Identification of High Explosives Using Single-Particle Aerosol Mass

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Anal. Chem. 2007, 79, 1918-1925

Identification of High Explosives Using Single-Particle Aerosol Mass Spectrometry Audrey N. Martin,†,‡ George R. Farquar,*,† Eric E. Gard,† Matthias Frank,† and David P. Fergenson†

Lawrence Livermore National Laboratory, Livermore, California 94550, and Michigan State University, East Lansing, Michigan 48824

The application of single-particle aerosol mass spectrometry (SPAMS) to the real-time detection of micrometersized single particles of high explosives is described. Dualpolarity time-of-flight mass spectra from 1000 single particles each of 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazinane (RDX), and pentaerythritol tetranitrate (PETN), as well as those of complex explosives, Composition B, Semtex 1A, and Semtex 1H, were obtained over a range of desorption/ionization laser fluences between 0.50 and 8.01 nJ/µm2. Mass spectral variability with laser fluence for each explosive is discussed. The ability of the SPAMS system to identify explosive components in a single complex explosive particle (∼1 pg) without the need for consumables is demonstrated. Explosives from military and industrial sources, as well as improvised explosive devices (IEDs), have been used worldwide in terrorist events. Sensitive and specific detection of explosives is an analytical issue that is important for the prevention and attribution of terrorist activities. The ideal explosive detector would be able to identify explosive traces in the air, on clothing, materials, and the body of the terrorist with enough warning time and physical distance to minimize the damage the explosive could cause. The National Research Council Report on Opportunities to Improve Airport Passenger Screening with Mass Spectrometry describes detection of explosives in terms of bulk detection versus trace detection, and stand-off detection versus point detection.1 Several bulk explosives detectors rely on imaging including X-ray tomography and microwave imaging to identify quantities of explosives that pose a threat.1 These systems focus on detecting the actual explosive device by visual characteristics such as the shape of the explosive material or detonator.2 Other bulk detection techniques rely on the chemical composition of the explosive, such * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (925) 424-4275. † Lawrence Livermore National Laboratory. ‡ Michigan State University. (1) Opportunities to Improve Airport Passenger Screening with Mass Spectrometry; The National Academies Press: Washington, D.C., 2004. (2) Existing and Potential Standoff Explosives Detection Techniques; The National Academies Press: Washington, D.C., 2004. (3) Garroway, A. N.; Buess, M. L.; Yesinowski, J. P.; Miller, J. B.; Krauss, R. A. Proceedings of SPIE-International Society for Optical Engineering, San Diego, CA; SPIE: Bellingham, WA, 1994; pp 139-149. (4) Liu, H. B.; Chen, Y.; Bastiaans, G. J.; Zhang, X. C. Opt. Express 2006, 14, 415-423.

