additivity

of mass spectra
https://doi.org/10.1351/goldbook.A00136
The process by which each chemical species present in the ion source at a certain partial pressure makes a contribution to the total mass spectrum which is the same as that which it would give if that chemical species alone were present in the ion source at a pressure equal to this certain partial pressure.
Source:
Orange Book, 2nd ed., p. 206 (http://media.iupac.org/publications/analytical_compendium/)