SRS Labs QMS 100 Series, User Manual

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2-14 Quadrupole mass filter
SRS QMS Gas Analyzer
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 free path of the ions.
The general principle of operation of the filter can be visualized qualitatively in the following terms:
One rod pair (X-Z plane) is connected to a positive DC voltage upon which a sinusoidal RF voltage is
superimposed. The other rod pair (Y-Z plane) is connected to a negative DC voltage upon which a
sinusoidal RF voltage is superimposed, 180 degrees out of phase with the RF voltage of the first set of
rods. The potentials are represented by the expression:
V
x/y
= +/- (U + V
0
cos
ω
t)
where,
U is the magnitude of the DC voltage applied to either pair of rods,
V
0
is the amplitude of the RF voltage applied to either set of rods, and
ω
is the angular frequency (=2
π
f) of the RF.
Light ions (low mass-to-charge ratio) are able to follow the alternating component of the field. For the X
direction , those ions will stay in phase with the RF drive, gain energy from the field and oscillate with
increasingly large amplitudes until they encounter one of the rods and are discharged. Therefore, the X
direction is a
high-pass mass filter : Only high masses will be transmitted to the other end of the
quadrupole without striking the X electrodes. On the other hand, in the Y direction , heavy ions will be
unstable because of the defocusing effect of the DC component, but some lighter ions will be stabilized
by the alternating component if its magnitude and amplitude are such as to correct the trajectory
whenever its amplitude tends to increase. Thus, the Y direction is a
low-pass mass filter : Only low
masses will be transmitted to the other end of the quadrupole without striking the Y electrodes.
The two directions together give a mass filter suitable for mass analysis : By a suitable choice of
RF/DC ratio the filter can be made to discriminate against both high and low mass ions to the desired
degree.
The RF voltage magnitude and frequency determine the mass of the ions that undergo stable trajectories
down the filter As the RF amplitude increases, heavier ions start to oscillate in phase with the RF and
collide with the rods. The DC/RF ratio determines the filter selectivity As the DC increases (at
constant RF) heavier ions are defocused by the negative DC component.
The attractive features of the quadrupole as a mass analyzer are evident from the above discussion:
The quadrupole provides a convenient filter which can be tuned to the desired mass by
varying the amplitude of the RF voltage; the mass selectivity (i.e. resolution) can also be
varied electronically by simply adjusting the DC/RF ratio.
Simultaneously varying the amplitude of
the DC and RF voltages allows entire mass spectra to be scanned.
Mass Range, Resolution and Throughput
The most important characteristics of a quadrupole are the mass range, its ultimate resolution and its
throughput characteristics .