mass spectrometry

Meaning: Mass spectrometry ionizes molecules, sorts the ions by mass-to-charge ratio, and records how many reach a detector at each value. In electron-impact ionization, a high-energy electron removes an electron from the molecule, giving a radical cation, the molecular ion (M⁺•). Extra energy often breaks it into fragments, and only the charged fragments are detected.

Equation: m/z = m ÷ z, where m is the ion's mass (kg in SI, usually reported in u) and z is the ion's charge number (unitless). Because z is usually +1, m/z is typically equal to the ion's mass in u.

On the MCAT: Spectra plot relative abundance versus m/z. The molecular ion peak gives the molecular mass. Gaps between it and fragment peaks reveal lost groups (M − 15 suggests loss of CH₃, M − 18 suggests loss of H₂O). Peaks at M and M+2 in about a 3:1 ratio suggest one chlorine; about 1:1 suggests one bromine.

Common confusion: The tallest peak is the base peak, which is set to 100% abundance and is a fragment in many spectra. The molecular ion is the peak at the highest m/z (ignoring small isotope peaks such as M+1 from ¹³C), even when it is short. Some molecules, such as alcohols, give a weak or missing molecular ion.