Tunable Generation and Adsorption of Energetic Compounds in the

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Anal. Chem. 2010, 82, 3389–3393

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 Karine Bonnot,† Pierre Bernhardt,‡ Dominique Hassler,† Christian Baras,† Marc Comet,† Vale´rie Keller,‡ and Denis Spitzer*,† NS3E-ISL-CNRS (Nanomate´riaux pour les Syste`mes Sous Sollicitations Extreˆmes) UMR 3208, French-German Research Institute of Saint-Louis, 5 rue du Ge´ne´ral Cassagnou, B.P. 70034, 68301 Saint Louis Cedex, France, and LMSPC-CNRS-UDS (Laboratoire des Mate´riaux, Surfaces et Proce´de´s pour la Catalyse) UMR 7515, ELCASS (European Laboratory for Catalysis and Surface Sciences), 25 rue Becquerel, 67087 Strasbourg Cedex, France Among various methods for landmine detection, as well as soil and water pollution monitoring, the detection of explosive compounds in air is becoming an important and inevitable challenge for homeland security applications, due to the threatening increase in terrorist explosive bombs used against civil populations. However, in the last case, there is a crucial need for the detection of vapor phase traces or subtraces (in the ppt range or even lower). A novel and innovative generator for explosive trace vapors was designed and developed. It allowed the generation of theoretical concentrations as low as 0.24 ppq for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in air according to Clapeyron equations. The accurate generation of explosive concentrations at subppt levels was verified for RDX and 2,4,6-trinitrotoluene (TNT) using a gas chromatograph coupled to an electron capture detector (GC-ECD). First, sensing material experiments were conducted on a nanostructured tungsten oxide. The sensing efficiency of this material determined as its adsorption capacity toward 54 ppb RDX was calculated to be five times higher than the sensing efficiency of a 54 ppb TNT vapor. The material sensing efficiency showed no dependence on the mass of material used. The results showed that the device allowed the calibration and discrimination between materials for highly sensitive and accurate sensing detection in air of low vapor pressure explosives such as TNT or RDX at subppb levels. The designed device and method showed promising features for nanosensing applications in the field of ultratrace explosive detection. The current perspectives are to decrease the testing scale and the detection levels to ppt or subppt concentration of explosives in air. Among various methods for landmine detection, as well as soil and water pollution monitoring,1-4 the detection of explosive * Corresponding author. Tel: +33 38969 5075. Fax: +33 38969 5074. E-mail: [email protected]. † NS3E-ISL-CNRS (Nanomate´riaux pour les Syste`mes Sous Sollicitations Extreˆmes) UMR 3208, French-German Research Institute of Saint-Louis. ‡ LMSPC-CNRS-UDS (Laboratoire des Mate´riaux, Surfaces et Proce´de´s pour la Catalyse) UMR 7515, ELCASS (European Laboratory for Catalysis and Surface Sciences). 10.1021/ac902930e  2010 American Chemical Society Published on Web 03/26/2010

