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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper III covers analytical and organic chemistry.

UPSC Combined Geo-Scientist, Mains · 2022 · Mains Paper III

Q1(b). Phases in GLC and choice of carrier gas

Question

Mention the stationary phase and mobile phase in Gas Liquid Chromatography (GLC). What factors are involved in selecting a carrier gas in GLC?

Answer in brief

Stationary phase: a non-volatile liquid film on an inert support or capillary wall; mobile phase: an inert carrier gas (He, N₂ or H₂), chosen for inertness, purity, detector, efficiency, speed, cost and safety.

Key points

  • Stationary phase: a non-volatile, thermally stable liquid as a thin film on an inert solid support (packed column) or on the wall of a fused-silica capillary.
  • Mobile phase: an inert carrier gas (He, N₂, H₂, also Ar) that only carries the vapour and does not interact with the solutes.
  • Detector decides the gas: thermal conductivity detector needs H₂ or He; flame ionisation works with N₂, He or H₂; electron capture needs N₂ or argon–methane.
  • Efficiency and speed (van Deemter): N₂ gives the lowest plate height only at low velocity; He and H₂ allow fast analysis.
  • Purity, dryness, cost, availability and safety: He is costly, H₂ is flammable.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Model answer to Q1(b), 2022 Mains Paper III: solution pages 3 and 4
Solution pages 3 and 4 of 38
Read the whole 2022 solution2022 question paper

Topic notes: chromatography and solvent extraction (plates, resolution, van Deemter, HPLC, GLC, ion exchange, Craig), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversgas liquid chromatographystationary phasecarrier gasvan Deemterthermal conductivity detector

All parts of Question 1