Predicting the persistence of explosives materials

Trace quantities of explosives left behind by those handling explosives materials present an opportunity to identify both the handlers and the objects they have contaminated. Understanding the evolution of these particles is critical for tailoring detection strategies of optical techniques as well as non-optical contact harvesting methods. We are working towards a complete particle persistence model that captures the contribution of environmental factors such as temperature, airflow, and humidity as well as physical factors such as vapor pressure, particle size and inter-particle spacing to predict particle lifetimes for explosives and other chemicals. Our approach involves depositing particles onto glass substrates using particle sizes and loadings known to be deposited by fingerprint deposition, and then studying their behavior in a custom flow cell with controlled airflow, humidity and temperature. Optical microscope images of the sample taken at fixed time intervals are analyzed to monitor particle sublimation, and those images used to determine the mass loss as a function of time. The data are then fit to a model and from the fitting constants the sublimation rate is calculated. We find that the measured sublimation rate exhibits the expected dependence on vapor pressure for a given material, and that the dependence on vapor pressure is largely material independent. We focus on the behavior of a model material, 2,4-dinitrotoluene and select explosive materials under controlled conditions. We are able to use the data from 2,4-dinitrotoluene to predict the behavior of 2,4,6-trinitrotoluene using the physical properties (e.g., vapor pressure) of the respective materials and compare it to experimental results.