UPSC Combined Geo-Scientist, Mains · 2018 · Mains Paper III
Q1(d). Source modulation in AAS
Question
Why is source modulation used in atomic absorption spectroscopy?
Answer in brief
It distinguishes transmitted lamp radiation from unmodulated emission by the flame or atomiser.
Key points
- Chop or electronically modulate the hollow-cathode-lamp intensity at a known frequency.
- The transmitted lamp beam retains this modulation after passing through the atom cloud. Much of the flame emission is steady.
- A phase-sensitive or frequency-selective detector extracts the modulated component, reducing emission interference and drift.
- Modulation alone does not remove molecular absorption of the source beam; that requires a suitable background-correction method.
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.
CoversSource modulation in AASAtomic absorption
All parts of Question 1
- 1(a) Carbonate titration endpoint5 marks
- 1(b) Medium for iodimetry5 marks
- 1(c) Magnesium and the EBT endpoint5 marks
- 1(d) Source modulation in AAS5 marks
- 1(e) Chromatographic zone broadening5 marks
- 1(f) ICP multielement capability5 marks
- 1(g) Alkaline glass-electrode error5 marks
- 1(h) Electron-capture selectivity5 marks