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Ion Transmission and Mass Analysis
Ion Detection Systems
Thermo Scientific Orbitrap Fusion Series Hardware Manual 43
Ion Detection Systems
The Orbitrap Fusion Series MS has a high-sensitivity, off-axis ion detection system with two
conversion dynodes and one electron multiplier (Figure 30). The ion detection system
produces a high signal-to-noise ratio (S/N), which is proportional to the number of ions
detected, and enables voltage polarity switching between positive and negative ion modes of
operation. The electron multiplier is located under the LIT. The conversion dynodes are
located on each side of the LIT.
A conversion dynode is a concave metal surface located at a right angle to the ion beam. The
ion detection system applies to the conversion node a high positive potential (12 kVdc) for
negative-ion detection or a high negative potential (–12 kVdc) for positive-ion detection.
Both conversion dynodes are physically connected and operate at the same voltage. An ion
striking the surface of the conversion dynode produces one or more secondary particles. The
curved surface of the conversion dynode focuses these secondary particles and the voltage
gradient accelerates them into the electron multiplier. The conversion dynode shields protect
the vacuum manifold from the electric field that the conversion dynode produces.
The electron multiplier samples the signal and includes an anode that collects the electrons
produced by the cathode. The data system records the current that leaves the electron
multiplier through the anode.
Because of the off-axis orientation of the ion detection system relative to the LIT, neutral
molecules from the trap tend not to strike the conversion dynode or electron multiplier. As a
result, the noise from neutral molecules is reduced.
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