HP 5973 MSD, Руководство по оборудованию

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32
Chemical Ionization Theory
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Electron capture
Electron capture, is the primary mechanism of interest in negative CI. Electron
capture (often referred to as high-pressure electron capture mass spectrometry, or
HPECMS) provides the high sensitivity for which NCI is known. For some samples,
and under ideal conditions, electron capture can provide sensitivity as much as 10
to 1000 times higher than positive ionization.
Note that all the reactions associated with positive CI will also occur in NCI mode,
usually with contaminants. The positive ions formed do not leave the ion source
because of the reversed lens voltages, and their presence can quench the electron
capture reaction.
The electron capture reaction is described by:
MX + e
-
(thermal)
→
MX¯
·
where MX is the sample molecule and the electron is a thermal (slow) electron
generated by the interaction between high energy electrons and the reagent gas.
In some cases, the MX¯
ˆ
radical anion is not stable. In those cases the reverse
reaction can occur:
MX
-
·
→
MX + e
-
The reverse reaction is sometimes called autodetachment. This reverse reaction
generally occurs very quickly. Thus, there is little time for the unstable anion to be
stabilized through collisions or other reactions.
Electron capture is most favorable for molecules that have hetero-atoms. For
example: nitrogen, oxygen, phosphorus, sulfur, silicon, and especially the halogens:
fluorine, chlorine, bromine, and iodine.
The presence of oxygen, water, or almost any other contaminant interferes with the
electron-attachment reaction. Contaminants cause the negative ion to be formed by
the slower ion-molecule reaction. This generally results in less sensitivity. All
potential contamination sources, especially oxygen (air) and water sources, must
be minimized.