2
Functional Description
MS Detector
30
Surveyor MSQ Plus Hardware Manual
Thermo Scientific
Figure 20.
Cross sectional view of the ion detection system, showing the electron multiplier and
the conversion diode
Typically, the electron multiplier is set to a gain of about 3 × 10
5
(which means that for each
ion or electron that enters, 3 × 10
5
electrons exit). The current that leaves the electron
multiplier by way of the anode is converted to a voltage by the electrometer circuit and
recorded by the data system.
The ion detection system of the Surveyor MSQ Plus MS detector increases signal while
maintaining a low level of noise. The high voltage applied to the conversion dynode results in
a high conversion efficiency and increased signal. That is, for each ion striking the conversion
dynode, many secondary particles are produced. The increase in conversion efficiency is more
pronounced for more massive ions than for less massive ions.
Because of the off-axis orientation of the ion detection system relative to the mass analyzer,
neutral molecules from the mass analyzer tend not to strike the conversion dynode or electron
multiplier. As a result, the noise from neutral molecules is greatly reduced.
Vacuum manifold
top cover plate
Anode
High voltage tube
High voltage ring
Cathode
Electron multiplier
Secondary particles
Conversion
dynode
Conversion dynode
shield tube
Mass
analyzer
Ions
Conversion dynode
feedthrough electrode