96 Advanced User Guide
5 Detectors
About the FPD
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The sample burns in a hydrogen-rich flame, where some species
are reduced and excited. The gas flow moves the excited species
to a cooler emission zone above the flame where they decay and
emit light. A narrow bandpass filter selects light unique to one
species, while a shield prevents intense carbon emission from
reaching the photomultiplier tube (PMT).
The light strikes a photosensitive surface in the PMT where a
light photon knocks loose an electron. The electron is amplified
inside the PMT for an overall gain of up to a million.
Figure 4 FPD
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gas flows
The current from the PMT is amplified and digitized by the FPD
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electronics board. The signal is available either as a digital
signal on the communications output or as a voltage signal on
the analog output.
The FPD
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should not be stored at temperatures above 50 °C,
based on the original manufacturer’s specifications for the PMT.
Valve
Frit
Frit
Valve
PS = Pressure Sensor
Column
PS
PS
PS
Valve
Frit
Restrictor Restrictor
Restrictor
EPC Module
H
2
Air
Wavelength filter
Window
Vent
Makeup
Emission Zone
Transfer line
Restrictor
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