Comprehensive Detection and Confirmation of Explosive Residues in Soil Using a High Performance Benchtop GC-TOFMS

Importance of the Topic

Reliable detection of explosive residues in soil is crucial for environmental safety, forensic investigations, counterterrorism efforts, and military demining. Soils can retain trace levels of nitroaromatic, nitroamine, and related explosive compounds long after deployment or accidental release. Methods that combine high sensitivity, broad applicability, and rapid analysis enable timely decision making in field and laboratory settings.

Study Objectives and Overview

This study demonstrates a fast, comprehensive analytical workflow for identifying and confirming fourteen commonly encountered explosive compounds in soil. A standard mixture of nitrobenzene, nitrotoluenes, dinitrotoluenes, trinitrobenzene, trinitrotoluene, RDX, HMX, tetryl, and amino-dinitrotoluenes was spiked into soil and analyzed in under five minutes. Both targeted and untargeted data processing approaches are showcased to illustrate the system's flexibility for routine screening and detailed investigation.

Methodology

A simple two-solvent extraction was applied to 0.40–0.50 g of soil using acetonitrile and dichloromethane with agitation at 30 °C. After drying over anhydrous calcium chloride, 1 mL of the extract was injected splitless into a gas chromatograph. Fast temperature programming ensured peak separation within a five-minute runtime. Deconvolution software was used to resolve coeluting components in the complex matrix.

Used Instrumentation

• Gas chromatograph: Agilent 7890 with Multimode Inlet and 7693 Autosampler
• Column: DB-5MS Ultra Inert, 15 m × 0.25 mm × 0.25 µm
• Carrier gas: Helium at 1.4 mL/min (constant flow)
• Injection: 1 µL splitless at 60 °C, ramped to 200 °C
• TOF mass spectrometer: LECO Pegasus BT, ion source at 250 °C, mass range 50–450 m/z, 20 spectra/s

Main Results and Discussion

The total analysis time was under five minutes, with retention windows from approximately 1.9 to 4.2 minutes for fourteen explosive standards. Twelve of fourteen compounds were detected with an average spectral similarity score of 880 out of 1000. Calibration curves for representative nitrotoluenes and TNT exhibited linearity from 0.1 to 100 ppb (r2 > 0.99), demonstrating reliable quantitation at trace levels.

Untargeted processing identified additional soil constituents such as polycyclic aromatic hydrocarbons, carbazoles, and dibenzothiophenes, reflecting the impact of a nearby asphalt parking area. Deconvolution effectively separated coeluting peaks, yielding clean mass spectra for compounds like 2-nitrotoluene and p-toluidine. A targeted analyte finding algorithm processed each dataset in under 12 seconds, confirming twelve explosives in the real sample.

Benefits and Practical Applications

• High throughput analysis with run times below five minutes
• Sub-ppt to low-ppt sensitivity for a wide range of explosive classes
• Combined untargeted and targeted data processing for both screening and quantitation
• Improved selectivity reduces false positives in complex matrices
• Minimal sample preparation simplifies laboratory workflows

Future Trends and Opportunities

Advances in database size and machine learning-driven spectral interpretation will enhance retrospective mining of archived data. Portable benchtop TOFMS systems may enable rapid in-field screening. Integration with surface extraction techniques and automated sample handling will further streamline high-throughput environmental monitoring and forensic analysis.

Conclusion

A straightforward extraction combined with high-performance GC-TOFMS and advanced data processing enables rapid, sensitive, and selective detection of explosives in soil. The approach delivers both comprehensive untargeted screening and efficient targeted quantitation, supporting diverse applications in environmental analysis, forensics, and security.

References

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