Diafiltration

Diafiltration is a membrane separation process that removes small solutes from a retained solution by adding solvent while filtering. It preserves retained macromolecules while exchanging the solution around them.

Tangential flow filtration: A membrane filtration method in which feed flows parallel to the membrane surface. Diafiltration commonly uses this flow pattern to limit membrane fouling while solvent and solutes pass through.
Ultrafiltration: A pressure-driven membrane process that retains macromolecules while allowing water and smaller solutes to pass. Diafiltration is commonly paired with ultrafiltration to wash a retained macromolecule solution.
Buffer exchange: Replacement of the dissolved components of a solution with those of a different buffer. Diafiltration removes the old buffer's small ions as incoming solvent supplies the new buffer.
Constant-volume diafiltration: Diafiltration performed while solvent addition balances permeate removal to keep retentate volume approximately constant. This is the standard washing mode when maintaining product concentration is desired.
Permeate: The fluid and dissolved species that pass through a membrane during filtration. Small solutes leave the retained solution in the permeate during diafiltration.
Semipermeable membrane: A barrier that permits some species to pass while restricting others. Selective passage through this barrier makes solute removal possible.
Protein purification: The isolation of a target protein from other molecules in a biological mixture. Diafiltration removes salts and small contaminants while retaining proteins.
Discontinuous diafiltration: Diafiltration in which solvent is added in batches between filtration steps. It contrasts with continuous solvent addition and can require less specialized control.
Retentate: The fluid and material retained on the feed side of a membrane. Diafiltration aims to keep the desired macromolecules in this stream.
Solvent: A substance that dissolves another substance to form a solution. Added solvent replaces permeate volume and carries dissolved small solutes through the membrane.

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