Abstract
Forensic fire debris analysis is an important part of fire investigation, and gas chromatography–mass spectrometry (GC-MS) is the accepted standard for detection of ignitable liquids in fire debris. While GC-MS is the dominant technique, comprehensive two-dimensional gas chromatography–mass spectrometry (GC×GC-MS) is gaining popularity. Despite the broad use of these techniques, their sensitivities are poorly characterized for petroleum-based ignitable liquids. Accordingly, we explored the limit of identification (LOI) using the protocols currently applied in accredited forensic labs for two 75% evaporated gasolines and a 25% evaporated diesel as both neat samples and in the presence of interfering pyrolysate typical of fire debris. GC-MSD (mass selective detector (MS)), GC-TOF (time-of-flight (MS)), and GC×GC-TOF were evaluated under matched conditions to determine the volume of ignitable liquid required on-column for correct identification by three experienced forensic examiners performing chromatographic interpretation in accordance with ASTM E1618-14. GC-MSD provided LOIs of ~0.6 pL on-column for both neat gasolines, and ~12.5 pL on-column for neat diesel. In the presence of pyrolysate, the gasoline LOIs increased to ~6.2 pL on-column, while diesel could not be correctly identified at the concentrations tested. For the neat dilutions, GC-TOF generally provided 2× better sensitivity over GC-MSD, while GC×GC-TOF generally resulted in 10× better sensitivity over GC-MSD. In the presence of pyrolysate, GC-TOF was generally equivalent to GC-MSD, while GC×GC-TOF continued to show 10× greater sensitivity relative to GC-MSD. Our findings demonstrate the superior sensitivity of GC×GC-TOF and provide an important approach for interlaboratory benchmarking of modern instrumental performance in fire debris analysis.
Highlights
The purpose of forensic science is to assist the court in assessing the significance of a piece of evidence by interpreting the findings of a scientific examination of exhibit material
Interpretation of 2D chromatographic data for fire debris analysis has been reported in the literature [30], there are no official ASTM guidelines specific to 2D data at this time
We applied the same general approach described by ASTM E1618-14 for interpreting 1D data when evaluating the 2D data, with the addition of using the structured retention information provided by the 2 D separation as an additional point of comparison to reference ignitable liquid standards
Summary
The purpose of forensic science is to assist the court in assessing the significance of a piece of evidence by interpreting the findings of a scientific examination of exhibit material. One of many considerations when weighing the results of a forensic examination is an understanding of how much target analyte must be present in an exhibit for conclusive identification of its presence (the sensitivity of the technique). Forensic fire debris analysis is the examination of exhibit material for the presence of ignitable liquids, and in this context an understanding of sensitivity is especially important. This is Separations 2018, 5, 58; doi:10.3390/separations5040058 www.mdpi.com/journal/separations. Ignitable liquids may be present in an exhibit due to the surrounding environment, since they are composed of volatile organic compounds which can evaporate from one location and condense in another (e.g., the collection of gasoline residues by materials exposed to automotive exhaust) [8,9,10,11]. Excessive sensitivity in an analysis can result in the assignment of inappropriately high significance to insignificant amounts of ignitable liquid [5]
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