Agilent Technologies 5977B Series, Troubleshooting And Maintenance Manual

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206 5977B Series MSD Troubleshooting and Maintenance Manual
7
Analyzer
Detector
The detector (See Figure 49 on page 207) 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 V for EI and PCI, and +10,000 V for NCI. 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 –3,000 V at its opening and 0 V 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 –3,000 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.