This article throws light upon the top four luminescence techniques used for monitoring pollutants. The techniques are: 1. UV Fluorescence SO2 2. Flame Photometry 3. Polarography Analyzer 4. Electctrocatalytic Analyzer.
Luminescence Technique # 1. UV Fluorescence SO2:
UV Fluorescence SO2 molecules present in exhaust gas are excited to SO2* by using UV light of wavelength 210 nanometer (nm). Excitation is the result of an atom’s absorption of this beam of electromagnetic radiation.
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The excited molecular state persists for only a few nanoseconds during which time some of the energy is lost through vibration transitions of the electrons. As the molecules return to its relaxed state, light of a longer wavelength, 350 nm, is released. The released light can be detected by the instrumentation within the monitor. Its intensity is then related to gas concentration.
Fluorescence monitors are affected by changes in the flue gas composition (%O2, %CO and %N2). These fluctuations in diluent gas concentration result in the relaxation of SO2* molecules through a process called quenching.
Quenching occurs when excited molecules return to their normal state by losing their extra energy through interaction with other gases instead of releasing their energy in the form of light; consequently, some of the excitation energy would not be detected. In order to reduce the problems caused by quenching, the background gas composition should be held relatively constant.
Luminescence Technique # 2. Flame Photometry:
Generally it is used to measure compounds that contain sulphur. In this method, the compounds are “burnt” in a hydrogen flame which results in the formation of excited diatomic sulphur molecules, S2*.
The conversion of the high energy S2* molecules to the lower energy ground state, S2, occurs with the emission of light. The intensity of this light is measured and related to the concentration of sulphur species in the sample.
Luminescence Technique # 3. Polarography Analyzer:
Paleographic analyzers can measure gases like O2, SO2, NO, NO2, CO and CO2. Different types of electrodes and electrolytes are required depending on which gas is selected for monitoring. The current of the gas across the cell is proportional to the rate of diffusion of the pollutant into the cell and also proportional to the pollutant concentration.
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The cells have a sensing electrode, an electrolyte polar graphic analyzer and a counter electrode. The polar graphic analyzers possess a thin-film membrane through which the pollutant must diffuse in order to initiate the electrochemical reaction and current flow.
Luminescence Technique # 4. Electro Catalytic Analyzer:
In this analyzer a solid electrolyte is used. A thin film, applied to the solid’s surface, catalyzes a reaction which allows gaseous molecules to migrate through the solid and generate a current.
In oxygen electro catalytic analyzers, a zirconium oxide (ZrO2) disc which has been coated with a thin film of platinum is heated to 850°C. A reference gas of about 21 percent oxygen is maintained on one side of the solid’, the sample gas is on the other side.
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Oxygen ions are generated at the platinum surface and then migrate through vacancies in the solid electrolyte. Electrons are released in the process as the system attempts to reach an equilibrium concentration of oxygen. The electron flow, or current, is then related to the concentration of oxygen in the sample.
An electro catalytic analyzer has also been developed for the measurement of SO2. This apparatus uses a potassium sulphate crystal and requires the simultaneous measurement of oxygen concentration in the sample.