compounds in air is becoming an important and inevitable challenge for homeland security applications, due to the threatening increase in terrorist explosive bombs used against civil populations.5-8 However, in the last case, there is a crucial need for the detection of vapor phase traces or subtraces (in the ppt range or even lower) leading to an increasingly active and challenging area of research.9-12 To develop detection devices allowing an even higher sensitivity to energetic compounds, it is necessary to develop explosive vapor generation devices to calibrate and test detectors at such low trace levels. So far, to our knowledge, the development of explosive vapor generators has been mainly focused on one type of energetic material, 2,4,6-trinitrotoluene (TNT) or 2,4-dinitrotoluene (DNT) nitroaromatics with relatively high vapor pressures, compared to other energetic materials which are much less volatile.13-16 Verkouteren et al. were the first in 2006 to focus their attention on the generation of low partial pressure explosive, such as pentaerythritol tetranitrate (PETN) or hexahydro-1,3,5(1) Alnaizy, R.; Akgerman, A. Water Res. 1999, 33, 2021–2030. (2) Cragin, J. H.; Leggett, D. C. U.S. Army Corps of Engineers, Cold Regions Research & Engineering Laboratory Report TR-0312; National Technical Information Service: Springfield, VA, 2003. (3) Walsh, M. E. Talanta 2001, 54, 427–438. (4) Jenkins, T. F.; Leggett, D. C.; Miyares, P. H.; Walsh, M. E.; Ranney, T. A.; Cragin, J. H.; Georges, V. Talanta 2001, 54, 501–513. (5) Liu, Y. S.; Ugaz, V. M.; North, S. W.; Rogers, W. J.; Mannan, M. S. J. Hazard. Mater. 2007, 142, 662–668. (6) Kannan, G. K.; Kappor, J. C. Sens. Actuators, B 2005, 110, 312–320. (7) Steinfeld, J. I.; Wormhoudt, J. Annu. Rev. Phys. Chem. 1998, 49, 203–232. (8) Gardner, J. W. Electronic Noses & Sensors for the Detection of Explosives; Kluwer Academic Publishers: Netherlands, 2004; pp 01-28. (9) Harper, R. J.; Almirall, J. R.; Furton, K. G. Talanta 2005, 67, 313–327. (10) Yinon, J. Trends Anal. Chem. 2002, 21, 292–301. (11) Staples, E. J. Electronic Noses & Sensors for the Detection of Explosives; Kluwer Academic Publishers: Netherlands, 2004, 235-248. (12) Pinnaduwadge, L. A.; Boiadjiev, B.; Hawk, J. E.; Thundat, T. Appl. Phys. Lett. 2003, 83, 1471–1473. (13) Pella, P. A. Anal. Chem. 1976, 48, 1632–1637. (14) Dionne, B. C.; Rounbehler, D. P.; Achter, E. K.; Hobbs, J. R.; Fine, D. H. J. Energ. Mater. 1986, 4, 447–472. (15) Oxley, J. C.; Smith, J. L.; Shinde, K.; Moran, J. Propellants, Explos., Pyrotech. 2005, 30, 127–130. (16) Rosen, J. M.; Dickinson, C. J. Chem. Eng. Data 1969, 14, 120–124.

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trinitro-1,3,5-triazine (RDX).17 They developed a system generating known vapor of explosives using the vaporization of explosive microdroplets on a heated ceramic plate. This system was developed to test explosive detection systems involving ion mobility spectrometry. The authors were able to generate concentrations of explosives as low as 290 fg/L and to calibrate systems at levels as low as 1 ng/L, i.e., 100 ppt.17 We report, herein, on a novel and innovative generator intended to produce controlled and known vapor concentrations of an explosive mixture at trace levels. The system we have developed is cheaper, easier to use, and ensures a better reproducibility of the vapor-generating experiments than the one designed by Verkouteren et al. in 2006.17 Therefore, our system employs a single-step generation procedure and uses the equilibrium law existing between a solid and its vapor at each temperature. The generated concentrations are independent of the mass of explosive used for producing vapors; those concentrations only depend on the temperature applied to the explosive solid. Verkouteren et al. used a ceramic heated plate to generate vapors. In the course of our experiments, air was pushed through the explosive solid, heated at a fixed temperature, in order to generate and drive the vapors into the testing system. Using a low airflow avoided the displacement of the solid-vapor equilibrium and, therefore, ensured the good reproducibility of the generated concentration of explosive in air. Low vapor pressure explosives, mostly used in bomb manufacturing, were employed: PETN, RDX, TNT, and 2,4-DNT. Here, the novelty was both the controlled generation of vapors of energetic materials having very low partial pressures, at least 100 times lower than the currently studied TNT, and which were representative of different kinds of existing energetic compounds (nitrate esters, nitramines, and nitroaromatics) and the use of the smoke properties of these explosive solids to test sensitive substrates for explosive sniffing. The objective was to reach very low-level concentrations of explosive vapors that could be representative of narrow air concentrations such as those expected in public places like airports or railway stations. This system could easily be used for calibrating and testing explosive vapor sensing devices with respect to a representative panel of vapor phase energetic compounds and for determining their sensitivity limits to traces. EXPERIMENTAL SECTION Explosive Vapor Generation System. The laboratory pilotscale explosive vapor generator (Figure 1) allowed the generation of explosive vapors at trace levels, based on the principle of a U-shaped reactor filled with explosive and maintained at a controlled temperature and, thus, reaching a known and constant partial pressure over the solid. By cooling the generation system down to temperatures as low as -15 °C and by dynamic dilution of the initially generated concentrations of the aforementioned pure or mixed explosives in order to produce trace vapor concentrations, it was possible to achieve very low final theoretical concentrations, between 0.24 ppq for RDX and 66 ppt for 2,4-DNT, depending on the compound and on the dilution ratio. The dependence of RDX, PETN, TNT, and 2,4-DNT vapor pressures on temperature was described using well-known literature (17) Verkouteren, R. M.; Gillen, G.; Taylor, D. W. Rev. Sci. Instrum. 2006, 77, 085104.

