Inficon Transpector CPM, Руководство по эксплуатации

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5 – 4
Transpector CPM Operating Manual
5.2.2 The Quadrupole Mass Filter
The ions produced in the ion source are injected into the mass filter, which rejects
all ions except those with a specific mass-to-charge ratio. Most ions contain only
one unit of charge. The mass filter is a quadrupole, to which is applied a
combination of RF and DC potentials. The RF frequency and amplitude determine
the mass. The RF/DC ratio determines the filter selectivity. (See
Figure 5-3 .)
Figure 5-3 The sensor’s quadrupole mass filter
The mass filter’s four rods (hence the term quadrupole ) are alternately charged to
direct ions of specific masses down through the center, deflecting all larger and
smaller masses (hence the term
mass filter ).
The mass filter consists of four parallel rods, or poles, in a square array. The rods,
and the insulators in which they are mounted, form an extremely precise
mechanical assembly. The distance between the center of the square array and the
closest rod surface is known as the quadrupole radius, r
0
. Ideally, the rod should
have a hyperbolic shape (towards the center of the assembly) rather than round. If
the ratio of the round rod radius to r
0
is equal to 1.148, the resulting electric field is
a reasonably good approximation of the desired hyperbolic shape.
Opposite rods are electrically connected together. The ions are directed into the
space between the poles in a direction nominally parallel to the length of the rods.
There the ions are separated according to their mass-to-charge ratios by the lateral
forces resulting from the potentials applied to the poles.
The applied potentials consist of an RF component and a DC component. The RF
potential on one set of rods is out of phase by 180° with respect to the RF potential
on the other set of rods, but all are the same amplitude. For one pair of rods, the
“X” pair, the DC potential is positive. For the other, the “Y” pair, the DC potential is
of the same magnitude but negative. The DC and RF potentials are referenced to
a
center voltage ( pole zero ).
X
Z
Y