6.2 Ignitable Liquid Sampling & Containers

6.2 Ignitable Liquid Sampling & Containers

Chemical & Physical Behavior of Ignitable Liquid Residues

Ignitable liquids (such as gasoline, diesel fuel, kerosene, lighter fluid, and paint thinners) are complex mixtures of hydrocarbons ranging from volatile low-molecular-weight alkanes ($C_4-C_9$) to heavier aromatics, cycloalkanes, and polynuclear aromatic hydrocarbons ($C_9-C_{23}$). When an ignitable liquid is poured as an accelerant at a fire scene, it undergoes rapid physical changes:

1. Evaporative Weathering: The lighter, highly volatile components evaporate rapidly due to ambient heat and fire exposure. This selectively depletes lower-boiling-point compounds, shifting the relative distribution toward higher-boiling-point components.
2. Substrate Absorption: Liquids gravity-drain into porous materials—such as carpet padding, floor joist seams, unsealed wood, concrete expansion joints, and soil—where they are partially protected from direct thermal destruction.
3. Thermal Pyrolysis Degradation: High heat breaks down synthetic carpet fibers (polypropylene, nylon), underlayment foam (polyurethane), and vinyl tile, producing background pyrolysis products (e.g., styrene, toluene, ethylbenzene, terpenes) that mimic or mask ignitable liquid compounds.

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Identifying Optimal Ignitable Liquid Sampling Locations

Sampling must be guided by pattern analysis rather than random selection. Under NFPA 921 Chapter 17, investigators should prioritize sampling from areas where ignitable liquids were protected from complete consumption during burning:

• Low Spots and Floor Seams: Seams between vinyl floor tiles, cracks in concrete slabs, baseboard-to-floor junctions, and floor drains.
• Protected Substrates: Carpet padding beneath furniture legs, bottom surfaces of unburned synthetic rugs, edges of pour patterns, and lower faces of wooden floor joists.
• Absorptive Materials: Wood flooring joints, porous grout lines, paper backings of drywall near floor level, and soil beneath exterior windows or porches where accelerants may have leaked or been discarded.
• Accelerant Detection Canines (ADC) & PID Screening: Utilizing trained Working Canines (certified under ASTM E2123) or Photoionization Detectors (PID) to pinpoint volatile hydrocarbon hotspots prior to destructive removal.

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Debris Container Selection & Technical Specifications

The selection of evidence packaging materials is dictated by the chemical characteristics of volatile hydrocarbons and the requirements of analytical laboratory extraction procedures.

+-----------------------------------------------------------------------------------+
|                         EVIDENCE CONTAINER SELECTION MATRIX                       |
+-----------------------+-----------------------+-----------------------------------+ 
| Container Type        | Target Evidence       | Volume & Sealing Rule             |
+-----------------------+-----------------------+-----------------------------------+ 
| Clean Unlined Can     | Solid fire debris     | Fill max 66% (2/3); leave 33%     |
| (Friction-top metal)  | (carpet, wood, tile)  | headspace for charcoal extraction |
+-----------------------+-----------------------+-----------------------------------+ 
| Specialty Nylon Bag   | Bulky items, soil,    | Gas-tight heat seal; do NOT use   |
| (Kapak / Polyolefin)  | large carpet rolls    | standard polyethylene (porous)    |
+-----------------------+-----------------------+-----------------------------------+ 
| Glass Jar w/ PTFE Cap | Liquid samples, neat  | Inert glass; Teflon cap liner;    |
| (Teflon-lined lid)    | fuel, concentrated IL | fill completely for liquids       |
+-----------------------+-----------------------+-----------------------------------+

