About The Fid - Agilent Technologies 7890 Series Advanced Operation Manual

About the FID

Advanced Operation Manual
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.
5
Detectors
169