Detection, Identification and Quantitation of Odorous Emissions Using Whole Air Sampling, Gas Chromatography/Mass Spectrometry and Semiquantitative Olfactometry
Abstract:Emission control strategies are commonplace in the operation of publicly owned sewage treatment plants, sanitary landfills, animal feeding operations and chemical plants. Emissions are often regulated under air toxics regulations, photochemical air pollution abatement regulations, or nuisance abatement regulations. Regulation and control of emissions under the first two categories usually involve control of specific compounds or classes of compounds. Odorous emissions are regulated under the latter type of rule. However, the control of odors is not as straightforward as the control of toxic or ozone-forming emissions. There is no one class of odorous compounds. Many sulfur- and nitrogen-containing compounds have strong odors, as do some oxygenated compounds such as carboxylic acids, esters, and aldehydes. Many hydrocarbons, including substituted aromatics and terpenes, can also produce strong odors. In addition, structurally similar compounds may have markedly different odor strengths.
Since an odor source can be very complex, an effective control strategy requires knowledge of the individual odorants in the gas emissions stream and their individual contributions to the overall perceived odor. If, for example, one or two high concentration components account for most of the odor, control of those compounds alone might suffice. However, if the overall odor is due to the contribution from many compounds that are present at relatively low concentrations, a quite different control strategy could be required. An analytical method for odor assessment must therefore be able to separate the compounds present in a gas stream, allow the compounds to be identified, and allow the odor strengths of the individual components of the mixture to be assessed. The method must also possess high sensitivity since many potential odorants have detection thresholds at parts-per-trillion levels.
Olfactometry has been paired with gas chromatography/mass spectrometry (GC-MS) in many instances to both determine the overall odor strength and identify odorants in gas stream samples. Odor strengths may be measured by ranking the overall odor of the sample on some arbitrary intensity scale, or more typically, by successively diluting the sample until the human subjects of an odor panel no longer detect the sample odor, thus providing a value for the odor threshold dilution ratio. The latter method is used more frequently because it is easier to perform, provides more reproducibility, and gives a numerical measure of odor strength. The primary drawback to this method is that it does not indicate how the odor is perceived at its original, undiluted concentration. GC-MS analysis can be used to identify the odorants present in a gas sample, but only if these odorants can be distinguished from non-odorant compounds that are also present. Many studies of this type have been performed to measure odor strengths and identify odorants from such sources as agricultural composts and sewage treatment plants (i.e., Noble, et al., 2001, Godlewski, et al., 2001, Zhuang, et al., 2001).
Olfactometric methods may also be directly applied to the outlet flow from a gas chromatograph (GC) column, allowing the individual odorants of a mixture to be characterized by a human subject and identified by chromatographic retention time and/or by mass spectral analysis. This technique has been used to successfully identify odorants in drinking water (Ginzberg, et al., 1998) and is commonly used in the food and fragrance industries to provide a “fingerprint” of an aroma in which odorant concentrations and odor intensities are known (Friedrich and Acree, 1998). We have adapted this method to the characterization and quantitation of odorants in whole air samples. Samples are taken in fused-silica-lined stainless steel canisters, followed by analyte concentration using a preconcentrator system equipped with cryogenic and adsorbant traps. The concentrated analytes are then thermally desorbed onto the column of a GC-MS system to effect the separation and identification of the components. Part of the column outlet flow is diverted to an olfactory detector, allowing a human subject to describe the specific odor characteristics and hedonic tone (agreeability) for each of the separated components. In addition, the subject can provide a quantitative evaluation of the odor intensities for the individual components. We have used this system to evaluate the effectiveness of different biofilter media in removing odorous compounds from air taken from the solids processing facilities of the Joint Water Pollution Control Plant (JWPCP) in Carson, California.
Document Type: Research Article
Publication date: 2004-01-01
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