Gas Chromatography - Tandem Techniques - The Basic Principles of Mass Spectroscopy > The Sector Mass Spectrometer > Chemical Ionization > Page 68

CH4 CH4+, CH3+, CH2+


CH4+ + CH4 CH5+ + CH3


CH3+ + CH4 C2H4+ + H2


Some side reactions occur that are not useful to the ionizing process but these are mostly in the minority. Interaction between positively charged ions with the uncharged sample molecules also occur in different ways, the four most common are as follows:


1. Proton transfer between the sample molecule and the reagent ion,


M + BH+ MH+ + B


2. Exchange of charge between the sample molecule and the reagent ion,

M + X+ M+ + X


3. Addition of the sample molecule to the reagent ion,


M + X+ MX+


4. Anion extraction,


AB + X+ B+ + AX


For example, (CH5+) ions (formed when methane is used as the reagent gas) will react with a sample molecule largely by proton transfer e.g.,


M + CH5 MX+ + CH4

Some reagent gases produce more reactive ions than others so, some reagent gases will produce more fragmentation.


(A) Reagent Gas Methane; (B) Reagent Gas Isobutane.


Figure 56. The Mass Spectrum of Methyl Stearate Produced by Chemical Ionization