# 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:

#### Worked example: Determining number of carbon atoms

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

### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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