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Figure 1. Explosive vapor generation device. The air flow entering the U-shaped reactor is controlled using a mass flow controller (MFC). The reactor is filled with explosive to be studied and glass beads serving two purposes: (i) a better diffusion of air through the overall volume of the U-shaped reactor before diffusing into the explosive solid and (ii) avoiding the dispersion of the explosive solid throughout the system. The explosive vapor exiting the reactor is heated. The temperature of the explosive is imposed by an electronic device using the Peltier effect to either cool or heat the silicone oil. A temperature probe measures the temperature of the bath in the neighborhood of the explosive U-shaped reactor.

Figure 2. Material sensing efficiency testing reactor. The explosive vapor enters either (i) the bypass to validate the vapor concentration or (ii) the tubular quartz testing reactor to test and calibrate the substrate under the explosive vapor flow conditions. The substrate to be tested (b) covers all the surface of a quartz porous plate (a) in the center of the tubular quartz reactor. Vapor is flowing from the bottom to the top of the reactor. Deactivated quartz wool over the substrate (not shown) prevents the substrate from spreading in the reactor. Using a 3-way valve, the two parts (bypass and reactor) can be alternately connected to the GC-ECD and the GC/MSD.

equations.13-16 The explosive vapors in air were then produced by circulating air through the explosive U-shaped bed. By setting exactly the flow rate and temperature, we were able to achieve a controlled and accurate concentration of the explosive vapor flow at a quasi-steady state. Low generation temperature levels, up to a maximum value of 120 °C for RDX, avoided the risk of fusion or partial degradation of the explosives in their solid form. Material Sensing Efficiency Testing Reactor. The material testing system (Figure 2) was connected to the laboratory pilotscale explosive vapor generator. As a steady state was reached, controlled by the analysis of the outlet gas, the vapors entered the heated adsorption zone in order to test adsorption capacity,

