Flaws in Chemical Bullet-Matching
April 24, 2002
The assumption that bullets found at a crime scene can be matched to those in a suspect's possession has helped convict countless murderers, robbers and armed felons in the U.S., Britain and elsewhere. Forensic scientists analyze lead bullets for traces of antimony, tin, arsenic, copper, bismuth, silver and cadmium.
The idea is that if two bullets have the same chemical signatures, they must have been made at the same time from the same batch of smelted lead.
However, Erik Randich, a metallurgist based at Lawrence Livermore National Laboratory discovered this assumption is wrong. An examination of detailed records held by manufacturers of the lead used to make ammunition shows it is impossible to prove that any two bullets come from the same batch.
The researchers examined records for 1998 to 2000 held by Sanders Lead Company in Alabama and Gopher Resources Corporation in Minnesota.
- They found many instances where it was impossible, using the Federal Bureau of Investigation's chemical profile standards, to distinguish between batches poured months apart.
- The researchers also found small but measurable differences in the composition of lead samples taken at the beginning and end of the same batch, probably due to oxidation of the trace elements.
- That means it is impossible to say whether any two bullets were made on the same day or come from the same box.
In 1999, the FBI commissioned its own research into the validity of bullet matching. A statistician at Iowa State University determined that while it is theoretically possible to determine the likelihood of a match, the FBI did not have enough data to do it. There is a high probability that a match is simply due to coincidence. No one knows how many verdicts have rested on such evidence.
Source: Robin Mejia and Ian Sample, "Chemical Matching of Bullets Fatally Flawed," New Scientist, April 20, 2002.
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