209 

 obtained from analyses with cryo-focusing. The drawback of this technique is, as 



mentioned, discrimination against polar compounds in addition to the acids; 



however, these compounds are still readily detected with single-stage cryo-focusing. 



Mass Spectrometry 



The historical significance and a brief description of CI and EI modes of 

 ionization were presented in chapter one. This section of Chapter 5 contains an 

 introduction to the more practical aspects of these ionization modes. The reagent 

 gases employed for CI throughout the course of this work have been methane, 

 carbon dioxide, and in a few cases, isobutane. Some of the work involving carbon 

 dioxide is presented in the appendix. Methane reagent gas was employed for all 

 experiments in this chapter; therefore, the presentation of CI below will be primarily 

 focused on methane as the reagent gas. The discussion of EI will be aimed at 

 specific aids (i.e. characteristic peaks) in the mass spectra used to predict the 

 compound class of peaks which may or may not have failed the library searching 

 process. 



PCI theory 



The reagent gas for chemical ionization is present in the ion source at higher 

 levels than the sample to be ionized by CI. Therefore, a greater probability exists 

 that the reagent gas instead of the sample will undergo electron ionization. Electron 

 ionization of methane produces the following ions [33,53,74]: 



