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The Mass Spectrometer

A concise revision guide to how a mass spectrometer works, including vaporisation, ionisation by electron bombardment, acceleration, deflection, detection and the interpretation of m/z values.

AS Level
Topic 1: Atomic Structure
9701 Papers 1 and 2
Dr. Mohammed Al-Fatah

Written by: Dr. Mohammed Al-Fatah

Chemistry specialist revision notes for A Level Chemistry.

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1

Where This Sits in the Cambridge Specification

The working of the instrument itself is not examined by Cambridge; the spectra it produces are (Topic 22.1). Use this page to understand where the peaks come from.

2

What a Mass Spectrometer Does

A mass spectrometer separates positive ions according to their mass-to-charge ratio, written as m/z.

It is used to identify isotopes, compare their relative abundance, and provide data used to calculate relative atomic mass. The sample must be converted into particles that can move through the machine and be detected as ions.

The main sequence is: vaporisation, ionisation, acceleration, deflection and detection.

Key idea: The mass spectrometer does not directly weigh atoms. It forms positive ions, separates them by m/z, and records detector signals that show relative abundance.

Labelled electron bombardment mass spectrometer with the five stages vaporisation, ionisation, acceleration, deflection and detection, and both ion paths reaching the detector

The detector records ions that pass through the selected path, allowing the computer to build a spectrum from their m/z values and abundances.

3

Why the Instrument Uses a Vacuum

The mass spectrometer operates at low pressure and under a vacuum.

This matters because air particles would otherwise collide with the ions, change their path, or become ionised themselves. If air particles reached the detector, they could produce extra signals and reduce the reliability of the spectrum.

Exam focus: A vacuum prevents collisions with air particles and prevents air particles from being ionised and detected.

Check your understanding

Quick Check: What If Air Leaks In?

Write a short explanation, then compare it with the mark points and the model answer.

4

Stage 1: Vaporisation and Ionisation

The sample must be in the gaseous state so that its particles can move through the mass spectrometer. If the sample is not already gaseous, it is first vaporised.

In electron bombardment, an electron gun fires high-energy electrons at the gaseous atoms or molecules. These electrons collide with the sample and remove an outer electron, forming a positive ion.

For an atom X, this can be represented as: X(g) + e– → X+(g) + 2e–.

Remember: Ionisation produces positive ions because an electron has been removed.

Mass spectrometer animation

Ionisation by electron bombardment

This animation shows a hot filament releasing a high-energy electron. The electron collides with the outer electron of a hydrogen atom, removing it and forming a positive H⁺ ion.

Ionisation chamber
Pause or play using the control below
1. The filament heats up and releases a high-energy electron. 2. The incoming electron knocks out the outer electron, forming H⁺ and two electrons.
filament energy increases
hot filament
H H⁺
positive ion formed: H⁺
H(g) + e⁻ → H⁺(g) + 2e⁻

Key idea: in electron bombardment ionisation, a high-energy electron knocks an electron out of an atom or molecule. Losing an electron forms a positive ion. For hydrogen, the process is H(g) + e⁻ → H⁺(g) + 2e⁻.

Hot filament The filament is heated, so electrons gain enough energy to be emitted into the ionisation chamber.
Electron collision The incoming high-energy electron collides with the electron in the hydrogen atom.
Positive ion forms The hydrogen atom loses its electron, leaving H⁺. Two electrons leave after the collision.
© Online Learning System. This educational animation, code structure, visual design and explanatory content are protected. Unauthorised copying, redistribution, resale, modification or republication is prohibited.
Check your understanding

Quick Check: Write the Ionisation Equation

Choose the equation that shows a water molecule being ionised by electron bombardment.

5

Stage 2: Acceleration

The positive ions are accelerated by an electric field. The field pulls the positive ions towards a negatively charged plate or region.

This forms a narrow beam of ions travelling through the instrument. Ions with the same charge are given the same kinetic energy as they are accelerated into the flight path.

Exam focus: Positive ions accelerate towards the negative plate because opposite charges attract.

Mass spectrometer animation

Acceleration of the positive ion

This animation shows the positive hydrogen ion being attracted towards a negatively charged disc with a small hole. The ion is accelerated through the hole and then travels quickly along the flight tube.

