# DBT JRF 2022 — Question 105

Exam: DBT JRF 2022 · Section: B · Topic: Bioprocess Engineering › Downstream Processing - Diafiltration · Difficulty: Medium

## Question

A diafiltration process is used to remove salts from a protein solution of 1L. After 1L of make-up buffer (without salts) has been added to this system, the residual salt concentration would be _____________ of the original.

## Options

- **A.** 50%
- **B.** 36%
- **C.** 10%
- **D.** 5%

## Answer

Option B

## Explanation

Continuous diafiltration: salt concentration after 1 volume exchange: Diafiltration replaces the solvent and removes small molecules (like salt) from a macromolecule solution by adding make-up buffer while simultaneously filtering. For continuous diafiltration, after adding $N$ diavolumes (volumes of make-up buffer equal to the initial retentate volume): $\text{Fraction remaining} = e^{-N}$ After 1 diavolume ( $N = 1$ ): $\text{Fraction of salt remaining} = e^{-1} \approx 0.368 = 36.8\%$ Interpretation: After adding 1 volume of buffer (1 diavolume), ~36.8% of the original salt remains. Comparison: After 2 diavolumes → $e^{-2} \approx 13.5\%$ ; after 5 → $e^{-5} \approx 0.67\%$ . For efficient desalting, typically 3–5 diavolumes are used to achieve >95% removal. Answer: Salt fraction remaining after 1 diavolume $= e^{-1} \approx 36.8\%$ .

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