Agilent Technologies 5975 Operation Manual page 187

Electron capture
Electron capture is the primary mechanism of interest in NCI. 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 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:
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
reaction can occur:
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.
5975 Series MSD Operation Manual for MassHunter
 MX
MX + e –
(thermal)
–
·
 MX + e –
MX –
·
–
·
radical anion is not stable. In those cases the reverse
Chemical Ionization Theory
A
187