Gunshot Residue Analysis at McCrone Associates
As one of only a handful of independent laboratories in the country providing gunshot residue analysis and consulting services to the forensic community, McCrone Associates has been proudly providing the services since the early 1990s. Projects are submitted from a variety of sources in this country—and more recently, from a few international countries—including police agencies, crime labs (not having their own capability), private detectives, and prosecutors or defense attorneys. We do not accept GSR projects from private individuals.
When a gun is fired, a smoke plume is produced that can deposit particulates on the shooters hands, clothing, or other objects in the vicinity of the fired gun. Most of the smoke is from the gunpowder, which is composed of mostly organic materials. Part of the smoke is particles produced from the primer cap chemicals, containing compounds of lead, barium, and antimony. These elements can recondense in any proportion, but particles containing all three elements are considered as characteristic of gunshot. Particles containing two of the three elements are considered to be consistent with a gunshot, but may also be found from other sources unrelated to a gunshot. Fireworks and some automotive brake pad dust are common sources of two component particles.
A scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometry (EDS) system is used to provide images and the elemental composition of particles during an automated search for gunshot residue produced from the primer components.
We follow the most current version of ASTM designation E-1588 “Standard Practice for Gunshot Residue Analysis by Scanning Electron Microscopy/Energy Dispersive X-Ray Spectrometry” for our GSR analysis.
Another very useful document was published by the Scientific Working Group for Gunshot Residue (SWGGSR) covering much more detail then ASTM E-1588. This document was intended to assist not only the GSR analysis laboratories, but also the legal community. This 100 page document is available as a free PDF download from the SWGGSR website.
Currently accepted submissions at McCrone Associates for GSR analysis are:
- Commercial GSR Collection kits for SEM analysis
- Commercial GSR Collection kits for SEM analysis typically contain two, three, or four SEM tape lift stubs.
- We sputter coat carbon onto all stubs to a thickness of ~5nm (50Å) to eliminate charging effects while exposed to the electron beam in the SEM. The coating also provides uniform conductivity over the stub surface.
- RedXDefense GSR-600 GSR field kit cards
- If the adhesive pulls back cleanly from the paper, then it can be mounted and analyzed directly in the SEM, either in low vacuum mode (if available), or carbon coated for use in high vacuum mode.
- If significant quantities of paper fibers come up with the adhesive, the GSR particles could be masked from detection. In this case, the adhesive is put into a ceramic crucible and ashed at 500° C for two hours. Upon cooling to ambient temperature, the bottom of the crucible is sampled with a commercial GSR collection kit stub, which is then carbon coated for the analysis in the SEM.
- Evidentiary clothing items
- We will have discussions with the client to develop a sampling strategy for various items of clothing depending on the specific circumstances of the case.
- We use the commercially available GSR collection kit stubs to obtain samples from the clothing.
- Evidentiary cotton-tipped swabs
- We will cut off the cotton tips from the shafts, and ash the samples in individual covered ceramic crucibles for two hours at 500°C. Upon cooling to ambient temperature, the bottom of each crucible is sampled with a commercial GSR collection kit stub, which is then carbon coated for analysis in the SEM.
- Evidentiary clear tape adhesive strips
- Occasionally clear tape adhesive strips, typically the two-inch-wide fingerprint tape, are used to collect samples from the interior of cars when impounded for a drive-by shooting. Each strip is cut into small pieces, which are placed into individual covered ceramic crucibles for ashing at 500° C for two hours. Upon cooling to ambient temperature, the bottom of the each crucible is sampled with a commercial GSR collection kit stub, which is then carbon coated for the analysis in the SEM.
Using automated analysis software, we scan approximately 95% of the total surface area of the SEM stubs or the clean GSR-600 adhesives in search of characteristic GSR particles produced from the primer cap compounds, i.e., particles containing lead (Pb), barium (Ba), and antimony (Sb). The GSR particles are located in the SEM by gray-level image threshold limits and then analyzed for elemental composition using the (EDS) detector. The EDS spectra from all detected particles are automatically stored for interpretation and future documentation needs. The quantification of the detected elements is provided with each EDS spectrum. In addition, the analyses provide particle shape and size (ECD—equivalent circular diameter) measurements which are used to further characterize GSR particles (usually <5 µm and spheroidal/ellipsoidal or irregularly shaped with molten appearance).
After the automated analysis has been completed, characteristic GSR particles are relocated for manual SEM electron micrograph imaging and confirmatory EDS analysis.
As part of our services, we also provide expert witness deposition and court testimony for an additional charge at hourly rates. The client is also responsible for any necessary travel or miscellaneous costs associated with the analyses or consulting (e.g., airfare, hotel accommodations, per diem, car rentals, photo documentation/displays for court testimony, etc.).
Also note that short courses for Gunshot Residue Identification, Scanning Electron Microscopy, and Advanced EDS, among many other microscopy courses, are taught at The McCrone Group’s Hooke College of Applied Sciences.