Checking for Leaks
A common issue in mass spectrometry is keeping the system as leak-tight as
possible. Air leaks may result in reduced sensitivity, tuning problems, and
decreased resolution; in addition, they may reduce the lifetimes of the capillary
column, filaments, turbomolecular pump, and the electron multiplier. Check the
system each day for air and water leaks before you begin running acquisitions.
Establishing Conditions Required for Leak Checks
To establish the conditions required to check for leaks, proceed as follows:
1. Activate C:\VarianWS\4000 MS Service\4000 MS (Int or Ext) Service.mth, go
to segment 1 and turn the trap on.
2. Verify the column flow rate is 1.0 mL/min.
3. Set the GCMS temperatures:
•
Trap temperature to 150 °C.
•
Transfer line temperature to 270 °C.
•
Manifold temperature to 35
°
C.
•
Source temperature to 150
°
C.
4. Set the column-oven and injector temperatures to 100 and 230 °C
respectively.
CAUTION
Often, major air leaks are accompanied by a hissing sound. These leaks may
be due to extremely loose fittings, improperly seated O-rings, or open valves. If
you suspect a major leak, do not turn on the electron multiplier, RF voltage, or
filament. Using the Diagnostics section, confirm that the turbomolecular pump
is operating at 100% speed. If it is not, there may be a major air leak.
•
If the ratio of the height of the peak of mass 18 (H
2
O
+
) to mass 19 (H
3
O
+
)
is about 10:1, there is little water vapor in your system.
•
If the ratio of peak height of mass 18 to mass 19 is less than 10:1 but
greater than 5:1, additional bakeout may be necessary.
•
If the ratio of the peak height of mass 18 to mass 19 is much less than
10:1, your system contains excess water vapor.
An Air/Water Spectrum Obtained from a System with No Significant Air Leaks
and Little Water Vapor is indicated by:
•
The peak at mass 18 (H
2
O
+
) may be the base (highest) peak. This is
dependent on the level of water vapor.
•
The ratio of the peak height at mass 18 (H
2
O
+
) to that at mass 19 (H
3
O
+
)
is greater than or equal to 10:1.
•
The base amount value is significantly less than 500.
•
The ratio of the peak height at mass 28 to that at mass 32 is about 4:1.
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Summary of Contents for 4000 GC
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