[MUSIC] We've said that when a gun is fired, the propellant and the primer are turned into gas and particles. These particles are known as gunshot residue or GSR. GSR comes out of the muzzle of the gun, it comes out of the breach of the gun, and it will be deposited on anything in the vicinity of gun when it's fired. It will deposited on the hands and the clothes of the gunman. Presumably not on the palm of the hand because that's on the gun, but on the back of the hand. And it may be deposited on the clothes of whoever is the target. The GSR spreads out to a distance of about one meter. So if the victim of the gunshot is one meter or less away, then you would expect GSR on the victim. If however, the victim is really close to the gun, that is the gun is being pressed against the victim's head, then there's not much GSR on the victim because it's been forced into the wound. So here's a case where the gun is not pressed against the victim's head and the victim will have GSR over him, but pressed against his head most of the GSR will go into the wound. Now, how can we detect GSR? Most explosives include nitro compounds. So GSR will contain nitrite, nitrate residues. Nitrite residues can be chemically detected by a process called the Griess test. However, the Griess test is not a specific test for GSR. It is a test for nitrite, which means that any other materials that may also contain nitrites, will also be positive to the Griess test. And this can be true from fertilizers, it can be true from urine, it can be true from cosmetics, it can be true from tobacco. Over reliance on the Griess test has caused wrongful convictions. For instance, the case of the Birmingham Six, who were convicted of being IRA terrorists, and carrying out a pub bombing in Birmingham in 1974, was based in part on Griess test, which was subsequently deemed to be unreliable. And in 1991, after a long time in prison, their conviction was overturned. Another test uses a chemical called sodium rhodizonate. Sodium rhodizonate is not sensitive to the nitrate. It's sensitive to the metals in gunshot residue. So gunshot residue may contain salts of metals such as lead, barium, and antimony. So this is sodium rhodizonate, and as you can see as a solid, it has this dark green color. So, what I'm going to do is to dissolve it in water. And it's an interesting material because it changes color when you dissolve it in water. So this is the color of an aqueous solution of rhodizonate, where the metal present is sodium. So sodium rhodizonate. But the color will change if we add this to materials that contain other metal compounds, such as the metals that are found in GSR. This is Barium. This is antimony. And this is lead. So if you can use sodium rhodizonate to detect these three elements, then you're probably dealing with GSR. Because this is a pretty rare combination of elements. But as we know from our lecture on spectroscopy and atomic structure, nowadays we really shouldn't rely on these old fashioned color tests. We have sophisticated instrumentation to do this analysis. And, one of the best ways to analyze gunshot residue is to use Sem-EDX, which we talked about in that lecture. SEM, the scanning electron microscopy component, we can actually see the particles of GSR, which are in the micron range. This is a GSR particle. The EDX component of the instrument, can then tell us what elements are present in that particle. This can be very powerful forensic evidence. In 1999, the British TV personality, Jill Dando was shot on her doorstep of her home in West London. After a long investigation, a man called Barry George was arrested, charged, and convicted. So in 2001, he was convicted of Jill Dando's murder. Part of the evidence for his conviction, was finding one particle of GSR in his coat pocket. Now, if you fire a gun or if a gun is fired at you and there's GSR deposited on you or your clothes, it's not going to last long. Within a few hours, depending on exactly what is done, that GSR will be gone. So GSR must be searched for as soon as possible after the crime is committed. In the case of Barry George, they were able to find a particle of GSR on him because it was in his coat pocket. And presumably was trapped in the coat pocket and therefore was not washed off in any way. Barry George however, appealed his conviction. And in 2007 his appeal was successful and he was released. He was considered innocent by the court of appeal, because, the GSR evidence was considered to be unreliable. What went wrong with the GSR evidence? The answer is GSR cannot be individualized. Just because you find a GSR particle which chemically matches any GSR found at the crime scene, doesn't mean it's linked because there's not much variation within GSR. Investigators involved in the Jill Dando case, categorized GSR into five types. GSR containing lead, barium, and antimony. GSR containing lead, barium, antimony, and aluminium. GSR containing lead, barium, antimony and tin. GSR containing lead, barium, calcium, tin, possibly antimony, and barium, calcium, silicon, tin and maybe lead. So if you only have five different types of GSR, it means that the individualization is not very effective. So GSR is strong evidence, but on it's own is not sufficient. So, arising from the internal ballistics, we have some important forensic methods. We can compare striations caused by rifling between different bullets. We can compare the mark caused by the impact of the firing pin onto the cartridge case. And we can also study the chemistry of GSR, and even though these methods have some limitations, it's still extremely important In firearms cases.
