Illustrated Glossary of Organic Chemistry

M: In mass spectrometry the molecular ion composed of isotopes with the lowest mass numbers, such as 1H (mass = 1 amu) instead of 2H (deuterium; mass = 2 amu) and 12C (mass = 12 amu) instead of 13C (mass = 13 amu).

Molecular ion region of the mass spectrum of 1-chlorodecane (C10H21Cl).
The relative abundance of M is due to 12C101H2135Cl.
The relative abundance of M+1 is due to 12C913C1H2135Cl + 12C101H202H35Cl.
The relative abundance of M+2 is due to 12C914C1H2135Cl + 12C813C21H2135Cl + 12C913C1H202H35Cl + 12C101H192H235Cl + 12C101H193H35Cl + 12C101H2137Cl.