Emily C. Lennert
pipe bomb, improvised explosive device, IED, PVC pipe, fragments, nitroglycerin, diphenylamine, ethyl centralite, smokeless powder, total vaporization, solid phase microextraction, SPME, gas chromatography, mass spectrometry, GC-MS, explosives
- Bors, D.; Goodpaster, J. Mapping smokeless powder residue on PVC pipe bomb fragments using total vaporization solid phase microextraction. Forensic Science International. 2017, 276, 71-76.
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.
Pipe bombs are a common improvised explosive device that may easily be created from readily accessible materials. Pipes and endcaps, either steel or PVC, may be purchased at sporting goods stores, while smokeless powders may also be purchased from sporting goods stores. Smokeless powders are a common low explosive that, when contained, may cause an explosion. In explosives research, it is important to understand how these devices work and how much residue remains for subsequent forensic analysis on the fragments of an exploded device. By understanding this, analysts will understand which fragments will provide the most residue for analysis and will be able to provide the most effective analytical process. The work presented in this study examined the distribution and amount of explosive residue on PVC pipe bomb fragments, after an explosion, using total vaporization solid phase microextraction gas chromatography/mass spectrometry (TV-SPME/GC/MS). TV-SPME/GC/MS is an analytical technique in which a sample, in the form of a solvent, is completely vaporized within a vial. The vapor is then sampled using SPME fiber. TV-SPME increases sensitivity of an analytical method, which allows for lower concentrations of compounds to be detected.
PVC pipe bombs were constructed for use in this study. Pipes were color coded into three sections to allow for the tracking of fragment origin: left endcap (white), middle section (black), and right endcap (orange). Approximately 50 g of smokeless powder was used in each device. Three PVC pipe bombs were constructed for use in this study. Bombs were initiated within a metal cage to contain the fragments; assembly, suspension, and initiation of the bombs were carried out by the Indiana State Police Bomb Squad. After explosion, fragments were collected and placed in paint cans, one can per device.
Fragments were cataloged as follows. First, the entire bomb’s fragments were photographed together. Then, fragments were individually photographed, weighed, and placed in individual plastic bags. Very small fragments were placed in batches weighing 10 mg each and extracted as a group. Each fragment was given an identification number. Fragments were sorted by color, with each color corresponding to a zone of the pipe. In fragments with more than one color, the dominant color was selected for sorting. Overall, each pipe produced 757-864 fragments. Each fragment was examined for the presence of intact smokeless powder particles, but none were found.
Bomb fragments were then extracted in acetone for 15 minutes on a shaker table. After acetone extraction, 55 μL of the acetone extract was transferred to a SPME vial. Samples were then incubated at 60 ˚C for 5 minutes, and a polyethylene glycol SPME fiber was inserted and exposed to the vapor for 20 minutes. After SPME fiber extraction, the SPME fiber was inserted into the GC inlet and desorbed for 1 minute for GC analysis. Additionally, calibration standards of nitroglycerin, diphenylamine, and ethyl centralite were prepared at concentrations ranging from 25 ppb to 1 ppm. These calibration standards were analyzed to create calibration curves for subsequent quantification of smokeless powder residues on the GC-MS.
Overall, the most concentrated areas of nitroglycerin were recovered from the endcaps of the devices as shown in Figure 4 within the study. Similarly, the most concentrated areas of diphenylamine were also recovered from the endcaps (see Figure 5 within the study). Both nitroglycerin and diphenylamine are common compounds in smokeless powders. The average total recovery of nitroglycerin from each device was determined to be approximately 1 mg. A significantly lower recovery was seen for diphenylamine, at 24 μg. Ethyl centralite had the lowest recovery, where is was only recovered from one device, at a level of 0.8 μg. The authors noted that this extraction method was optimized for nitroglycerin recovery, so the low yields of diphenylamine and ethyl centralite are not unexpected. For each fragment, the most commonly found amount of nitroglycerin per fragment ranged from 25-50 μg, as seen in Figure 6 within the study. The authors noted that, if a fragment were to be extracted in 1 mL of solvent, it would yield an extract concentration ranging from 25-50 μg/mL, which should be detectable by conventional GC-MS. When adjusted for weight, the yield of nitroglycerin was most commonly 15-30 μg/g, as seen in Figure 7 within the study. This is higher than reported in a previous study in which steel pipes were studied; however, PVC pipes have a much lower density and hence lower mass than steel pipes, so a higher yield would be expected.
- The amount of explosive residue that remains on the fragments of a PVC pipe bomb is approximately 1.0 mg nitroglycerin and 24 μg diphenylamine.
- For a single fragment, the typical amount of nitroglycerin recovered ranged 25-50μg.
- The bulk of the residue was concentrated around the endcaps of the devices; therefore, fragments from the endcaps will be most valuable for analysis.
The mapping and quantification of residue may help analysts to better process the given evidence.
- Fragments from pipe bomb endcaps may provide the most valuable evidence.
- A single fragment may yield enough residue to determine whether nitroglycerin or other smokeless powder components are present.