Mapping smokeless powder residue on PVC pipe bomb fragments using total vaporization solid phase microextraction.

Quantitating post-blast explosive residue is not a common practice in crime labs as it is typically not legally relevant. There is value in quantitation, however, if the distribution of residues on Improvised Explosive Devices (IEDs) can help guide future sample collection and/or method development. Total vaporization solid phase microextraction gas chromatography mass spectrometry (TV-SPME/GC/MS) was used to quantify residues of double-base smokeless powder (DBSP), which includes nitroglycerin (NG), diphenylamine (DPA), and ethyl centralite (EC) on post-blast PVC pipe bomb fragments. The analytical method could separate the three constituents in under 5min with a detection limit under 1ppb, which demonstrates high throughput while maintaining high sensitivity. The method was optimized for nitroglycerin, as it is the most indicative of DBSP. The average mass of nitroglycerin recovered from an entire PVC device was 1.0mg. The average mass of diphenylamine recovered was much lower (24µg) and only one device had detectable levels of EC. The typical concentration of NG on any given fragment was approximately 15-30ppm (µg NG/g fragment). However, there was no correlation between the mass of a fragment and the mass of residue upon it. Instead, the residue was distributed such that the highest concentration of residues was found on end cap fragments.

Optimisation of recovery protocols for double-base smokeless powder residues analysed by total vaporisation (TV) SPME/GC-MS.

The investigation of explosive events requires appropriate evidential protocols to recover and preserve residues from the scene. In this study, a central composite design was used to determine statistically validated optimum recovery parameters for double-base smokeless powder residues on steel, analysed using total vaporisation (TV) SPME/GC-MS. It was found that maximum recovery was obtained using isopropanol-wetted swabs stored under refrigerated conditions, then extracted for 15min into acetone on the same day as sample collection. These parameters were applied to the recovery of post-blast residues deposited on steel witness surfaces following a PVC pipe bomb detonation, resulting in detection of all target components across the majority of samples. Higher overall recoveries were obtained from plates facing the sides of the device, consistent with the point of first failure occurring in the pipe body as observed in previous studies. The methodology employed here may be readily applied to a variety of other explosive compounds, and thus assist in establishing 'best practice' procedures for explosive investigations.

Design and optimization of a total vaporization technique coupled to solid-phase microextraction.

Solid-phase microextraction (SPME) is a popular sampling technique in which chemical compounds are collected with a sorbent-coated fiber and then desorbed into an analytical instrument such as a liquid or gas chromatograph. Typically, this technique is used to sample the headspace above a solid or liquid sample (headspace SPME), or to directly sample a liquid (immersion SPME). However, this work demonstrates an alternative approach where the sample is totally vaporized (total vaporization SPME or TV-SPME) so that analytes partition directly between the vapor phase and the SPME fiber. The implementation of this technique is demonstrated with polydimethylsiloxane-divinylbenzene (PDMS-DVB) and polyacrylate (PA) coated SPME fibers for the collection of nicotine and its metabolite cotinine in chloroform extracts. The most important method parameters were optimized using a central composite design, and this resulted in an optimal extraction temperature (96 °C), extraction time (60 min), and sample volume (120 µL). In this application, large sample volumes up to 210 µL were analyzed using a volatile solvent such as chloroform at elevated temperatures. The sensitivity of TV-SPME is nearly twice that of liquid injection for cotinine and nearly 6 times higher for nicotine. In addition, increased sampling selectivity of TV-SPME permits detection of both nicotine and cotinine in hair as biomarkers of tobacco use where in the past the detection of cotinine has not been achieved by conventional SPME.

The anatomy of a pipe bomb explosion: the effect of explosive filler, container material and ambient temperature on device fragmentation.

Understanding the mechanical properties of different piping material under various conditions is important to predicting the behavior of pipe bombs. In this study, the effect of temperature on pipe bomb containers (i.e., PVC, black steel and galvanized steel) containing low explosive fillers (i.e., Pyrodex and double-base smokeless powder (DBSP)) was investigated. Measurements of fragment velocity and mass were compared for similar devices exploded in the spring (low/high temperature was 8°C/21°C) and winter (low/high temperature range was -9°C/-3°C). The explosions were captured using high speed filmography and fragment velocities were plotted as particle vector velocity maps (PVVM). The time that elapsed between the initiation of the winter devices containing double-base smokeless powder (DBSP) and the failure of their pipe containers ranged from 5.4 to 8.1 ms. The maximum fragment velocities for these devices ranged from 332 to 567 m/s. The steel devices ruptured and exploded more quickly than the PVC device. The steel devices also generated fragments with higher top speeds. Distributions of fragment masses were plotted as histograms and fragment weight distribution maps (FWDM). As expected, steel devices generated fewer, larger fragments than did the PVC devices. Comparison to devices exploded in the spring revealed several pieces of evidence for temperature effects on pipe bombs. For example, the mean fragment velocities for the winter devices were at or above those observed in the spring. The maximum fragment velocity was also higher for the winter steel devices. Although there were no significant differences in mean relative fragment mass, the fragment weight distribution maps (FWDMs) for two winter devices had anomalous slopes, where lower energy filler caused more severe fragmentation than higher energy filler.

The anatomy of a pipe bomb explosion: measuring the mass and velocity distributions of container fragments.

Improvised explosive devices such as pipe bombs are prevalent due to the availability of materials and ease of construction. However, little is known about how these devices actually explode, as few attempts to characterize fragmentation patterns have been attempted. In this study, seven devices composed of various pipe materials (PVC, black steel, and galvanized steel) and two energetic fillers (Pyrodex and Alliant Red Dot) were initiated and the explosions captured using high-speed videography. The video footage was used to calculate fragment velocities, which were represented as particle velocity vector maps. In addition, the fragments were weighed. The results demonstrate a correlation between the type of energetic filler and both the size and velocity of the fragments. Larger fragments were produced by Pyrodex filler indicating a less complete fragmentation, compared with smaller fragments produced by double-base smokeless powder. Additionally, higher fragment velocities were seen with Alliant Red Dot filler.