Chemiluminescence

Chemiluminescence is the emission of light energy that results from a chemical reaction. It was found in the late 1960s that the reaction of NO and ozone (O3) produced infrared radiation from about 500 to 3000nm.
NO + O3 → NO2* + O2
NO2* → NO2+ hv
Where NO2* symbolises the molecule in excited state, v is the light frequency and h is Planck’s constant.
Monitors that measure NO concentrations by detecting chemiluminescent radiation select only a narrow region of the total light emission. A filter is used to select light in the region from about 600 to 900nm.
Nitrogen dioxide (NO2) does not undergo this reaction and must be reduced to NO before it can be measured by this method. The PG 250  analyser contains a converter that catalytically reduces NO2 to NO. The NO (converted from NO2) plus the original NO in the sample is then reacted with O3 as described above to give a total NO + NO2 (NOx) reading. In the PG 250 Ozone is generated and is supplied in excess to the reaction chamber to ensure complete conversion of NO to NO2.

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