Description Type of error
Adaptation and cross adaptation
When exposed continually to a background concentration of an odor, the subject is unable to detect the presence of that odor at low concentrations. When removed from the background odor concen- tration, the subject’s olfactory system will recover quickly. Ultimately, a subject with an adapted olfactory system will be unable to detect the presence of an odor to which his system has adapted.
Sample modification Both the concentration and composition of odorous gases and vapors
can be modified in sample collection containers and in odor detection devices. To minimize problems associated with sample modification, the period of odor containment should be minimized or eliminated, and minimum contact should be allowed with any reactive surfaces.
Subjectivity When the subject has knowledge of the presence of an odor, random
error can be introduced in sensory measurements. Often, knowledge of the odor may be inferred from other sensory signals such as sound, sight, or touch.
Synergism When more than one odorant is present in a sample, it has been
observed that it is possible for a subject to exhibit increased sensitivity to a given odor because of the presence of another odor.
are shown on Fig. 2–18. Field olfactometers are very useful for making odor determina- tions over a large area surrounding a treatment plant. Often a mobile odor laboratory, which contains several types of olfactory and analytical equipment in a single van type vehicle, is used for field sites.
Fixed Olfactometers. Equipment used in a laboratory setting to analyze odors includes (1) the triangle olfactometer, (2) the butanol wheel, and (3) a variety of other specialized olfactometers. The triangle olfactometer enables the operator to introduce the odorous air sample at different concentrations at five or six different cups each equipped with three sampling ports [see Fig. 2–19(a)]. At each cup, two ports contain purified air, and one port contains a diluted sample. Each odor panel member (usually six) then sniffs each of the three ports and must select the port he or she believes contains the sample [see Fig. 2–19(b)]. The procedure is repeated at the remaining four or five cups. The concentration of the odorous air is increased in successive cups, typically doubling in each successive cup (ASTM, 2004). The results are analyzed using a standardized statistical program based on signal detection theory (Green and Swets, 1966).
The Butanol Wheel is a device used to measure the intensity of an odor against various concentrations of n-butanol. The device comprises eight sampling ports located on a rotatable disk [see Fig. 2–19(c)]. Dilutions of n-butanol that increase by a factor of two are delivered to
Figure 2–18
Examples of field hand-held olfactometers used for field odor studies: (a) Scentometer® schematic and front view looking at nose pieces (5 in 3 6 in 3 2.5 in, from Barnebey & Sutcliffe Corp.) and (b) Nasal Ranger® schematic and pictorial view (from St Croix Sensory Inc.).
Odorous air Graduated series
of orifices Purified air for dilution Nosepieces
Activated carbon bed
(a)
Odorous air Purified air
for dilution
(b)
Air
Air Activated
carbon cartridges
Graduated series of orifices rotated in sequence to control air blend Blended air
to nasal mask
Dilution-to- threshold (D/T) dial
2–4 Inorganic Nonmetallic Constituents 109
each successive port. Each odor panel member first sniffs the odorous sample being tested and then compares it to the various dilutions of n-butanol starting with Port 1 [see Fig. 2–19(d)].
The test is continued until the panelist identifies the n-butanol dilution which most closely matches the intensity of the odorous sample. The results are reported in ppmv n-butanol odor intensity. Application of Butonal Wheel test results is illustrated in Example 2–8.
Figure 2–19
Examples of fixed olfactometers:
(a) schematic of dynamic forced- choice triangle olfactometer, (b) panel member sniffing one of the three sample ports, (c) view of Butanol wheel, (d) panel member sniffing one of sample ports. [Figs. (b), (c) and (d) courtesy of RK & Associates, Inc.]
34 56 2 1 2 3 4 5 6 1
1 2 3 4 5 6
Air to n-butanol vessel
Liquid n-butanol in vessel Sampling cup equipped with three sampling ports
(c) (d)
(a) (b)
EXAMPLE 2–8 Determination of Relative Persistence Intensity measurements were made at
different dilutions for two odor samples. Using the data provided, determine which of the two odors is more persistent.
n-butanol odor intensity, ppmv
Dilution-to-threshold, D/T Sample A Sample B
10,000 0 0
100 25 3.2
10 316 10
0 3160 32
Solution
1. Linearize Eq. 2–54 and log transform the given data.
a. The linearized form of Eq. 2–54 is log I 5 log k 1 n log C
b. The log transformed data are:
Log I
log D/T Sample A Sample B
3 0 0
2 1.4 0.5
1 2.5 1.0
0 3.5 1.5
3. Plot log I versus log C and determine the slope n to determine which sample is more
persistent.
a. The required plot is given below
0 0.5 1 1.5 2 2.5 3 3.5 4
0 0.5 1 1.5 2 2.5 3 3.5
Dilution to threshold, D/T
n-Butanol odor intensity, ppm
Sample A
n = –0.84
Sample B
n = –2.0
b. The slopes for the two samples are
Sample A 20.84 Sample B 22.0
c. Based on the slopes, Sample A is more persistent than Sample B.
Many of the specialized laboratory olfactometers are designed to work in conjunction with instrumental methods of analysis. For example, the Gerstel ODP2® is used in conjunc- tion with a GC or MS chromatograph for the detection of compounds that as they elute from the separation column. Thus, there is a simultaneous instrumental and olfactory characterization of an odorous compounds (Agus et al., 2011).
Instrumental Measurement of Odors. It is often desirable to know the specific compounds responsible for odor. Although gas chromatography has been used success- fully for this purpose, it has not been used as successfully in the detection and quantification