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The collection of gunshot residues (GSR) from a suspect's hands is a routine forensic technique widely utilized by police investigators. Presently the most widely used method for the collection and analysis of GSR samples is by swabbing the hands with dilute nitric-acid-moistened cotton swabs and determining the barium and antimony levels by flameless atomic absorption spectrophotometry (FAAS). These techniques of collection and analysis have acquired wide acceptance because of their low cost, uncomplicated procedures, and overall reliability when compared with alternate methods.

Several methods have been developed by law enforcement agencies for determining whether or not an individual has recently handled or discharged a weapon. When a suspect has been apprehended following a shooting, detection of gunshot residues on his hands may provide significant evidence. The value of such a test is substantial in the investigation of alleged suicides, armed assaults, poaching, and other violations involving the use of firearms.

External beam proton-induced X-ray emission (PIXE) analysis has been used to verify the presence of lead in the finger bone of a murder victim. The deceased, who had been buried several years, was known to have suffered a bullet wound to his right hand several yers before death. X-ray radiographs of the right second proximal phalanx revealed the possible presence of metal fragments below the surface of the bone. To verify the presence of lead in a nondestructive manner, the bone was scanned with a 1.5-MeV proton beam. PIXE analysis showed that lead was present only in the vicinity of the fragments previously detected in the radiographs. A study of gunshot residue in bone shows that the distribution of lead around the bullet hole is independent of the firing distance for distances greater than 0.6 m.

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.

The well-established method of gunshot residue (GSR) particle detection using scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) is generally employed in both the examination of the suspect and the target area. In the present work, it was demonstrated that GSR particles can be consistently found on the bottoms of discharged bullets, including those severely deformed on impact or having undergone other severe conditions. In addition, a proposed mechanism for this finding was investigated. Because this phenomenon is found useful in several types of cases, it should be well remembered by criminalistic related examiners that the bottoms of discharged bullets contain valuable information not to be overlooked.

The detection of primer and gunshot residues is important to the criminalist. Several techniques to detect trace materials have been proposed as providing definitive proof that primer or gunshot residue is present [1–8]. While the dermal nitrate-nitrite test has been shown to yield misleading information [1], tests based on the detection of one or more of the metals lead, barium, antimony, and copper appear to be much more reliable [8].

Methods of extraction of gunshot residue (GSR) swabs for determination of antimony (Sb), barium (Ba), and lead (Pb) by flameless atomic absorption spectrophotometry (AAS) have been studied. Optimum extraction parameters were determined using recovery studies of Sb, Ba, and Pb added to swabs at amounts representative of actual GSR and handblank levels. Two difficulties with most extraction-AAS procedures are incomplete extraction of Sb and incorrect Ba results arising from improper matching of sample and standard matrices before AAS determinations. Utilization of standards made by spiking analyte elements on swabs and extraction along with samples by an efficient procedure minimizes errors in Sb, Ba, and Pb determination.

Intuitively, a hand held in close proximity to a firearm at the instant of discharge will intercept a significant amount of gunshot residue, even though the hand did not actually come into contact with the weapon. There is, however, little information specifically described in the forensic science literature concerning the residue levels which might be encountered in such an instance. The present work confirms that antimony levels consistent with an individual having fired or handled a firearm may be intercepted by a hand held in close proximity.

The identification of gunshot residues (GSR) is generally accomplished by bulk analysis of complete residue samples (by atomic absorption spectroscopy or neutron activation analysis) or by residue particle analysis (by scanning electron microscopy and energy dispersive X-ray analysis). The limitations and advantages of the two techniques are discussed. Two systems for the automation of GSR particle search have become commercially available recently. A detailed operational description and the results of 50 experimental search runs are reported for 1 of these systems. These results are also compared with results of 16 experimental search runs conducted with the second of the automation systems commercially available.

