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
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.
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
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
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)
Example Absorption spectrum
NO3
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
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