Review: A Forensic Investigation on the Persistence of Organic Gunshot Residues

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Emily C. Lennert

 

Category

Chemistry

Keywords

organic, gunshot residue, GSR, OGSR, persistence, ultra performance liquid chromatography, UPLC, LC, mass spectrometry, tandem, MS, LC-MS/MS

Article Reviewed

Maitre, M.; Horder, M.; Kirkbride, K.P.; Gassner, A.; Weyermann, C.; Roux, C.; Beavis, A. A forensic investigation on the persistence of organic gunshot residues. Forensic Science International. 2018, 292, 1-10.

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

Traditionally, gunshot residue (GSR) is characterized by the presence of spherical particles containing lead, barium, and antimony, collectively known as inorganic GSR (ISGR). However, with the advent of lead free or heavy metal free primer mixtures, GSR may lack these characteristic particles. To address this challenge, organic GSR (OGSR) may be analyzed. OSGR originates from the organic smokeless powder present in ammunitions. Both lead containing and lead free ammunitions contain smokeless powders, and therefore will produce OGSR. Additionally, the amount of smokeless powder present in a round of ammunition is much larger than that of the primer, and thus it is possible that more residues may be obtained from the smokeless powder, i.e. OGSR, than from the primer mixture, i.e. IGSR. Smokeless powders are usually observed as either single or double base in commercial ammunitions. Single base powders contain nitrocellulose as the base charge, whereas double base contains nitrocellulose and nitroglycerin. In addition to the base charge, additives are present in smokeless powders that serve as stabilizers, plasticizers, and flash inhibitors. These organic components may be observed in OGSR. Ethyl centralite (EC) and methyl centralite (MC), which are restricted for use in smokeless powder production, are highly characteristic of a smokeless powder. Diphenylamine (DPA) is also commonly found in smokeless powders; however, DPA has many industrial uses. Diphenylamine is considered characteristic of OGSR only when observed in a mixture with its nitrated derivatives, e.g. N-nitrosodiphenylamine (N-nDPA). The persistence of OGSR is an important consideration for casework. Knowledge of the persistence of OGSR may provide a window for acceptable collection time after shooting, or provide insight to the potential value of GSR collection after a period of time has passed between the shooting and collection. The goal of this study was to determine whether OGSR was detectable on the hands of a shooter for up to four hours following shooting, as previously reported by other means of analysis.

Four analytes of interest were selected for this study: EC. MC, DPA, and N-nDPA. These were selected due to their common presence in smokeless powders and relevance, i.e. compounds indicative of smokeless powders. An internal standard of deuterated DPA (D10-DPA) was also selected for normalization purposes. Standards of each analyte of interest were analyzed as reference for identification and for method validation. A standard curve was also created and analyzed with each sample run as a quality control measure.
Two firearms of different calibers were selected for the shooting experiments. The first firearm was a .40 caliber Glock 22® S&W (.40 S&W), and the second was a .357 Magnum S&W Revolver model 686 (.357 Mag). A lead-free ammunition was used for the .40 S&W, while a traditional ammunition was used for the .357 Mag. A lead-free and traditional ammunition was selected to demonstrate the utility of OGSR analysis as a complementary method to IGSR analysis. The shooting method is given in Figure 3 within the study. A single shooter was used for the experiment. The shooter washed their hands to decontaminate the sampling surface, then blank samples were obtained from the dominant and non-dominant hands. Following blank collection, the firearm was discharged three times. After the selected time period (0, 0.5, 1, 2, and 4 h), samples were collected from the shooter’s dominant and non-dominant hands. In the time between shooting and collection, the shooter was instructed to go about daily activities without washing their hands. For each firearm, experiments were repeated four times for all time points, with the exception of the 4 h time point, which was repeated in triplicate.

