210 

 CH4 — * CH4 , CH3 , CH2 * 5-1 



These ions further react with CH4 neutrals in the ion source to produce CH;"^, 



C2H5*, and CjHs"^, which are formed via the ion/molecule reactions: 



CH4*' + CH4 -^ CH5* + CHj* 5-2 



CH3* + CH4 -* C^Hj^ + H2 5-3 



CH,^' + CH4 ^ CjHj^ + H2 + H' 5-4 



C^H,* + CH4 -^ CjHj^ + H2 5-5 



The major ions formed with methane reagent gas are discussed later in the text; the 



ion/molecule reactions available for CI are discussed below. 



There are four categories of CI reactions which can occur between positive 



reagent ions, X^ or XH"^, and sample molecules, M [33,74]. These are: 



XH^ + M ^ MH^ + X proton transfer 5-6 



X^ -f- M ^ [M-H]"" + XH anion (hydride) abstraction 5-7 



X* + M -» MX^ adduct formation 5-8 



X** -1- M -» M*' + X charge exchange 5-9 



Proton transfer (Eq 5-6) involves the transfer of a proton from the reagent 



gas ion to the sample molecule. This will occur if the proton affinity (PA) of the 



sample molecule is greater than that of the conjugate base of the reagent ion. In 



this case, the conjugate base (CH4) of the CH5* reagent ion has a proton affinity of 



552 kJ/mol [33,74]. Additionally, the conjugate base of the C2H5"^ ion has a PA of 



682 kJ/mol, which is lower than that of isobutane and of water [33]. Therefore, the 



reagent ions formed from CH4 will readily yield exothermic proton transfer reactions 



