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Gunshot Residue and Distance Determination (Distance Determination (Shot…
Gunshot Residue and Distance Determination
Powders and Residues
propellant and other residues expelled from muzzle during firing
componenets
primer residues from priming mixture
residue from burning propellant
material from interaction of bullet and barrel
unburned and partially burned powder
Examination
Examination and Documentation
visual, microscopic, and chemical examinations per SOP; written and image documentation
Visual and Microscopic Examination
preliminary visual and microscopic exams priority to chemical
start with low power stereomicroscope; IR may be used to see heavy soot on dark/bloody clothing
physical characteristics
holes; ring around perimeter (bullet wipe); ripped material in star-shape (stellate) from contact shot; burned or singed material
melted tips of artificial fibers; particulate other than gunpowder
heavy vaporous lead residue (smoke); burned, partially burned, or melted gunpowder; type of gunpowder particles
Chemical Testing
order: Modified Griess Test, Dithiooxamide Test, Sodium Rhodizonate Test
Modified Griess Test
Chemistry - convert nitrite into bright orange dye; preserved in emulsion-coated test medium; nitrite exposed to acetic acid and heat to make nitrous acid; nitrous acid combines with sulfanilic acid to make diazonium comopund of sulfanilic acid; diazonium compound couples with alpha-naphthol to make orange azo dye
Reagents and Test Media - requires nitrite sensitive emulsion-coated test paper (captures azo dye pattern), nitrite test swabs, glacial acetic acid, deionized water, nitrite compound-free cheesecloth
chromophoric; tests for nitrite compounds from burned smokeless powder; took place of original Griess test (used Marshall's reagent - may be carcinogenic)
Procedure - confirm paper nitrite sensitivity; mark reference points of evidence on test paper; create layers of reaction items; heat reaction layers; examine and interpret results; label and retain test paper
Reverse Modified Griess Test - used for nonporous or thick surfaces that do not let acetic acid steam through; altered procedure to accomodate; confirm nitrite sensitivity, mark reference points, apply acetic acid solution to test paper, create layers of reaction items, examine and interpret results, label and retain test paper
Dithiooxamide Test
tests for cuprous materials; identifies bullet wipe or bullet splash caused by copper-bearing particulate from jacket
Chemistry - material from bullet hole perimeter exposed to ammonia solution; expose same material to solution of dithiooxamide dissolved in ethanol; turns gray-green if copper present; turns blue-pink if nickel present
Reagents and Test Media - dithiooxamide, ethanol, concentrated ammonium hydroxide, deionized water, white filter paper, known copper source (positive control) cotton swab, eye dropper
Test Procedure - confirm performance of preliminary examinations and tests (Modified Griess Test first); place test mark on item to be tested; position area to be tested; perform color test; examine and interpret results; dry test item
Sodium Rhodizonate Test
chromophoric test for lead (vaporous, particulate, primer, bullet, shot pellet wipe); particulate lead random and nonpredictable; vaporous lead found at closer ranges (testable for particular substrate)
Chemistry - questioned area spraeyd with saturated sodium rhodizonate in dionized water solution; buffer (pH 2.8) with sodium bitartrate and tartaric acid in dionized water sprayed on area; pink may indicate lead or other heavy metals; dilute HCl sprayed on area; if pink changes to blue-violent, lead is confirmed
Reagents and Test Media - saturated sodium rhodizonate in dionized water solution; buffer (pH 2.8) with sodium bitartrate and tartaric acid in dionized water; 5% HCl in dionized water; known lead source (positive control)
Test Procedure - do in hood due to aerosolized reagents; confirm presence of preliminary examinations; place test mark on item to be tested; position area to be tested; perform color test
Bashinski Transfer
same chemistry as sodium rhodizonate; specifically for dark surfaces that could cover color change
position area to be tested; place test mark on filter paper; prepare filter paper; lift residues from item; perform color test
Distance Determination
Significance of Results
interpretation is given as presence or absence of residues; the three tests should not conflict with each other or physical effects
absence of residues cannot determine distance - other explanations
Contact Shot
rip, tear, burn, and/or singe cloth; melt tips of artificial fibers; heavy vaporous lead deposits
because contact was made, no suspected firearm needed to determine distance
Nitrite Residues
patterns of different size and density around bullet hole as distance increases
not visible - need Modified Griess Test
can be reproduced with suspect firearm to determine distance
Visible Gunpowder - not possible without chemical confirmation; allows additional measurement beyond powder residue
Vaporous Lead Residues
deposited at closer ranges
detected via Sodium Rhodizonate Test
useful for determining max distance of smoke deposits and corroborating Modified Griess Test results
Residues Consistent with Passage of Bullet - may not have particulate but still look like bullet hole; check for copper and lead residues (dithiooxamide, Sodium Rhodizonate)
Residues Consistent with the Discharge from a Firearm - deposit lead around bullet hole; can get deposits from droplets from erosion of bullet as it passes through barrel, ejecta from dirty barrel, or lead shavings from bullet; residues chemically detectable and can be used to determine if firearm was discharged but not at what distance
Interpretation Challenges
minimal quantity of residue from rough handling and environment; reduces distance specificity
emergency personnel (density loss); wet weather (degrade water soluble nitrites); dried blood may mask lead and nitrite; high temperature and humidity can degrade nitrite; outdoors may eliminate nitrites, but lead still likely present; transfer of residues across garment during packaging and transport reduces value of nitrite tests
Reproduction of Results
suspect firearm must be available
size and density of pattern depends on firearm, ammunition, barrel length, caliber, powder type, and powder charge
sources of testing: evidence ammunition from case, purchased ammunition, reference ammunition
may purchase duplicate fabric to do reproduction testing
Nitrite Residue Patterns
shoot nitrite patterns both larger and smaller than evidence pattern to determine range
best distance determination is max distance to which those residues can be projected
at least three test shots per distance
Shot Patterning
pellet pattern can be measured for large distance
requires suspect firearm; must identify and duplicate ammunition used by suspect
may have elongated shape due to angle relative to surface (can't really be calculated)
Chemical Testing
same procedures as suspected bullet holes with some changes
Modified Griess and Sodium Rhodizonate performed because shot pattern may conceal bullet hole and other residues
shot pattern is determining factor for distance (even though smoke and nitrite residue present at close ranges)
pellet wipe and lead randomly deposited by impact of wadding materials can be seen with testing
Shot Pattern Reproduction
use same firearm and ammunition as was used in incident
shotgun should be used with same choke it had when received
shot pattern may vary when used by same firearm - may get same diameter at different distances
use larger range at greater distance (larger margin of error)
repeated testing to ensure reliability
Source: Savage K, Freed G. Firearm Examiner Training. National Forensic Science Technology Center. [accessed 2019 Jan 25].