How mass spectrometry works
How mass spectrometry works
The simple TOF mass spectrometer
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A time-of-flight mass spectrometer identifies ions by timing their journey to a detector. Follow what happens to the particles at each stage, rather than learning five names without their meanings.
- Ionise: make positive ions using electrospray ionisation or electron impact, as described below.
- Accelerate: an electric field accelerates the positive ions. Ions with the same charge gain the same kinetic energy when accelerated through the same potential difference.
- Drift: the ions travel through an evacuated, field-free tube, keeping the same kinetic energy they gained during acceleration. For ions of the same charge, this means equal kinetic energies but different speeds. The vacuum prevents collisions with gas particles.
- Detect: positive ions gain electrons at the detector. This electron movement produces a current. More arriving ions produce a larger current.
- Analyse: flight time is used to find m/z, while current gives relative abundance.
Ionisation context
- Electrospray ionisation
- The sample is dissolved in a volatile solvent and passed through a fine needle at high voltage to form charged droplets. The solvent evaporates and molecules gain a proton, forming [MH]⁺ ions. Useful for large or thermally unstable molecules; generally causes little fragmentation.
M the original neutral molecule; [MH]+ the protonated molecule: M with an extra H⁺ attached; H+ a proton; (g) gas.
- Electron impact
- The sample is vaporised and bombarded with high-energy electrons. A collision knocks an electron from a gaseous atom or molecule, forming a positive ion. Commonly used for small volatile molecules; molecular ions may fragment.
M a gaseous atom or molecule of the sample; M+ the singly charged positive ion formed by electron loss; e− an electron; (g) gas.
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Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
The stages below are not in the correct order: acceleration, data analysis, detection, drift, ionisation.
- Put the stages in the correct order. [2]
- State how the ions are accelerated and the energy condition produced. [2]
- Explain why the drift region is evacuated. [1]
Total: 5 marks
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- Ionisation → acceleration → drift → detection → data analysis. [2]
- An electric field accelerates the ions [1], giving ions of the same charge the same kinetic energy. [1]
- To prevent ions colliding with gas particles. [1]
Describe what happens when a positive ion reaches the detector and explain how the detector signal is related to abundance.
Total: 2 marks
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- The positive ion gains an electron, producing a current. [1]
- The size of the current is proportional to the abundance of that ion. [1]
- Write an equation, including state symbols, for the formation of a singly charged positive ion from a gaseous atom Y by electron impact. [1]
- Explain how the ion produces a detector signal and how it gives relative abundance. [3]
Total: 4 marks
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- Y(g) + e⁻ → Y⁺(g) + 2e⁻. The net ionisation equation Y(g) → Y⁺(g) + e⁻ also shows the loss of one electron. [1]
- The ion gains an electron at the detector [1]. Electron movement produces a current [1]. A larger current indicates greater abundance [1].
A large organic molecule M is analysed using electrospray ionisation.
- Describe how [MH]⁺ ions are produced. [4]
- Write the equation for forming [MH]⁺. [1]
- A singly charged [MH]⁺ peak is at m/z 587.4. Determine Mᵣ of M. [1]
Total: 6 marks
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- Dissolve M in a volatile solvent [1]; inject through a fine needle at high pressure [1]; apply high voltage to form charged droplets [1]; M gains a proton as the solvent evaporates, forming [MH]⁺ [1].
- M + H⁺ → [MH]⁺. [1]
- Mᵣ = 587.4 − 1 = 586.4. [1]
Why lower-m/z ions arrive first
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For ions with the same charge, acceleration through the same potential difference gives equal kinetic energy. Equal kinetic energy does not mean equal speed: a heavier ion moves more slowly than a lighter ion.
- Using the kinetic-energy equation, KE = ½mv²: if mass is larger while KE stays the same, speed must be smaller.
- Both ions travel the same distance. The slower, heavier ion therefore reaches the detector later.
For ions with the same charge, lower mass means lower m/z, so lower-m/z ions arrive first.
KE kinetic energy of one ion, in J; m mass of one ion, in kg; v speed of the ion, in m s⁻¹.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
Two singly charged ions are given the same kinetic energy. They travel the same flight distance. One ion is heavier. Explain why the heavier ion reaches the detector later.
Total: 3 marks
Show answer & marking points
- Both ions have the same kinetic energy. [1]
- From KE = ½mv², the heavier ion must have a lower speed. [1]
- It takes longer to travel the same distance. [1]