Other Reagent Gases - Agilent Technologies 7200 Operation Manual

Other Reagent Gases

This section describes the use of isobutane or ammonia as the reagent gas. You
should be familiar with operating the CI-equipped 7200 Q-TOF GC/MS with
methane reagent gas before attempting to use other reagent gases.
Do not use nitrous oxide as a reagent gas. It radically shortens the life span of the
C A U T I O N
filament.
Changing the reagent gas from methane to either isobutane or ammonia
changes the chemistry of the ionization process and yields different ions. The
principal chemical ionization reactions encountered are described in general
in the Agilent 7200 Accurate-Mass Q-TOF GC/MS Concept Guide. If you are
not experienced with chemical ionization, we suggest reviewing that material
before you proceed.
Isobutane CI
Isobutane (C
fragmentation is desired in the chemical ionization spectrum. This is because
the proton affinity of isobutane is higher than that of methane; hence less
energy is transferred in the ionization reaction.
Addition and proton transfer are the ionization mechanisms most often
associated with isobutane. The sample itself influences which mechanism
dominates.
Ammonia CI
Ammonia (NH
fragmentation is desired in the chemical ionization spectrum. This is because
the proton affinity of ammonia is higher than that of methane; hence less
energy is transferred in the ionization reaction.
Because many compounds of interest have insufficient proton affinities,
ammonia chemical-ionization spectra often result from the addition of NH
and then, in some cases, from the subsequent loss of water. Ammonia reagent
ion spectra have principal ions at m/z 18, 35, and 52, corresponding to NH
NH
7200 Accurate-Mass QTOF Operation Manual
H ) is commonly used for chemical ionization when less
4 10
) is commonly used for chemical ionization when less
3
) + , and NH
(NH (NH
4 3 4
Operating in Chemical Ionization (CI) Mode
+ .
)
3 2
4
+
4
+ ,
4
63