Which fragment produces the base peak of 2-butanone?

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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 the mass spectrometry of 2-butanone, the base peak—the most intense peak in the spectrum—occurs at m/z = 43. This fragment is produced by the loss of the ethyl group (CH $_{3}$ CH $_{2}$ , with a mass of 29) from the molecule, leaving behind the acetyl cation (CH $_{3}$ CO $^{+}$ ).1

While the molecular ion of 2-butanone appears at m/z = 72, the molecule characteristically breaks at the bond where the alkane chain joins the carbonyl group. The loss of the lighter methyl group (CH $_{3}$ ) results in a fragment at m/z = 57, but the cleavage of the heavier ethyl group is the process that generates the base peak.1