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Sampling and Measurement for Volatile Organic Compounds.

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Presentation on theme: "Sampling and Measurement for Volatile Organic Compounds."— Presentation transcript:

1 Sampling and Measurement for Volatile Organic Compounds

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

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

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

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

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

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

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

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

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

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

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

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