Variety of new techniques for ionizing organic molecules were briefly sammarized in relation to the internal energies of molecular ions. The principle of chemical ionization (CI) and the characteristics of CI mass spectra were described to present the advantage of CI spectra. The influences of proton affinities and hydride affinities of reagent gases and sample molecules were shown in the CI spectra which also give the information about chemical reactions. Isotopically labelled compounds like D₂O can be useful reagent gas to obtain the informations about proton affinities of radicals, extent of H - D exchange reaction and structure of compounds.
The principle and characteristics of KE mass spectrum (mass spectrum of ions having excess kinetic energy) were also described. Ions with excess kinetic energy (KE ions) are formed mainly from direct cleavage of highly excited parent molecules, so that it is easy to interpret KE mass spectra which give the infomations about functional groups, such as C₂H₅, CH₂OH, CH₃CO, CH₃CHNH₂, CH₂CN and also about fragmentation mechanism, effect of internal and translational energies and structure of compounds.