Comment pubs.acs.org/ac
Comment on “Tunable Generation and Adsorption of Energetic Compounds in the Vapor Phase at Trace Levels: A Tool for Testing and Developing Sensitive and Selective Substrates for Explosive Detection” Jay W. Grate,* Robert G. Ewing, and David A. Atkinson Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States
Anal. Chem. 2010, 82 (8), 3389−3393. DOI: 10.1021/ac902930e DOI: 10.1021/ac400141d
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material at Texas Tech University, leading to severe injury, has led to increased awareness of safety hazards associated with laboratory experimentation using energetic materials; this accident was investigated by the U.S. Chemical Safety Board and it has renewed concerns over laboratory safety practices.11−13 The use of explosives dispersed on solid supports is often preferred today over neat solid materials as vapor sources. Dispersed explosives provide a higher surface area to promote the saturation of the carrier gas with vapor, while at the same time promoting safety by reducing the possibility of sustaining a detonation. In fact, explosives dispersed on solid quartz sand have been developed as safe standards for testing detectors and canines14,15 and have been used in vapor generators for sensor testing.16 These have been called NESTT materials, meaning Non-Hazardous Explosives for Security Testing and Training,14 and are available commercially. The generation of explosive vapors from solid samples at controlled temperatures goes back over 3 decades.16−29 Thus, there is a substantial history in the development of explosives vapor generators,30 although some of this literature is in government or institutional reports rather than the peerreviewed literature.31 Vapor generators have also been described based on known masses or mass flows derived from the vaporization of solution standards of known concentration or the thermal desorption of known masses originally deposited from solution.32−35 Some references for explosives vapor generators have been summarized in Moore’s review of explosives detection instrumentation1 and Yinon’s book on explosives detection.36 A comprehensive review of vapor generators for explosives has been published recently.30 Vapor pressures of explosives compounds were reviewed in two recent articles.37,38 In 1976, Krzymien described a continuous flow vapor generator using a neat solid 2,4-DNT sample as the source to generate saturated vapor.17 In the same year, in Analytical Chemistry, Pella et al. described vapor generation from solid explosives (TNT, 2,4-DNT, 2,6-DNT) dispersed on a Chromosorb solid support.18 Dispersed liquid EGDN was also used as a source. Exiting vapor concentrations could be tuned by adjusting the temperature of the explosives vapor source, the fraction of the generated vapor that entered a
he security risks associated with explosive compounds have existed for decades and continue to be a major driver in the development of explosives sampling and detection technologies.1,2 Both the evaluation of developed technologies3 and research on new detection approaches4 require the ability to generate explosive vapors in the gas phase. Vapor generators are needed in such diverse applications as determining detection limits of commercial explosives detectors, assessing sampling schemes in walk-though explosives detection portals, and research into new analytical preconcentrators and sensors. Generating vapors of most explosives compounds at known calibrated concentrations is challenging due to their quite low vapor pressures and the fact that most exist as solids rather than liquids. In 2010 in Analytical Chemistry, Spitzer et al. described a vapor generator based on the passage of a carrier gas through a solid explosive sample in a U-tube held at an adjustable constant temperature, such that explosive vapor exited this source at “a quasi-steady state” (presumably saturated) concentration.5 The source output could be diluted downstream using a flow system, and the detection limits were described for an online gas chromatography (GC) system that was included to directly monitor vapor generator outputs. The U-tube was “filled with explosive”; neither the quantity of