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How does 2-butanone fragment during 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, 2-butanone undergoes fragmentation primarily at the bonds where the alkane chains are attached to the carbonyl group.12

The fragmentation process occurs as follows:

Molecular Ion: The molecule is initially ionized to form a molecular ion at m/z = 72.12
Loss of Methyl Group: The loss of a CH₃ group (mass 15) from the molecular ion generates a fragment at m/z = 57.12
Loss of Ethyl Group: The loss of the heavier CH₃CH₂ group (mass 29) generates a fragment at m/z = 43. This specific fragment corresponds to the base peak of the spectrum, which is the most intense peak recorded.12