selectivity, and detection abilities of several specific materials with respect to RDX and TNT explosive molecules. These materials consisted of the explosive sniffer systems to be tested and evaluated. Before entering this area, the explosive stream was diluted by means of a known and controlled airflow so as to decrease the explosive concentration by 1 order of magnitude. This area was maintained at a constant temperature of the order of 130 °C in order to prevent the condensation, stacking, or degradation of the explosive molecules in the system.18 Control and Validity of the Overall Device. Both systems were coupled to an online gas chromatograph with an electron capture detector (GC-ECD). This analysis technique, which is the most sensitive current analyzer for nitro-organic explosive molecules,1-4,19-22 was only used as an end-of-process technique to analyze the exhaust gas in order to validate both the generation and the testing systems. The GC-ECD theoretically makes it possible to reach detection limits (LODs) as low as 100 ppt for TNT and DNT, 300 ppt for RDX, 250 ppt for HMX, and between 1 and 4 ppb for the nitrate esters like PETN using manual injection port.19-22 Experimentally, using an automatic valve injection port without any concentration device, we achieved real LODs of 125 ppt for TNT and 595 ppt for RDX, both with a good reproducibility, and 1.4 ppb for PETN with a poor reproducibility. Although the GC-ECD analytical device is not sensitive enough to detect traces of explosive vapors under a 100 ppt level, at over-LOD level conditions of explosive concentrations, it allowed both (i) the calibration of the generation system and the verification of its accuracy and reproducibility and (ii) the calibration of sensitive substrates toward one or more explosive vapors and consequently the discrimination between these substrates. However, the explosive generator device could be easily used for generating lower concentrations to test materials at nanoscale. We have recently developed a protocol to perform accurate measurements at lower levels than GC-ECD LOD concentrations. This will be described in more details in a future article. The fraction of the flow which did not enter the GC-ECD was directed toward a GC with a mass spectrometer detector (GC/ MSD) so as to follow the signatures of the selected explosives23 and the products issued from the possible reaction or degradation of explosive molecules on the sensing materials under test. Experimental Testing Conditions. Preliminary experiments were conducted at the laboratory scale and consisted of studying the adsorption of a single explosive vapor at ppb and subppb levels. The aim was to validate the ability of our generation device to compare and discriminate between adsorption affinities and sensitivities of different solids exposed to trace levels of explosive vapor. Two solids were used for this validation: a porous tungsten trioxide (Aldrich, 31.6 m2/g mean particle diameter of 26.6 nm) as a sensing substrate and glass microbeads (Potters-Ballotini S.A., near-zero specific surface, mean particle diameter of (18) Peterson, P. K. Proc. Int. Symp. Anal. Detect. Explos. 1983, 391–395. (19) Wadell, R.; Dale, D. E.; Monagle, M.; Smith, S. A. J. Chromatogr., A 2005, 1062, 125–131. (20) Oxley, J. C.; Smith, J. L.; Kirschenbaum, L. J.; Schinde, K. P.; Marimganti, S. J. Forensic Sci. 2005, 50, 826–831. (21) Walsh, M. E.; Ranney, T. A. U.S. Army Corps of Engineers, Cold Regions Research & Engineering Laboratory Report SR 99-12; National Technical Information Service: Springfield, VA, 1999. (22) Walsh, M. E. Talanta 2001, 54, 427–438. (23) Syage, J. A.; Hanold, K. A. Trace Chemical Sensing of Explosives; John Wiley & Sons Inc.: Hoboken, NJ, 2007, pp 219-244.

Figure 3. Validation of the explosive vapor generating system using a GC-ECD. Peak 1 corresponds to the signal of CO2 in air. Peaks 2, 3, and 5 were attributed to 2,4-DNT, TNT, and RDX, respectively. Peak 4, corresponding to PETN, has a low signal reproducibility.24,25 The sixth peak is assumed to be an analysis artifact. The Y axis is not labeled as it is expressed in terms of ECD signal intensity (arbitrary unit).

106-212 µm) as a reference material as they were supposed to have no sensing efficiency. Vapor concentrations up to 54 ppb for TNT and 608 ppb for RDX were generated and pushed through the solid using a 50 cm3/min air flow. Adsorption experiments on either TNT or RDX vapors were conducted independently on these two substrates. A known mass of the studied material was introduced into the tubular quartz reactor. Before the adsorption experiment took place, the reactor and material were conditioned at 120 °C during 3 h in order to remove adsorbed water and impurities. Adsorption took place at a controlled temperature of 130 °C, preventing both the potential adsorption of the explosive molecules on the reactor walls and the degradation or modification of the adsorbing solid. We chose to end each adsorption experiment when the material was saturated with explosive molecules adsorbed on its porous structure and surface. As we had previously taken care not to be limited by diffusion phenomena, we were able to determine different characteristics related to the adsorption efficiency of the material and its selectivity and affinity with respect to one or a mixture of explosive vapors. After each set of experiments testing one explosive at several concentrations, the overall system was flushed under air flow conditions during one night in order to ensure the cleansing of the device before testing another explosive. RESULTS AND DISCUSSION Validation of the Generation System. Figure 3 shows the GC-ECD signal spectra of a mixture of 2,4-DNT, TNT, PETN, and RDX generated using the developed explosive vapor generation system. To ensure an optimal level of response from the ECD for each component and an accurate visibility of the overall explosives on the aforementioned figure, the mixture was heated up to 120 °C. Six peaks were observed. Peak 1 corresponds to the signal of CO2 in air. The simultaneous analysis by GC/MSD allowed the (24) Sigman, M. E.; Ma, C.-Y. ORNL Report ORNL/TM-1999/315; National Technical Information Service: Springfield, VA, 1999. (25) Sigman, M. E.; Ma, C.-Y. J. Forensic Sci. 2001, 46, 6–11.