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as nuclear quadrupole resonance (NQR)3 and THz spectroscopy.4 THz spectroscopy is particularly appealing because the energy of the THz waves is much lower than that of X-rays and is thus safer for personnel.4 The main disadvantage of bulk detection systems is that they only trigger an alarm if an explosive device is present and cannot detect the residual quantities of an explosive that may be present on the clothing or skin of a perpetrator. Alternatively, trace detection is advantageous in that small amounts of explosives that may have been transferred to the skin or a bag of a would-be terrorist may be detected before the actual explosive device is used, but the detection of the actual device is limited. Stand-off detection, where the detector operates at a given distance from the target, provides additional time and distance to respond to the presence of an explosive before it is within an effective destruction range. Raman spectroscopy has been applied to the standoff detection of high explosives in a silica matrix from up to 50 m away.5 NQR detection has also been modified for standoff detection.6 Point detection, such as that used in airports, allows for analysis of a specific targeted region in space. For such systems, sampling must be simple, the analysis must be quick, and a low rate of false positives and false negatives in a variety of backgrounds is required. Trace detection of explosive compounds has recently been reviewed by Moore.7 Explosives are commonly identified using UV absorption,8,9 laser-induced fluorescence (LIF),10,11 immunoassay,12 ion mobility spectrometry (IMS),13-17 and mass spectrometry (ion trap and time-of-flight (TOF)).18-29 It should be noted that (5) Carter, J. C.; Angel, S. M.; Lawrence-Snyder, M.; Scaffidi, J.; Whipple, R. E.; Reynolds, J. G. Appl. Spectrosc. 2005, 59, 769-775. (6) Anferov, V. P.; Mozjoukhine, G. V.; Fisher, R. Rev. Sci. Instrum. 2000, 71, 1656-1659. (7) Moore, D. S. Rev. Sci. Instrum. 2004, 75, 2499-2512. (8) Oehrle, S. A. J. Chromatogr., A 1996, 745, 233-237. (9) Bailey, C. G.; Yan, C. Anal. Chem. 1998, 70, 3275-3279. (10) Bailey, C. G.; Wallenborg, S. R. Electrophoresis 2000, 21, 3081-3087. (11) Wallenborg, S. R.; Bailey, C. G. Anal. Chem. 2000, 72, 1872-1878. (12) Anderson, G. P.; Moreira, S. C.; Charles, P. T.; Medintz, I. L.; Goldman, E. R.; Zeinali, M.; Taitt, C. R. Anal. Chem. 2006, 78, 2279-2285. (13) Buryakov, I. A. J. Chromatogr., B 2004, 800, 75-82. (14) Ewing, R. G.; Miller, C. J. Field Anal. Chem. Technol. 2001, 5, 215-221. (15) Eiceman, G. A.; Stone, J. A. Anal. Chem. 2004, 76, 390A-397A. (16) Ewing, R. G.; Atkinson, D. A.; Eiceman, G. A.; Ewing, G. J. Talanta 2001, 54, 515-529. (17) Tam, M.; Hill, H. H. Anal. Chem. 2004, 76, 2741-2747. (18) Asano, K. G.; Goeringer, D. E.; McLuckey, S. A. Anal. Chem. 1995, 67, 2739-2742. (19) Syage, J. A.; Hanold, K. A.; Hanning-Lee, M. A. Proceedings of the 42nd Annual Meeting of the Institute of Nuclear Materials Management; Indian Wells, CA; INMM: Northbrook, IL, 2001; pp 1-8. 10.1021/ac061581z CCC: $37.00

