Nitrogen rule

The nitrogen rule states that organic compounds containing exclusively hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and the halogens either have 1) an odd nominal mass that indicates an odd number of nitrogen atoms are present or 2) an even nominal mass that indicates an even number of nitrogen atoms are present in the molecular ion. The nitrogen rule is not a rule, per se, as much as a general principle which may prove useful when attempting to solve organic mass spectrometry structures.