Semester

Spring

Date of Graduation

2025

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Forensic and Investigative Science

Committee Chair

Glen P. Jackson

Committee Co-Chair

Shikha Sharma

Committee Member

Shikha Sharma

Committee Member

Jacqueline Speir

Abstract

ABSTRACT

Refinement of a thermodynamic model to explain the weathering patterns of ignitable liquids on household substrates at elevated temperatures

Chaney A. Ganninger

In 2021, arson caused more than 4,361 deaths in the United States. To aid in the differentiation between arson and accidental or natural causes, investigators often analyze fire debris for the presence of ignitable liquid residues using headspace concentration followed by gas chromatography-mass spectrometry (GC-MS). Classification of ignitable liquids can be confounded by weathering, which is the uneven evaporation of compounds in ignitable liquid residues. The Jackson group, among others, has developed a thermodynamic model to help understand the effects of substrates and elevated temperatures on the weathering of ignitable liquids. Previous work in the Jackson group was restricted to a simple artificial gasoline mixture to facilitate model development. The current research used commercial gasoline as the basis for experiments. Weathering of gasoline was conducted on different substrates at 210 °C, and traditional headspace concentration was compared with solvent extraction as alternative methods to extract and concentrate residues from the substrates. Headspace concentration of gasoline residues replicates the methods commonly used in laboratory casework. Modeling this data required several accommodations to the original thermodynamic model, including: 1) a calibration method to predict vapor pressures from retention indices, 2) retention time alignment to facilitate chromatographic comparisons between modeled and measured data, 3) an adjustment for a proportion of unobserved volatiles, 4) a correction factor to address the resistance to mass transfer of gasoline with different substrates, and 5) the effect of headspace concentration on the chromatographic abundances. The substrates included nylon carpet, laminate wood flooring, vinyl flooring, polyurethane foam, and drywall.

Experimental extents of weathering ranged from 0 to 99.9% (w/w) across all substrates. To achieve a final measurable volume after weathering of approximately 200 µL for all samples, different initial volumes ranged from 250 µL to 1000 µL. The extents of evaporation were determined by the mass of gasoline before and after evaporation. After the implementation of all changes, the thermodynamic model was able to predict the extent of weathering of gasoline using the total ion chromatograms (TICs). When residues were extracted using solvent extraction, the absolute error in the extent of weathering was approximately ±3%. This accuracy was achieved with Pearson product moment correlation (PPMC) values from 0.90-0.99 between the modeled and measured chromatograms. For gasoline residues extracted using headspace concentration, the model was less accurate at predicting the extent of evaporation. Our residue volumes were on the order of 200 mL, which exceeded the capacity of the activated charcoal strips and caused a strong bias against the more volatile components. Passive headspace concentration samples also provided larger random variance between modeled and measured chromatograms because of subtle differences in the extraction efficiencies of aromatic versus aliphatic compounds. The predictions of this thermodynamic model provide physical and chemical explanations for how and why gasoline weathered to 90% or more can still appear to be unweathered when using headspace concentration and analyzed using GC-MS. Liquid extractions of gasoline evaporated on drywall showed the counterintuitive result that volatiles can become enriched relative to fresh gasoline and therefore appear to be less weathered than the original liquid.

Download

Share

COinS