299 GUNSHOT RESIDUE INVESTIGATIONS USING TXRF Alexander Wastl 1, Bettina Bogner 2, Peter Kregsamer 1, Peter Wobrauschek 1, Christina Streli 1 1 Atominstitut, Vienna University of Technology, Vienna, Austria 2 Police Command Vienna, Vienna, Austria ABSTRACT When a firearm is fired, gunshot residues (GSRs) are dispersed in the area around the shooter. It is very important for a police investigation at a crime scene to gain as much information from such residues as possible. Due to the minute amounts of residue available, total reflection x-ray fluorescence analysis (TXRF) seems well suited for GSR analysis. For this research, three shooting experiments were conducted. For each shot, sample carriers were distributed in the area in front of the shooter and were analyzed afterwards using TXRF. Due to the heterogeneity of the GSRs no reproducible results could be obtained. The elemental composition of the gunshot residue particles as well as the total amount being deposited showed high variation, even when the shot and the position were the same. Because of this variability, these results must be classified as statistically not very reliable. Results found with TXRF in the used configuration can neither be used neither to determine weapon or cartridge specific elemental profiles nor to estimate the shooting distance. Nevertheless Pb from the GSRs could be detected at a distance of up to 8 m from the muzzle. INTRODUCTION Gunshot residues consist of particles with a size of a few µm (Mou et al., 2008). Those particles are molten elements of the primer of the ammunition (Dalby et al., 2010) produced due to the high temperature (up to 3600 C) and high pressure (up to 3 10 5 kpa) (Romolo et al., 2001) when firing.
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300 Fig. 1: Different particle shapes by AFM (Mou, et al., 2008) Different manufacturers of ammunition use different compositions of elements. The main goal of this research was to find out whether it is possible to detect weapon and cartridge specific element combinations in the GSRs. The state of the art investigation method is the collection of GSRs with adhesive tape, which is afterwards analyzed in the scanning electron microscopy combined with energy dispersive X-ray analysis (Schwoeble et al., 2000). Typical elements which are searched for are Pb, Ba and Sb (Romolo et al, 2001). As TXRF is a powerful analytical method for analyzing minute amounts of sample, it seems suited to investigate GSRs. TXRF can investigate larger areas at the same time (the used detector has an effective detection area of 50 mm²) and detects several elements simultaneously. EXPERIMENTAL SETUP Three shots were conducted using the Austrian standard police hand gun Glock 17 with the ammunition GECO 9 mm LUGER Parabellum, 8.0 g full metal jacket to test the reproducibility of the results. The experiment was performed at the indoor shooting range of the Police Command Center in Vienna.
301 Before shooting the weapon has been cleaned carefully by the police officer and all sample carriers were cleaned and measured for contaminations. For this experiment quartz-reflectors were used as sample carriers. At every measurement location two reflectors were positioned one blank and one with 50 ng cobalt as internal standard for quantification. The geometry of the positions is shown in figure 2. All reflectors are placed along the projectile trajectory, 20 cm above the ground facing upwards. The distances are 0.5 m, 2.0 m, 3.5 m, 5.0 m, 6.5 m and 8.0 m. Fig. 2: Measurement setup All 36 reflectors were measured using the TXRF-spectrometer Atomika 8030C with a Mo/W- X-ray-tube, 50 kv / 47 ma and 500 seconds measurement time.
302 Fig. 3: Picture of shooting experiment RESULTS Fig. 4: First shot, spectra of reflectors at 0.5 m, with and without internal standard
303 Figure 4 shows the spectra of the reflectors at 0.5 m distance from the muzzle. The blue spectrum is the result of the measurement of the reflector with the internal standard (Co), the black is the result of the reflector without internal standard. Obviously Pb and Ba from the primer have been deposited on the reflectors. So the first result is that GSRs can be detected with TXRF. Fig. 5: Spectra of all three shots, reflectors of the position in 0.5m Figure 5 is a plot of three spectra of the three shots including the reflectors in 0.5 m distance to the muzzle. The three reflectors did not carry an internal standard. It is easy to see that the ratios of Pb, Ba and Cu are not constant in all shots. In the first shot, the Pb peak is the highest, but for the third shot the Ba peak is the highest. So it was not possible to show that this weapon-cartridge-combination has a characteristic spectrum or element intensity ratio. This effect can be seen in other spectra of reflectors in different distances too. The fact that the amount of GSRs deposited on the reflectors is changing randomly can also be seen in Figure 6. The graph shows the Pb-intensity of the three shots at different distances. As the setup has been the same all the time (same layout, same weapon, same ammunition and same officer shooting), the differences between the lines were supposed to be much
304 smaller. An accurate estimate of the distance between the muzzle and the reflectors cannot be made. Fig. 6: Pb-intensity of the three shots in different distances However, it is possible to detect at least Pb up to 8 m distance as it is shown in figure 7. Fig. 7: Spectrum of the second shot, position at 8.0 m distance
305 CONCLUSION Due to (a) the high variations of the elemental intensities between the shots and (b) the variation of the ratios between the elemental intensities, it has to be concluded that the results found with this test configuration are not reproducible. So neither quantitative results nor weapon or cartridge specific elemental profiles ("finger prints") can be deduced. As these problems may result from the minute amount of sample on the sample carrier, further research should consider this fact. Maybe other sample carriers or larger investigated areas per measurement point can increase the amount of sample and improve the statistics concerning the reliability. Another point of further research is that Pb still could be found at 8.0 m distance to the muzzle. This is a large distance compared to other GSR analysis methods. Accordingly, TXRF may prove as a more sensitive method for investigating medium shooting distances. REFERENCES Dalby O., Butler D., and Birkett J. (2010). "Analysis of gunshot residue and associate materials". Journal of Forensic Science. 55, 924-943 Mou Y., Lakadwar J., and Rabalais J.W. (2008). "Evaluation of shooting distance by AFM and FTIR/ATR analysis of GSR". Journal of Forensic Science. 53, 1381-1386 Romolo F., and Margot P. (2001). "Identification of gunshot residue". Forensic Science International. 119, 195-211 Schwoeble A.J. and Exline D. (2000). Current Methods of Forensic Gunshot Residue analysis (CRC Press, Boca Raton)