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Table 1. Adsorption Efficiencies of 9 mg of WO3 toward TNT 12, 33, and 54 ppb Vapors and toward RDX 168, 323, and 608 ppb Vaporsa TNT b P

V CAdsc CMold

12 0.019 0.0016

33 0.039 0.0033

RDX 54 0.052 0.0044

168 0.74 0.064

323 1.40 0.120

608 1.69 0.146

a The adsorption efficiency of the substrate with respect to an explosive vapor was expressed either as the mass of explosive adsorbed per unit mass of substrate or as the number of molecules that could adsorb on one square nanometer active surface. b Vapor pressure of TNT or RDX in ppb. c Mass adsorption capacity of the material with respect to explosive vapor in µg/mg of solid. d Number of molecules adsorbed per nm2 of specific surface.

Figure 4. Breakthrough curves showing the explosive concentration in the air exiting the adsorption reactor during the experiments. The Y values were expressed in terms of ECD signal intensity, i.e., the number of counts (arbitrary unit). These experimental curves were obtained after performing adsorption tests of explosive vapors of (a) TNT at 12 ppb (cross), 33 ppb (full circles), and 54 ppb (empty triangles); and of (b) RDX at 168 ppb (cross), 323 ppb (full circles), and 608 ppb (empty triangles) on WO3.

potential identification of unknown compounds and of major specific ions for each molecule. Peaks 2 to 5 were attributed to 2,4-DNT, TNT, PETN, and RDX respectively, and after the calibration of the GC-ECD with known samples of each pure explosive. Peak 4, corresponding to PETN, had a low signal reproducibility. Sigman and Ma had already worked on the detection of PETN using both an ECD and an MSD.24,25 They found a higher relative standard deviation (RSD) for PETN than for the other explosives. They attributed this RSD value to its very weak thermal stability and low lifetime while retained on a support. The weak and nonreproducible signal of PETN was supposed to be due to its partial degradation in the system and in the retention column. The sixth peak was observed whatever the generated atmosphere and was attributed to an analysis artifact. The reproducible generation of explosive LOD levels, as described in the experimental part using an automatic valve injection port, was also verified while generating single explosive vapors. Substrate Sensing Efficiency for Explosive Detection. Figure 4 compares the experimental breakthrough curves obtained on the reference WO3 for either TNT or RDX flowing vapor exposure. The breakthrough curve is defined as the front formed by the outlet concentration exiting the sensing material when it begins to be saturated by adsorbed molecules.26 Breakthrough curves are characteristic of the saturation state of an adsorbent material and can be described by three different stages: (i) the solid is still adsorbing, i.e., there is no molecule exiting it; (ii) the solid begins to be saturated, resulting in an ascending phase called “breakthrough front”, at which point only a fraction of molecules driven through the solid can still adsorb on its active areas, while other molecules directly exit the solid without any adsorption or reaction on its surface; (iii) the solid is saturated, i.e., there is no more adsorption and a steady state is reached, at which time all the molecules entering the system directly exit without any adsorption. By integrating the area under the experimental breakthrough curve, it is (26) Bonnot, K. PhD Thesis, National Polytechnic Institute of Lorraine, 2003.