1. Clean, Unused, Unlined Metal Paint Cans

• The Industry Standard: New, unlined steel paint cans with friction-fit lids are the universally preferred container for solid fire debris samples.
• Why Unlined Cans Are Mandatory: Standard commercial paint cans are frequently coated with internal epoxy or phenolic resin linings to prevent rusting when storing water-based paints. Epoxy linings contain organic solvents and plasticizers (e.g., phenol, toluene, isophorone) that dissolve when exposed to ignitable liquid vapors. During laboratory heating, these lining compounds off-gas into the headspace, creating false chromatographic peaks that corrupt mass spectrometry analysis. Investigators must verify that cans are certified "unlined steel."
• Headspace Volume Requirement: Containers must be filled to a maximum of two-thirds (66%) capacity, leaving the upper one-third (33%) as empty headspace volume. This air gap is chemically required for laboratory passive headspace extraction (ASTM E1412), where an activated charcoal strip is suspended above the debris inside the sealed can to adsorb volatile vapors during oven heating.
• Sealing Protocol: Ensure can rims are clean of debris, seat the friction lid, and tap down firmly around the entire circumference using a rubber mallet. Apply tamper-evident tape across the lid-to-can seam.

2. Specialty Gas-Tight Nylon Bags (Kapak / Polyolefin)

• Bulky Evidence: Used for items that cannot fit into standard 1-gallon or 5-gallon metal cans (e.g., large carpet sections, clothing, bedsheets).
• Material Specification: Must be constructed of multi-layer nylon, polyester, or polyolefin film specifically rated for volatile solvent containment.
• Polyethylene Warning: Standard household or commercial polyethylene garbage bags or ziplock bags MUST NEVER be used. Polyethylene is chemically porous to volatile hydrocarbons; ignitable liquid vapors readily permeate polyethylene walls within hours, resulting in sample loss and ambient cross-contamination.
• Sealing Protocol: Thermal impulse heat-sealer to create a continuous, leak-proof weld.

3. Glass Jars with PTFE (Teflon)-Lined Screw Caps

• Neat Liquid Samples: Reserved for free liquids recovered from containers, fuel lines, or liquid pools at the scene.
• Lid Specifications: Screw caps must feature a Polytetrafluoroethylene (PTFE / Teflon) liner. Standard rubber, paper, or plastic lid liners contain phthalates and softeners that dissolve into fuel samples.

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Comparison / Control Sampling Methodology

Collecting comparison (control) samples is a mandatory requirement under NFPA 921 Chapter 17 whenever fire debris is submitted for ignitable liquid testing.

Purpose of Comparison Samples

Many synthetic building materials (carpet fibers, rubber underlayments, floor adhesives, linoleum backing, asphalt shingles) are petroleum-derived polymers. When burned or pyrolyzed under fire conditions, these materials decompose into volatile hydrocarbons—including toluene, styrene, ethylbenzene, xylenes, and alkylbenzenes—that are chemically identical to compounds found in refined ignitable liquids.

A comparison sample consists of an unburned, unimpregnated piece of the exact same matrix material taken from an unburned area of the scene (e.g., carpet cut from an undamaged closet or adjacent hallway). By analyzing the comparison sample alongside the burn debris sample, the forensic laboratory analyst can perform baseline subtraction, subtracting the substrate's natural pyrolysis product profile from the sample chromatogram to definitively verify whether an added ignitable liquid is present.

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Soil and Liquid Sampling Protocols

Soil Evidence Handling

• Microbial Biodegradation: Soil contains active populations of hydrocarbon-utilizing bacteria and fungi (e.g., Pseudomonas, Rhodococcus). At room temperature, soil microbes rapidly metabolize $n$-alkanes and aromatic compounds, destroying ignitable liquid evidence within 24 to 48 hours.
• Preservation Standard: Soil samples collected in unlined metal cans or nylon bags must be chilled immediately with ice packs and frozen at -20°C (-4°F) until laboratory extraction.

Liquid on Water Surface

• When ignitable liquids float on fire suppression water, collect the surface layer using a sterile glass jar or by utilizing hydrophobic, oleophilic absorbents (e.g., pure PTFE strips or sterile cotton gauze), which selectively absorb hydrocarbons while repelling water.