Bullet Lead Analysis
Bullet Lead Analysis
Crime scene bullets are sometimes too mutilated or fragmented to be useful for normal ballistics analysis—that is, the bullet has no markings that investigators can compare with those produced by a gun connected to a suspect. Bullet lead analysis, sometimes called compositional bullet lead comparison or comparative bullet lead analysis (CBLA), allows forensic investigators to identify the elemental composition and characteristics of a bullet. This information can show if the bullet matches that of a bullet whose source is known, whether two bullets likely came from the same source (a single manufacturer's lot), or possibly even the same box. CBLA is widely accepted in the courts to show that a bullet recovered from a crime scene matches other bullets found in the possession of a suspect, strongly suggesting that the suspect fired the bullet in question. CBLA can also be used to exclude suspects.
The procedure, which is generally carried out by the Federal Bureau of Investigation (FBI ), was developed in the 1960s by researchers under a federal grant to develop uses for neutron activation analysis (NAA). With the aid of a nuclear reactor, these researchers were able to show that a billet, or ingot, of lead poured from a pot of molten lead was homogenous in its elemental composition. Thus, any bullets manufactured from that ingot were analytically indistinguishable. Conversely, bullets manufactured from billets poured from different pots of molten lead were analytically distinguishable because the concentrations of elements in them were different. Since about 1995, researchers have replaced NAA with a process called inductively coupled plasma-optical emission spectroscopy (ICP-OES). This process enables researchers to measure the concentrations of up to 70 elements simultaneously. The relevant elements in bullet lead are antimony, arsenic, copper, bismuth, silver, tin, and cadmium.
Bullets are generally manufactured from lead obtained from secondary lead smelters, which obtain most of their lead from recycled automobile batteries. The smelter separates the lead and melts it in kettles with a capacity of up to 100 tons, often supplementing the recycled lead with virgin lead. Elements such as antimony may be added to produce hardened lead, which is generally used in non-jacketed bullets. Soft lead, generally used in jacketed bullets, contains little or no antimony. Other elements are likely to be present in varying trace amounts. The lead is then processed into ingots ranging in weight from 65–80 pounds (29.5–36.2 kilograms); billets, from 100–300 pounds (45.3–136 kilograms); or sows, which weigh a ton. The bullet manufacturer then re-melts the lead, often adding scrap lead. The lead is then poured into a mold, where it is allowed to cool and harden before being extruded into "wires" that are cut into slugs, which are then formed into bullets by a process called swaging, then tumbled to make them smooth, and loaded into cartridges with gunpowder. The cartridges are then placed into boxes, which are stamped with a packing code or lot number.
CBLA is regarded as reliable forensically because each batch of lead that comes from secondary smelters is unique in its elemental composition. Differences in composition are tolerated as long as they are with in acceptable quality limits. At the bullet manufacturer, rejected bullets, rejected lead from previous runs, trimmings, and any other source of lead may be recycled into a particular batch, creating similar variances that give each batch a unique elemental fingerprint. The result is that each batch, and the bullets manufactured from it, is homogeneous, while elementally distinguishable from all other batches. The FBI states that ICP-OES technology is able to detect narrow compositional ranges of 0.01–0.05 percent of each of the seven relevant elements. The result is that ICP-OES can detect millions of distinguishable lead compositions.
The scientific validity of CBLA has been challenged in the courts. Specifically, defense attorneys have challenged the "same composition, same molten lead" theory of CBLA under Rule 702 of the Federal Rules of Evidence , claiming that the premises of CBLA—that lead and bullet samples are representative and that lead sources are compositionally uniform and unique—have not been sufficiently tested. CBLA has withstood these challenges, notably in United States v. Jenkins (1997) and in a 1998 New York case, People v. McIntosh. CBLA is likely to come under increased scrutiny as a result of a 2004 report released by the National Research Council of the National Academies of Science (NAS). The report was the result of a 12-month study conducted by the Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, which found weaknesses in the research base that underpins CBLA. Following release of the report, though, the FBI maintained that CBLA "will still be useful in linking individuals to a crime as well as to exclude others."
see also Ballistics; FBI crime laboratory; Federal Rules of Evidence; Laser ablation-inductively coupled plasma mass spectrometry; Spectroscopy.