Functional Description
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___________________________________________________________________ MS Detector
_______________________ Finnigan LTQ Hardware Manual _____________________
2-21
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ratios become unstable and are ejected through the slots in the exit rods. Many
of these ions are focused toward the ion detection system where they are
detected.
Ion Detection Systems
The LTQ is equipped with a two high sensitivity, off-axis ion detection
systems that produce a high signal-to-noise ratio and allows for voltage
polarity switching between positive ion and negative ion modes of operation.
Each ion detection system includes a 15-kV conversion dynode and a channel
electron multiplier. The ion detection systems are located on opposite sides of
the mass analyzer. See Figure 2-11.
The conversion dynode is a concave metal surface that is located at a right
angle to the ion beam. A potential of +15 kV for negative ion detection or
-15 kV for positive ion detection is applied to the conversion dynode. 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. These secondary
particles are focused by the curved surface of the conversion dynode and are
accelerated by a voltage gradient into the electron multiplier. The conversion
dynode shield, tube, and disk shield the vacuum manifold from the electric
field produced by the conversion dynode.
The electron multiplier is mounted on the top cover plate of the vacuum
manifold next to the mass analyzer. See Figure 2-11 and Figure 2-16. The
electron multiplier includes a cathode and an anode. The cathode of the
electron multiplier is a lead-oxide, funnel-like resistor. A potential of up to
-2.5 kV is applied to the cathode by the high voltage ring. The exit end of the
cathode (at the anode) is near ground potential. The cathode is held in place
by the high voltage ring, two support plates, the electron multiplier support,
and the electron multiplier shield. A spring washer applies a force to the
cathode to hold it in contact with the electron multiplier shield. The electron
multiplier support is attached to the top cover plate of the vacuum manifold
by two screws.
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 top cover 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. Due to the funnel shape of the
cathode, the ejected electrons do not travel far before they again strike the
inner surface of the cathode, thereby causing the emission of more electrons.
