Background Subtraction
Definition
The analytical practice of comparing fire debris EIPs against comparison sample EIPs from unburned substrate of the same type. Features present in both are attributed to substrate background; features present only in the debris are candidate accelerant signals.
- Field
- Fire debris analysis, ignitable liquid residue
- Compared items
- Debris EIP vs. unburned substrate comparison sample
- Purpose
- Separates substrate pyrolysis products from accelerant
Common questions
Why is an unburned comparison sample essential to this technique?+
Many building and furnishing materials produce hydrocarbon patterns when they pyrolyse in a fire that closely resemble ignitable liquid patterns, so without a genuinely unburned comparison sample of the same substrate an analyst cannot reliably tell whether a peak in the debris extract came from an accelerant or from the material itself burning.
What happens if a truly unburned comparison sample cannot be obtained?+
The analyst has to rely on reference libraries of known substrate pyrolysis patterns or interpret the debris extracted ion profile with greater caution, since without a matched comparison sample there is a higher risk of either a false accelerant identification or of missing a genuine accelerant masked by substrate interference.
How does background subtraction relate to ASTM E1618 classification?+
ASTM E1618 classifies ignitable liquids by their extracted ion profile pattern, and background subtraction is the step that confirms the pattern seen in the debris extract is not simply the pyrolysis signature of the substrate, which is a prerequisite for a defensible ASTM classification call.
Related terms
- 4-Vinylcyclohexene (4-VCH)
- A pyrolysis product of styrene-butadiene rubber (SBR) carpet backing, appearing at m/z 108 in the GC-MS. Elevated m/z 108 in a fire...
- Alkene Triplet Pattern
- The GC-MS signature of polypropylene pyrolysis: a repeating triplet of n-alkane, 1-alkene (terminal double bond), and diene peaks at each carbon number...
- Comparison (Control) Sample
- A sample of unburned substrate collected from the same room or proximate unburned area as the fire debris sample, of the same...
- Principal Component Analysis (PCA)
- A multivariate statistical method used in fire debris research to reduce chromatographic data matrices to principal components that capture major variance. Used...
- Pyrolysis
- The thermal decomposition of cellulose in paper at approximately 150 to 300 degrees Celsius, disrupting glycosidic bonds and releasing water and carbon...
- Substrate Interferent
- A pyrolysis product from building or furnishing material that produces a GC-MS chromatographic pattern overlapping with or resembling an ignitable liquid category...
- SWGFEX Type C Interferent List
- A documented list maintained by the Scientific Working Group for Fire and Explosion Investigation (and continued by OSAC) of substrates known to...
- Type C Interferent (E1618)
- ASTM E1618 designation for a substrate whose pyrolysis products can produce a chromatographic pattern meeting the criteria for an ignitable liquid category....
- Unresolved Complex Mixture (UCM)
- The broad hump of hundreds of co-eluting cycloalkane and isoalkane compounds in the TIC of heavy petroleum products (diesel, heavy fuel oil,...