Diafiltration

Ultrafiltration and microfiltration are able to prepare reasonably concentrated samples of  macromolecules, but they permit only a limited separation of the retained solutes from the smaller or more permeable components present in the solution needing to be filtered. In order to remove such smaller components more efficiently from the retained species, the feed solution can be washed in a process known as diafiltration. In this process, water or an appropriate buffer solution is added to the retentate during filtration, with the membrane-permeating species being removed as the excess fluid or buffer is filtered. Such a process may be undertaken in two different modes: continuous, in which the wash fluid is added throughout filtration, or discontinuous, in which the feed solution is first concentrated by a predetermined amount using standard ultrafiltration techniques, the wash fluid is then added, and the process repeated any number of times until the final solute concentration is achieved. When using continuous diafiltration, the amount of solute remaining in the retentate may be determined from a mass balance and expressed as

Where C_R is the concentration of the retentate, C₀ the initial concentration, N_D is the number of diavolumes, defined as the total volume of the wash fluid divided by the initial volume of the solution, and S is the solute sieving coefficient.

For discontinuous diafiltration, it is given by 

Where n corresponds to the number of discontinuous diafiltration procedures.