Electron-Impact Ionization & Fragmentation

Molecule

Electron Beam

Internal energy 30 eV

Current ion

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Ions detected

Base peak (m/z)

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Last m/z

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How it works

A high-energy electron strikes the neutral molecule and knocks out one of its own electrons, leaving a radical cation — the molecular ion M⁺•. That ion carries the leftover collision energy internally. At low internal energy it usually survives the trip to the detector whole, registering a peak at its full mass. Raise the energy and it's increasingly likely to snap apart at its weakest bond before it gets there; only the charged half of the split is steered onward and counted, while the neutral half is invisible to the detector. Each molecule has its own weakest bonds, so the same physical process draws a different fingerprint of peaks for each compound.

Internal energy — shifts the odds from "stays as M⁺" toward "breaks into a smaller charged fragment."
Molecule — switches which bond is weakest, and therefore which m/z peaks dominate the spectrum.
Acetone at high energy shows a second cleavage: its first fragment ion can itself lose a neutral CO before reaching the detector.

Carbon Oxygen Hydrogen Electron beam Cation (detected)

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