Forensic Analysis of Fire Samples Flashcards
1
Q
How are fire scene residues analysed?
A
- Common accelerants are volatile liquids which evaporate quickly.
- Portable hydrocarbon detectors
- Organic vapours affect the electrical conductivity of semi-conductor detectors. Not susbtance specific.
- Trained dogs are also used
2
Q
How does a photo-ionisation detector work?
A
- This uses photons in UV range to break down molecules into positively charged ions.
- The gas becomes electrically charged and the ions produce a current. This gives an instantaneous reading with DL down to PPB. Data is automatically logged.
- This is a portable detector, for use at fire scene. Used when searching through large amounts of clothing for residues of ignitable liquids.
3
Q
What are the three classes of samples obtained at the fire scene?
A
- Fire sample: a sample from as near as possible to the seat of fire, tested for presence of accelerant.
- Comparative control sample - same material as fire sample taken from elsewhere in the room and thus uncontaminated with accelerant.
- Negative control samples - tools and empty containers of the same type used in the sampling process.
- Samples from suspect (if there is one).
4
Q
How should fire residues be packaged?
A
- Airtight and retain volatile long-chain hydrocarbons and aromatic molecules such as toluene and xylene
- Dual-bagging in nylon (inner) and polypropylene (outer). Closed by swan neck seal with cable tag.
- Glass jars can be used with a metal lid and must withstand vapour pressure of volatile liquids.
5
Q
How is a swan neck seal made?
A
- The bag should be sealed when no more than 75% full
- The neck is twisted firmly and is then doubled over.
- Hold the twist firmly and pass the seal over the neck of the bag.
- Tighten seal manually to create an effective seal.
- Accurate labelling of all samples is essential
- Good record keeping is essential to ensure continuity of evidence.
6
Q
What samples should be obtained in the laboratory?
A
- A positive control is obtained with a volatile flammable liquid on it. This is packaged in same manner as fire scene sample
- Usually analysed at end of analytical sequence to reduce risk of contamination.
- Additional negative control samples should be obtained from within the analysis laboratory, to guard against accelerant traces arising from contamination of laboratory equipment.
7
Q
What is passive headspace analysis?
A
- In passive headspace analysis, the vapour in the sealed container is exposed to activated charcoal, a chemical adsorbant, where it is trapped for later analysis.
- Temperature is between 60-80oC
- Time = 2 hours
8
Q
What is dynamic headspace analysis?
A
- The vapour is drawn from the sample through an adsorbent using a flow of carrier gas such as dry nitrogen.
- This method was developed from methods used for environmental monitoring of ultra-trace quantities.
- The adsorbent is activated charcoal or TENAXTM - a porous polymer which traps organic volatiles but has a low affinity for water.
- DL as low as 1 microlitre of accelerant.
- Applicable to wide range of accelerants, inc alcohols/ketones.
9
Q
How is gas chromatography implemented after headspace analysis?
A
- The samples containing volatile solutes are injected into a high temperature port and are vaporised.
- The sample is carried into a separating column by a stream of inert gas.
- GC column separates solutes by adsorption on an active column material.
- Each solute has its own characteristic retention time depending on the setup of the experiment.
10
Q
How does a flame-ionising detector detect the separated components in the gas column?
A
- The gas stream carrying the separated components passes through a detector, which responds to the solute molecules
- A flame ionisation detector (FID) uses a small hydrogen-oxygen flame to burn sample solutes as they emerge from the column.
- A high voltage is applied across electrodes close to the flame. Most carbon-containing compounds produce charged molecular fragments when burnt. These fragment ions allow an electrical current to flow between the electrodes.
11
Q
What are the limitations/advantages of using GC for fire residue analysis?
A
- Not many ions produced by functional groups so GC detects solute molecules by their hydrocarbon skeleton. It is not selective for the class of compound
- Method is more sensitive to heavier hydrocarbons which produce a higher yield of ions. This is not very important in ID, since compounds have already been separated by the column, but it does affect quantitative analysis.
12
Q
What information can be obtained from interpretation of GC?
A
- Retention time depends primarily on molecular mass
- It depends secondarily on structure of molecule
- For samples with a broad boiling point range, programmed temperature increase of the column is used.
- Recognition of chromatographic patterns of common flammable liquids.
- Recognition of mixtures.
- Weathering of samples due to ageing at scene and heating.
13
Q
What GC pattern is typical of distillate fuels?
A
- The spread of components with different numbers of carbon atoms (resembling Gaussian distribution is characteristic of distillate fuels.
- This allows these fuels to be distinguished from mixtures of unrelated single substances that might be present in a sample, such as pyrolysis products.
14
Q
What is typical of weathered samples on the Gas Chromatogram?
A
- The most volatile components are missing from fire samples as they have evapourated before samples could be collected.
- The most volatile components have a lower molecular mass, among compounds of the same type.
- This does not necessarily apply if low molecular mass oxygenate additives are being compared with higher molecular mass hydrocarbons e.g.
15
Q
What other substances may be present in the GC?
A
- Combustion/pyrolysis of carpets, polymers, adhesives, wood products and clothing
- Fire retardents e.g. brominated hydrocarbons (remove free radicals, preventing combustion)
- Surfactants and fire suppression agents from foams:
- Phos-Chek WD 881 foaming agent contains long-chain sulfonated alkenes, alcohols and limonene (interferes with GC analysis of accelerants. Also in perfumes etc.)
- F500 - long aliphatic acids, alcohols and ethers.
