Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Ionizer 3-5
SRS Residual Gas Analyzer
The ECU contains all the necessary high voltage and current supplies needed to bias the
ionizer’’s electrodes and establish an electron emission current. The ionizer settings can
be directly controlled and monitored by the user through the RGA’’s high level command
set.
The following table summarizes the ionizer settings available to the user, including
default factory values, parameter ranges and minimum acceptable increments:
Ionizer settings:
Units Default Range Min. Incr.
Ion Energy
eV 12 8(low) or 12(high) n.a.
Electron Energy eV 70 25 to 105 1
Focus Voltage V -90 0 to -150 1
Emission Current
mA 1.00 0 to 3.5 mA .02
The average electron energy , expressed in eV, is equal to the voltage difference between
the filament and the anode grid and can be set anywhere in the range of 25 to 105 eV.
For electrons to produce ionization of gas molecules by bombardment, they must have a
certain minimum kinetic energy. This minimum energy is called the
ionization potential
and is different for every molecule. Above the threshold, the ionization efficiency
increases linearly with the electron energy until a maximum is reached. For most
molecules, this maximum is in the range of about 50 -100 eV, and for electron energies
above the maximum, the ionization efficiency slowly decreases with electron energy.
The kinetic energy of the ions as they move down the ion filter, simply referred to as the
ion energy, and expressed in eV, is equal to the voltage biasing of the anode grid. Two
ion energy settings (i.e. anode grid voltages) are: 8 (low) or 12 eV (high). The ion energy
setting affects the magnitude of the ion signals collected (i.e. sensitivity of the
spectrometer) and limits the ultimate resolution of the mass filter. Newly formed ions are
attracted by the negative potential of the focus plate and passed to the ion filter where
they are decelerated by the ground potential at the central axis of the quadrupole rod
assembly. Hence, the anode grid voltage defines the kinetic energy of the ions as they
enter the ion filter. Ion energy determines the time spent by the ions in the filter and
hence, limits the resolution that can be obtained. It is well established that the resolution
limit is governed by the number of cycles of RF field to which the ions are exposed
before they reach the detector. In practice, the minimum resolution,
'
m
10%
, is mass
independent, linearly related to the ion energy, and inversely proportional to the square of
the product of the quadrupole length and frequency. The two available ion energy
settings correspond to ultimate resolutions of approximately 0.3 and 0.5 amu (well under
the 1 amu factory default setting). Ion energy also determines the time spent by the ions
in the fringing fields at the entrance and exit points of the filter. Ions passing through the
fringing fields can collect high transverse velocities and are more likely to collide with
the quadrupole rods and never be collected at the detector. As a result, ion signals (i.e.
sensitivity) generally increase with ion energy.
The focus plate negative potential can be adjusted to any value within the range of 0 to
-150 V. In general, the voltage is selected so as to optimize the ion signals. The plate
serves the double purpose of drawing the ions away from the anode grid, and containing
the ionizing electrons inside the source. Electron leakage into the filter is only detectable