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How does electron impact ionization differ from chemical ionization?

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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 mass spectrometry, electron impact and chemical ionization differ primarily in their mechanism of ion formation and the resulting degree of molecular fragmentation.1

Electron Impact Ionization: This method uses a beam of electrons, typically extracted from a glowing filament, to bombard gaseous sample molecules. This process often causes the molecule to break apart into a range of fragments. While this fragmentation can be seen as a disadvantage, it is highly valuable for organic compound identification because the resulting mass spectrum acts as a unique "fingerprint" for the source molecule.1
Chemical Ionization: This technique is employed when the fragmentation caused by electron impact is undesirable. In this process, a reagent gas, such as methane, is mixed with the sample and subjected to electron bombardment. The ionized reagent gas then reacts with the sample molecules through proton or charge transfer to produce ions. Because this method is gentler, it results in significantly less fragmentation and can sometimes increase the mass of the formed ion by one unit.1