Accelerant Detection: ASTM E1387 / E1618 and Passive Headspace Concentration

The passive headspace concentration method works by exploiting the equilibrium partitioning of volatile organic compounds (VOCs) between a heated solid or liquid debris matrix and the enclosed gas phase above it. The ASTM E1412 procedure requires:

A sealed, airtight metal container (typically a clean, unlined paint can of one-litre or half-litre capacity) into which the fire-debris sample is placed. The can must not be lined with any organic coating that could contribute VOC contamination. New, solvent-rinsed cans are used for each exhibit. The sealed container is brought to the laboratory on the same day of collection where possible, kept cold during transport (but not frozen), and logged into the evidence management system.

A activated charcoal strip (also called a carbon strip, or a Carbotrap strip in some US laboratory SOPs) is prepared from Carbograph-1 (Leco Corporation), coconut-charcoal granules, or equivalent characterised adsorbent. The strip typically measures approximately 1 x 3 cm, containing 40 to 100 mg of activated charcoal. The strip is suspended inside the sealed can using stainless steel wire or a heat-resistant clip, positioned in the headspace above the debris but not in contact with it or with the can walls. Contact with wet debris contaminates the strip with water and substrate extract.

The sealed can is placed in an oven at 60 to 80°C for 16 hours (overnight). At this temperature, VOCs in the debris phase equilibrate into the headspace at elevated concentrations, and those VOCs then adsorb onto the charcoal strip. The oven temperature must be below the flash point of the anticipated residues and below the point at which significant pyrolysis of unburned substrate begins. Sixty to eighty degrees Celsius is chosen to balance adsorption efficiency against the risk of generating additional pyrolysis products in the oven.

After 16 hours, the can is opened in a fume cupboard and the strip removed with solvent-rinsed tweezers. The strip is placed in a small glass vial and eluted with 150 to 200 microlitres of carbon disulfide (CS2), the solvent of choice for most activated-charcoal applications because it has minimal UV-absorption and does not produce a large GC-FID peak that would obscure early-eluting compounds. After 15 to 30 minutes of elution, the CS2 eluate is transferred to a GC vial and injected. Some laboratories use diethyl ether as an alternative eluent for charcoal types that show poor CS2 desorption efficiency. The choice of eluent must be documented in the laboratory SOP and validated for the specific charcoal type used.

Active headspace (ASTM E1413, Standard Practice for Separation of Ignitable Liquid Residues from Fire Debris Samples by Dynamic Headspace Concentration) is an alternative for high-yield samples, where a purge-and-trap or Tenax tube concentrates headspace vapours by active gas flow. Active headspace offers higher sensitivity for trace residues but introduces the risk of over-concentrating common substrate volatiles and is more instrument-dependent.

ASTM E1387 (Standard Test Method for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography) was the original analytical standard for fire debris GC analysis, first published in 1990. It was withdrawn in 2010 without replacement as a standalone standard, its functions having been incorporated into and superseded by ASTM E1618. Any fire-debris analytical report that cites ASTM E1387 as the controlling method standard for analysis conducted after 2010 is citing a withdrawn document. In the US context, this may be relevant to challenges under Daubert to the reliability of the methodology (Federal Rules of Evidence Rule 702). In the UK, the equivalent challenge would be framed under the Criminal Procedure Rules Part 19 expert-witness requirements.

ASTM E1618 (Standard Test Method for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography-Mass Spectrometry) is the current active standard, first published in 1997 and periodically revised, with the 2025 edition (E1618-25) current. E1618 requires gas chromatography with mass spectrometric detection (GC-MS) rather than GC with flame ionisation detection (GC-FID) alone. The mass spectrometer provides two critical capabilities that GC-FID cannot: compound identification by spectral matching against a mass spectral library (NIST Mass Spectral Library, Wiley Registry), and selective ion monitoring (SIM) or extracted ion profiling that allows co-eluting compounds to be distinguished by their mass spectral ions.

E1618 defines the classes of ignitable liquid residue by GC-MS pattern, not by compound identification. A fire-debris extract is classified into one of several petroleum product classes based on the chromatographic pattern of compound groups: the aromatic fraction, the branched-alkane fraction, the n-alkane fraction, and the cyclic fraction. The classification is pattern-based because fire destroys some fraction of all compound classes, and the surviving pattern rather than the absolute compound inventory determines classification.

