Agilent Technologies 7000 Triple Quadrupole GC/MS, Maintenance Manual

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Analyzers and Collision Cell
7
Troubleshooting and Maintenance Manual 161
Detector
The detector ( Figure 33 ) in the MS analyzer is a high energy conversion
dynode (HED) coupled to an electron multiplier (EM). The detector is located
at the exit of the rear analyzer. It receives the ions that have passed through
the mass filter. The detector generates an electronic signal proportional to the
number of ions striking it. The detector has three main components: the
detector ion focus, the HED and the EM horn.
Detector ion focus
The detector ion focus directs the ion beam into the HED, which is located off
axis. The voltage on the detector focus lens is fixed at –600 V.
High-energy dynode
The HED operates at –10,000 volts for EI. It is located off-axis from the center
of the quadrupole mass filter to minimize signals due to photons, hot neutrals,
and electrons coming from the ion source. When the ion beam hits the HED,
electrons are emitted. These electrons are attracted to the more positive EM
horn. Do not touch the insulator.
EM horn
The EM horn carries a voltage of up to –3000 volts at its opening and 0 volts at
the other end. The electrons emitted by the HED strike the EM horn and
cascade through the horn, liberating more electrons as they go. At the far end
of the horn, the current generated by the electrons is carried through a
shielded cable outside the analyzer to the signal amplifier board.
The voltage applied to the EM horn determines the gain. The voltage is
adjustable from 0 to –3000 VDC. Use the EM voltage found in autotune as a
baseline for the EM voltage setting.
• To increase signal gain, increase the EM voltage.
• For concentrated samples where less signal gain is needed, decrease the EM
voltage.