Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Quadrupole mass filter 3-11
SRS Residual Gas Analyzer
It is well established that the resolution attainable by a quadrupole is limited 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%
) value attainable 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 of the RGA correspond to
ultimate resolution values of approximately 0.3 amu
(8 eV) and 0.5 amu (12 eV) which are well under the factory default setting, and more
than adequate to separate ions which differ in mass by 1 amu or less.
The resolving power, R, at a mass M is strictly related to the DC/RF voltage ratio. An
increase in the resolving power usually results in a decrease in the effective
throughput
of the filter As the DC/RF ratio is increased the amplitude of the ion oscillations within
the filter increase and a greater fraction of the ions are lost to collisions with the analyzer
rods. The throughput of the quadrupole affects the overall
sensitivity of the spectrometer
to the mass being filtered. The exact relationship between resolution and sensitivity is
very complex as it depends on the concentration and divergence of the ion beam leaving
the source. It is complicated further by the defocusing action of the fringing fields
between the ion source and the rods. However, as a general rule of thumb
Sensitivity
decreases at 1 to 1.5 times the rate of resolving power increase.
The inverse relationship between sensitivity and resolving power is also responsible for a
very important feature of quadrupole mass filters operated at constant resolution:
The
throughput of the analyzer decreases with mass. (Since
'
M
10%
is constant, the
resolving power, R= M/
'
M
10%
, increases with mass reducing the effective throughput of
the filter for high masses. This effect must be taken into account when calculating partial
pressure sensitivities from ion currents.)
Zero Blast Suppression
When the applied potentials are small (or zero) as at the beginning of a scan, ions
entering the filter may be transmitted even though their trajectories are mathematically
unstable. This gives rise to an output signal at the beginning of mass scans called the
““Zero Blast””. In the SRS RGA the Zero Blast is suppressed preventing the DC voltage
from reaching zero under 1.5 amu. Zero Blast suppression is standard practice in all
commercially available RGAs and is responsible for the lack of reliable readings for
hydrogen at 1 amu.