Odor detection

In some of the alternative energy processes, such as in fuel cell control and optimization, where the fuel gas is generated by wastewater digesters or garbage dumps, odor can indicate both the type and the concentration of these gases. These concentrations can be in the parts per billion (ppb) and parts per trillion (ppt) ranges, so extremely sensitive detectors are needed to measure them. 

Odor is a sensation associated with smell, which can be hard to quantify. The same quantities of different materials cause different odor intensities. The unit of odor intensity (W) is based on the odor of tertiary butyl mercaptan or TBM (W = 1.0). Using that reference, H2S, for example, has an odor intensity of W = 0.08, or 8% of TBM. Most odorant substances contain sulfur. Table 3.31 lists a number of odorant substances and their relative odor intensities (W). 

Recent improvements in technology and increased research in the area of instrumentation have made dramatic improvements in the creation of instruments that are capable of surpassing the human olfactory system. In fact, the instruments of today are approaching the sensitivity of the canine olfactory system, which is thought to be as much as a million times more sensitive than that of humans. 

Electronic nose technology relies on the use of solid-state sensors, which can either be chemoresistors, chemodiodes, or electrodes. Primary odors are 

Post-Oil Energy Technology After the Age of Fossil Fuels 

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Elemental fluorine and the fluoride ion are highly toxic. The free element has a characteristic pungent odor, detectable in concentrations as low as 20 ppb, which is below the safe working level. The recommended maximum allowable concentration for a daily 8-hour time-weighted exposure is 1 ppm. 

The odor detection-threshold values of organic compounds, water, and mineral oil have been determined by different investigators (Table 2 and 3) and may vary by as much as 1000, depending on the test methods, because human senses are not invariable in their sensitivity. Human senses are subject to adaption, ie, reduced sensitivity after prolonged response to a stimulus, and habituation, ie, reduced attention to monotonous stimulation. The values give approximate magnitudes and are significant when the same techiriques for evaluation are used. Since 1952, the chemistry of odorous materials has been the subject of intense research (43). Many new compounds have been identified in natural products (37—40,42,44—50) and find use in flavors. 

Table 2. Odor Detection Threshold Levels in Water and Mineral Oil, ppm...

Table 3. Odor Detection Thresholds of Organic Compounds ...

Formaldehyde causes eye, upper respiratory tract, and skin irritation and is a skin sensitizer. Although sensory irritation, eg, eye irritation, has been reported at concentrations as low as 0.1 ppm in uncontrolled studies, significant eye/nose/throat irritation does not generally occur until concentrations of 1 ppm, based on controlled human chamber studies. Odor detection has commonly been reported to occur in the range of 0.06—0.5 ppm (133—135). 

While discussing ethers we should mention that the presence of unreacted anisoles or methyl anisoles is highly undesirable in the manufacture of phenol-formaldehyde resoles. These materials tend to be unreactive relative to phenol under normal resole conditions. They are also volatile and have odors detectable at very low concentrations. They have been the source of worker complaints and costly claims in the wood products industry. Benzophenones and methyl phenyl ketones are also common phenol contaminants that are problematic in this regard. 

Table XL Taste/Odor Detection of Additives in Cola Beverage Minimum Detectable in Detection by ...

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