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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 · 2023 · Mains Paper III

Q1(d). Graphite-furnace AAS compared with flame AAS

Question

What is the advantage and disadvantage of GF-AAS as compared to Flame-AAS?

Answer in brief

Advantage: far higher sensitivity (absolute detection limits 100 to 1000 times lower, picograms) from only 10–50 µL of sample. Disadvantage: poorer precision, slow analysis, stronger background and matrix interference.

Key points

  • GF-AAS: 10–50 µL of sample is dried, ashed and atomised inside an electrically heated graphite tube in argon.
  • Atomisation is nearly 100 % efficient (a flame atomises as little as 0.1 %); absolute detection limits are 100 to 1000 times lower: picograms.
  • Very small sample (microlitres); solids, slurries and viscous samples are taken directly.
  • Poorer precision: about 5–10 % against about 1 % for the flame; slow, one element at a time.
  • Stronger background and matrix effects: background correction (D₂ lamp or Zeeman) and matrix modifiers are needed.

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(d), 2023 Mains Paper III: solution pages 5 and 6
Solution pages 5 and 6 of 40
Read the whole 2023 solution2023 question paper

Topic notes: spectroscopic instruments (Beer–Lambert law, UV–vis and IR instruments, Job's method, AAS and flame emission), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversgraphite furnace AASelectrothermal atomisationflame AASdetection limitatomic absorption

All parts of Question 1