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For most of the
thirty years or so this author has been involved
with fire debris analysis, investigators have been insisting that lab methods for
volatile accelerants in fire debris are not
sensitive enough and that results would be a lot
more accurate (and presumably a lot more reflective of their conclusions about the fire) if
only the lab techniques were more sensitive. Not all that many years ago, the human nose
was the standard technique for identifying the presence of ignitable liquids in fire debris.
It was pretty sensitive (20ppm) and could be trained to be quite discriminating, but it
was subject to variables (person-to-person variations, colds, allergies, fatigue) and
somewhat subjective (ask five different people what
a sample of fire debris smells like and you are likely to get two or three different answers).
It was not until 1962 that gas chromatography (GC) was first suggested for fire
debris analysis. GC was pretty crude 10 or
12 peaks was considered a good result but
you still had to get the volatile part of the
sample from the can into the GC. This usually
meant solvent extraction for heavy compounds, heated headspace for light, volatile ones,
or steam distillation for a wide variety. Each technique had its own limit of detection.
Headspace sampling was simple and non-invasive but it was not very sensitive (with
12cc of vapor sampling, a lower limit was on the order of 2050ppm in the air sample)
and then only for liquids volatile enough to evaporate significantly at the temperatures used.
Steam distillation and solvent extraction had the potential to be more sensitive but
were limited by the physical limits of sample handling. Adding a dollop of solvent could increase the volume of a tiny amount of
distillate such that it could be recovered from
a distillation condenser receiver but that added to the risk of contamination. Evaporating
the solvent in a solvent extraction increases the sensitivity, but one runs the risk of
driving off the sample itself with too much
evaporation. All this meant that the same
concentration of sample that could be isolated and
identified by GC was about the same as a well-trained human nose could detect. Some
investigators complained that if they could not smell anything, the lab never identified
anything anyway, so why bother with the lab? There was always a demand for more
sensitivity for arson debris.
In the late 1970s and early 1980s. GCs
got better, as did extraction methods. Stealing the concept from the environmental
sciences of concentration by trapping on activated charcoal , it was found that exposing a
small quantity of activated carbon to a container
of fire debris would allow trapping of the volatile species, leaving the water, ash, and
debris behind. Drawing a volume of air from (or through) a container of debris would
allow concentration of the volatiles into the trapping medium (adsorbent). The
volatiles could then be eluted from carbon by a
small quantity of solvent or driven off from a
polymeric trapping medium like Tenax thermally. During the 1980s, these techniques
were tested and refined, improving the sensitivity and the simplicity until very sensitive,
very reproducible extraction methods were established. "Dynamic" extraction methods
were more sensitive, particularly to heavier compounds but ran the risk of destroying
samples in a "one-shot" analysis if the sample got
too hot or the extraction carried out for too
long. "Passive" methods were found to me
most sensitive to light- and mid-range volatiles
but did not interfere with the sample (other than opening the container) and could be
repeated if the extraction went awry. These
methods were shown to be 10 to 100 times more sensitive than the old "bulk" (daresay
"bucket chemistry") methods.
Fortunately, GCs became more sensitive
with the advent of capillary columns and more sensitive detectors. Older GC columns were
1/8 to 1/4 inch in diameter tubes, 6 to 10 feet
long. In gas chromatography, length means better separation, so research grade columns
were 100 to 200 feet long. They were capable of showing 150 or more compounds in
gasoline but they took a long time for each
analysis and took a lot of sample volume.
Capillary columns today are typically 40 to 60 feet
long, but only 0.01 inch in internal diameter.
They have no packing, the active ingredient to achieve separation is coated on the inside
of the column so they allow very rapid analysis (very short run times) with great
resolution and very small sample volumes. These
improvements meant that very small traces of volatiles could yield excellent GC results.
As the sensitivity increased, we started
seeing some interesting things. Just like centuries ago when telescope optics improved,
suddenly the sky was filled with faint stars no one ever expected to be there, more
sensitive extraction and analysis methods meant we could see traces of volatiles in all sorts of
fire debris samples. Most of these were pyrolysis products, the partially disintegrated
molecules of the solid fuels in the debris, that could be sorted out by the lack of a
"pattern" to the GC peaks and their retention
times, which did not match those of hydrocarbons from petroleum-based fuels. But we
also started seeing traces of volatiles in
unburned materials like athletic shoes, newspapers,
carpet tiles and even the packaging we used for fire debris evidence (like the "old" Kapak bags and paint cans lined with
solvent-based varnishes). Many of these volatiles were
petroleum products, residues of the manufacturing processes. So we learned to be
more careful in interpretation and to check our
containers for contaminants before use. But for the most part, we could recognize
pyrolysis product or product contamination and
distinguish it from "real" accelerants.
