What can diafiltration be used for?

Correct answer: To change buffer or remove small solutes

The direct answer is A: changing the buffer or removing small dissolved solutes. Diafiltration is a membrane-based downstream operation, commonly performed with ultrafiltration. The product is retained by the membrane while buffer and small solutes pass through. Fresh buffer is added as filtrate leaves, so unwanted salts or small molecules are washed out and the product becomes equilibrated in the new buffer. Option A is correct because these are the main uses of diafiltration. Option B is wrong because coloring a fermenter is unrelated to membrane separation. Option C is impossible and irrelevant; air cannot be made into metal by diafiltration. Option D is wrong because the process does not name cells; identification is done by records or analytical methods. A useful cue is: ultrafiltration concentrates, while diafiltration washes and exchanges buffer, although both may be used together.