Ultra-Violet Spectroscopy in the Detection of Nitrogen Dioxide Air Pollutants

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Nitrogen Oxide 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
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.
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UV 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
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Energy Level Diagram. Electrons from the p orbital overlap to form
the pi binding orbital, which is of lower energy than the contributing
p orbitals.  Excitation causes an electron to be promoted to the pi
antibonding orbital of a higher energy.  


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Blank vs. Sample.  Blank cell filled with pure solvent (above)
absorbs less than the sample cell (below).  Blank cells have
higher transmitted intensity than sample cells. 

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- 800 nm) is passed through cell
  • Intensity of light transmitted is detected
  • Wavelength varied to test absorption at different energies

UV VIS Theory
T = (I/I0)
where I=Light intensity and 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)
           

Example Absorption Spectra
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Atomic Spectrum of NO3 showing characteristic peaks for NO3 (Wayne et. al. 1991)

Gas Chromatography with UV-Vis Detection
  • 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
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Block Diagram of a Typical Gas Chromatograph.  Sample evaporate
into gas phase and dissolved into the carrier gas.  Sample travels
through column interacting with stationary phase elements to different
degrees.  Detection by UV-Vis spectroscopy.
source: http://www.sfu.ca/bisc/bisc-429/GLC.html


Advantages

Disadvantages

References

L. Lagesson-Andrasko, V. Lagesson, J. Andrasko.  " The Use of Gas-Phase UV Spectra in the 168−330-nm Wavelength Region for Analytical Purposes. 1. Qualitative Measurements."  Analytical Chemistry (1998).   70: 819-826.  

R. P. Wayne, I. Barnes, P. Biggs, J.P. Burrows, C. E. Canosa-Mas, J. Hjorth, G. Le Bras, G. K. Moortgast, D. Perner, G. Poulet, G, Restelli, and H. Sidebottom.  "The Nitrate Radical: Physics, Chemistry, and the Atmosphere." Atmospheric Environment Part A General Topics (1991).  25: 1-203. 

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

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

http://www.chem.ucalgary.ca/courses/350/Carey/Ch13/ch13-uvvis.htmlhttp://www.epa.gov/air/nitrogenoxides/