Mass Spectrometry : Examples involving acylium ions, [RCO]⁺

Ions with the positive charge on the carbon of a carbonyl group, C=O, are also relatively stable. This is fairly clearly seen in the mass spectra of ketones like pentan-3-one.

The base peak, at m/z=57, is due to the [CH₃CH₂CO]⁺ ion. We've already discussed the fragmentation that produces this.