explosive material nor the size of the U-tube containing it, were reported. Spitzer et al. did not discuss the detonation hazard, if any, of the solid explosive materials in their vapor generator source.5 It was noted that maximum temperatures were limited to avoid “... the risk of fusion or partial degradation of the explosives in their solid form”.5 The quantity of material that can sustain a detonation depends on the grain diameter, the geometry, and whether it is a loose powder, a compressed powder, or a cast solid. The critical diameters, meaning the minimum size that can propagate a detonation wave, have been reported to be approximately 2 mm for TNT and 0.5 mm for RDX.6 These diameters correspond to masses of approximately 6.6 mg for TNT and 0.11 mg of RDX, assuming spherical solid particles and densities of 1.58 and 1.70 g/cm3, respectively. These numbers are offered solely as qualitative numbers to illustrate the potential hazard of small quantities of neat explosives and are not intended to indicate safe levels. Other sources provide substantially different values for critical diameters.7−10 There is recent enhanced interest in peroxidebased explosives; some of these are particularly sensitive to shock and thus easy to initiate. An accident with an energetic © 2013 American Chemical Society
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Comment
supports. Often there is a method to switch the final output between the diluted explosive vapor and a clean carrier gas. Vapors are generated from more volatile explosives such as 2,4DNT or TNT and from less volatile explosives such as RDX and PETN. The recent vapor generator described by Spitzer et al.5 shares a number of these characteristics. The vapor generator by Spitzer et al. was designed to yield very low explosive vapor concentrations. Unfortunately, the dilution flow system was not described; details of explosives flow systems are very important30 due to the possibility of depositing vapors by adsorption on surfaces and condensation on cold spots. It was projected in Spitzer’s paper that theoretical concentrations as low as 0.24 ppq could be generated for RDX, based on the vapor pressures, flow rates, and dilution. Using an online GC-ECD system, without a concentration device, the authors achieved real limits of detection of 125 ppt for TNT and 595 ppt for RDX.5 For concentrations above these levels, the GC-ECD detector could be used to calibrate and verify the generator. The results of these calibrations, and the range of temperatures and flow rates under which the source can provide a sustained saturated vapor stream, were not reported. Experimental work at 54 ppb for TNT and 608 ppb for RDX in the study of adsorbents was described. More recently, experimental work with TNT at 5− 10 ppb using this generator has been reported.40 Historically, several methods have been used to analyze output vapor streams. Adsorbent collection tubes or cold traps have been used to accumulate the output for analysis using GC.17,18,20 Similarly, collection tubes or cold traps have been used with subsequent high performance liquid chromatography (HPLC) analysis.27,28 The use of collection tubes provides an average output over time and may provide material for compositional analysis, but it may not reveal real timedependent dynamics in the generator output. The weight losses of permeation bags or diffusion tubes also provide average mass flow rates over time and only provide data on the source, not the diluted output flow.26,27 IMS for output analysis has been used in a number of formats. In some cases it has been used to analyze explosives accumulated on collection tubes25 or vapors supplied by a preconcentrating sampler that is part of the IMS instrument.32 Vapor generator outputs have also been interfaced directly to IMS instruments.22,25,27 An IMS device coupled to a tandem mass spectrometer was used for compositional analysis.27 Spitzer et al.5 instrumented their vapor generator for output analysis with a GC/MS system in addition to the GC-ECD and noted with supporting citations that GC-ECD is among the most sensitive analysis methods for nitro-containing organic compounds. The analysis of the concentration and composition