UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III
Q1(b). Stationary-phase film thickness in gas chromatography
Question
In gas chromatography, what is the effect of stationary-phase film thickness on gas chromatograms?
Answer in brief
A thicker film raises the phase ratio β⁻¹, so retention (k), sample capacity and retention time increase; peaks are broader and column bleed is higher. A thin film gives fast, sharp peaks with low capacity.
Key points
- k=(K_D)/(β)=(2K_Dd_f)/(r): k rises in direct proportion to the film thickness d_f. A thick film (1–5 μm) retains volatile analytes (gases, solvents), separating them without sub-ambient cooling.
- more stationary phase holds more sample before overload, so peaks do not front: suitable for concentrated samples.
- the Cₛ term of the van Deemter equation grows with d_f², so thick films give broader peaks and a lower plate number for high-boiling analytes. Bleed: higher, with a noisier baseline.
- elutes high-boiling, high-molar-mass analytes at lower temperature, sharp peaks, low bleed, but overloads easily.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- Retention: k=(K_D)/(β)=(2K_Dd_f)/(r): k rises in direct proportion to the film thickness d_f. A thick film (1–5 μm) retains volatile analytes (gases, solvents), separating them without sub-ambient cooling.
- Capacity: more stationary phase holds more sample before overload, so peaks do not front: suitable for concentrated samples.
- Efficiency: the Cₛ term of the van Deemter equation grows with d_f², so thick films give broader peaks and a lower plate number for high-boiling analytes. Bleed: higher, with a noisier baseline.
- Thin film (0.1–0.25 μm): elutes high-boiling, high-molar-mass analytes at lower temperature, sharp peaks, low bleed, but overloads easily.
Coverscapillary GCStationary-phase film thickness in gas chromatography
All parts of Question 1
- 1(a) Classification of samples by size5 marks
- 1(b) Stationary-phase film thickness in gas chromatography5 marks
- 1(c) Percentage of amine from the absorbance of its picrate5 marks
- 1(d) Principle of vapour-generation AAS5 marks
- 1(e) Purge gases in thermogravimetric analysis5 marks
- 1(f) X-ray fluorescence in X-ray diffraction5 marks
- 1(g) Argon in ICP-MS; acids for arsenic5 marks
- 1(h) MnO₂ in pyrolusite by back titration5 marks