Gunshot Residue

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, in 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.

Concepts

• Firearm chemistry
• Lead testing
• False negative
• Nitrate testing
• False positive
• Presumptive test

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 that gunpowder of years gone by. In modern gunpowders, 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 build into the demonstrations. 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

Safety Precautions

Diphenylamine contains concentrated sulfuric acid and glacial acetic acid. Sulfuric acid is severely corrosive to eyes, skin and other tissue, extremely hazardous when in contact with finely divided materials such as carbides, chlorates, nitrates and other combustible materials. Acetic acid is corrosive to skin and tissue and a moderate fire risk. It is also moderately toxic by ingestion and inhalation. Diphenylamine is moderately toxic by ingestion. Sulfuric acid is severely corrosive to eyes, skin and other tissue. It is also toxic by ingestion. Avoid all body tissue contact. It can cause damage to most materials. Lead nitrate is moderately toxic by inhalation and ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Copper nitrate and sodium rhodizonate may be disposed of according to Flinn Suggested Disposal Method #26b, lead nitrate by Method #27f, and diphenylamine by Method #24b.

Prelab Preparation

1. Prepare fresh diphenylamine solution the day of the demonstration. Mix diphenylamine into sulfuric acid in proportion of 1 g diphenylamine: 100 mL of 18 M sulfuric acid (e.g., 0.5 in 50 mL). Label the bottle of diphenylamine with all the same precautions as concentrated sulfuric acid.
2. Prepare a fresh saturated solution of sodium rhodizonate. Add sodium rhodizonate 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:

1. Pipet lead nitrate solution onto a piece of filter paper and then allow it to dry. Label “Known Lead Nitrate.”
2. Pipet copper nitrate solution onto a piece of filter paper and then allow it to dry. Label “Known Copper Nitrate.”
3. Pipet lead nitrate solution onto a piece of filter paper and then allow it to dry. Label “Crime Scene.”

Procedure

1. Incorporate evidence into your simulated crime scene or create a “mini-crime scene” for these demonstration tests. Example: 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 are 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.

Gun Shot Residue Tests

2. Dip a cotton swab into a known nitrate solution (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.
3. 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.
4. Repeat the nitrate test for all of the swabs of the suspects and see which suspect(s) had evidence of nitrates on their hands.
5. 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.

Bullet Metal Test

6. Sometimes bullet makeup can be detected by testing the area around a bullet hole 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 hole. Utilize the soaked filter papers for this part of the demonstration.
7. Wet a piece of filter paper with distilled water. Conduct the negative test for lead by placing several drops of sodium rhodizonate solution onto the filter paper. The filter paper should be similar in color to the sodium rhodizonate solution (i.e., no color change).
8. 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.
9. Test the “Known Copper Nitrate” filter paper by adding sodium rhodizonate 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.
10. Test the crime scene bullet hole 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.

Teacher Tips

• This kit contains enough chemicals to perform the demonstrations many times. The kit contains: 35 cotton swabs, 100-mL concentrated sulfuric acid, 1 g of diphenylamine, 35 Beral-type pipets, 1 g of sodium rhodizonate, 25 mL of 0.5 M lead nitrate and 25 mL of 0.5 M copper nitrate.

• Be sure to mix all chemicals fresh the day of the demonstrations. If chemicals are stored after the first day, run tests beforehand to be sure the chemicals are still viable.
• Be especially careful with the concentrated sulfuric acid throughout the test procedure. When the test solution is placed on the swabs, be sure to set the tested swabs only on a noncorrosive surface (e.g., in a beaker). Be aware of every drop of the test solution and where it goes!