33. Back diffusion occurs mostly in:
A. Size exclusion chromatography
B. Gas chromatography
C. HPLC
D. HPTLC
Back diffusion occurs mostly in Gas Chromatography (GC), making option (B) correct. This is due to GC’s gaseous mobile phase where diffusion rates are significantly higher than in liquid systems.
Option Analysis
Size exclusion chromatography (A): Separates by molecular size through pores. Minimal back diffusion impact since separation relies on physical exclusion, not partitioning.
Gas chromatography (B): Correct. Gaseous mobile phase has diffusion coefficients ~10,000x higher than liquids (B/u term dominates van Deemter equation at low flow rates). Primary band broadening mechanism.
HPLC (C): Liquid mobile phase has much lower diffusion rates (D_m ~ 10<sup>-5</sup> cm²/s vs 10<sup>-1</sup> in GC). Mass transfer (C term) dominates over longitudinal diffusion.
HPTLC (D): Planar chromatography with stationary mobile phase during development. Diffusion occurs but separation completes before significant back diffusion affects resolution.
Back diffusion in gas chromatography dominates band broadening due to rapid molecular diffusion in gaseous mobile phases. The B/u term in van Deemter equation shows longitudinal diffusion ∝ 1/flow rate, critical at optimal GC velocities.
Van Deemter Analysis
H = A + B/u + Cu
↑ ↑ ↑
Eddy Longitudinal Mass transfer
diff. diffusion
GC: B term dominates (D_m gas ≈ 0.1-1 cm²/s)
HPLC: C term dominates (D_m liquid ≈ 10<sup>-5</sup> cm²/s)
Technique Comparison
| Technique | Mobile Phase | D_m (cm²/s) | Dominant Broadening |
|---|---|---|---|
| GC | Gas | 0.1-1 | Back diffusion |
| HPLC | Liquid | 10<sup>-5</sup> | Mass transfer |
| SEC | Liquid | 10<sup>-5</sup> | Steric exclusion |
| HPTLC | Liquid (static) | Minimal | None during run |
Why GC? Gas molecules diffuse ~200x faster than liquids, causing analyte bands to spread longitudinally against flow direction during column transit.
Exam Strategy
GATE Life Sciences tests mobile phase physical properties:
-
Gas = high diffusion → back diffusion
-
Liquid = low diffusion → mass transfer
-
Planar = static phase → minimal diffusion impact
Key Formula: Minimum plate height occurs where dH/du = 0, balancing B/u (back diffusion) vs Cu (mass transfer).
1 Comment
Vanshika Sharma
February 3, 2026Back diffusion mostly occur in gas chromatography