Molecular ion
When visible, it supports molecular mass and formula candidates.
Spectroscopy study
Mass spectrometry
Mass spectra are strongest when interpreted alongside formula constraints and functional-group evidence.
Base peak
The tallest peak marks the most abundant ion, not necessarily the parent molecule.
Isotope patterns
Chlorine and bromine produce especially recognizable M+2 patterns.
Nitrogen rule
For many organic molecules, an odd nominal mass can indicate an odd number of nitrogens.
Fragments
Common losses and stable cations should fit the functional groups present.