Concentration-detection functions for the odor of homologous n-acetate esters

Abstract

Using air-dilution olfactometry, we measured concentration-response functions for the odor detection of the homologous esters ethyl, butyl, hexyl, and octyl acetate. Stimuli were delivered by means of an 8-station vapor delivery device (VDD-8) specifically designed to capture odor detection performance by humans under environmentally realistic conditions. Groups of 16-17 (half female) normosmic (i.e., having a normal olfaction) non-smokers (ages 18-38) were tested intensively. The method involved a three-alternative forced-choice procedure against carbon-filtered air, with an ascending concentration approach. Delivered concentrations were confirmed by gas chromatography before and during actual testing. A sigmoid (logistic) model provided an excellent fit to the odor detection functions both at the group and individual levels. Odor detection thresholds (ODTs) (defined as the half-way point between chance and perfect detection) decreased from ethyl (245 ppb by volume), to butyl (4.3 ppb), to hexyl acetate (2.9 ppb), and increased for octyl acetate (20 ppb). Interindividual threshold variability was near one and always below two orders of magnitude. The steepness of the functions increased slightly but significantly with carbon chain length. The outcome showed that the present thresholds lie at the very low end of those previously reported, but share with them a similar relative trend across n-acetates. On this basis, we suggest that a recent quantitative structure-activity relationship (QSAR) for ODTs can be applied to these and additional optimized data, and used to describe and predict not just ODTs but the complete underlying psychometric odor functions.

Figure 1

Group psychometric odor functions (left) and confidence ratings as a function of concentration (right) for the four acetates. For ethyl, hexyl, and octyl acetate, each point represents the outcome of 560 trials made by 16 subjects. For butyl acetate, each point represents the outcome of 595 trials made by 17 subjects. In both graphs, bars depict standard error (SE).

Figure 2

Individual psychometric odor functions for ethyl acetate fitted by the sigmoid Equation (2). Each point represents the outcome of 35 trials made by that subject.

Figure 3

Same as in Figure 2 but for butyl acetate.

Figure 4

Same as in Figure 2 but for hexyl acetate.

Figure 5

Same as in Figure 2 but for octyl acetate.

Figure 6

Comparison between the odor detection thresholds for acetates from Cometto-Muñiz and Cain, 1991 and those from the present study. Carbon chain length refers to the number of carbon atoms in the variable section of the molecules (e.g., 2 = ethyl acetate, 4 = butyl acetate, etc.). Bars depict standard error (in the case of the present data they are mostly hidden by the symbol).

Figure 7

Representing the odor detection thresholds reported for the acetates in each of the studies compiled by Devos et al., 1990 (empty symbols) and by van Gemert, 1999 (filled symbols). (Values from the different studies in each compilation are spread out along the x-axis for clarity.) The crosses represent the ODTs measured in the present investigation.

Figure 8

Comparison between observed (experimental) and calculated (from Equations (3) and (4)) odor psychometric plots for acetates, alcohols, and ketones. To facilitate visual comparison of relative potency across odorants, the vapor concentration range (x-axis) is the same in all plots.