Chemiluminescence detector (CLD)
A molecule can pass of the electronic ground state to an electronic excited state through energy absorption. The absorbed energy can be emitted, in the transition to an energetically lower state, on the one hand in form of heat (non-radiative deactivation) and on the other by the emission of light (luminescence) again.
Chemiluminescence refers to light which is emitted after excitation of a chemical reaction.
It is being used in the analysis of nitrogen gases. Nitric oxide reacts with ozone to excited nitrogen dioxide. The subsequently emitted light is amplified and measured by a photomultiplier.
In order to analyze higher nitrogen oxides, they have to be reduced with a catalyst to nitrogen monoxide. Only then they can be, as described above, captured.
If all nitrogen oxides should be measured in a single sample gas stream, the content of nitrogen monoxide must be measured first. This measurement is carried out without the catalyst. Subsequently, the gas stream is passed over the catalyst and thus all nitrogen oxides (for example, NO2) are reduced to NO. The content of all nitrogen oxides is measured now and the following difference between the first and the second measurement indicates the concentration of the other nitrogen oxides.
Nitrogen oxides arise in particular during combustion, thats why this analysis method finds application in the exhaust gas measurement technology of power plants, in the automotive industry as well as in environmental technology.
Chemiluminescence detector (CLD) bright in chamber