SRS Labs QMS 100 Series, User Manual

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2-12 Quadrupole mass filter
SRS QMS 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.
Figure 5 Quadrupole Mass Filter Components
In modern RGA’s, round electrodes are a common replacement for hyperbolic ones 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
µ
in tolerance) and are held in quadrature and parallel to each other, to tolerances better
than 300
µ
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 350
°
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)