127. A diafiltration process is used to remove salts from a protein solution of 1L. After 1L of makeup buffer (without salts) has been added to this system, the residual salt concentration would be
________ of the original.
A. 50%
B. 36%
C. 10%
D. 5%

Introduction:

Diafiltration is a widely used technique in bioprocessing to remove low molecular weight solutes like salts from protein solutions. It leverages a semipermeable membrane to separate smaller molecules from larger ones such as proteins. A common question in this process is: How much salt remains after a certain amount of buffer has been added to the system?

This article explores how much salt remains after one volume exchange of buffer in a diafiltration process and why 36% of the original salt concentration is retained.

What Is Diafiltration?

Diafiltration is a membrane-based separation technique that removes small solutes by washing them out with a clean make-up buffer. As buffer is added, small solutes like salts pass through the membrane, while larger molecules like proteins are retained.

Understanding Volume Exchange:

Let’s define:

• Initial volume = 1 L of protein solution containing salt

• Buffer added = 1 L of salt-free buffer

• Membrane = Allows passage of salts, retains proteins

This setup is a constant volume diafiltration, where as buffer enters, an equal volume of filtrate exits. The reduction in salt concentration follows an exponential decay model:

C=C0×e−NC = C_0 \times e^{-N}C=C0​×e−N

Where:

• $C$ = final salt concentration

• C0C_0C0​ = initial salt concentration

• $N$ = number of diavolumes (volume of buffer added ÷ original volume)

For N = 1:

C=C0×e−1≈C0×0.368⇒about 36.8% of the original salt remainsC = C_0 \times e^{-1} \approx C_0 \times 0.368 \Rightarrow \text{about } 36.8\% \text{ of the original salt remains}C=C0​×e−1≈C0​×0.368⇒about 36.8% of the original salt remains

Correct Answer:

B. 36%