Gunshot
residue (GSR), also known as cartridge
discharge residue (CDR), gunfire residue (GFR), or firearm
discharge residue (FDR), consists of all of the particles that are
expelled from the muzzle of a gun following the discharge of a bullet. It is
principally composed of burnt and unburnt particles from the explosive primer, the
propellant (gunpowder), stabilisers and other additives. The
act of firing a bullet incites a highly pressurised, explosive reaction
that is contained within the barrel of the firearm, which expels the bullet.
This can cause the bullet, the barrel, or the cartridge to become
damaged, meaning gunshot residue may also include metallic particles from
the cartridge casing, the bullet jacket, as well
as any other dirt or residue contained within the barrel that could have become
dislodged.
Law
enforcement commonly uses swabbing, adhesives and vacuums with very fine
filters to collect GSR. They commonly swab the web of the non-firing hand
to look for gunshot residue if they are suspected of having discharged a
firearm themselves or were in close contact with one at the time of discharge.
Hair and clothing also accumulate GSR; typically, a double-sided adhesive is
used to sample areas that may have been exposed to such residue. It is also
possible to use a swab moistened with 5% nitric acid for
collection.
To
determine if GSR is present in an area, presumptive tests,
such as the modified Griess test and the sodium rhodizonate test, are
performed. Any presumptive GSR samples are collected for confirmatory testing
using instruments such as Scanning Electron Microscopy, dispersive X-ray
spectrometry (SEM-EDX), Flame or Graphite Furnace Atomic
Absorption Spectroscopy.
There are both inorganic and organic
components in GSR. Organic GSR (OGSR) consists of organic compounds such as
Nitroglycerine. Organic compounds can originate from the primer,
propellants, lubricants or other additives used by manufacturers. Analysis of
OGSR is not done with the same instrumentation as stated above; instead,
techniques like Gas Chromatography- Mass Spectroscopy are
used.
A
graphic representation of the GSR left on a target when fired upon from varying
ranges.
History
The
detection of nitrates and nitrites for GSR has been around since the early
1900s. The first recorded use of paraffin wax as a lifting medium was done by
Dr. Iturrioz in 1914 and was popularized in 1933 by Teodoro Gonzalez of the
Mexico City Police Laboratory. The aptly named paraffin test is also
referred to as the diphenylamine test, dermal nitrate test and the Gonzalez
test. This test consisted of coating a suspect's hands with paraffin wax,
allowing it to solidify and peeling it away before adding a diphenylamine/sulfuric
acid reagent.
The presence of dark blue spots is said to indicate a positive result. This
is no longer used in casework due to the high number of false positives caused
by the commonality of nitrates and nitrites in a variety of mundane products
such as fertilisers.
In
1971 John Boehm presented some micrographs of gunshot
residue particles found during the examination of bullet entrance holes using
a scanning
electron microscope. If the scanning electron microscope is
equipped with an energy-dispersive X-ray
spectroscopy detector, the chemical
elements present
in such particles, mainly lead, antimony and barium,
can be identified.
In
1979 Wolten et al. proposed a classification of gunshot residue based on
composition, morphology, and size. Four compositions were
considered characteristic:
- Lead, Antimony and Barium
- Barium, calcium,
and silicon
- Antimony
- Barium
The
authors proposed some rules about chemical elements that could also be present
in these particles.
Wallace
and McQuillan published a new classification of the gunshot residue particles
in 1984. They labelled as unique particles those that contain
lead, antimony, and barium, or that contain antimony and barium. Wallace and
McQuillan also maintained that these particles could contain only some chemical
elements.
SEM-EDX
results
A
positive result using SEM-EDX spectroscopy will generate x-ray spectra
characteristic of GSR, likely containing combinations of metals such as Pb-Sb-Ba or
Sb-Ba. Spectra may also indicate the presence of Ca, S and Si but
is not always indicative of GSR. GSR may be present when an
individual discharged a firearm or was close by when a discharge
occurred. GSR has been observed to undergo both secondary and tertiary
transfers, meaning the presence of GSR may be attributed to the persistence of
the residue and the unpredictability of human interaction.
A
negative result on someone could mean they were near it but not close enough
for gunshot residue to land on them, or it can mean that the gunshot residue
deposited on them wore off. Gunshot residue can also be removed from
surfaces by washing, wiping, or brushing it off, so a negative result cannot
fully rule out a gun was not fired by the tested object or area. Expelled
gunshot residue does not travel very far from the muzzle because the particles
lack momentum. Depending on the type of fire arm and ammunition used, it will
typically travel no farther than 3–5 feet (0.9–1.5 meters) from the muzzle of
the gun.
