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School of Chemistry
MM4EOX: Electron-optical and X-ray Techniques
Pages 1&2: Forensic investigation: trace evidence analysis for gun-shot residue
Page 3: Phase identification : characterization of nanostructured materials
Page 4: Enviromental safety: characterization of fine scale particle distribution
Forensic investigation: trace evidence analysis for gun-shot residue
The ability to identify gunshot residue has provided solution to the problems encountered in the resolution of forensic science, legal medicines, and for shooting distance determination. Visually comparing the casework powder residue pattern on the garment or on the skin of the victim with the pattern obtained from a series of test firings at known distances, using the same gun and ammunition has been used as a method for routinely estimating gunshot range. Burned and unburned powder grains, carbonaceous particles, bullet jacket debris, shaving and dirt have been shown to be examples of materials ejected from the bore of the weapon around the entrance hole of the bullet and have been used as materials for analytical determination in order to obtain a more and accurate result.1
An issue that has become very important in criminal investigation is trace evidence. Very often it come into place after the commission of a crime, well after the charges have been filed and well after the completion of forensic examination. Trace evidence can usually take the form of fibers, paint chips, soil, building materials, glass, gunshot residue, seeds, feathers, animal hair, human hair, wood fragments and other material. It has been proved that these substances are usually transferred between individuals during physical contacts and can also be transferred from individuals to environments and from environment to individuals.2
This report shows how complementary analytical techniques ranging from batch injection analysis (BIA) method, based on differential pulse anodic stripping voltammetry (DPASV), scanning electron microscopy/ energy dispersive X-ray analysis (SEM/EDX), capillary electrophoresis, can been used to identify trace evidence for gun-shot residue.
Batch Injection Analysis
Batch injection analysis (BIA) method, based on differential pulse anodic stripping voltammetry (DPASV) can been use to carry out forensic analysis of lead in gunshot residues (GSR). The design consist of a simple “J” shaped adaptor which is being used to direct the flux of the analyte injected with a micropipettor onto the hanging mercury drop electrode of any commercial electrode stand. Lifting with adhesive tape is usually selected for field use and pasting of the tape is done on polyethylene screens and stored in capped vials. Sampling is usually done with multiple strips and thus provides coarse mapping of the distribution of the Lead on the shooter's hand following the dissolution/extraction step with chloroform/aqueous HCl. Certain volume of this aqueous phase is then injected for few seconds for accumulation of the Lead on the HMDE at a certain volt (vs. Ag/AgCl). A detection limit of 20 ng/mL of Pb(II), outreaching for GSR analysis can be achieved without oxygen removal, at a frequency of 20 injections per hour. Quantitative analysis has shown the detection of over 90% of lead residues.3
Scanning electron microscopy/ energy dispersive X-ray analysis
Elemental analysis using SEM/EDX can be carried out in order to understand bullet structure and major elemental composition. Data's produced from elemental composition of bullets can be used in deciding the exact methods most appropriate for the identification of bullet hole and the determination of firing distance. Bullets and shots are usually collected from unfired cartridges and subsequently brushed with detergents and cleaned using tap water, distilled water, and acetone. Double-sided carbon tape can be used as a means of mounting jackets, lead cores and lead bullets and shot onto the sample stub on subjection to SEM/EDX analysis. The external surface, cross section of lead core, lead bullet, or lead shot can be subjected to elemental analysis using EDX. The internal surface and cross section of the jacket can also be subjected to elemental analysis in order to understand its structural composition. EDX measurement conditions can be set from the SEM unit with regards to spectral measurement, multi-point measurement, mapping, and display of analysed elements on the SEM monitor. The image data obtained from the SEM can be used as basic data for the EDX, while the setting conditions for the SEM units are automatically transferred to the EDX unit. The function which provides the strongest backup for elemental distribution is the Smart Map. The Smart Map operates by recording the X-ray spectral data for all analysis points on the test sample together with the positions of analysis, thus providing the user to recall valuable data as needed. Backscattered electron imaging mode in SEM is able to reveal layers of metals on the jacket's cross-section and its subsequent compositions while on the other hand, EDX analysis is able to reveal the coating elements detected on the external surface of lead bullet.4-5 At the moment, this technique is widely accepted due to the morphological (SEM) and elemental (EDX) determination of the metal residue.