© 2007 American Chemical Society Published on Web 01/24/2007

research into explosives detection is performed on gas-, liquid-, and/or solid-phase explosives and that the sampling methods and concepts of operation vary widely between the different techniques. The limits of detection for the various techniques are dependent on the sampling method and sample phase and therefore prevent a direct comparison. Ion mobility spectrometry (IMS) is commonly used as a detection system at airport security screening; however, it is limited in its application by sampling requirements. Samples are introduced to the IMS by wiping the exterior of a passenger’s baggage with a small wipe. This introduction method assumes that explosive residue would have been in contact with the wiped surface, that a sufficient quantity of explosive is transferred to the wipe, and that this quantity is within the limits of operation of the IMS instrument.1 General Electric’s EntryScan3 and Smiths Detection’s Ionscan Sentinel II are portal-based IMS systems currently deployed in airports that sample the air around a person standing in the portal, eliminating this need for a sample wipe to contact the explosive residue.30,31 In general, IMS lacks the ability to concurrently screen for a wide variety of explosive compounds, due to the optimization of several parameters, e.g., ionization conditions, for a particular set of compounds. IMS is also limited in its sensitivity, due to false positives, which requires the instrument to be operated at a higher detection threshold.1 Mass spectrometry has been a successful detection method for trace high explosives in the vapor phase and on surfaces. Perr et al. demonstrated increased selectivity and picogram detection limits for several explosives using gas chromatography-positive chemical ionization MS/MS with an ion trap.25 One limitation of such analyses is that chemicals important for forensic attribution of an explosive event can be destroyed in the heating step required for analysis. McLuckey et al. coupled glow discharge ionization with a quadrupole ion trap to perform MS/MS analysis of several vapor-phase explosives, using a tailored waveform to selectively accumulate target ions in the trap (see refs 18 and 29 and references therein). Ions attributed to 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazinane (RDX), and pentaerythritol tetranitrate (20) Evans, C. S.; Sleeman, R.; Luke, J.; Keely, B. J. Rapid Commun. Mass Spectrom. 2002, 16, 1883-1891. (21) Zhang, M.; Shi, Z.; Bai, Y.; Gao, Y.; Hu, R.; Zhao, F. J. Am. Soc. Mass Spectrom. 2006, 17, 189-193. (22) Holmgren, E.; Carlsson, H.; Goede, P.; Crescenzi, C. J. Chromatogr., A 2005, 1099, 127-135. (23) Mullen, C.; Irwin, A.; Pond, B. V.; Huestis, D. L.; Coggiola, M. J.; Oser, H. Anal. Chem. 2006, 78, 3807-3814. (24) Takats, Z.; Cotte-Rodriguez, I.; Talaty, N.; Chen, H.; Cooks, R. G. Chem. Commun. (Cambridge) 2005, 1950-1952. (25) Perr, J. M.; Furton, K. G.; Almirall, J. R. Talanta 2005, 67, 430-436. (26) Cotte-Rodriguez, I.; Takats, Z.; Talaty, N.; Chen, H.; Cooks, R. G. Anal. Chem. 2005, 77, 6755-6764. (27) Cody, R. B.; Laramee, J. A.; Durst, H. D. Anal. Chem. 2005, 77, 22972302. (28) Hankin, S. M.; Tasker, A. D.; Robson, L.; Ledingham, K. W. D.; Fang, X.; McKenna, P.; McCanny, T.; Singhal, R. P.; Kosmidis, C.; Tzallas, P.; Jaroszynski, D. A.; Jones, D. R.; Issac, R. C.; Jamison, S. Rapid Commun. Mass Spectrom. 2002, 16, 111-116. (29) McLuckey, S. A.; Goeringer, D. E.; Asano, K. G.; Vaidyanathan, G.; Stephenson, J. L., Jr. Rapid Commun. Mass Spectrom. 1996, 10, 287-298. (30) All Clear; General Electric: http://www.ge.com/stories/en/20349.html, Apr. 25, 2005. (31) IONSCAN SENTINEL II; Smiths Detection: http://trace.smithsdetection.com/products/Default.asp?Product)24, 2002. (32) Gillen, G.; Mahoney, C.; Wight, S.; Lareau, R. Rapid Commun. Mass Spectrom. 2006, 20, 1949-1953.

