The purpose of the nitrogen rule is to assist with deciphering how many nitrogen atoms are present without any prior information on the molecular formula. Depending on the ionization mode, an odd nominal mass indicates an odd number of nitrogen atoms, e.g. 1,3,5, whereas an even nominal mass indicates an even number of nitrogen atoms, e.g. 0,2,4.
The nitrogen rule can only be applied under the following conditions:
1. the m/z value is the molecular ion,
2. the unknown is an organic compound with any combination of hydrogen, carbon, oxygen, nitrogen, phosphorous, silicon, sulfur, fluorine, chlorine, bromine, iodine, and
3. the MS data is nominal data.
For a better way to tell if nitrogen is present, please visit this link to Fiehn’s Lab.