Vapour Loss
Definition
The escape of low-boiling hydrocarbon fractions from a sealed fire debris sample through an imperfect container seal, or by volatilisation during warm storage. Selectively removes lighter components and can alter the apparent GC-MS pattern class.
- Field
- Fire debris and accelerant analysis
- Mechanism
- Escape of low-boiling hydrocarbon fractions through an imperfect seal
- Effect
- Selective loss of lighter components skews the GC-MS pattern
- Mitigation
- Prompt sealing in unlined metal cans and cool storage
Common questions
Why does vapour loss matter for classifying an ignitable liquid?+
Standard classification schemes such as ASTM E1618 rely on the relative proportions of light, medium, and heavy hydrocarbon fractions. Because vapour loss removes lighter components preferentially, it can shift a sample's pattern toward a heavier classification than the liquid actually was when the fire started.
How can an examiner tell whether vapour loss has occurred rather than the sample never containing an accelerant?+
Examiners look for an internally consistent but truncated pattern, such as a normal-alkane series missing its earliest members while later members and characteristic branched or aromatic markers remain intact. A genuinely accelerant-free sample instead shows the pyrolysis products of the burned substrate rather than a truncated petroleum pattern.
Related terms
- Accelerant Detection Canine (ADC)
- A dog trained by operant conditioning to alert on the vapour signature of ignitable liquids. Certified under ASTM E2323; used as a...
- ASTM E1492
- Standard Practice for Receiving, Documenting, Storing, and Retrieving Evidence in a Forensic Science Laboratory: establishes container requirements for fire debris (metal can...
- ASTM E2323
- Standard guide for forensic engineering investigations of buildings affected by fire. Specifies minimum competency requirements for accelerant detection canine teams, including vapour...
- Combustible Gas Indicator (CGI)
- An instrument that measures vapour concentration as a percentage of the lower explosive limit (LEL) using a catalytic bead sensor. Used for...
- Comparison Sample
- A sample of the same substrate type collected from an unburned area of the same room or adjacent space. Provides the substrate...
- Handler Cueing
- The unintentional communication of handler expectation to a detection dog through subtle postural, gestural, or vocal cues. Identified by Lit et al....
- NFPA 921
- US National Fire Protection Association Guide for Fire and Explosion Investigations. The internationally accepted methodology Indian fire investigators and SFSL fire divisions...
- Photoionisation Detector (PID)
- An electronic instrument that uses a UV lamp (typically 10.6 eV) to ionise organic vapour molecules and measures the resulting ion current....
- SWGFEX
- Scientific Working Group for Fire and Explosion Investigation. A US-based multidisciplinary body that developed best practice guides for fire debris collection, canine...