base peak
/ BAYSS peek /
Imagine an audience of people clapping at different volumes, and you decide to call the loudest clap '100%', then describe everyone else relative to it — 'half as loud', 'a tenth as loud'. In a mass spectrum, the base peak is that loudest clap: the tallest peak, which is set to 100% so every other peak can be measured against it.
Formally, the base peak is the most intense (most abundant) peak in a mass spectrum. Its height is defined as 100% relative abundance, and all other peaks are scaled to it as percentages. This is purely a way of normalizing the chart so spectra can be compared on the same footing regardless of how many ions were detected overall.
It matters because using relative intensities makes spectra comparable from run to run and instrument to instrument, even though the absolute number of ions varies. The honest caveat is that the base peak is simply the most abundant ion, which is not necessarily the molecular ion — often it is a stable fragment that the molecule readily breaks into.
In the electron-ionization spectrum of toluene, the base peak sits at m/z 91 (a stable fragment), not at m/z 92 (the molecular ion) — so the tallest peak is a piece of the molecule, not the whole of it.
The base peak is the tallest peak, set to 100% to scale the rest.
Base peak and molecular ion are easily confused but distinct: the base peak is defined by intensity (tallest), the molecular ion by identity (the whole molecule). They coincide only when the intact molecule is also the most abundant ion.