Agilent Technologies 5975T LTM GC/MSD, Troubleshooting And Maintenance Manual

Page: 210 / 270

210 Troubleshooting and Maintenance Guide
13
Analyzer
Detector
The detector ( Figure 33 ) in the MSD analyzer is a high energy
conversion dynode (HED) coupled to an electron multiplier
(EM). The detector is located at the exit end of the quadrupole
mass filter. 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.
As the EM horn ages, the voltage (EMVolts) required increases
over time. If the EM voltage must always be set at or near
–3000 VDC to complete Autotune, with no other probable cause,
it may need to be replaced. Check your tune charts for gradual
degradation, which indicates wearing out. Select
View Tunes