In simple terms
A friendly intro before the formal notes — no formulas yet.
The Molecular Weighing Scales
Mass spectrometry is a technique that sorts ions based on their mass. It allows us to find the precise mass of individual atoms and molecules, revealing the different isotopes of an element or the structure of a compound.
Imagine you're at a bowling alley, but instead of one type of ball, you have bowling balls of different weights. You roll them all with the same initial force towards a giant fan blowing from the side. The lightest balls will be pushed far off course, while the heaviest ones will barely deviate. A mass spectrometer does this with ions: it fires them through a magnetic field (the 'fan'), and how much their path bends tells us their mass.
- 1
First, the sample is vaporised and bombarded with high-energy electrons. This knocks an electron off each atom or molecule, creating a positive ion.
- 2
Next, these positive ions are accelerated by an electric field, ensuring they all have the same kinetic energy before the next stage.
- 3
The fast-moving ions then enter a magnetic field, which deflects them. Lighter ions are deflected more than heavier ions.
- 4
Finally, a detector counts the ions at each deflection angle. This data is used to generate a mass spectrum, plotting abundance against mass-to-charge ratio ().
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
1 more simulation for this topic — run them in the Simulations section below
Simulations
Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.
1 simulation
- PhETCore9701 22.2
Isotopes and Atomic Mass
Mix isotopes of an element and read the percentage of each alongside the average atomic mass.
Try this
- On Mixtures choose boron and select Nature's Mix; read the percent composition of ¹⁰B and ¹¹B.
- Sketch the mass spectrum those abundances would give: peaks at m/e 10 and 11 with heights in that ratio.
- Calculate Aᵣ from your sketch and check it against the average atomic mass shown.
Look for An element's mass spectrum is exactly this information — one peak per isotope, height proportional to abundance — and Aᵣ = Σ(isotopic mass × % abundance) ÷ 100. The same idea gives chlorine's 3:1 M and M+2 peaks.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
Key formulas
Tap any symbol to reveal exactly what it means and its units.
Full topic notes
Formal explanation with the rigour you need for the exam.
The Four Stages of Mass Spectrometry
A conventional mass spectrometer operates through four key stages. The sample is first introduced and converted into positive ions, which are then accelerated, separated according to their mass-to-charge ratio, and finally detected.
1. Ionisation: The sample is first vaporised and then injected into the ionisation chamber. Here, it is bombarded with a stream of high-energy electrons from an 'electron gun'. These electrons knock off electrons from the atoms or molecules of the sample, creating positive ions. For a molecule M, this is represented as .
2. Acceleration: The newly formed positive ions are accelerated by a series of negatively charged plates, creating a fine beam of ions all having the same kinetic energy.
3. Deflection: The beam of ions passes into a strong magnetic field, which is perpendicular to their direction of travel. The magnetic field deflects the ions into a curved path. The degree of deflection depends on the mass-to-charge ratio (). Lighter ions are deflected more than heavier ions, and more highly charged ions are deflected more than singly charged ions.
4. Detection: By varying the strength of the magnetic field, ions of different ratios can be directed towards a detector. When an ion hits the detector, it accepts an electron, generating a tiny electrical current. The size of the current is proportional to the number of ions arriving, giving the relative abundance of that ion.
Interpreting Mass Spectra of Elements
When an element is analysed, the mass spectrum shows the different isotopes present. The x-axis represents the mass-to-charge ratio (), which for singly charged ions is simply the isotopic mass. The y-axis shows the relative abundance of each isotope. From this data, we can calculate the relative atomic mass () of the element.
Mass Spectrometry of Molecules and Fragmentation
When a molecule is passed through a mass spectrometer, the peak with the highest value is called the molecular ion peak (). This is formed when the molecule loses one electron but remains intact. The value of this peak gives the relative molecular mass () of the compound. The high energy of the ionisation process often causes the molecular ion to break apart into smaller pieces, a process called fragmentation. This gives rise to other peaks at lower values, creating a unique fragmentation pattern that can be used like a 'fingerprint' to help identify the molecule's structure.
