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Transpector XPR 3+ Operating Manual
between the source exit aperture and the quadrupole. Imbalances in the amplitude
of the two phases of RF applied to the rod pairs, and of the DC voltages also
applied, result in a further modification of the ion energy.
The mass of the ions passed by the filter is determined by the RF amplitude, the
RF frequency, and the quadrupole radius, as shown by the following equation:
[3]
where:
V is the peak-to-peak RF amplitude in Volts,
M the mass of the ion in atomic mass units (AMU) per electron charge,
f the RF frequency in megahertz,
r
0
the quadrupole radius in centimeters.
The mass of ions transmitted (
M) is directly proportional to the RF amplitude
(provided
f is constant). As the RF amplitude is increased, progressively higher
mass ions will be made to oscillate in phase with the RF field and thus gain
sufficient energy to strike the poles. Of course, the DC voltage must also be
increased to maintain the high-mass rejection properties of the filter. A mass
spectrum can therefore be obtained by sweeping the RF amplitude, along with the
DC voltage.
The variation in the efficiency of transmission of ions through the filter with mass is
discussed in section 3.5 on page 3-10. Following that, section 3.6 on page 3-10
discusses the behavior of the filter at very low masses where the applied voltages
approach zero.
3.4.3 The Ion Detector
The ion detector region of the sensor consists of the quadrupole exit lens and the
detector itself. Often, the quadrupole exit aperture is biased negatively with respect
to the anode, focusing ions that have been transmitted through the quadrupole into
the detector element. The detector can be a simple Faraday Cup (FC), an Electron
Multiplier (EM), or a combination of both. Transpector XPR 3+ is a combination of
Faraday Cup and Electron Multiplier.
3.4.3.1 The Faraday Cup Detector
The Faraday Cup detector is typically a metal plate or a cup-shaped electrode, on
which the ion beam impinges. Ions strike the detector and are neutralized, thus
drawing a current from the circuitry connected to the electrode. Usually, the current
flow that results is exactly equal to the incident ion current. In the Transpector
family of instruments, the Faraday Cup is at ground potential.
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