9 - Theory of mass spectrometry

Summary

INTRODUCTION

A thorough understanding of mass spectrometric processes is severely hampered by poor knowledge of the structures and electronic states of ions. The fragmentation of ions depends on their structures and the excess of internal energy that they contain and this last is governed mostly by three factors: the thermal energy in the molecule immediately prior to ionization, the energy gained during ionization and the subsequent environment of the ion, which determines whether or not it collides with molecules. These energies are depicted schematically in figure 9.1, from which it can be seen that ions formed by removal of an electron (M^-· in figure 9.1(a)), addition of a proton ([M + H]⁺ in figure 9.1(b)) or addition of an electron (M^-· in figure 9.1(c)) require different energy inputs to achieve minimum energy levels of ionization. Such ions have no excess of internal energy and will be stable towards fragmentation. However, ions can gain extra internal energy in several ways, some of which are shown in figure 9.1(d), and this extra energy (excess of internal energy) may well be sufficient to induce decomposition not only in the first-formed ions (primary fragmentation) but also in those arising from this initial fragmentation (secondary fragmentation). The very act of heating a sample to vaporize it into the ion source adds extra thermal energy (E_v) to the molecules even before the ionization step; additionally, some sources such as those for El are usually hot, so that collision of the sample molecules with the walls of the source before ionization also adds further excess of thermal energy.