74.
Which one of the following techniques CANNOT be used to remove salt from a protein solution

• (A) Ultrafiltration
• (B) Ion exchange chromatography
• (C) Gel filtration chromatography
• (D) Dialysis

  The correct answer is A. Ultrafiltration.
  Ultrafiltration retains proteins (>10-100 kDa) using size-based membranes but allows salts (<1 kDa) to pass through only if the solution is diluted repeatedly; it cannot efficiently remove salts without significant protein loss or impractical dilution.

  Option Explanations

  A. Ultrafiltration uses semi-permeable membranes (MWCO 3-100 kDa) to concentrate proteins but retains salts unless diafiltration (buffer exchange) is performed 10-20x, making it inefficient for salt removal compared to other methods.

  B. Ion exchange chromatography binds proteins selectively while salts flow through or are washed away; elution with low-salt buffer completes desalting.

  C. Gel filtration chromatography separates salts (small molecules) from proteins via size exclusion—salts elute in total volume (Vt), proteins in void volume (V0).

  D. Dialysis equilibrates protein solution across a membrane (MWCO 1-14 kDa) against low-salt buffer; salts diffuse out over 12-24 hours with multiple changes.

  Protein Desalting Principles

  Techniques cannot remove salt from protein solution ultrafiltration highlights a common GATE Life Sciences misconception. Ultrafiltration concentrates proteins but requires diafiltration for salt removal, unlike true desalting methods.

  Method Comparison for Salt Removal

  Technique	Salt Removal Mechanism	Efficiency	Time
  Ultrafiltration (A)	Diafiltration (buffer exchange)	Poor without 10-20x dilution	Fast
  Ion Exchange (B)	Selective protein binding	High	1-2 hrs
  Gel Filtration (C)	Size exclusion	High (>95%)	30-60 min
  Dialysis (D)	Diffusion equilibrium	Excellent	12-24 hrs

  Why Ultrafiltration Fails

  UF membranes retain both proteins AND salts unless retentate is diluted repeatedly with salt-free buffer. Example: 1 M NaCl solution needs ~7 diafiltration volumes (700% buffer) for 99% salt removal. Protein recovery drops with each cycle.

  GATE Strategy

  PYQs test mechanism understanding: dialysis (diffusion), gel filtration (size), ion exchange (charge). Ultrafiltration = concentration, not desalting. Remember: “UF retains what dialysis removes.”