The scale is 0 to 5 , and extrapolations to 0 are used to predict the ODTC50. This approach has led to good agreement with ODTC50 values from the literature. Another odor-intensity measurement system is used more widely than OPM. ASTM Standard E544 uses various vials with dilutions of n-butanol in water to assign n-butanol-equivalent concentrations to the intensity of a given sample. The upper limit of this equivalency scale appears to be the saturation limit of n-butanol in water rather than a sensory upper limit. The intensity of the odor sample is expressed in mg/L of n-butanol, with a larger value of indicating a stronger odorant. Whole-sample, undiluted total odor analysis is relatively straightforward. Determining the odor notes can lead to indications of the source and even a subset of the individual odorants. An odor-intensity rating of the overall mixture, however, is more controversial and less useful. Duration information can be added to a decision-making matrix, too. For example, a field panel made observations using OPM at three off-site and four on-site locations at a trash transfer station .As an odor mixture is diluted, the odor notes and hedonic tones change and eventually, after sufficient dilution, the concentration of the final detectable odorant drops below its odor detection threshold so the diluted sample becomes “odorless” . The amount of dilution required to reach this point is considered an indicator of the odor intensity of the initial, undiluted sample, which is problematic because the final detectable odorant may not be indicative of the odorant that dominated the odor of the undiluted sample . Relying on dilution quantities to indicate the intensity of the total odor is crude at best and misleading at worst. Further, presenting a dilution quantity as a measure related to the mass of odorants in the undiluted sample,mobile vertical rack when in reality it measures only the final detectable odorant, adds an unknown amount of uncertainty to such claims. Portable dilution instruments can be used by field investigators . In the United States, such instruments include the Nasal Ranger® and Scentroid SM100 .
Both mix the ambient air being sampled with odorless air at variable ratios.This term tends to be reserved for field measurements, while OU tends be reserved for indoor panels. Both are dilution levels and not mass-based concentrations. A mixture of odorants was tested using both devices . The Nasal Ranger®, which has settings from 2 to 60 dilutions, performed well between 3 and 30 dilutions. For the Scentroid SM100, which has set points from 3 to 101 dilutions, the settings were about half what the test actually showed, possibly due to odorant sorption to internal surfaces. Field dilution devices avoid the need for sample collection, storage and transport. They may have sorption issues, however, and appear to be better suited for low odor concentrations.Field odor measurements may also involve a panel, such as the use of OPM at an impacted school near a landfill site in California , a trash transfer station in California and a landfill in France . For the landfill study , an abbreviated version of the grid and plume methods discussed next were applied on a single day as well as OPM. The dominant odor notes were “rotten vegetable” and “rancid,” which had high or medium odor intensities. According to the landfill odor wheel, the associated odorants were fatty acids and sulfur compounds , which have very low odor thresholds. Confirmation of odorants by GC-MS-sensory was not performed. In Europe, field panels are central to the grid and plume methods . Figure 3.6 includes general guidance for such a field panel, Figure 3.7 shows how the grid method is applied, and Figure 3.8 shows how the plume method is applied. Both methods require trained panelists to decide whether they recognize an odor note selected from a list. The grid method is applied over a sufficiently long period of time to provide a representative map of the exposure of the population to recognizable odors. Field panelists write down their observations every 10 seconds for 10 minutes . If 6 of those observations are a recognized odor note, then the label “odor hour” is applied . The plume method is used to determine the area in which an odor plume can be perceived under specific meteorological conditions.
The odor-plume boundary is where the odor no longer is detectable, and panelists mark yes/no on a map as they walk through and out of the plume. Adding OPM to the grid or plume method provides an intensity scale and can indicate suspected odorants from odor wheels .To confirm and support the sensory analysis of environmental odor exposures, traditional analytical chemistry air monitoring methods are used. A substantial review of analytical and sensory methods for odor measurement was conducted previously . The methods that have advanced since then are the focus of this section . The other methods are covered briefly for completeness. The comparative advantages and disadvantages of these methods are discussed in Section 4.4. Chemical analysis is most appropriate in cases where known single odorants are responsible for an odor, as opposed to diverse mixtures of odorants. The list of odorous compounds that may be measured is virtually endless. For example, over 400 odorants were detected from swine facilities . Although the level of each odorant was low, the overall mixture led to extremely strong odor intensities. In this case and others, sensory measurements lead to better estimates of odor intensity than analytical measurements. Analytical measurements are only performed for risk assessment when method detection limits are sufficiently low to be below the hazard benchmarks of concern. To achieve such, the human nose is typically required for odor assessment. Although the identification and quantification of specific odorants does not directly indicate the potential odor nuisance, the information is useful for identifying and tracking odor sources . Further, it can help indicate the reactions leading to odorant formation, especially microbial reactions at WWTPs, landfills and composting sites.Gas chromatography, which separates and quantifies odorants, is useful for complex mixtures of chemicals at trace levels, especially on-site where concentrations are higher. Recent advances include two-dimensional and multidimensional gas chromatography , which decrease the analytical problems associated with peak overlap.
