Thermo Finnigan LCQ Deca XP Plus, Руководство по оборудованию

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Functional Description
Ion Detection System _________________________________________________________
1-20
___________________ LCQ Deca XP Plus Hardware Manual ________________
1.4 Ion Detection System
The XP Plus is equipped with a high sensitivity, off-axis ion detection system
that 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. The ion detection system is located at the rear of the vacuum
manifold behind the mass analyzer. See Figure 1-8.
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. 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 a
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.