Forensic Science provides analysis in organic and inorganic analytical chemistry, nuclear science, biochemistry, and genetics useful for supporting law enforcement. It involves numerous techniques and analysis to provide justification and proof of evidence to a crime. This paper is focused on analysing gunshot residues using atomic absorption and Inductively Coupled Plasma methodology
Combustion of the primer and powder of the cartridge is caused by firing a weapon. Unburned primer of powder components, or the residue of the combustion products, can be used to detect a fired cartridge. Residue can be found on the skin or clothing of a person who fired a gun. It can also be found on an entrance wound of a victim, or even on the target materials at the scene. The discharge of a firearm, particularly a revolver, can deposit reside even to an individual that is close to a person who fired the gun (Thomton, 1986).
Lead (Pb), barium (Ba), or antimony (Sb) was the major primer elements. A smaller amount of elements contain aluminium (Ai), sulphur (S), Calcium (Ca), tin (Sn), Chlorine (Cl), silicon (Si), or Potassium (K). This element varies depending on who and where the ammunition is manufactured. A mercury-fulminates based primer may be found in ammunition manufactured in Eastern Europe and Used in the Middle East (Zeichner, et al, 1992) Primer elements may be easier to detect in residues because they do not get as hot as the powder, and compounds may be detectable. (Tassa et al, 1982). Current gunpowder, or the so-called “Smokeless” powder, can contain up to 23 organic compounds (FBI).
On the other hand, samples must be extracted from the skin surfaces of a victim at the scene. Samples must be obtained immediately, washing of the body prior to autopsy or even movement will diminish or destroy gunshot residues (Kitty, 1975). Atomic absorption spectroscopy or was applied determining the amount of copper, zinc and nickel in entrance skin wounds and cloth injuries made by different type of jacketed bullets. This method measures trace element signatures, but it is the oldest techniques used in forensic today. In order to analyse a combination of elements, such as those found in gunshot residues, a solution of the mixture is vaporized by a flame, through which is shone light thought to be characteristic of an element in the mixture. If there are any of these elements present, it will absorb the light, leaving dark lines in the absorption spectrum.
One of the key methods for detecting primer residues is Atomic Absorption (AA) or atomic absorption spectroscopy (AAS). By definition, atomic absorption (AA) is an analytical technique, used to determine the elemental composition and concentration of many metals and other inorganic elements. The material being analysed, generally in solution, is atomised, or broken up into individual atoms, usually by the action of extreme heat in a flame or small furnace. The ability of the atomised material to absorb characteristic wavelengths of visible or ultraviolet light is then measured using a spectrophotometer.
Atomic absorption (AA) spectroscopy requires light absorption for measuring the attentiveness of gas-phase atoms. Samples are usually in liquids or solids, analyte atoms ions or atoms must be vaporized in a flame or graphite furnace. Atoms take up ultraviolet light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Due to the fact that atomisation efficiency is variable from the sample matrix, and the presence of non-uniformity of concentration and path length of atoms, applying the Beer-Lambert law directly in AA spectroscopy is difficult. The need to calibrate the instrument with standard of known concentration is needed. Concentration measurement is usually determined in a working curve. These working curves are obtained by measuring the signal from the series of standard of known concentration.
The Beer-Lambert law (or Beer’s law) is the linear relationship between absorbance and concentration of an absorbing species. The general Beer-Lambert law is usually written as: A = a(lambda) * b * c; where A is the measured absorbance, a(lambda) is a wavelength-dependent absorptive coefficient, b is the path length, and c is the analyte concentration. When working in concentration units of molarity, the Beer-Lambert law is written as: A = epsilon * b * c where epsilon is the wavelength-dependent molar absorptive coefficient with units of M-1 cm-1.
Inductively Coupled Plasma Spectrometry or ICP replaced the atomic absorption method. However, due to the fact that AA or AAS is relatively simple and low cost, has guaranteed a place in laboratory bench for the predictable future. ICP is an analytical technique used for the detection of trace metal in environmental samples. The primary goal is to get elements to emit characteristic wavelength specific light, which can then be measured. This technology method was first employed in the early 1960’s, it is intended in improving crystal growing techniques. Since then, ICP has been refined and used in conjunction with other procedures for quantitative analysis.
