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1.2.4Additional lessons

Interpreting Mass Spectra (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 7 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5How a Mass Spectrometer Works

1.2.4 Interpreting Mass Spectra (HL)

A mass spectrometer separates ions by their mass-to-charge ratio.

1
The sample is vaporised and then ionised, usually forming ions with a charge of +1 by removing one electron.
2
The positive ions are accelerated by an electric field so that they all gain the same kinetic energy.
3
A magnetic field deflects the ions, with lighter ions deflected more than heavier ones.
4
Only ions with the right mass-to-charge ratio reach the detector for a given field strength.
5
The detector records the abundance of each type of ion, giving the mass spectrum.

Part 2 of 5Reading the Spectrum

Each peak represents one isotope of the element.

Horizontal axis
  • The m/z value is the mass-to-charge ratio, which equals the mass number for singly charged ions.
Vertical axis
  • The height of a peak shows the relative abundance of that isotope.
Number of peaks
  • Each peak corresponds to a different isotope present in the sample.
Ar calculation
  • Ar = Σ(m/z × relative abundance) ÷ Σ(relative abundance).
m/zRelative abundance %
2478.99
2510.00
2611.01

Part 3 of 5Using Spectra

Using Spectra
Identify isotopes
  • The peak positions identify the mass numbers of the isotopes present.
Find abundance
  • Peak heights give percentage abundance when scaled to a total of 100.
Molecules
  • Compounds give a molecular ion peak at the molar mass and further peaks from fragments.
Tool
  • Mass spectrometry is used in forensic science, drug testing and archaeology.

Part 4 of 5Worked Example

Question Using the table for magnesium, calculate its relative atomic mass.

  1. Multiply each m/z by its abundance: 24 × 78.99 = 1895.76; 25 × 10.00 = 250.0; 26 × 11.01 = 286.26.
  2. Total = 2432.02; divide by total abundance 100.00.

Answer Ar = 24.32

Part 5 of 5Exam Checklist & Tips

1.2.4 – Key Points for the Exam

  • m/z The mass-to-charge ratio of an ion.
  • Mass spectrum A plot of relative abundance against m/z.
  • Ionisation The removal of electrons to form positive ions.
  • Molecular ion The ion formed when a molecule loses one electron without fragmenting.

Common Mistakes

  • Averaging the m/z values without weighting by abundance.
  • Reading peak height as the mass of the isotope.
  • Forgetting that a charge of 2+ would halve the m/z value.
  • Stating that the heaviest ion is the most abundant.

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