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Making up for Missing Pieces: SEM-EDS Gunshot Residue Analysis of Human Cranial Bone.

Background and Introduction

Forensic anthropologists rely on characteristic features related to trauma mechanism (e.g. blunt, sharp, projectile) and impact velocity (i.e. high versus low) to interpret skeletal trauma. Fractures produced by handguns and rifles are indicative of the high velocity with which a projectile strikes bone. The biomechanical properties of bone subjected to high velocity impacts resemble a brittle material, so the bone shatters like glass. The resulting fragments can be reassembled easily, and the reconstruction closely approximates the bone's original form. Other characteristic defects of gunshot trauma are internal and external beveling associated with entrance and exit defects. Low velocity impacts allow time for bone to deform prior to failure, producing warped fragments that prohibit reconstructing the skeletal element to its original form. Thus, even when remains are severely comminuted and lack definitive impact sites, the manner in which the fragments fit together may provide information about the type of trauma.

Accurate trauma interpretation is problematic when numerous fragments are not recovered and diagnostic features are not evident on the inventoried remains. One option to interpret trauma is to test bone fragments for residues and particles uniquely associated with weapons. Metal residues have been detected on bones impacted with blunt and sharp metal objects using scanning electron microscopy (SEM) coupled with energy dispersive x-ray spectroscopy (EDS or EDX) (Bai et al. 2007; Bajanowski et al. 2001; Gibelli et al. 2012; Pechnikova et al. 2012; Vermeij et al. 2012). A recent study successfully detected window and mirror glass particles on bone using SEM-EDS (Montoriol et al. 2017). Gunshot residues have also been detected on bone, including bone forcibly stripped of its periosteum (Berryman et al. 2010), cremated bone (Amadasi et al. 2012), skeletal remains after decomposition (Amadasi et al. 2015; Taborelli et al. 2012), buried and macerated bones (Taborelli et al. 2012), and remains in marine environments (Lindstrom et al. 2015).

Gunshot residue (GSR) is a combination of particles from the primer, bullet, and cartridge case. Ignited primer and gunpowder exit as a gaseous plume from open surfaces of the gun. Primer mixtures often contain three inorganic compounds: lead styphnate (initiator), antimony sulfide (fuel), and barium nitrate (oxidizer) (Wolten et al. 1979b). Sulfur, charcoal, and potassium nitrate are common organic compounds in gunshot residue derived from the gaseous plume. Gunshot residue particles have a homogenous spherical shape due to the high temperatures and pressures associated with their explosive discharge from the weapon (Kara et al. 2015; Vermeij et al. 2009).

Most work on gunshot residue detection has been accomplished with scanning electron microscopy (SEM) coupled with energy dispersive x-ray spectroscopy (EDS or EDX), which enables the examination of particle morphology and elemental composition of the inorganic compound (Saverio Romolo et al. 2001). A scanning electron microscope scans the surface of an object with a focused beam of electrons. The electrons interact with the atoms in the sample and produce an image of the sample's surface topography. The signals produced by the interaction of the electrons and the atoms in the sample include x-rays, which can be used to interpret information about the sample's composition. An energy dispersive spectrometer (EDS) measures the number and energy of the x-rays, indicating the atomic structure of the emitting element and elemental composition of the specimen. An SEM equipped with an energy dispersive spectrometer provides a non-destructive analytical technique that offers the "maximum specificity" in GSR detection (Saverio Romolo et al. 2001).

Wolten et al. (1979b) published one of the most extensive works on GSR analysis using SEM-EDS which included criteria for elemental compositions unique to GSR and those that are consistent but not unique. These criteria were later revised by Wallace and McQuillan (1984) after five years of experience with SEM-EDS analyses on 868 samples from hands, face, clothing, and spent cartridges. The two unique elemental combinations specific to primer discharge are (1) lead (Pb), antimony (Sb), and barium (Ba), and (2) Sb and Ba. A variety of other combinations were found to be indicative of primer discharge but not unique. Limited data is also available on several types of lead-free ammunition, but these profiles have not been tested and validated as rigorously as those from lead-based ammunition (Haag 1995; Saverio Romolo et al. 2001).

