Chapter 14
Radioanalytical Chemistry in the Courtroom
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J. D a v i d R o b e r t s o n Department of Chemistry and Research Reactor, University of Missouri, Columbia, MO 65211
Two examples of the use of particle-induced X-ray emission (PIXE) analysis to forensic science are presented. In the first, P I X E was used as a non-destructive method to search for signatures of gunshot residue on clothing. In the second, P I X E was used to investigate the authenticity of gold coins from the New World.
Introduction The first application of radiochemical methods to forensic analysis is, most probably, found in the popular anecdote told of Georg Hevesey who was awarded the Nobel Prize in Chemistry in 1943 "for his work on the use of isotopes as tracers in the study of chemical processes." The story is told that Hevesey used a dose of P b for a bit of detective work at his austere boarding house where his frugal landlady continued to serve the same old pudding, disguised by the addition of fruits and nuts, for more than one week. The first application of forensic activation analysis, published in 1963, was the determination of arsenic in human hair in relation to possible case of poisoning ( i ) and the first U.S. court case involving activation analysis occurred in March of 1964 (2). From this point forward, there was much optimism of the tremendous sensitivities of high-flux neutron activation analysis for the detection 212
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© 2004 American Chemical Society In Radioanalytical Methods in Interdisciplinary Research; Laue, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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207 for large numbers of elements in very small samples for solving problems in forensic science. From the literature and other sources of information, it is evident that the use of neutron activation, and other radioanalytical methods, in forensic analysis has gradually diminished over the last two decades. For example, in the early 1970s, one forensic activation analysis lab alone was processing more than 1,000 gunshot residue hand swabs per month but a 1996 survey of 80 forensic science labs found that only 2% of those labs surveyed still used activation analysis for gunshot residue anlaysis (3). This decline is due, in part, to the emergence of powerful, non-nuclear techniques for trace element analysis over the last three decades. Not only do these competing techniques perform well in respect to analytical characteristics, but they also have the advantages that they may often be carried out in house, enable a substantial throughput with short turn-around times, and do not require access to radioactive materials, a nuclear reactor, or particle accelerator. There are, however, still instances in which the unique characteristics of radiochemical analytical methods lead to their application in forensic analysis. Two such cases are described in this work.
Particle-Induced X-ray Emission (PIXE) The technique of using an accelerated particle beam for X-ray emission analysis was first introduced at the Lund Institute of Technology in 1970 (4). Like other X-ray spectroscopic techniques used for elemental analysis, P I X E utilizes the X-rays that are emitted from the atoms in a sample when that sample is exposed to an excitation source. The energies of the X-rays are characteristic of the elements from which they are emitted and the number of X-rays of a given energy is proportional to the mass of that corresponding element in the sample. The use of a high-energy proton beam as an excitation source offers several advantages over other X-ray techniques including a higher rate of data accumulation across the entire periodic table and better overall sensitivities, especially for the lower atomic number elements. In the case of electron excitation, the better sensitivity is due to a lower bremsstrahlung background and, in the case of X-ray fluorescence analysis, the better sensitivity is due to die lack of a background continuum across the entire spectrum. O f course, the chief disadvantage of P I X E is the reason that it is considered a radioanalytical technique; it requires the use of a particle accelerator. For the interested reader, detailed descriptions of the theory and application of PDCE can be found in references (5) and (6). In this work, I describe the application of P D Πto gunshot residue and coin analysis. Other forensic applications of P I X E include the analysis of bone (7), ink (
