Dissociative Electron Capture; Ion Pair Formation; Ion-Molecule Reactions - HP 5973 MSD Hardware Manual

Dissociative electron capture

Dissociative electron capture is also known as dissociative resonance capture. It is
a process similar to electron capture. The difference is that during the reaction, the
sample molecule fragments or dissociates. The result is typically an anion and a
neutral radical. Dissociative electron capture is illustrated by the reaction equation:
-
MX + e
(thermal)
This reaction does not yield the same sensitivity as electron capture, and the mass
spectra generated typically have lower abundance of the molecular ion.
As with electron capture, the products of dissociative electron capture are not
always stable. The reverse reaction sometimes occurs. This reverse reaction is
sometimes called an associative detachment reaction. The equation for the reverse
reaction is:
·
+ X¯ → MX + e
M

Ion pair formation

Ion pair formation is superficially similar to dissociative electron capture. The ion
pair formation reaction is represented by the equation:
-
MX + e
(thermal)
As with dissociative electron capture, the sample molecule fragments. Unlike
dissociative electron capture, however, the electron is not captured by the
fragments. Instead, the sample molecule fragments in such a way that the electrons
are distributed unevenly and positive and negative ions are generated.

Ion-molecule reactions

Ion-molecule reactions occur when oxygen, water, and other contaminants are
present in the CI ion source. Ion-molecule reactions are 2 – 4 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 has been 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.
·
→ M
+ X¯
-
-
→ M
+
+ X¯ + e
 Chemical Ionization Theory
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