The sniffer

In the most general sense, a sniffer is a physical device that can record, or digitize, odorants. In other words, it takes chemical data and turns it into numbers. Upon the introduction of an odorant in its inlet, the sniffer produces a numerical output, which becomes (usually after some further manipulation) a representation, or a fingerprint, of the odorant. To be useful in our odor communication system, we shall further require from a sniffer to be sufficiently discriminatory, in that it produces unique fingerprints for all odorants. Moreover, we would like its fingerprints to exhibit some correlations with the smell perception of their sources. Any instrument that quantifies a certain property of chemicals in a unique and reproducible way suffices. In principle, an apparatus capable of measuring the boiling point of an odorant could become a sniffer. However, we can expect the correlation of boiling points with odor perception to be rather difficult.


A more realistic example is the combination of a gas chromatograph (GC) and a mass spectrometer (MS). The GC/MS combination is very popular in analytical chemistry, and is used to precisely identify the compounds of a mixture. However, we doubt that it would make a good sniffer, since we have no reason to believe that the output it produces has anything to do with smell perception. From a commercial point of view, GC/MS suffers from additional disadvantages: it is expensive, it is large and bulky, and it is complicated to use, requiring carefully-trained operators. Moreover, analyzing its results is time consuming, and often sample preparation is tedious too. In our opinion, the best candidates to serve as sniffers are the instruments collectively grouped under the term electronic noses (e Noses). These are analytic devices, whose main component is an array of non-specific chemical sensors, i.e., sensors that interact with a broad range of chemicals with varying strengths. Consequently, an incoming analyte stimulates many of the sensors in the array, and elicits a characteristic response pattern. These patterns are then further analyzed for the benefit of the specific application. The fact that the biological smelling system also relies on an array of non-specific receptors, gives hope that we may be able to find significant relationships between the biological nose and its artificial counterpart. The usual chemical sensors are replaced by biosensors that are supposed to work in essentially the same way as the biological receptors in the nose. From a commercial point of view, e Noses enjoy several desired properties: they can be made small and cheap; they are easy to use, fast to operate, and for most applications they do not require any special sample preparation.

In the electronic realm, as in the biological one, the desire for sensitivity does not always go well with the desire for non-specificity. Sensors (or receptors) that are designed to respond to an assortment of stimuli are normally characterized by low sensitivity. Indeed, e Noses are typified by relatively high detection thresholds, on the order of 1-10ppm. Although seemingly problematic, this is not a true stumbling block for an odor communication system. First, many odor sources release higher concentrations than this in their immediate vicinity. Second, a preliminary step of concentration enrichment can be always carried out if necessary.