Comparison samples of carpet were strongly
suggested but most accelerants could be identified
in their absence.
And then came the 1990s when
petroleum chemists came up with a whole universe
of new products that were no longer petroleum distillates and so no longer had the
distinctive pattern of their GC peaks that made
them readily identifiable. Refinements in instrumentation meant that mass
spectrometry could be added to bench-top GC's for
nearly anyone to use (and thus were no longer the complex, expensive machines that
were solely the province of specialists). Mass
spectrometry made it possible to identify individual components in a collection of
peaks rather than just look at the retention times
and peak patterns. GC/MS systems could be asked to look just for particular
molecular types (just the aromatics, for instance)
and plot those out separately from the alkanes. "Target" compounds were identified for
gasoline and other common accelerants and they could be searched for amidst the forest
of peaks of different species found in pyrolysis products. These advances meant that
possible accelerants could be identified even at
very low concentrations even when lots of volatile pyrolysis products were present. As
petroleum products (at least in the U.S.) changed, GC/MS became the only way
to identify the aromatic blends, isoparaffinic products, naphthenic/aliphatics,
oxygenated compounds, and all the rest for which
GC/FID was inadequate. As the minimum detection level dropped, we saw just what a
petroleum-based world we live in. From the volatile residues in roofing paper and
newspaper print, the solvents in glues and adhesives
used in floor coverings and footwear, to residues of dry cleaning solvents, insecticides,
and even cleaning agents (like the limonene used in those "green" grease removers), we can
detect petroleum products in all manner of consumer goods today. Without specific
identification of the product and comparison
against a reference sample, the mere presence of a volatile petroleum product in fire debris
may well be meaningless.
Much of the sensitivity issue came to
light when canine detection was being debated. Yes, a properly trained canine nose can
detect petroleum products at very low levels with a high degree of accuracy, sometimes
at concentrations below even what a qualified lab could analyze. Yes, there may be
samples where a canine correctly detects traces of gasoline that the lab cannot confirms if it adheres to the accepted standard practices.
But the issue is not simply sensitivity, there are far too many other possible sources of
petroleum products, it is the accuracy of the identification of the actual product present that
is essential. A canine can only signal yes or no, it cannot make the identification as to
whether that is a trace of the insecticide (with its
aromatic blend solvent) the owner used the week before or gasoline that an arsonist used to
set the fire. Only the lab with its verified and accepted methods and criteria can
distinguish those products. The canine cannot
discriminate between the residues of carpet glue
used to repair a section of carpet from the
residues of isoparaffinic solvent used as an
accelerant. Only a properly equipped and
qualified lab can make such calls. Another issue
of extreme sensitivity is erroneous reconstruction. If someone steps in a puddle of
gasoline leaking from a Bob Cat or a spilled from a fuel can outside the scene and then
walks through the scene, traces might be found in several locations. A blind reliance on
mere presence of an "accelerant" could lead to
the conclusion that gasoline was used in several places throughout the scene. We must ask:
" Why was this found in these concentrations, and how else might it have gotten there?"
Dr. John DeHaan has been a criminalist for some 32 years. He has worked at county, State, and Federal forensic labs.
He is a native of Chicago and his Bachelor of Science degree in physics was from the University of Illinois at Chicago.
He has been involved with fire and explosion investigations for over 30 years, and has authored dozens of papers on fires,
explosions, and their investigation and analysis. He is probably best known as the author of the textbook Kirk's Fire
Investigation (now in its Fourth Edition). His doctorate (in 1995), from the University of Strathclyde in Glasgow, Scotland,
was on the Reconstruction of Fires Involving Flammable Liquids.
He is a member of NFPA, and served on its 921 Technical Committee from 1991-1999. He is a member of the IAAI and serves on
its Forensic Science Committee. He holds a diploma in Fire Investigation from the Forensic Science Society (United Kingdom)
and one from the Institution of Fire Engineers (U.K.). He is a Fellow of the American Board of Criminalistics in Fire Debris
Analysis and a member of the Institution of Fire Engineers. He retired from the California Department of Justice in December
1998 and is now the president of his own consulting firm, Fire-Ex Forensics, Inc., Based in Vallejo, California, where he now
serves as a consultant in fire and explosion cases all over the U.S., Canada and overseas.
Other articles in the Our Changing World Series
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