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).
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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 such as a camera. The image detected is displayed on a computer.
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SEM is a technique that uses a beam of electrons to produce magnified images of sample by detecting secondary electrons which are emitted from the surface due to excitation by a primary electron beam. The electron beam is scanned across the surface of the sample with detectors building up an image by mapping the detected signals.The beam that impacts with the sample produces 3-dimensional (3D) images of the surfaces at high levels of magnification. SEM can also reveal the actual surface details of the particles with known examples of GSR, the large particles of partially burnt powder and spheres of residues analysed can appear to be from contaminated materials and not just from the specimen. In SEM, backscattered electrons (BSE) form due to collisions between incoming electron and the nucleus of the target atom where electrons are being knocked off. BSE are used to detect contrast between areas with different chemical compositions where heavy metal elements appear brighter in the images formed and lighter metal elements appear darker. SEM has the ability to be coupled with energy dispersive x-ray spectroscopy (EDS or EDX) to provide information about the elemental composition of the sample being analysed.
To date the most successful technique is SEM/EDX, which concentrates on the inorganic particles of GSR. This technique not only allows much of the elemental composition of individual particles to be produced but to also enable images that show morphology and the characteristics of GSR (Jackson et al, 2011: 317) . This is important because these two techniques allow GSR particles to be uniquely identified and the discovery of a suspect may appear to be guilty of a crime. Particles will either be identified as possibly being GSR or shown not to have fired the weapon. The advantage of using this technique has the ability to analyse individual particles of GSR typically the three elements lead, barium and antimony can be identified easily using this technique.(Jackson et al, 2011: 315) The EDX technique detects x-rays emitted from the sample during bombardment by an electron beam to characterize the elemental composition of the analysis, features as small as 1 μm or less can be analysed. When the sample is bombarded by the SEM’s electron beam, electrons are knocked off from the atoms to the sample’s surface.
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The electrons from the ground state are filled by electrons from a higher state which creates an electron hole so that an x-ray is emitted to balance the energy difference between the two electron states. The number and energy of the x-rays emitted from a specimen can be measured by an energy dispersive spectrometer; this gives information direct to the energy difference. The data can be interpreted in various forms from its composition to the x-ray spectrum. SEM/EDX cannot determine whether a person discharged a weapon on any occasion.Thedisadvantages of using this technique are that it’s cost effective, limited availability and a significant amount of training is required to operate the machine as it’s considered to be a specialised piece of equipment (Bell, 2006: 447).
SEM examines specific particulates under high magnification whereas EDX allows elemental analysis of samples. SEM/EDX is the preferred method of GSR analysis as it shows in current study that the use of this technique has increased from 21% to 26% (Ronald et al, 1996: 195) this shows how reliable and accurate it is to use.
Positive results are always produced and when providing evidence in court it is less frequently challenged by the judge. Over 72% of laboratories that analyse GSR use SEM/EDX and search more than 50% of the stub; this is composed of aluminium and is an electrically charged conducting tab which is directly placed into the SEM/EDX machine without sample pre-treatment (Ronald et al, 1996: 197) to start the analysis.EDX extends the usefulness of SEM in that elemental analysis which can be performed within regions as small as a few cubic micrometres. All elements from the periodic table can be detected with this type of method.
There are many other methods that have been applied to identify both organic and inorganic GSR analysis but it depends on which method you use. Time of Flight-Secondary Ion Mass Spectrometry (TOF-SIMS), x-ray micro-fluorescence, colour/spot testing, inductively coupled plasma (ICP), neutron activation analysis (NAA), gas chromatography (GC) and atomic absorption spectrometer (AAS).
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For this method to be effective the interviewer is required to ensure the interviee fully understands the reasons for the interview to avoid such interviews to be viewed as efficiency evaluation which may hinder them to describe their job accurately. The method can be utilized efficiently for top management job analysis for example the position of the managing director of a company. This would ...
TOF-SIMS was determined to have many advantages over SEM/EDX but because of its lack of high resolution imaging it could not reach its level of expectation. TOF-SIMS analyses smokeless black powders because of the high vacuum conditions inside the instrument but was reported to be unsuitable for volatile components such as nitro-glycerine (NG), which is a liquid substance made from glycerol, nitric and sulphuric acid.(Oliver et al, 2010)
References
Books
1. Suzanne Bell (2006).
Forensic Chemistry. USA: Pearson Education Inc. 447. 2. Andrew R.W Jackson and Julie M. Jackson (2011).
Forensic Science. 3rd ed. London: Pearson Education Inc. 311-317. 3. Ian K. Pepper (2005).
Crime Scene Investigation: Methods and Procedures. 2nd ed. United Kingdom: McGraw-Hill Company. 118. Journals
4. Ronald L. Singer, 1 M.S.; Dusty Davis, 2 B.S.; and Max M. Houck, 3 M.A. (1996).
Journal of Forensic Science. A Survey of Gunshot Residue Analysis Methods. 41 (2), 195-198. 5. Oliver Dalby, B.Sc, David Butler, MSc and Jason W. Birkett Ph.D. (2010).
Analysis of Gunshot Residue and Associated Materials-A Review. Journal of Forensic Sciences. 55 (4), 924-926 930-931.
Websites
6. http://www.jic.ac.uk/microscopy/intro_em.html – accessed Thursday 13th December 2012 at 11.12pm 7. http://mee-inc.com/eds.html – accessed Wednesday 19th December 2012 at 3.09pm
