CIE A Level Chemistry复习笔记4.1.4 Molecular Ion Peak & Fragmentation

Mass Spectrometry: Deducing Molecular Formula

  • Each peak in the mass spectrum corresponds to a certain fragment with a particular m/e value
  • The peak with the highest m/e value is the molecular ion (M+) peak which gives information about the molecular mass of the compound
  • The molecular ion is the entire molecule that has lost one electron when bombarded with a beam of electrons


  • The [M+1] peak is a smaller peak which is due to the natural abundance of the isotope carbon-13
  • The height of the [M+1] peak for a particular ion depends on how many carbon atoms are present in that molecule; the more carbon atoms, the larger the [M+1] peak is
    • For example, the height of the [M+1] peak for an hexane (containing six carbon atoms) ion will be greater than the height of the [M+1] peak of an ethane (containing two carbon atoms) ion

Worked example: Analysing mass spectra




The mass spectrum corresponds to propanal as the molecular ion peak is at m/e = 58

Propanal arises from the CH3CH2CHO+ ion which has a molecular mass of 58

Butanal arises from the CH3CH2CH2CHO+ ion which has a molecular mass of 72

Identifying Molecules using Fragmentation

  • The molecular ion peak can be used to identify the molecular mass of a compound
  • However, different compounds may have the same molecular mass
  • To further determine the structure of the unknown compound, fragmentation is used
  • Fragments may appear due to the formation of characteristic fragments or the loss of small molecules
    • For example, a peak at 29 is due to the characteristic fragment C2H5+­­
    • Loss of small molecules give rise to peaks at 18 (H2O), 28 (CO), and 44 (CO2)


  • Simple alkanes are fragmented in mass spectroscopy by breaking the C-C bonds
  • M/e values of some of the common alkane fragments are given in the table below

m/e values of fragments table



Mass spectrum showing fragmentation of alkanes


  • Halogenoalkanes often have multiple peaks around the molecular ion peak
  • This is caused by the fact that there are different isotopes of the halogens


Mass spectrum showing different isotopes of the halogens in the molecular ion


  • Alcohols often tend to lose a water molecule giving rise to a peak at 18 below the molecular ion
  • Another common peak is found at m/e value 31 which corresponds to the CH2OH+­­ fragment
  • For example, the mass spectrum of propan-1-ol shows that the compound has fragmented in four different ways:
    • Loss of H• to form a C3H7O+ fragment with m/e = 59
    • Loss of a water molecule to form a C3H6+ fragment with m/e = 42
    • Loss of a •C2H5 to form a CH2OH+ fragment with m/e = 31
    • And the loss of •CH2OH to form a C2H5+ fragment with m/e = 29


Mass spectrum showing the fragmentation patterns in propan-1-ol (alcohol)

Worked example: Ion fragmentation



The correct answer is 4 as bromomethane (CH3Br) will fragment into 3 peaks

    • CH381Br → [CH381Br]+ + e at m/e 96
    • CH379Br → [CH379Br]+ + e at m/e 94
    • CH3Br → [CH3]+ + •Br at m/e 15

The last two peaks (which correspond to the molecular ion peak) therefore are equal in size and occur at m/e values of 94 and 96


Worked example: Alcohol fragmentation



The correct answer is 4 because a line at m/e = 43 corresponds to an ion with a mass of 43 for example:

    • [CH3CH2CH2]+
    • [(CH3)2CH]+

2-butanol is not likely to have a fragment at m/e = 43 as it does not have either of these fragments in its structure.