(PETN) were identified in a sub-parts-per-billion vapor sample.29 Recently, the Cooks group has developed desorption electrospray ionization (DESI) using an ion trap and have applied it for the sampling and detection of explosives from material surfaces.24,26 In DESI, an explosive sample on a solid is exposed to an electrospray of solvent, typically water/methanol, which creates secondary ions that are introduced into the mass analyzer. This technique has achieved limits of detection in the laboratory setting as low as 5 pg for RDX on a paper sample.26 A significant benefit of DESI is that the sampling region can operate at atmospheric pressure which is advantageous for point detection; however, it requires consumables in the form of the electrosprayed solvents and is not capable of directly analyzing aerosolized particles. Gillen et al. analyzed explosive particles deposited on a silicon surface using secondary ion mass spectrometry.32 By using a C8 cluster primary ion beam, they were able to detect parent ions of TNT as well as adducts of TNT, RDX, and PETN. Hankin et al. have applied femtosecond ionization to the detection of explosives using a time-of-flight mass analyzer.28 By desorbing a solid sample with a 5 ns, 266 nm laser pulse, followed by an 80 fs, 800 nm ionization laser pulse, they were able to generate an increased amount of intact molecular ion versus nanosecond ionization schemes. Mullen et al. have employed single-photon ionization using 118.2 nm light from the frequency-tripled third harmonic output of an Nd:YAG laser for the detection of several gas-phase explosives using a time-of-flight mass analyzer.23 Analysis of TNT yielded the parent molecular ion almost exclusively; however, the technique is limited to molecules with ionization energies below 10.49 eV (118.2 nm).23 Syagen Technology, Inc. has collaborated with Sandia National Laboratory to combine a preconcentrating particle sampling portal, similar to those used with IMS and described above, with an air discharge ionization quadrupole ion trap-TOF (QitTOF) MS for the point detection of trace explosives. Detection limits in the picogram range were reported for the QitTOF-MS using TNT, RDX, and PETN.19 On-line single-particle aerosol mass spectrometry (SPAMS) has been reviewed by Noble and Prather33 beginning with the initial work of Davis.34 Over the past decades, SPAMS has evolved to include particle sizing,35 analysis in various environments,36,37 and simultaneous acquisition of positive ion and negative ion spectra.37 Dale et al. demonstrated the detection of 2,4,6-trinitrotoluene (TNT) adsorbed on silicon carbide beads using a single-particle quadrupole ion trap mass spectrometer.38 Time-of-flight SPAMS at Lawrence Livermore National Laboratory (LLNL) in the form of bioaerosol mass spectrometry (BAMS) has been applied to the detection of various species of Bacillus spores,39-45 viruses,39 toxin simulants,39 fungi,41 and Mycobacteria,45 with and without the use of a chemical matrix during ionization. Matrix-free systems are (33) Noble, C. A.; Prather, K. A. Mass Spectrom. Rev. 2000, 19, 248-274. (34) Davis, W. D. Environ. Sci. Technol. 1977, 11, 587-592. (35) Sinha, M. P. Rev. Sci. Instrum. 1984, 55, 886-891. (36) Noble, C. A.; Prather, K. A. Environ. Sci. Technol. 1996, 30, 2667-2680. (37) Gard, E.; Mayer, J. E.; Morrical, B. D.; Dienes, T.; Fergenson, D. P.; Prather, K. A. Anal. Chem. 1997, 69, 4083-4091. (38) Dale, J. M.; Yang, M.; Whitten, W. B.; Ramsey, J. M. Anal. Chem. 1994, 66, 3431-3435. (39) Steele, P. T.; Srivastava, A.; Pitesky, M. E.; Fergenson, D. P.; Tobias, H. J.; Gard, E. E.; Frank, M. Anal. Chem. 2005, 77, 7448-7454. (40) Srivastava, A.; Pitesky, M. E.; Steele, P. T.; Tobias, H. J.; Fergenson, D. P.; Horn, J. M.; Russell, S. C.; Czerwieniec, G. A.; Lebrilla, C. B.; Gard, E. E.; Frank, M. Anal. Chem. 2005, 77, 3315-3323.

