SHOT THROUGH THE HEART - What Gunshot Residue Can and Can't Tell

Gunshot residue tests used to range from the now-discredited paraffin test, which essentially tested for the nitrates from the gunpowder, to atomic absorption. Atomic absorption entails swabbing the suspect’s hands with cotton swabs, then soaking them in an acid solution. The solution is exposed to high temperatures, and a light reads the absorption rate of the atomized solution. It’s a time-consuming and time-limited process. First, the swabs have to be ‘digested’, then spun, then diluted with deionized water (in different concentrations for barium and antimony) in tiny sample cups which are placed in an automated machine, a spectrometer. It will take it from there, running for hours unless a sample is too concentrated, or the printer paper goes off track, or something happens that it simply doesn’t like. Then you start all over again and test for the other element. It is, frankly, a pain in the butt.

Atomic absorption can only tell us that the element is present, but not in what form. Barium and antimony can also be found in firecrackers, paint, and some industrial settings. Finding these elements on someone’s hands does not prove that they fired a gun, or even that they handled a gun. It only ‘indicates’ that they were in the vicinity when a gun was fired. GSR is, obviously, composed of extremely tiny particles, which can easily be removed from the skin. All the shooter has to do is wash his hands thoroughly, although normal human movement and activity can dislodge the particles even if he can’t get to a sink. This is a large reason why dead victim’s hands are bagged (in paper! not plastic as you see on TV), to preserve the GSR particles. Most labs will not test samples collected (from live people) more than 6 hours after the shooting. This doesn’t apply to dead humans, as they’re not moving around or using their hands after death. Not usually, anyway.

From this comes the maxim that while the presence of GSR doesn’t prove someone did fire a gun, the absence of GSR doesn’t prove they didn’t. There are too many factors involved—weather (including wind), sweating skin, dry skin, putting hands in pockets, washing. This is where attorneys and forensic scientists often conflict. Prosecutors want us to say that GSR on the defendant’s hands proves that he IS the killer. Defense attorneys want us to say that the absence of GSR

Go To Page: 1 2 3