Analysis of Petroleum Products and Fire Accelerants

Crude oil is fractionally distilled into cuts that the forensic analyst will see again and again as accelerant residues. Memorise the ladder by carbon range, by common name, and by where each fraction sits on a GC-FID chromatogram.

The light petroleum distillates sit at C4 to C9: pentane, hexane, light naphtha, petroleum ether, some camping fuels and lighter fluids. They are dominated by short-chain n-alkanes and branched alkanes, and they elute early on a non-polar GC column. The medium petroleum distillates sit at C8 to C12: mineral spirits, paint thinner, charcoal-starter fluid, some kerosene-range solvents. Gasoline is its own class because the refinery blends in aromatics and oxygenates that give a fingerprint pattern, not a simple distillate curve. Kerosene and jet fuel A sit at C9 to C16, with the n-alkane envelope peaking around C11 to C12. Diesel is the heavy petroleum distillate band at C10 to C25 with the envelope peaking around C13 to C16. Lubricating oil sits beyond C20 as a smooth, late-eluting hump that almost never appears in arson cases on its own.

For NET MCQ purposes, three facts carry most of the marks. Gasoline is uniquely aromatic-rich (toluene, xylenes, C3- and C4-alkylbenzenes, naphthalenes). Kerosene is a clean n-alkane envelope centered around C11. Diesel looks like kerosene shifted to heavier carbon numbers with a wider envelope. Pyrolysis hydrocarbons from carpet and foam overlap the kerosene-to-diesel range, which is exactly why ASTM E1618 demands GC-MS pattern recognition, not GC-FID alone.

ASTM E1387 (now withdrawn but still examinable) was the original colour-coded classification of ignitable-liquid residues. ASTM E1618 (current) is the modern standard and the one NTA tests. E1618 places every ignitable liquid into one of eight classes by combining boiling-range data with chemical-class identification on a GC-MS total ion chromatogram (TIC) supplemented by extracted ion profiles.

The eight E1618 classes, in the order MCQs usually list them:

1. Light petroleum distillate (LPD). C4 to C9 n-alkanes plus branched alkanes. Examples: pentane, hexane, petroleum ether, some lighter fluids.
2. Gasoline. Distinct aromatic-rich pattern: toluene, ethylbenzene, m+p-xylene, o-xylene, C3- and C4-alkylbenzenes, plus methylnaphthalenes. Oxygenate additives (MTBE in older fuel, ethanol in modern Indian E10 petrol) can be diagnostic.
3. Medium petroleum distillate (MPD). C8 to C12 paraffinic and naphthenic hydrocarbons. Examples: mineral spirits, paint thinner, charcoal-starter fluid.
4. Kerosene-range product. C9 to C16 n-alkane envelope. Examples: kerosene, jet fuel A, lamp oil. In India the dominant arson accelerant in homicide and dowry-death cases.
5. Heavy petroleum distillate (HPD). C9 to C23+ n-alkane envelope shifted heavy. Examples: diesel, fuel oil number 2, heating oil.
6. Isoparaffinic products. Branched alkanes only, almost no n-alkanes or aromatics. Examples: some specialty solvents and odourless mineral spirits used in dry cleaning and printing.
7. Aromatic products. Toluene, xylenes and trimethylbenzenes dominate, with little to no aliphatic component. Examples: industrial aromatic solvents, some specialty thinners.
8. Naphthenic-paraffinic products. Cycloalkanes (naphthenes) plus n- and iso-alkanes, almost no aromatics. Examples: insecticide carriers, some lamp oils, some copier toner solvents.
9. Normal-alkane products. Sub-class with a clean homologous series of n-alkanes only.

Fire-debris sampling is one of the most-tested practical bits of Unit VI, because the container choice decides whether the analyst sees an accelerant signal at all. The order of preference for Indian SFSL field kits is:

1. Unlined metal cans (clean paint cans, lever-lid). The gold standard. Steel walls do not absorb hydrocarbons and the lever-lid seals tight. A 1-litre or 1-gallon paint can holds enough debris and enough headspace for a single passive-headspace extraction.
2. Glass jars with PTFE-lined caps. Acceptable when metal cans are unavailable. PTFE liner prevents absorption into the cap. Risk: glass breaks in transit.
3. Nylon polymer bags (heat-sealed). Acceptable, nylon is impermeable to most petroleum volatiles in the short term.
4. Polyethylene zip bags. Forbidden. Polyethylene is permeable to light hydrocarbons; pentane and hexane diffuse out within hours, and even C9 to C12 components are lost over days. The first MCQ trap in this topic is "which bag must NOT be used for fire debris" and the answer is polyethylene.

