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We provide a succinct explanation of electron ionization for GC–MS and illustrate the fundamentals of ion formation.
In an electron ionization (EI) source, analyte ions in the gas phase encounter a stream of thermionic electrons with 70 electron volts (eV) of energy, emitted from the surface of a heated metal filament. The energy from these electrons is transferred, in part, to an analyte (no collisions are involved!), which causes the ejection of an electron from an atom within the analyte molecule, forming a radical (odd electron) cationic species.
The amount of energy required to remove an electron from smaller organic molecules typically ranges from 8 to 12 eV, and any excess energy imparted by the ionizing electron may cause bond breakage and the formation of fragments (note that not all of the remaining electron energy is necessarily transferred to the analyte molecule).
The site of ionization is perhaps the first consideration when attempting to better understand the ionization process, and, in general, the following series indicates the energy required (and therefore favorability) of ionization site:
So, if an analyte were to contain a heteroatom, for example, containing non-bonding electrons, one might first begin to elucidate the spectrum obtain by assuming that the charge is cited on the heteroatom, and so on.
In electron ionization mass spectrometry, the intensity of the spectral signal for a molecular ion or fragment is dependant upon the energetic favorability of that species being formed, which is usually closely linked to the ability of the resulting ion to stabilize the charge which it carries, as well as the stability of the radical or molecular product (that is, the stability of all products must be considered). For this reason, highly unsaturated or aromatic species, which are able to stabilize charge, tend to have their most intense fragments at the higher molecular weight end of the spectrum (the right hand end), and alkane species, which are less able to stabilize the charge on the molecular ion, tend to fragment more readily, and the most intense fragments will lie to the lower molecular weight end (the left hand end of the spectrum).
Identifying the molecular ion within the spectrum is important, as it provides the molecular weight of the analyte, which is obviously very helpful for analyte identification. If a very weak molecular ion is suspected, one may reduce the energy of the ionizing electrons, from 70 eV down to around 25 eV, before signal intensity becomes too weak to distinguish from noise. This tends to promote the intensity of the molecular ion, and helps us confirm the suspected molecular ion.
When considering the nature of the fragments formed, and thus the chemical nature of the analyte, there are some "typical" mechanisms and pathways with which fragments are formed, typically to stabilize the charge on the analyte molecule, and these are illustrated in Figure 1.
Remember that only the charge products are seen within the mass spectrometer, as we cannot guide the neutral specie through the mass spectrometer toward the detector.
There are many more "tools" that can be used in spectral interpretation and described above represent just some the fundamental considerations. For further information see: https://www.chromacademy.com/mass-spec-training.html