|Robert L. Watters, Jr., is the chief of the Measurement Services Division at the National Institute of Standards and Technology. He has over 28 years of experience at NIST in SRM development and international metrology comparisons. Watters is the NIST representative to the International Organization for Standardization’s Committee on Reference Materials and to the U.S. Pharmacopoeia.
Nancy Parrish is a measurement services product specialist at the National Institute of Standards and Technology. She has eight years federal government experience in finance and budget policy and planning. In 2002, she joined NIST’s Measurement Services Division, where she is responsible for business operations, policies, and procedures related to the NIST Standard Reference Materials, Standard Reference Data, and Calibration programs.
NIST Standard Reference Materials
Supporting Metrology and Traceability for the Forensic Science Community
The ability to measure precisely and consistently is the great enabler of modern technology. The reliability, safety and interoperability of today’s devices and systems, the foundation of international trade and global manufacturing, even the veracity of evidence presented in court, can be traced to the fact that a volt is measured by the same standard everywhere across the power grid, that a gram in a laboratory in Leipzig is identical to one on an assembly line in Lansing, that a degree Celsius is a degree Celsius whether measured in a cleanroom or a courtroom.
We stand on this pinnacle thanks to more than a century of determined effort by thousands of talented men and women, many associated with organizations such as ASTM International and the National Institute of Standards and Technology. A non-regulatory agency of the U.S. Department of Commerce, NIST was established by Congress as the National Bureau of Standards in 1901 to improve the competitiveness of American industry by setting and maintaining the nation’s measurement standards and advancing the measurement science that underpins those standards. Within this charter, NIST’s mission has expanded to promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life. Today, the institute and its 3,000 employees among them many of the world’s leading scientists and engineers, including Nobel laureates, and more than 1,500 annual guest researchers enjoy a worldwide reputation as both a premier metrology research team and an unbiased referee in disputes involving measurements and standards.
Among NIST’s measurement services programs, that of developing and providing reference materials is the largest. The International Vocabulary of Metrology, published by the International Organization for Standardization (ISO), defines a reference material as one or more materials or substances whose properties are sufficiently homogeneous and well established to be used for calibrating apparatuses, assessing measurement methods, or assigning values to materials. The vocabulary defines certified reference material as a reference material, one or more of whose property values are certified as traceable to an accurate realization of the unit in which the property values are expressed, and for which each certified value is accompanied by an uncertainty at a stated level of confidence.1 A NIST Standard Reference Material® is a certified reference material that meets NIST-specific certification criteria and is issued with a certificate of analysis that details its characteristics and provides information on its appropriate uses. NIST certificates of analysis contain certified values and uncertainties in which NIST has the highest confidence because it has fully investigated or accounted for all known or suspected sources of bias. (See the sidebar about NIST’s SRM Program.)
A Case Study: NIST Standard Bullets and Casings
NIST has served as a technical consultant to the U.S. criminal justice system since 1912 and even trained the Federal Bureau of Investigation’s first forensic scientists when the bureau opened its crime laboratory in 1924. In 1971, NIST established its Office of Law Enforcement Standards (OLES) to provide law enforcement and public safety agencies with measurements, standards and technical guidance.
A NIST SRM 2460 standard bullet is mounted on a blue stub. One of the six highly uniform, land engraved areas (trapezoidal in shape) is clearly visible.
OLES is particularly active in the forensic sciences, developing tools and techniques that help investigators collect, manage and analyze material from crime scenes in ways that meet evidentiary requirements established by the courts. Among these tools are a number of SRMs, including materials for:
• Calibrating instruments and methods used to measure blood-alcohol levels;
• Verifying methods used to detect drugs of abuse in urine and hair;
• Identifying residues of additives in smokeless gunpowder and residues of ignitable liquids used by arsonists; and
• Standardizing forensic DNA analyses and providing quality control of polymerase chain reaction procedures and the sequencing of human mitochondrial DNA.
Among the forensic SRM efforts that have drawn the most attention recently is the NIST standard bullets and casings project.
The FBI reports that, in 2004, firearms were used in the commission of more than 1.2 million crimes in the United States. This makes solving firearm-related crimes a primary national concern, and ballistic comparisons that match ammunition to firearms are among the most useful tools in this effort.
When a firearm is discharged, the gun barrel striates the surface of the bullet and the gun’s firing pin, breech face, and ejector imprint the bullet casing with distinct markings. These markings comprise a “bullet signature” and a “casing signature” that are unique to each firearm. By analyzing these signatures, forensic examiners can connect a firearm to a criminal act.
In the early 1990s, the Bureau of Alcohol, Tobacco and Firearms and the FBI each established its own database system for imaging, analyzing, and storing the signatures of bullets and casings found at crime scenes. These separate systems limited the ability of state and local law enforcement agencies to compare bullets and casings found at different crime scenes, establish connections, and build cases against suspects.
