Stanford Research Systems RGA100, Operating Manual And Programming Reference

Page: 124 / 362

6-20 Programming the RGA Head
SRS Residual Gas Analyzer
process may result in a delay from the time the scan trigger is received to the time it
actually starts. Using the HS1 command and waiting until the whole scan data stream
is transmitted back to the host computer will minimize the problems that are
associated to this feature.
Single Mass Measurements
Single mass measurements are triggered with the MR command. The parameter is the
integer mass number (mass-to-charge ratio in amu units) at which the measurement is
performed.
The type of detector and noise-floor settings to be used by the measurement must be
selected in advance with the NF and HV commands.
The precision and duration of the measurement are totally determined by the NF
parameter value. 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. The command execution time includes some initial time spent
waiting for the Quadrupole Mass Filter and the electrometer response to settle.
As usual, a compromise must be made between signal-to-noise and measurement time.
The ion current is expressed in the usual format: 4 byte long, 2's complement integer in
units of 10
-16
A, with Least Significant Byte transmitted first.
Important:
x
During a Single Mass Measurement the RGA performs a ““ Miniscan”” around the
mass requested, and the maximum current value measured is sent out over RS232 .
The scanning procedure, referred to as Peak-Locking , is designed to measure peak
currents for individual masses in a mass spectrum without being affected by drifts in
the mass scale calibration. The Miniscan covers a 0.6 amu range centered at the mass
requested, and selects the maximum current from 7 individual measurements
performed at 0.1 amu mass increments.
x
The detector settings (i.e. type of detector and noise-floor setting) to be used during
the measurement must be selected in advance with the NF and HV commands;
otherwise, the measurements are performed with the detector settings that are present
at the time the scan is triggered.
x
The ion detector can be zeroed, prior to the measurements, performing an analog or
histogram scan or using the CA command, under the same detector settings (An
offset-correction factor is calculated and stored in memory that it is then
automatically used by the Miniscan procedure to correct all ion currents measured).
The CA command will also update the internal scan parameters to assure 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 during the Miniscan.
Single Mass Measurement example
The following list of commands can be used to perform a single mass measurement at 28
amu. First, the electron multiplier is turned off and the FC is connected to the