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Transpector MPH Operating Manual
3.4 The Quadrupole Mass Filter
The ions produced in the ion source are injected into the mass filter, which rejects
all ions except those of a specific mass-to-charge ratio. Most ions contain only one
unit of charge. In Transpector MPH, the mass filter is a quadrupole type, to which
a combination of RF and DC potentials is applied. The RF frequency and amplitude
determine the mass/charge ratio, and the RF/DC ratio determines the filter
sensitivity. (See Figure 3-3.)
Figure 3-3 The Sensor’s quadrupole mass filter
The mass filter’s four rods 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, with the symbol r
0
. If the
ratio of the round rod radius to r
0
is made equal to 1.148, the resulting electric field
generates the maximum quadrupole field.
Opposite rods are electrically connected together. The ions are directed down the
central axis of the assembly in a direction nominally parallel to the length of the
rods. There the ions are separated by their mass-to-charge ratios by the lateral
forces resulting from the potentials applied to the poles.
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