Will Ashby and Dr. Lawrence B. Rees, Physics and Astronomy

History of CABL

Bullets often mutate or fragment upon impact. This can erase physical markings such as rifling. Until the development in the mid 1900s of techniques for precisely analyzing elemental compositions of materials such as lead, forensics scientists could do nothing to compare bullets obtained from a suspect to mutated or fragmented bullets from a crime scene. Around 1960 forensic scientists began performing the compositional analysis of bullet lead (CABL). The compositions of the bullets from a suspect were compared to the composition of the mutated bullet or fragment from a crime scene in an attempt to identify the guilty suspect [1]. Since the 1980’s CABL has been used in approximately 2,500 investigations, and in 500 of these cases the analyses were presented as evidence in court [2].

The current CABL protocol that most forensic scientists follow consists of using inductively-coupled plasma optical emission spectroscopy (ICP-OES) to analyze seven elements: antimony, copper, arsenic, silver, tin, bismuth, and cadmium. The concentrations of these seven elements from different bullets or fragments are compared to each other in an attempt to connect a suspect to a crime. This connection rests upon the assumptions that bullets manufactured at the same time have identical compositions and that this compositional “signature” is never repeated when other bullets are manufactured. As I began doing experiments and continued searching through the literature on CABL, I discovered that three years ago Randich et. al. showed that both these assumptions are seriously flawed [3]. However, it was too late for me to restart and redirect my research so I continued onward.

The ICP-OES technique is well suited to the forensic lab because it takes less space than a particle accelerator and is relatively easy to use. However, particle-induced x-ray emission (PIXE) spectrosopy can analyze more elements simultaneously with similar precicion to ICP-OES. This is because ICP-OES calibration requires a standard containing known amounts of almost all the desired elements (currently seven). In PIXE just one known elemental standard is needed for calibration. (Often yttrium is used.) Another advanatage is that PIXE does not require dissolution of the sample and can be performed non-destructively.

Conclusion

After analyzing 20 9mm Federal Hydrashock bullets and two samples from a National Institutes of Standards and Technology (NIST) standard reference material, I found that a lot of work needed to be done to improve the results. The certified values of the standard reference material did not match our initial PIXE analysis as hoped for. Because of this I had to calibrate the data we had gatherd via PIXE to the certified values of the standard reference material instead of using a single element like yttrium. This greatly reduced the number of elements that could be included in the results from 35 to 11.
Despite this difficulty, the results did suggest that manganese, nickel, sulfur, selenium, and zinc may also be useful in CABL. These elements had compositions that varied significantly between each bullet when compared to the standard deviations for each element.

References

1. Ernest R. Peele, Donald G. Havekosi, Charles A. Peters, John P. Riley, and R. C. Halberstam, “Comparison of bullets using the elemental composition of the lead component,” in Proceedings of the International Symposium on the Forensic Aspects of Trace Evidence [microform], (Forensic Science and Research Training Center, FBI Academy, Quantico, Virginia, USA, 24-28 June 1991).
2. Alexandra Goho, “Forensics on Trial: Chemical matching of bullets comes under fire,” Science News 165(13), 202 (2004).
3. Erick Randich, Wayne Duerfeldt, Wade McLendon, and William Tobin, “A metallurgical review of the interpretation of bullet lead compositional analysis,” Forensic Science International 127, 174-191 (2002).