Elemental composition fragmentation pathways

To be reliable the scheme should represent ions of known elemental composition, while the mentioned fragmentation pathways should be confirmed by metastable ion... 

Note It is important to assign the correct charge and radical state to all species encountered and to carefully track them through a fragmentation scheme. Otherwise, impossible fragmentation pathways may be formulated, thereby misleading the assignment of elemental composition and molecular constitution. 

Although convenient at first sight, the lack of fragment ion peaks in FI spectra also means a lack of structural information. If more than an estimate of the elemental composition based on the isotopic pattern is desired, collision-induced dissociation (CID, Chap. 2.12.1) can deliver fragment ions for structure elucidation. Fortunately, the fragmentation pathways of IVT ions in CID are the same as in EI-MS (Chap. 6). 

Photolytically degraded PBO was re-examined by GC-EIMS at a higher mass spectrometer resolution (5000) with PFK in the mass spectrometer source as an internal mass calibrant. This procedure permits the accurate mass measurement of molecular and key fragment ions and hence the determination of their elemental compositions and fragmentation pathways. This information, along with the molecular weight information (GC-CIMS) and the low-resolution GC-EIMS spectrum, allows a structure to be proposed for each of the degradation compounds observed. 

A brief inspection of the bar-graph spectral presentation can tell the experienced interpreter a substantial amount about the unknown molecule. You have already seen how the mass and relative abundance of the molecular ion indicate the size and general stability of the molecule (Section 3.6). In addition, the number of abundant ions in the spectrum and their distribution in the mass scale are indicative of the type of molecule and the functional groups present. For example, a glance at the spectrum of Unknown 5.1 should tell you that this is a highly stable molecule. Deduce its elemental composition this should correspond to a value for rings plus double bonds which is consistent with this stability. Not only are there no weak bonds in the molecule, but apparently the main fragmentation pathways are of approximately the same low probability. What is a possible structure ... 

Structure (the indene nucleus) was suggested based on its elemental composition determined as QHg. In contrast, the dissociation pathway to the fragment ion at m/z 70 was suggested to include the formation of a 1-methyl-azetidine residue and the loss of C13H9O2 (-215 Da, route... 

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