Acceleration chamber and flight tube
Pause or play using the control below
1. The positive H⁺ ion is attracted towards the negatively charged disc. 2. The ion passes through the hole and accelerates rapidly into the flight tube.
negatively charged disc −
ion speed increases
flight tube
positive ion accelerated by electric attraction → enters flight tube at high speed

Key idea: the ion formed in the ionisation chamber is positive, so it is attracted towards a negative plate. As it moves through the electric field and passes through the small hole, it is accelerated and enters the flight tube with high kinetic energy.

Positive ion The hydrogen ion is H⁺, so it is attracted to a negatively charged plate.
Accelerating disc The disc has a small hole. The ion is pulled towards it and passes through the hole as it speeds up.
Flight tube After acceleration, the ion travels very quickly through the hollow tube towards the next stage of the mass spectrometer.
© Online Learning System. This educational animation, code structure, visual design and explanatory content are protected. Unauthorised copying, redistribution, resale, modification or republication is prohibited.
Check your understanding

Quick Check: What Gets Accelerated?

Decide whether the statement about atoms that were not ionised is true or false.

6

Stage 3: Deflection

The beam of positive ions passes through a magnetic field. The magnetic field deflects the ions so that their path bends.

The amount of deflection depends on the mass-to-charge ratio, m/z. Ions with a smaller m/z are deflected more. Ions with a larger m/z are deflected less.

If the ions all have a charge of 1+, then lighter ions have a smaller m/z and are deflected more than heavier ions. If the charge is different, m/z must be considered directly rather than mass alone.

A 2+ ion forms when a second electron is knocked out during electron bombardment. Its m/z is half that of the 1+ ion of the same mass, so it is deflected more. Most ions are 1+ because removing a second electron needs much more energy.

Ion propertyEffect on m/zEffect on deflection
Lower mass with the same chargeSmaller m/zMore deflection
Higher mass with the same chargeLarger m/zLess deflection
Higher positive charge with the same massSmaller m/zMore deflection

Example: An ion with mass 28 and charge 1+ has m/z = 28. An ion with mass 56 and charge 2+ also has m/z = 28, so both can be detected under the same magnetic field setting.

Mass spectrometer animation

Deflection of positive ions

This animation shows two positive ions entering a magnetic field on the same trajectory. The smaller blue ion travels first, followed by the larger green ion behind it. Inside the magnetic field region, their paths split because the smaller ion is deflected more strongly.

Deflection chamber
Pause or play using the control below
1. The ions travel along the same initial path, with the small blue ion first and the larger green ion behind. 2. Their paths split inside the magnetic field because the lighter ion is deflected more strongly.
magnetic field region
N
S
same initial trajectory
Deflection pattern The path only splits after the ions enter the magnetic field. The smaller blue ion curves more than the larger green ion.
same charge and speed: lighter ion curves more • heavier ion curves less
Small blue ion: lower mass-to-charge ratio, so it is deflected more strongly.
Large green ion: higher mass-to-charge ratio, so it is deflected less strongly.
Magnetic field: causes positive ions to follow different curved paths.

Key idea: in the deflection stage, moving positive ions are deflected by a magnetic field. Before entering the magnetic field, both ions follow the same trajectory. Once they are inside the field, the lighter ion is deflected more, while the heavier ion is deflected less.

Same initial trajectory The blue ion is shown first, with the larger green ion behind it. Both travel along the same path before the magnetic field.
Magnetic deflection The magnetic field changes the direction of motion of the ions, so their paths begin to split inside the field region.
Mass-to-charge ratio With the same charge and speed, the smaller mass-to-charge ratio gives greater deflection. The larger ion follows a less curved path.
© Online Learning System. This educational animation, code structure, visual design and explanatory content are protected. Unauthorised copying, redistribution, resale, modification or republication is prohibited.
Check your understanding

Quick Check: Compare the m/z Values

Work out m/z for each ion, then answer five quick questions on deflection.

7

Stage 4: Detection

The deflected ions reach a detector plate. When a positive ion hits the detector, it gains an electron and becomes neutral.

Electrons flow from the detector to the positive ions, creating a small electrical current. This signal is amplified and sent to a computer.

The size of the detector signal is proportional to the number of ions reaching the detector. This is why the peak height or intensity in a mass spectrum shows relative abundance.

Exam focus: More ions hitting the detector produce a larger current, so the signal represents greater abundance.

Mass spectrometer animation

Detection of ions and abundance

This animation shows separated positive ions reaching the detector. When each ion hits the detector plate, it gains an electron and becomes neutral. Each hit produces a signal, and the signal grows with the number of ions detected.

