Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Quadrupole mass filter 3-7
SRS Residual Gas Analyzer
Quadrupole mass filter
Positive ions are transferred from the ionizer into the quadrupole where they are filtered
according to their mass-to-charge ratios. Ions that successfully pass through the
quadrupole are focused towards the detector by an exit aperture held at ground potential.
Description
The quadrupole mass filter is an electrodynamic quadrupole operated by a combination of
DC and RF voltages. It is constructed of four electrically-conducting, cylindrical rods
accurately held in place by a set of two high-purity alumina insulators.
Ribbon connectors
Quadrupole rods
Alumina spacers
Spring loaded
perforated screws
Figure 5 Quadrupole Mass Filter Components
Round electrodes are a common replacement for hyperbolic ones in modern RGA’’s and
result in reduced cost and ease of construction. The stainless steel (type 304) rods are
4.5”” long, have a 0.250”” diameter (100
P
in tolerance) and are held in quadrature and
parallel to each other, to tolerances better than 300
P
in, by the precision ground alumina
spacers. Spring loaded, perforated screws fasten the rods to the spacers. The whole
assembly is self-aligning, can easily be taken apart for cleaning and it can withstand
repeated cycling to 300
q
C for bakeout. The exact placement of the rods is optimized to
minimize the distortion of the electrodynamic quadrupole field by canceling higher order
(multipole) potential terms. The radius of the circle inscribed by the rods is 0.109””. The
frequency of operation is f=2.7648 MHz.