Since the 1968 discovery of SEM-EDS by the Metropolitan Police Forensic Laboratory in New Scotland Yard, England, the method has become the gold standard for detecting GSR on the skin of shooters and on fleshed remains (Saverio Romolo et al. 2001; Wolten et al. 1979a, 1979b). Experimental research has shown that the technique is also applicable to skeletal remains of extended postmortem intervals and in various taphonomic conditions (Amadasi et al. 2012, 2015; Berryman et al. 2010; Lindstrom et al. 2015; Taborelli et al. 2012). This report details a case in which SEM-EDS analysis of cranial bone fragments supported the forensic anthropologists' suspicion of gunshot trauma in a cranium lacking the diagnostic morphological features associated with gunshot wounds.

Case Report

On October 18, 2013 hunters walking through a wooded area discovered a shallow hole in the ground containing skeletal remains. The shallow grave appeared to contain ribs, some long bones, and clothing. The skull and a femur were discovered approximately 20 feet from the rest of the remains, apparently moved there by scavengers. The local coroner's office responded to the scene and conducted a recovery of the remains.

Autopsy Findings

Upon pathological examination, the remains were found to have areas of residual intact skin on the torso which were desiccated but devoid of injury. Evidence of animal and insect predation was present on the soft tissues, and the majority of the internal organs were absent. The remaining organs, including the right lung and liver, showed severe decomposition with no evidence of natural disease or injury. The pelvic organs were partially intact, but no defining sex organs could be identified. An intact hair mat was also discovered at the scene, but the skull was fragmented and incomplete due to trauma. The remains were sent to an anthropologist for a biological profile and trauma analysis. The remainder of this case report details the trauma findings.

Anthropological Findings

The skeletal inventory revealed a nearly complete skeleton and partial skull (Figure 1). Postmortem scavenging was evident on several elements, including the lumbar vertebrae, sacrum, coccyx, iliac crests, and the epiphyses of some long bones. The occipital bone was absent, and the right parietal, sphenoid, left and right temporal, and vomer were partial. Linear fractures consistent with perimortem trauma were present on the right parietal, left temporal, and right hard palate:

* Fractures A, B, and C are linear fractures on the endocranial surface of the right parietal bone measuring approximately 6 cm, 4 cm, and 4 cm long, respectively (Figure 2).

* Fracture D is a 3.5 cm linear fracture on the right maxilla and right palatine bone that traverses the transverse palatine suture and terminates at the median palatine suture (Figure 2).

* Fracture E is a linear fracture on endo- and ectocranial surfaces of the right parietal bone that terminates ectocranially at the sagittal suture (Figures 2-4).

* Fracture F is a diastatic fracture that follows the squamosal and sphenoparietal sutures of the left temporal and is more visible on the endocranial surface (Figure 3).

A large portion of the cranium was not recovered from the scene, so the etiology of these fractures could not be determined with certainty (e.g. it was unclear if they were caused by high velocity or low velocity trauma). The pathologist and anthropologist strongly suspected gunshot trauma, so several cranial fragments were submitted for SEM-EDS analysis.

SEM-EDS Analysis

Five fragments from the right parietal (n= 4) and left temporal (n= 1) and a control specimen from a thoracic vertebral spinous process were analyzed for elemental composition of inorganic substances and foreign particulate morphology to determine if GSR was present on the skull. Fragments were collected from the lateral aspect of the right parietal (n=2), posterior right parietal in the area of the lambdoid suture (n = 1), left posterior parietal (n = 1), and left temporal (n = 1). Fragments were harvested from the margins of fractures with an autopsy saw. The bone fragments were mounted onto stubs and sputter coated with gold using the Hummer VI-A sputter system in order to view them with the SEM. Fragments were then analyzed using a Hitachi TM-3000 microscope equipped with a Bruker Quantax 70 EDS system. Energy dispersive x-ray spectrometry was used to determine the surface elemental composition, while backscatter SEM was used to analyze spherical particulate morphological abnormalities. Each bone fragment was scanned ten times in various locations to achieve the most accurate elemental composition. Bone fragments with detectable elemental percent composition of barium (Ba), antimony (Sb), and lead (Pb) were recorded and examined under the SEM for spherical-shaped particles.

EDS analysis detected trace amounts of primer or gunpowder signature elements characteristic of gunshot residue on the cranial bone fragments but not on the vertebral spinous process (Figure 5). Barium (Ba), antimony (Sb), and lead (Pb) were detected on three of the parietal bone fragments and the temporal bone fragment. This elemental combination is unique to GSR and confirms the presence of gunshot residue on these fragments (Figure 6) (Wallace and McQuillan 1984). The presence of barium (Ba) and lead (Pb) but absence of antimony (Sb) on one of the parietal fragments does not allow positive GSR identification on this fragment (i.e. this elemental combination is indicative of GSR but not unique). SEM analysis of bone surface morphology revealed the presence of characteristic GSR spherical deposits on three of the fragments (two parietal and one temporal) (Figure 7). The combined presence of signature trace elements and spherical surface deposits on the cranial fragments is consistent with the presence of gunshot residue.

