Stanford Research Systems RGA100, Operating Manual And Programming Reference

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3-8 Quadrupole mass filter
SRS Residual Gas Analyzer
Principle of operation
The following figure schematically represents the quadrupole mass filter and its
connections.
+
+
–
–
X
Y
Z
+
ions
Quadrupole
axis
+
+
– –
U+V
o
cos
ω
t
X
Y
-(U+V
o
cos
ω
t)
Figure 6 Quadrupole Connections
During operation, a two dimensional (X-Y) quadrupole field is established between the
four cylindrical electrodes with the two opposite rods connected together electrically.
Ions enter the filter along the Z-axis and start oscillating in the X- and Y- directions. The
ions are separated based on their mass-to-charge ratio by lateral forces resulting from the
electric potentials applied to the rods. A combination of DC and RF voltages control the
quadrupole filter operation. Successful separation of a specific mass species requires
setting the RF and DC values such that only the ions of interest have stable trajectories
down the quadrupole rod assembly. By simultaneously varying the amplitude of the DC
and RF voltages an entire mass spectrum can be scanned.
The RGA is completely calibrated at the factory so that the ECU automatically programs
the RF and DC voltages on the rods to the necessary levels during scans or individual
mass measurements. The calibration parameters can be accessed and modified by the user
in case recalibration or a change in resolution is required. (Please see the RGA Tuning
chapter for details.)
The quadrupole mass filter must be operated in a vacuum. Linear operation can be
expected up to pressures of the order of 10
-4
Torr. In general quadrupoles can be operated
at relatively high pressures compared to other types of analyzers. The upper limit of
useful operation is determined by the collisions between the ions and the neutral gas
molecules. In order to avoid collisional scattering it is necessary to maximize the mean