Do not confuse the molecular ion peak with the base peak. The molecular ion peak is the one with the highest value. The base peak is the tallest peak in the spectrum, representing the most abundant (and often most stable) fragment. They can be the same, but often are not.
Characteristic Isotopic Patterns
Some elements have very distinctive isotopic abundances, which lead to characteristic patterns in the mass spectra of molecules containing them. The most important examples for A-Level are chlorine and bromine.
Chlorine (): Has two main isotopes, (~75%) and (~25%). This gives a 3:1 abundance ratio. A molecule containing one chlorine atom will show an M peak and an M+2 peak in a 3:1 ratio. A molecule of chlorine, , will have three possible molecular ions: at , at , and at . The relative abundance ratio of these peaks is approximately 9:6:1.
Bromine (): Has two main isotopes, (~50%) and (~50%). This gives a 1:1 abundance ratio. A molecule containing one bromine atom will show an M peak and an M+2 peak in a 1:1 ratio. A molecule of bromine, , will have three possible molecular ions: at , at , and at . The relative abundance ratio of these peaks is approximately 1:2:1.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
The mass spectrum of a sample of zirconium shows five peaks with the following values and relative abundances:
| Relative Abundance | |
|---|---|
| 90 | 51.5 |
| --- | --- |
| 91 | 11.2 |
| 92 | 17.1 |
| 94 | 17.4 |
| 96 | 2.8 |
Calculate the relative atomic mass of zirconium to one decimal place.
- 1
Calculate the sum of (mass × abundance) for each isotope:
The mass spectrum of propan-1-ol, , is shown. The molecular ion peak is at . Suggest the chemical formula for the fragments responsible for the peaks at , , and .
- 1
First, confirm the of propan-1-ol: . This matches the molecular ion peak.
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- fragmentation
The high energy of the ionisation process often causes the molecular ion to break apart into smaller pieces, a process called fragmentation. This gives rise to other peaks at lower values, creating a unique fragmentation pattern that can be used like a 'fingerprint' to help identify the molecule's structure.
- Mass spectrometer
An instrument that measures the mass-to-charge ratio () of ions, allowing for the determination of atomic and molecular masses and isotopic abundances.
- Molecular ion peak ()
The peak with the highest value in the mass spectrum of a compound. It represents the intact molecule that has lost one electron. Its value gives the relative molecular mass () of the compound.
- Base peak
The peak with the greatest relative abundance in a mass spectrum (the tallest peak). It is assigned a relative intensity of 100% and corresponds to the most stable positive ion formed in the spectrometer.
- fragmentation in mass
The process where the high-energy molecular ion breaks apart into smaller, positively charged fragments and neutral species (like radicals). Only the charged fragments are detected.
- must a mass spectrometer
To prevent the ions from colliding with air molecules. Such collisions would interfere with their flight path from the ion source to the detector, invalidating the results.
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
If you can explain it simply, you own it — gaps here are marks you’d lose.
Teach it back
Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
1. Ionisation: The sample is first vaporised and then injected into the ionisation chamber. Here, it is bombarded with a stream of high-energy electrons from an 'electron gun'. These electrons knock off electrons from the atoms or molecules of the sample, creating positive ions. For a molecule M, this is represented as .
2. Acceleration: The newly formed positive ions are accelerated by a series of negatively charged plates, creating a fine beam of ions all having the same kinetic energy.
3. Deflection: The beam of ions passes into a strong magnetic field, which is perpendicular to their direction of travel. The magnetic field deflects the ions into a curved path. The degree of deflection depends on the mass-to-charge ratio (). Lighter ions are deflected more than heavier ions, and more highly charged ions are deflected more than singly charged ions.
4. Detection: By varying the strength of the magnetic field, ions of different ratios can be directed towards a detector. When an ion hits the detector, it accepts an electron, generating a tiny electrical current. The size of the current is proportional to the number of ions arriving, giving the relative abundance of that ion.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Test your knowledge on mass spectrometry
Test your knowledge on mass spectrometry
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Test your knowledge on mass spectrometry on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.
Discuss Mass spectrometry
Ask, share and discuss with other Chemistry students