Fragment Finder

Ion Parity

Almost all molecules have an even number of electrons. If you remove an electron to form an ion in the source of a mass spectrometer, then a molecular ion with an odd number of electrons will result. For example, the ionization of a molecule in the source gives:
M + e- -> M+• + 2e-
Molecules or ions with an odd number of electrons are called radicals. Molecular ions usually fragment by the loss of a neutral radical:
M+• -> F+ + R•
Therefore, fragment ions are usually even electron species. However, the loss of a stable neutral molecule is also a common feature of fragmentation of ions. Therefore, fragments resulting from the loss of stable neutrals like H2, H2O, CO, CO2, HCN, N2, HF, HCl, HBr will leave the ion produced by primary fragmentation of the molecular ion an odd electron radical. For example, alcohols often loose H2O to form odd electron fragments:
R-OH+• -> [R-H]+• + H2O

Even electron ions rarely fragment to form odd electron ions. Therefore, once an even electron fragment forms, subsequent fragmentations produce even electron fragments. The following rules then result(1).
In the absence of nitrogen atoms or for an even number of nitrogen atoms:

For an odd number of nitrogens the rules are reversed.

Common stable molecule losses include

In summary the following rules are normally observed:

Reference: 1. Robert A. W. Johnstone, Malcolm E. Rose, Mass spectrometry for chemists and biochemists, 2nd. Ed., Cambridge University Press, Cambridge, Great Britain, 1996.

Colby College Chemistry, 1999