SRS Labs QMS 100 Series, User Manual

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3-14 Calibration
SRS QMS Gas Analyzer
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The effect is more significant at the higher masses and that is why we do this adjustment second
after the width has already been modified by the change in DI.
•
If the mass-to-charge ratio of the low mass gas is real low this adjustment will have a small
effect on the width of its peak.
Iterations: In most cases it will be necessary to repeat the two width adjustments one or two more
times until both low and high mass peaks show the desired widths.
Note: Changes in the peak positions will also be observed, at a rate of -0.40 amu per amu of increase
in peak width, and a Peak Position Tuning procedure will be needed to put the peak positions back
where they belong.
Important : The Peak Width Tuning Procedure can be used to adjust the resolution of the RGA to the
ultimate theoretical limit of the mass filter. See the Quadrupole Mass Filter section of the RGA Probe
chapter of this manual for details on that limit.
Temperature Effects on the Mass Scale Calibration
As the temperature of the RGA Head changes two different effects affect the calibration of the mass
scale:
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For small temperature changes : Drift in the voltage output of the RF Driver that controls the
RF power supply can cause the mass peaks to shift their position in the mass spectrum. In
order to correct against this effect, the RF driver output is checked at the beginning of each
analog and histogram scan at 0 and 128 amu. The internal calibration parameters, used by the
firmware to step the RF during scans, are updated so that the control levels specified by the RI
(0 amu) and RS (128 amu) parameter values are correctly set at the present temperature.
•
For large temperature changes: The sensitivity of the RF power supply to its controlling
voltages might be affected or, more fundamentally, the relationship between mass and RF levels
in the filter might change. In this case a Peak Tuning procedure will be necessary to reestablish
the mass axis scale.