Capillary electrophoresis can be used for the analysis of organic and inorganic components of gunshot residue in order to study sampling methodology, selectivity, reproducibility, quantification and the enhancement of the bulk analysis. A typical P/ACE MDQ Beckman capillary electrophoresis system is being used with polymide bare fused silica capillaries. A diode detector is usually used as a means of carrying out direct UV detection. A temperature of 25oC, with a positive voltage of 30KV and hydrodynamic injection of 5s and 0.5 p.s.i is usually used. The conditioning of new capillaries is usually done by rinsing with ethanol, HCL, NaOH at specific time and temperature. Between runs, the capillary is usually rinsed with deionised water, NaOH, and again with deionised water and background electrolyte at specific times. The sample is usually ejected into the capillary by temporal replacement of one of the buffer reservoir (usually at the anode) with a sample reservoir upon application of either an electric potential or external pressure for a few seconds. Upon replacement of the buffer reservoir, an electric potential is applied between the capillary and the separation is performed. Optical UV-detection of the separated GSR component can be obtained directly through the capillary wall near the opposite end (usually near the cathode). Swabbing technique has been used as a means of obtaining samples from gunshot. This technique is however not too good in detecting important organic residue such as barium and antimony. Figure 1, shows a typical zone of sampling for gunshot residue.6-7
Figure1 Typical zones of sampling for gunshot residue.
(A) web and (B) palm.7
Phase identification: characterization of nanostructured materials
Complementary analytical techniques such as X-ray diffraction (XRD), electron microscopies such as TEM & SEM, and EDX spectrometry can be employed as a set of tools in characterizing a one-dimensional inorganic nanostructure in order to investigate the crystal structure, particle size distribution, morphology, composition and aggregate state.
The technique of XRD can be used to deduce the lattice parameter of inorganic nanostructures which can be used to provide information on the thermal properties of the material, strain state or an analysis of the defect structure. The diffraction pattern of the material can be indexed appropriately if the crystal structure of the material is known. For example, in a cubic system, the d spacing which correspond to each diffraction pattern is related to the lattice parameter a following the equation a2 = d2 / (h2 + k2 + l2) in which hkl are the miller indices. This is however used in indexing the diffraction pattern. This technique is however subject to systematic error in the position of the diffraction peaks and random error in the individual calculation of the lattice materials.8 The calculated lattice parameter value is usually compared with the experimental value and this can further be used for appropriate interpretation of result.
Nanostructured materials can be characterized by SEM integrated with an EDX analyzer in order to determine the particle morphology and chemical composition of the sample. The SEM column forms a focused probe of electrons on the sample while the beam current and probe current are usually adjusted as required. An image is formed by scanning the probe in a raster pattern on the sample, detecting some excited radiations from the sample, and storing the result either as a pattern of varying intensity levels on a cathode ray tube (CRT) screen or an a pattern of digital values in electronic memory for later manipulation and display. SEM images are usually formed by detecting either the secondary (low-energy) electrons emitted from the sample, or the backscattered (high-energy) electrons.9 Secondary electron images can provide information on the sample topography thus revealing information about the grain size distribution of the material.
TEM/EDX investigation of nanostructured materials can provide a more detailed information about the smallest particle. When the selection area diffraction (SAD) pattern is projected onto the viewing screen, we can use this pattern to perform the two most basic imaging operations in the TEM. In order to form an image in TEM, we either use the central spot, or we use some or all of the scattered electrons. The way we choose which electron forms the image is to insert an aperture into the back focal plane of the objective lens, thus blocking out most of the diffraction pattern except that which is visible through the aperture. If the direct beam is selected the resultant image is a bright-field (BF) image and if the scattered electrons are selected then the resultant image is the dark-field (DF) image.10 The dark-field imaging and digitization of particles can be used to extract size distribution of the grains through thresholding and measurement of the projected areas.
Enviromental safety: characterization of fine scale particle distributions
Appraisal of the fine scale particle distributions emitted from a waste disposal furnace can be carried out using SEM/EDX to investigate the particle morphology, composition and chemistry, while TEM/EDX can be carried out to give a more detailed information about the particle size distribution and SIMS to determine isotopic ratios.
SEM equipped with an energy dispersive X-ray (EDX) spectrometer can be used determine the elemental composition, morphology and chemistry of emitted particle in a waste disposal furnace. In order to determine the emitted particle containing the heavy metals, the sample is subjected to backscattered electron imaging while emitted particle containing salts can be imaged in the secondary electron mode. The detected backscattered electrons originate from the larger volume of the sample than do secondary electrons, and thus form an image of lower resolution. Focusing of a small spot on the high Z area followed by analysis of the X-ray signal with EDX allows the resulting X-ray lines to be detected and the elemental composition determined.
TEM images of the sample which comprises DF and BF can be coupled with EDX analysis and chemical mapping can be carried out. Both DF and BF imaging in TEM can be used to investigate the metal speciation in the fine fraction of the emitted particle furnace. DF imaging and digitization of the particle can be used to extract size distribution through thresholding and measurement of project areas. Elemental composition of the emitted particle could be detected in small aggregates upon analysis by EDX and selected area electron diffraction.
Secondary ion mass spectrometry (SIMS) can be used to determine isotopic ratios of the heavy metals from within emitted particles. It operates by switching between masses and it possesses two microfocus ion sources. Sputtering is usually done with a primary O2+ and the intensity controlled by tuning the primary ion beam. The instrument can operate with a mass resolution power (MRP) of 25000.11
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