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.

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
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.  

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
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
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




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.