Characterizing Organic Gunshot Residues with Low-Frequency Raman and Terahertz Vibrational Spectroscopies

Area of Science:

• Forensic Science
• Analytical Chemistry
• Materials Science

Background:

• Solid-state organic gunshot residues (OGSRs) are crucial trace evidence in criminal investigations.
• Low-frequency vibrational spectroscopy (10-300 cm^-1) probes lattice vibrations, offering unique spectral fingerprints for compound identification.
• Terahertz time-domain spectroscopy (THz-TDS) and low-frequency Raman spectroscopy (LFRS) are advanced techniques for analyzing these vibrations.

Purpose of the Study:

• To analyze and differentiate structurally similar OGSRs, specifically ethyl centralite and methyl centralite, using LFRS and THz-TDS.
• To interpret spectral data using solid-state density functional theory (DFT) simulations.
• To evaluate the sensitivity and applicability of LFRS and THz-TDS for OGSR detection and quantification.

Main Methods:

• Acquisition of LFRS and THz-TDS spectra for pure ethyl centralite and methyl centralite.
• Computational analysis using solid-state density functional theory (DFT) to understand spectral origins.
• Measurement of binary mixtures to assess differentiation capabilities.

Main Results:

• Distinct LFRS and THz-TDS spectra were observed for ethyl centralite and methyl centralite, despite their structural similarity.
• DFT simulations indicated that phenyl ring torsional motions, not intermolecular translations, dominate the strongest spectral features.
• Low-frequency Raman spectroscopy demonstrated higher sensitivity, identifying characteristic peaks at 98.8 cm^-1 for ethyl centralite and 111.7 cm^-1 for methyl centralite.

Conclusions:

• Low-frequency vibrational spectroscopy, particularly LFRS, is a powerful tool for the reliable detection and quantification of specific OGSRs like alkylated diphenylureas.
• The identified spectral markers provide practical utility for future forensic analyses.
• This study highlights the significant, yet underexplored, potential of low-frequency vibrational spectroscopy in forensic science applications.