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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 · 2026 · Mains Paper III

Q1(g). Argon in ICP-MS; acids for arsenic

  • 5 marks
  • Topic: Atomic mass spectrometry
  • Answered
Question

(i) What is the purpose of using argon gas in ICP-MS? (ii) Why is HCl or HClO₄ not recommended for sample preparation for arsenic analysis by ICP-MS?

Answer in brief

(i) Argon (IE 15.76 eV) forms the plasma (6000–10 000 K) that atomises and ionises the sample and acts as nebuliser and coolant gas; it is inert. (ii) ⁴⁰Ar³⁵Cl⁺ has m/z=75, the same as ⁷⁵As⁺: a polyatomic interference; HCl and HClO₄ supply the chlorine.

Key points

  • (i) 2 marks: the plasma gas of the torch; its ionisation energy (15.76 eV) exceeds the first IE of nearly all elements (not F, Cl, Ne, He), so analytes give singly charged ions; it is inert, giving no oxide or compound lines;…
  • (ii) 3 marks: arsenic is monoisotopic, ⁷⁵As, so no other isotope can be used. In HCl or HClO₄ the plasma makes ⁴⁰Ar³⁵Cl⁺ (m/z 74.93 against 74.92 for As): a quadrupole cannot resolve them. Use HNO₃ (or collision/reaction-cell…

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

  • (i) 2 marks: the plasma gas of the torch; its ionisation energy (15.76 eV) exceeds the first IE of nearly all elements (not F, Cl, Ne, He), so analytes give singly charged ions; it is inert, giving no oxide or compound lines; the argon plasma is stable and cheap.
  • (ii) 3 marks: arsenic is monoisotopic, ⁷⁵As, so no other isotope can be used. In HCl or HClO₄ the plasma makes ⁴⁰Ar³⁵Cl⁺ (m/z 74.93 against 74.92 for As): a quadrupole cannot resolve them. Use HNO₃ (or collision/reaction-cell mode with He or O₂ mass shift to AsO⁺ at 91).
Model answer to Q1(g), 2026 Mains Paper III: solution page 7
Solution page 7 of 30
Read the whole 2026 solution2026 question paper

CoversICP-MSArgon in ICP-MSacids for arsenic

All parts of Question 1