Molecular Ion Peak & Fragmentation (CIE A Level Chemistry)

• 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

MOLECULE  MOLECULE⁺• + e^–

• 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

• 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 gives rise to peaks at 18 (H₂O), 28 (CO), and 44 (CO₂)

Alkanes

• 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

Straight chain alkanes show characteristic peaks 

Halogenoalkanes

• 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 for a bromine containing species

Mass spectra show different halogen isotopes in the molecular ion

Alcohols

• 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 of propan-1-ol

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