The properties of a compound relating to human olfactory perception includes its odour quality, threshold and intensity as a function of its concentration.
A psychometric function with a log concentration–perceived intensity plot is characterised by its sigmoidal shape, with its initial baseline representing the compound at concentrations below its threshold, a slow rise in response around the inflection point representing the threshold, an exponential rise in response as the concentration exceeds the threshold, a deceleration of the response to a flat region as the zone of saturation or the point at which the change in intensity is no longer perceived is reached.
On the other hand, a log concentration–log perceived intensity plot, using Steven's power law, forms a linear line with the exponent characterising the relationship between the two variables.
[1] The apparatus consists of a gas chromatograph equipped with an odour port (ODP), in place of or in addition to conventional detectors, from with human assessors sniff the eluates.
The odour port is characterised by its nose-cone design connected to the GC instrument by a transfer line.
[5] It is generally placed 30–60 cm away from the instrument, extending from the side such that it is not affected by the hot GC oven.
The deactivated silica transfer line is generally heated to prevent the condensation of less-volatile compounds.
[5] In a GC-O analysis, various methods are used to determine the odour contribution of a compound or the relative importance of each odorant.
The GC-O analysis is carried out by a panel of 6–12 assessors to count the number of participants who perceive an odour at each retention time.
It is also presumed to relate to the intensity of the odorant at the particular concentration, based on the assumption that individual detection thresholds are normally distributed.
The detection frequency method benefits from its simplicity and lack of requirement for trained assessors, as the signal recorded is binary (presence/absence of odour).
However, with a trained panel of assessors, the analysis can be done in a relatively short amount of time with high precision.