Forensic analysis and differentiation of black powder and black powder substitute chemical signatures by infrared thermal desorption DART-MS
Thomas P. Forbes, Jennifer R. Verkouteren
The trace detection and forensic analysis of fuel-oxidizer mixtures, specifically targeting black powders and black powder substitutes, directly from wipe-based sample collections was demonstrated using infrared thermal desorption (IRTD) coupled with direct analysis in real time mass spectrometry (DART-MS). Discrete 15 s heating ramps were generated, creating a thermal desorption profile that desorbed more volatile species (e.g., organic and semivolatile inorganic compounds) at lower temperatures and nonvolatile inorganic oxidizers at high temperatures (450 °C to 550 °C). Common inorganic components of black powders (e.g., sulfur and potassium nitrate) as well as the alternative and additional organic and inorganic components of common black powder substitutes (e.g., dicyandiamide, ascorbic acid, sodium benzoate, guanidine nitrate, and potassium perchlorate) were detected from polytetrafluoroethylene-coated fiberglass collection wipes with no additional sample preparation. IRTD-DART-MS enabled the direct detection of intact inorganic salt species as nitrate adducts (e.g., (KClO4)NO3-) and larger clusters. The larger ion distributions generated by these complex mixtures were differentiated using principal component analysis (PCA) of the mass spectra generated at two points during the thermal desorption profile (low and high temperatures), as well as at high in-source collision induced dissociation (CID). The PCA framework generated by the analysis of two black powders and five black powder substitutes was used to classify samples collected from a commercial firecracker containing both flash powder and black powder. The coupling of IRTD- DART-MS and multivariate statistics demonstrated the powerful utility for identification and discrimination of trace fuel-oxidizer mixtures.