Quantitative calibration of vapor levels of TNT, RDX, and PETN using a diffusion generator with gravimetry and ion mobility spectrometry.

A prototype generator for creating a continuous stream of explosive vapor was referenced quantitatively both to a standard weight from the National Institute of Standards and Technology (NIST) and to the response of an ion mobility spectrometer. Vapors from solid explosive, in a precision bore glass tube at constant temperature, diffuse into an inert gas flow. Mass output rates were determined by (1) sample temperature, and (2) sample tube dimensions (length and cross-sectional area). A reference to NIST was achieved gravimetrically though a microbalance calibrated with a reference weight; mass output rates were obtained for 2,4,6-trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX) and pentaerythritol tetranitrate (PETN) at three or more oven temperatures between 79 degrees C and 150 degrees C. The mass output rate was stable over hundreds of hours of continuous operation and the output was adjustable from a few picograms per second to several nanograms per second through variation of the oven temperature. An independent calibration of the vapor generator for TNT at 79 degrees C using an ion mobility spectrometer matched exactly the gravimetric-based findings. In most instances, measured mass output rates compared favorably with theoretically calculated mass output rates, with discrepancies in a few cases resulting primarily from uncertainties in terms (vapor pressures and diffusion coefficients) used to perform the calculations. Agreement is generally not good for PETN, where molecular decomposition contributed to higher than expected measured mass outputs.