Gunshot Residue (GSR) Tests: What Do They Really Prove?
Contrary to what you may believe; when evaluating the suspect, gunshot residue test results can be misleading. There is a common misconception that a positive GSR test means that the individual fired a gun (false positive). Worse yet, believe they fired the gun in question. It’s also wrong to assume that a negative test result means a person didn’t fire a gun (false negative). That’s because these tests are primarily affected by time, activity, and the condition of the testing surface.
When a modern cartridge is fired; particles of lead, antimony, and barium are expelled from the cartridge primer with the explosive gases from the fired cartridge.These particles travel through the air and may end up landing on your suspect’s hands and face. At the crime scene a CSI will use small plastic discs coated with an adhesive to sample the hands and face (one for each hand, one for the face, and one as a control).
Its not enough though to simply find these particles on your suspect. Many crime labs require that a certain quantity of lead antimony and barium are present before concluding they are consistent with gunshot residue. That’s because these same particles can be found in other environments. One recent study found moderate levels of each on laundered shop towels in a metal fabrication shop.
There are other considerations too. If your suspect has GSR on his hands did he get them from firing a gun? What if the arresting officer (the one who put him in handcuffs) also fired a gun during the incident? Could there be a transfer of the particles? What if the person was simply in the vicinity of a gun going off? Maybe they were fighting over the gun when it went off. One instance in which a positive reaction does carry some weight is if the suspect says they didn’t fire a gun and haven’t been around one since last they washed up.
There are reasons for false negative reactions as well. Obviously if the suspect wore gloves, washed their hands and/or discarded their clothes you may not find enough GSR particles (or any). I once had a case where a mentally disturbed suspect executed his brother with a gunshot to the head. When I got to him he was sitting in a cell licking his fingers and eating his brother’s brains and blood from them. Not pretty. CSIs will also take samples from the bodies of suicide victims but if the face and hands are contaminated with blood then the test may be inconclusive.
So if you plan to mention GSR tests in your novel keep in mind that there are a number of factors that can affect the test results. Many criminalists are hesitant to testify with certainty about such results other than the positive or negative findings. Interpretation of those findings can be very problematic depending on the specifics of the case information. You can also use that to your advantage by throwing a wrench into an otherwise solid case.
Posted on July 11, 2011, in General, The Crime Scene and tagged autopsy, cartridge, Crime Scene, detective, fiction, forensics, GSR, gunshot residue, mystery, police, thriller, tom adair. Bookmark the permalink. 28 Comments.