Odor - The Electronic Nose Artificial Olfaction Technology - page 94

odors can differ along many dimensions. One way to evaluate quality is to ask
panelists for judge similarity between a test sample and various reference samples,
using a five-point category scale. For certain applications, reference odorants can
be chosen to represent only that portion of the qualitative range relevant to the
odor problem under investigation (e.g., animal odors). Another procedure is to ask
panelists to assess the degree of association between a test sample's quality and
certain verbal descriptors (e.g., sweaty, woody, chalky, sour). An odor can be
characterized either by an open-ended word description or by multidimensional
scaling. Multidimensional scaling is based on similarity and dissimilarity judg-
ments in comparison to a set of standard odors or to various descriptors. In some
instances, the interest may be merely whether an odor's quality has changed as a
result of some treatment (e.g., use of a bacteriostat). Under these circumstances,
samples of air taken before and after treatment can be compared directly (using a
simple scale of similarity) or indirectly (with appropriate verbal descriptors).
4.5.6 Odor Unit
Odor concentration can be expressed as the number of unit volumes that a unit
volume of odorous sample occupies when diluted to the odor threshold with non-
odorous air. If a sample of odorous air can be reduced to threshold by a tenfold
dilution with pure air, the concentration of the original sample is said to be 10 odor
units. Hence, odor units are equivalent to multiples of threshold concentrations.
Odor units are not units of perceived magnitude. The ''odor unit'' seems to be the
most common index for odor emission control. A number of states in the US have
a source emission standard. However, there are problems in using the odor unit as
a standard: (1) because of the variability of people, who serve as the detectors for
generation of the odor unit, data vary from laboratory-to-laboratory, and (2) the
odor unit includes no measure of the importance of the odor.
The European Odor Unit (OUE) is defined in terms of N-butanol.
To calculate the European Odor Units [ 17 ]:
1. Determine concentration of n-butanol at its Odor Detection Threshold (ODT b ).
This is the Odor Detection Concentration for n-butanol (ODC b ).
2. Determine the Odor Units for the ''mixed sample'': this is the Odor Detection
Threshold of the unknown sample adjusted to the Odor Detection Concentra-
tion for n-butanol
OUE = (ODT X ODC b )/40 ppb
OUE = European Odor Units
ODT = Odor detection threshold (ratio) of the sample
ODCb = Odor concentration of n-butanol at its detection threshold
40 ppb = the ''definition'' of 1 OUE in terms of n-butanol