Screening and Analytic Techniques for Identification of Gunpowder and Primer Residue
Andrea Barreto

Since the invention of guns, there have been millions of criminal cases where guns are involved. According to US Department of Justice research, "in 2007, offenders used firearms in 68 percent of the Nation's murders, 42.8 percent of robberies, 21.4 percent of aggravated assults." ( Murders and assaults are the most common, but in some parts of America, guns are a common item in the household. The constitution states that every American citizen has a right to bear arms and the accessibility to a gun in recent times in amazing. A license is required for concealed guns and certain models, but many guns are illegally sold or parts are easily bought on the internet and can quickly modify different models. When a gun is fired, gunpowder and primer residue is left behind, and when analyzed can sometimes be matched or traced back. Due to the amount of cases involving and residue, forensics have developed various methods of analyzing gunpowder and primer residue to assist in investigations.

Method of Analysis:

Firing the Gun:
When a gun is fired, a series of events happens within the gun that propels the bullet out. Everything begins when the person pulls the trigger and the firing pin of the cartridge and the primer compound ignites. A flame is sent into the cartridge case and the gunpowder inside changes from a solid to a gas. The sublimation causes pressure which shoots the bullet out at a great speed. When the bullet is shot out however, the gunshot residue is also blown from different areas of the gun.
Gunshot Residue being blown out while firing. <>
Gunshot Residue being blown out while firing. <>

Residue Makeup and Primer:
Most residues are made up of gunpowder and lead components, though now newer types of lead-free gunpowder are being made. These components are what help forensic scientists identify the guns with the powder. The lead residues can come from a variety of places within the cartridge but usually come from the primer. A primer is a cap containing a compound that is used for firing powder. Primers usually contain lead styphnate, barium nitrate, and antimony sulfide compounds, though some newer lead or barium free primers are being manufactured. If a gun contains a lead based primer, a cloud of vaporous lead residue is expelled when the bullet is fired.
Modern gunpowder and black powder contain nitrate compounds, and black powder also contains charcoal and sulfur. Smokeless powders can be single-based, containing nitrocellulose as the main compound or double-based containing nitroglycerin as the main compound. When the gunpowder residue is found, it is either unburned, partially burned, or completely burned into, a nitrate-based compound.
Another form of gunpowder residue is small lead particles that are shaved from the sides of the bullets as they are propelled from the gun called lead particulate. It has more mass than the vaporous form and travels further.

Location and Collection of Residue:
The amount of residue expelled depends on the type of gun, type of ammunition, and force on the bullet. Residue is composed of various
Gunshot resiude magnified 200 times. <>
Gunshot resiude magnified 200 times. <>
very small elements and is extremely hard to locate. Luckily, the metallic elements that makeup gunpowder residue keep it from breaking down in a normal environment. It also comes in various different shapes such as balls, discs, flakes, etc. The distance from the gun to its target is very important in many cases, such as one in which the suspect pleads that they shot in self defense. If the shot is at a peculiar distance, such as a long distance range, foul play can be proven. As a bullet travels farther and farther away, the broader and less concentrated together the residue is. Certain types of guns such as short-barreled firearms and low velocity cartridges will not send the residue as far as higher velocity rifles can. The closer a gun is to its target; the most easily the gunpowder is seen.

There are various collection techniques, and they are used depending on the surface on which the gunshot residue is located. Gunpowder is usually found on the body (especially hands of the person who fired the gun), clothing, and surrounding surfaces in the location of the shooting. . Gunshot residue is easily rubbed off or moved by washing hands or even sweat, so samples from a suspect needed to be taken as quickly as possible. There are no standards or rules for collections, only that the collector must wear sterile gloves to try to prevent cross-contamination. The two most common collection techniques are a cotton swab with a small amount of nitric acid solution or an adhesive discs. The adhesives are used to collect evidence for Scanning Electron Microscope (SEM) and the swabs can be used for various other tests such as the Atomic Absorption (AA) and Neutron Activation Analysis (NAA).

Identification & Analysis Techniques:
There are various ways to identify and analyze samples of gunpowder residue around the world. There are no national or international standards used when identifying the residue so it is up to forensic scientists to make the decide whether there is a match.