Matching
gunshot residue to a specific source
If
the ammunition used was specifically tagged in some way by special elements, it
is possible to know the cartridge used to produce the gunshot residue.
Inference about the source of gunshot residue can be based on the examination
of the particles found on a suspect and the population of particles found on
the victim, in the firearm or in the cartridge case, as suggested by the ASTM
Standard Guide for gunshot residue analysis by scanning electron
microscopy/energy dispersive X-ray spectrometry. Advanced analytical techniques
such as ion beam analysis (IBA), carried out after scanning electron
microscopy, can support further information allowing one to infer about the
source of gunshot residue particles. Christopher et al. showed as the grouping
behaviour of different makes of ammunition can be determined using multivariate
analysis. Bullets can be matched back to a gun using comparative ballistics.
Organic
gunshot residue
The
abbreviation OGSR is often used to distinguish the organic residues found after
a discharge. Organic residues can come from propellants like nitrocellulose and trinitrotoluene,
plasticisers like triacetin, stabilisers like diphenylamine and
possible reaction products of said compounds. The persistence of these
residues is quite low compared to inorganic GSR, with very little quantities of
carryover (if any). Detection of OGSR becomes difficult a mere hour after the
firing. The persistence of OGSR is subject to environmental factors like wind
as well as the substrate it clings to. Organic gunshot residue can
be analysed using methods such as micellar electrokinetic capillary
electrophoresis (MEKC),
High- performance liquid chromatography and gas-chromatography-
mass spectroscopy.
Where
is it found?
GSR
mainly had to be looked at-
·
The hands, mainly present on the thumb,
trigger finger and on the web on hands.
·
On the clothes of the victim if the person
is being targeted.
·
Intermediate targets may also have GSR
disposition.
·
Inside of Cartridge cases and barrels can
have GSR. The surface of the barrels can also be examined for GSR.
·
Fired projectiles must also be examined
for GSR.
Collection
of GSR-
The
Gunshot Residue are being collected by several methods-
·
Dry method
·
Wet method
·
Collecting Of Organic Residue
1.
Swabbing
2.
Tape Lifting
3. Vaccum
Lifting
Presumptive
tests
Sodium
rhodizonate lead test sticks
Presumptive
testing always precedes analysis of a questioned sample. Most presumptive tests
involve a chemical reaction that results in a colour change that is detectable
with the plain eye. It is important to note that thorough documentation of the
scene through notes, photographs, etc., must be done before any presumptive or
confirmatory testing in order to maintain the chain of custody and avoid
contamination.
The
Griess test and the Walker test are two presumptive tests that can be used
to determine if a questioned sample contains nitrites. The Walker test is used
to determine GSR area on clothing using naphthylamine-sulfanilic
acid soaked
photograph paper. Red colouration appears when nitrite ions are present. A
variant of the Griess test reagent is sulfanilamide and napthylamine in
an acidic medium. The Modified Griess test detects nitrite compounds,
which are a by-product of the combustion of gunpowder. Forensic examiners use
this test to determine the gun to target distance. This test is performed first
because it does not interfere with the later sodium rhodizonate test. The
presence of nitrite ions is what triggers the colour change, and therefore, we
do not consider this test to be indicative of GSR.
The
sodium rhodizonate test can detect the presence of lead and barium; it results
in a red or purple colour when lead is present in the tested area, and
a reddish-brown colour when exposed to barium. It is an extremely
sensitive, specific, and efficient method as it can obtain information on the
origin of particulate debris, and it can be done on surfaces or objects. This
test can't determine the precise distance of gun to target; however, it is
often used around holes to determine if it is consistent with the passage of a
bullet.
The
Harrison and Gilroy method was introduced in 1959. It is a colorimetric test
used to verify the presence of antimony, lead and/or barium. The test involves
dampening a cloth with 0.1M hydrochloric acid (HCl),
swabbing the item being analysed and allowing that to dry before subjecting it
to various reagents. The sensitivities of the reagents used makes this test
very unreliable and unrealistic for crime scene analysis.
Author:
Ms. Shreya Dutt
Intern
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𝐒𝐜𝐢𝐞𝐧𝐜𝐞𝐬 (AFRS)
Indore,
Madhya Pradesh, India.
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