Emily C. Lennert
explosives, improvised explosive device, IED, smokeless powder, DART-MS, direct analysis in real time, mass spectrometry, gas chromatography, GC-MS, dynamic headspace concentration, headspace, vapor, sorbent-coated wire mesh, adsorbent
Li, F.; Tice, J.; Musselman, B. D.; Hall, A. B. A method for rapid sampling and characterization of smokeless powder using sorbent-coated wire mesh and direct analysis in real time – mass spectrometry (DART-MS). Science and Justice. 2016, 56, 321-328.
2ASTM E1413-13 Standard Practice for Separation of Ignitable Liquid Residues from Fire Debris Samples by Dynamic Headspace Concentration. ASTM International, West Conshohocken, PA, 2013.
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.
Smokeless powder is a readily available low explosive that is often found in improvised explosive devices (IEDs), such as pipe bombs. Smokeless powders contain nitrocellulose as the base charge, as well as nitroglycerin in double base powders. In addition to the base charge, smokeless powders contain additives, such as plasticizers, stabilizers, deterrents, and flash suppressants. These additives serve to prolong the shelf life of a smokeless powder as well as to partly control the burn rate. After an explosive event, such as the explosion of an IED, smokeless powder particles or residues may be recovered. Currently, the primary methods of collection for trace explosive residues are vacuuming and swabbing. However, headspace sampling may offer an advantage to these techniques. Headspace sampling involves the generation of headspace vapors, which are collected and concentrated onto an adsorbent material. This technique often offers increased sensitivity for target compounds. Dynamic headspace methods use continuous flow of gas or a vacuum to flush the headspace vapors through the adsorbent, which results in a faster and more sensitive method when compared to static headspace methods such as solid phase microextraction (SPME). Due to the speed and sensitivity, dynamic headspace is an ideal method for rapid sample preparation. Additionally, the technique can be employed in the field, through the use of portable samplers. The authors of this study developed a rapid method for in field collection of trace explosive material using dynamic headspace methods, followed by subsequent analysis by direct analysis in real time – mass spectrometry (DART-MS).
The rationale for this dynamic headspace method was derived from the American Society for Testing and Materials (ASTM) International E 1413-13 standard practice for separation of ignitable liquid residues from fire debris samples by dynamic headspace concentration.2 A negative pressure system was employed, i.e. a small handheld vacuum pump, to draw headspace through the adsorbent material. The adsorbent consisted of Carbopack-X coated wire mesh placed inside a cassette to hold the adsorbent. A diagram depicting the dynamic headspace setup can be found in Figure 1 within the study. Samples were placed inside a paint can, held within a portable heating mantle, and heated to release headspace vapors, which were then collected on the adsorbent material.
Hodgdon Lil’ Gun (HLG) smokeless powder was used in this study, and obtained from a local sporting goods store. HLG smokeless powder was selected due to the variety of additives present in the sample. The National Institute of Standards and Technology (NIST) RM 8107 reference standard was also used in this study. Prior to DART-MS analysis, the dynamic headspace parameters were optimized. Temperature, flow rate, and sampling time were optimized using HLG smokeless powder to create a procedure for dynamic headspace sampling. Keeping time and flow rate constant, at 5 min and 3 L/min, respectively, the temperature was varied to determine optimal temperature. Temperatures of 148 ˚C, 160 ˚C, and 172 ˚C were examined. Using 5 mg og HLG smokeless powder, all target compounds were detected at 148 ˚C. At 172 ˚C, thermal degradation of the powder was observed. Therefore, the authors selected the temperature of 148 ˚C to reduce the potential for thermal degradation of the sample. With a constant temperature of 148 ˚C and flow of 3 L/min, sampling time was varied between 1.25, 2.5, 5, 7.5, and 10 min. All target compounds were observed with sampling times of 5 min or higher. Diphenylamine, ethyl centralite, dibutyl phthalate, and nitroglycerin were observed after 1.25 min of sampling. However, ethyl centralite and dibutyl phthalate were in low abundance and could not be distinguished from the background noise until sampling time reached 5 min. Akardite II was not observed until 2.5 min of sampling. At 10 min, the smokeless powder sample was reduced to a residue. Therefore, a sampling time of 5 min was selected. Finally, flow rate was evaluated at 3, 4, and 5 L/min, with constant temperature and sampling time of 148 C and 5 min, respectively. Signal intensity of ethyl centralite and dibutyl phthalate, i.e. the low abundance compounds, was observed to decrease with an increased flow rate. Therefore, the lowest flow rate was selected, 3 L/min, due to the higher ion abundance observed.
The optimized dynamic headspace concentration method was then validated using the NIST RM 8107. Samples were analyzed by DART-MS by placing the entire length of the wire mesh into the ion stream of the DART source. A NIST RM 8107 sample was prepared using the optimized dynamic headspace concentration method, followed by analysis via DART-MS. All components of the standard were successfully detected with DART-MS, in less than a minute, following dynamic headspace concentration using the optimized parameters. Results were confirmed by gas chromatography – mass spectrometry (GC-MS).
Confirmatory analysis, as well as analysis for comparison, was performed using separate samples prepared for GC-MS. These samples were prepared by dissolving two flakes of either HLG smokeless powder or NIST RM 8107 in 1 mL of acetone, followed by analysis on GC-MS, which was an approximately 30 min run-time. Results were compared between GC-MS and DART-MS to determine whether similar results were observed between techniques. Similar results for HLG smokeless powder were reported, with the exception of two compounds that were detected by DART-MS and not by GC-MS. Akardite II and N-nitrosodiphenylamine were detected by DART-MS and not by GC-MS. Similarly, the N-nitrosodiphenylamine in NIST RM 8107 was detected by DART-MS and was not detected by GC-MS. This is likely due to the thermally labile nature of Akardite II and N-nitrosodiphenylamine; thermally labile compounds may be more difficult to detect on the GC-MS. N-nitrosodiphenylamine may be detected by GC-MS; however, it decomposes into diphenylamine and therefore cannot be distinguished from diphenylamine also present in the sample.
- An optimized dynamic headspace concentration method is presented for subsequent analysis by DART-MS: heating temperature 148 ˚C, flow rate 3 L/min, sampling time 5 min.
- The optimized method was validated using NIST RM 8107, in which all stated compound in the reference standard were identified during DART-MS analysis following dynamic headspace concentration.
- Akardite II and N-nitrosodiphenylamine were detectable by DART-MS, which were not detectable by GC-MS.
Dynamic headspace concentration offers an alternative sampling method which may provide more sensitive analysis.
Dynamic headspace concentration followed by DART-MS analysis may be a viable alternative to the current sampling and analysis techniques used for explosive residue collection and analysis.