Ion sources

In the ion source, sample molecules become ionised to form a beam of charged molecules. Today, the most widely used source is the electron impact (EI) source. In the EI source, neutral sample molecules in the gas phase collide with energetic electrons and form a large range of ions (molecular ions, quasi molecular ions and fragments). Fragmentation takes place, especially with energies of around 70 eV, and produces a typical fragmentation pattern for each sample compound. Whereas this is an advantage for identification and structure elucidation of small unknown compounds, it is difficult to obtain intact molecule ions of large molecules, in particular of biopolymers like nucleic acids or proteins. For this reason, soft ionisation techniques like fast atom bombardment (FAB), plasma desorption (PD), secondary ion MS (SIMS), electrospray ionisation (ESI) and matrix assisted laser desorption/ionisation (MALDI) were developed. These soft ionisation techniques have in common, that the sample molecule becomes ionised in a very gentle way producing intact molecular ions, normally [M$+$H$^+$]$^+$ or [M$-$H$^+$]$^-$. Thus in the last decade, MALDI and ESI ion sources gained recognition, especially in biochemical applications.