Stanford Research Systems RGA100, Operating Manual And Programming Reference

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7-6 Peak Tuning Procedure
SRS Residual Gas Analyzer
their known mass-to-charge ratio (This is needed to make sure the Peak-locking
algorithm used for single mass measurements always finds the mass peak within its
search window). The peak width,
'
m
10%
, must be a constant, and less than 1 amu
throughout the whole scan range.
Example : The following figure shows the result of peak tuning the RGA based on the
18
H
2
O
+
(low mass= 18 amu) and
86
Kr
+
(high mass=86 amu) calibration peaks. All peaks
are at their correct mass settings and show absolute resolution values of about 0.9 amu
amu. Also note a 20 amu peak in the low mass spectrum corresponding to
40
Ar
++
at 20
(i.e. 40/2) amu.
Low Mass High Mass
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
15 16 17 18 19 20 21 22 23 24 25
H
2
0
+
Ar
++
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
80 81 82 83 84 85 86 87 88 89 90
86
Kr
+
Users writing their own computer code can write Peak Tuning Commands for their own
programs using the Tuning Commands of the RGA Command Set and the instructions of
the following two sections.
Peak Position Tuning Algorithms
The magnitude of the RF determines the mass-to-charge ratio of the ions that can pass
through a quadrupole mass filter without striking the rods (i.e. with stable oscillations). A
linear relationship between mass and RF amplitude is one of the most attractive features
of these type of filters
. The regulated output of the RF source that powers the RGA’’s
quadruple rods is controlled by, and linearly related to, the voltage output of an RF
Driver circuit. The RF Driver uses an 18 bit digital-to-analog converter and some
additional electronics to program its output voltages.
The purpose of the Peak Position Tuning Procedure is to determine the voltages that the
RF Driver must output at 0 and 128 amu so that all the peaks in an analog spectrum
appear in the right place in the mass axis.
The calibrated voltage settings, in mV, are saved in the non-volatile memory of the RGA
Head (RI and RS commands respectively) and used by the firmware to generate the
internal scan parameters used to step the RF during scans and single mass measurements.
amu/e amu/e