Bullet-lead analysis
Bullet-lead analysis is a forensic technique used to examine trace elements in lead bullets to facilitate comparisons between bullets and potentially identify their sources. Developed in the 1960s, this method involves dissolving a bullet fragment, vaporizing the solution, and analyzing the emitted light spectrum to determine the presence and quantities of trace elements such as antimony, arsenic, and copper. The composition of these elements can vary depending on the origin of the lead and the materials used in bullet production, which often includes recycled lead from sources like automobile batteries.
While this analysis has been employed to link suspects to crimes—using bullet fragments found at crime scenes or bullets recovered from suspects—its reliability has been heavily scrutinized. Critics argue that a chemical match does not definitively indicate the source of the bullets, as large batches of lead can produce many bullets that share similar elemental compositions. Furthermore, there are concerns about the accuracy of experimental error calculations and the problematic practice of "chaining," where matches between multiple bullets are assumed without direct evidence. Following significant legal challenges and criticisms, the FBI announced in 2005 that it would discontinue the use of bullet-lead analysis, marking a notable shift in forensic practices.
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Bullet-lead analysis
Definition: Examination of the amounts of trace elements in lead bullets to enable comparison of bullets and determination of their sources.
Significance: Analysis of the trace elements present in bullets can be conducted on bullet fragments found at a shooting scene and on any bullets found in the possession of suspects; comparison of the findings can link a suspect to a crime.
Bullets found at crime scenes may be compared with test bullets fired from weapons suspected to have been used in the crimes, but such analysis is not possible when only bullet fragments are recovered or no weapons are found. To address such situations, the technique of bullet-lead analysis was developed during the 1960’s. In this process, a fragment of a bullet is dissolved, the solution is vaporized, and the vapor is heated until it glows. By examining the spectrum of light from the glowing vapor, the analyst can determine what trace elements are present in the lead and the amount of each element. Lead typically has traces (1 percent or less) of antimony, arsenic, bismuth, cadmium, copper, silver, and tin. The amounts of these elements vary with where the lead was mined and what kinds of scrap lead have been added. (Most modern bullets are made from recycled lead taken from automobile batteries.)
The use of bullet-lead analysis has been criticized by some. In one case involving the analysis of bullet lead, Michael Behm was convicted in 1997 of murdering a man in South River, New Jersey. The only physical evidence presented in court that linked Behm to the murder was a chemical match between the amounts of trace elements found in bullet fragments from the crime scene and the amounts of those elements found in ammunition recovered from Behm’s home. The prosecution led the jury to believe that this meant that the crime scene bullet fragments must have come from the box of ammunition in Behm’s possession. This was a serious misuse of bullet-lead analysis because such a match does not necessarily pinpoint a bullet’s source. A large batch (or melt) of lead might produce millions of bullets, so hundreds of people in each of dozens of towns might have had bullets matching the crime scene fragments.
Another serious problem with bullet-lead analysis is the question of how the level of experimental error should be calculated. For example, the amount of antimony in a sample might be reported as 0.85 percent � 0.15 percent, meaning that the amount of antimony is between 0.70 percent and 1.0 percent. If the level of experimental error (in this instance �0.15 percent) is estimated from too few measurements, confusion can result. Suppose that the antimony content of a different lead sample is reported as 0.51 percent � 0.20 percent, so that it lies between 0.31 percent and 0.71 percent. Because this range overlaps with the antimony range given for the first lead sample, it appears that the antimony contents in the two lead samples “match” or are “analytically indistinguishable.” Had the second measurement been reported as 0.51 percent � 0.10 percent, no overlap, and thus no match, would have resulted.
A questionable practice sometimes used in the presentation of evidence involving bullet-lead analysis is called chaining: Because bullet A is found to be a chemical match to bullet B, which in turn matches bullet C, the claim is made that bullet C matches bullet A, regardless of whether A and C are chemical matches. Chaining was used in convicting Behm. On March 7, 2005, an appellate judge ruled this interpretation of the bullet-lead analysis results invalid and overturned Behm’s conviction. On September 1, 2005, after extensive review, the Federal Bureau of Investigation (FBI) announced that it would discontinue the use of bullet-lead analysis.
Bibliography
Boyce, Nell. “Do Bullets Tell Tales?” U.S. News & World Report, November 24, 2003, 60-61.
Goho, Alexandra. “Forensics on Trial: Chemical Matching of Bullets Comes Under Fire.” Science News, March 27, 2004, 202.