Residuo de pólvora: la pista invisible

During an early morning armed robbery of a convenience
store, the sole clerk is shot. A suspect is arrested 20 minutes
later, several blocks away without a weapon. On his hands,
however, is gunshot primer residue (GSR), an invisible clue that
could be used by investigators in this and most other crimes
involving a firearm. Unfortunately, in many such instances,
this valuable evidence would not be made available to
investigators or jurors. Why not? There are various reasons,
including an unfamiliarity with proper procedures for collecting
GSR for analysis. This article addresses the strengths and
weaknesses of these processes and offers suggestions for more
effective use of this often overlooked evidence.
The explosion inside a firing cartridge burns the gunpowder
so completely that no analytical technique has yet been developed
that consistently identifies the remaining trace quantities of
unburned powder on the hands or clothing of the shooter.
However, several procedures to accomplish this have been tried
over the years. In the first attempts to associate an
individual with a firearm, the hands were coated with a film of
paraffin in order to lift off residual nitrites. This residue
then could be visualized with diphenylamine.
This procedure was abandoned over 20 years ago, however,
because nitrites do not provide sufficient specificity, and
because large deposits are necessary to yield an adequate color
development. Still, even today, many investigators erroneously
refer to the «paraffin test» when discussing modern gunshot
primer residue analysis.
Continued investigation into applications of neutron
activation analysis identified two noncombustible primer mixture
components, barium and antimony, as detectable residues from the
discharge of most ammunition. (1) It was this discovery that
led to the reliable tests available to the law enforcement
community today.
In the most common analytical protocol, cotton swabs
moistened with diluted nitric acid are wiped over the web and
palm areas of each hand. Neutron activation analysis (NAA) or
atomic absorption spectroscopy (AA) is used to determine the
quantities of barium and antimony on the swabs from both areas
of each hand. Since neither barium nor antimony is unique to
GSR, it is necessary to find both elements in amounts within the
range found on the hands of persons who are known to have
recently fired a weapon (a control group).
In another method, technicians use adhesive disks to pick
up microscopic particles of GSR from the hands. A scanning
electron microscope (SEM) equipped to conduct energy dispersive
X-ray analysis (EDXA) is used to detect particles containing
barium and antimony. SEM-EDXA produces a visual image of
particles, thereby providing the analyst with useful size and
shape information. Additionally, the barium and antimony are
shown to occur specifically within these particles, as opposed
to being part of general background contamination. This
technique has gained support in recent years due to the
development of automated systems that simplify and eliminate
much of the lengthy and tedious searching process.
There are variations and combinations of these methods.
However, they all rely, at least in part, on finding barium and
antimony as presumptive evidence of GSR.
Gunshot primer residue is much like chalk on the hands of a
school teacher using a blackboard. The minute the teacher walks
away from the board, chalk loss starts through mechanical
actions, such as rubbing the hands together, putting them in
pockets, rubbing them against clothing, or handling objects.
Therefore, officers are instructed to collect GSR evidence
immediately upon making an arrest. Generally, there is little
hope of finding adequate quantities of barium and antimony to
associate an individual with a weapon after 3 hours of normal
hand activities. And, washing the hands removes essentially all
GSR deposits.
Unfortunately, ideal GSR collection procedures are at odds
with the fundamental precept of immediately handcuffing
arrestees hands behind their backs. This cuffing procedure can
greatly decrease the amount of GSR because the outer webs of the
hands are pressed against the body. Any improper procedures
should be addressed by arresting officers and crime scene
personnel since they could lead to elimination or contamination
of this potentially valuable evidence.
GSR collection kits are available at police supply stores
and through catalogs. The deceptively simple appearance of
these kits implies that acceptable substitutes can be made from
standard drugstore items. However, this practice can introduce
multiple errors into the collection process. These errors can
be avoided by using collection kits and questionnaires prepared
commercially or by knowledgeable laboratory personnel.
The real value of the GSR test is that it can associate an
individual with a firearm. It is important, however, to note
that this does not identify that person as the shooter. GSR can
settle on any hand placed near a weapon as it is fired. A
person can pick up GSR simply by handling a dirty weapon or
discharged ammunition components. It is also possible, but very
unlikely, that residue would be deposited on hands by other
means. Thus, placing an individual in an environment of GSR
generally puts that person in the presence of a firearm.