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ideal for real-time and on-line detection systems, as no sample preparation and thus very few reagents are needed for the analysis. The BAMS system has the ability to detect a small number of particles against a large background, creating the selectivity of the method. SPAMS also provides real-time detection via an automated data analysis system which could allow for early warning of a potential threat. By detecting single particles in the micrometer size range (picograms) with high throughput, these systems are capable of detecting trace quantities and allowing differentiation of signal from the background environment. An additional advantage of the SPAMS system is that it can be applied to the detection of biological agents, chemical agents, and now high explosives without modification, making it a single instrument suitable for point detection and capable of monitoring an environment for a wide variety of threats. The work presented here focuses on the application of SPAMS to the detection of single particles of high explosives for use as a trace point detector. Optimal experimental parameters for biological samples were used for the initial analyses, followed by individual studies of several explosives at various laser energies. TNT was studied as crushed dry particles and nebulized samples, demonstrating the reliability of nebulized samples as a surrogate for dry samples in terms of particle size and mass spectral response. The production of TNT particles by crushing is a real world pathway for explosives particle production and demonstrates the applicability of the technique to real world detection. Finally, TOF mass spectra were obtained from mixed explosives samples to determine if the SPAMS explosives detection system could identify individual explosives in a complex mixture. MATERIALS AND METHODS Sample Preparation. 2,4,6-Trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazinane (RDX), pentaerythritol tetranitrate (PETN), Composition B (Comp B), Semtex 1A, and Semtex 1H were obtained as solids from the High Explosives Applications Facility (HEAF) at LLNL. Stock solutions were made by dissolving approximately 10 mg of explosive in 1 mL of acetone (reagent grade, Sigma-Aldrich, St. Louis, MO). For aerosolization, a 25 µL aliquot of the stock solution was diluted in 10 mL of methanol (Burdick & Jackson, biotechnology grade 99.9+%, Muskegon, MI) in a glass Collison nebulizer (BGI Inc., CN-25, 27). A control blank solution was made by the addition of 25 µL of acetone (Burdick & Jackson, HPLC grade 99.9+%) to 10 mL of methanol. The aerosols were generated by connecting the Collison nebulizer to a house nitrogen line. (41) Fergenson, D. P.; Pitesky, M. E.; Tobias, H. J.; Steele, P. T.; Czerwieniec, G. A.; Russell, S. C.; Lebrilla, C. B.; Horn, J. M.; Coffee, K. R.; Srivastava, A.; Pillai, S. P.; Shih, M.-T. P.; Hall, H. L.; Ramponi, A. J.; Chang, J. T.; Langlois, R. G.; Estacio, P. L.; Hadley, R. T.; Frank, M.; Gard, E. E. Anal. Chem. 2004, 76, 373-378. (42) Steele, P. T.; Tobias, H. J.; Fergenson, D. P.; Pitesky, M. E.; Horn, J. M.; Czerwieniec, G. A.; Russell, S. C.; Lebrilla, C. B.; Gard, E. E.; Frank, M. Anal. Chem. 2003, 75, 5480-5487. (43) Czerwieniec, G. A.; Russell, S. C.; Tobias, H. J.; Pitesky, M. E.; Fergenson, D. P.; Steele, P.; Srivastava, A.; Horn, J. M.; Frank, M.; Gard, E. E.; Lebrilla, C. B. Anal. Chem. 2005, 77, 1081-1087. (44) Tobias, H. J.; Pitesky, M. E.; Fergenson, D. P.; Steele, P. T.; Horn, J.; Frank, M.; Gard, E. E. J. Microbiol. Methods, in press. (45) Tobias, H. J.; Schafer, M. P.; Pitesky, M.; Fergenson, D. P.; Horn, J.; Frank, M.; Gard, E. E. Appl. Environ. Microbiol. 2005, 71, 6086-6095.

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Instrument Parameters. The SPAMS instrument used for these experiments has been described previously.41 Briefly, the aerosol particles created with the Collison nebulizer pass through a diffusion drier to remove moisture and allow the solvent time to evaporate from the particles. Nebulization was chosen in an attempt to keep the particle size distribution narrow. Particles are then introduced to the top of the SPAMS via a converging nozzle, pressure flow reducer, and an aerodynamic focusing lens which focus the particles into the center of the instrument and reduce the pressure for introduction to the mass analyzer. The particles then pass through a tracking region which consists of up to six lasers (continuous wave (CW), 660 nm) and photomultiplier tubes, three of which were used in the current study, which measure the light scattered off a particle as it passes through the beam of each consecutive laser. The time between light scattering events is used to determine the particle’s velocity and aerodynamic diameter via software and a field-programmable gate array. Using the velocity information, the electronics trigger the desorption/ ionization (D/I) laser to fire when the particle has reached the source region of the mass spectrometer. The D/I laser is a Q-switched frequency-quadrupled Nd:YAG laser (Ultra CFR, Big Sky Laser Technologies, Inc.) operating at 266 nm with 7 ns (full width at half-maximum (fwhm)) pulses. The D/I laser energy is adjustably attenuated prior to reaching the ion source region by passing through a rotatable half-wave plate and vertical polarizer. The transmitted light is focused by a 10 cm plano-convex focusing lens to an approximately 330 µm diameter spot at the point where it intersects the particle beam. Laser pulse energy is controlled by adjustment of the rotation of the half-wave plate and is measured at the exit of the ion source chamber with a laser power meter (Coherent, Inc., J25LP-MUV) with digital readout. Prior to the presented experiments, the laser power meter was calibrated with and without the laser exit optical window to account for its absorbance. Power measurements are determined for each experiment and are reported as an average of 50 pulses and corrected for the measured energy loss in the back window. Fluence is a measure of the laser energy per unit of the beam and was calculated by dividing the net laser energy by the crosssectional area of the laser beam (330 µm diameter). Both positive and negative ions were generated in the source region and simultaneously introduced in opposite directions to two reflectronTOF mass analyzers. Ions were detected via microchannel plates (Burle Technologies, Inc.), and the signal was digitized (Signatec, Inc., PDA1000) at 8 bits with 256 arbitrary units (au) on a 330 mV scale. The mass spectra were analyzed using software developed in-house. Safety Considerations. All explosives used in this study are secondary explosives and should be handled with care. No more than 10 mg of each explosive was present in the laboratory, and whenever possible, samples were used and stored in solution. RESULTS AND DISCUSSION TNT, RDX, and PETN are common explosives that were selected for analysis based on their structural class: nitroaromatic, nitramine, and nitrate ester, respectively (Figure 1). Although the explosives were aerosolized from a methanol solution, the diffusion drier installed before the sample introduction port of the SPAMS removed the moisture and allowed the solvent time to evaporate. The single dried particles of explosive

