Sampling and Measurement for Volatile Organic Compounds

1. Sampling and Measurement for Volatile Organic Compounds

2. Types of analytical instruments • Personal Monitors • Light weight monitors • Can be conveniently carried or worn by a person • Portable monitors • Can be moved during sampling • Stationary monitors • Have to be operated from a fixed location Volatile Organic Compounds

3. Types of Sampling • Active Sampling • Contaminated air is directly drawn into a sensor or collector • Provides a real time analysis of the sample • Passive Sampling • Air diffuses into the collector • Longer period of time must be allowed for accurate results Volatile Organic Compounds

4. Sampling methods • Principles of Sampling Collectors 1. Air displacement 2. Condensation 3. Gas washing or absorption 4. Adsorption Volatile Organic Compounds

5. Air Displacement Sampling Method • Evacuated flasks or plastic bags are used • Samples are collected by opening the inlet to the contaminant-laden air • Sample is subsequently analyzed using a suitable analytical method Volatile Organic Compounds

6. Condensation Sampling Method • Air is passed through a U-tube or a suitable container • Sample is subsequently cooled to below the boiling point of the pollutant • In most cases a liquid nitrogen cryogenic trap is used. • Condensed liquid is eventually analyzed Volatile Organic Compounds

7. Gas washers/ Absorbers Sampling Method • Air containing the pollutants is bubbled through a liquid contained in an impinger • An aliquot of the liquid is analyzed to determine the characteristics and concentrations of the pollutants • Distilled water is commonly used for readily soluble gases • A liquid medium that reacts with the pollutants should be avoided Volatile Organic Compounds

8. Gas washers/ Absorbers Sampling Method • This method is suitable only for a few specific pollutants such as Formaldehyde, Phenol and Ethylene Oxide • Collection Efficiency can be increased by: • Decreasing the flow rate • Improving the distribution of gas phase in the liquid medium • Increasing the residence time • Using two or more collectors in series Volatile Organic Compounds

9. Adsorption Sampling Method • Used for measurement of emissions from materials. • Emissions are measured either in an environmental chamber or through “head space” analysis • Environmental Chamber Studies: • Material is placed in a chamber • Environmental parameters of the chamber (temperature, humidity, air exchange rates) are controlled precisely • Air samples are collected in solid adsorbent tubes for later analysis with a gas chromatograph Volatile Organic Compounds

10. Adsorption Sampling Method • “Head-Space” analysis • Small samples of the material are placed in a container • Head space gases are collected and analyzed by a gas chromatograph • Sampling time depends on the emission rate from the material • Solid adsorption media are more popular • Pollutants are allowed to adsorb for a predetermined period of time on a solid • Most frequently used solid adsorbent is activated carbon and silica gel Volatile Organic Compounds

11. Adsorption Sampling Method • Factors for successful use of adsorbent cartridges • Careful cleaning of the sorbent tube to remove background contamination • Accurate determination of sampling rate • Cleaning of cartridges • Heat them at a high temperature under vacuum or by flowing inert gas through the cartridge • Flow of inert gas should also continue during cooling • Another method is to wash the cartridge with a solvent and dry it at a temp. of 100-150 oC Volatile Organic Compounds

12. Analysis Methodologies • Colorimeters and Spectrophotometers • Used to quantify a specific pollutant • Utilize liquid reagents, chemically treated papers and glass indicating tubes • Chromotropic acid method • Most frequently used to determine the concentration of Formaldehyde • Measured volume of sample air is drawn through distilled water into an absorber • Collection efficiency greater than 95% is achieved by using two absorbers in series Volatile Organic Compounds