Review: The Effect of Skin Debris on Gunshot Residue Sampling and Detection

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Emily C. Lennert

 

Category

Chemistry

Keywords

gunshot residue, GSR, scanning electron microscopy, energy dispersive x-ray, SEM-EDS, SEM-EDX, skin debris, carbon adhesive stub

Article Reviewed

Burnett, B. R. The effect of skin debris on gunshot residue sampling and detection. Journal of Forensic Sciences. 2016, 61 (6), 1632-1638.

Disclaimer

The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.

Summary

Gunshot residue (GSR) is commonly collected on carbon adhesive stubs, which are dabbed on the skin’s surface to collect GSR from suspected shooters’ hands. During the sample collection process, skin debris composed of epithelial cells and skin oils may collect on the surface of the carbon adhesive. The author states that, in their experience, skin debris typically covers 25 – 100% of the sampler surface. Skin debris may obscure GSR particles on the sampler for scanning electron microscopy – energy dispersive x-ray analysis (SEM-EDS). The purpose of this study was to provide preliminary observations and experiments regarding the effect of skin debris on the detection of GSR particles.

Experiment 1

Experiment 1 was designed to test the adherence of particles on the sampler surface after multiple dabs. A volunteer shooter fired one shot from a 357 revolver, with both hands on the revolver. After firing, his right hand was dabbed 30 times with the sampler and an SEM image of a small area of the sampler was taken. His right hand was dabbed another 30 times (total 60 times) and the same area of the sampler was imaged. The dabbing was repeated 40 times, for a total of 100 dabs, followed by imaging. Then, the left hand was dabbed 50 times, for a total of 150 dabs, and the sampler was imaged a final time.
Between 30 and 60 dabs, there was no observed addition of GSR particles on the sampler surface. One particle was lost. At 100 dabs, two particles were added to the area imaged. Additionally, after 150 dabs, another two particles were added to the imaged area. Particles adhered to skin debris on the sampler surface. The single lost particle was lost from the adhesive surface. Thus, the authors state that skin debris presents an “attractive surface” for GSR particle adherence.

Experiment 2

Samplers were dabbed on the surface of two volunteers’ hands to collect skin debris. Images of samplers were taken before and after bleach treatment to determine the efficacy of removal of skin debris by the bleach solution. The authors sought to remove skin debris in an effort to uncover previously obscured GSR particles present on the stub, which was addressed in Experiment 3. A bleach solution made of 6% sodium hypochlorite, i.e. bleach, and calcium hypochlorite was used for washing of the samplers in an effort to remove skin debris. Calcium was present to lower the solubility of GSR particles in water, therefore preventing GSR particle loss during the washing process. It was concluded that the bleach solution was successful in removing skin debris from the sampler surface.

Experiment 3

Backscatter electron images were taken prior to and following bleach treatment to determine whether the washing would reveal GSR particles previously hidden by skin cells/debris. GSR was collected from the firing of a 22 caliber revolver. Vellum paper was taped over the right side of the cylinder gap of the revolver to collect GSR particles. After firing, the vellum paper surface was dabbed ten times with a carbon adhesive sampler to collect GSR particles on the sampler surface. The sampler was then dabbed 50 times on the back of the author’s hand to collect skin debris. Five samplers were prepared in this manner. Following collection, samplers were imaged by secondary and backscatter electron imaging on the SEM. Samples were then washed according to the bleach protocol in Experiment 2 and reimaged on the SEM.

After washing, some GSR particles were revealed that had previously been obscured by skin debris. The author noted that the size of the GSR particles were unaffected by the bleach treatment. Additionally, particles were only occasionally removed from the sampler surface as a result of the bleach wash. These particles were particles that were adhered to skin debris. Particles adhered to the sampler surface directly were not removed by the bleach treatment.

Experiment 4

Backscatter electron images were collected at 20 kV and 30 kV to determine whether the greater voltage would result in a difference in imaging. Previous studies have suggested that higher voltages should result in improved sensitivity. Samples were imaged before and after bleach treatment.

When no skin debris covered the GSR particle surface, 20 kV and 30 kV images were nearly equivalent. When skin debris covered the particle, the images generated at 20 kV had a lower quality than the images generated at 30 kV. The bleach treatment resulted in poor samples for SEM imaging; the rinse used was too long, which caused erosion of GSR particles. Therefore, only results from before treatment were reported.

Scientific Highlights

  • GSR particles adhere to skin debris present on the carbon adhesive sampler.
  • The bleach solution presented in this study resulted in the successful removal of skin debris from sampler surfaces.
  • Bleach treatment resulted in the loss of GSR particles that were adhered to skin debris, but not the loss of particles adhered to the carbon adhesive.
  • GSR particles are obscured by skin debris, but may be revealed by bleach treatment.

Relevance

GSR is commonly collected from shooters’ hands by carbon adhesive stubs. However, in this process, skin debris also accumulates on the sampler surface. Therefore, the effect of skin debris must be understood to allow for better processing of GSR evidence.

Potential Conclusions

After initial imaging, if no particles are discovered, bleach treatment may be useful in revealing obscured GSR particles.