CIE AS Chemistry (9701) exams from 2022

Revision Notes

4.1.5 M + 1 Peak

Determine Number of Carbon Atoms Using M+1 Peak

  • The [M+1] peak is caused by the presence of the carbon-13 (13C) isotope in the molecule
  • Carbon-13 makes up approximately 1.1% of all carbon atoms
  • Therefore, the [M+1] peak is much smaller than the M peak as the isotope is less common
    • The ratio of 13C to 12C is approximately 1:99
  • Thus, the greater the number of carbon atoms present in a molecule the greater the height of the [M+1] peak
  • The number of carbon atoms, n, in a compound can be deduced using the [M+1] peak and the following formula:

Determine Number of Carbon Atoms Using M+1 Peak equation 1

Worked example: Determining number of carbon atoms

Analytical Techniques Worked example - Determining number of carbon atoms, downloadable AS & A Level Chemistry revision notes

Analytical Techniques Spec 1_Determine Number of Carbon Atoms Using M+1 Peak, downloadable AS & A Level Chemistry revision notes

Answer

The M+ ion peak is at m/e 58 with a relative abundance of around 85

The [M+1] peak is at m/e 59 with a relative abundance of 3

Therefore, the number of carbon atoms (n) is:

Determine Number of Carbon Atoms Using M+1 Peak worked example equation 1

There are therefore 3 carbon atoms present in compound X

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