• 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:

Worked example: Determining number of carbon atoms

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:

There are therefore 3 carbon atoms present in compound X