Fluorescence techniques for environmental monitoring

Dr. Deepak

Sampling of a polluted water sample

Fluorescence spectroscopy provides a fast and precise means of monitoring of environmental pollutants in atmosphere and water. The main applications relate to monitoring of atmospheric gases such as oxides of nitrogen collectively referred to as NOX and SO2 as well as levels of polycyclic aromatic hydrocarbons in drinking and waste waters. In this article fluorescence used for monitoring of such pollutants are briefly covered.

Oxides of Nitrogen

Oxides of nitrogen NOX are highly reactive group of gases present in the atmosphere. The main sources are vehicular and power plant emissions. Exposure is particularly harmful for asthmatics, children and elderly. The gases can react with ammonia and moisture to form small particles which can aggravate respiratory diseases

PAN or peroxyacetyl nitrate has been identified as a major component of photochemical smog. It is produced by the photochemical oxidation of hydrocarbons in presence of oxides of nitrogen. PAN is a powerful eye irritant and also contributes to skin cancer.

NO on reaction with ozone forms the excited molecule [Latex]NO_2*[/Latex] which gives fluorescence on relaxation. On the other hand if [Latex]NO_2[/Latex] is to be determined in absence of NO then it is oxidised to [Latex]NO_2[/Latex] approved by passing to a catalytic reactor before it is excited to [Latex]NO_2*[/Latex] by reaction with ozone and the fluorescence is measured.

PAN CH3C(O)OONO2 is detected in the ppt range by thermally decomposing it to [Latex]NO_2[/Latex] which gives luminiscence on reaction with alkaline luminol solution

Sulphur Dioxide

Sulphur dioxide is the major component of gaseous sulphur oxide gases. Like NOX group oxides of sulphur also lead to aggravation of asthma and other respiratory complications in vulnerable sections of population which include elderly and children

Sulphur dioxide emissions result from fossil fuel combustion in power plants and industrial operations such as extraction of metals from ores.

The fluorescence method excites the [Latex]SO_2[/Latex] molecule to [Latex]SO_2*[/Latex] by absorption from a pulsed UV source. The intensity of fluorescence on relaxation of the excited molecule at low pressure is proportional to the [Latex]SO_2[/Latex] concentration. The method serves to determine [Latex]SO_2[/Latex] in the ppm level range.

Polycyclic Aromatic Hydrocarbons (PAH’s)

Polycyclic aromatic hydrocarbons contribute to water and soil pollution and result from incomplete combustion of coal oil or gas. They are also present in vehicle exhaust, coal tar, eating foods cultivated in contaminated soils or consuming highly grilled meat products. Most polycyclic aromatic hydrocarbons exist in the environment for extended periods and do not break down easily. PA H’s are known for their toxic and carcinogenic properties. The fluorescence properties of fused rigid polycyclic rings are well known.
Chromatographic separation procedures followed by fluorescence detection offers a highly sensitive technique for separation and identification of polycyclic aromatic hydrocarbons.