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Ultraviolet-Visible Spectroscopy in the Detection of Nitrogen Oxide Air Pollutants.

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Presentation on theme: "Ultraviolet-Visible Spectroscopy in the Detection of Nitrogen Oxide Air Pollutants."— Presentation transcript:

1 Ultraviolet-Visible Spectroscopy in the Detection of Nitrogen Oxide Air Pollutants

2 Nitrogen Oxides Pollution NO x forms from emissions from vehicles, power plants, and off-road equipment Adverse Effects: – Formation of ground level ozone in the presence of heat of sunlight – Airway inflammation and increased respiratory symptoms in asthma patients Nitrogen Dioxide Ozone

3 EPA Regulation EPA sets national standard for nitrogen oxide ambient air concentrations to 53 ppb (annual average) – Decreased by more than 40% since 1980 – Expected to decrease further as mobile source regulations that are taking effect Ultraviolet-visible (UV-Vis) spectroscopy can determine the concentration of nitrogen oxides.

4 Ultraviolet-Visible Spectroscopy Molecular absorption due to excitation of bonding electrons – Can identify functional groups – Can quantify compounds with absorbing groups The lowest energy transition is the HOMO-LUMO gap in the ground state (  E). If energy of light exactly matches  E, photon can be absorbed More conjugated systems have smaller HOMO-LUMO gap – Have lower  E and absorb longer wavelength of light

5 UV-Vis measurements Sample dissolved into a non-absorbing solvent Sample placed in cell – A cell of pure solvent is also analyzed as control Monochromatic light (190 nm- 800nm) is passed through cell Intensity of light transmitted is detected – Wavelength varied to test absorption at different energies

6 Ultraviolet-Visible Spectroscopy Light transmitted (T) through the sample: T = (I/I 0 ) I=Light intensity I 0 =Initial light intensity Beer-Lambert Law of Absorbance (A): A = -log(I / I 0 ) = εbc ε= molar absorptivity (L/mol*cm) b=pathlength of sample cell (cm) c=concentration of compound (mol/L)

7 Example Absorption spectrum NO 3

8 Gas Chromatography-UV-Vis Sample evaporated into gas phase Sample injected into column Analytes interact with stationary phase in column to different extents – Allows separation of different analytes Detection by UV-Vis

9 Advantages/Disadvantages UV-Vis detection for GC Advantages: – General indicator of functional groups – Minimal damage to sample – Good quantitation – Using GC eliminates solvent effects Disadvantages: – Lack of sensitivity & selectivity – Limited to UV-Vis absorbing compounds

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