Internal energy effects in mass spectrometry

The internal energy of an ion has a very large effect on the appearance of the mass spectrum. The first part of this paper gives the theoretical background: (i) dependence of the reaction rate on internal energy; (ii) correspondence between the reaction rate and fragment ion abundances; (iii) role of the residence time of an ion in various parts of the instrument; (iv) the breakdown curve; and (v) the degrees of freedom or number of oscillators affected. In the second part internal energy distributions are discussed: those of molecular ions produced by various ionization techniques and of precursor ions in tandem mass spectrometry. The latter topic is closely connected with secondary excitation, and the particular case of collisional activation is discussed in some detail. In the final part, practical examples of internal energy effects in mass spectrometry are given, including high- and low-energy electron impact, chemical ionization using various reagent gases, metastable and collision-induced dissociation, keV and low-energy collision induced dissociation and matrix-assisted laser desorption/ionization.