To plot the MS detector’s response, the most abundant ion, called the base peak, is given an arbitrary abundance or intensity of 100. The application reports other peaks as a percentage of the size of the base peak. After this normalization, the data system can compare spectra directly.

The following figure is an example of a NIST library spectrum showing the fragmentation of acetone C3H6O (molecular weight = 58 Da).

70 eV electron ionization (EI) mass spectrum of acetone
70 eV electron ionization (EI) mass spectrum of acetone

The m/z labels appear above the spectrum peaks that represent the most abundant ions. In this example, the molecular ion (58 Da) is not the most abundant ion. The most abundant ion is the acetyl ion, CH3CO (molecular weight = 43 Da).