Effect of evaporation on the limit of detection of explosive traces by active spectral imaging

When developing methods for standoff detection of explosive traces, an important issue is determination of the limit (minimum) amount of a substance available for detection. This limit depends on the features of the measuring complex realization, on the physicochemical properties of the substance, and on the dynamics of sublimation (evaporation) of the substance from the surface. In this work, we consider the problem of detection limit for a measuring complex based on the method of active spectral imaging. In realization of this method, the distance to the object and the measurement time after the appearance at the surface of a substance imprint are important. How long this trace amount of substance can be detected, taking into account its evaporation? An explosive imprint is a thin layer on a surface (usually considered as a fingerprint with traces of the substance). We theoretically consider the dynamics of the change in the area of this imprint on the basis of a mathematical model of evaporation of a thin layer. Based on mathematical modeling, we can estimate the amount of a specific substance available for detection under the given conditions and the availability time. We identified the main parameters that affect the rate of film evaporation and the range of parameter change and calculated the kinetic parameters of sublimation of thin films of some explosives. The performed calculations allowed us to draw conclusions about the detection limit of substances for the active spectral imaging method depending on the distance to the measurement object and the time after the appearance of a surface trace with an initial surface concentration and area specific to a fingerprint.