mass spectrometer

Meaning: A mass spectrometer ionizes a sample, often by knocking off an electron to give a radical cation (M⁺•) that may fragment. The ions are accelerated, separated by mass-to-charge ratio (m/z), and detected. The spectrum plots relative abundance against m/z. Most ions carry a +1 charge, so m/z usually equals the mass.

Equation: r = mv/(qB), where r is the radius of the ion's path (m), m is mass (kg), v is speed (m/s), q is charge (C), and B is magnetic field strength (T). For ions with the same speed and charge, heavier ions curve less.

On the MCAT: The molecular ion peak, the main peak at highest m/z, gives molar mass. Fragment losses suggest structure: M − 15 points to loss of CH₃, and M − 18 to loss of H₂O. Br gives M and M+2 peaks of about equal height; Cl gives about a 3:1 ratio. A small M+1 peak typically comes from ¹³C.

Common confusion: Students often call the tallest peak the molecular ion. The tallest peak is the base peak, the most abundant ion, which is often a stable carbocation fragment. The molecular ion can be weak. Also, mass spectrometry gives mass and fragmentation patterns, not a direct list of functional groups the way IR does.