Advances in sublimation studies for particles of explosives

Abstract

When handling explosives, or related surfaces, the hands routinely become contaminated with particles of explosives and related materials. Subsequent contact with a solid surface results in particle crushing and deposition. These particles provide an evidentiary trail which is useful for security applications. As such, the opto-physico-chemical characteristics of these particles are critical to trace explosives detection applications in DOD or DHS arenas. As the persistence of these particles is vital to their forensic exploitation, it is important to understand which factors influence their persistence. The longevity or stability of explosives particles on a substrate is a function of several environmental parameters or particle properties including: Vapor pressure, particle geometry, airflow, particle field size, substrate topography, humidity, reactivity, adlayers, admixtures, particle areal density, and temperature. In this work we deposited particles of 2,4-dinitrotoluene on standard microscopy glass slides by particle sieving and studied their sublimation as a function of airflow velocity, areal particle density and particle field size. Analysis of 2D microscopic images was used to compute and track particle size and geometrical characteristics. The humidity, temperature and substrate type were kept constant for each experiment. A custom airflow cell, using standard microscopy glass slide, allowed in-situ photomicroscopy. Areal particle densities and airflow velocities were selected to provide relevant loadings and flow velocities for a range of potential applications. For a chemical of interest, we define the radial sublimation velocity (RSV) for the equivalent sphere of a particle as the parameter to characterize the sublimation rate. The RSV is a useful parameter because it is independent of particle size. The sublimation rate for an ensemble of particles was found to significantly depend on airflow velocity, the areal density of the particles, and the particle field size. To compare sublimation studies these parameters must be known.

Persistence of explosives under real world conditions

“…To a first approximation, the sublimation results for DNT can be applied to TNT or other explosives by using a relative vapor pressure correction factor. Our previous report confirmed that, as expected, increasing airflow resulted in increased sublimation rates, as vapor molecules removed from near the surface of the particle increased the net flux of molecules escaping the surface layer 12 . We discovered a linear relationship between the airspeed and a particle-size-independent term we defined as the radial sublimation velocity (RSV), which is defined in the following section.…”

“…The persistence, or lifetime, of particles of explosives on a surface is a limiting factor for security applications. A range of environmental parameters and particle properties, shown in Figure 1, influence particle sublimation including: inherent chemical vapor pressure, particle size and geometry, airflow, substrate topography, relative humidity, reactivity, adlayers, admixtures, particle areal density and temperature [9][10][11][12] . In a previous work 12 , we have specifically studied the effects of areal coverage, airflow velocity and particle field size on sublimation of 2,4-dinitrotoluene (DNT) particles on glass substrates.…”

“…The airflow for the studies reported in this paper was set to 0.5 mph, a value calculated to equal the human thermal plume generated under static ambient conditions 12,[14][15][16] . A separate gas line was available to introduce a controlled amount of water vapor into the flow channel.…”

“…However, it cannot be relied upon completely without confirmation using alternative means. We have developed a simple, non-destructive method for quantifying individual test coupons using custom software to analyze optical micrographs 12 . Trying to determine the three-dimensional volume (and mass) of an ensemble of particles based on just a two-dimensional photograph of the sample may seem fraught with error, but as we have previously shown, it can be performed with good results 8,12 .…”

“…We have developed a simple, non-destructive method for quantifying individual test coupons using custom software to analyze optical micrographs 12 . Trying to determine the three-dimensional volume (and mass) of an ensemble of particles based on just a two-dimensional photograph of the sample may seem fraught with error, but as we have previously shown, it can be performed with good results 8,12 . The software incorporates corrections for calculating the volume of non-spherical particles and for discrimination the boundaries of particles in contact with each other.…”