Compositional analysis of bullet lead (CABL) is chemical analysis of some (generally seven) of the elements found in lead alloy used to make bullets.1 These elements may be present in lead ore but not completely removed in smelting, present in recycled lead used for bullet manufacture, or, as in the case of antimony, added to bullet lead to control such properties as hardness. In bullet manufacture, the concentrations of the elements in the lead alloy are specified only within broad ranges or below a maximum concentration, so given volumes of lead have differing elemental compositions.
The Federal Bureau of Investigation (FBI) has recognized and exploited that characteristic of bullet lead by using CABL. CABL allows bullets or bullet fragments found at a crime scene2 to be compared with unused bullets found in the possession of a suspect.3 Comparison is accomplished by using an analytical method that employs inductively coupled plasma-optical emission spectroscopy (ICP-OES).
ICP-OES is an instrumental method that is capable of determining the concentration of elements in solution. Each lead sample must be dissolved in an
acidic solution before analysis. The measurements of element concentrations obtained are compared to the measurements of element concentrations in a National Institute of Standards and Technology Standard Reference Material to determine the actual concentration of elements measured. If the concentrations of all seven elements in the bullet lead from a crime scene are determined by FBI examiners to statistically match the concentrations of the same seven elements in the bullet lead from a suspect, FBI examiners conclude that the bullets are “analytically indistinguishable.” The results can be used by prosecutors as circumstantial evidence in a trial.
Some oppose the use of CABL. Questions have been raised as to the homogeneity of a source of lead, the uniqueness of a source of lead, the definition of a source of lead, the distribution of bullets and loaded ammunition, and the validity of specific statements made in court by expert witnesses.
CABL assumes that a “source” of bullet lead is homogeneous. Opponents of CABL point to purported inadequate mixing of the lead melt in the manufacturing process as new materials are added, to the microscale separations that may occur during cooling of the bulk solid after the melt is poured, and to the migration of less-soluble elements to the interior of the solidifying lead as it cools after the melt is poured. If a source is not homogeneous, no bullet can be representative of the source.
CABL also assumes that each lead source has a unique composition. Published data have shown that two lead sources prepared twelve years apart had compositions that were analytically indistinguishable4 (Ref. 1).
Analytically indistinguishable samples of bullet lead are said to come from the same source. There is some confusion about the definition of source and to which volume of lead in the manufacturing process it refers. The volume of lead affects the number of bullets that can be considered to come from one source.
Although the major bullet manufacturers distribute their products nationally and even internationally, some regional distributors might receive and distribute many bullets from the same compositionally indistinguishable source. That would increase the probability of finding a match between a crime-scene bullet and a bullet in the possession of an innocent person.
A wide variety of statements have been made in court by FBI examiners about the significance of CABL results. Some of these statements may have been exaggerated and may foster misinterpretation of the meaning of laboratory analyses.
The issues that have been raised by opponents to CABL are not trivial. To determine whether and how the use of CABL should be continued, the FBI
wanted to address those issues and others to an independent, unbiased institution. Thus, the National Research Council (NRC) was called on to evaluate CABL scientifically, statistically, and legally. The questions in the statement of task accepted by the NRC with respect to CABL were as follows:
Analytical method. Is the method analytically sound? What are the relative merits of the methods currently available? Is the selection of elements used as comparison parameters appropriate? Can additional useful information be gained by measurement of isotopic compositions?
Statistics for comparison. Are the statistical tests used to compare two samples appropriate? Can known variations in compositions introduced in manufacturing processes be used to model specimen groupings and provide improved comparison criteria?
Interpretation issues. What are the appropriate statements that can be made to assist the requester in interpreting the results of compositional bullet lead comparison, for both indistinguishable and distinguishable compositions? Can significance statements be modified to include effects of such factors as the analytical technique, manufacturing process, comparison criteria, specimen history, and legal requirements?
The Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison is composed of 14 experts in analytical chemistry, statistics, forensic science, metallurgy, and law. It met four times in Washington, D.C. The meetings allowed the committee to hear from experts in lead manufacturing, statistics, and use of CABL in court. At each meeting, the committee received presentations from FBI employees who research, use, or testify about CABL. The committee also used background information, such as scientific journal articles (both those provided to the committee by individuals outside the committee, and those found by the committee in its own search of relevant literature), published statistics on lead, court transcripts, and the expertise and experience of its members. Members of the committee visited the FBI Laboratory, Eldorado Cartridge Corporation/PMC, and the SHOT Show to gather data. The deliberations of the committee on the questions in the statement of task and on other related issues led to this report.
Chapter 2 addresses the analytical chemistry portion of CABL. It discusses the analysis of lead with ICP-OES and compares it with other, previously used instrumental methods and with potentially useful technology untested for this application. The elements that are measured with ICP-OES and compared to determine a match are also assessed. The chapter evaluates the entire written analytical protocol of the FBI and draws conclusions about the protocol’s appropriateness and application.
Chapter 3 presents and critiques the statistical protocol used by the FBI for
bullet matching. The chapter recommends alternate tests to be used in place of the FBI’s current procedure.
The process of CABL culminates in its use as circumstantial evidence in court. The first half of Chapter 4 provides basic information about lead refining and bullet manufacturing to further an understanding of their significance in the interpretation of CABL data. It also offers some statistics on bullet production and the various volumes of liquid and solid lead that are eventually used to form bullets. Sections on the homogeneity of lead volumes and on the definition of source are integral to the committee’s findings. The second half of the chapter introduces the admissibility of scientific evidence, relevance, and how CABL evidence has been used in trials. It discusses inconsistencies and changes in CABL-related testimony, laboratory reports, and printed handbooks and discusses the importance of these inconsistencies and changes. The chapter includes the rules governing pretrial discovery of reports and summaries of expert testimony, and the use of expert witnesses.
1. Randich, E.; Duerfeldt, W.; McLendon, W.; and Tobin, W. Foren. Sci. Int. 2002, 127, 174–191.