Molecular Formulas and Isotopic Abundances

In every organic compound (natural or synthesized from natural sources), the will result in a peak that is one mass number greater than the mass of the molecular ion. This C-containing peak is generally designated as (M + 1). For example, with methane, CH4, the MW is 16, assuming that carbon has an atomic weight of 12 (i.e., = 12 + 4 = 16). Let the abundance of this peak be 100. 

If our molecule contains only carbon and hydrogen, since there is 1.1% relative natural abundance of compared to 100% C, and our mass spectrum shows a 1.1% abundance of (M + 1) to M, only one carbon atom can be present in the molecule. That is, if we are dealing with a hydrocarbon and only one carbon atom is present, then the ratio (M + 1)/M = 1.1%. If two carbons are present in the molecule, as there are in ethane, the probability of being present is twice as great and (M + 1)/M = 2.2%. 

---