Illustrated Glossary of Organic Chemistry

Base peak: The most intense (tallest) peak in a mass spectrum, due to the ion with the greatest relative abundance (relative intensity; height of peak along the spectrum's y-axis). Not to be confused with molecular ion: base peaks are not always molecular ions, and molecular ions are not always base peaks.

The electron impact ionization mass spectrum of PhCH₂Cl, in which the base peak is a fragment ion having m/z = 91.


The electron impact ionization mass spectrum of methane, in which the base peak is M having m/z = 16.