Based upon the recently developed glue-lift collection of gunshot residue particles for examination in the scanning electron microscope with energy dispersive X-rays, this laboratory has undertaken a research program to determine if reconstruction of gunshot deaths is feasible. Because undisturbed conditions of suicide victims may help in securing unambiguous results and high success rates, the program has been carried out to reconstruct suicides only. Data obtained from 13 firearms suicides and their laboratory reconstruction which involved primarily shotguns and handguns, indicate that reconstruction can be immensely useful to interpretation of the gunshot residue distributions on a suspect's or victim's hands. This reports outlines the basic experiments performed to relate the residue emission from the gun to the deposits found on the firing hand(s) and a description of the reconstruction technique that uses a target to simulate a human body. The technique can determine the specifics of a victim's hand position at the time of firing and which hand was used to pull the trigger. Because the technique is sensitive to the nature of the grasp of the firing hand and of the supporting hand, in the case of a shotgun, it has been possible in all cases to date, to determine which one of the potential eight hand positions existed at the time of shooting.

The efficiency of the filtration process for concentrating small populations of gunshot residue (GSR) particles (for detection by scanning electron microscopy/energy dispersive X-ray analysis [SEM/EDX]) was examined with a novel method using GSR particle suspensions. Methods of concentrating and segregating GSR particles on small areas (for example, 7 mm2) using both filtration and heavy liquid separations with centrifugation were examined from the practical point of view. It is shown that, in real cases, there may be an intrinsic difficulty in concentrating GSR particles as a result of a pile up of extraneous material or clogging of the collecting filter or both. Collection of GSR particles from clothes using a suction method was compared to the direct glue-lift technique. The suction method involved trapping the GSRs in an organic solvent and recovering them on a membrane filter. The method proved less efficient than using the glue-lift technique. From the findings in this study, it seems that the most efficient way of detecting GSR particles by SEM/EDX is by using the recently developed automated systems directly on glue-lift samples without any pretreatment.

Critical comparisons of Ba and Sb in firearms discharge residue were made on samples collected by three independent collection technqiues. Collection materials studied were transparent adhesive tape, (Scotch Brand), a solution of cellulose acetate in acetone ("Film Lift"), and plastic-shafted cotton swabs wetted with dilute nitric acid. Flameless atomic absorption analyses were performed with a Jarrell-Ash Model 810 instrument equipped with a tantalum strip atomizer. Tape and cotton swabs gave comparable positive indications of residue, with frequencies of 90 and 80%, respectively. The plastic Film Lift gave fewer positives, with a frequency of 50%. With the transparent tape lift, gunshot residue particles are discernible, making nondestructive microscopic identification possible prior to destructive elemental analysis.

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.

The presence of lead, copper, and antimony (Sb, Cu, and Pb) was shown on the left back and right palm by use of anodic stripping voltammetry (ASV). These data are consistent with a cross overhand held weapon (left over right). The use of ASV for gunshot residue (trace elements) is an alternative approach in that it allows observation of the entire three element (Sb, Cu. Pb) pattern.

Neutron activation analysis (NAA) for antimony and barium has been described as the most effective technique for the detection of gunshot residues on the hands of a suspect [1,2]. The method, however, is not used routinely for a number of reasons. Among the reasons are the complex nature of the statistical treatment of the data and the difficulty experienced by individual laboratories of collecting the voluminous background data required for these calculations. One of the important aspects of the technique is that the sample has to be collected with extreme care so that the contamination is minimal. Because the amounts of the elements analyzed are at microgram levels, specially designed procedures and training are required to be able to collect the samples without contamination. The existing techniques, such as paraffin lift and cotton swabbing, are found to be unsatisfactory in this respect. By taking repeated samples from hands by these procedures, it was found that three or four collections are required for complete removal of the trace elements. Thus, a single collection by these methods is not quantitative, and therefore, any subsequent calculation would be in error.

The results of gunshot residue tests in more than 800 criminal cases are described. A combination of neutron activation and atomic absorption analyses was found to be the most efficient method for the determination of trace amounts of antimony and barium on swabs. A statistical treatment was used as a basis for the evaluation of individual test results.

