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.”