Troy J Ernst1, Scott J Campbell2, John H Moncur2
1Michigan State Police Grand Rapids Forensic Laboratory, Michigan, USA.
2SpectralWorks Limited, Cheschire, WA7 4EB, United Kingdom.
First Published: ASMS 2019 (Atlanta, GA).
Forensic laboratories routinely analyze samples collected from fire scenes for the presence of ignitable liquid residues (ILRs). ILRs are often recovered from fire debris using a passive heated headspace extraction, with adsorption of volatile components on active carbon followed by solvent or thermal desorption. Fire debris cases present many analytical challenges, including interfering compounds from the substrate, pyrolysis products, changes in the composition of an ignitable liquid due to evaporation, chromatographic complexity of many ignitable liquids, and overlap of compounds among the classes of ignitable liquids (Figure 1). This presentation demonstrates an automated search method to evaluate complex chromatograms for the presence and classification of ignitable liquid residues using target compounds and extracted ion profiles (EIPs).
Figure 1. Fire Debris and Select ILR Chromatograms.
Materials and Methods
GC-MS data from a set of 120 reference ignitable liquids, reference standards, and interfering products were collected using the following equipment and parameters:
- CTC Analytics PAL autosampler, injection volume 1µl
- Thermo Trace GC 2000, Zebron ZB-1MS column (30m plus 5m guard, 0.25mm ID, film thickness 1µm), injection temperature 220°C, transfer temperature 250°C, split flow 100 ml/min, split ratio 50:1, flow 2 ml/min, initial temperature 40°C, 3.5 minute hold, 12.5°C/min ramp to 170°C, 30°C/min ramp to 280°C, 2.43 min hold
- Thermo DSQ mass spectrometer, ion mode CEI, ion source 250°C, scan range 30-500 amu
- Thermo Xcalibur for data collection
- SpectralWorks AnalyzerPro with ProfileAnalyzer for data processing and analysis
The reference ignitable liquids included representatives of each ignitable liquid class described in ASTM E16181 as well as various evaporation levels of select ignitable liquids. Sixty-nine target compounds (Table 1) were identified as indicators of ignitable liquid class or of interfering compound based on their presence and intensities. Chromatographic deconvolution software was used to reduce the complexity of the MS of the target compounds to 2-5 ions for search efficiency (Figure 2). Chromatographic fingerprints (spectragrams) of the target compounds for each reference sample for use in comparisons to fire debris samples and unknown liquids were created (Figure 3).
Table 1. Target Compounds
Figure 2. Benzene - actual and deconvoluted mass spectra.
Figure 3. Reference ignitable liquids with target component spectragams.
The workflow of data processing to reporting for casework is shown in Figure 4.
Figure 4. Casework Application Workflow.
Typical chromatograms and target component spectragrams of two fire debris samples are shown in Figure 5.
Figure 5. Fire Debris Chromatograms and Spectragrams.
These chromatograms and spectragrams can be compared to specific reference liquids and assigned a specific score as shown in Figure 6.
Figure6. TIC and selected EIPs of fire debris and reference liquids (mirror view).
Where necessary, individual components can be examined further using MS library searches (Figire 7).
Figure 7. Individual Component MS Library Searches.
GC-MS data from fire debris and unknown liquids from over 400 criminal investigations have been collected, processed, and compared to the reference samples using the described equipment and methods. Because ignitable liquid classification is accomplished using comparisons of TICs and EIPs of the sample data and reference ignitable liquids, a straightforward search and display workflow as demonstrated here is valuable for the forensic analyst.
The automated searching and report printing options have significantly reduced the amount of time spent analyzing the GC-MS data and preparing reports. The search program consistently selects the appropriate reference sample as the top hit. Interfering compounds from the substrate and pyrolysis are overcome by automated generation of appropriate EIPs. The necessary information for forming conclusions regarding presence and classification of ignitable liquids, including identifications of individual components and comparisons of TICs and EIPs of various retention time windows, is readily accessed. This information is displayed in reports that are information-rich yet easily comprehended, allowing for efficient evaluation of complex data and well-supported conclusions.
 ASTM E1618-17, Standard Test Method for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography – Mass Spectrometry.