Q1(g). Interference of calcium in the ICP–MS determination of iron

Answer in brief

Calcium forms polyatomic ions in the plasma with the same nominal mass as the iron isotopes: ⁴⁰Ca¹⁶O⁺ falls on ⁵⁶Fe⁺ (m/z 56), so the iron result is too high; the heavy Ca matrix also suppresses the signal and clogs the cones.

Key points

• Spectroscopic (polyatomic) interference: Ca combines with O, H and N; the molecular ions have the same nominal m/z as iron isotopes, unresolved by a quadrupole.
• Overlaps: ⁴⁰Ca¹⁶O⁺ on ⁵⁶Fe⁺, ⁴⁰Ca¹⁶O¹H⁺ on ⁵⁷Fe⁺, ⁴⁰Ca¹⁴N⁺ on ⁵⁴Fe⁺, ⁴²Ca¹⁶O⁺ on ⁵⁸Fe⁺: positive error.
• Matrix effects: signal suppression by space charge and the easily ionised matrix; CaO deposits on the cones cause downward drift.
• Remedies: collision/reaction cell (He with KED; H₂ or NH₃), high-resolution instrument (m/Δm about 2500), dilution, matrix-matched standards, internal standard.