Evidence
The Evidence domain tests identification, collection, packaging, chain of custody, and laboratory submission. It rewards knowing which container belongs with which sample and what a laboratory result does and does not establish.
Categories and authentication
Evidence is physical, documentary, testimonial, or demonstrative, and may be direct or circumstantial. Every item needs a chain-of-custody record: who collected it, when, where, in what condition, and every transfer afterward. Gaps invite exclusion.
Collection and packaging
- Ignitable liquid residue samples: carbonaceous debris from the suspected area, in clean, unused, airtight metal cans or approved heat-sealed sample bags. Never reuse containers and never use a container that held a petroleum product.
- Always collect a comparison (control) sample from a comparable, unaffected area.
- Do not submit an entire appliance when a representative sample of the damaged area will answer the question; photograph before removal.
Laboratory analysis
Gas chromatography-mass spectrometry following ASTM E1618 is the recognized method for ignitable liquid residues. A positive result establishes that residue is present; it does not, by itself, establish that a liquid caused or accelerated the fire. Background contamination, pyrolysis products, and common household products can produce responses, which is exactly why the comparison sample matters.
Worked example
A living-room floor shows a burn pattern consistent with poured liquid. The investigator collects charred flooring and subfloor plus a comparison sample from an unaffected part of the same room. The lab reports a medium petroleum distillate in the fire sample and only light background in the comparison. Reasoning: the comparison shows the room was not uniformly contaminated, so the result is meaningful. Even so, the investigator must still show the pattern, the depth of charring, and eliminate a spilled lamp oil as a source before calling the fire incendiary.
Traps that cost marks
Destroying the item by testing in place; skipping the comparison sample; packaging in a container previously used for fuel; photographing but never logging; failing to preserve evidence that undercuts your own hypothesis - it will surface anyway and damage your credibility.