molecular ion

Meaning: In electron-impact mass spectrometry, a high-energy electron knocks one electron out of a molecule, giving a radical cation written M⁺•. An electron's mass is negligible, so the ion has essentially the molecule's mass. With a charge of +1, its m/z equals the molecular mass. Many molecular ions then break into fragments, which appear at lower m/z.

Equation: M + e⁻ → M⁺• + 2e⁻, and m/z = m ÷ z, where m is the ion's mass (u, or kg in SI units) and z is the charge number (dimensionless, usually +1).

On the MCAT: A passage shows a spectrum and asks for the molar mass or identity of the compound. Find the molecular ion peak at the far right of the main cluster. Then subtract fragment peaks from it to identify what was lost, such as 15 for CH₃ or 18 for H₂O. A small M+1 peak comes from ¹³C. An M+2 peak about one third the height of M suggests one Cl, and one about equal to M suggests one Br. An odd molecular mass suggests an odd number of nitrogens.

Common confusion: The molecular ion is not necessarily the tallest peak. The tallest is the base peak, often a stable fragment such as a carbocation. The molecular ion peak can be small or missing for easily fragmented molecules, such as many alcohols. Peaks at M+1 and M+2 are isotope peaks of the molecular ion, not impurities.