Recovery of TNT from post-blast soil samples: effects of soil type, extraction methodology, and storage conditions

Traditionally, determination of explosives and their decomposition products in soil samples involves a 24-hour ultrasonic solid-liquid extraction (SLE) using acetonitrile followed by a "salting out" liquid-liquid extraction (LLE). Alternatively, aqueous extracts of soil can undergo solid-phase extraction to isolate target analytes prior to liquid chromatography. Although well suited for environmental analysis and standardized by the Environmental Protection Agency (EPA), these methods can be overly labor- and time-intensive for a forensic chemist working a post-blast investigation. This highlights the need for rapid, simple, and effective alternatives. In addition, the extent to which the post-blast residues of explosives can biodegrade due to soil bacteria is not fully characterized. In this study, we introduce total vaporization-solid phase microextraction (TV-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) as an alternative to traditional methods for environmental and forensic samples. To benchmark TV-SPME, more established methods including modified-immersion SPME GC-MS and immersion-SPME GC-MS were evaluated for their ability to identify TNT in both spiked and post-blast soils. Three soils with varying composition and organic matter content were spiked with TNT and analyzed to assess the impact of matrix on TNT recovery. In addition, three different storage temperatures (freezer, fridge, and room temperature) were examined to understand TNT degradation in post-blast soil. The chemical kinetics of the TNT degradation process were also explored. TV-SPME showed the greatest response for TNT present in soil. Among the spiked soils, those with higher organic content exhibited greater degradation over time. There is clear evidence of microbial degradation of TNT in post-blast soil over several weeks, which argues for immediate analysis of such samples or their storage at temperatures of at least -18 °C. These findings emphasize the importance of soil composition, extraction methodology and storage conditions when analyzing explosives in forensic and environmental samples.