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Why is methane used as a reagent gas in 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, chemical ionization is used as an alternative to electron bombardment when the extensive fragmentation of a sample molecule is undesirable. Methane (CH₄) is employed as a reagent gas in this process because it produces fewer fragments than direct electron impact.1

When methane is mixed with the sample gas and subjected to electron bombardment, it becomes ionized. This ionized methane reacts to form CH₅⁺, which subsequently ionizes the sample gas through proton or charge transfer. This technique is valuable because it provides a gentler ionization method that preserves the molecular structure, sometimes increasing the mass of the resulting ion by one unit.1