Stanford Research Systems RGA100, Operating Manual And Programming Reference

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6-16 Programming the RGA Head
SRS Residual Gas Analyzer
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The scan rates and signal-to-noise ratios for the different NF settings of the
electrometer are listed in a table in the Electrometer section of the ““RGA Electronics
Control Unit”” chapter.
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When using the RGA Windows program to operate the RGA, the Scan Speed
parameter setting available in the Scan Parameter Setups of the Scan menu is used to
set the NF parameter value in the RGA Head according to the equation:
NF =
ScanSpeed - 1.
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Use the CA command to readjust the zero of the detector every time the detector
settings are changed (i.e. after changing the NF parameter setting or changing
detector type). This is particularly important if the new settings have not been used in
a long time or since the unit was turned on or recalibrated with the CL command.
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Follow all recommended procedures for the operation of the CDEM. Consult the
RGA Maintenance chapter for complete CDEM Care and Handling information.
Setting up Analog Scans
Analog scanning is the most basic operation of the RGA as a quadrupole mass
spectrometer. During analog scanning the quadrupole mass spectrometer is stepped at
fixed mass increments through a pre-specified mass-range. The ion current is measured
after each mass-increment step and transmitted to the host computer over RS232.
Analog scans are triggered with the SC command. The scan parameter can be set for
single, multiple and continuous scanning operation. The mass range for the scan is set in
advance with the commands MI (Initial Mass) and MF (Final Mass) and the mass-
increments are fixed with the command SA. Scan rate and detection limits are pre-
programmed by the NF (Noise Floor) setting. A current value is transmitted for MI and
after each mass increment through MF for a total of (MF-MI )*SA+1 measurements (See
AP query command). For maximum data throughput, the ion currents measured are
represented as integers in units of 0.1 fA, and transmitted directly in Hex format ( four
byte integers with 2's complement format and Least significant byte first).
The detector settings (i.e. detector type and electrometer’’s noise-floor setting) to be used
during the scans must be selected in advance with the NF and HV commands. The zero of
the RGA’’s ion detector is automatically readjusted at the beginning of each analog scan
so that the baseline is centered around zero. (The offset correction factor calculated and
stored in memory, will then be automatically used to correct all currents measured (i.e.
including those from single mass and total pressure measurements) under the same
detector settings)
The internal scan parameters used by the firmware to step the RF during the scan, are
checked and corrected at the beginning of the scan to make sure that the correct RF levels
(i.e. as specified by the last Peak Tuning procedure) are programmed on the RF rods as a
function of mass.