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Revolutionizing Fire Debris and Arson Analysis: Dynamic Headspace vs. Traditional Techniques

Arson vs. Fire Debris

Arson is the deliberate act of setting fire to property to cause damage. In contrast, fire debris broadly refers to any material recovered from a fire with or without criminal intent. While arson requires proof of criminal intent, such as incendiary devices, multiple points of origin, or, most importantly, ignitable liquids, fire debris encompasses all evidence regardless of intent.

The Art of Arson

Arson is often calculated, involving materials like timers, magnifying glasses, and ignitable liquids. These items are easily purchased from local stores, making the crime accessible to the average person. The arsonist can inflict maximum damage while avoiding detection, often not even needing to be present when the fire occurs. Arson is one of the largest contributors to property damage and financial loss, yet it remains one of the most under-prosecuted crimes, with an estimated conviction rate of less than 1% [1]. The destructive nature of fires often leaves investigators lacking substantial evidence. What does remain are ignitable liquid residues (ILRs), one of the few remaining traces that help link a perpetrator to the crime.

Ignitable Liquids

An ignitable liquid, also known as an accelerant, is any easily obtainable and volatile substance that, when ignited, allows fires to spread rapidly. Due to consumer availability, common ignitable liquids include gasoline, kerosene, and diesel fuel. American Society for Testing and Materials (ASTM) 1618 classifies ignitable liquids into nine classes with three subcategories [2]. The nine classes are gasoline, petroleum distillates, isoparaffinic products, aromatic products, naphthenic paraffinic products, n-alkane products, de-aromatized distillates, oxygenated solvents, and other/miscellaneous. The subcategories are light, medium, or heavy and indicate an ignitable liquid’s carbon range and relative volatility. ASTM 1618 summarizes the nine classes, providing investigators with a general reference to each ignitable liquid’s chromatographic fingerprint after analyzing fire debris evidence.

Traditional Techniques

Crime labs typically rely on well-established techniques to extract and identify ILRs from fire debris. One of the most common methods utilized is passive headspace extraction, outlined in ASTM 1412 [3]. This methodology employs paint cans as extraction vessels, activated charcoal strips as sorbents, and an elution solvent. Fire debris is placed in the vessel with a suspended charcoal strip, and the vessel is sealed and heated to upwards of 80 °C for several hours. Then, the charcoal strip is removed, analytes are eluted with solvent, and the solvent is injected into a gas chromatograph-mass spectrometer (GC-MS) for subsequent analysis. While extremely reliable, ASTM 1412 is time-consuming, calls for harmful solvents, and suffers from volatility biases due to the nature of the passive headspace extraction. As outlined in other ASTMs, alternative methods, including solid phase microextraction (SPME) and active sampling with sorbent-filled tubes and sampling pumps, offer some improvements [4-5]. However, ASTM 1412 remains the most widely used due to its simplicity and proven effectiveness.

ASTM 1412 – Passive Headspace Extraction using Activated Charcoal Strips

Dynamic Headspace

GERSTEL’s Dynamic Headspace 3.5+ (DHS 3.5+) module offers significant advantages over ASTM 1412 for extracting ILRs from fire debris. These advantages include:

Reduced extraction times – DHS reduces analysis time to a few minutes, compared to several hours required for passive headspace extraction.

Lower limits of detection – Achieved through an exhaustive extraction by continuously purging the sample headspace and sweeping analytes onto sorbent-filled tubes.

Improved analyte retention and minimized breakthrough – Ensured by using thermal desorption tubes with larger sorbent capacities, effectively trapping light and broad carbon range ignitable liquid subcategories.

Automation – Reduces user error and variability among forensic chemists, enhancing reproducibility and reliability.

Less environmental impact- Extracts analytes without the use of harmful solvents.

Diagram of the DHS Process

DHS in Action: GERSTEL AppNote 269

To demonstrate the efficacy of the DHS 3.5+, AppNote 269 highlights how the module was used to extract residues from three ignitable liquids on mock fire debris evidence. The ignitable liquids of choice were gasoline, automotive parts cleaner, and diesel fuel, as these are readily available and cover all three ignitable liquid subcategories:

Mock Evidence Preparation

Adhesive floor tile squares were used as the fire debris substrate. A 10 cm x 10 cm tile square was burned, but not to completeness, as typical fire debris evidence from an arson scene has minimal charring to ensure the highest probability of recovering ILRs. Approximately 1 cm x 1 cm cuttings of each tile square were used as a representative sample. Cuttings were placed in 20 mL screw-capped vials, and 100 µL of ignitable liquid (diluted 1:1000 in ethanol) was spiked into the respective vials. The mock evidence was spiked with an ignitable liquid after burning to ensure sufficient sample was deposited for classification and identification.

ILRs Extraction

The samples were incubated at 100 °C for 3 minutes and extracted for 15 minutes at the same temperature under 50 mL/min helium flow for a total trap volume of 750 mL onto a Tenax® TA sorbent-filled tube. 100 °C was deemed the best temperature, as illustrated in the temperature optimization study in AppNote 269.

Gasoline’s Chromatographic Fingerprint

Gasoline is a complex accelerant containing several aromatic hydrocarbons in the light to medium range, each useful in characterizing it as an ignitable liquid. Its unique chromatographic fingerprint categorizes it as its own ignitable liquid class. According to reports from American forensic laboratories, gasoline is also the most commonly identifiable ignitable liquid [6]. 

ASTM 1618 describes gasoline as containing an abundance of aromatics, alkanes, cycloalkanes, and naphthalene-related compounds. Due to its prevalence in arson investigations and subsequent discussion in expert witness testimony, gasoline’s target compounds are often informally named into groups to facilitate a comprehensible explanation to a jury. In this study, all of gasoline’s target compounds were accurately identified in the corresponding chromatogram (Figure 1), fitting into their informally named groups.

Figure 1: Total ion chromatogram of gasoline with extracted ion insets for informally named target groups

Conclusion: Advancing Arson Investigations with DHS

The findings described in GERSTEL’s AppNote 269 highlight many key advantages of the DHS 3.5+. By providing a fast, solvent-free, exhaustive, and automated alternative to traditional methods, like ASTM 1412, the DHS 3.5+ addresses many challenges faced when analyzing fire debris evidence. Its ability to continuously purge ignitable liquid analytes from fire debris sample headspace onto a large sorbent-filled trap prevents analyte breakthrough and effectively mitigates the volatility biases associated with passive headspace methods. 

References

  1. Burnette, G. E. (n.d.). The Anatomy of an Arson Case. InterFIRE online.
  2. ASTM 1618 Standard Test Method for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography-Mass Spectrometry.
  3. ASTM 1412 Standard Practice for Separation of Ignitable Liquid Residues from Fire Debris Samples by Passive Headspace Concentration with Activated Charcoal.
  4. ASTM 2154 Standard Practice for Separation of Ignitable Liquid Residues from Fire Debris Samples by Passive Headspace Concentration with SPME.
  5. ASTM 1413 Standard Practice for Separation of Ignitable Liquid Residues from Fire Debris Samples by Dynamic Headspace Concentration onto an Adsorbent Tube.
  6. Excerpts from The Pocket Guide to Accelerant Evidence Collection. InterFIRE online. (1999).

For a deeper understanding of DHS and the determination of ILRs from the remaining two ignitable liquids spiked onto mock evidence, download AppNote 269 for the full details of this study: