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Functional Description
Mass Spectrometer
Thermo Scientific TSQ Series Hardware Manual 35
The voltage applied to each of the lenses can vary from about –300 to +300 V. Typically,
however, the voltage applied to the first and third elements of the L2x lens set is somewhat
greater than the quadrupole offset voltage applied to Q1. Because the Q1 quadrupole offset
voltage is generally set to about ±5 V (depending on the charge of the ions of interest), the
voltage applied to lenses L21 and L23 is typically about –10 V for positive ions and +10 V for
negative ions. The voltage applied to the central lens of the L2x lens set is typically about
±225 V.
In the Q3MS scan mode, the voltage applied to the lenses of the L3x lens set is about the same
as that applied to the corresponding lens in the L2x lens set. Note, however, that in the
MS/MS scan modes, the voltage applied to the L3x lens set automatically varies with the
quadrupole offset voltage applied to Q3. As the Q3 quadrupole offset voltage ramps, the
voltages applied to the lenses ramp correspondingly.
Lens L4 is located between Q3 and the ion detection system. L4 is held at ground potential.
Its purpose is to shield Q3 from the high voltage applied to the ion detection system and to
shield the ion detection system from the high rf voltages applied to Q3.
Ion Detection System
The TSQ mass spectrometer is equipped with a high-sensitivity, off-axis ion detection system.
This system produces a high signal-to-noise ratio and allows for voltage polarity switching
between positive ion and negative ion modes of operation. The ion detection system includes
a 15 kV conversion dynode and a channel electron multiplier. See Figure 29. The ion
detection system is located at 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 TSQ mass spectrometer applies to the conversion dynode a potential of +15 kV for
negative ion detection or –15 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.
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
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