The four basic techniques for collecting gaseous pollutants are absorption, adsorption, condensation (freeze out), and grab sampling:
1. Absorption Sampling:
It is the process by which a gaseous containment in air is removed by bringing the contaminant into close contact with a liquid chemical with which it reacts to form a non-gaseous substance.
A standard chemical solution is prescribed for each gaseous contaminant to be collected. In this technique, it is important to ensure that all the gaseous contaminants come in close contact with the chemical absorbing solution, and are retained long enough to allow a complete reaction to occur. This is to ensure high collection efficiency.
Image Source : upload.wikimedia.org
2. Adsorption Sampling:
This technique utilizes the phenomenon by which gases are attracted to the surface of a solid and retained there. The total amount of the gaseous pollutant adsorbed is related to the surface area of the adsorbent, the pressure and temperature maintained in the sampling train, and the physical and chemical characteristics of the adsorbent used.
The different adsorbents generally used are activated carbon, silica gel, activated alumina, and a molecular sieve which is synthetic sodium or calcium aluminosilicate shows a sample collecting device and a typical container.
3. Freeze Out or Condensation Sampling:
This method is used to collect hydrocarbons, radioactive gases and insoluble or non-reactive vapours. Air pollutants, existing as gases, can be trapped or removed by the freeze out or condensation method; trapping implies collecting a pollutant, and removal implies freeing unwanted gas contaminants from the gas stream.
The mechanics of the freeze out condensation process is as follows: Air is drawn through collection chambers at a progressively lower temperature; if the chamber temperature is equal to or less than the boiling point of the gas, the gas will change into a liquid.
ADVERTISEMENTS:
This liquid or condensate is collected in the chamber where the phase change from gas to liquid occurs. Sampling trains, using the free-out principle, have the ability to collect several gases at the same time when several collection chambers are provided. A typical freeze-out sampling train.
4. Grab Sampling:
This is another technique for collecting gaseous pollutant samples. A grab sample is taken out at a particular time, within an interval of a few seconds to a minute.
The pollutant-absorbing solution is placed in the sampling apparatus, and the sample collected is allowed to come to equilibrium with the absorbing solution available. A deflated plastic bag evacuated flask gas/displacement collector or liquid/displacement collector are used.