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Chemical Ionization Options
Ion Intensities for Standard CI Reagents
The CI Adjust function gives recommendations of an acceptable level of CI
reagent ions for each of the five standard CI reagents. The general principles
used in implementing these tests are:
Methane
Adjust the reagent gas pressure so that the peak
heights at m/z 17 (CH
5
+
) and 29 (C
2
H
5
+
) are about
equal. The ratio of the ions at m/z 17 to m/z 16 should
be about 10:1. The ion at m/z 41 (C
3
H
5
+
) should be
visible.
Isobutane
Adjust the reagent gas pressure so that the peak
heights at m/z 57 [(CH
3
)
3
C
+
] and m/z 43 [(CH
3
)
2
CH
+
] are
about equal. There may also be an intense reagent ion
at m/z 41 (C
3
H
5
+
).
Ammonia
Adjust the gas pressure so that the ratio of the peak
heights at m/z 18 [(NH
3
)H
+
] to m/z 17 (NH
3
+
) is about
10:1.
Acetonitrile
Adjust the reagent gas pressure so that the ion at m/z
42 [CH
3
CNH
+
] is about 10 times higher than at m/z 41.
The valley between the 41/42 ions should reach a
minimum at less than half the height of the m/z 41 ion.
The m/z 54 ion [CH
3
CHCNH
+
] will be present at 10 -
15% the height of m/z 42. Too much acetonitrile in the
trap can cause early filament failures.
d3-Acetonitrile Adjust the reagent gas pressure so that the ion at m/z
46 [CD
3
CND
+
] is about 10 times higher than at m/z 44.
The m/z 58 ion [CD
3
CDCND
+
] will be present at 10 -
15% the height of m/z 46.
Methanol
The ion at m/z 33 [(CH
3
OH)H
+
] will dominate the
spectrum. No ion is observed at m/z 32, but a small
peak is observed at m/z 31 and m/z 47.
In each case, by following these guidelines, the reagent gas pressure in the ion
trap will be approximately 1 to 2 x 10
-5
Torr (about 1.3 to 2.6 x 10
-3
Pa). The CI
reagent molecules comprise about 1% of the gas pressure in the ion trap. He
atoms from column flow are present at 100 times this pressure.
Setting CI Gas Pressure in a Diffusion Pump System
For the diffusion pump system, the CI Reagent Gas delivery pressure should be
set to only 5 psi as indicated on the pressure regulator. The foreline pressure
should then be checked:
•
Start System Control and click on the Diagnostics tab.
