Quantitative analysis of smokeless powder particles in post‐blast debris via gas chromatography/vacuum ultraviolet spectroscopy (GC/VUV)

Abstract Forensic analysis of smokeless powder particles recovered from the debris of an improvised explosive device can provide information about the type of smokeless powder used and can aid investigation efforts. In this study, quantitative methods were used to yield information about the difference in the chemical composition of the particles pre‐ and post‐blast. The technique, gas chromatography/vacuum ultraviolet spectroscopy (GC/VUV), was able to quantify nitroglycerin, 2,4‐dinitrotoluene, diphenylamine, ethyl centralite, and di‐n‐butyl phthalate in pre‐ and post‐blast smokeless powder particles using heptadecane as an internal standard. Post‐blast debris was obtained via controlled explosions with assistance from the Indiana State Police Bomb Squad. Two galvanized steel and two polyvinyl chloride pipe bombs were assembled. Two devices contained single‐base smokeless powder and two contained double‐base smokeless powder. 2,4‐dinitrotoluene and diphenylamine were successfully quantified in the single‐base smokeless powder post‐blast debris while nitroglycerin, diphenylamine, and ethyl centralite were successfully quantified in the double‐base smokeless powder post‐blast debris. Compounds were detected at concentrations as low as 9 μg of 2,4‐dinitrotoluene per mg, <3 μg of diphenylamine per mg, 131 μg of nitroglycerin per mg, and <3 μg of ethyl centralite per mg. Concentration changes between pre‐ and post‐blast smokeless powder particles were determined as well as microscopic differences between pre‐ and post‐blast debris for both smokeless powders in all devices. To our knowledge, this is the first use of GC/VUV for the quantification of explosives.


| INTRODUC TI ON
Bombings in the United States are on the rise for the first time since 2016, with a 71% increase in bombing incidents from 2019 to 2020 [1]. Of the 428 bombings in the United States in 2020, Improvised Explosive Devices (IEDs) were utilized in 177, and of the known main charges used, commercial explosive propellants were one of the three most common [1]. The most prominent type of commercial explosive propellants used in IEDs is smokeless powders (SPs) [2]. Smokeless powders are low explosives that are readily available to purchase for use as reloading powder [3,4], but when used as the filler in IEDs they can cause extensive damage [5].
Smokeless powders are classified based on their chemical composition. Nitrocellulose (NC) is the energetic material in single-base SP, NC and nitroglycerin (NG) in double-base SP, and NC, NG, and nitroguanidine in triple-base SP [6]. In addition to the energetic material, several organic compounds are added to the powder as stabilizers, plasticizers, flash suppressants, deterrents, opacifiers, and dyes [7]. In the analysis of post-blast debris, the presence of these compounds indicates that an SP was likely used [8], and from this, it may be possible to identify the explosive manufacturer [9]. The collection and subsequent analysis of post-blast debris can not only lead to explosive identification, but the data produced can potentially be used as evidence in a court of law or to aid investigation efforts [10,11].
Smokeless powders have been widely studied using a variety of chemical and instrumental techniques. In general, the most common instruments used to analyze SPs and their components are liquid and gas chromatography. Liquid chromatography has been coupled with QTOF-MS [12], MS/MS [13], DAD [8], and UV-Vis spectrometry [2] for the qualitative detection of intact, unburned SP samples.
Gradient reversed-phase LC-ESIMS has been used for the quantification of organic additives differentiate between SPs [14]. HS-SPME has been paired with GC/FID [15], GC/MS [4], and GC-micro-ECD for the identification and quantification of organic additives in intact, unburned SP samples [4]. Total Vaporization-SPME (TV-SPME) GC/MS has been used for the quantification of SP residues in real post-blast debris samples from both PVC and steel IEDs [16,17].
Cryofocusing capillary microextraction has been paired with GC/MS as a HS extraction device for qualitative analysis of intact SPs [18].
GC/VUV has been used to study the thermal degradation of nitrate ester and nitramine explosives [22,44,46,47] and an optimized method for explosives analysis with application to post-blast debris has been reported [45]. Cruse et al have also identified SP organic additives in both unburned SP [22] and real post-blast debris [45] and added the subsequent spectra to the VUV spectral library. The compounds identified were nitroglycerin, diphenylamine, ethyl centralite, and di-n-butyl phthalate.
In contrast to previous studies, this study will focus on the quantitative analysis of SP. Although the legal question in explosives investigations is which explosive was used (not how much), the quantitation of the components of SPs will allow limits of detection (LODs) to be calculated and determine the sensitivity required of the GC/VUV to detect these compounds [16]. The analysis of SP particles in post-blast debris can also yield information about the difference in chemical composition of the particle pre-and post-blast, and how this differs based on the container used.
This work quantifies nitroglycerin, 2,4-dinitrotoluene (2,4-DNT), diphenylamine (DPA), ethyl centralite (EC), and di-n-butyl phthalate in pre-and post-blast SP particles using heptadecane as an internal standard. To our knowledge, this is the first use of GC/VUV for the quantification of explosives.

| Chemicals
Nitroglycerin (1000 μg/ml in methanol) was purchased from Restek. Diphenylamine and n-heptadecane were purchased from Acros Organics. Dinitrotoluene was purchased from Spectrum Chemicals. Ethyl centralite was purchased from Aldrich Chemistry.
Di-n-butyl phthalate was purchased from Supelco. Acetone was

| Post-blast debris generation and collection
Post-blast debris was obtained via controlled explosions with assistance from the Indiana State Police Bomb Squad. Two galvanized steel and two polyvinyl chloride (PVC) IEDs were assembled. For both materials, one device contained IMR 4064 and the other contained Alliant Red Dot. Each device was placed in a vented 2′ × 2′ × 2′ steel box to help retain the post-blast debris for collection.

