Ion Formation in Mass Spectrometry: Ionization and Fragmentation

Abstract

Formation of ions is the first step of mass spectrometric analysis. Neutral species are ionized in an ion source giving primary ions. These ions act as precursors of secondary ones that are produced mainly by fragmentation. Fundamental ionization processes include ejection or capture of an electron, charge exchange, removal of an anion, proton transfer, and formation of adducts or clusters. In a particular ion source, one or more of these processes can be effective for a given analyte. For example, protonated molecules and sodium adducts are simultaneously observed after the electrospray ionization. The ionization is also related to the pressure in the ion source. Under high vacuum, the loss of electron is predominant, while ion-molecule reactions become more important with increasing pressure. Ionization techniques significantly influence the applicability of mass spectrometry. They have been developed to achieve ionization of still less volatile substances, more polar and larger molecules. Nevertheless, universal ionization technique has not been introduced yet. Careful selection of the technique appropriate for the given analysis is still a crucial step.