Unit 2 Section 4 Mass Spectrometry

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Finding Relative Molecular Mass

  • Remember - to find the relative molecular mass of a compound you look at the molecular ion peak (the M peak) on the spectrum.
  • Molecular ions are formed when molecules have electrons knocked off.
  • The mass/charge value of the molecular ion peak is the molecular mass (assuming the ion has 1+ charge, which it normally will have).
  • For most organic compounds the M peak is the one with the second highest mass/charge ratio.


  • Its M peak is 72 - so the compounds Mr is 72.
  • The smaller peak to the right of the M peak is called the M+1 peak - It's caused by the presence of the carbon isotope C-13.
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  • The bombarding electrons make some of the molecular ions break up into fragments.
  • The fragments that are ions show up on the mass spectrum, making a fragmentation pattern which can be used to identify molecules and their structures.


  • First you need to identify the fragments. 15 - CH3, 17 - OH, etc. To find the other fragments just add combinations of C-12, H-1 and O-16.
  • The next step is to piece them all together to form a molecule with the correct Mr (72 for this example is CH3CH2CH2CH2CH3+).
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