Gunshot Residue Demonstration/Lab
Introduction
When a gun is fired, violent chemical reactions occur and chemicals are quickly
dispersed into the surrounding environment. The chemicals are likely to be on the gun, on
the shooter, in the air, on nearby objects, and on objects struck by the bullet. Being able to
detect some of these chemicals can be helpful to crime scene investigators.
Background
When the firing pin of a gun strikes the primer cap of a bullet, the compound in the
primer ignites, sending a flame into the bullet casing. Gunpowder in the bullet casing
starts to burn, causing it to change from a solid material to a gas. This violent chemical
reaction releases a huge burst of energy and the sudden change of state creates a
tremendous pressure inside the bullet cartridge. The pressure forces the actual bullet
projectile out of the casing, down the barrel of the gun and ultimately out of the end of the
barrel. The pressure is only finally released when the bullet leaves the end of the barrel of
the gun. The bullet projectile acts like a cork in a champagne bottle that has been shaken.
The gas pressure blows the cork out of the bottle with great force and speed!
The residue of the unburned gunpowder is usually a mixture of sulfur, charcoal, and
potassium nitrate. Modern gunpowder is not drastically different than gunpowder of years
gone by. In modern gun powders, the substances have been refined and purified to get
maximum expansion with minimal unused residue. Modern guns project less unburned
gunpowder residue than older guns. As a rule, approximately 15% of the gunpowder is not
completely combusted in the explosion and is ejected through the mechanism of the gun.
Multiple shots fired one after another will result in more total residue than a single shot.
Though modern weapons produce less residue, the residue still exists and continues to be
useful to the forensic scientist.
The muzzle-to-target distance can vary considerably depending upon the firearm
and the type of ammunition being used. Short-barreled firearms (pistols) will not throw
residue as far as long-barreled firearms (rifles). At shorter distances, however, the residue
concentration might be greater. Gunshot residues can be collected from the hands and
clothing of a suspect, from the weapon itself, from objects in the line of fire, and from
objects penetrated by the bullet.
Modern smokeless gunpowder and black powder contain nitrate compounds. The
most common type of gunpowder is a combination of potassium nitrate, charcoal, and
sulfur. Single-based gunpowder contains nitroglycerin. When either of these types of
gunpowders burns, the residue left behind will be in the form of a nitrate-based compound.
It is the nitrate-based compounds that are the basis for some “on-site” testing procedures
used by criminalists. Residue samples can be tested right at the crime scene to determine
possible shooters and discharged firearms. The tests used in this demonstration kit are
similar in principle to actual tests used by crime scene investigators.
Gunshot residue tests are presumptive tests only. A gunshot residue test can lead
to a false positive result since nitrates are relatively common compounds and can be
present on clothing and skin from other sources. Also, if a person were to wash his or her
hands and clothing prior to testing, the results could indicate a false negative result. For
these reasons, the tests are not considered definitive nor do they ever constitute “proof” in
a case. They can, however, be very helpful in pointing criminalists in the right direction
and coupled with other evidence can be useful in solving a crime. The false positive and
false negative concepts are very important to understand in this demonstration. Of course,
these tests only represent a fraction of the ballistics tests that are utilized by crime scene
investigators. Neutron activation analysis, flameless atomic absorption spectrophotometry,
bullet “finger printing,” and scanning electron microscopes would likely follow the residue
testing procedures.
In addition to the nitrates produced from the burning of gunpowder, the lead found
in most bullets also leaves a tell-tale trail. Thus, testing for lead around bullet holes and
other items in the crime scene can often reveal clues about the origin, direction, and
distance of bullet paths. Simple lead tests can also be used to determine what type of
bullet was used.
