Abstract
Hazard detection systems must be evaluated with appropriate test material concentrations under controlled conditions in order to accurately identify and quantify unknown residues commonly utilized in theater. The existing assortment of hazard reference sample preparation methods/techniques presents a range of variability and reproducibility concerns, making it increasingly difficult to accurately assess optically- based detection technologies. To overcome these challenges, we examined the optimization, characterization, and calibration of microdroplets from a drop-on-demand microdispenser that has a proven capability for the preparation of energetic reference materials. Research presented herein focuses on the development of a simplistic instrument calibration technique and sample preparation protocol for explosive materials testing based on drop-on-demand technology. Droplet mass and reproducibility were measured using ultraviolet-visible (UV-Vis) absorption spectroscopy. The results presented here demonstrate the operational factors that influence droplet dispensing for specific materials (e.g., energetic and interferents). Understanding these parameters permits the determination of droplet and sample uniformity and reproducibility (typical R2 values of 0.991, relative standard deviation or RSD ≤ 5%), and thus the demonstrated maturation of a successful and robust methodology for energetic sample preparation.
Highlights
The development of systems capable of detecting and identifying explosive materials on surfaces at range is a priority for the United States Army
It was important to have a working knowledge of how the drop-on-demand instrument settings, analyte concentration, and inkjet printing solvent viscosity, influence the character of droplets produced
Under the conditions tested it was found that in order to maintain a consistent drop with a high velocity, several variables needed to be optimized, including the pressure set point, dwell time, period, and voltage applied to the dispensing device
Summary
Evaluation of systems based on optical detection techniques that allow for ranged sensing is complicated by spatial dependencies and a lack of a reliable means to generate calibrated reference samples containing the explosive materials. A variety of techniques that offer temporary alternatives has been employed, including spray deposition [2] and drop-and-dry (dropcasting) methods; it is often observed that there is material waste or uneven sample coverage. In the latter case, material loading fluctuations over a given surface area may result in significant signal variance from laser-based detection systems that have beam diameters on a similar length scale.
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