Diafiltration procedure

DiajUtration After concentration has removed permeable components from the system, a new buffer can be added to dilute the retained product back to its original volume. Repeated application of this procedure, known as batch diafiltration, is used to exchange one buffer for another and is conveniently implemented at lab scale. 

A minimalist extraction buffer is often recommended for protein purification [25], since most additives provide only a marginal improvement in yields, but will increase costs significantly. The removal of phenolics, for example by tangential-flow ultrafil-tration/diafiltration [25], is an important step that should be carried out as early as possible in the purification procedure, since these molecules can become covalently linked to amino acid side chains and can oxidize certain residual groups [123]. 

Diafiltration is a variation of ultrafiltration, in which fresh solvent is added to the feed solution to replenish the volume ultrafiltered, and in the process washes small molecules such as salts away from the retained macromolecules. Using appropriate replenishing solutions, diafiltration is a common procedure to perform buffer exchange of proteins. Alternatively, a dilute solution may be first ultrafiltered to concentrate the feed material, then diafiltered to purify the retentate. It is sometimes possible to fractionate a mixture of macrosolutes by sequential diafiltration with a series of membranes of progressively lower molecular weight cutoff ratings. 

The use of the charts is straight forward. For each recovery, there are two corresponding charts. One determines the optimum time cycle and the other determines the optimum diafiltration volume. For the case where the initial volume, membrane flux, desired recovery, and the time cycle are specified or known, the required membrane area can be determined from the corresponding Time Cycle Chart. The procedure is to first calculate P and Q based on an assumed area. Then, the time cycle is found from the chart. Finally, the area is adjusted until the time cycle read from the chart matches the specified time cycle. Once the area is determined, the optimum relative diafiltration volume can be found from the corresponding relative diafiltration Volume Chart. 

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