Materials
 Cotton Swabs
 Porcelain spot plate
 Beral-type pipets
 Diphenylamine solution
 Sodium rhodizoate solution, saturated
 Lead nitrate solution 0.5 M
 Copper nitrate 0.5 M
Preparation
1. Prepare fresh diphenylamine solution the day of the demonstration. Mix
diphynylamine into sulfuric acid in proportion of 1 g diphenylamine: 100 mL of 18 M
sulfuric acid. (0.5 in 50 mL, etc.) Label the bottle of diphenylamine with all the same
precautions as concentrated sulfuric acid.
2. Prepare as concentrated solution of sodium rhodizoate. Add sodium rhodizoate to
5 mL of distilled water until it stops dissolving (less than 0.1 g).
3. Prepare three or four test swabs and label them with suspects’ identities: Mr.
Gunshot, Mrs. Leadaby, Mr. Bullet, and Miss Hotshot. Dip one or two of the swabs
in lead nitrate solution the night before class.
4. Prepare three pieces of soaked and dried filter paper prior to class.
a. Pipet lead nitrate solution onto a piece of filter paper and then allow it to dry.
Label “Known Lead Nitrate.”
b. Pipet copper nitrate solution onto a piece of filter paper and then allow it to
dry. Label “Known Copper Nitrate.”
c. Pipet lead nitrate solution onto a piece of filter paper and then allow it to dry.
Lable “Crime Scene.”
Procedure
1. Incorporate evidence into your simulated crime scene or create a “mini-crime
scene” for these demonstration tests.
Mini-crime scene: Police have just arrived at a murder crime scene. Shots
were heard by witnesses but no gun was at the scene. All individuals
detained at the crime scene claim to have no knowledge of the murder and
the firing of a gun. All individuals’ hands were swabbed to collect any residue
on their hands and filter paper swabs are used to soak up residue near the
bullet hole on the victim’s clothing, etc.
2. Gun Shot Residue Tests
a. Dip a cotton swab into a known nitrate (copper nitrate or lead nitrate) for
students to see. Then demonstrate a “positive nitrate test.” Use a Beral-type
pipet to carefully place a drop of diphenylamine solution onto the tip of the
swab over a porcelain spotting plate. (Warning: This is concentrated sulfuric
acid!) When the diphenylamine solution comes in contact with a nitrate, it will
turn a dark blue/black color.
b. Dip a cotton swab into some distilled water for students to see. Demonstrate
a “negative test” by using a Beral-type pipet to add a drop of diphenylamine
to the tip of the swab. Do the test over the porcelain plate. The tip of the
swab should not turn the dark blue color.
c. Repeat the nitrate test for all the swabs of the suspects and see which
suspect(s) had evidence of nitrates on their hands.
d. Discuss the test results. Include in the discussion the need for controls and
the presumptive nature of a positive nitrate test. False negatives and false
positives should be clarified during the discussion.
3. Bullet Metal Test
a. Sometimes bullet makeup can be detected by testing the area around a
bullet hold even if the bullet cannot be recovered. A piece of absorbent paper
(filter paper) is placed over the hole and allowed to absorb particles from the
area surrounding the bullet hold. Utilize the soaked filter papers for this part
of the demonstration.
b. Wet a piece of filter paper with distilled water. Conduct the negative test for
lead by placing several drops of sodium rhodizoate solution onto the filter
paper. The filter paper should be similar in color to the sodium rhodizoate
solution, i.e., no color change.
c. Test the “Known Lead Nitrate” filter paper prepared before class. When the
sodium rhodizonate is added to the filter paper, it should turn a red-purple
color indicating the presence of lead. This would also indicate that the bullet
contained lead and may not have been coated with copper or other lead
shield.
d. Test the “Known Copper Nitrate” filter paper by adding sodium rhodizoate
solution. The filter paper should not test positive for lead, indicating that the
lead bullet was encased in a shield or made of something other than lead.
e. Test the crime scene bullet hold for lead. Relate the results to your
simulated crime scene or story. Discuss the potential usefulness of this
presumptive test. Be sure to discuss what can be concluded from the test
and what cannot be concluded.