There are many applications for the detection of trace amounts of specific organic substances, which require a combination of high sensitivity, high specificity and a short time of measurement. Included in these applications are the detection of illicit drugs, concealed explosives and chemical agents, etc. For some applications it is advantageous to simultaneously detect the presence of one or more of a common family of vapours, (e.g. of organonitrate explosives, or of illicit drugs, etc).
Some of the substances referred to above have a sufficiently high vapour pressure to produce detectible levels of their vapours at room temperature (e.g. EGDN and TNT, etc.). Other substances have very low volatility (e.g. cocaine, heroin and plastic explosives) and must be gathered in the form of particulates, and then heated to produce vapours for detection and analysis.
Ultimately, however, all detection devices for these applications operate on a vapour phase detection basis wherein the required threshold sensitivity of detection is commonly in the parts-per-billion, or even part-per-trillion level, in the original sample of air. Since the ultimate detector for the target organic vapours may not have sufficiently high intrinsic sensitivity, it is common practice to use a preconcentrator device. This device serves to raise the level of the target vapours by collecting them over a period of time, from a relatively large volume of air, and then releasing them into a much smaller volume of air or carrier gas for detection.
Essential characteristics of a preconcentrator for specific target vapours are a high efficiency in the collection of these vapours from the incoming volume of air, and a high degree of specificity, (i.e. preference for the collection of the target vapours), in the presence of a large quantity of extraneous substances in the original sample volume. In addition, where a short time of measurement is essential for a specific application, the time of release (desorption) of the target vapours from the preconcentrator must be kept very short.
Adsorbers of vapours are well known in the art of trace vapour analyzers, and are commonly employed for the purposes of preconcentration of selected vapours. Examples of such prior art adsorbers are disclosed in References 1-8 listed in Appendix A to this disclosure.
All such adsorbers rely on the absorption of the target vapours in a substantial volume of the adsorbent material. This creates a problem in respect of the speed of desorption of the vapours, because of the appreciable thermal mass of the adsorber material. A large thermal mass requires an appropriately large infusion of heat to raise it to the proper temperature of desorption. This also means that the adsorber will require a relatively long time to cool down to an effective adsorption temperature after desorption, thus creating unproductive time delays between successive measurements.