211 

 with most components in the sample. The drawback to such exothermic reactions 



is the extensive fragmentation which results for compounds of much higher proton 



affinity than methane. 



Hydride ion abstraction (Eq 5-7) can occur if the hydride ion affinity (HIA) 

 of the reagent gas ion is greater than the corresponding HIA of the sample [M-H]^ 

 ion. The HIA values of CH5* and C2H5+ are 1127 kJ/mol and 1135 kJ/mol, 

 respectively [74]. Hydride ion abstraction will occur with the C3H5* ion and possibly 

 the CjHj^ ion. 



Adduct formation (Eq 5-8) typically occurs when the sample proton affinity 

 is slightly less than that of the conjugate base of the reagent ion. This can occur for 

 both the C2H5"^ ion and the CjH;"^ ion. Due to the much lower PA of the conjugate 

 base of CH5*, this species would rather transfer a proton, as discussed previously, 

 than form an adduct ion. 



The final reaction (Eq 5-9), is charge exchange. This process occurs typically 

 for reagent gases that do not have a hydrogen available for proton transfer reactions. 

 Charge transfer can occur to produce the M*' molecular ion of a compound. The 

 fragmentation is El-like with increased relative abundance of the molecular ion 

 compared to that of conventional EI. The extent of fragmentation is dependent 

 upon the internal energy of the sample ion formed, which is given by the difference 

 of the recombination energy of the reagent gas ion and the ionization energy of the 

 sample molecule (see appendix). The recombination energies of the CHs^ and 

 CjHs"^ ions are 7.9 eV and 8.3 eV, respectively [74]. These energies are very low 



