Mass Spectrometry

Tom Squire
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Mass spectrometry is an analytical tool used to measure the mass-to-charge ratio (m/z) of a sample. The results are typically presented as a mass spectrum, a plot of intensity vs the mass-to-charge ratio. A mass spectrum is commonly used by researchers as a good way of identifying the purity of a sample as well as if the sample is that which is predicted. This is usually done though identifying if the molecular ion peak (M+) produced matches the predicted mass of the molecule analysed. 

Mass spectrum of CO2 (image adapted from Chiu & Muddiman 2001).  

However, an actual mass spectrum is a lot more complicated than just one molecular ion peak. Peaks smaller than the predicted molecular mass will be seen as the molecule fragments. Peaks at a greater mass than the molecular ion peak may also be seen if the molecule analysed is a salt (e.g. a peak of the molecular weight plus 23 relating to a Sodium ion can be seen). Also if an atom in a molecule has multiple natural isotopes, the molecular ion peak appears to be split into multiple peaks. E.g. if a molecule contains Chlorine such as CClH3, where the Chlorine has natural isotopes of 35Cl and 37Cl, the molecular ion peak will actually be split in two at 50 and 52 (m/z). The ratio of the two peaks will also approximately match that of the isotopes, therefore the peaks will be at an approximate ratio of 75:25 as this is the natural abundance of the Chlorine isotopes 35Cl and 37Cl. 

Schematic of mass spectrometer
Schematic of mass spectrometer (source and for information see

The process that generates the mass spectrum can be broken down into four sections;

  1. Ionisation; A vaporised sample is ionised by knocking one or more electrons off the molecules to give positive ions.
  2. Acceleration; The positive (gas-phase) ions are then accelerated so that all the ions have the same kinetic energy. 
  3. Separation; The ions are then separated by their mass-to-charge ratio, whereby the heavier ions take longer to reach the detector. 
  4. Detection; The beams of ions are then detected electronically with the resulting information used to produce a mass spectrum. 

Page Author; Thomas Squire

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