The Ignitable Liquid Reference Collection (ILRC), maintained by the National Centre for Forensic Science at the University of Central Florida, is the reference database underlying E1618 pattern comparisons. The ILRC contains chromatographic data for over 600 ignitable liquid products including gasolines from multiple suppliers and seasons, kerosene, diesel, aviation fuel, charcoal lighter fluids, paint thinners, and specialty solvents. The database is updated as new products enter the market. In the UK, the Forensic Science Service (now disbanded) maintained an equivalent reference collection; work on a European standardised reference collection is ongoing under the ENFSI (European Network of Forensic Science Institutes) Fire and Explosion Working Group. In India, the Central Forensic Science Laboratory (CFSL) and State Forensic Science Laboratories (SFSLs) maintain local reference collections of Indian market petroleum products, which are essential because product formulations vary between markets.

ASTM E1618 classifies ignitable liquid residues into eight product classes based on GC-MS pattern. Each class has a characteristic carbon number range (the range of n-alkane chain lengths that dominate the chromatogram), a characteristic aromatic fraction composition, and a characteristic branched-alkane distribution.

Light petroleum distillate (LPD): Carbon number range approximately C4 to C9. The chromatogram shows a cluster of peaks in the light-end portion of the run eluting within the first few minutes. The compound distribution includes pentane, hexane, heptane, and octane with variable branched-alkane content. LPD products include charcoal lighter fluid (lighter grades), some paint thinners, and camping fuel. The LPD class overlaps significantly with the range produced by polyethylene pyrolysis, which is the primary source of false-positive LPD identifications.

Gasoline: Carbon number range approximately C4 to C12, with a complex multicomponent pattern dominated by aromatic compounds: benzene, toluene, ethylbenzene, xylene isomers (the BTEX group), and trimethylbenzene isomers (1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene). Branched alkanes (isooctane / 2,2,4-trimethylpentane, the reference octane compound) are also prominent. Gasoline has the most complex and distinctive GC-MS pattern of any ignitable liquid class. The aromatic compound profile is the key discriminator. Gasoline also contains oxygenate blending components: methyl tert-butyl ether (MTBE) in most US and European market gasolines; ethanol (at up to 10 per cent by volume in most EU gasolines under the Fuel Quality Directive 2009/30/EC and EN 228, and targeting 20 per cent in India under the National Biofuel Policy 2018 as amended in 2022). MTBE, where present, is a useful marker for gasoline because it is not produced by common substrates.

Medium petroleum distillate (MPD): Carbon number range approximately C8 to C13. Includes mineral spirits, some paint thinners, and charcoal lighter fluids of the heavier grades. The aromatic fraction is lower than gasoline and the n-alkane pattern is smoother.

Kerosene: Carbon number range approximately C9 to C16, with a dominant n-alkane backbone peaking around dodecane (C12) and tridecane (C13), pentadecane (C15) at the trailing edge. Kerosene is the most important accelerant class in forensic chemistry globally because it is cheap, widely available, and sold without restriction in most jurisdictions. The broader petroleum products and lubricants chemistry topic covers the refinery-process distinctions between kerosene, diesel, and transformer oil that determine their chromatographic signatures. In India, kerosene has historically been subsidised for domestic cooking (the public distribution system kerosene, colloquially "mitti ka tel") and is a common accelerant in both accidental and deliberate fires. In many Sub-Saharan African countries and South Asian markets, kerosene remains the most common accelerant encountered. The chromatographic signature of kerosene survives fire well because the C12-C16 n-alkane backbone is relatively involatile compared to LPD and gasoline components.

Diesel / heavy petroleum distillate (HPD): Carbon number range approximately C9 to C20+, with a prominent n-alkane backbone extending to heptadecane (C17), octadecane (C18), and nonadecane (C19). The carbon-number distribution is broader than kerosene. Diesel fuel (automotive gas oil) is common in commercial, industrial, and transport-related arson scenes. The prismatic (UCM, unresolved complex mixture) hump under the n-alkane peaks is characteristic of diesel and HPD but not of kerosene.

Isoparaffinic, naphthenic-paraffinic, normal-alkane, and aromatic classes cover more specialised petroleum products (Isopar solvents, cycloalkane-rich products, mineral oil-based products, and aromatic solvents such as toluene and xylene respectively).

ASTM E1618 uses two complementary chromatographic display strategies: total ion chromatogram (TIC) for the full-pattern overview, and extracted ion profiles (EIP, also called target compound chromatograms or TCCs) for class-specific compound identification.

