Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Peak Tuning Procedure 7-7
SRS Residual Gas Analyzer
Please consult the Tuning Commands List in the RGA Programming chapter of this
manual for details on the RS and RI commands.
As described above, the peak position tuning procedure requires the introduction of two
known gases into the vacuum system. A
low mass gas (1-20 amu recommended) is used
to adjust the low mass end of the mass axis, a high mass gas, with a mass-to-charge ratio
close to the upper limit of the scanning range of the RGA, is used to adjust the high mass
end of the mass scale. Several analog scans are performed, and the values of RI (RF
Driver output @ 0 amu) and RS (RF Driver output @ 128 amu) are adjusted until all
mass peaks appear at the correct position in the mass scale. An increase in RI causes the
low end of the analog spectrum to displace towards lower masses (A small effect is seen
at the high masses). An increase in RS results in the spacing between peaks in a scan to
decrease (with the largest effect seen at the high mass end).
The formulae used to correct the calibration parameters during peak position adjustment
are presented next:
Low Mass Peak Position Adjustment : To displace a low mass peak by a distance
'
m
amu in the mass axis the value of RI must be modified from its original value RI
0
according to:
RI = RI
0
-
'
m
.
(RS / 128)
Notes:
x
The new RI value must fall within the acceptable parameter range of the RI
command.
x
This change mostly affects the position of the peaks at the low end of the spectrum.
x
A decrease in RI shifts the low mass peaks to the right (peaks show up at higher
masses) , an increase in RI shifts the same peaks to the left (peaks show up at lower
masses).
High Mass Peak Position Adjustment : To displace a high mass peak, m
0
, by a distance
'
m amu in the mass axis modify the value of RS from its original value RS
0
to:
RS = RS
0
.
[ m
0
/(m
0
+
'
m)]
Notes:
x
The new RS value must fall within the acceptable parameter range of the RS
command.
x
Modifying RS affects the spacing between peaks in the spectrum. An increase in RS
results in the peaks getting closer together, and a decrease in RS results in the peaks
getting further apart from each other.
x
The effect is more significant at the higher masses and should have very little effect
on the position of the low mass peaks (that is why you do this adjustment second).
x
The lower the mass-to-charge ratio of the low mass gas the less this adjustment will
effect the position of its peak.
x
See that a decrease in RS results in the high mass peaks moving to higher masses
(right shift), and vice versa.