Weathering of ignitable liquids at elevated temperatures: A thermodynamic model, based on laws of ideal solutions, to predict weathering in structure fires

• NCJ Number: 303289
• Journal: Forensic Chemistry Volume: 18 Dated: 2020
• Author(s): I. C. Willis; et al
• Date Published: 2020
• Grant Number(s): 2018-DU-BX-0225
• Agencies: NIJ-Sponsored
• Award: Investigating Kinetic and Thermodynamic Approaches to Predict Evaporation of Gasoline at Elevated Temperatures for Fire Debris Applications

Annotation

This article reports experimental evidence and a strong theoretical basis that weathering at elevated temperatures (up to 210 °C) results in significantly different distributions of the weathered residues compared to room temperature weathering, especially when the extent of weathering is held constant. 

Abstract

A nine-component artificial gasoline mixture enabled quantitative comparisons between the residues predicted by a mathematical model and those measured in temperature-controlled evaporations. The simple mathematical model employs iterative fractional losses (e.g. 5 percent each step) of the mixture components in proportion to their theoretical partial pressures. The partial pressures of the constituents were determined using either: 1) Raoult’s law and Antoine constants from the literature, or 2) Henry’s law. The model supports the experimental observations in that the composition of weathered residues as a function of time—or extent of weathering—is significantly different at different temperatures; for example, toluene falls below the limits of detection at 90 percent weathering and 30 °C but is still readily observable at ~1 percent of the total ion chromatogram (TIC) at 98 percent weathering and 210 °C. Such behavior could help explain why ignitable liquids that are highly weathered at elevated temperatures in structure fires are likely to resemble those weathered in the laboratory to a lesser extent at room temperature. Given a chromatogram of a pristine ignitable liquid, the model based on Raoult’s law predicts the peak area of each weathered compound with a root mean squared error of prediction (RMSEP) of ~3 percent when the liquid is  percent weathered up to 98 percent and 210 °C.

(publisher abstract modified)