Stanford Research Systems RGA100, Operating Manual And Programming Reference

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7-8 Peak Tuning Procedure
SRS Residual Gas Analyzer
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 (Slope), stored in the non-volatile
memory of the RGA, to calculate the 8 bit settings of the DAC according to the linear
equation:
DAC8 (m) = DS
.
m + DI (DC_Tweek (m) = (DAC8(m) - 128)
.
19.6 mV)
where m is the mass in amu, and DAC8(m) is the 8 bit setting at that mass.
The purpose of the Peak Width Tuning Procedure is to determine the values of DI
and DS so that all the peaks in an analog spectrum have the desired peak width
(typically
'
m
10%
=1 amu). The calibrated parameters are saved in the non-volatile
memory of the RGA Head (DI and DS commands, respectively) and used by the
firmware to generate the internal scan parameters used to adjust DC_Tweek during scans
and single mass measurements. Please consult the Tuning Commands List in the RGA
Programming chapter of this manual for details on the DS and DI commands.
Note : The sensitivity of the peak widths to the DC_TWEEK voltage can accurately be
approximated to: -1 amu per 550mV (28 bits). In other words, a 550 mV increase in
DC_Tweek voltage causes a 1 amu decrease in the width of any peak. The DC_Tweek
voltages span from -2.5 to +2.5 V with increments of 19.6 mV per bit (255 total bits).
This corresponds to a peak width adjustment range of +/- 4.5 amu and minimum
increments of 0.036 amu per bit.
As described above, the peak width 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 peak widths at the low end of the mass scale, 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 widths at the high end of the mass scale. Several analog scans are performed on the
sample and the values of DI ( nominally 128) and DS (nominally 0) are adjusted until all
peaks appear at their correct setting.
The formulae used to correct the calibration parameters during peak position adjustment
are described next:
Low Mass Peak Width Adjustment : To increase the low mass peak width by an amount
'
m amus modify the value of DI from its original value, DI
0
, to: