13C

Carbon-13 (¹³C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes, it makes up about 1.1% of all natural carbon on Earth.

A mass spectrum of an organic compound will usually contain a small peak of one mass unit greater than the apparent molecular ion peak (M) of the whole molecule. This is known as the M+1 peak and comes from the handful of molecules that contain a ¹³C atom in place of a ¹²C. A molecule containing one carbon atom will be expected to have an M+1 peak of approximately 1.1% of the size of the M peak, as 1.1% of the molecules will have a ¹³C rather than a ¹²C. Similarly, a molecule containing two carbon atoms will be expected to have an M+1 peak of approximately 2.2% of the size of the M peak, as there is double the previous likelihood that any molecule will contain a ¹³C atom.

In the above, the mathematics and chemistry have been simplified, however it can be used effectively to give the number of carbon atoms for small- to medium-sized organic molecules. In the following formula the result should be rounded to the nearest integer:

${\displaystyle C={\frac {100Y}{1.1X}}}$

C = number of C atoms X = amplitude of the M ion peak Y = amplitude of the M+1 ion peak

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