In 1997, ATF and the FBI attempted to join the two systems into a National Integrated Ballistics Information Network, or NIBIN. Soon the two agencies realized that selecting one of the systems, the Integrated Ballistics Identification System, known as IBIS, was the only workable solution to create the NIBIN. This approach ensured that two key requirements were fulfilled: to unify ballistics measurements among the hundreds of forensic laboratories nationwide, and to establish a reliable method for tracing measurements and ensuring their accuracy through the system. Based on its long-standing research in realizing the SI unit of length, NIST undertook the task of helping the criminal justice community harmonize ballistics measurements nationwide. NIST Standard Bullets and NIST Standard Casings are being developed to help fulfill these requirements.
NIST Standard Bullet (SRM 2460) is in its final certificate review and user acceptance testing. The SRM consists of both a physical standard and a virtual standard. The physical material is the bullet itself, the tool for verifying the accuracy of imaging equipment and signature comparisons.2 The virtual standard is a set of six digitized bullet profile signatures that provides the information used to machine the bullet signatures on the physical standard.3,4 The digitized profiles serve as the template for producing the physical SRM bullets. The SRM is intended for use in calibrating IBIS equipment and verifying ballistic measurements.
At the outset of the project, NIST established four basic technical requirements for its standard bullets:
• Bullet surface characteristics The shape, size, material and color of the physical standard bullets must be as close as possible to those of real bullets.
• Repeatability and reproducibility The signatures on the SRM standard bullets must show such a high degree of repeatability and reproducibility that when the bullets are distributed nationwide for instrument calibrations, they function as a single bullet.
• Measurement traceability NIST’s measurements of the SRM bullet signatures must be traceable to the SI unit of length. Bullet signature measurements performed at local laboratories must be traceable to measurements of SRMs maintained at the ATF and FBI central laboratories.
• Information technology The bullet signatures for the SRM standard bullets must be digitized so that they can be used to produce identical standard bullets at any time.
The development phase of the work began in 1998 when NIST’s Precision Engineering and Manufacturing Metrology Divisions machined the first two prototype standard bullets, each bearing six reference bullet signatures. Tests showed the signatures on the manufactured bullets to be highly uniform and reproducible. From 1998 to 2001, NIST collected comments on these prototype standard bullets from firearm examiners at ATF and the FBI, and improved both the design and manufacturing process for the physical standard bullets. Further work was necessary to establish a traceable measurement parameter that would account for the small differences between the two-dimensional virtual standard and its three-dimensional realization on the standard bullets.
The development phase of this work continued with the ATF National Laboratory Center and the FBI Central Laboratory each providing three master bullets, which had been fired from different guns under standardized firing conditions. NIST’s Surface Calibration Laboratory used a commmercial stylus instrument to perform a profile measurement of each bullet, and the resulting set of six digitized two-dimensional bullet signatures was stored in a NIST computer as the 2D virtual bullet signature standard.
To produce the SRM, the virtual bullet signature standards were used to control the tool path of a numerically controlled diamond turning machine at the NIST Instrument Shop. Six bullet signatures were machined on each of the SRM 2460 standard bullets.4 The material is OFHC copper rod with about a 1-mm thick pure-copper coating. After machining, the NIST Metallurgy Division applied commercial corrosion protection to the surface of the SRM bullets.
A similar process is being used to develop and manufacture standard casings from five master casings supplied by the ATF National Laboratory Center. From these masters, two different manufacturers produced 21 prototype standard casings. IBIS was used to test these prototypes, with comparisons made between markings from firing pins, ejectors, and breech faces. The results demonstrated a high degree of reproducibility and close agreement between compared images. Based on these initial results, NIST is improving the design in preparation for fabrication and distribution.
NIST Standard Reference Materials have long played a central role in industry and commerce, and more recently in areas such as criminal justice, public safety and homeland security. NIST has earned this role by consistently matching design, development and production to customer needs, incorporating innovation, ensuring quality, and providing outstanding follow-up support. And seeing the growing need for reference materials in industry, commerce, and beyond NIST is already developing capabilities that will lead to the next generation of measurement standards. //
1 International Vocabulary of Basic and General Terms in Metrology (VIM), 2nd Edition; BIPM/IEC/ IFCC/ISO/IUPAC/IUPAP/OIML, International Organization for Standardization (ISO), 1993.
2 Ma, L., Song, J., Whitenton, E., Zheng, A., Vorburger, V. and Zhou, J., “NIST Bullet Signature Measurement System for SRM (Standard Reference Material) 2460 Standard Bullets,” J. Forensic Sci., 49, 4, July 2004, pp 649-659.
3 Song, J., Vorburger, T. and Ols, M., “Establishment of a Virtual/Physical Standard for Bullet Signature Measurements,” Proc. 2001 NCSL, Washington, D.C., August 2001.
4 Song, J., Whitenton, E., Kelley, D., Clary, R., Ma, L., Ballou, S. and Ols, M., “SRM 2460/2461 Standard Bullets and Casings Project,” J. Res. Natl. Inst. Stand. Technol. 109, 6, 2004, pp 533-541.