June 10, 2008
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
For MS data, the ionization method will dictate how the nitrogen rule is applied.
For example, if the ionizer is Electron Impact (EI) ionization, then the nitrogen rule is to be applied to the molecular ion [M]+* as follows:
-an odd nominal mass indicates an odd number of nitrogen atoms, e.g. 1,3,5
-an even nominal mass indicates an even number of nitrogen atoms, e.g. 0,2,4.
If the particles are charged using ElectroSpray Ionization (ESI), for example, then the nitrogen rule is to be applied to the molecular ion [M+H]+ or [M-H]- as follows:
-an odd nominal mass indicates an even number of nitrogen atoms
-an even nominal mass indicates an odd number of nitrogen atoms.
The chemical structure for Etifenin is shown below; the differences between the structures are displayed in red. The nominal mass for the [M+H]+ structure, shown on the left side, is odd and therefore the nitrogen rule dictates an even number of nitrogen atoms. For the M+* structure shown on the right side, the even nominal mass dictates an even number of nitrogen atoms.
