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.