Illustrated Glossary of Organic Chemistry

Nitrogen rule: When m/z for M has an even mass (even number of amu), the corresponding molecular formula has an even number of nitrogen atoms (0, 2, 4, etc.). When m/z for M has an odd mass (odd number of amu), the corresponding molecular formula has an odd number of nitrogen atoms (1, 3, 5, etc.). The rule applies when the molecule in question has only hydrogen, carbon, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. It may or may not work when other elements are present.

Molecule:	Ammonia		Hydrazine		TNT		Caffeine
Formula:	NH3		N2H4		C7H5N3O6		C8H10N4O2
m/z (M):	17		32		227		194
	One nitrogen m/z (M) is odd		Two nitrogens m/z (M) is even		Three nitrogens m/z (M) is odd		Four nitrogens m/z (M) is even