Review: Characterization of Smokeless Powders Using Multiplexed Collision-Induced Dissociation Mass Spectrometry and Chemometric Procedures

Emily C. Lennert

 

Category

Chemistry

Keywords

smokeless powder, liquid chromatography, time of flight, mass spectrometry, atmospheric pressure chemical ionization, LC-APCI-TOFMS, LC-MS, nitroglycerin, ethyl centralite, diphenylamine, akardite II, N-nitrosodiphenylamine, dibutyl phthalate, principal component analysis, hierarchical cluster analysis, PCA, HCA

Article Reviewed

1. Reese, K. L.; Jones, A. D.; Smith, R. W. Characterization of smokeless powders using multiplexed collision-induced dissociation mass spectrometry and chemometric procedures. Forensic Science International. 2017, 272, 16-27.

Disclaimer

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.

Summary

Smokeless powders are common low explosives that are present in ammunitions and often used to make improvised explosive devices, such as pipe bombs. Currently, the analysis of unburned smokeless powder is used to determine if the substance is, in fact, a smokeless powder. Research in the smokeless powder area has focused on differentiating unburned powders from one another based on their chemical profiles, derived from the organic compounds present in the smokeless powder. To date, little work has been done regarding the comparison of post-fire smokeless powder residues, i.e. organic gunshot residue, to the original unburned powder. Some work has been accomplished in which post fire smokeless powders have been screened for target analytes. The disadvantage of a targeted approach is if compounds are present, which are not part of the target group, the compounds will not be identified. The study presented here offers a non-targeted approach for the analysis of unburned and post-fire smokeless powders. Statistical approaches were employed to examine discrimination between unburned smokeless powders as well as to determine the association between post-fire residue and its respective unburned powder.

Ammunitions of several brands, calibers, primer compositions (i.e. leaded or lead-free), and age were selected for use in this study. Five cartridges from each ammunition were selected and the smokeless powder removed and placed in individual vials. 25 mg of smokeless powder from each vial was extracted separately in 1 mL of acetone, then dried under a stream of nitrogen. Samples were extracted again by three subsequent 0.5 mL aliquots of dichloromethane for 10 minutes per aliquot. The three dichloromethane extracts were then combined and dried under nitrogen. Then, 0.5 mg of the extracted material was reconstituted in 1 mL of a 1:1 mixture of acetonitrile and water containing 25 μM 4-aminodiphenylamine and 10 mM N-(3,5-dinitro-2-pyridinyl)phenylalanine internal standards. Samples were then analyzed by liquid chromatography – atmospheric pressure chemical ionization – time of flight mass spectrometry (LC-APCI-TOFMS).

Post-fire residues were collected by firing samples and collecting the cartridges. Cartridges were collected by placing a plastic zip-lock style bag over the ejection port of the firearm to collect each spent cartridge. Cartridges were then transferred to individual vials, upright, to prevent the loss of residue; five cartridges were collected per ammunition. Firearms were cleaned between ammunitions. Cartridges were then extracted in a similar manner to that of the unburned powders (described above).

Using a non-targeted approach, two peaks were identified that were not included in the reference standards analyzed. A summary of reference standards is provided in Table 2 within the study. These two unknowns would not have been recognized in a targeted approach, and were identified as two relevant compounds: akardite II and N-nitrosodiphenylamine. Akardite II is a stabilizer and N-nitrosodiphenylamine is a byproduct of the stabilizing properties of diphenylamine. The chemical composition of each unburned powder is summarized in Table 4 within the study.

Principal component analysis (PCA) and hierarchical cluster analysis (HCA) were used to analyze the data further. PCA scores and loadings plots were generated for the unburned smokeless powders to determine how well the powders were discriminated from one another and which compounds were responsible for differentiation of groupings among the powders. On the scores plot, powders did not group by ammunition, but rather by the most abundant compound in the group. Five compounds were determined to be responsible for the grouping of samples, based on the loadings plot: akardite II, ethyl centralite, dibutyl phthalate, diphenylamine, and N-nitrosodiphenylamine. The scores and loadings plot can be seen in Figure 3 within the study. HCA was performed and the resulting dendrogram is given in Figure 5 within the study. Based on the dendrogram, five clusters were identified. Cluster 1 was formed based on the high abundance of dibutyl phthalate in the samples. Cluster 2 samples shared a high abundance of akardite II. Cluster 4 formed due to high abundance of ethyl centralite. In cluster 3 and cluster 5, grouping was due to the relative abundance of diphenylamine and N-nitrosodiphenylamine. In cluster 3, samples with higher diphenylamine and lower N-nitrosodiphenylamine were found; whereas, in cluster 5, samples with higher abundances of N-nitrosodiphenylamine were found.

Using HCA, the association of fired residues with their respective unburned smokeless powder was tested. One fired residue was tested at a time to determine whether the sample would classify among the cluster containing the unburned samples. For 10 of the 17 powders investigated, none of the five replicates showed association with the respective unburned smokeless powder. Overall, post-fire residues retained the same compounds as the original unburned powder but did not maintain the relative proportions of compounds. For the remaining 7 powders, some degree of association was seen. This association was attributed to the conservation of the dominant compound after firing. For example, 5 of the samples had profiles dominated by akardite II in both the unburned and post-fire results; these samples showed association. Similarly, for the remaining 2 samples, ethyl centralite was the dominant compound before and after firing, and association was observed. Overall, in samples where akardite II and ethyl centralite dominated, association was observed. However, in samples where dibutyl phthalate, diphenylamine, and N-nitrosodiphenylamine dominated the unburned smokeless powder profile, the post-fire residue did not show this dominance and therefore association was not observed.

Scientific Highlights

• Unburned smokeless powders were classified into smaller groups based on the most prevalent compound present in samples within the group.
• Using PCA, important compounds responsible for the differentiation of groups were identified: akardite II, ethyl centralite, dibutyl phthalate, diphenylamine, and N-nitrosodiphenylamine.
• Post-fire smokeless powder residues showed some association with the unburned smokeless powders, particularly in samples in which the major component was conserved after firing, e.g. ethyl centralite and akardite II.

Relevance

The findings of this study may be applied to gunshot residue analysis. Post-fire residues may potentially be linked to the unburned powder, which may provide an evidentiary link between a suspect and crime.

Potential Conclusions

• Using statistical methods, characteristic compounds may be identified in smokeless powders that allow for the differentiation of samples.
• Post-fire smokeless powder residues may be correlated to the unburned smokeless powder, dependent on composition.