a, Formula to calculate spectral entropy with results for mock-up spectra. b, Spectral entropy calculated for example spectra from MassBank.us with precursor ion peaks in gray. Spectrum of guanosine is generated from Orbitrap with 35 eV; spectrum of lactitol is generated from TripleTOF with 35 eV; spectrum of l-valine is generated from QTOF with 55 eV; and spectrum of juarezic acid is generated from Q exactive HF with 65 eV. c, relationship between spectral entropy and number of MS/MS fragment ions. d, Distribution of spectral entropies in the NIST20, MassBank.us and GNPS databases.