22.2.5—The number of carbon atoms, n, in a compound

The M+1 peak mainly reflects molecules containing one ¹³C instead of ¹²C. For a simple organic compound, n ≈ 100 × abundance(M+1)/(1.1 × abundance(M)).

Use the ratio of peak abundances, not their m/e positions. The result should be close to a whole number and is an estimate subject to other minor isotopes.

If M+ has abundance 50 and M+1 is 5.5, n ≈ 100×5.5/(1.1×50)=10 carbon atoms.

Do not use the base peak in place of M+, and do not round a physically implausible result without checking peak assignment.