Gunshot residue is made of particles that form when gasses coming out of a gun hit a surface and instantly cool and condense. The presence or absence of gunshot residue can suggest whether a person fired the weapon or was the victim. There are many tests to show whether or not gunshot residue is present on a surface. The techniques and methods of testing have gotten much more scientifically advanced and more sensitive to minor details. There have also been many experiments to disprove the concerns of gunshot residue testing, such as false positives, transferability, and destruction of evidence. These facts alone disprove many of the arguments that gunshot residue is unreliable and should not be used as a source of evidence.
Strengths and Importance of Gunshot Residue as Evidence in Court Cases
Firearms are not a rare commodity in the United States, or the world for that matter, and so a basic understanding of what happens when the trigger of a gun is pulled is necessary. Many people know that when the trigger of a weapon is pulled the hammer strikes the back of the bullet casing, which ignites the primer, and creates pressure and heat in the barrel. This pressure buildup is what propels the projectile down the barrel and towards wherever the gun is pointing. The knowledge of what else comes out of the barrel and what happens with it that is not quite as well known.
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When the primer is struck, the intense heat causes the chemicals in the primer to vaporize and get mixed in with the gasses that are building up. When the projectile is pushed out of the barrel the gasses and the burning and unburned grains of gunpowder travel with the bullet. These gasses hit a surface such as the hands of the shooter, the victim, or surface that is being fired at. The gasses then condensate on the surface, leaving particles that are composed of the chemicals in the primer. This condensation of chemicals is referred to as gunshot residue, or GSR (Wolten & Nesbitt, 1980).
Gunshot residue has been used for many years as a source of evidence to not only suggest if a person has fired a gun or how far from a surface a gun was fired, but also if a case was a homicide or a suicide. However, there have been disputes over whether or not GSR is a reliable source of evidence. The points brought up in this argument are that gunshot residue tests can have false positives and false negatives, GSR can be transferred from person to person or surface to surface, and that test results can be different and sometimes inconsistent (Wolten & Nesbitt, 1980).
Over the years the methods of testing for gunshot residue have dramatically improved and become much more scientific. There are much less false positives due to the increased sensitivity of the tests. Research has been done that shows that even though GSR may transfer, investigators can still tell if a person fired a weapon, or just came in contact with it (DiMaio, 1999). There are also many other uses for gunshot residue analysis other then knowing if a person came in contact with a weapon, such as range determination (Saferstein, 2006). The purpose of this paper is to show the strengths and importance of gunshot residue analysis as substantial evidence in criminal court cases.
In the detection of GSR, DiMaio (1999) states that scanning electron microscope-energy dispersive x-ray spectrometry (SEM-EDX) has a much higher sensitivity because it uses a scanning electron microscope to view questionable GSR particles at a high magnification. Torre, Mattutino, Vasino, and Robino (2004) agree with using SEM-EDX because the technique can distinguish between GSR and brake lining particles. By using an adhesive lifting method the SEM-EDX is even more effective (Nesbitt, Wessel, & Jones, 1976). Bird, Agg, Barnett, and Smith (2007) disagree with the use of SEM-EDX. They say that time resolved x-ray fluorescence should be used.
On the topic of transferability of gunshot residue, Gialamas, Rhodes, and Sugarman, (1995) states that police officers are very unlikely to transfer GSR to suspects. Vinokurov, Zeichner, Glattstein, Koffman, Levin, and Rosengarten (2001) agree that GSR is not transferred or destroyed very easily with an experiment on the destruction of GSR due to machine washing or brushing. Havekost, Peters, and Koons (1990) state that the investigator also has to look at where the GSR is located on a person to tell if the particles have been transferred or not.
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Firing distance determination is a common factor in investigations. Saferstein (2006) states that using the Greiss Test method provides a more contrasted view of GSR on a surface. DiMaio (1999) states that using Greiss Test results can help determine whether a case is a homicide or a suicide. Brazeau and Wong (1997) say that using GSR tests can also help determine whether a bullet wound is an entrance or an exit wound.
Gunshot residue detection tests first came to the United States in 1933 in the form of a paraffin test, which was used by covering the hands with paraffin wax and using a color-changing reagent on the wax. Swabs were used instead of wax starting in 1959, but in the 1980s neutron activation and flameless automatic absorption spectrometry (FAAS) were the methods used most commonly. The above methods were effective for the detection of the three main elemental components in GSR, antimony, barium, and lead, but came up with many false positives and negatives (DiMaio, 1999).
