Detectors 5
Advanced User Guide 67
About the FID
The FID passes sample and carrier gas from the column through
a hydrogen-air flame. The hydrogen-air flame alone creates few
ions, but burning an organic compound increases the number of
ions produced. A polarizing voltage attracts these ions to a
collector located near the flame. The current produced is
proportional to the amount of sample being burned. This
current is sensed by an electrometer, converted to digital form,
and sent to an output device.
The FID uses three supply gases (hydrogen, makeup gas, and
air) and two supply lines. Air flows through one supply line,
while hydrogen mixed with the makeup gas flows through the
other. All three supply gases use:
• A filter frit to protect the flow path and limit the flow rate
• A proportional valve to control the pressure
• A pressure sensor and restrictor to control the valve
The hydrogen and makeup mix outside the flow module and
enter the detector at the base of the jet. Air enters above the jet.
PS = Pressure Sensor Column
PSPS
EPC/EPR Module
H
2
AirMakeup
Frit Frit Frit
PS
Valve
Frit
Valve
Frit
Valve
Frit
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