In the examination by scanning electron microscopy of hair and polyester fabric that had been shot with a 9-mm pistol at muzzle-to-target distances from 10 to 70 cm, it was discovered that part of the gunshot residue (GSR) deposited on the target is molten at impact. Impact of these molten GSR particles results in flattening, splattering, or some modification of the otherwise spherical form. In the case of hair, microtopography and the probable rapid movement of the hair in the muzzle blast influence the form of GSR. A sample of hair shot within 30 cm often results in GSR finding its way under hair scales. The detection of these form-modified GSR particles on fabric or on (and under) scales of hair indicates a close-proximity shot.

Particle analysis techniques provide much more information useful for identification of gunshot residue than the conventional analytical techniques that measure only the concentration of elements averaged over the entire specimen. By combining the morphological information by microscopy with elemental analysis by X-ray fluorescence, the SEM provides definitive identification of residue particles. Therefore, the particle analysis technique should be more revealing in situations where conventional methods fail as the quantity of residue approaches the background level.

A photoluminescence technique, which detects lead and antimony in gunshot residue, was evaluated for use in investigations of apparent gunshot suicides. The study was conducted in conjunction with forensic science laboratories in five U.S. cities. Samples were collected by the adhesive lift method from the backs of the hands of 67 gunshot suicide victims, 41 subjects who died of other causes, and 31 live subjects with high occupational exposure to lead and antimony. Tentative simultaneous threshold levels of 0.85μg for lead and 0.01 μg for antimony were selected as criteria for presuming the presence of gunshot residue on samples from the hands of these suicide victims. Although blood decreases the detectability of lead and antimony in the samples, 48% of the suicide cases involving handguns other than .22 revolvers exceeded the threshold levels for both lead and antimony. Methods are suggested for eliminating the effects of blood, which should significantly increase the success rate for cases involving these guns. A much lower success rate was obtained for cases involving .22 revolvers and long guns, as expected from the sparse amounts of residue found in previous test firings of these guns.

Rapid, convenient detection of gunshot residue on the hands of a suspect, following a shooting, can be accomplished by the photoluminescence determination of the presence of lead (Pb) and antimony (Sb), which originate in the cartiridge primer. Following the firing of a gun, the backs of both hands are washed in a stream of distilled water. Each hand washing is filtered, and the residue collected on a membrane filter is dissolved in hydrochloric acid. Lead(II)and Sb(III) form chloride ion complexes with the acid, which luminesce strongly upon selective ultraviolet excitation at low temperature. Upon excitation, the Pb and Sb complexes emit light with maxima at wavelengths characteristic for the two metallic elements. By the use of this procedure, it is possible to detect as little as 1.0 ng of Pb and 10 ng of Sb. The total time for sample collection and analysis is less than 30 min.

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.

A rapid yet reliable method to detect the presence of antimony, barium, and lead in gunshot residue is employed to deal with the ever-increasing use of firearms in criminal cases. Since a reduction in analysis time and operator attention is realized, a more systematic approach can be used to deal with the significance of these three elements and their relation to gunshot residue. Residues are collected with cotton swabs and 5% nitric acid and are leached overnight in nitric acid. Enhanced reproducibility is also achieved when an automatic injector system is used instead of traditional manual pipet dispenser.

Scanning electron microscopy combined with energy dispersive X-ray analysis (SEM-EDX) was used to determine new structural criteria to further substantiate the view that gunshot residue (GSR) particles have a characteristic structure. Because GSR particles are formed by rapid cooling from extreme temperatures and high pressures, they should contain features of condensates not only on their surfaces, but also within their interiors. Both the surfaces and the cross sections of GSR were examined for X-ray mapping of elements and for topographic analysis. Vaporized lead, antimony, and barium may condense uniformly and concurrently, or irregularly and discontinuously, or as a layer of lead around a nucleus of barium and antimony. These three modes of GSR formation may correspond to the equilibrium state, the increasing temperature state, and the decreasing temperature state of the explosion gas mixture of the priming compound. Most GSR smaller than 10 $#X3BCm are formed as droplets at equilibrium. Larger spheres grow by coalescence of the smaller droplets. These residues pass through various metastable forms and then freeze. Only a few semisolid spheroids of barium and antimony may capture lead vapors of the etched bullet and burnt residues, and these appear as “peeled oranges.”

