Neutral Fragments Expelled by Simple Cleavage

TABLE 2.10 Neutral Fragments Expelled by Simple Cleavage  

Fragment Nominal Mass Fragment Nominal Mass 

Note that several of the mass differences can be acconnted for by more than one possible strnctnre, or even composition. An M-31 ion may be indicative of a methoxy substituent (as in the example of methyl benzyl ether), bnt the same atoms may be lost if there is a primary alcohol function in a molecule (Equations 2.96 and 2.97). 

Simple cleavage with the loss of 43 mass units may be indicative of an acetyl or a propyl unit in the molecule. Isotope peak intensities may help to identify the correct composition, but of greater value in this respect is high-resolution mass measurement. From the accurate masses of the molecular and fragment ions, the elanental composition of a neutral fragment can be determined. More about this important technique is discussed in Chapter 3. 

The choice becomes clear when the next-lower fragment peak is considered, for only the benzoyl ion loses 28 mass units (as CO) readily (Equation 2.98). The compound is therefore benzoic acid. 

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In the first case we would detect the EE" " ion and in the second case the OE " ion. The neutral species lost on fragmentation can be identified by the difference between the mass of M and the fragment ion. Common losses from the molecular ion are given in Table 10.9. Tables 10.10 and 10.11 list more common neutral fragments expelled by simple cleavage and by multicentered fragmentations. 

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