UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III
Q1(d). Principle of vapour-generation AAS
Question
What is the principle used in VG-AAS? Give the names of two elements that are measured with better detection limit using VG-AAS than by flame AAS.
Answer in brief
The element is converted by NaBH₄ in acid into a volatile hydride (or, for Hg, a vapour by SnCl₂ reduction), swept by argon into a heated cell and atomised. The analyte reaches the light path with 100 % efficiency (flame: 5–10 %). Better detection limits: As and Se (also Sb, Bi, Te, Sn, Hg).
Key points
- (i) nearly all the analyte is transferred (nebuliser: 5–10%); (ii) separation from the matrix; (iii) a long residence time in the quartz cell; (iv) no flame absorption of the lines at 193.7 nm (As) and 196.0 nm (Se), where the…
- Detection limits fall from 0.1–1 mg L⁻¹ (flame) to 0.1–1 μg L⁻¹.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
As^(III) + 3BH₄⁻ + 3H⁺ + 3H₂O → AsH₃ ↑ + 3H₃BO₃ + 3H₂^(IV)→H₂Se
- Why the limit is lower: (i) nearly all the analyte is transferred (nebuliser: 5–10%); (ii) separation from the matrix; (iii) a long residence time in the quartz cell; (iv) no flame absorption of the lines at 193.7 nm (As) and 196.0 nm (Se), where the air–acetylene flame absorbs strongly.
- Detection limits fall from 0.1–1 mg L⁻¹ (flame) to 0.1–1 μg L⁻¹.
Covershydride generationPrinciple of vapour-generation AAS
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