Figure 1. Chemical structures of 2,4,6-trinitrotoluene (TNT), 1,3,5trinitro-1,3,5-triazinane (RDX), and pentaerythritol tetranitrate (PETN).

were then sampled into the instrument. A dual-polarity mass spectrum of an individual particle was obtained in a fraction of a second, allowing for a large number of particles to be analyzed in a short time. Figure 2a shows the concatenated positive and negative mass spectra obtained from five consecutively analyzed single particles of TNT (aerodynamic diameter, 1.08 µm; relative standard deviation (RSD), 5.0%) with a laser fluence of 0.70 nJ/ µm2. These particles were analyzed in a total time of 2 s. Although the mass spectra were obtained over a large m/z window, only the m/z range containing observed peaks is shown (except inset). Slight shot-to-shot variations in peak height are present from particle to particle, but the diagnostic peaks for TNT are present in all spectra and are discussed below. Figure 2b shows a spectrum obtained from a single solid particle of TNT (aerodynamic diameter, 1.28 µm) with a laser fluence of 0.75 nJ/µm2, directly introduced to the instrument from a crushed powder without the use of a solvent. This mimics more closely the manner in which explosives particles would be detected in a real-world application, such as airport baggage screening, where particles would be directly sampled. The particle size is similar to that of the nebulized sample, and as seen in the figure, the single solid particle spectrum contains the same peaks in the negative spectrum that are used to identify TNT as the nebulized particles, as well as several peaks in the positive ion spectrum (m/z 197, 210, 226) that are not present in the nebulized sample but are also indicative of TNT. Because of the similarity of the spectra and the suitability of either sample for the evaluation of highexplosives detection using SPAMS, either aerosolization method could have been used for subsequent experiments. For reasons of safety, ease of aerosolization, and particle size consistency, particles from nebulized solutions were used for the remainder of the experiments. Mass spectra were obtained for 1000 single particles of TNT with a laser fluence of 0.70 nJ/µm2. The average aerodynamic diameter of the 1000 detected TNT particles was 1.06 µm (RSD, 38.4%). Aerodynamic diameter is reported for all compounds due to the unknown density of the composite explosives analyzed below. The particle size histogram was fit to a Gaussian curve (R2 ) 0.960) and z values were used to eliminate 27 outlier particles at a 99.99% level of confidence. Removal of these particles from the size analysis yielded an average aerodynamic diameter of 1.00 µm (RSD, 10.8%). To compare the consistency of the spectra, the peak area for the [TNT-H]- peak at m/z -226 was calculated for each particle over a three m/z window to include all parent ion contributions as an average of 1468.3 au, RSD ) 38.2%, resolution ∼ 430 (1208.9 au, RSD ) 37.8% with outliers removed). One possible source of some of this error could be due to the variation in the energy present in the laser beam. Previous research in our group has shown that the optics used

Figure 2. (a) Single-particle mass spectra of five consecutive single TNT particles (aerodynamic diameter 1.08 ( 0.05 µm) obtained at a laser fluence of 0.70 nJ/µm2. These spectra for successive particles are shown to present the typical shot-to-shot variation of the technique (the following figures contain averages). (b) Single-particle mass spectrum of a single TNT particle introduced into the SPAMS system from a crushed powder with no solvent, obtained at 0.75 nJ/µm2. The nebulized TNT particles contain information similar to that of the TNT particle introduced to the system from powdered TNT.

