On Feb 7th, 2012 Gateway Analytical hosted a live webinar titled “Forensic Gunshot Residue Analysis for Police & Attorneys”. During the webinar our presenters, David Exline and Cara Plese, discussed what gunshot residue (GSR) is, how to collect it and control contamination, the historical and current methods of analytical testing of GSR, as well as a discussion on how terminology in the GSR community has changed and why.
The webinar was well attended and many of those attendees commented that they found the content very useful. In case you missed the webinar, here are the questions and answers from the Q&A session at the end of the webinar.
For anyone that missed the webinar, you can watch the replay and download the presentation slides.
Q&A Session with David Exline:
Q: You talked about "deposited" particles, but a Czech expert found that GSR hovers in the air for more than 8 minutes after the shooting. Do you have a reference for that fact?
A: I do not have a particular reference; I’m not familiar with the paper that you are referring too. But I am very interested in it, and I will follow up on it after the webinar to get that reference. The only thing I can reference for that, is that there is a text book called “Current Methods in Forensic Gunshot Residue Analysis”. Where there were some statistics and some calculation made based on the particle size and chemistry and density of gunshot residue particles of what the likely hood or what the theoretical time would be that one would remain in the air before being deposited. I would point you in that direction to that reference and I will look into the other reference and if I find anything else out about it, I will send an update out to everyone that attended the webinar.
Q: Can you do GSR testing on the gloves, or clothes, that a shooter wore? Also, can you comment on the persistence of GSR on clothes and gloves?
A: Certainly you can collect GSR on clothing and hands we talked a lot about doing it on hands, but gloves are a very valuable tool. Anything that is in the immediate vicinity of a discharged fire arm is a reasonable surface to collect GSR analysis, and we use the same types of samplers and stubs as we would use to do off of hands. Also a company, RJ Lee Group manufactures a nice set of sampling media for looking at clothing and fabrics and those types of materials, and we can use those and collect data using the Scanning Electron Microscope as well.
Now to address the second part of the question, the persistence is affected directly based on the type of material you are dealing with. If you have a very smooth surface, gunshot residue is likely to fall off of that surface, but if you have for example a pair of wool gloves the particles may get caught up inside the weave. So, depending on the recipient type of material or garment, that’s going to directly affect the amount of persistence of gunshot residue particles as well as the likely hood that it may transfer onto something. So, all of those things have to be taken into account when you are evaluating the collection, the persistence as well as the transfer of gunshot residue.
Q: What are the most commonly encountered non-firearm sources of Antimony, Barium and Lead together?
A: I believe Cara covered this during the presentation. I think the two most common that we deal with or that are in the literature that have gotten the most press have been fireworks and brake pads. These are two different things that utilize Antimony, Barium and Lead elements in there chemical compositions. These are things that create high temperatures and this is an environment where you could have potentially particles that are of those elemental compositions that might be fused together.Other things that we deal with are nail guns and vehicle air bags. And these are because they have cartridges, so in essence they have primers. So they may have the same type of particulate that might be formed when you are not directly talking about a firearm but something that actually uses cartages that are actually being detonated. Those are the types of areas that are commonly at the for-front of other environmental sources for these particulate.
Q: How did your agency come to deciding on a four hour window for collection of GSR? Was it internal policy based on research done at your facility or research done external to your agency? Do you accept samples beyond the four hour window?
A: Well we are a private analytical testing laboratory, so we pretty much accept samples and analyze samples for the presence of GRS and we typically do not collect the GSR samples. We base our interpretation on the analysis as to presence or not. Some departments say less than two hours, some say less than four, and some are less than a day. It varies from department to department.The important thing is that each department has its own protocol and its own standard operating procedures. But again, the collection and the persistence of GSR depends on so many factors; environmental factors, substrate factors, type of ammunition, and the type of firearm that people put general polices into place, but there needs to be some flexibility in case there are exceptions to those rules. But that zero to four hour time frame is based on the literature and based on experience that we have seen from different police agencies and their operating procedures.
Q: Has X-Ray Diffraction been used for analysis?
A: I am not aware of X-Ray Diffraction being used for gunshot residue analysis. X-Ray Diffraction is typically a bulk method. In order to do particles that small, theoretically you could do single crystal X-Ray Diffraction. However, you would run into the same problems as you did, in the old days with SEM, where you have to automate and conduct the analysis in a reasonable amount of time and it doesn’t seem practical. I can’t remember ever hearing a reference of using X-Ray Diffraction for gunshot residue analysis.