Mass Spectrometry: GCSE Chemistry Guide

What is a mass spectrometer?

A mass spectrometer is an instrument that:

1. Converts atoms or molecules into ions (charged particles)
2. Accelerates those ions through an electric field
3. Deflects them using a magnetic field — lighter ions deflect more, heavier ions deflect less
4. Detects the ions at a detector

The output is a mass spectrum: a bar chart showing the relative abundance of each ion at each mass-to-charge ratio (m/z).

At GCSE, all ions are assumed to carry a charge of +1 (one electron removed), so the m/z value equals the mass number of the isotope.

How does it work step by step?

The four stages of a simple mass spectrometer:

1. Ionisation. The sample is bombarded with high-energy electrons, which knock electrons off atoms and produce positive ions: X → X⁺ + e⁻.
2. Acceleration. An electric field accelerates the positive ions towards the detector. All ions gain the same kinetic energy.
3. Deflection. A magnetic field deflects the beam of ions — lighter ions are deflected more than heavier ions at the same charge.
4. Detection. Ions that reach the detector generate a signal. The intensity of each signal is proportional to the number of ions of that mass — i.e. their relative abundance.

What does a mass spectrum look like?

A mass spectrum for an element shows peaks at the mass number of each naturally occurring isotope. The height (or area) of each peak gives the percentage abundance of that isotope.

Example: chlorine

Chlorine has two stable isotopes:

The mass spectrum shows two peaks: a taller peak at m/z = 35 and a shorter peak at m/z = 37, in a ratio of approximately 3:1.

How do you calculate relative atomic mass from a mass spectrum?

Relative atomic mass (Aᵣ) is the weighted mean of the masses of all the isotopes of an element, taking their natural abundances into account.

Formula: Aᵣ = Σ (isotope mass × percentage abundance) ÷ 100

Worked example: chlorine

Aᵣ = (35 × 75) + (37 × 25) = 2625 + 925 = 3550

Aᵣ = 3550 ÷ 100 = 35.5

This explains why chlorine's Aᵣ is 35.5 — not a whole number — because it is a weighted average of two isotopes.

Worked example: boron

Boron has two isotopes: ¹⁰B (20% abundance) and ¹¹B (80% abundance).

Aᵣ = (10 × 20) + (11 × 80) = 200 + 880 = 1080

Aᵣ = 1080 ÷ 100 = 10.8

What are isotopes?

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, and therefore different mass numbers. All isotopes of an element have identical chemical properties (because they have the same electron configuration) but slightly different physical properties (because they have different masses).

The existence of isotopes is why relative atomic masses are often not whole numbers.

What are the uses of mass spectrometry beyond GCSE?

At GCSE you only need to know about elemental analysis and Aᵣ calculations. However, mass spectrometry is also used to:

• Identify unknown organic compounds (by comparing fragmentation patterns)
• Test athletes for banned performance-enhancing drugs
• Date ancient artefacts (carbon-14 dating uses the ratio of ¹⁴C to ¹²C)
• Analyse the composition of stars and planetary atmospheres from their light spectra

Frequently asked questions

Why are relative atomic masses not always whole numbers?

Elements in nature occur as a mixture of isotopes. The relative atomic mass is a weighted average of the masses of all isotopes, weighted by their natural abundance. If an element has two isotopes — one of mass 10 (20% abundant) and one of mass 11 (80% abundant) — the Aᵣ is 10.8, not 10 or 11. The only elements with whole-number Aᵣ values in practice are those where one isotope is overwhelmingly dominant (e.g. fluorine, which is 100% ¹⁹F, has Aᵣ = 19.0).

How does mass spectrometry differ from using the periodic table?

The periodic table gives the Aᵣ values already calculated. Mass spectrometry is the instrument that actually measures these values from a real sample: you would use it if you wanted to determine the Aᵣ of an unknown element, to confirm the isotopic composition of a sample, or to detect whether an element in a sample has an unusual isotopic ratio (e.g. as a result of nuclear activity or geological age).

Why does the mass spectrometer need the sample to be ionised first?

Only charged particles (ions) can be accelerated by an electric field and deflected by a magnetic field. Neutral atoms pass through both fields unaffected and would never reach the detector. By knocking one electron from each atom to form M⁺ ions, the instrument ensures that every particle in the beam can be manipulated and detected by the electromagnetic fields.

What is the difference between mass number and relative atomic mass?

Mass number is always a whole number — it is the total number of protons plus neutrons in one specific nucleus of a specific isotope (e.g. carbon-12 has a mass number of exactly 12). Relative atomic mass is the weighted average mass of all naturally occurring isotopes of an element relative to ¹²C, and it is therefore not generally a whole number (e.g. Aᵣ of chlorine is 35.5). On the periodic table, the numbers shown are Aᵣ values, not mass numbers.

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