At the same time, failure to find GSR on the hands does not
mean that a person tested did not handle or fire a weapon. For
example, many test firings under controlled conditions in the
FBI Laboratory do not deposit sufficient quantities of the
material to allow identification. A firearm may produce
deposits on five consecutive firings but not on the sixth. A
weapon may simply not be sufficiently dirty or not handled
enough to effect a transfer.
As noted earlier, GSR could have been deposited but later
removed through washing or normal use of the hands. A finding
of inconclusive amounts of barium and antimony simply means that
the analyst can offer no opinion of value associating a tested
individual with a firearm. The situation is analogous to a
fingerprint analyst having no opinion concerning a particular
person’s presence at a crime scene if print analysis is
The tests using neutron activation analysis (NAA) or atomic
absorption spectroscopy (AA) for determining the total barium
and antimony in each sample does not constitute an unequivocal
identification of GSR. When elevated levels of both elements
are found in a sample, the results are reported as being
consistent with those obtained from persons known to have
discharged a firearm. It is unlikely, but possible, to get
independent environmental contamination of both elements in one
or more of the four specimens collected from each person tested.
Barium and antimony can be found in trace amounts on most
hands, and it is not uncommon to detect elevated levels in
samples from a nonshooters hands. In a recent study, the FBI
Laboratory analyzed samples from the hands of persons who had
not been near a firearm. Of 267 sets of hand samples analyzed,
9 (3 percent) had significantly elevated levels of both elements
and most of these were eliminated as being consistent with GSR
by other parameters relevant to GSR tests. (2)
Analysis of GSR on the victim has little value in a
suicide-homicide situation and should not be used routinely on
the victim as an investigative tool. More gunshot residue goes
out of the weapon’s barrel with the bullet than escapes near the
handle. If the victim of a close range shooting attempts to
grab the gun or instinctively shields the head, significant
deposits can be left on the hands. Laboratory analysis cannot
reliably determine whether the deposit was made in this manner
or was the result of a self-directed firing.
Likewise, suspects at the crime scene should only be
sampled if they do not admit to or cannot otherwise be
associated with a weapon at the approximate time of the
shooting. The person who just returned from a hunting trip or
claims to have struggled with the victim (or assailant) over the
weapon before the shooting, for example, generally should not be
tested for GSR.
Accurate identification of GSR largely depends on the prior
experiences of the laboratory performing the analysis to
determine what is expected from specific areas of the hands
after handling weapons. Such information is not generally
available, except for these specifically defined and studied
areas of the hands. Thus, surfaces, such as automobile windows,
clothing, and parts of the body other than these specific areas
of the hands, are usually not suitable for GSR examinations.
Several factors can affect the analysis of unfamiliar
surfaces, including environmental barium and antimony
contamination and the potential for previous exposure to GSR.
The latter concern is significant because GSR is not volatile
and will generally remain on a surface until it is mechanically
removed. Thus, GSR on the clothing of a suspected shooter can
be explained by that person handling a weapon while wearing the
garment several weeks earlier.
The detection of gunshot primer residue on the hands of an
individual confirms that this person was in an environment of
the material within a few hours preceding the collection of
samples. This would likely result from firing a weapon,
handling a weapon or ammunition, or being in close proximity to
a weapon as it is discharged by another person.
Failure to detect GSR on the hands indicates that the test
offers no information of value in determining whether an
individual had been in the presence of the material. With the
exception of very few well-defined situations, nothing more
should be inferred from the results of GSR tests.
To avoid useless analysis, officers should not collect
samples if:
* The person can be associated recently with a firearm by
a witness,
* The hands were washed or more than a few hours have
elapsed since the shooting,
* The ammunition used in the shooting does not contain
both barium and antimony.
Setting these parameters saves time and eliminates much of the
misunderstanding and confusion surrounding GSR tests. Like any
analytical process, certain conditions must exist to ensure a
useful GSR analysis.
(1) «Special Report on Gunshot Residues Measured by
Neutron Activation Analysis,» U.S. Atomic Energy Commission
Report GA 9829, National Technical Information Service, U.S.
Department of Commerce, Springfield, Virginia, 1970.
(2) D.G Havekost, C.A. Peters, and R.D. Koons, «Barium
and Antimony Distributions on the Hands of Nonshooters,»
Journal of Forensic Science, JFSCA, vol. 35, No. 5, September

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