Review: Stability of Smokeless Powder Compounds on Collection Devices
Emily C. Lennert
smokeless powder, gun powder, gunshot residue, organic, GSR, OGSR, swab, stub
Taudte, R.V.; Roux, C.; Beavis, A. Stability of smokeless powder compounds on collection devices. Forensic Science International. 2017, 270, 55-60.
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.
For more information on SEM-EDX, refer to Review: Determining the Effect of Cartridge Case Coatings on GSR Using Post-Fire Priming Cup Residue.
For more information on UHPLC-MS, refer to Review: The Role of Ultra High Performance Liquid Chromatography with Time of Flight Detection for the Identification of Synthetic Cannabinoids in Seized Drugs.
Detection of smokeless powder components may be of particular importance in the investigation of shootings or explosive events. Organic gunshot residue (OGSR), burned smokeless powder residues, and unburned smokeless powders may be analyzed. These samples are commonly collected by swabbing or using GSR stubs. Few studies exist that have examined the effect of storage on OGSR degradation. When a sample is collected, it may be stored for days or weeks, or the sample may undergo inorganic GSR (IGSR) analysis, i.e. detection of lead, barium, and antimony, by scanning electron microscopy – energy dispersive x-ray (SEM-EDX) prior to OGSR extraction. The authors of this study examined the effect of storage time on the degradation of smokeless powder compounds spiked onto two sample collection devices: swabs and stubs.
Several possible OGSR compounds were selected for analysis: resorcinol, 1,3,5-trinitroperhydro-1,3,5-tiazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), 4-amino-2,6-dinitrotoluene (4-A-2,6-DNT), nitroglycerin (NG), 1,3,5-trinitrobenzene (TNB), m-dinitrobenzene (m-DNB), diphenylamine (DPA), N-nitrosodiphenylamine (N-nDPA), and ethyl centralite (EC). An internal standard of 2-naphthol at a concentration of 20 ppm was used. An internal standard is a compound at a known concentration that is used to improve quantitative precision by providing a known reference within a sample. Calibration curves were created for each individual compound to allow for quantification for unknown samples.
Samples were prepared by spiking the swab or stub with 10 ng of a mixed standard solution containing all of the selected OGCR compounds. After spiking, samples were allowed to dry, then stored in a refrigerator at 4 °C until extraction. Time interval between sample preparation and extraction were varied to study the effect of time on the degradation of smokeless powder components. Extractions were performed at 0, 1, 2, 4, 8, 15, 22, 29, 40, 49, and 63 days. Swabs were extracted by sonication for 5 minutes in methyl tert-butyl ether (MTBE), followed by drying under a stream of nitrogen and reconstitution in an acetonitrile (ACN):methanol (MeOH) 50:50 solution and 20 ppm 2-naphthol. Stubs were extracted by sonication in acetone for 5 minutes, followed by drying under a stream of nitrogen and reconstitution in ACN:MeOH 50:50 with 20 ppm 2-naphthol. Extracts were analyzed immediately after preparation by ultra high performance liquid chromatography – mass spectrometry (UHPLC-MS). After analysis, calibration curves were used to quantify each component and the concentration was converted to a percentage of the original amount.
Degradation varied across compounds; however, some trends were observed. The greatest degree of degradation occurred within the first week for all compounds except 2,4-DNT, with most of the degradation occurring in the first 4 days. This may be problematic for OGSR analysis, particularly when OGSR analysis follows IGSR analysis or if the lab has a high volume of casework and requires samples to be stored for a lengthy period of time prior to OGSR analysis.
Additionally, for most compounds, with the exception of TNT and EC, compound recovery on stubs were higher than that from swabs. This may be attributable to the collection device matrix, where swabs may form stronger interactions with the target compounds or could potentially promote degradation. On swabs, OGSR may be solubilized during the swabbing process and evaporate over time; whereas, stubs collect particles that are more stable.
Compounds commonly indicative of OGSR, e.g. EC, NG, and DPA, showed high degrees of degradation over the course of the 63 day experimental period after sample collection. EC was observed to degrade 70% on stubs and 52% on swabs and NG degraded 49% on stubs and 33% on swabs. This means that once the swabs or stubs collect the OGSR, if the samples are allowed to sit undisturbed for 63 days, most of the NG and EC will have degraded and thus cannot be analyzed to indicate the presence of OGSR on the suspect. Additionally, the authors noted that NG, which is an important indicator of OGSR and is high unstable, was not observed on any swab after day 15; all of the compound had degraded. However, at the time of collection, there was a minute amount of NG initially and thus a result of 33% degradation yielded all of the NG degraded. DPA was observed to degrade 16% on stubs and 75% on swabs but after 63 days DPA was still observed in the GC-MS data. Conversely, several compounds were observed to be stable, i.e. degradation less than 16%, over the course of the 63 day period. RDX, HMX, TNB, m-DNB, resorcinol, 4-A-2,6-DNT, N-nDPA, and 2,4-DNT were all considered stable and thus could be observed after a 63 storage period.
Another observation made by the authors is the apparent increase in concentration of some components: m-DNB, 4-A-2,6-DNT, and 2,4-DNT. These increases may be attributable to the formation of degradation products from other components. For example, the degradation of TNT may form 2,4-DNT, which would account for the increase in 2,4-DNT concentration. The authors note that further research regarding the degradation patterns of OGSR is necessary.
- Most of the degradation of smokeless powder components occurs within the first four days of storage.
- Common OGSR targets (i.e. NG, EC, and DPA) degrade more rapidly than energetic materials commonly found in explosives (i.e. RDX and HMX).
- Some compounds, such as TNT or DPA, may form degradation products that complicate the interpretation of OGSR results.
OGSR is becoming more relevant, due to the increasing use of lead-free ammunitions, which eliminate the possibility of traditional IGSR analysis.
OGSR samples should be stored in a refrigerator and analyzed soon after collection to avoid the degradation of organic compounds present in the sample.