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Secondary ion mass spectrometry (SIMS) is a widely used analytic method with demonstrated applications in forensic science. An incident ion beam ejects (sputters) material from the specimen. Some of the emitted atoms are ionized (i.e., have an electrical charge) and can be therefore be manipulated by electric and magnetic fields and thus identified by measuring their mass (actually the ratio of mass to electric charge).

One powerful new mass spectrometer at Aerospace (a Cameca Model IMS 5F) is of the magnetic sector type with cesium and oxygen ion sources. This instrument has very high mass resolution and sensitivity and is particularly suited to trace element analysis and thus to the individualization of certain kinds of samples. The Cameca instrument is usually termed an ion microprobe, because it can actually display images showing the distribution of specific masses on a specimen with sub-micron spatial resolution. The mass resolution is sufficiently high that species with nominally the same atomic mass (e.g., the oxygen molecule ion and atomic sulfur) are readily distinguished.

Bullet individualization is one potentially interesting, novel application for SIMS in forensic science. Bullets often deform or fragment upon impact. The classical methods of "ballistic analysis" in which microscopic marks on bullets possibly from the same weapon are compared in an optical microscope frequently can not be applied in police investigations. However, lead is one of several elements in which the isotopic composition is not constant. Lead has an atomic number (the number of protons in the nucleus) of 82 and four stable isotopes with atomic masses (i.e., the number of protons and neutrons in the nucleus) of 204, 206, 207 and 208. Masses 206, 207, and 208 are formed as the end product of radioactive decay. The isotopic variability of lead results because the elements from which these isotopes form were not evenly distributed in ore bodies. The ion microprobe can measure lead isotope ratios and can at the same time measure the concentrations of trace elements in even very small bullet fragments.

Related Sites

SIMS WWW Server
SIMS Online Tutorial


The first study of the use of lead isotope analysis to individualize bullets was carried out at The Aerospace Corporation by Dr. Gary Stupian. More work is needed to determine if lead isotope analysis (combined with trace element analysis) can be more routinely used in forensic investigations. However, several additional papers have been published that support the conclusions drawn in the original Aerospace effort. In one paper, x-ray analysis was used to locate bullet fragments in clothing.

Mention should also be made of "static" SIMS in which the intensity of the incident ion beam is kept low enough that the specimen surface is not appreciably disturbed. Large molecules and big fragments of large molecules are removed then from the surface. Mass spectrometers of the "time-of-flight" with gallium ion sources type are particularly suited to the characterization of complex molecular species. There have not as yet been applications to forensic science, but the technique appears to have considerable potential.

Lead Isotope Analysis for Bullet Individualization

G. W. Stupian, "Lead Isotope Ratio Measurements: A Potential Method for Bullet Identification", Journal of the Forensic Science Society, vol. 15, 161-164 (1975).

Keisch and R. C. Callahan, "Potential Uses of Lead Isotope Ratios in Gunshot Cases", Journal of the Association of Official Analytical Chemists, vol. 61, 520-525 (1978).

Andrasko, I. Kopp, A. Abrink, "Lead Isotope Ratios in Lead Smears and Bullet Fragments and Application in Firearm Investigations", Journal of Forensic Sciences, vol. 38, 1161-1171 (1993).