The sample is collected from the suspected seat of fire and from a control area away from the burn pattern. The control catches the substrate's background pyrolysis hydrocarbons (carpet fibres, polyurethane foam, vinyl flooring) so the analyst can subtract them. Every can is labelled with case number, location, date, collector name and seal number; the chain-of-custody register is started at the point of collection per BNSS Section 176(3) for offences punishable with seven years or more. Further fire-scene context lives in the book chapter on fire and burn pattern interpretation, which covers V-patterns, low burns and pour patterns alongside accelerant evidence.

Four extraction families are in the NET-testable repertoire. Memorise them by ASTM number, principle and trade-off.

Passive headspace concentration on activated charcoal strip (ASTM E1412). The workhorse method, developed by John Lentini and colleagues in the 1980s and now the default in accredited labs worldwide. A small activated-carbon strip (Albrayco or equivalent) is suspended by a wire from the lever-lid of a sealed paint can holding the debris. The can is heated in an oven at 60 to 80 degrees Celsius for 12 to 24 hours. Volatile hydrocarbons partition from the debris into the headspace, are adsorbed onto the carbon, and concentrate over time. The strip is then eluted with about 0.5 mL of carbon disulfide (CS2), which displaces the hydrocarbons. The CS2 extract is injected onto a GC-FID and, for confirmation, a GC-MS. Strengths: non-destructive (sample can be re-extracted), high sensitivity, no pyrolysis bias because of the gentle oven temperature, validated standard. Weakness: overnight turnaround.

Solid-phase microextraction (SPME) headspace. A polymer-coated fused-silica fibre (PDMS or PDMS-DVB, 100 micrometre coating) is exposed to the heated headspace of a smaller vial holding the debris. After 10 to 30 minutes, the fibre is withdrawn and thermally desorbed directly in the GC injector at 250 degrees Celsius. Strengths: fast (under an hour), solvent-free, very low detection limits for light volatiles. Weaknesses: lower total capacity than charcoal strip (the fibre saturates with heavy debris), and the sample cannot be re-extracted.

Dynamic headspace concentration (Tenax tube). Air is actively drawn through the headspace of the warmed debris and over a Tenax (porous polymer) adsorbent tube. Tenax is then thermally desorbed onto the GC. Reserved for trace-level cases where passive methods fail. Operationally more complex; rare in routine Indian casework.

Solvent extraction and steam distillation. Historical methods. Debris is soaked in pentane or carbon disulfide, or steam-distilled into a Likens-Nickerson apparatus. Both are destructive, both introduce solvent peaks that obscure light volatiles, and both have been displaced by passive headspace. They remain on the NET syllabus as MCQ distractors.

Once the extract is in a vial, the instrumental side is straightforward but conceptually rich. The standard analytical chain is GC-FID for pattern matching against the lab's reference collection, then GC-MS for confirmation. The book chapter on gas chromatography (GLC) and detectors is the deep dive on the column, oven program, FID flame chemistry and detector selectivity; the hyphenated GC-MS, LC-MS and GC-FTIR techniques chapter covers the EI source, quadrupole filter and SIM / MRM modes used for confirmation.

The column is almost always a 30 metre, 0.25 mm internal diameter, 0.25 micrometre film non-polar 5 percent phenyl methyl polysiloxane (DB-5 or equivalent). The oven program ramps from 40 to 280 degrees Celsius over 25 to 35 minutes, which captures everything from pentane to C25 in a single run.

Gasoline pattern recognition. On the TIC, gasoline shows the characteristic alkylbenzene envelope: toluene, ethylbenzene, m+p-xylene, o-xylene, then a cluster of C3-alkylbenzenes (1,3,5-, 1,2,4- and 1,2,3-trimethylbenzene), C4-alkylbenzenes, and finally the methylnaphthalenes and dimethylnaphthalenes. Pulling extracted ion profiles at m/z 91 and 105 highlights the alkylbenzenes; m/z 128, 142 and 156 highlight the naphthalene family. The pattern is so distinctive that even severely weathered gasoline, which has lost everything lighter than C8, is recognisable from the C9 to C12 aromatic envelope alone. Oxygenate additives are diagnostic too: MTBE (m/z 73) in older or imported fuel, ethanol (m/z 31, 45) in modern Indian E10 petrol.