of output vapors from an explosives vapor generator are continuing challenges for researchers in the explosives detection field. Most available instrumental methods cannot measure concentrations as low as those that may be generated. Some generators based on saturated sources are simply assumed to produce vapor concentrations in accord with certain published vapor pressure equations, e.g., those by Dionne et al.20 Such assumptions do not reveal if the generator source provides saturated vapors or if the output is being trapped downstream by surface adsorption or condensation in cold spots. In addition, the vapor pressure equations in the literature for a given explosive, as reported by different laboratories, are variable.37 (Recent reviews provide critical guidance on the most representative vapor equations to use.37,38) It should be noted that vapor pressure equations
dilution stage, and the flow rate of the regulated dilution gas. This dynamic gas flow system could deliver vapor concentrations as low as 0.05 ppb. Generator outputs were measured using collection tubes and a gas chromatography (GC) instrument with electron capture detection (ECD). Careful studies were reported to validate that exiting vapor concentrations were indeed saturated by analyzing the exiting vapor concentration as a function of flow rate. Equilibrium vapor concentrations as a function of source temperature were also reported. In 1986, Dionne et al. described vapor generation from several solid explosive vapor sources including less volatile materials such as RDX and PETN; these studies determined vapor pressures over a range of temperatures and developed vapor pressure equations.20 These equations have been used in several subsequent studies on vapor generation and detection. In 1992, Kenna et al. discussed vapor emission from dispersed sources in some detail and reported results for the generation of RDX vapors from coated beads and coated screens.22 Output RDX concentrations versus source temperature as well as outputs vs carrier gas flow rate were reported. In 1993, Lucero et al. described a vapor generator using a neat explosives sample contained within a thin-walled Teflon bag as a permeation source.26 Using RDX as an example, a 1 L/min carrier flow rate over the permeation bag provided a source concentration that was approximately 1500 times lower concentration than the RDX saturated vapor pressure at 75 °C. Using an additional permeation barrier and carrier gas for dilution, the generator output of RDX was stated to be as low as 2 × 10−3 ppt, although validating analytical measurements were not provided. Eiceman described a diffusion tube-based explosives vapor generator for TNT, RDX, and PETN in Talanta in 1997.27 Output vapor concentrations and compositions were characterized by a variety of methods including a commercial ion mobility spectrometry (IMS) instrument and a custom IMS system coupled to a tandem mass spectrometry (MS) system. Evidence for thermal decomposition of PETN was presented. The diffusion tube source temperature could be varied to tune the output vapor concentration range. A vapor generator based on vaporization of explosives solutions delivered from an inkjet source onto a heated ceramic surface has been described, using TNT, RDX, and PETN.32 There is thus a significant record of work in generating vapors from low volatility explosives such as RDX and PETN, in addition to more common work with TNT or 2,4-DNT. A number of pulsed vapor generators for RDX and PETN have also been described,23−25,29,33,34 including a generator from Idaho National Engineering Laboratories that was described in Analytical Chemistry in 1993.25 This last example used explosives dispersed on a solid support as a source to generate saturated vapor concentrations that could be delivered in 5-s pulses using an automated flow control system. The elapsed time between saturated pulses was a few minutes. This generator has been used in the development of microsensors for explosives detection, as seen, for example, in Pinnaduwage et al.39 Common features of explosives vapor generators include the following: controlled-temperature sources whose temperatures can be varied to tune the range of output vapor concentrations, flow control and dilutions systems, multiple controlledtemperature zones, output tubes or nozzles that are separately heated, and sources that are most frequently either neat solid explosives or solid explosive materials dispersed on solid 3014