The cause of death was certified as disruptive head trauma, and given the circumstances of the case the manner of death was certified as homicide. A missing person fitting the biological profile provided by the anthropologist was found, and identity was confirmed via dental comparison. A suspect has not been identified as of this writing.

Discussion

This case report provides an example of a multidisciplinary effort to ascertain the etiology of cranial fractures. Definitive morphological features were not available to determine the type of trauma with certainty, but the forensic pathologist and anthropologist strongly suspected that the linear fractures were caused by gunshot trauma. SEM-EDS analysis of several cranial fragments and a control specimen from an area of the body with no trauma (thoracic vertebra) confirmed this suspicion. The presence of the unique elemental composition of GSR (Pb, Ba, Sb) plus spherical particles on four cranial fragments versus only Pb and Ba on the fifth fragment may be related to the proximity of the tested area to the lesion or due to the effects of decomposition on the GSR profile (Taborelli et al. 2012). Taborelli and colleagues (2012) found variable elemental profiles in experimental studies of decomposed remains and suggest that the liquefaction of soft tissues may cause spreading or loss of GSR, modifying the GSR profile recovered from decomposed material. The contextual information for proper taphonomic interpretation that forensic anthropologists can glean during the recovery of severely decomposed remains may provide a means to tease out some of this information (Dirkmaat 2012; Dirkmaat et al. 2008). For example, forensic anthropologists can reconstruct the original position and orientation of the body and discern the role of human intervention versus other "natural" agents at a scene (Dirkmaat et al. 2008).

While SEM-EDS analysis was able to confirm the presence of GSR on the cranium, caution should be exercised when interpreting distance based on residue patterns. Attempts to interpret distance must consider the many variables that can affect the pattern of residues, including ammunition, barrel length, caliber, and powder type/charge. The Scientific Working Group for Firearms and Toolmarks advises that similar weapon and ammunition be used for distance tests to reproduce residue patterns accurately (Scientific Working Group for Firearms and Toolmarks). Berryman et al. (2010) recovered GSR from bones shot from a distance of up to six feet; however, more research is needed on the effects of distance, as well as other taphonomic variables, on GSR detection on bone.

The SEM-EDS method is used frequently in crime laboratories to identify GSR on a shooter's skin and clothing, but it is rarely used to confirm the presence of GSR on skeletal remains. Although the analysis requires time and a qualified technician, it provides an additional line of evidentiary support to the forensic anthropologist's assessment of skeletal trauma. While more obvious cases do not require further evidence to establish the nature of the traumatic event(s), SEM-EDS analysis provides valuable evidence to support potentially refutable or contested conclusions.

Acknowledgements

The authors would like to thank the Georgia Bureau of Investigation and the Paulding County District Attorney's Office for permission to publish this case report. The opinions expressed in this report are the authors' own.

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Natalie R. Langley (a*) * James Joseph West (b) * Stan Kunigelis (c) * Cassie Boggs (d)

(a) Mayo Clinic Arizona, Department of Anatomy, Ringgold Standard Institution, 13400 East Shea Blvd., Scottsdale, AZ 85259, USA

(b) Lincoln Memorial University, Ringgold Standard Institution, 6965 Cumberland Gap Parkway, Harrogate, TN 37752-8245, USA

(c) Lincoln Memorial University, DeBusk College of Osteopathic Medicine, Ringgold Standard Institution, Harrogate, TN, USA

(d) Cobb County Medical Examiner's Office, Marietta, GA, USA

(*) Correspondence to: Natalie Langley, Mayo Clinic Arizona, Department of Anatomy, 13400 East Shea Blvd, Scottsdale, AZ 85259, USA

E-mail: nlangley@gmail.com

Received 20 June 2017; Revised 19 August 2017; Accepted 13 September 2017

DOI 10.5744/fa.2018.0007
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Title Annotation:CASE REPORT; scanning electron microscopy with energy dispersive x-ray spectroscopy
Author:Langley, Natalie R.; West, James Joseph; Kunigelis, Stan; Boggs, Cassie
Publication:Forensic Anthropology
Article Type:Case study
Date:Jan 1, 2018
Words:2858
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