Sampling and Measurement for Volatile Organic Compounds

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

Volatile Organic Compounds 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

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

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

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

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

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

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

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

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

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