Proceso para en análisis de trazas de disparo

Clothing submitted to the laboratory will be examined to determine if a pattern of gunshot residues are present. There are basically three steps taken by the firearms examiner to process an item for the presence of gunshot residue.
The first step is to visually and microscopically examine the evidence. The presence of any gunshot residues found around the bullet hole as well as the shape and appearance of the hole will be documented.
The next two steps involve chemically processing the exhibit for gunshot residues.
The first chemical test conducted is called the Modified Griess Test. The Modified Griess Test is performed first on the exhibit because it will not interfere with later tests for lead residues.
The Modified Griess Test is a test to detect the presence of nitrite residues. As described earlier, nitrite residues are a by-product of the combustion of smokeless gunpowder. When a firearm is discharged nitrite particles are expelled from the muzzle of a firearm and can be imbedded in or deposited on the surface of a target. The Modified Griess Test is the primary test used by firearms examiners to determine a muzzle-to-garment distance.
The Modified Griess Test is performed by first treating a piece of desensitized photographic paper with a chemical mixture of sulfanilic acid in distilled water and alpha-naphthol in methanol. Desensitized photographic paper is obtained by exposing the paper to a hypo solution. The photographic paper will no longer be light-sensitive but will be reactive to the presence of nitrite residues.
The exhibit being processed is placed face down against a piece of treated photo paper, with the bullet hole centered on the paper.
The back of the exhibit being examined is then steam ironed* with a dilute acetic acid solution in the iron instead of water.
The acetic acid vapors will penetrate the exhibit and a reaction takes place between any nitrite residues on the exhibit and the chemicals contained in the photographic paper. The resulting reaction will appear as orange specks on the piece photographic paper.
A close-up of the results can be seen below.
The last test conducted on the exhibit is called the Sodium Rhodizonate Test. This chemical test is designed to determine if lead residues are present on the exhibit.
The Sodium Rhodizonate Test is performed by spraying the exhibit with a weak solution of a mixture of Sodium Rhodizonate and distilled water. This solution has a dark yellowish/orange color.
The exhibit is then sprayed with a buffer solution which causes the background color to disappear.
The Sodium Rhodizionate reacts with any lead that may be present and turns the lead a very bright pink. The pink color is only an indication of the presence of lead residue and to confirm the presence of lead residue the area can be treated with a diluted Hydrochloric Acid solution.
If the pink turns to a blue then the presence of lead is confirmed.
The above images show a car’s headrest with a sooty deposit around a bullet entrance hole. The right image shows the headrest after processing with Sodium Rhodizonate and a buffer solution.
Here’s a brief case history on the headrest above. A subject was holding a gun to the head of his estranged girlfriend who was sitting in the driver’s seat of her car. As police approached her car the subject fired several shots through the windshield of the car at the officers. He then turned to shoot his ex-girlfriend as a police officer returns fire through the windshield. The subject fires a shot off, missing the victim’s head, shooting through the headrest above. The victim now has opened the car’s door and started to roll out of the drivers seat. The suspect manages to get off one more shot, striking the victim in the «lower back side». The assailant is struck several times by shots fired from the officer and later dies. The pattern of residue found on the headrest certainly shows the intent by the suspect to kill his ex-girlfriend. She later fully recovered from her injuries. Her car had sustained fatal injuries from multiple gunshot wounds.
The steps taken to conduct the tests described above have been shortened slightly. Only the main procedures have been described. Additional quality control procedures are normally taken but have been omitted to simplify the descriptions.
Once a pattern of gunshot residues is found on the evidence it’s time to test the submitted firearm to determine at what distance it will deposit a similar pattern of gunshot residues. Click the «Next» button below to learn about Gunshot Residue Distance Standards.
* Steam Ironing of the back of the garment is a variation of the test that I described in a paper published in the Association of Firearm and Tool Mark Examiners (AFTE) Journal in May of 1987. The test in its original form called for soaking pieces of cheesecloth with dilute acetic acid, placing the cheesecloth on the back of the questioned garment and then ironing the back of the cheesecloth. Myself and others have used the «steam iron» method since that time with great success.

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