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(16) Houser, E. J.; Mlsna, T. E.; Nguyen, V. K.; Chung, R.; Mowery, R. L.; McGill, R. A. Talanta 2001, 54, 469−485. (17) Krzymien, M.; Elias, L. J. Phys. E: Sci. Instrum. 1976, 9, 584−586. (18) Pella, P. A. Anal. Chem. 1976, 48, 1632−1637. (19) Carter, C.; Williams, W. D.; Conrad, F. J.; Lucero, D. P. Proc.New Concepts Symp. Workshop Detect. Identif. Explos. 1978, 45−47. (20) Dionne, B. C.; Rounbehler, D. P.; Achter, E. K.; Hobbs, J. R.; Fine, D. H. J. Energ. Mater. 1986, 4, 447−472. (21) Jenkins, A.; McGann, W.; Ribeiro, K. Extraction, Transportation and Processing of Explosives Vapor in Detection, DOT/FAA.CT-92/11, FAA Technical Center: Atlantic City, 1992. (22) Kenna, B. T.; Conrad, F. J.; Hannum, D. W. Explosive Vapor Emission, DOT/FAA.CT-92/11, FAA Technical Center: Atlantic City, 1992. (23) McGann, W.; Jenkins, A.; Ribeiro, K. A Thermodynamic Study of the Vapor Pressures of C-4 and Pure RDX, DOT/FAA.CT-92/11, FAA Technical Center: Atlantic City, 1992. (24) Davies, J. P.; Blackwood, L. G.; Davis, S. G.; Goodrich, L. D.; Larson, R. A. Adv. Anal. Detect. Explos., Proc. Int. Symp., 4th 1993, 513− 532. (25) Davies, J. P.; Blackwood, L. G.; Davis, S. G.; Goodrich, L. D.; Larson, R. A. Anal. Chem. 1993, 65, 3004−3009. (26) Lucero, D. P.; Roder, S. R.; Jankowski, P.; Mercado, A. Adv. Anal. Detect. Explos., Proc. Int. Symp., 4th 1993, 485−502. (27) Eiceman, G. A.; Preston, D.; Tiano, G.; Rodriguez, J.; Parmeter, J. E. Talanta 1997, 45, 57−74. (28) Rana, P.; Kannan, G. K.; Bhalla, R.; Kapoor, J. C. Def. Sci. J. 2003, 53, 415−423. (29) Miller, C. J.; Kaser, T. G.; Rodriquez, J. Sens. Imaging 2007, 8, 101−110. (30) Grate, J. W.; Ewing, R. G.; Atkinson, D. A. Trends Anal. Chem. 2012, 41, 1−14, DOI: 10.1016/j.trac.2012.08.007. (31) Reports on vapor generators have appeared in journal articles, edited books, conference proceedings, and in reports from government agencies or research institutions. (32) Verkouteren, R. M.; Gillen, G.; Taylor, D. W. Rev. Sci. Instrum. 2006, 77, 085104/1−085104/6. (33) Reiner, G. A.; Heisey, C. L.; McNair, H. M. J. Energ. Mater. 1991, 9, 173−89. (34) Bromberg, E. E. A.; Dussault, D.; MacDonald, S.; Curby, W. A. Adv. Anal. Detect. Explos., Proc. Int. Symp., 4th 1993, 473−84. (35) Poziomek, E. J.; Almeer, S. H. Proc. SPIE 1998, 3575, 392−402. (36) Yinon, J. Explosive Vapor Generators. In Forensic and Environmental Detection of Explosives; John Wiley & Sons: New York, 1999; pp 79−87, Chapter 2.11. (37) Ewing, R. G.; Waltman, M. J.; Atkinson, D. A.; Grate, J. W.; Hotchkiss, P. TrAC, Trends Anal. Chem. 2013, 42, 35−48. (38) Ö stmark, H.; Wallin, S.; Ang, H. G. Propellants Explos. Pyrotech. 2012, 37, 12−23. (39) Pinnaduwage, L. A.; Yi, D.; Tian, F.; Thundat, T.; Lareau, R. T. Langmuir 2004, 20, 2690−2694. (40) Bonnot, K.; Siegert, B.; Cottineau, T.; Keller, V.; Spitzer, D. Sens. Actuators, B 2012, 166−167, 829−832.
generally have limited temperature ranges and generators may operate with source temperatures outside those ranges. Sensitive online measurements to characterize vapor generator outputs, as described by Spitzer et al., is a significant advantage. Design of explosives vapor generators, validation of the concentration of the output vapors, analysis of the composition of the generated vapors, and delivery to test devices without condensation or adsorptive losses, using practical approaches, remain as challenges for researchers developing materials and devices for explosives sampling and detection.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge funding from the Laboratory Directed Research and Development funds administered by Pacific Northwest National Laboratory (PNNL) and the U.S. Department of Energy (DOE). PNNL is a multiprogram national laboratory operated for the DOE by Battelle Memorial Institute.
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REFERENCES
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