in focusing the D/I laser create a laser profile that has a roughly “flat top” distribution of energy;39 however, particles that are focused near the edges of the beam may experience a lower D/I energy.42 Also, the laser fluence of 0.70 nJ/µm2 is an average value of 50 consecutive laser pulses, so variation exists shot-to-shot, although it is typically less than 5%. The effect of laser energy on the desorption/ionization of TNT was investigated. Average mass spectra of 1000 individual TNT particles, each obtained with laser fluences of 0.70, 1.51, 2.51, 5.21, and 6.99 nJ/µm2, are shown in Figure 3. The average particle’s aerodynamic diameter was 1.05 µm (RSD, 7.2%) for all laser powers. As expected for a nitro aromatic explosive, the molecular ion is present in the spectra.19 At 0.70 nJ/µm2, the [TNT-H]- is most prevalent, seen at m/z -226. As the laser power is increased, this parent ion is fragmented, resulting in a decreasing parent ion peak intensity. At high fluences, the [TNT-H]- ion is not the major peak in the spectrum; however, it is still visible in the spectrum obtained, even at 6.99 nJ/µm2. The increased fragmentation is apparent in the higher laser energy spectra, where numerous peaks appear due to loss of nitrate groups (m/z -46, +30, +63, +72, +89) and the breaking of the aromatic ring (m/z -197, -66, -42, -26, +81). Interestingly, the spectra taken at 0.70 nJ/µm2 also contain peaks at m/z well above the mass of TNT (inset, Figure 3). These peaks are possibly due to the dimerization of TNT: [2(TNT-H) + Na]Analytical Chemistry, Vol. 79, No. 5, March 1, 2007

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Figure 3. Single-particle mass spectra of aerosolized TNT at laser fluences of 0.70, 1.51, 2.51, 5.21, and 6.99 nJ/µm2 (note differences in y-axes). Each spectrum is the average of 1000 single-particle spectra. The average particle was 1.05 ( 0.08 µm in aerodynamic diameter. Note the [TNT-H]- peak at m/z -226 is present at all laser fluences.

Figure 5. Single-particle mass spectra of aerosolized PETN at laser fluences of 0.57, 1.33, 2.88, 5.39, and 7.95 nJ/µm2 (note differences in y-axes). Each spectrum is the average of 1000 single-particle spectra. The average particle’s aerodynamic diameter was 1.10 ( 0.12 µm. Note the [PETN + NO3]- peak at m/z -378 is present at all laser fluences.

Figure 6. Single-particle mass spectra of aerosolized TNT, Comp B (63% TNT, 35% RDX), and RDX at 0.70, 0.50, and 0.57 nJ/µm2, respectively (note differences in y-axes). Each spectrum is the average of 1000 single-particle spectra. Dashed lines indicate mass peaks that are present in multiple compounds. Note the presence of peaks from TNT and RDX in the Comp B spectrum. Figure 4. Single-particle mass spectra of aerosolized RDX at laser fluences of 0.57, 1.39, 2.63, 4.83, and 7.80 nJ/µm2 (note differences in y-axes). Each spectrum is the average of 1000 single-particle spectra. The average particle was 1.00 ( 0.03 µm in aerodynamic diameter. Note the [RDX + NO3]- peak at m/z -284 is present at all laser fluences.

