Summary

This Application Note describes the analysis of ignitable liquid residues (ILRs) from fire debris, using static or dynamic headspace sampling onto sorbent tubes, in conjunction with thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS). Key points demonstrated are the ease with which samples can be compared and conditions optimised, the benefits of sample re-collection for repeat analysis, and the very low levels of carryover (even for heavy petroleum distillates).

Introduction

Analysis of fire debris offers vital evidence to forensic studies, because the detection of accelerants in ignitable liquid residues (ILRs) can indicate that the fire was not accidental but the result of arson. Various methodologies are used for such analyses, and some of these are described in ASTM methods.^1–5 The oldest method, direct solvent extraction,¹ as well as having the drawbacks of solvent use (such as cost and risk of sample contamination) involves the destruction of the sample. Methods that preserve the sample are therefore increasingly favoured, especially those using static² or dynamic³ collection of sample headspace onto activated charcoal strips,⁴ SPME fibres,⁵ PLOT columns⁶ or sorbent tubes.^7–9 Sorbent tubes are the most versatile of these sampling methods, and are widely used for sampling headspace in many fields, in conjunction with thermal desorption (TD) pre-concentration and gas chromatography–mass spectrometry (GC–MS) analysis. Key benefits of sorbent tubes for forensic applications are the low detection limits (due to the large sorbent area), and quantitative analysis of C₃ to C₄₄ without analyte bias. There are also several practical advantages – the use of sorbent tubes completely eliminates the issues of solvent cost and disposal, they are much less likely than SPME fibres to be damaged due to operator error or incorrect storage, and automated sequential desorption of up to 100 tubes is easily achieved using modern instrumentation. In this Application Note, we illustrate the advantages of sorbent-tube sampling and TD–GC–MS analysis for the investigation of ILRs in fire debris. Specifically, we highlight the ability of TD to interface with a variety of sampling devices – in this case, collection of static headspace vapours using a manually-operated pump, and dynamic sampling with a microchamber device.

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