217 

 some cases, this can be explained by high EA of the resulting anions or high bond 



dissociation energies of the C-H and O-H bonds. In this dissertation, the distinction 



is not of particular importance. What is sought is the MX ion or the [M-H]^ ion 



in order to deduce the r^ of the compound. 



The analysis of NCI spectra consisted of determining the m/z of either the 

 MX ion or the [M-H]~ ion, regardless of whether generated by ECNCI or true 

 NCI, in the mass spectrum to aid in the determination of relative molecular mass. 

 Additional confirmation came from the presence of the following ions: [M+CH3']~, 

 [M-H+CH3]~, and [M+CjHs]^ [76]. Additionally, manual inspection of mass spectra 

 for compounds suspected of being esters or alcohols was conducted; esters fragment 

 to yield the carboxylate ion while alcohols yield an [M-H-Hj]" ion [72]. 

 Characteristic ion fragmentations (EI) 



Electron ionization generally employs a distribution of electron energies 

 centered around 70 eV emitted by the filament in a low-pressure ion source. These 

 conditions maximize fragmentation and structural information. As described above, 

 negative ions formed by EI are low molecular weight fragments of quite low 

 intensity. In the positive ion mode, ions retain their charges longer and can 

 distribute excess internal energy more efficiently, thus undergoing less fragmentation. 



The purpose of this introduction to EI is not to provide a complete set of 

 rules for EI fragmentation; these may be found elsewhere [79,109]. For the most 

 part, EI mass spectra acquired in this dissertation were evaluated by library searching 

 on the TSQ70 with a NBS library database of over 49,000 compounds. In some 



