21 

 Sample ionization 



Mass spectrometric analysis requires that compounds of interest form gas- 

 phase ions. Ionization can be accomplished in a number of ways. For the work in 

 this dissertation, ionization of volatiles (already in the gas phase) is accomplished 

 either by electron ionization (EI) or chemical ionization (CI). 



Electron ionization fEI) . The first EI source is credited to work in 1921 by 

 Dempster; however, the precursor to the modern EI source was pioneered by Nier 

 in 1947 [33]. Electron ionization involves direct ionization and fragmentation of 

 volatile sample molecules in an electron beam. The fragmentation pattern provides 

 structural information about the original sample molecules. 



Chemical ionization fCI) . The technique of chemical ionization (positive 

 chemical ionization or PCI) is credited to Munson and Field; a series of papers from 

 1956-1966, including the work of Franklin and Lampe, cover the development of this 

 technique [34-53]. Chemical ionization is accomplished via a reagent gas of choice. 

 This reagent gas is in greater abundance in the ion source than the sample, and at 

 a pressure high enough (approximately 1 torr) to favor ion-molecule reactions. The 

 reagent gas is ionized by electron ionization; the resultant reagent ions ionize the 

 sample molecules via ion-molecule reactions, typically this is by proton transfer. The 

 result is that less fragmentation of the sample molecule will occur due to the transfer 

 of less energy from ionized reagent ions than from electrons given off by the 

 filament. The greater abundance of intact molecular species relative to 

 fragmentation allows for molecular weight determination. In addition to positive ion 



