Portable Raman Spectroscopy in Forensics: Explosive Residues and Inflammable Liquids

Significance of the Topic

Portable spectroscopic tools are transforming forensic investigations by enabling rapid, in situ chemical identification. Raman spectroscopy, long valued in forensic laboratories to identify explosives, drugs, fibers and inks, gains field portability with modern devices. This advance supports faster decision-making at crime scenes, minimizing evidence degradation and enhancing safety for first responders.

Objectives and Study Overview

This study evaluates the performance of a portable 785 nm Raman spectrometer (i-Raman Pro) in two challenging forensic applications:

• Identification of pre-blast and post-blast explosive residues including organic explosives (TNT, TATP, PETN) and inorganic salts (ammonium nitrate, potassium nitrate, sodium chlorate).
• Non-invasive standoff analysis of flammable liquids (gasoline, diesel, ethanol, acetone) inside intact glass bottles.

Methodology and Instrumentation

The i-Raman Pro uses a cooled detector (–25 °C) for improved signal-to-noise and spectral resolution. Two sampling modes are applied:

• Probe mode for direct analysis of bulk or liquid samples.
• Microscope-coupled mode for micro-trace analysis of small particles.

Optimized measurement parameters for explosive residues: five scans, 0.5 s acquisition, 10 % laser power at 785 nm. Distance regulator removal on the probe enabled deeper laser penetration for bottle analysis.

Main Results and Discussion

– Explosives and salts: Clear Raman signatures were obtained for TNT, TATP, PETN and for inorganic oxidizers. The cooled spectrograph resolved the 7 cm⁻¹ difference between ammonium and potassium nitrate, overcoming limitations of prior portable systems. Principal component analysis (PCA) demonstrated distinct clustering of all compounds with >90 % confidence.

– Post-blast residues: Microscopic unburnt particles from dynamite, ANFO and chlorate-based compositions were detected and correctly identified via the microscope-coupled mode. Key spectral features matched reference libraries.

– Flammable liquids: Raman spectra of gasoline, diesel, ethanol and acetone inside sealed glass bottles showed minimal glass interference. PCA separation exceeded 95 % confidence, confirming reliable non-invasive standoff identification.

Benefits and Practical Applications

– In situ identification reduces evidence relocation and handling risks.
– Portable microscope mode enables micro-trace detection at blast sites.
– Non-invasive bottle scanning enhances safety when screening incendiary devices.
– Direct field analysis accelerates investigative workflows and supports real-time decision making.

Future Trends and Opportunities

– Expansion of portable spectral libraries to cover emerging threats.
– Integration with chemometric algorithms for automated compound classification.
– Miniaturization and wireless connectivity for remote standoff detection.
– Combined multimodal field platforms (Raman, FTIR, fluorescence) for comprehensive evidence profiling.

Conclusion

The portable i-Raman Pro delivers laboratory-grade Raman performance in the field, enabling rapid, sensitive and safe identification of explosives and flammable liquids. Its cooled detector and optional microscope enhance spectral resolution and microsampling capabilities. This tool strengthens forensic workflows by supporting on-scene analysis and minimizing sample manipulation.

Reference

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