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Isobutane CI
Isobutane (C
H
) is commonly used for chemical ionization when less
4
10
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
) is commonly used for chemical ionization when less fragmentation
3
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
4
ion spectra have principal ions at m/z 18, 35, and 52, corresponding to NH
+
NH
(NH
)
, and NH
4
3
To adjust your MSD for isobutane or ammonia chemical ionization, use the following
procedure:
1 3HUIRUP D VWDQGDUG 3RVLWLYH &, DXWRWXQH ZLWK PHWKDQH DQG 3)'7'
2 8QGHU WKH
PHQX FOLFN
Setup
This will change the menus to use the selected gas, and select appropriate default
tune parameters.
3 6HOHFW DQ QHZ WXQH ILOH QDPH RU ORDG DQ H[LVWLQJ 3&, WXQH ILOH IRU WKH
VSHFLILF JDV
4 7XUQ
RQ
Gas B
After the amber light stops flashing and the Purge light goes off, set the gas flow to
20%.
5 &OLFN
Isobutane (or Ammonia) Flow Adjust
There is no CI autotune for isobutane or ammonia in PCI.
6 ,I \RX ZLVK WR UXQ 1&, ZLWK LVREXWDQH RU DPPRQLD ORDG
H[LVWLQJ 1&, WXQH ILOH IRU WKH VSHFLILF JDV
+
(NH
)
.
4
3
2
Select Reagent Gas
RQ WKH 6HWXS PHQX
CI operation
7R XVH RWKHU UHDJHQW JDVHV
DQG VHOHFW
RU
Isobutane
.
RU ORDG DQ
NCICH4.U,
+
,
4
Ammonia
65
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Troubleshooting
