 

Some dialysis membranes are asymmetric, i.e. they allow solutes to pass through easily from one side, but much more difficult from the other side. Thus, Xie and Christian 

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