Thermo Scientific TSQ Series Hardware Manual page 52

2
Functional Description
Mass Spectrometer
The electron multiplier includes a cathode and an anode. The cathode of the electron multiplier is
a lead-oxide, funnel-like resistor. The high voltage ring applies a potential of up to -2.5 kV to the
cathode. The exit end of the cathode (at the anode) is near ground potential.
The anode of the electron multiplier is a small cup located at the exit end of the cathode. The
anode collects the electrons produced by the cathode. The anode screws into the anode
feedthrough in the base plate.
Secondary particles from the conversion dynode strike the inner walls of the electron multiplier
cathode with sufficient energy to eject electrons. The ejected electrons are accelerated farther into
the cathode, drawn by the increasingly positive potential gradient. The funnel shape of the cathode
causes the ejected electrons not to travel far before they again strike the inner surface of the
cathode, which causes the emission of more electrons. A cascade of electrons is then created that
finally results in a measurable current at the end of the cathode where the anode collects the
electrons. The current collected by the anode is proportional to the number of secondary particles
striking the cathode.
Figure 29. Ion detection system, showing the electron multiplier (top) and conversion dynode (bottom)
Typically, the electron multiplier is set to a gain of about 3 × 10
that enters, 3 × 10
circuit converts the current that leaves the electron multiplier via the anode to a voltage and the
data system records the voltage.
The ion detection system of the TSQ mass spectrometer increases signal and decreases noise. The
high voltage applied to the conversion dynode results in a high conversion efficiency and increased
signal. That is, for each ion striking the conversion dynode, many secondary particles are
produced. The increase in conversion efficiency is more pronounced for more massive ions than it
is for less massive ions.
Because of the off-axis orientation of the ion detection system relative to the mass analyzer, neutral
molecules from the mass analyzer tend not to strike the conversion dynode or electron multiplier.
As a result, noise from neutral molecules is reduced.
34 TSQ Series Hardware Manual
electrons exit) in MS mode and 2 × 10
5
5
(that is, for each ion or electron
6
in MS/MS mode. The electrometer
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