Swabs or Stubs? It’s Not Even A Question

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  • 1:09PM Sep 19, 2017

Gunshot Residue Analysis

Gunshot residue (GSR) has long been collected during criminal investigations.  Currently, the standard method for collecting primer GSR is using adhesive stubs which can then be analyzed via scanning electron microscopy (SEM).  Prior to SEM analysis, the most common method of analyzing primer GSR was atomic absorption which utilizes cotton swabs for collection.  Atomic absorption is referred to as a “bulk analysis” because it collects all potential primer particulate from an area and concentrates it in one sample leading to one final result.  So why has this method been largely replaced by scanning electron microscopy, and which method gives a more accurate representation of the particles found on a subject?
 
Gunshot residue, in the broadest sense of the term, is the particulate expelled from a firearm when it is discharged.  When the trigger is pulled, the firing pin strikes the primer cap of the cartridge, setting off a gaseous reaction within the cartridge.  A vaporous cloud of particulate is expelled from the firearm creating a plume.  Condensed particles within the plume then land on the surrounding area of the discharge.  This substance is then collected.
 
Typically, the expelled residue is composed of lead, antimony and barium, originating from primer based on the Sinoxid formulation.  Nitrogen-based particulate (nitrocellulose, nitroglycerine) originating from the propellant, or gun powder, along with particulate originating from the firearm itself may also be found in the plume.  Other elements such as tin, aluminum, copper, and/or zinc, for example, may also be found in primer GSR.  Due to the way primer GSR particles are formed, these particles can fall under three different categories, when primer originates from the sinoxid formula:
 

 
According to the Criminal Forensics Department at RJ Lee Group, there are other sources that create particulate that is similar to gunshot reside such as brake pads, fireworks, or airbag exhaust.  Typically, particles originating from these sources also include elemental indicators that are not typically found in a population of gunshot residue.  One cannot say that particles consistent or commonly associated with GSR are specific to the discharge of a firearm.  However, when these particles are present in conjunction with particles classified as characteristic of GSR, they support the opinion that a population of gunshot residue is being observed.  When particles characteristic of GSR are confirmed on an individual, it means that he or she either discharged a firearm, were in close proximity to a firearm when it was discharged, or came into contact with a surface that had GSR on it.  These scenarios are all equally possible of having occurred.
 

GSR Collection Using Swabs and Atomic Absorption

Atomic Absorption (AA), which utilizes cotton swabs as a collection media to gather particulate, has been used for years prior to the introduction of the Scanning Electron Microscope (SEM) for GSR analysis.  Crime scene investigators wipe the cotton swabs over the hands of a subject, package the swabs appropriately, and finally send them to the laboratory for AA analysis.  This method may have several disadvantages.
 
After collection, the GSR particulate could fall off the cotton swabs, especially if the packages are jostled, resulting in a potential loss of evidence before it even reaches the laboratory. The analysis itself is able to account for the presence of GSR metals (lead, barium and antimony) and it is able to quantify the amounts of those metals.  However, it lacks the capability to determine if those elements were ever present in combination with one another in a single particle.  According to the forensic scientists at RJ Lee Group, it is necessary to know whether or not lead, antimony, and barium were ever combined in one particle.  Those characteristic particles are highly specific to the discharge of a firearm and are necessary to confirm whether or not GSR is present.  Atomic absorption analysis lacks that ability and can therefore only indicate the possible presence of particles characteristic of GSR but cannot confirm them.  It is this lack of specificity that has led to fewer and fewer laboratories offering AA analysis as a method for analyzing GSR.
 

GSR Collection Using Stubs and SEM Analysis

Stub kits, which are used to collect primer GSR samples for SEM analysis, are composed of aluminum pin mounts containing an adhesive material and are protected within the confines of a plastic vial. The cap is removed for the investigator to collect samples from a subject using an up-and-down dabbing motion, and then snapped back into place to protect the evidence. The stubs are then sent to the laboratory, where confirmatory SEM analysis is conducted.  SEM analysis is a confirmatory technique because it allows scientists to view the elemental composition of individual particles and determine whether or not all important primer metals are present and condensed together in one particle.
 
The SEM also allows the scientist to confirm the shape or morphology of each particle.  The particles should look rounded or molten as is the result of high heat reaction from the firearm’s discharge.
 
This type of analysis is both quantitative and qualitative, meaning the SEM determines how many particles are present and of what they are composed. The SEM indicates the specific makeup of each particle by showing elemental profiles exhibited as peaks on a spectrum. For instance, lead (Pb) is confirmed by the presence of at least three peaks at specific locations on an x-ray energy axis, as can be seen in Figure 1. An analyst utilizing an SEM is able to successfully determine if particles characteristic of GSR are present… something that atomic absorption analysis cannot do.
 

Figure 1: Backscattered electron image and elemental spectrum of GSR particle detected on a stub. Three peaks of lead, or Pb, can be seen in each.

The evolution from atomic absorption to SEM analysis allows an analyst to affirm the presence of particles characteristic of gunshot residue on a subject, his or her clothes, or even his or her automobile. While atomic absorption analysis can still be performed when swabs are used during an investigation instead of stubs, RJ Lee Group no longer offers atomic absortion analysis of gunshot residue samples.  However, the scientists in the forensic department have developed a method for extracting particulate from swabs so that they may be analyzed by the SEM.  Results from the SEM may provide law enforcement officers with a forensic tool that allows them to reach objective conclusions within criminal investigations.
 
To learn more, or for more information regarding extraction of swab samples, please contact our Criminal Forensics department at 1.866.860.1775, option 1, then 3, or click here. To purchase GSR hand and clothing stubs, click here.

 

Article co-authored by Allison Laneve, Stephanie Horner, Joe Coiro, and Tarah Helsel of the RJ Lee Group Forensic Science Department.