AQA Chem4: Mass Spectrometry

Revision notes on mass spectrometry from AQA chem 4

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  • Created by: anna
  • Created on: 07-06-13 12:30

AS Revision

5 stages of mass spectrometry:

  • vapourisation - this turns the sample into a gas
  • ionisation - electron gun knocks electron out of molecule to make an ion
  • acceleration - accelerates molecule with electric field
  • deflection - magnet used to deflect ions different amounts
  • detection - charged plate detects ions


Definition for relative atomic mass - the average mass of an atom  compared to 1/12 of carbon 12

Ar = sum of (m/z x relative abundance) / sum of relative abundance


Definition of relative molecular mass - the average mass of an entity compared to 1/12 of carbon 12

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Fragmentation of Molecular Ion

  • the molecular ion is positively charged and has one unpaired  electron shown as M+.
  • the high energy ionising beam from the electron gun causes the ionised molecule to fragment
  • this involves breaking covalent bonds in the ionised molecule
  • this produces an ion and a free radical
  • the general equation is - M+. (g) --> X+ (g)  +  Y. (g)
  • further fragmentation of X+ is possible to give another ion and free radical
  • this results in a characteristic fragmentation pattern
  • the molecular ion (M+.) fragments at weaker bonds first
  • dominant peaks are associated with the most stable fragments (best able to have a positive charge)
  • the most stable ions are carbocation and acylium ions (e.g. HCO+) 
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Common Fragments


m/z = 15        possible ion: CH3+

m/z = 29        possible ion: CH3CH2+ or HCO+

m/z = 31        possible ion: CH3O+

m/z = 43        possible ion: (CH3)2CH+ or CH3CO+

m/z = 57        possible ion: CH3CH2CO+

m/z = 77        possible ion: C6H5+

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Spectra of Haloalkanes

  • both chlorine and bromine have isotopes
  • compounds containing either Cl or Br will have more than one molecular ion peak



  • has 2 isotopes, chlorine 35 and 37 in the ratio of 3:1
  • compounds with one Cl atom will have 2 molecular ion peaks and the ratio of the heights of the peaks will be 3:1



  • has 2 isotopes, bromine 79 and 81 which exist in equal amounts
  • compounds with one Br atom will have 2 molecular ion peaks of the same height
  • compounds with 2 Br atoms will have 3 molecular ion peaks
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