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Technical Description: System
Ion Detection
and depends on the voltage applied. Operation of the MCP is
described below. Electron current exiting the MCP is picked up on
the detector electrode which connects directly to the amplifier
mounted on the front right face of the vacuum housing.
Figure B–9
illustrates an MCP that consists of an array of millions
of glass capillaries, each of which acts as secondary electron
multipliers to form a large secondary electron multiplier array.
Each channel has an internal diameter of 25 µm and are fused
together to form a thin disk 0.8 mm thick. The inside wall of each
channel is coated with an electron emissive material the ends of each
channel is covered with a thin metal film to form an electrode. Each
channel is therefore an independent secondary electron multiplier.
When a voltage is applied across the ends of the capillaries, an
electric field is generated along the axis of each channel. When an
ion strikes the entrance wall of the channel, secondary electrons are
produced. The secondary electrons are accelerated by the electric
field and travel along the parabolic trajectories determined by their
initial velocity. When these electrons strike the opposite wall of the
channel, additional secondary electrons are produced. This process is
repeated many times along the channel and as a result, the electron
current increases exponentially towards the output of the channel.
The capillary bundles are sliced into thin disks with a slice angle
chosen to ensure that primary electrons cannot pass through the
channel without subsequently colliding with the channel wall.
MCP Operation
The MCP should not normally be operated at voltages exceeding
1200 V. Optimum results are normally obtained at a gain setting of
1000. At this value detector noise is negligible and the life of the
MCP assembly is prolonged.
Caution! Damage to the MCP!
-12
Ion currents in excess of 1 x 10
A should never be applied to the MCP
as irreversible damage, requiring replacement of the MCP, will occur.
For normal analysis the MCP should only be used for peaks less than
100 ppm.
Calibration of the MCP requires that the voltage needed to produce a
certain gain is established. This is achieved by measuring the ratio of
peak intensity on the Faraday and MCP array as follows.
Thermo Fisher Scientific
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
B-11
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