Negative Ci Theory - HP 5973 MSD Hardware Manual

 Chemical Ionization Theory
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Negative CI theory

Negative chemical ionization (NCI) is performed with analyzer voltage polarities
reversed to select negative ions. There are several chemical mechanisms for
negative chemical ionization. Not all mechanisms provide the dramatic increases in
sensitivity often associated with negative chemical ionization. The four most
common mechanisms (reactions) are:
• Electron capture
• Dissociative electron capture
• Ion pair formation
• Ion-molecule reactions
In all of the cases except the ion-molecule reactions, the reagent gas serves a
function different from the function it serves in positive chemical ionization. In
negative CI, the reagent gas is often referred to as the buffer gas. When the reagent
gas is bombarded with high energy electrons from the filament, the following
reaction occurs:
Reagent gas + e
If the reagent gas is methane, the reaction is:
-
CH + e
(230eV)
4
The thermal electrons have lower energy levels than the electrons from the
filament. It is these thermal electrons that react with the sample molecules.
There are no negative reagent gas ions formed. This prevents the kind of
background that is seen in PCI mode, and is the reason for the much lower detection
limits of NCI. The products of negative chemical ionization can only be detected
when the MSD is operating in negative ion mode. This operating mode reverses the
polarity of all the analyzer voltages.
Carbon dioxide is often used as a buffer gas in negative CI. It has obvious cost,
availability, and safety advantages over other gases.
30
-
→ Reagent ions + e
(230eV)
+ -
→ CH
+ 2e
(thermal)
4
-
(thermal)