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

2. 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
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

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 (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

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/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)

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

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
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