Topic 2 · page 3 of 15

The mass spectrometer

A mass spectrometer sorts ions by their mass-to-charge ratio and counts them. The result is a spectrum with one peak for each isotope, and its height shows how much of that isotope there is.

13 min read2.62.6(i)

Read first: Isotopes

The five stages 2.6

You cannot see an atom, but you can weigh it. A mass spectrometer does this by turning atoms into ions, which can be steered with electric and magnetic fields. The heavier the ion, the harder it is to steer. There are five stages.

A mass spectrometer in five stages. The sample is vaporised, ionised by an electron gun, accelerated by an electric field, deflected by a magnetic field, and detected. Ions with a smaller m/z are deflected too much and ions with a larger m/z are deflected too little, so only ions of the right m/z reach the detector.
The five stages. The whole instrument is kept under a vacuum.
  1. Vaporise. The sample is turned into a gas, so that its particles are free to move.
  2. Ionise. A beam of high-energy electrons from an electron gun knocks one electron off each particle. This makes positive ions:
X(g)+eX−→XX+(g)+2 eX− \ce{X(g) + e- -> X+(g) + 2e-}
  1. Accelerate. Negatively charged plates attract the positive ions and speed them up into a narrow beam.
  2. Deflect. A magnetic field bends the path of the beam. Ions with a smaller mass-to-charge ratio are deflected more; ions with a larger one are deflected less. Only ions with the right ratio follow the curve through the tube and reach the detector. The strength of the field is changed gradually, so that each type of ion reaches the detector in turn.
  3. Detect. Ions that arrive at the detector gain an electron and make a tiny electric current. The more ions arrive, the larger the current.

The inside of the instrument is under a vacuum, so that the ions do not collide with air molecules on the way.

Reading a mass spectrum 2.6(i)

A mass spectrum is a bar chart. The horizontal axis is m/z. The vertical axis is relative abundance, which tells you how much of each ion the detector counted. Because the detector current is proportional to the number of ions, the height of a peak shows how common that ion is.

For an element, each peak is one isotope. Almost every ion carries a 1+ charge, so the m/z is the mass number of the isotope.

Mass spectrum of magnesium with three peaks: 24Mg+ at m/z 24 with abundance 79.0 per cent, 25Mg+ at m/z 25 with 10.0 per cent, and 26Mg+ at m/z 26 with 11.0 per cent.
Magnesium, a schematic spectrum. One peak for each isotope.

To deduce the isotopic composition, read off each peak:

m/z Ion Isotope Relative abundance
24 ²⁴Mg⁺ magnesium-24 79.0%
25 ²⁵Mg⁺ magnesium-25 10.0%
26 ²⁶Mg⁺ magnesium-26 11.0%

So magnesium has three isotopes, and about four out of every five atoms are ²⁴Mg.

Worked exampleIsotopic composition of a sample of silicon

A sample of silicon gives peaks at m/z 28, 29 and 30. Their heights are 92.2, 4.7 and 3.1. These add up to 100.0, so they are percentages.

  1. Three peaks, so there are three isotopes: ²⁸Si, ²⁹Si and ³⁰Si.
  2. ²⁸Si is 92.2%, ²⁹Si is 4.7% and ³⁰Si is 3.1%.

Check yourself

  1. Why must the sample be vaporised and then turned into positive ions?
  2. Write an equation, with state symbols, for the ionisation of a chlorine atom in a mass spectrometer.
  3. A beam contains ³⁵Cl⁺ and ³⁷Cl⁺ ions in a magnetic field of the same strength. Which are deflected more?
  4. An element gives peaks at m/z 63 with height 69 and at m/z 65 with height 31. State its isotopic composition.
  5. What is the ion responsible for the peak at m/z 25 in the magnesium spectrum?
  6. In what way does the detector show that one isotope is more abundant than another?

Answers cover these until you have tried the questions

  1. A gas has particles that are free to move, so they can form a beam. Ions are needed because only charged particles can be accelerated by an electric field and deflected by a magnetic field. They are positive because electrons are knocked off.
  2. Cl(g)+eX−→ClX+(g)+2 eX−\ce{Cl(g) + e- -> Cl+(g) + 2e-}
  3. ³⁵Cl⁺. Its m/z (35) is smaller than that of ³⁷Cl⁺ (37), so it is deflected more.
  4. Two isotopes, ⁶³X and ⁶⁵X (this is copper). The heights add up to 100, so they are percentages: ⁶³X 69% and ⁶⁵X 31%.
  5. ²⁵Mg⁺
  6. The current at the detector is proportional to the number of ions arriving, so a more abundant isotope gives a larger current, which is drawn as a taller peak.