Forensic Chemistry - CH6058
Gunshot Residue Analysis (GSR)
When a firearm is discharged, residues from the bullet’s force, the primer, cartridge case, firearm itself and the powder from the propellant are expelled from gaps in the guns working parts (Pepper, 2005: 118). These particles are known as gunshot residue (GSR) or firearm discharge residue (FDR) and are composed of partially burnt and un-burnt propellant powder, particles from the ammunition primer, smoke, lubricants, grease and metals from the cartridge. GSR’s are mainly deposited on skin, the hand in which the weapon was fired, on clothing, the entrance wound of victim or any other local target materials present at the crime scene. The major primer elements are lead (Pb), barium (Ba) and antimony (Sb) usually all three elements are present on the hand after firing. Less common elements include aluminium, zinc, copper, silicon and potassium. Organic and inorganic compounds are also present in GSR’s. Organic compounds are found in propellant powder and primer mixtures whereas inorganic compounds derive from primer mixtures. GSR particles are formed when the pin strikes, the primer cap and mixture is ignited creating high levels of temperature and pressure within the cartridge. The increase of temperature melts the primer mixture and the three elements are produced by vaporisation. As the primer mix ignites the propellant powder, an increase in pressure and temperature occurs and the bullet is released from the firearm barrel. The particles involved in this process form as liquid droplets which instantly solidify due to the quick increase of temperature followed by instant cooling. The discharge of firearm can deposit residues on all persons present and not just the person that fired the gun, the analysis as to who fired the weapon must be made with precaution because any hand or body part that was close to the fired weapon may have residue appearing consistent with having fired the weapon. The cartridge case, bullet and bullet coating contain specific elements that can also be detected such as zinc (Zn) and copper (Cu). There are three classifications of GSR particles which take on characteristic morphologies of GSR, regular spheroids, irregular particles and particles that form a layer of lead surrounding an inner core. The collection of GSR should be carried out instantly because GSR evidence can easily be destroyed just by washing your hands and once examined it is generally spherical in shape. Strong evidential value of all three elements must be found by specialised equipment to confirm a gun had been fired by specific suspect(s). Electron microscope (EM) is a type of microscope that uses a beam of electrons to create an image of the specimen being analysed, it is capable of much higher magnifications and is able to see much smaller objects in detail. They are large, expensive pieces of equipment, and require a lot of skill and training to operate EM. All electron microscopes use electromagnetic and electrostatic lenses to control the path of electrons. The design of an electromagnetic lens is a coil of wire around the outside of a tube through which one can pass a current, creating an electromagnetic field. The electron beam passes through the centre of the coil of wire and down the column of the EM towards the sample. Electrons are very sensitive to magnetic fields and are controlled by changing the current through the lenses. There are two types of EM, Transmission Electron Microscope (TEM) and Scanning Electron Microscopy (SEM). Transmission electron microscopy involves a high voltage electron beam emitted by a cathode and formed by magnetic lenses. The electron beam that has been partially transmitted through the thin specimen carries information about the structure of the specimen.This information is then magnified by a chain of magnetic lenses until it is recorded by hitting a fluorescent screen, photographic plate, or light sensitive sensor...
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