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possible to define the stoichiometric breakthrough time tS as the mean value of explosive breakthrough times. The value tS is directly related to the adsorption capacity of the solid, as described by eq 1.26

[

tS ) t0 1 +

FS ∆q · ε ∆C

]

(1)

t0 is the time necessary for air to break through the solid (s), FS is the density of the solid (kg m-3), ε is the apparent porosity of the bed, ∆C is the molar quantity of explosives in air (mol m-3), and ∆q is the solid adsorption capacity (mol kg-1). For none of the two tested explosives, glass microbeads showed adsorption during the 2 h long experiments, as was expected. On the contrary, the WO3 solid clearly showed adsorption but with a different trend for TNT (Figure 4a) and RDX (Figure 4b). It must be mentioned that the concentrations of TNT vapors were deliberately much lower than for RDX. These values were chosen first to ensure a high resolution of the RDX signal and, consequently, were limited for TNT to the partial pressure corresponding to the fusion of the solid, occurring at 70-80 °C. The comparison of the adsorption trends of TNT and RDX on the reference WO3 showed that TNT was poorly retained on the porous solid, whereas RDX was more strongly adsorbed. Despite the specific experimental conditions, it has been proven that the nano-WO3 solid had a weaker affinity with TNT than with the RDX adsorption. To verify this graphic assumption and to avoid the mass effect of the material in the reactor, the adsorption capacities of the WO3 reference solid with respect to TNT and RDX vapors were calculated using eq 1. Using these data, the number of explosive molecules that could theoretically adsorb on a square nanometer surface of porous WO3 was also determined (Table 1). The adsorption capacities showed nonlinear dependence on the explosive concentration with a correlation coefficient of 99.4% and 99.8% for RDX and TNT, respectively, according to a polynomial fit of the data. The obtained curves were consistent with a Langmuir-type adsorption behavior occurring in the fixed bed.26 Using the polynomial fit curve of the RDX data, the estimated value for the adsorption of a 54 ppb RDX vapor was 0.285 µg/mg, i.e., at least 5 times higher than the TNT adsorption capacity at the same concentration, as mentioned in Table 1.

By taking into account the graphical and calculated results, it has been demonstrated that the reference WO3 showed a higher selectivity and sensitivity for RDX than for TNT molecular sensing. To confirm that the mass of sensitive material used had no effect on the adsorption results, we carried out an experiment for a 323 ppb vapor of RDX on 5 mg of WO3 (instead of 9 mg in the previous ones) and compared the results with the values obtained with 9 mg (Table 1). The adsorption capacity and number of molecules adsorbed per unit area were 1.53 µg/mg and 0.132 molecule/nm2, respectively, and showed an RSD of 9.6% compared to the reference set. We assumed that this RSD was not significant enough to be attributed to a dependence of the adsorption capacity on the mass of sensitive material. CONCLUSIONS In conclusion, we developed, optimized, and validated in this study a system capable of generating controlled vapor concentrations in air of low vapor pressure explosives. The novelty consisted of (i) the calibration of the system at ppb level using existing analytical methods, thus allowing, after extrapolation, the generation of traces of explosive vapors in the ppt range or lower; (ii) the generation of energetic compound vapors less commonly studied than TNT and 2,4-DNT, such as RDX and PETN; and (iii) the possibility of discriminating between different kinds of adsorbing solids in terms of sensitivity and selectivity with respect

to explosive vapors, consequently allowing an easy and rapid screening of potential materials for explosive detector devices. This constitutes a valuable enabling technology for testing and calibrating explosive vapor detectors. To our knowledge, it is also the first time that such a complete and sensitive tool has been developed and has proved to be suitable for the calibration and testing of existing or future explosive sensing and sniffing material devices under real conditions for trace level concentrations of explosive mixtures. Furthermore, we are currently studying tools and methods using this complete system at lower levels. The aim would be to achieve the testing and calibration of nanosniffing detectors based on substrates usable as highly sensitive and selective portable detection devices which would allow the detection of very low partial pressure explosives in complex environments. ACKNOWLEDGMENT The authors are thankful to the Conseil Re´gional d’Alsace and to the DGA (Direction Ge´ne´rale de l’Armement) for providing financial support to this research (Contract REI DGA/MRIS 2009.34.024).

Received for review December 22, 2009. Accepted March 9, 2010. AC902930E

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