Olfactory detectability of homologous n-alkylbenzenes as reflected by concentration-detection functions in humans

Abstract

As part of our systematic exploration of chemical determinants for the olfactory potency of vapors towards humans, we measured concentration-detection functions for the odor of the homologous n-alkylbenzenes toluene, ethylbenzene, butylbenzene, hexylbenzene, and octylbenzene. A vapor delivery device based on dynamic olfactometry and calibrated by gas chromatography, served to test groups of 16 to 17 participants. Subjects were young adults from both genders, normosmics, and nonsmokers. Odor functions were tightly modeled by a sigmoid (logistic) function, both at the group and the individual level. Odor detection thresholds (ODTs), defined as the concentration producing a detectability halfway between chance and perfect detection, decreased with alkyl chain length from toluene (79 ppb) to butylbenzene (2.5 ppb), and then increased form butyl to octylbenzene (89 ppb). The "U"-shaped trend of ODTs as a function of alkyl chain length indicated a loss of odor potency beyond a certain molecular size, a phenomenon recently described for chemosensory irritation (chemesthesis) and that will need consideration in structure-activity models of chemosensory potency. Interindividual ODTs' variability for any single odorant amounted to one order of magnitude, in agreement with recent studies of other homologous series but quite smaller than commonly depicted.

Figure 1

Plots of group psychometric odor detection functions (left graph) and confidence ratings as a function of concentration (right graph) for each alkylbenzene. For toluene, butyl benzene, and hexyl benzene, each point represents the outcome of 560 trials provided by 16 subjects. For ethyl and octyl benzene, each point represents the outcome of 595 trials provided by 17 subjects. Bars indicate standard error (SE). Detectability functions (left graph) were fitted by the sigmoid equation (2).

Figure 2

Plots of individual psychometric odor functions for toluene, fitted by the sigmoid equation (2). Each point in a graph represents the outcome of 35 trials provided for that concentration by that subject. Each subject was given a unique number, so one can follow the performance of participants tested on more than one alkylbenzene.

Figure 3

Same as in Fig. 2 but for ethyl benzene.

Figure 4

Same as in Fig. 2 but for butyl benzene.

Figure 5

Same as in Fig. 2 but for hexyl benzene.

Figure 6

Same as in Fig. 2 but for octyl benzene.

Figure 7

Comparison of ODTs along homologous alkylbenzenes obtained in this study (2009) with those obtained in a previous study (1994) that delivered vapors via static olfactometry (squeeze bottles) and employed a conservative, fixed-performance criterion (i.e., five out of five correct in an ascending concentration approach) (Cometto-Muñiz and Cain, 1994). Bars indicate standard error (SE) (they are covered by the symbol in the case of the present data). Also shown are the trends in ODTs along homologous n-alcohols (Cometto-Muñiz and Abraham, 2008b), 2-ketones (Cometto-Muñiz and Abraham, 2009), and acetate esters (Cometto-Muñiz et al., 2008) (see text).

Figure 8

Illustration of the range of ODTs for n-alkylbenzenes in air reported in the literature, as compiled by van Gemert (filled circles) (van Gemert, 2003) and as compiled and standardized by Devos et al. (empty circles) (Devos et al., 1990). (Values from the studies listed in each compilation are spread out along the x-axis for clarity.) We also show ODTs obtained by Nagata (triangles) (Nagata, 2003), and those obtained in the present study (crosses).