212 

 compared to commonly employed charge exchange reagent gases, and thus, charge 



exchange is not expected to be an abundant process with methane reagent gas. 



The analysis of PCI mass spectra consisted of determination of the relative 

 molecular mass (r^) by identifying specific ions. This information is complementary 

 to the structural information obtained from EI. Therefore, the focus is mainly on 

 determination of r^ for specific peaks rather than employing CI for structural 

 elucidation. The first two ions examined for are the even-electron [M + H]^ ion 

 produced by proton transfer from CH^^ , CjHj"^, or the [M-H]^ ion produced from 

 from hydride abstraction with CjH;^ and C3H5*. These ions bracket the r^^i of the 

 compound of interest. Additional confirmation can be obtained from the presence 

 of ions at [M-l- 29]"^ and[M+41]'^, representing adduct formation of CjHs"^ andC3H5"^ 

 to the sample molecule, respectively. Although not examined in the work, ions 

 produced by CI may exhibit some informative rearrangements [103,104]. Due to the 

 availability of EI mass spectra along with on-line libraries of EI data, fragmentation 

 information was derived solely from this mode of ionization. It should be pointed 

 out that the actual CI analysis consisted of acquisition by PPINICI, where the PCI 

 and NCI data are acquired as consecutive scans into a single file [70]. This allows 

 for using the negative ion data, specifically the [M-H]~ ion, to aid in determination 

 of the compound relative molecular mass. 

 NCI theory 



When bombarded by electrons in the ion source, methane produces various 

 positive reagent ions as discussed in the previous section. The collision of an 



