The Investigation of Arson Samples by GCxGC-TOFMS

Importance of the topic

The detection and identification of accelerants in fire debris plays a pivotal role in arson investigations. Challenges such as low residual accelerant concentration, thermal degradation, and matrix interferences demand advanced analytical techniques. GCxGC-TOFMS offers enhanced separation capacity and detailed chemical fingerprinting, enabling reliable recognition of accelerant use even in complex fire debris.

Objectives and study overview

This study evaluates comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GCxGC-TOFMS) for the analysis of arson samples containing gasoline as an accelerant. By comparing a gasoline standard with a real fire debris sample, the work aims to demonstrate pattern recognition capability and confirm accelerant presence despite fire-induced alterations and matrix effects.

Methodology and instrumentation

A gasoline standard and an arson sample provided by the National Metrology Laboratory (Pretoria, South Africa) were analyzed under identical GCxGC-TOFMS conditions. Characteristic ions representing aliphatic hydrocarbons, substituted benzenes, and alkylated aromatics were extracted to generate targeted chromatographic slices and compare compositional patterns.

Instrumentation

• LECO Pegasus 4D GCxGC-TOFMS system
• GC Columns: First dimension 30 m × 0.25 mm × 0.25 µm Rtx-5 Sil MS; second dimension 2 m × 0.18 mm × 0.20 µm Rtx-200
• Inlet: 1 µL injection, split 1:100 at 200 °C
• Oven program: 35 °C (2 min), 2 °C/min to 175 °C; secondary oven offset +15 °C, modulation period 6 s
• Transfer line: 200 °C; carrier gas: helium at 1.0 mL/min
• TOFMS: source 200 °C, acquisition rate 100 spectra/s, mass range 45–350 u

Main results and discussion

Both total ion chromatograms (TIC) of the standard and arson sample exhibit similar global profiles despite fire exposure effects. Extracted ion chromatograms for three ion groups (i) m/z 57+71+85 (alkanes), (ii) m/z 91+105+134 (alkylbenzenes), and (iii) m/z 128+142+156+170 (alkylnaphthalenes) reveal matching patterns in retention space, confirming gasoline fingerprints. GCxGC dramatically reduces coelution and enhances visualization of trace accelerant components within complex debris matrices.

Benefits and practical applications

• Improved discrimination of accelerant-derived compounds from pyrolysis products
• Robust pattern recognition for definitive accelerant identification
• High throughput screening with targeted ion slices for rapid comparison
• Potential integration into forensic workflows for reliable court-admissible evidence

Future trends and potential applications

• Extension to other common accelerants such as kerosene and diesel fuel
• Implementation of automated chemometric libraries and machine learning for fingerprint matching
• Development of portable GCxGC-TOFMS systems for on-site forensic analysis
• Expansion of mass spectral databases to encompass diverse fire debris scenarios

Conclusion

GCxGC-TOFMS provides a powerful analytical platform for arson debris analysis, delivering enhanced separation and clear chemical fingerprints despite fire-induced alterations. The method enables confident confirmation of gasoline use as an accelerant and suggests broad applicability to forensic investigations involving complex matrices.

References

• B-J de Vos. Thesis submitted for the degree of Master of Science, Chemistry, University of Pretoria, Pretoria, South Africa, February 2005.