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How are gaseous molecules turned into ions in electron-ionization mass spectrometry?

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Figure 1: An electron bombardment ion source in cross section. An electron beam is drawn from the filament and accelerated across the region in which the ions are formed and toward the electron trap. An electric field produced by the repeller forces the ion beam from the source through the exit slit. — mass spectrometry Figure 1: *An electron bombardment ion source in cross section.* An electron beam is drawn from the filament and accelerated across the region in which the ions are formed and toward the electron trap. An electric field produced by the repeller forces the ion beam from the source through the exit slit.

In electron-ionization mass spectrometry, gaseous molecules are converted into ions through a process known as electron bombardment.1

In this technique, electrons are extracted from a glowing filament to create a beam of electrons. When these electrons collide with neutral gas molecules, they possess sufficient energy to force electrons out of the molecules. This interaction results in the production of ion pairs, which consist of a positive ion and a detached negative electron.12

For organic compounds, this process often results in the ionization of the whole molecule as well as the creation of various fragments. These fragments are highly valuable in chemical analysis, as they produce a mass spectrum that acts as a unique identifier for the source molecule. If fragmentation is undesirable for a specific experiment, alternative methods such as chemical ionization may be employed instead.1