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Why does 2-butanone have a molecular-ion peak at m/z 72?

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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, the peak at m/z 72 for 2-butanone represents the molecular ion.12

This peak occurs because, within the ionization chamber of the mass spectrometer, the 2-butanone molecule is bombarded by a beam of high-energy electrons. This process removes a single electron from the molecule, creating a positively charged molecular ion. For 2-butanone, which has the molecular formula $C_{4} H_{8} O$ , this ion has a mass-to-charge ratio (m/z) of 72. Because the ion carries a single positive charge (z = 1), the m/z value directly corresponds to the molecular mass of the compound.123