The occurrence of false negatives and positives is one of the main reasons that gunshot residue is sometimes considered a risky or an unreliable source of evidence. Since the previous tests only tested for the presence of barium, antimony, and lead, any other substance including those elements had the potential to give a false positive result. Defense attorneys could use these false positives as defense tactics to suppress evidence.
In the late 1980s a new GSR test, scanning electron microscope-energy dispersive x-ray spectrometry (SEM-EDX), started to be used. SEM-EDX has a much higher sensitivity, because this technique uses a scanning electron microscope to view questionable GSR particles at a high magnification and look at the size and shape of the particles. After particles are found under the microscope, x-ray waves are used to identify the elements on and inside the particles (DiMaio, 1999).
Since SEM-EDX allows a person to look at the size and shape of a particle, GSR particles can be distinguished from other environmental or chemical particles that may also appear on the tested surfaces. Being able to differentiate between sources of particles diminishes the false positives to a very few occurrences, if any. This also means that gunshot residue tests and results cannot be as easily disputed in court.
The theory of having less false positives has been tested on different occasions to show that using SEM-EDX makes GSR tests more reliable. Research by Torre et al. (2004) shows the results of tests involving particles and residue from the hands of people who work with automobiles. Particles from the brake linings and other moving parts of a car contain barium, lead, and antimony similar to GSR. This experiment proved that SEM-EDX successfully differentiates between gunshot residue and automobile particles using blind tests, which are tests where the person running using the SEM-EDX does not know where the sample came from (Torre et al., 2004). Other tests and experiments included testing to see if SEM-EDX can differentiate between leaded gasoline, which has particles most similar to GSR, and gunshot residue. The experiment was also done in a blind test fashion and was completely successful in further proving the reliability of SEM-EDX (Nesbitt, Wessel, & Jones, 1976).
Another positive benefit of the scanning electron microscope tests is the methodology of the collection of the samples that was used. Instead of swabbing the hands an adhesive lift is used (Nesbitt et al., 1976). Since an adhesive lift collects the particles in their relative spots it is possible to determine the ratio of particles in a particular surface area. This gives a more accurate distribution and concentration ratio than swabbing a surface and analyzing the number of particles on the swab.
The scanning electron microscope-energy dispersive x-ray spectrometry method allows a much longer testing window from the time the gun was fired. With SEM-EDX positive results can be received up to twelve hours after the shooting (DiMaio, 1999). This is because SEM-EDX combines visual inspection of individual particles as well as a mass calculation of the elemental concentrations. There is also another test that can have positive results for as long as thirty-six to forty-eight hours after the gun was fired. This is done with the trace metal detection technique (TMDT), which uses reagents that change colors under a ultraviolet light after they have come in contact with the elements in GSR (DiMaio, 1999).
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With the newest technological advances, x-ray fluorescence microscopy allows for an even more precise look at GSR particles. This method uses the excited state of particles due to x-rays and investigators observe these particles underneath high powered microscopes. The particles fluoresce and appear brighter then the surface (Bird, Agg, Barnett, & Smith, 2007). The fluorescing particles make the visualization of GSR particles much easier and allows for a more specific determination of the spread of the residue.
Destruction and Transferability
Some people may say that allowing more time to pass between the firing of the weapon and when the sample is collected is a detrimental thing. The extra time allows people to wash their clothes or hands or try to at least wipe them off. This is another point argued by people who say GSR is unreliable. There is always the possibility that a suspect can wash their hands and clothes after firing a weapon. The fear is that once that has been done that there will no longer be particles left to detect. In the experiment published by Vinokurov, et al. (2001), tests were done on clothes that had been machine-washed and other tests on clothes that had been brushed with another piece of material. The tests showed that even though a majority of the GSR particles had been removed, there were still enough particles in some circumstances to get a positive GSR detection (Vinokurov, et al., 2001). The results of this experiment proved that even though investigators may allow more time before testing, there are still chances that investigators can get results even after evidence is washed.
Besides washing clothes and hands, there is also the possibility that GSR particles can be transferred to another person or surface by direct contact, or if a person is within a close distance when a gun is fired. Gunshot residue is easily rubbed off or transferred to someone else, which sometimes can make deciding what really happened difficult, but not impossible. Even though gunshot residue can be found on a person who did not fire a weapon, there will be certain circumstances in order to prove they didnt fire the gun.
