To clarify whether and when a handgun was fired, forensic scientists search for and analyze Gunshot Residue (GSR) deposits. One highly interesting aspect of GSR is the presence of volatile organic chemical residues, which can offer clues as to when a weapon was fired. An innovative extraction technique based on Headspace Sorptive Extraction (HSSE) and Gerstel Twister has now been shown to extract additional information from spent cartridges.
When investigating a crime that was carried out using a firearm, it is often of critical importance to determine whether gunshot residue (GSR) can be found at the crime scene or on weapons potentially used. Such a finding can enable detectives to identify — or rule out — a firearm as the weapon used and potentially link it to a suspect. Further, given the right technique, chemical analysis can shed light on the time elapsed since a weapon was last fired.
Widely used analysis methods based on gas chromatography with mass spectrometry detection (GC/MS) and Solid Phase Micro-Extraction (SPME) have been evaluated by collaborating scientists at the University of Lausanne, Switzerland; the Sapienza University in Rome, Italy; and King’s College in London, England. In addition, a novel method using Headspace Sorptive Extraction (HSSE) followed by thermal desorption-GC/MS (TD-GC/MS) was compared with the SPME based method. The scientists were pleased with the results .
The Content of Gunshot Residue
Every firearm emits gunshot residue when discharged. The compounds involved are released at explosive speed and deposited or adsorbed on the hand, body and clothing of the shooter. Depending on the distance to the firearm, residue can also be found on the victim and, obviously, on the weapon used and on spent cartridges that were ejected. Sources of GSR are the ignition- and propellant charge powders in the cartridge. In addition, metallic powder is formed by abrasion from the bullet and cartridge and added to the mix.
GSR consists of various inorganic and organic chemicals. Due to aging processes and volatilization, changes in the concentrations and amounts of organic chemicals as well as certain compound to compound ratios of amounts can be helpful in determining the time of discharge of a weapon according to Gallidabino et al. as reported in their paper in Analytical Chemistry .
The compounds identified as GSR from literature references are, among others, nitroglycerine, diphenylamine, ethylcentralite, dibutylphthalate and 3-Ethyl-1-Hexanol as well as organic reaction byproducts, especially derivatives of benzene, and polycyclic aromatic hydrocarbons (PAHs). To determine these, the SPME technique has been widely used in combination with GC/MS. To perform GSR-based dating, Gallidabino et al. performed repeat extractions of analytes from the interior of a firearm and/or spent ammunition with the aim of establishing an aging profile, which can be compared with reference profiles. Naphthalene and decomposition products of nitrocellulose were proposed as target reference compounds.
The problem with handguns — and the solution
SPME is an established technique for time estimates concerning discharge of ammunition from rifles and other larger firearms. However, its usefulness for forensic dating of firing of smaller handguns is more limited. The repeatability is insufficient, as reported by the scientists, and degradation curves for the target analytes quickly fall below their limits of detection with SPME, as Gallidabino et al.  report. For these reasons, SPME is not the ideal technique for the investigation of GSR in combination with handguns.
In their search for an alternative extraction and analysis method with significantly higher sensitivity for the target analytes, the scientists’ attention was caught by Headspace Sorptive Extraction (HSSE). HSSE is based on the Gerstel Twister, a glass coated magnetic stir bar with a PDMS sorption phase.
The Twister is most often used for Stir Bar Sorptive Extraction (SBSE) of aqueous samples. In HSSE, the Twister is suspended in the Headspace above the sample inside a sealed 20 mL headspace vial. HSSE is thus very similar in principle to Headspace Solid Phase Micro-Extraction (HS-SPME). The main difference is that the Twister offers a much larger sorption phase volume than SPME fibers which consist of a thin sorption phase coating on a metal or glass fiber contained within a syringe needle. Consequently, SBSE provides better recovery, better repeatability, and lower limits of detection.
After the extraction step, analytes are released from the Twister by thermal desorption in the Gerstel Thermal Desorption Unit (TDU 2) immediately followed by GC/MS determination of the analytes. These steps are fully automated using the Gerstel Multi Purpose Sampler (MPS) under Maestro software control. “The Twister can provide up to 1000 times higher sensitivity than SPME, depending on the application”, says Oliver Lerch, Ph.D., Application Scientist from Gerstel. SBSE can be performed with different Twisters to cover a wide range of analytes from non-polar to polar. Desorption can be performed by thermal desorption — as is most often the case — or by liquid desorption combined with either GC/MS or LC/MS.
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