The molecular ion peak is usually observable, although it can be weak.
An important fragmentation process for aldehydes involves a-cleavage as shown in figure 32.
Figure 32. a-cleavage.
A second common mode of fragmentation involves b-cleavage (see figure 33).
Figure 33. b-cleavage.
McLafferty rearrangement can take place for aldehydes with at least 4 carbons. A fragment ion of m/e = 44 is formed, and is considered quite characteristic of an aldehyde (see figure 34).
Figure 34. McLafferty rearrangement of an aldehyde.
The complete mass spectrum for hexanal is given in figure 35, which illustrates most of these points.
Figure 35. Mass spectrum for hexanal.
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