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

Nitrogen Oxides Pollution NOx 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 http://www.epa.gov/air/nitrogenoxides/ Children, the elderly, people with lung diseases such as asthma, and people who work or exercise outside are at risk for adverse effects from ozone. These include reduction in lung function and increased respiratory symptoms as well as respiratory-related emergency department visits, hospital admissions, and possibly premature deaths. Ozone

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. http://www.epa.gov/air/nitrogenoxides/

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 (DE). If energy of light exactly matches DE, photon can be absorbed More conjugated systems have smaller HOMO-LUMO gap Have lower DE and absorb longer wavelength of light http://www.chem.ucalgary.ca/courses/350/Carey/Ch13/ch13-uvvis.html

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 http://www.chemistry.ccsu.edu/glagovich/teaching/316/uvvis/uvvis.html

Ultraviolet-Visible Spectroscopy Light transmitted (T) through the sample: T = (I/I0) I=Light intensity I0=Initial light intensity Beer-Lambert Law of Absorbance (A): A = -log(I / I0) = εbc ε= molar absorptivity (L/mol*cm) b=pathlength of sample cell (cm) c=concentration of compound (mol/L) http://www.chemistry.ccsu.edu/glagovich/teaching/316/uvvis/uvvis.html

Example Absorption spectrum NO3 Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies

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 http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm

Advantages/Disadvantages UV-Vis detection for GC 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