UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper III
Q1(f). Quadrupole mass analyser in ICP–MS
Question
Explain the working principle of quadrupole mass analyzer in ICP–MS.
Answer in brief
Four parallel rods carry a dc voltage U and an RF voltage V cos ωt; for given U and V only ions of one m/z reach the detector, all others strike the rods.
Key points
- Ions (mostly M⁺) formed in the argon plasma pass the sampler and skimmer cones and the ion lens into the quadrupole under high vacuum.
- Opposite rods are connected: one pair at +(U + V cos ωt), the other at −(U + V cos ωt).
- For given U and V only one m/z has a stable, bounded trajectory (Mathieu equation); all other ions strike the rods and are lost.
- The positive pair acts as a high-mass pass filter, the negative pair as a low-mass pass filter: together a narrow band-pass.
- Raising U and V together at constant U/V brings successive m/z values to the electron multiplier; unit mass resolution, very fast scanning.
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.
Coversquadrupole mass analyserICP-MSmass filterMathieu equationRF and dc voltage
All parts of Question 1
- 1(a) Sample mass for a sampling standard deviation of ±5 %5 marks
- 1(b) van Deemter equation and its significance5 marks
- 1(c) Non-absorbing analyte by UV-vis spectrophotometry5 marks
- 1(d) Flame absorption profiles of magnesium and silver5 marks
- 1(e) Heating rate, particle size and heat of reaction in TGA5 marks
- 1(f) Quadrupole mass analyser in ICP–MS5 marks
- 1(g) Gravimetric estimation of nickel in steel5 marks
- 1(h) Lattice spacing from Bragg’s law5 marks