Detectors 5
Advanced User Guide 103
FPD linearity
Several mechanisms produce sulfur emission. The excited
species is diatomic, so that emission intensity is approximately
proportional to the square of the sulfur atom concentration.
The excited species in the phosphorus mode is monatomic,
leading to a linear relationship between emission intensity and
atom concentration.
FPD Lit Offset
The default Lit Offset is 2.0 pA.
Starting Up and Shutting Down the FPD
The FPD creates a great deal of water vapor when the flame is
on. This could condense in the vent tube on top of the detector
and drop onto the flame, possibly extinguishing it. To avoid this,
turn the heaters on, wait 20 minutes for the vent to heat up, and
then ignite the flame. Water vapor will now make it over the top
of the vent tube before condensing.
For similar reasons, extinguish the flame before turning the
heaters off.
FPD photomultiplier protection
The PMT is extremely sensitive to light. Always turn the PMT
voltage off (which turns off the high voltage to the PMT) before
removing the PMT housing or opening the emissions chamber.
Failing to do this can destroy the PMT.
Even with the PMT voltage off, protect the PMT from room light.
Cap the housing when removed, place it end down to exclude
light, reduce room light level before exposing the PMT, and so
on. A brief exposure (always with the PMT voltage turned off)
will not damage it but prolonged exposure will cause a gradual
loss of sensitivity.
FPD optical filters
The filters are marked on the edge with the transmission
wavelength. Each filter has a small arrow on its side which must
point toward the PMT when installed.
The sulfur filter is silvery on both sides and transmits at
393 nanometers.
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