  1. (Modified) Griess Test- This test is the most common procedure that can identify relative distance and detect any trace of lead residue that might be around the bullet hole. The examiner first presses photographic paper onto the target with a hot iron which makes the nitrite particles visible when chemically treated. The pattern of the particles is then compared to known patterns from
    external image
    external image
    test firings at difference distances to determine the shooting distance. Another test can be made by spraying the questionable surface with a sodium rhodizonate solution followed by a series of acid solution sprays which makes the particles light up with a pink and then blue-violet color indicating a positive result.
  2. SEM-The Scanning Electron Microscope is the primary tool forensic laboratories use to identify compounds in gunshot resdiue. The microscope is used with an X-Ray and is prefered because it can characterize particles by their size and shape as well as by their chemical makeup. The X-Rays are created when the SEM strikes the target with an electron beam. The system then analyzes the emitted X-Rays and sorts them by their distiguishing energy values to create a photo of the elemental distribution and identify the sample's elements. The concentration of the specific element can also be measured by looking at the intensity of the X-Ray emission. Forensic scientists use this machine to look for and identify lead, nitrate, antimony, and barium particles found in primers. The scientists can also use the specific elements and concentrations to match the residues at crime scenes with evidence collected from suspects.
  3. NAA-Neutron Activation Analysis is a nondestructive technique used to identify and quantify elements. NAA uses a nuclear reactor to shower the sample with neutrons, creating a radioactive isotope. The energy and intensity emitted by the gamma rays are then measured to identify the element and concentration. This process is does not destroy the sample and can analyze 20 to 30 elements at the same time. The only disadvantage is the price. Nuclear reactors and analyzers are not a common tool in forensic laboratories.
  4. AA-In Atomic Absorption, the atomic absorption spectrophotometer is a simple and low cost technique used to measure small amounts of elements in physical evidence such as gunshot residue. The spectrophotometer vaporizes the specimen's atoms and then exposes them to radiation. Then a monochromator detects the radiation frequencies and sends them to a detector which converts them to an electrical signal. A discharge lamp can be created from the same element and radiation is made and emits frequencies of light consistent only with those present in the element. Forensic scientists use these lamps of the elements they are looking for with a spectrophotometer to see if the sample absorbs the light. If it does, then the sample contains the element in the discharge lamp.
  5. Burleson's Test is a new method developed in early 2008 that has captured the intrest of the scientific community in the new method that extracts and identifies almost all of the gunpowder's components rather than looking for certain kinds such as lead or antimony. The new test uses gas chromotography with a nitrogen phosphorus detector to seperate and identify components. It is expected to become especially popular because it can detect components in the newer types of eco-friendly gunpowders that do not contain the metals looked for in the other tests. Burleson's test only requires a very small amount of sample and takes under an hour to yeild results.
Scanning Electron Microscope. <>
Scanning Electron Microscope. <>
Particle under NAA. <>
Particle under NAA. <>

Problems/Sources of Error:
There are various problems and sources of error with the techniques used to identify and analyze gunshot residue. One prominent problem is cross-contamination and the loss of evidence. Residue is easily rubbed or washed off and the evidence is easily lost. Residue also can be transfered onto the clothes and hands of an innocent person who brushed up against the perpetrator or who went into the room where the shooting took place. It moves around so easily, sometimes it is very hard to prove exactly who had the residue on them. Another problem is that other things such as fireworks and disc brake pads create tiny specks of lead, antimony, and barium that resemble residue, but are not. Many car mechanics and auto electricians have these particles on them. Certain tests such as the Modified Griess Test or the SEM only look for certain components, and can be to ambiugous.The fact that there are no set standards of analysis in the world or even the country raises questions when it comes to making matching decisions and presenting evidence in court.

1929-A German chemist named Goroncy created a test using an acetic acid solution to detect nitrate compounds. The intensity of the red color after adding the acetic acid indicated the approximate distance. However, process gave no pattern of residue on clothes and destroyed the evidence.
1932-Teodoro Gonzales introduced "Dermal Nitrate" or "diphenyl-amine test" in Criminal Identification Labratory in Mexico. The test was the most popular for many years in the identification of unburned gunpowder or nitrates in the primer. Required the application of hot wax/paraffin on the suspect's hands. Once the substance was cool, it was removed and tested with diphenulamine. A blue color was taken as a positive for nitrates, but has recently been unaccepted because of its "lack of specificity."
1937-Joseph Walker improved upon Goroncy's technique replacing the acetic acid with C-acid or H-acid and using photographic paper rather than samples directly on the clothing.
1955-Started use of Atomic Absorption Spectrophotometer to quanitfy elements in a sample.
1959-Harrison and Gilroy introduced the calorimetric chemical test that detected barium, antimony, and lead on the hands of suspects.
1970s-The chemical alpha-naphthylamine used in Walker Test was proved dangerous and the FBI Laboratory substituted it with a chemical in one of Fritz Fiegl's Spot Tests in Organic Analyisis known as the Griess Test. The combination of both techniques was named the Modified Griess Test.
1974-Team at Aerospace Corporation (Wessel, Jones, Kwan, Nesbitt, and Rattin) created the scanning electron microscope with electron dispersive X-Rays that could specifically detect gunshot residue.
2008-Garrett Lee Burleson and chemist Jorn Chi Chung Yu from Sam Houston State University recently created a new method of detecting various elements in gunshot residue, not just looking for one thing such as lead. The Burleson test can use a tiny amount of residue and uses a new method that can even identify components in newer eco-friendly ammunitions that do not contain lead or the metals other tests used for identifying residue.

Case Applications:
Forensics Gone Wrong:
The Tyree Wright murder case is an example of forensics gone wrong. On June 24, 1998, fifteen-year-oldTyree Wright was approached by two men while talking with his family in his East Baltimore home. One of the men, identified as wearing a grey shirt, shot and killed Tyree while his brother Emmanuel and uncle run away. Less than an hour later police arrested Tyrone Jones who seemed to fit the description and he was swabbed for gunshot residue on his hands after he was arrested and taken into an interview room. He was swabbed after being in the police car, interview room, and in handcuffs (all of which have a high probability of containing gunshot residue on its own). After analysis he was charged for murder and brought to court. According to the notes, no residue was found on his dominant right hand, but on his left, one unique gunshot residue particle was found and fifteen lead particles which by themselves are not considered gunshot particles. At trial however, the jury was told that 17 gunshot particles were found, and he was found guilty. Jones claims that he did not know Wright and he had a clean record. Jones was sentenced to life in prison and is currently trying to get an appeal.

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