A person standing within a close range can have GSR on them. Although a person will test positive the location of the GSR and the concentrations will be different then if that person pulled the trigger and fired the weapon. For instance, if a person puts their hand out in self-defense of a shooter, there will be residue found on the palm of the hand in but very little if any on the back of the hand. If the person fired the gun, there would be a high concentration on the back of the hand (Havekost, Peters, & Koons, 1990).
Another situation is one that has been argued by defense attorneys. Defense attorneys say that the GSR that was found on the suspect could have been transferred from the hands of the police officer that arrested them. In theory, this may sound possible, but most officers do not even touch their gun on a daily basis, let alone fire it. In a study published by Gialamas, Rhodes, and Sugarman (1995) police officers that had not fired their weapon over a certain period of time were tested for gunshot residue. Forty-three officers were tested, and out of those officers twenty-five showed absolutely no particles that even resembled GSR. Seventeen officers were found to have particles similar to GSR, but were only environmental contaminates, and three officers were found to have only one particle of GSR (Gialamas, et al., 1995). Even though a couple of the officers showed a particle of GSR, there would have to be a much higher concentration of particles in order to conclude that that officer had fired a weapon. Even if the officer had a GSR particle on them, although possible, the likelihood of touch transfer is extremely small. Even if that particle did transfer when an officer touched a person, the particle would be in a place inconsistent with firing a weapon, such as the shoulder, wrists, back of the neck, etc.
Gunshot residue analysis can be used for purposes other then determining if a suspect was holding the gun that was fired. Gunshot residue can be used to determine how far away from an object the gun was when it was fired. This is done by the GSR pattern left on the surface of the target. There are other tests that can be used to better develop and lift the residue pattern from a surface.
One of the methods that can be used is the Greiss test. This test involves the use of a chemically treated gelatin-coated photograph paper. The paper transfers the residue pattern by reacting with the nitrates in the gunshot residue. After the pattern is transferred off of the target surface test fires are done to match the spread and distribution of the GSR and determine the relative distance of the shooter (Saferstein, 2006). Since the Greiss test can be used on clothes and other target surfaces this technique suggests that gunshot residue is very valuable in the determination of distance.
Using gunshot residue as a distance determination can also help determine whether the case is a homicide or a suicide (DiMaio, 1999). Sometimes a homicide can be staged to look like a suicide, usually by placing the gun in the hands of the victim. By using the gunshot residue pattern to determine the distance of the weapon when fired, investigators can tell whether or not the victim was holding the weapon. It is only physically possible for a human to hold a gun aimed at themself a certain distance away from their own body and still be able to pull the trigger. This distance is directly related to the victims arm length. If the range determination suggests that the distance between the victim and the gun was much greater then the victims arm length, then the crime was more then likely a homicide (DiMaio, 1999).
Gunshot residue cannot only be used to determine the distance that the weapon was fired at, but also whether or not a wound is an entrance or an exit wound. A GSR test can be done on the edges of the wound to see if there is residue present (Brazeau & Wong, 1997). A medical examiner may need help determining whether a wound is an entrance or an exit in a couple of circumstances. Sometimes a bullet can ricochet off of another object before hitting the target, which can cause the wound to look different then the normal entrance wound. The medical examiner may also consider using a GSR test to determine if the wound is an entrance wound if the projectile has entered and exited the body multiple times due to the way the victims body is positioned (Brazeau & Wong, 1997).
Another use of gunshot residue tests around the edges of a wound is to see if the wound is in fact from a firearm. Sometimes wounds can look like a gunshot wound but are actually from other sources. One of the sources for wounds that can appear as a gunshot wound is the hole some insects will make while they are feasting and laying eggs on a dead body (Brazeau & Wong, 1997).
There are no two cases that are completely and indisputably the same. That is why each case has to be looked at individually. Investigators need to take the time to evaluate the results of any tests and evidence, including gunshot residue. Just because a gunshot residue comes back negative does not necessarily mean there was never and residue there. The same goes for a positive result, just because a test comes back positive does not necessarily mean that that person was the shooter or held the gun.
With all the new advances in the technology used to test for gunshot residue the downfalls and errors previously associated with GSR have almost completely been eliminated. Many different studies and experiments have disproved many of the concerns over the tests and results of GSR tests being unreliable. GSR also has many other helpful uses in solving cases such as distance determination and the difference between a homicide and a suicide. While gunshot residue may have been made out to have many downfalls and disadvantages, and while it is not absolutely accurate, nothing in the scientific world is absolutely error proof, and therefore GSR is extremely helpful and is reliable enough to be considered substantial evidence not only in cases but also in court.