The application of X-ray diffraction complemented by scanning electron microscopy and energy dispersive X-ray analysis to the characterization of gunshot residues is reported. Lead in the metallic form is found to be the main constituent of all gunshot residues tested. Several mechanisms are proposed to explain the appearance of lead in the metallic state as well as the formation of gunshot residue.

The proton-induced X-ray emission (PIXE) technique was applied to the identification and analysis of gunshot residues. Studies were made of the type of bullet and bullet hole identification, firearm discharge element profiles, the effect of various target backings, and hand swabbings. The discussion of the results reviews the sensitivity of the PIXE technique, its nondestructive nature, and its role in determining the distance from the gun to the victim and identifying the type of bullet used and whether a wound was made by a bullet or not. The high sensitivity of the PIXE technique, which is able to analyze samples as small as 0.1 to 1 ng, and its usefulness for detecting a variety of elements should make it particularly useful in firearms residue investigations.

A quick, convenient, and reliable test for detecting gunshot residue is needed to cope with the increasing number of crimes involving handguns. Chemical color tests for nitrates [1] and for antimony, barium, and lead [2] have been used, but these methods lack the required sensitivity and specificity for forensic purposes. Two instrumental techniques, neutron activation analysis and atomic absorption, have found some acceptance by crime laboratories. Neutron activation analysis has been used to detect antimony and barium in hand samples [3], but it suffers from the drawbacks that samples must be sent out for analysis and that it is insensitive to lead. Atomic absorption has been used to detect lead by flame atomization [4], and antimony and barium have been detected with a flameless atomizer [5]. This approach has extended the availability of gunshot residue analysis. However, the costs still limit the number of laboratories with the required equipment. Perhaps a more serious limitation of any technique that is based on the bulk amounts of antimony, barium, or lead on a person's hands is the possibility of environmental or occupational sources of these elements on the hands. This limitation requires setting a threshold often higher than the amount deposited by firing. Furthermore, the amount of residue on the hand declines very rapidly with time.

In the analysis of hand samples for barium and antimony levels indicative of gunshot residue, severe problems are encountered with swabs contaminated by blood, dirt, or grease. These problems originate from two main sources: decreased effectiveness of sample leaching and high instrumental backgrounds. When the flameless atomic absorption spectroscopy (FAAS) method developed in our laboratory [1] is used, the choice of sample collection materials is restricted to those that can be easily separated from the elements of interest. In our approach, a sample such as a cotton swab is leached with dilute nitric acid to extract any residues present. Surface contamination may make the swab impervious to solvent penetration and, therefore, inefficient leaching occurs. Contaminants such as blood, dirt, or grease on the sample can coat any primer residue particles present and prevent their dissolution in the acid, resulting in the amount of the elements of interest being lower in the solution than expected. The measured level of these elements will thus be lower than that actually present in the sample. Increasing the contact time between solvent and sample, or rigorous agitation to improve solvent attack, usually results in enhancement of the background response during the atomization phase of the AA analysis with no improvement in analytical results. This enhanced background, especially notable at the 217.6- and 217.9-nm (2176- and 2179-A) lines used for antimony detection, may be attributed to light scattering by organic materials not completely destroyed during the drying and ashing cycle of the AA determination. Destruction of the matrix by ashing of the samples prior to analysis significantly decreases the analytical background and eliminates leaching difficulties.

A recent review of neutron activation analysis (NAA) applications in forensic science by Guinn has again focused attention on the utility of barium and antimony determinations on residues removed from the hands as indication of recent gun firing by the individual. This method has been well established and is in routine use by state, local, and federal law enforcement groups in the United States and by governmental agencies in other countries. Increasing use of this technique has made reduction in sample analysis time imperative.