FORENSIC FILES FOR WRITERS

SHOT THROUGH THE HEART WHAT GUNSHOT RESIDUE CAN (AND CAN'T) TELL YOU Part I I can assure you from personal experience that guns are still the weapons of choice for most American murders. Assuming one figures into your plot, what kind of gunshot residue evidence can be used against your villain/hero/red herring? Let's start with how a gun works, for those of you who aren't familiar with them. If you look at the back of any bullet larger than a .22, you'll see a small circle in the center (known as centerfire). This is a short, sealed cylinder containing primer. When the trigger is pulled, the hammer of the gun hits this cylinder, and the shock of that blow causes the primer to explode. This explosion blows out into the large chamber of the casing, where the gunpowder is, causing the gunpowder to catch fire. The burning of the gunpowder is what expels the slug away from the casing, down the barrel of the gun and out the open end. (Some small caliber bullets, like .22s, may have the primer crimped into the rim of the bullet instead of in a center cylinder, and the hammer can strike the bullet anywhere on the rim to fire the slug. This is known as rimfire.) Tiny particles of this primer material (gunshot residue or GSR) are expelled from the front of the gun. They also leak from the breechblock area when the gun is fired, either from the open back of the chamber in revolvers, or when chamber opens and the cartridge is ejected in automatics. This way, GSR can settle on the shooter's hand. Primer is generally made up of barium nitrate and antimony sulfide; however, most .22 ammunition does not have antimony and rare brands have neither antimony or barium. It also contains lead styphnate, but unfortunately for all of us, lead is so commonly found in the environment that the presence of it on someone's hands is not considered significant. Therefore most GSR detection techniques concentrate on barium and antimony. In all ammunition the ratio of barium to antimony is not known and manufacturers do not see fit to release that information. It is, understandably enough, their livelihood and they are not interested in giving their formulas out. In addition, as in any other manufacturing process, they are often changing, adjusting and experimenting with their batches. Most gunpowder used today is double-base, which means it is a combination of nitroglycerin and cellulose nitrate (commonly and incorrectly known as nitrocellulose). 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 the swabs 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. 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 victims 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 on the defendant's hands proves that he's NOT the killer. As the legal community gets more and more spoiled with the yes and no of DNA results, they don't want to hear about indications. Some labs have discontinued GSR analysis for that very reason. To Be Continued in Shot Through the Heart Part II