Question

Back diffusion occurs mostly in __________.

• A. Size exclusion chromatography
• B. HPLC
• C. Gas chromatography
• D. HPTLC

Hint:

Because molecules move vastly quicker in gases than in liquids, longitudinal (back) diffusion (\(B\) term) is a significant factor in Gas Chromatography, demanding higher optimal carrier gas flow velocities to minimize band spreading.

Answer 1

The Correct Option is C

Concept: Band broadening in chromatographic columns is mathematically described by the standard Van Deemter equation, which links the height equivalent to a theoretical plate (\(\text{HETP}\)) to the linear velocity (\(u\)) of the mobile phase: \[ \text{HETP} = A + \frac{B}{u} + C \cdot u \] where:
• \(A\) represents Eddy diffusion (multiple pathways).
• \(B\) represents Longitudinal diffusion (back/longitudinal diffusion along the flow axis).
• \(C\) represents Resistance to mass transfer.

Step 1: Analyze Longitudinal (Back) Diffusion The \(B\) term describes the natural thermodynamic tendency of solute molecules to diffuse away from the concentrated center of a band out into regions of lower concentration, moving both forward and backward along the primary line of flow. The longitudinal diffusion coefficient is directly dependent on the diffusion rate of the solute within the specific mobile phase matrix (\(D_m\)): \[ B = 2 \cdot \gamma \cdot D_m \]

Step 2: Contrast Gas vs. Liquid phases In Gas Chromatography (GC), the mobile phase is a gas. Molecular diffusion rates of solute components within a gaseous medium are roughly $10^4$ to $10^5$ times faster than diffusion rates inside a liquid mobile phase (as used in HPLC, HPTLC, or SEC). Because gases offer minimal resistance to molecular displacement, axial back-diffusion occurs to a high degree in GC, making the \(\frac{B}{u}\) term a major factor in band broadening, especially at lower carrier gas flow rates.