Gaseous Pollutants - Chemiluminescence

Figure: Chemical reaction to determine oxides of nitrogen by chemiluminescence

Chemiluminescence methods for determining components of gases originated with the need for highly sensitive means for determining atmospheric pollutants such as ozone, oxides of nitrogen, and sulfur compounds. Chemiluminescence is based upon the emission spectrum of an excited species that is formed in the course of a chemical reaction. Oxides of nitrogen can be determined by the gas phase reaction of NO with ozone (O3). An ambient air sample is mixed with excess ozone in a special sample cell. A portion of the NO present is converted to an activated NO2 which returns to a lower energy state and in the process emits light. This phenomenon is called chemiluminescence. The intensity of this light can be measured with a photomultiplier tube and is proportional to the amount of NO in the sample. A second reaction measures the total oxides of nitrogen in the air sample and in turn, the concentration of NO2 can be calculated.

Another important chemiluminescence method is used for monitoring atmospheric ozone. In this instance, the determination is based upon the luminescence produced when ozone reacts chemically with the dye rhodamine-B absorbed on an activated silica gel surface or with ethylene. The chemical reaction creates light pulses, which are detected and counted by a photomultiplier tube. The concentration of ozone is determined by comparing the number of light pulses created by the sample with the number of light pulses created by a sample having a known concentration of ozone.