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SCD and NCD User Manual
3
System Description
Reaction cell and photomultiplier tube (PMT)
The ozone generator discharges ozone into the reaction cell.
This ozone reacts with any SO or NO to generate SO
2
* and NO
2
*
respectively. As the species return to ground state though
chemiluminescence, the photomultiplier tube produces a
current proportional to the intensity of emitted light. A
bandpass filter is used to optimize the detector for either sulfur
or nitrogen detection.
EPC modules
The detector controls hydrogen, oxidizer (air or oxygen), and
ozone supply (oxygen) gas flows using two electronic pressure
control modules.
Vacuum pump
A two-stage, oil-sealed rotary vacuum pump provides an
operating pressure between 3 and 10 Torr in the reaction cell.
This vacuum helps transfer combustion gases from the burner
to the reaction cell, as well as transferring the ozone from the
ozone generator into the reaction cell. The vacuum pump also
reduces non-radiative collisional quenching of the emitting
species in the reaction cell.
Ozone destruction trap
A chemical trap between the detector exhaust and the vacuum
pump destroys ozone, converting it to diatomic oxygen.
Unconverted ozone reduces pump life.
Oil coalescing filter
The oil-sealed rotary vacuum pump uses a partially-open gas
ballast to aid in the elimination of water produced in the burner
and transferred to the pump. As a result of the open gas ballast
and the relatively high flow rates of gases, oil vaporized in the
pump can escape through the pump exhaust. To minimize oil
loss, the pump includes an oil coalescing filter on the pump
exhaust to trap vaporized oil and to return this oil to the
vacuum pump oil reservoir.
FID adapter (optional)
The SCD burner normally mounts onto the GC oven directly as a
stand-alone detector. However, some applications also require