User guide

Detectors 10
Advanced User Guide 301
FID automatic reignition (Lit offset)
Lit offset is the expected minimum difference between the FID
output with the flame lit and the output with the flame off. The
GC checks this value during runs and when loading a method.
During a run, if the output falls below the Lit offset value, the FID
will attempt to reignite three times. If after the third attempt
the output does not increase by at least this value, the detector
shuts down all functions except temperature and makeup gas
flow.
When loading a method that includes a Flame On setting, the GC
performs a similar check. If the detector output is less than the
Lit offset, it will attempt reignition after reaching method
setpoints.
The default setting for Lit offset is 2.0 picoamps. This is a good
working value for all but very clean gases and systems. You may
want to lower this setpoint if the detector attempts to reignite
when the flame is still on, thus producing a shutdown.
To change Lit offset:
1 Press [Config][Front Det] or [Config][Back Det].
2 Scroll to Lit offset.
3 Enter the new value and press [Enter].
Recommended starting conditions for new FID methods
See Table 66 for guidelines and rules to select initial detector
settings for new methods.
Table 66 Recommended starting conditions
Combustible gas mix Make sure that the final hydrogen-to-air ratio is between 8% and 12%
Detector temperature Set to 20 °C above the highest oven temperature, depending on the column type.
A temperature of 300 °C provides a good starting point and easier ignition, and
minimizes water condensation.
The GC will not attempt to ignite the flame at a temperature <150 °C.
Carrier gas flow (hydrogen, helium,
nitrogen)
Packed columns Suggest 10 to 60 mL/min
Capillary columns Suggest 1 to 5 mL/min