SRS Labs QMS 100 Series, Руководство пользователя

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3-12 Calibration
SRS QMS Gas Analyzer
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:
•
The new RI value must fall within the acceptable parameter range of the RI command.
•
This change mostly affects the position of the peaks at the low end of the spectrum.
•
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:
•
The new RS value must fall within the acceptable parameter range of the RS command.
•
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.
•
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).
•
The lower the mass-to-charge ratio of the low mass gas the less this adjustment will effect the
position of its peak.
•
See that a decrease in RS results in the high mass peaks moving to higher masses (right shift),
and vice versa.
Iterations: In most cases it will be necessary to repeat the two position adjustments one or two more
times until both low and high mass peaks show up at their known positions.
Peak Width Tuning Algorithms
Constant absolute resolution (
∆
m
10%
) in a quadrupole mass filter requires DC voltages linearly related to
the mass, with a slight negative offset at low masses (i.e. negative intercept).
The RGA Head adjusts the DC levels of the quadrupole filter during measurements so that constant
mass resolution is automatically available throughout the entire mass range of the spectrometer. The bulk
of the DC voltage is supplied by a DC power supply whose output is linearly related to the RF
amplitude. The rest of the DC voltage (DC_Tweek) is provided by the output of an 8 bit digital-to-
analog converter (DAC). The firmware uses two Peak Tuning Parameters : DI (Intercept) and DS