DNA probe 81What a Crime Lab Does, It Does
with Chemistry
by Harry K. Garber
Much as a car runs on gasoline, a crime lab runs on physi-
cal evidence. The O. J. Simpson murder trial in 1995 put a
nationwide, if not a worldwide, spotlight on many aspects of
what a crime lab does. Physical evidence encompasses any
and all objects that can either establish that a crime has
been committed or provide a link between a crime and its
victim or perpetrator. Forensic scientists in a crime lab ana-
lyze this physical evidence; however, much of the analysis
is in fact chemical analysis.
Let’s briefly examine physical and biological analysis
first. The hit-and-run death of a pedestrian can involve
both. Fragments of broken headlight glass are collected
from the scene. Samples of the victim’s blood are collected
from the autopsy. Later, a suspect car is located. Evidence
collected from the car includes the broken headlight, more
fragments of broken headlight glass from the headlight
well, and dried bloodstains from the undercarriage. The
stains are collected by rubbing with cotton swabs moist-
ened with purified water; the swabs are dried and submit-
ted to the crime lab. Having first kept careful track of which
fragments of glass came from the car (the “known” evi-
dence) and which came from the crime scene (the “ques-
tioned” evidence), forensic scientists attempt to reassemble
part of the headlight in jigsaw-puzzle fashion. Any jigsaw fit
that includes both questioned and known fragments of
glass shows that the two were originally part of the same
headlight and therefore links the car to the crime scene.
This is an example of physical analysis. And if DNA profiles
of the victim’s blood (the “known” evidence) and the blood
on one or more swabs (the “questioned” evidence) match,
then the car is linked to the victim. This is an example of
biological analysis.
A crime lab is typically divided into several sections.
Due to the degree of expertise required to perform the nec-
essary analysis, a forensic scientist is typically hired to
work in just one section. However, cross training sometimes
occurs, depending upon a lab’s needs. In many labs, a sin-
gle forensic scientist analyzes a given case in a given sec-
tion. In some labs, scientists team up. In either instance,
peer review by a scientist not involved precedes issue of
the final report in many labs for quality assurance. And
what is considered one “case” by an agency bringing evi-
dence to the lab may require analysis by more than one
section.
A scientist in the trace-evidence section might have per-
formed the physical analysis in the hit-and-run case men-
tioned earlier (whereas one in the bioscience section might
have performed the biological analysis). If a jigsaw fit cannot
be obtained, chemical analysis can be performed to deter-
mine whether the questioned and known glass could have
had a common source. Properties compared may include
ultraviolet fluorescence, refractive index, density, and ele-
mental analysis. The more properties that agree, the more
likely it is that the two at least had a common source. Con-
versely, a mismatch in any property rules out a common
source. Trace-evidence scientists also examine fire debris for
the presence of accelerants, paint chips (from auto crashes
or burglaries) for common source (can compare paint vehicle
itself, pigments, elemental analysis), tear gas samples (to
identify the active agent), explosive residues (to identify either
traces of parent explosive or products of the explosion), and
headlight filaments to determine whether a broken headlight
was on or off at the instant of a crash. This last analysis is
actually physical (simply examination under an ordinary
microscope) but is in part a result of chemistry. An intact
headlight is filled with an inert gas, since air would oxidize the
white-hot filament. A headlight that was off at impact has a
shiny, silver-colored filament. A headlight that was on at
impact has a dark blue- to black-colored filament; when the
glass breaks, the admitted air forms a layer of metal oxide(s).
Another feature is often evident on such a filament—micro-
scopic globules of glass sticking to its surface. These globules
arise from small glass fragments melting on contact and then
freezing into position as the filament cools off.
A primary function of the questioned-documents sec-
tion is to determine whether the same person executed
both questioned or known handwriting. For example, a
murderer might attempt to conceal the crime by writing a
suicide note. Comparison of handwriting in the note to
known samples of both the victim’s and the suspect’s hand-
writing would expose the ruse. This is not chemical analy-
sis. However, if the note was typed or computer printed,
both physical and chemical analysis could compare prop-
erties, such as letter size and shape or chemical composi-
tion of the paper and ink, to see if the note matches either
a typewriter, a computer printer, or paper accessible to
either the victim or the suspect. If a document is suspected
of being altered, modern instrumentation allows it to be
examined in a matter of minutes for inks with different light
absorption or emission properties. Sometimes, determining
whether a document was executed on an alleged date is
desired. For example, a document alleged to be executed
in 1980 could not be authentic if analysis reveals that it
contains either paper or ink that did not exist until 2000.(continues)