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Functional Description
Mass Spectrometer
38 TSQ Quantum XLS Series User Guide Thermo Scientific
Ion Detection System
The TSQ Quantum XLS Series mass spectrometers are equipped with a high-sensitivity,
off-axis ion detection system. See Figure 25. This system produces a high signal-to-noise ratio
and provides for voltage polarity switching between positive ion and negative ion modes of
operation. The ion detection system includes a 10 kV conversion dynode and a channel
electron multiplier. The ion detection system is located on the back of the vacuum manifold
behind the mass analyzer.
The conversion dynode is a concave metal surface located at a right angle to the ion beam.
The mass spectrometer applies to the conversion dynode a potential of +10 kV for negative
ion detection or –10 kV for positive ion detection. When an ion strikes the surface of the
conversion dynode, one or more secondary particles are produced. These secondary particles
can include positive ions, negative ions, electrons, and neutrals. When positive ions strike a
negatively charged conversion dynode, the secondary particles of interest are negative ions and
electrons. When negative ions strike a positively charged conversion dynode, the secondary
particles of interest are positive ions. The curved surface of the conversion dynode focuses
these secondary particles and a voltage gradient accelerates the particles into the electron
multiplier.
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.
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