Detection chamber
Signal size shows abundance
1. Separated ions continue from the deflection stage towards the detector. 2. At the detector, each positive ion gains one electron and becomes neutral. 3. More ion hits produce a larger signal, showing greater abundance.
blue isotope signal
green isotope signal
Positive ion + electron → neutral atom. Each detector hit adds to the signal.
Relative abundance Four green ions hit the detector compared with two blue ions. The green signal becomes twice as large as the blue signal.
Blue ions: 2 detector hits, producing the smaller signal.
Green ions: 4 detector hits, producing twice the signal.
Electron gain: each positive ion gains one electron and becomes neutral.

Key idea: in the detection stage, positive ions hit the detector and gain electrons to become neutral atoms. Each collision creates a signal. The more ions that hit the detector, the larger the signal. Here, the green ion produces 4 hits compared with 2 blue hits, so the green peak is twice as large and has twice the relative abundance.

© Online Learning System. This educational animation, code structure, visual design and explanatory content are protected. Unauthorised copying, redistribution, resale, modification or republication is prohibited.
Check your understanding

Quick Check: Reading the Detector Current

Use two detector readings to make a deduction about a boron sample.

8

Changing the Magnetic Field

For a fixed magnetic field strength, only ions with a particular m/z value follow the correct path through the slit and reach the detector.

The magnetic field strength is then changed so that ions with different m/z values can be detected. Ions with a larger m/z are deflected less, so a stronger magnetic field is needed to bend them onto the detector. A weaker field brings ions with a smaller m/z onto the detector. The computer combines these detector signals to produce the mass spectrum.

Key idea: Changing the magnetic field lets the instrument scan across different m/z values.

Check your understanding

Quick Check: Scanning to a Heavier Ion

Decide how the magnetic field must change to bring a heavier ion onto the detector.

Check your understanding

Quick Check: Follow a Sample Through the Instrument

Put the seven events in the order in which they happen.

9

Common Exam Points

Mass spectrometry questions often test precise sequencing, the role of the vacuum, the meaning of m/z, and the link between detector current and abundance.

Use the correct sequence

Vaporise the sample if needed, ionise it, accelerate the positive ions, deflect them in a magnetic field, then detect them.

State why a vacuum is needed

A vacuum prevents collisions with air particles and prevents air particles from becoming ionised and detected.

Do not use mass alone when charge can vary

Deflection depends on m/z, not only mass. A higher charge lowers the m/z value.

Link current to abundance

A larger current is produced when more ions reach the detector, so signal intensity is proportional to abundance.

✓

QuickSnap

In a mass spectrometer, the sample is first vaporised if needed, then ionised by electron bombardment to form positive ions. The positive ions are accelerated by an electric field and deflected by a magnetic field according to their m/z value. Ions with smaller m/z values are deflected more, while ions with larger m/z values are deflected less. When ions reach the detector, they gain electrons and produce a current. The size of this current is proportional to the abundance of the ions reaching the detector.

Ionisation

High-energy electrons remove electrons from gaseous particles, forming positive ions.

Acceleration

Positive ions are attracted towards a negative electric field and form a beam.

Deflection

The magnetic field separates ions by m/z. Smaller m/z means greater deflection.

Detection

Positive ions gain electrons at the detector, producing a current proportional to abundance.

The Mass Spectrometer FAQs

Use these quick answers to secure the key wording for Cambridge International AS & A Level Chemistry.

What does a mass spectrometer measure?

It separates and detects positive ions according to their mass-to-charge ratio, m/z.

Why must the sample be vaporised?

The sample must be in the gaseous state so that its particles can move through the instrument and be ionised.

How are positive ions formed in electron bombardment?

High-energy electrons collide with gaseous atoms or molecules and remove electrons, producing positive ions.

Why is a vacuum needed in a mass spectrometer?

A vacuum prevents collisions with air particles and prevents air particles from becoming ionised and detected.

What determines the amount of deflection?

The amount of deflection depends on m/z. Ions with smaller m/z values are deflected more.

What does the detector signal show?

The detector signal is a small current. Its size is proportional to the abundance of ions reaching the detector.

Related Topics

Continue with the linked Topic 1 revision pages to connect mass spectrometry with isotopes, relative atomic mass and ionisation energy.

RAM Calculations Predicting Diatomic Mass Spectra Ionisation Energy
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Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.