Extracted ion profiles isolate a specific mass-to-charge ratio (m/z) from the full mass spectral data and plot intensity against retention time only for ions at that m/z. Because different chemical classes produce different diagnostic ions, the extracted ion profile effectively acts as a class-selective filter on the complex chromatographic mixture.

The principal diagnostic ions used in ASTM E1618 analysis are:

m/z 57: The major fragment ion for n-alkanes (produced by C4H9+ or C3H5O+ fragments). Plotting the ion at m/z 57 produces a chromatogram showing the n-alkane envelope, with each peak corresponding to an n-alkane. The spacing between n-alkane peaks is even, and the distribution shape (bell-curve, shoulder, skew) characterises the product class.

m/z 91: The tropylium cation (C7H7+), the characteristic fragment ion for alkylbenzenes (toluene, xylenes, ethylbenzene, trimethylbenzenes, C3-benzenes). Plotting m/z 91 isolates the aromatic fraction and is the primary gasoline indicator.

m/z 105: The methyltropylium cation (C8H9+), from C2-benzenes (xylenes, ethylbenzene) and C3-benzenes. Used alongside m/z 91 to characterise the alkylbenzene distribution.

m/z 119: C3-benzene fragment (trimethylbenzenes, propylbenzenes, 1,2,3-trimethylbenzene). The 1,2,4-trimethylbenzene / 1,3,5-trimethylbenzene / 1,2,3-trimethylbenzene ratio in the m/z 119 extracted ion profile is a sub-class indicator for the aromatic fraction of gasoline and MPD products.

A complete ASTM E1618 analytical report includes: the TIC, the m/z 57 extracted ion profile (n-alkane pattern), the m/z 91 extracted ion profile (aromatic pattern), the m/z 105 and m/z 119 profiles, and a verbal description matching the observed pattern to the relevant class in the ILRC database or the E1618 class descriptions. The analyst also reviews the m/z 71 (for light-end alkanes), m/z 43 (for oxygenated compounds including MTBE at its characteristic retention time), and m/z 77 (benzene ring) profiles as needed.

The central analytical challenge in fire-debris chemistry is distinguishing ignitable liquid residue from pyrolysis products of the substrate materials burned in the fire. The challenge is not theoretical: it has produced wrongful arson convictions in the United States (the Willingham case is the most cited example), in the United Kingdom (several cases reviewed by the now-defunct Forensic Science Service), and almost certainly in other jurisdictions where post-conviction science review is less well developed.

The substrate pyrolysis problem arises because the hydrocarbon products of burning building materials overlap in GC retention time and mass spectral character with the hydrocarbon products of petroleum ignitable liquids. The overlap is not complete, but it is significant:

Polyethylene flooring, cable insulation, and packaging materials produce n-alkane and 1-alkene series covering C3 to C20. This distribution overlaps the LPD class (C4-C9) and can extend into the MPD and kerosene ranges. The 1-alkene series (1-hexene, 1-heptene, 1-octene, etc.) produces characteristic ions at m/z 55, 69, and 83 that are present in some petroleum distillates but not others. A skilled analyst can use the alkene-to-alkane ratio in the extracted ion profiles to suggest substrate-pyrolysis contribution, but the discrimination is not absolute.

Carpet backing materials, particularly SBR (styrene-butadiene rubber) latex, produce aromatic pyrolysis products including toluene, xylenes, and styrene, which at low concentrations can add to the m/z 91 extracted ion profile in a way that resembles a trace gasoline residue. Understanding which pyrolysis products each substrate generates requires the combustion-chemistry foundation in fire chemistry, pyrolysis and the arson investigation framework. Identifying the aromatic profile source requires the analyst to know the carpet backing composition, which requires scene documentation before sampling.

NFPA 921 addresses this directly in Chapter 22 (the accelerant-detection chapter): the fire investigator and the laboratory analyst must collectively document all substrate materials present in the area of interest before collection of debris samples, and the analyst must explicitly evaluate whether the observed chromatographic pattern can be explained by substrate pyrolysis alone. When that documentation is absent or inadequate, the analyst's confidence in an accelerant identification should be reduced.

In the US, the Scientific Working Group for Fire and Explosions (SWGFEX, now the Organization of Scientific Area Committees for Fire and Explosives (OSAC) under NIST) has published guidance on substrate pyrolysis considerations. In the UK, the Chartered Society of Forensic Sciences and the National Fire Chiefs Council reference the same NFPA 921 / ASTM E1618 framework. In India, the CFSL SOP for fire-debris analysis references substrate documentation as a required pre-analytical step.

