4.1.5 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:

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