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Functional Description
MS Detector
Thermo Scientific
Surveyor MSQ Plus Hardware Manual
29
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 entrance of the cathode by the high voltage ring. 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 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.
Thus, a cascade of electrons is created that finally results in a measurable current at the end of
the cathode where the electrons are collected by the anode. The current collected by the anode
is proportional to the number of secondary particles striking the cathode.