The design of the ICP hardware is focused on generating plasma, whish is a gas in which atoms are present in an ionised state. It has tree concentric tubes, which is made of silica. These tubes, were termed Intermediate loop, inner loop and outer loop, and it makes up the torch of the ICP. The torch is positioned within a water-cooled coil of a radio frequency generator. The R.F. field is activated and the gas in the coil region is made electrically conductive as flowing gases are introduced into the torch. Plasma is formed in this sequence of events.
Plasma formation depends upon adequate magnetic field strength and the pattern of the gas streams follows a scrupulous rotationally symmetrically pattern. Plasma is maintained by inductive heating of the flowing gases. High frequency annular electric current within the conductor is generated by induction of a magnetic field. The conductor, in return, is heated as the results of its ohmic resistance.
To prevent possible short-circuit and meltdown, the plasma must be insulated from the rest of the instrument. Concurrent flow of gasses through the system achieved insulation. There are three gasses that flows through the system, the inner or carrier gas, the intermediate gas and the outer gas. Argon and Nitrogen are the outer gas. The outer gas serves several purposes that includes stabilizing the position of the plasma, thermally isolating the plasma from the outer tube, and maintaining the plasma. Normally argon gas is used for both intermediate gas and inner or carrier gas and the purpose of the carrier gas is to convey the sample to the plasma
The ICP typically includes the following components; sample introduction system (nebulizer), ICP torch, high frequency generator, transfer optics and spectrometer and a computer interface. An ICP requires that the elements, which are to be analysed, be in solution. A liquid solution is preferred over an organic solution, due to the reason that organic solution will require special treatment prior to injection to the ICP. Clogging of the instrument will occur if solid samples are used, therefore it is discouraged. The nebulizer transforms the aqueous solution into an aerosol. Electrical signal can be measured quantitatively by means of converting light emitted by the atoms of an element into an electrical signal. Resolving the light into its component radiation and measuring the light intensity with the photo-multiplier tube at the specified wavelength for each element line accomplish this conversion. The photo-multiplier in the spectrometer converts the light emitted by the atoms or ions in the ICP into electrical signals. Then the intensity of the electron signal is compared to previous measured intensities of known concentration of the element and a concentration is computed. Note that each element will have many specific wavelengths in the spectrum, which could be used for analysis. Therefore, selection of the best line analytical application in hand requires considerable experience of ICP wavelengths
Today inductively coupled plasma is used in forensic by means of combining ICP with Atomic Emission Spectroscopy. This is advantageous over the other method mentioned above, since AES require that sample to be in a gaseous form proceeding to injection into the instrument. Thus, using ICP in combination with these instruments eliminates any sample preparation time, which would be required in the absence of an ICP.
Advantages of using an ICP include its ability to identify and quantify all elements with the exception of Argon (Traci Bradford, 1997); since many wavelengths of diverse sensitivity are available for determination of any one element, the ICP is appropriate for all concentrations from ultra trace levels to major components; recognition limits are normally low for most elements with a typical range of 1 – 100 g / L. most likely the largest advantage of employing an ICP when performing quantitative analysis is the fact that multi-elemental analysis can be achieved, and quite rapidly. A whole multi-element analysis can be accomplished with in 30 seconds, consuming only 0.5 ml of sample solution. It is much obvious that this is a better method than the other.
ICP methodology coupled with Atomic Emission Spectroscopy is now commonly used in forensic science. One possible usage of the said method is the determination of toxic metal ions in beverages. If the toxic ions are mixed with the beverage in a criminal case, it is required to make precise determination and detection of these ions. Since ICP-AES is a proven practice to detect these toxic ions such as Cu, Cd, Tl and Pb it will be advantageous to use this methods than Atomic Absorption analysis.
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