SRS Labs QMS 100 Series, User Manual

Page: 115 / 280

Quadrupole mass filter 2-13
Technical Reference
potential terms. The radius of the circle inscribed by the rods is 0.109”. The frequency of operation is
f=2.7648 MHz.
Principle of operation
The following figure schematically represents the quadrupole mass filter and its connections.
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