The first paper in this series [1] described the application of particle analysis to the identification of gunshot residue. In the course of developing this application, gunshot residue was identified in each of several hundred samples collected. In many instances, the nonfiring hand was sampled as a control, or “handblank” samples were taken from persons who had not fired a gun [2]. Some of these many tests were performed “blind,” that is, without prior knowledge by the analyst of the nature of the sample. No false positives were obtained from nonfiring samples. Nevertheless, it was thought that better proof of the uniqueness of gunshot residue particles was required and that an investigation should be made of the possibility that particles in the natural environment or particles produced by man could closely resemble gunshot residue.

Several methods of bulk elemental analysis, such as fiameless atomic absorption, flame emission spectroscopy, neutron activation analysis, and photoluminescence spectroscopy, can reliably and quantitatively determine the amounts of antimony and barium (antimony and lead in the case of photoluminescence) removed from the hand. The information thus furnished, however, in most cases is not sufficient to constitute presumptive evidence of the presence of gunshot residue. Many analyses are inconclusive because the amounts of antimony and barium (lead) are less than certain “thresholds” considered necessary because these elements are not unique to gunshot residue. Firings from .22 caliber weapons, unless the cartridge is known to be a Federal brand, are not usually analyzed because domestic rimfire primers, except Federal, do not contain antimony. Understandably, there has been great interest in alternative analysis methods capable of furnishing additional information of potentially higher specificity for gunshot residue.

The results of the present study are consistent with the hypothesis that gunshot residue is formed by the condensation of vaporized bullet and primer materials that segregate into metallic and compound particles. Some of the metallic vapors are oxidized and “scavenged” by oxygen and sulfur-rich primer compounds, among which the sulfur compounds appear to be more effective. The “bullet” particles of the residue thus originate in the bullet material, and the “primer” particles are a mixture of primer-derived and bullet-derived materials.

Current methods used to determine whether or not an individual has fired a handgun are based on analyses of samples taken from various regions of the hand for total content of antimony, barium, and often lead. High amounts of these elements are considered to be characteristic of gunshot residue [1–11]. However, the ability to identify residue conclusively has been severely restricted by the environmental occurrence of these same elements. Results of the studies described in this paper indicate that, by combining information about the morphology of gunshot residue particles with elemental analysis of individual particles [obtained by using an X-ray analyzer with the scanning electron microscope (SEM)], it is possible to reach a conclusion of increased significance concerning the presence of residue as distinguished from environmental contamination. The results of extensive studies of residue particles and analyses of numerous hand samples are presented here. It has become apparent that the new technique is a major improvement and that it is moderately rapid when performed by a trained microscopist.

This is the second paper in a series describing the photoluminescence technique for detection of gunshot residue. It presents results of a substantial number of analyses of samples obtained for a variety of handguns and cartridges and collected under various conditions. A primary objective of the second study has been to evaluate factors which affect the utility of the method, such as the persistence of residue on the hand after firing, the effect of activity on retention of residue, and the possible presence of identifiable residue on surfaces other than the hand. The data represent an extensive addition to existing information on lead in gunshot residue and samples from several environmental conditions. A new method of residue collection, based on an adhesive layer, was used to collect samples from hands. It was found to be superior to the wash procedure used in the previously reported work [1]. For background information on the luminescence method the reader is referred to the earlier report.