Bullet Lead Analysis - Defendants not Told of Flaws in Fbi Technique

Hundreds of innocent people nationwide have been convicted with the help of an FBI forensic tool that was discarded over two years ago, and the FBI has yet to alert the affected defendants or courts, even as the window for appealing convictions is closing.

The Basis for Compositional Bullet Lead Analysis (Comparisons)

Background

When the physical markings of a fired bullet recovered from a crime scene are too mutilated for visual comparison or the firearm used in the crime is not recovered, the bullet can be compared with other bullets associated with a suspect by its elemental composition.

When a crime-scene bullet contains the same analytical elemental concentrations (i.e. match in composition) as the bullets from known cartridges, a single source of these bullets cannot by excluded. During the manufacturing processes, thousands of lead specimens (bullets and bullet cores) are produced with analytically indistinguishable compositions.

However, those lead specimens that share the same composition are generally packaged within the same box of cartridges, or in boxes of cartridges of the same caliber and type at the same manufacturing plant, on or about the same date.

When the differences in element concentrations are small but analytically significant, a comparative examination can be used to differentiate among bullets made of different alloys or to exclude a single source for bullets of the same alloy.

Comparative bullet lead analysis was developed in the early 1960s by researchers at General Atomic (now General Activation Analysis, Inc. located in Encinitas, California) under a federal grant to develop uses for neutron activation analysis (NAA).

Researchers developed procedures for analyzing such materials as gunshot primer residues, glass, paint and bullet lead. The results of their research were published in U.S. Atomic Energy Commission Reports (Lukens et al. 1970), the Journal of Radioanalytical Chemistry (Guinn 1982; Guinn et al. 1987), and the Journal of Forensic Sciences (Lukens and Guinn 1971).

In one research effort, the group acquired and analyzed samples from bullet lead manufacturers. The results of these analyses confirmed that a cast billet poured from a pot of molten lead is relatively homogeneous, but that leads poured from separate molten batches are distinguishable.

As a result, comparative bullet lead analysis has been adopted by laboratories and accepted by courts internationally (Andrasko et al. 1993; Blacklock and Sadler 1978; Brandone and Piancone 1984; Capannesi and Sedda 1992; Cohen et al. 1988; Desai and Parthasarathy 1983; Dufosse and Touron 1998; Gillespie and Krishnan 1969; Guy and Pate 1973; Kishi 1987; Krishnan 1973; Krishnan and Jervis 1984; Sankar Das et al. 1978; Screenivas et al. 1978; Suzuki and Yoshiteru 1996).

The NAA technique used at many laboratories has been replaced by inductively coupled plasmaoptical emission spectroscopy (ICP-OES), previously known as inductively coupled plasmaatomic emission spectroscopy (ICP-AES) (Peters and Koons 1988).

OES was adopted because people confused AES with auger electron spectrometry (Boss and Fredeen 1997). Since the 1970s, ICP-OES has been widely accepted and is the method of choice for most inorganic analysis (Koons 1993; Montaser and Golightly 1987).

One advantage of ICP-OES is its ability to determine the concentrations of as many as 70 elements simultaneously in some samples. ICP-OES instrumentation is used in environmental, manufacturing, research and forensic laboratories throughout the world and has been used by the FBI Laboratory in casework for the past 12 years.

The ICP-OES procedure currently used in the FBI Laboratory can determine the concentrations of seven elements (antimony, arsenic,copper, bismuth, silver, tin and cadmium) in most bullet leads. The main disadvantage of ICP-OES is that it is a destructive technique, requiring acid digestion of approximately 60 milligrams of each replicate sample of bullet lead.