Calibration of an Explosives Vapor Generator Based on Vapor Diffusion from a Condensed Phase

In the field of explosives detection there is currently a need for a calibrated source of explosives vapor. Such a source could be used to test and calibrate explosives detection systems which identify explosives via the collection of vapor or airborne particulate matter. This paper describes the principles of operation and evaluation of one such explosives vapor generator. This generator is based on the diffusion of vapor horn a condensed phase (i.e. solid or liquid) in a source resavoir, and the output has been tied to a National Institute of Standards and Technology (NIST) llMSS standard. We discuss results of the calibration of this generator using the exqdosives 2,4,6- trinitrotoluene (TNT) and cyclonite (R.DX). The mass output of this generator is stable over hundreds of hours of continuous operation, and is adjustable from the low picograms(pg)hec range to at least 10 nanograms(ng)kc. In the case of TNT, the mass output correlates well with predictions based on gas phase diffhsion themy. In the case of RDx the agreement with themy is less good. This maybe attributable to a variety of factors, possibly including inaccuracies in the published data on RDX vapor pressure as a fu&tion of temperature.