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UGA Series Universal Gas Analyzers
Calibration and Input Design
5 –
The peak value, I
40
, can be extracted from a spectral scan or mea-
sured directly using the single mass measurement mode of the
RGA. For example, a 0
-9
amp peak value corresponds to 9.8 × 0
-9
Torr of Ar. Note that equation (4) is a particular case of equation
(3), and that the fragmentation factor for the principal peak of Ar is
one by definition.
The UGA has been calibrated at the factory to measure the partial
pressure of nitrogen correctly. For many purposes this will be suit-
able. Overtime the calibration can change or operating conditions
may change. There are many factors involved in calibrating the
UGA and interpreting the mass spectra. To make accurate mea-
surements, the following conditions need to be met:
•
The total pressure needs to be known.
•
The main sensitivity factor needs to be calibrated.
Sensitivity factors change as a factor of time due to aging
and periodic recalibration is necessary.
•
For careful quantitative analysis, it is important that the
sensitivity of the RGA be determined for every gas which
may be a component of the system. Each gas component
in the sample will fragment differently and will have
slightly different sensitivities.
•
Correct calibration of the mass scale is essential during
qualitative analysis for the correct assignment of mass
numbers to the different peaks. The mass scale will affect
the peak height if it is more than 0.3 amu out of calibration.
•
The mass resolution of the quadrupole mass filter, ∆m
0%
,
must be kept at or under amu to avoid overlap between
adjacent peaks. Changes in ∆m
0%
during the
measurements (caused by aging, severe contamination and
large temperature changes) will cause variations in the
sensitivity of the instrument and the shapes of the
fragmentation patterns of the molecules, affecting all quan-
titative measurements.
•
The gain of the electron multiplier is mass dependent and
needs to be determined prior to performing measurements
with the device. The gain characteristics of the multiplier
change with time and periodic recalibrations are very im-
portant.
5.1.5 Calibration