OGSR samples were collected via carbon-coated adhesive stubs, which were used to sample the thumb-index finger area of the palm and back of the hand as well as the wrist. Carbon-coated adhesive stubs have been demonstrated in literature to provide higher OGSR recoveries than alcohol swabs, another common sampling method. After sampling, stubs were sealed in tubes and placed in storage at 4˚C. Extraction was performed within 24 hours of collection to prevent sample degradation. Samples were extracted via solvent extraction using acetone, which was then filtered and dried under nitrogen. The dried extract was then reconstituted in methanol and acetonitrile (1:1) and spiked with the internal standard for a concentration of 20 ppm D10-DPA. Samples were analyzed via Ultra Performance Liquid Chromatography – Tandem Mass Spectrometry (UPLC-MS/MS) using a triple quadrupole mass spectrometer system and electrospray ionization source. For more information on LC-MS/MS, refer to Review: Rapid Analysis of Drugs of Abuse and their Metabolites in Human Urine Using Dilute and Shoot Liquid Chromatography – Tandem Mass Spectrometry.

The analytical method was validated based on specificity, linearity, accuracy, precision, and robustness. Repeatability was assessed over the course of two days. Robustness was evaluated by altering the chromatographic method. A summary of the results of method validation is provided in Table 4 within the study. The method was determined to be fit for purpose. Quantification was not performed in this study, as the initial amount of each analyte in the fired cartridge was unknown for samples taken after t=0. Additionally, the combustion and deposition of OGSR is highly variable and unpredictable, and thus the quantification of the fired residues was deemed uninformative.

Analytes were considered detected when the signal observed was above the limit of detection determined during method validation (see Table 4 within the study). Although previous research had indicated that hand washing would remove all traces of OGSR, it was observed that some sample blanks contained OGSR. Thus, if the subsequent sample contained less OGSR than the blank, the sample was treated as negative for OGSR. Positive samples were observed for 72% of .40 S&W samples and 89% of .357 Mag samples. Three of the four target analytes were detected: EC, DPA, and N-nDPA. MC was not detected; a propellant usually uses either EC or MC, and therefore it was reasonable to expect that MC would not be present in samples containing EC. After four hours, the three detected analytes were still observed at significantly larger abundances than the limit of detection for each, suggesting that OGSR may be detectable beyond the four hour mark. A large decrease in OGSR was observed within the first hour, as seen in Figure 5 within the study. At the four hour mark, an average between both hands of 14% N-nDPA, 23% DPA, and 35% EC was detectable (see Table 5 within the study for detailed data). Figure 6 within the study showed the trend of OGSR loss for each hand separately. It was observed that, for the .40 S&W, a larger amount of OGSR was detected on the dominant hand. This was attributed to the firearm, which had an ejection port of the right, i.e. dominant hand, side. The .357 Mag had a more uniform distribution of OGSR across both hands, likely due to the revolver construction.

Overall, the trends observed for OGSR were similar for those previously reported for IGSR, with the majority of loss occurring in the first hour after discharge. However, the loss of IGSR over time appeared more significant than the loss of OGSR, which appeared to be retained on the skin. Additionally, a large variation in the amount of OGSR detected at each time point was observed, as demonstrated by the error bars in Figure 5 in the study. Discharge-to-discharge variability, i.e. variations between firings of the weapon, as well as external factors such as air flow led to high variability in the samples. The authors note that this study has inherent limitations. The study reflects an ideal situation, in which a shooter refrains from hand washing after firing the weapon. The authors suggested further persistence studies.

Scientific Highlights

  • Three of the four compounds of interest were detected in over 70% of samples obtained up to four hours after firing.
  • The greatest loss of OGSR was observed in the first hour after firing.
  • The loss of OGSR was less rapid than the loss of IGSR reported by other studies, suggesting better retention of OGSR on skin.

Relevance

Knowledge about the persistence of OGSR may help in the prioritization of cases for investigators.

Potential Conclusions

OGSR may be detected up to, and potentially after, four hours on a shooter’s hands.