Kerosene and diesel. Both show clean n-alkane envelopes. Extracted ion profile at m/z 57, 71 and 85 (the n-alkane fragments) gives the classic "picket fence" of C9, C10, C11, C12 ... peaks evenly spaced. Kerosene peaks around C11; diesel peaks around C13 to C16 with a wider envelope.

Interpretation pitfalls. Carpet, polyurethane foam, vinyl flooring and tyre rubber all release hydrocarbons when they burn or smoulder. These pyrolysis products overlap the kerosene-to-diesel range and can fool a naive GC-FID read. The analyst always runs a control sample from an unburned area of the same substrate and subtracts that background. GC-MS pattern recognition (the aromatic envelope shape, the EIP picket fence) is what defeats pyrolysis interference. The book chapter on

NTA's syllabus says "petroleum products and other incendiary materials", and the second half matters. Half a dozen non-petroleum accelerants and incendiaries are testable.

Alcohols. Ethanol and methanol are common in arson involving country-liquor stocks or hand-sanitiser stores. They show up cleanly on GC-FID at very short retention times, well before the petroleum range. The E1618 oxygenated-solvent class catches them.

Turpentine. Pinene-rich, distinct GC pattern dominated by alpha-pinene, beta-pinene and limonene. Used historically in paint and varnish work and occasionally in arson. Falls outside the petroleum classes; reported as "turpentine" by reference pattern.

Organic solvents. Acetone, methyl ethyl ketone (MEK), ethyl acetate, isopropanol. Common in industrial arson involving paint shops, garages, dry cleaners. Oxygenated-solvent class on E1618.

Thermite. Iron oxide plus aluminium powder, ignited by a magnesium ribbon. Reaches around 2500 degrees Celsius and burns through steel. Almost no organic residue; investigators look for unreacted iron oxide and aluminium dust at the seat and for the molten-iron slag bead. SEM-EDX is the confirmation tool.

Magnesium chips and ribbon. Bright white flame, very high temperature, used as an igniter rather than a bulk accelerant. Residue is white magnesium oxide ash.

White phosphorus. Self-igniting on air, burns at around 800 degrees Celsius producing dense white smoke (P2O5 fume). Residue is phosphoric acid on adjacent surfaces. Forensically distinctive because white phosphorus stains exposed materials in characteristic patches, and the phosphate residue is confirmable by colour test (ammonium molybdate) and ICP-OES.

Each of these is recognised by a combination of scene observation (slag bead, white ash, phosphoric-acid stain) and laboratory confirmation (SEM-EDX, ICP, colour test) rather than by GC-MS alone.

The Indian institutional frame for fire-debris analysis sits in two layers. CFSL Hyderabad (under DFSS, MHA) operates the central fire-debris division with a full GC-MS, SPME and dynamic-headspace setup, and handles cases referred from across the country in NIA, CBI and high-profile state matters. State SFSLs (Maharashtra at Kalina, Tamil Nadu at Chennai, Karnataka at Bengaluru, Uttar Pradesh at Lucknow and Agra, and so on) run their own fire-debris wings, mostly using charcoal-strip extraction and GC-FID with GC-MS confirmation where available.

Three Indian arson patterns recur in the casework literature and are worth knowing as MCQ context.

Petrol-bottle Molotov. Glass bottle of petrol stoppered with a cloth wick, lit and thrown. Common in communal-violence and political-violence cases. Debris from the seat shows a heavy gasoline signature plus glass shards from the bottle. Useful corroboration when the suspect's hands or clothing are also tested for trace gasoline by SPME-GC-MS.

Kerosene in homicide-arson. Kerosene is cheap, widely available in rural India, and burns long enough to consume a body partially. Debris from suspected homicide-arson scenes regularly returns a clean C9 to C12 kerosene envelope. The forensic question is whether the kerosene was present as a normal household accelerant (lamp, stove fuel) or was poured. The pour pattern, the V-pattern, low burns and concentration of accelerant at the body are read together.

Dowry-death "stove burst" cases. A long-standing forensic and legal dispute. The defence narrative is that a kerosene stove malfunctioned and the burning fuel splashed onto the victim. The forensic counter-narrative looks for accelerant distribution that is inconsistent with a single-point stove burst: kerosene on the back of the victim, on multiple separated parts of the room, or in patterns that match deliberate pouring. CFSL and SFSL fire wings have given expert opinions in many such cases under Section 304B IPC and now BNS Section 80 (dowry death).