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34 Operation and Maintenance Manual
directly in front of the photomultiplier tube (PMT). A red cut-off filter between
the reaction cell and the PMT selectivity transmits the light from the nitric
oxide and ozone reaction.
Efficient combustion in the ceramic tubes and the red cut-off filter eliminate
interference from non-nitrogen containing analytes (sulfur dioxide, alkenes,
olefins) that have chemiluminescence reactions with ozone.
A background signal is typically present as a result of ozone-wall interactions
and low level nitrogen contamination from carrier and detector gases. The
background signal is a useful troubleshooting aid.
NCD Photomultiplier Tube and Cooler
In the NCD, a red-sensitive PMT detects emissions from the nitric oxide and
ozone reaction. A thermoelectric cooler cools the PMT 40 °C below ambient
temperature to approximately -15 to -20 °C to reduce background noise from
the PMT. The cooler operates whenever the Detector is connected to AC power.