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Analyzer
7
Detector
Detector
The detector in the MSD analyzer is a high energy conversion dynode (HED) cou-
pled 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 fil-
ter. The detector generates an electronic signal proportional to the number of ions
striking it. The detector has three main components: the detector focus lens, the
high energy dynode, and the electron multiplier horn.
Detector focus lens
The detector focus lens directs the ion beam into the HED, which is located off
axis. The voltage on the detector focus lens is fixed at -354 V.
High energy dynode
The high energy dynode (HED) operates at -10,000 volts, attracting the positive
sample ions exiting the quadrupole. The HED 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 electron multiplier
horn.
Electron multiplier horn
The electron multiplier 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 electron multiplier horn determines the gain. The volt-
age is adjustable from 0 to -3000 V dc. Use the electron multiplier voltage found
in autotune as a baseline for the electron multiplier voltage setting.
• To increase signal strength, increase the electron multiplier voltage.
• For concentrated samples where less signal strength is needed, decrease the
electron multiplier voltage.
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