1. Scene documentation
   Before any sample is collected, the fire investigator photographs and records all substrate materials present in the sampled area: flooring type, carpet backing, cable insulation, furniture upholstery, stored products. This information accompanies the evidence to the laboratory as a substrate inventory.
2. Evidence collection
   Debris samples are collected from areas of suspected accelerant use and from comparison areas (same substrates, no suspected accelerant). The [sample collection and chain-of-custody rules for chemical evidence](/topics/forensic-chemistry/sample-collection-packaging-and-chain-of-custody-for-chemical-evidence) govern container selection, labelling, and continuity documentation for these exhibits. Both are packaged in clean, unlined metal paint cans immediately after collection. Labels include case number, exhibit number, area description, date, collector identity.
3. Passive headspace concentration
   Activated charcoal strips suspended in sealed cans, oven at 60-80°C for 16 hours. One strip per can. Comparison substrate samples run in parallel to establish the pyrolysis baseline for those substrates.
4. CS2 elution and injection
   Strip removed under fume extraction, eluted with 150-200 uL CS2 in sealed vial. 1-2 uL injected onto GC-MS (typically Agilent 7890B / 5977B or equivalent). Column: 30 m x 0.25 mm DB-5 or equivalent, temperature programme from 40°C to 280°C.
5. Pattern recognition
   TIC reviewed for hydrocarbon distribution. Extracted ion profiles at m/z 57, 91, 105, 119 generated. Pattern compared against ILRC database. Substrate contribution evaluated by comparing to comparison-sample chromatogram. Classification reached per ASTM E1618 class definitions.
6. Report and conclusion
   Conclusion stated as a finding of consistency with a named ignitable liquid class, with caveats about substrate contribution where relevant. A finding of no ignitable liquid detected is reported if the pattern is within substrate-pyrolysis baseline. Report includes the TIC and all extracted ion profiles as figures.

The composition of ignitable liquid residue recovered from fire debris is not the same as the composition of the original ignitable liquid. Fire, heat, and water change it in predictable ways that the analyst must account for.

Evaporative weathering is the primary transformation. Volatile components of the ignitable liquid (benzene, toluene, the C5-C7 n-alkane fraction of LPD, ethanol in blended gasolines) evaporate preferentially from the ignitable liquid pool before ignition and during the fire event. Post-fire, they continue to evaporate from any residual liquid trapped under flooring material or absorbed into porous substrates. What the charcoal strip captures 24 to 72 hours after fire suppression reflects evaporatively weathered residue. LPD residue collected more than 24 hours post-fire may show significant loss of the C5-C6 light end, making it resemble an MPD or a weathered gasoline rather than a fresh LPD.

Gasoline weathering is particularly well characterised in the ILRC database. Fresh gasoline has a broad C4-C12 pattern with prominent BTEX aromatics. As weathering progresses, the lighter components (benzene, toluene, C4-C6 alkanes) diminish, and the pattern shifts toward the C8-C10 trimethylbenzene region. Heavily weathered gasoline can be misclassified as MPD. The trimethylbenzene signature and the MTBE presence (where applicable) are more weathering-resistant than benzene and toluene and are the most reliable gasoline sub-class indicators in heavily weathered samples.

Fire suppression water adds a further complication. Firefighting water washes water-soluble components from debris (shorter-chain alcohols, water-soluble glycol components of some specialty fuels) and may physically redistribute liquid residues across the scene. The analyst who receives a sample from an area where the fire brigade used high-volume hosing is receiving a sample from a washed and diluted matrix. The passive headspace method, by concentrating all VOCs above the debris regardless of water content, is relatively robust to water-washing compared to liquid extraction methods, but the overall residue concentration will be lower.

Biodegradation is relevant only in debris samples stored for extended periods (weeks to months) before analysis. In routine casework this is uncommon, but in complex litigated cases where original samples must be re-analysed years later, biodegradation of the more labile aromatic fraction (particularly benzene and toluene, which are substrates for common soil bacteria) can shift the classification from gasoline toward a weathered-gasoline or MPD pattern.

The SWGFEX / OSAC guidance and the ASTM E1618 commentary address weathering by requiring the analyst to document the time elapsed between fire suppression and sample collection, the storage conditions, and to note in the report that the characterised pattern may represent an evaporatively weathered product rather than a fresh ignitable liquid residue.