Both aid in odorant identification. Although detection reaches ppb levels , trace odorants still go undetected, as do odorants that are unstable during sample collection and transport . Identification of unknown peaks from gas chromatography is typically by mass spectrometry and its libraries of thousands of known compounds. However, even knowing the identity of an odorant does not tell how it contributes to the overall odor of a mixture. Such instruments are expensive, as is there operation and maintenance.A new, albeit even more expensive, instrument has been used for odor investigations called “selected ion flow tube mass spectrometry” . It is transportable and can detect and quantify the concentrations of 20 to 50 odorants real-time,vertical grow rack even if the levels are changing rapidly. The SIFT-MS instrument directly measures components of the air by first using chemical ionizing agents on the sample followed by mass spectrometry . The chemical ionizing agents include cations and anions . The ions are generated at the inlet by a microwave-powered ion source of moist air. The analyte concentration is found from the ratio of the product ion counts to the reagent ion counts, the flow rate, and instrument calibration. Low-ppb detection has been achieved.Gas chromatography with a sensory port , often performed in tandem with mass spectrometry , is a hybrid technique that brings together the separation of odorants and the sensitivity of the human nose. The sensory port allows the analyst to smell the eluting compounds at the same time the instrument detector makes a reading. When successful, it can indicate which odorants contribute to the total odor. Recent advances in GC-sensory methods include improved GC-port interfaces, increasing the number of simultaneous panelists , bi-dimensional GC techniques and sophisticated data processing . Disease detection is an emerging use of GC-sensory.Gas-specific sensors can target key odorants but not the total odor. They are often portable, relatively inexpensive, and continuously log data. The most common gas-specific sensors are for hydrogen sulfide and ammonia. Detection is through chemical, electrochemical, catalytic or optical signals. Some can reach ppb levels. Hydrogen sulfide, however, does not account for the entire odor nuisance. At WWTPs, hydrogen sulfide levels can be well controlled and monitored continuously, yet nuisance odor complaints persist . Benzene, a carcinogen, is a problem emitted from oil refineries and gasoline stations, as well as from the semiconductor industry. Advanced sensors using metal-oxide detectors have been developed that can work in various levels of humidity and interferants .Improved sensor technologies and advanced computational techniques have merged to produce non-specific gas-sensor arrays that try to mimic the human sense of smell. Often called an “electronic nose” or “e-nose,” a bank of up to 30 sensors generates a complex electronic signal that is processed through computer algorithms.
The result is a reading – but not a true “fingerprint” – for a known odor that then can be compared to signals from future samples to see if they match. When properly calibrated, e-noses should continuously detect the presence of odors in ambient air, estimate concentrations of odors, and attribute the odor to a specific odor source . The sensors are typically a variety of metal oxides, conducting polymers and oscillating quartz crystals; however, new sensor materials are under development continually. As with all sensors, they are subject to the effects of temperature and humidity, degradation, poisoning and the need for frequent re-calibration to address drift. It is difficult to find e-noses used outside of research laboratories , which confirmed the observation by Muñoz et al. that their initial promotion had been overly optimistic. Nonetheless, e-noses developed within laboratories, plus accompanying field tests, have led to numerous publications and several recent reviews of the emerging field. Under controlled situations, e-noses have monitored odors. Australian researchers observed that the e-nose for chicken odor worked in-shed yet was unreliable beyond the shed . Today’s e-noses function well for the context for which they are designed but do not yet cover broad environmental odor monitoring . A workgroup in Europe was launched in 2015 to develop a standard for e-noses . One area under development is stack monitoring, where the conditions are more predictable than ambient monitoring yet harsh on the equipment. Producing minimum performance standards and other essential criteria will help guide the field. Unlocking the molecular features that trigger our sense of smell may someday lead to improved e-noses. Keller et al. supplied chemoinformatic data and sensory data on 407 molecules to teams so they could develop predictive algorithms. The algorithms were tested on 69 molecules, and the results were favorable for 8 odor notes out of 19 total. With successful reverse-engineering of the smell of a molecule and then combining that with appropriate sensors, a true e-nose that fully mimics the human nose may be achieved some day. Other technologies have adopted the “e-nose” name, such as portable, fast gas chromatographs or mass spectrometers . Even “electronic mucosa” is under development. In the nasal cavity, natural mucosa acts like the stationary phase of a gas chromatography column to differentially apportion odorants. “Electronic mucosa” consists of multiple sensor arrays, each separated by gas-chromatograph-like micro columns. The rich data set obtained could predict the presence of odorants at low concentrations .No single approach can successfully address nuisance odor complaints . Human panels provide some of the strongest information yet, due to the variable perception of odors, yield inconsistent results. Chemical analysis provides confirmation of exposure to specific odorants yet may miss the key odorants. Instead of a single method, the right mix of sensory and analytical methods needs to be used . The Odor Profile Method followed by GC-sensory conformation provides one of the strongest tools today. The use of standard odorants to calibrate panelists has been advocated since the 1970s . The use of n-butanol, however, has not led to transferability of results to non-butanol odors according to a review of 412 odor measurements .