Page 13 - A Burning Question Fire Debris

normally shield the floor, and inverted cone-shaped (charged atoms or molecules), and molecules of
burn patterns on vertical surfaces. gasses, liquids, or dissolved solids.

To test fire debris for the presence of an ignitable In activated charcoal passive headspace sampling,
liquid, it is important to sample from areas likely activated charcoal is suspended in the headspace
to contain traces of the liquid after burning. These of the sample container. As the sample container
areas include the lowest regions of the burned is heated, the ignitable liquid present in the fire
area where ignitable liquids will run to and pool, debris is volatilized and collects in the headspace
insulated areas within the pattern that would be of the container. The volatilized ignitable liquid
protected from heavy burning, porous substrates then adsorbs onto the activated charcoal. After
(such as cloth/paper/cardboard) in direct contact heating, the activated charcoal is removed from the
with the pour pattern that would tend to absorb container and the ignitable liquid adsorbed onto the
ignitable liquids, seams or cracks where ignitable charcoal is then removed, or desorbed, by solvent
liquids would tend to settle, and the lightly burned extraction, generally with carbon disulfide. Carbon
edges of the pour pattern, as the center of a pour disulfide is frequently the solvent of choice because
pattern is often too heavily charred to generate it produces excellent desorption of most accelerants,
good results and ignitable liquids poured onto that is highly volatile, and generates low detector
area have likely burned completely or degraded. response when analyzed by a gas chromatograph
utilizing a flame ionization detector (FID). Carbon
Once an appropriate area of the fire debris at a disulfide, however, has a strong, unpleasant odor, is
scene has been identified for sampling, samples extremely flammable, and, at high levels, represents
must be properly collected to allow for later a significant health hazard to the nervous system.
analysis. Fire debris samples must be collected
in tightly sealing containers that will not interfere After solvent extraction of an activated charcoal
with future testing. These containers are frequently adsorbed fire debris sample, the presence of
glass jars or clean, unlined metal paint cans. If at all ignitable liquids is either confirmed or disproved
possible, enough debris should be collected to fill through analysis by a gas chromatograph-mass
approximately 2/3 of the container. The remaining spectrometer, or GC-MS. A GC-MS is an instrument
top 1/3 of the container, the space above the that combines a gas-liquid chromatograph with a
sample, is called the headspace and is required for mass spectrometer. A gas-liquid chromatograph
sampling. This sampling is generally performed is an instrument in which a liquid mixture sample
through a technique known as passive headspace is injected for analysis, carried through the
sampling. In passive headspace sampling, the instrument by an inert carrier gas (called the
container of fire debris is heated to volatilize any mobile phase), often helium, and separated into
ignitable liquids remaining in the debris, which then its components by the time it takes for those
collect in the headspace of the container. While the components to flow through a GC column. This
headspace may be sampled directly with a syringe time is retarded by the affinity of the components
for analysis, the most common forensic technique for a microscopic layer of liquid or polymer
involves the use of activated charcoal, often in the (called the stationary phase) coating the inside
form of strips or pellets. Activated charcoal is a of the column. The components come off of the
form of carbon that has been specifically processed column separately in a process known as elution,
to be extremely porous and to have a large surface and then enter the mass spectrometer. The
area capable of adsorption, the binding of particles mass spectrometer then further separates these
to a surface. These particles include atoms, ions components based upon the mass-to-charge ratio

THE MYSTERY OF LYLE AND LOUISE 13