| Sample preparation
Acetone was chosen as the solvent for this study so that it would effectively dissolve any nitrocellulose present and thus effectively disrupt the matrix of the sample. An internal standard solution of 100 ppm heptadecane in acetone was made and used as the solvent for all experiments. A mixture of nitroglycerin, 2,4-dinitrotoluene, diphenylamine, ethyl centralite, and dibutyl phthalate in internal standard solution was made. First, the nitroglycerin in methanol (1000 μg/ml) was concentrated using a nitrogen blow down apparatus and reconstituted in internal standard solution. After this step, each of the remaining components were added to the mixture at the same concentration as nitroglycerin. Using this mixture, calibrants were made for subsequent analysis by GC/VUV. Three calibration curves were used to quantify the data discussed in this paper. All

| Gas chromatography
1 μl of each sample was injected into an Agilent 7890B gas chromatograph equipped with an Agilent 7693 autosampler and a multimode inlet. The sample was introduced using hydrogen carrier gas at 2.4 ml/min. The multimode inlet was in splitless mode with a temperature program beginning at 50°C and ramped to 200°C at 900°C/ min. The analytes were then separated by a 15 m × 320 μm × 0.25 μm Restek Rtx®-5MS column. The oven temperature began at 50°C for 30 s, and was then ramped at 20°C/min to 250°C. The same oven temperature program was used for all samples. The GC and VUV parameters used here were previously optimized [45].

| Vacuum ultraviolet spectroscopy
After separation in the GC, the analytes were introduced into a VUV Analytics VGA-101 VUV spectrometer at a transfer line and flow cell temperature of 300°C. Samples were analyzed using a scan rate of 4.5 Hz, a deuterium lamp at 1.2 V, a make-up gas pressure of 0.00 psi, and a spectral range of 120-430 nm [45]. To improve the signal-tonoise ratio, the resulting chromatograms were analyzed using absorbance data from 125-240 nm.

| Calculations
Signal-to-noise ratios were calculated using a wavelength filter from 125-240 nm. Peak height was used to define signal. The standard deviation of the 20 baseline points preceding the peak was defined as noise. Nine calibration concentrations ranging from 3-1118 ppm were used for calculations. Limits of detection were calculated by plotting log(S/N) versus log(concentration) and determining the concentration of analyte that yielded a S/N of 3.

| GC/VUV calibration for quantification
Three sets of calibrants were used for quantification because the debris was analyzed on three separate days. The linear ranges, limits of detection, and mass on column reported in Table 1 Figure 1 is an example chromatogram with corresponding VUV spectra. All chromatograms were generated by averaging absorbance values from 125-240 nm to improve S/N. Figure 1 is the VUV spectrum of nitroglycerin (box 1). Unlike the other VUV spectra included, this spectrum is rich in spectral details and contains narrow absorption bands. These spectral details are a result of the thermal degradation of nitroglycerin in the VUV flow cell into a mixture of nitric oxide, carbon monoxide, formaldehyde, oxygen, and water [22]. The nitroglycerin spectrum produced is a combination of the VUV spectra of each of the decomposition products. The complete thermal degradation of nitroglycerin occurs at a flow cell temperature of 280°C. At temperatures below this, the molecule is still intact that results in a broad VUV spectrum consistent with the VUV spectrum of water and oxygen [22].

| IMR 4064
IMR 4064 is a single-base rod-shaped smokeless powder designed to burn from the inside out. This phenomenon is illustrated in Figure 2.
When both pre-and post-blast particles were viewed vertically, a small opening was seen in the center of the particle. Both PVC and steel post-blast particles were smaller in overall size, but the size of this opening increased more in the steel particles than the PVC particles. This is likely because the steel device reached a higher temperature before failing, so more of the particle was consumed in the explosion.

| Alliant red dot
Alliant Red Dot is a double-base disc-shaped smokeless powder with red identifiers. If red particles are found in post-blast debris, it is a clear indication that Red Dot was used. The particles recovered from both the PVC and steel devices appear to be partially burned and have less of a circular shape than the pre-blast particles (Figure 3).

| Quantification of pre-and post-blast particles via GC/VUV
Due to the nature of these of experiments, only one device of each type was analyzed (e.g., one steel IED filled with IMR 4064), so the SP replicates are particles from the same device. The conclusions made about concentration changes in the particles from pre-to post-blast would be strengthened if more devices of the same type were utilized. This would allow SP particles to be compared between devices to ensure the concentration changes observed from pre-to post-blast particles are due to container type.

| IMR 4064
The components of interest in IMR 4064 were 2,4-DNT and DPA, both of which were detected and quantified in all IMR 4064 samples.  Table 2). This change in concentration was found to be statistically significant. The decrease in 2,4-DNT in the particles collected from the steel device is likely because the steel device reached higher temperatures before failure, so the compound was consumed in the reaction. Although not statistically significant, there was a relative increase in concentration of 2,4-DNT and DPA from the pre-blast particles to PVC post-blast particles. This is likely because the size of the particle decreased, but neither the 2,4-DNT or DPA were consumed in the reaction so there was a relative increase in their concentration. The relative increase in concentration found in pre-and post-blast intact particles is reported in Table 3.
There were a limited number of intact particles from the steel device, and because of this only one run was able to be performed (no  These values are the average of three replicates PVC post-blast particles is likely because the size of the particle decreased, but the EC was not consumed in the reaction so there was a relative increase in its concentration.