Installation manual

Chemical Ionization Theory A
5975 Series MSD Operation Manual 159
Ion-molecule reactions
Ion-molecule reactions occur when oxygen, water, and other contaminants are
present in the CI ion source. Ion-molecule reactions are two to four times
slower than electron-attachment reactions and do not provide the high
sensitivity associated with electron capture reactions. Ion-molecule reactions
can be described by the general equation:
M + X
–
 MX
–
where X
–
is most often a halogen or hydroxyl group that was created by
ionization of contaminants by electrons from the filament. Ion-molecule
reactions compete with electron capture reactions. The more ion-molecule
reactions that occur, the fewer electron capture reactions occur.