(m/z -475), [2TNT-NO2-H + K]- (m/z -446), and [2TNTNO2-2H]- (m/z -406). Several other high mass peaks, attributed to [TNT-CH3]-, [TNT-OH]-, and [TNT-NO]- (m/z -212, -210, and -197, respectively) that are distinct in the low-energy TNT spectra are greatly reduced at high energies, presumably 1922 Analytical Chemistry, Vol. 79, No. 5, March 1, 2007

due to the extensive fragmentation. Thus, spectra obtained with lower D/I laser fluences allow for the more facile identification of TNT, due to the presence of the parent ion and other high mass peaks. Average mass spectra of 1000 individual RDX particles at laser fluences of 0.57, 1.39, 2.63, 4.83, and 7.80 nJ/µm2 are shown in Figure 4. The average particle’s aerodynamic diameter was 1.00 µm (RSD, 2.7%) for all laser powers. The parent ion is not present in either the positive or the negative spectrum; however, several peaks can be used to identify RDX. The [RDX + NO3]- ion is the

Table 1. Summary of Mass Spectral Data from Comp B, Semtex 1A, and Semtex 1H Obtained at Laser Fluences of 0.50, 0.51, and 0.57 nJ/µm2, Respectivelya Comp B m/z TNT

-475 -446 -406 -226 -212 -210 -197

Semtex 1A

compound

m/z

compound

[2(TNT-H) + [2TNT-NO2-H + K][2TNT-NO2-2H][TNT-H][TNT-CH3][TNT-OH][TNT-NO]RDX fragment [RDX-2(NO2) + H]RDX fragment [RDX-CH2NNO2-NO2][RDX-3(NO2) + 2H]+ RDX fragment RDX fragment

PETN

-378 -333

[PETN + NO3][PETN + OH]-

Other

-147 -131 -62 -46 117

PETN fragment PETN fragment NO3NO2unknown

a

-62 -46 178

m/z

compound

Na]-

RDX -147 -131 -115 -102 86 104 108

Semtex 1H

NO3NO2TNT or RDX

-284 -268 -147 -131

[RDX + NO3]RDX + NO2]RDX fragment [RDX-2(NO2) + H]-

-102 86

[RDX-CH2NNO2-NO2][RDX-3(NO2) + 2H]+

-378 -333 -317 -147 -131 -62 -46 117

[PETN + NO3][PETN + OH][PETN + H]PETN fragment PETN fragment NO3NO2unknown

Peak position (m/z) and the suggested origin of the peak are shown.

Figure 7. Single-particle mass spectra of aerosolized PETN and Semtex 1A at laser fluences of 0.57 and 0.51 nJ/µm2, respectively (note differences in y-axes). Each spectrum is the average of 1000 single-particle spectra. Dashed lines indicate mass peaks that are present in both compounds.

most prominent in the 0.57 nJ/µm2 spectrum (m/z -284), but its intensity decreases as the D/I laser energy increases. The same trend is seen for [RDX + NO2]- and [RDX + Cl]- (m/z -268, -257, respectively). Possibly, as the laser energy increases, less RDX remains intact to react and form these RDX adducts. At a laser fluence of 7.80 nJ/µm2 the peak at m/z -257 is not present; however a new peak at m/z -255 is seen. The source of this peak it unknown, although it is not due to a chlorine adduct since no isotopic pattern is present. The peak at m/z -102, due to [RDXCH2NNO2-NO2]-, is typically seen in the mass spectra of nitramines19 and is present in the RDX spectra obtained with all laser fluences. The most prominent peak in the positive ion spectra is attributed to [RDX- 3(NO2)]+ (m/z +86). The intensity of this peak increases almost a factor of 4 from laser fluence 0.57 to 1.39 nJ/µm2 and then decreases as the D/I laser energy is increased further. It is possible that there is not enough energy at the lowest laser setting to completely remove all the nitro groups from the

Figure 8. Single particle mass spectra of aerosolized PETN, Semtex 1H, and RDX at a laser fluence of 0.57 nJ/µm2 (note differences in y-axes). Each spectrum is the average of 1000 single particle spectra. Dashed lines indicate mass peaks that are present in multiple compounds. Note the presence of peaks due to PETN and RDX in the Semtex 1H spectrum.

RDX. This hypothesis is supported by the increase in intensity of the [NO3]- and [NO2]- as laser fluence is increased. The decrease in intensity of the m/z +86 peak as the laser power is further increased may be due to more extensive fragmentation. Several other peaks show the same trend, such as [RDX-2(NO2) + H](m/z -131). A peak at m/z -46 due to the characteristic NO2- of explosives is present in the negative spectrum of RDX, as well as in the negative spectrum of TNT. Average mass spectra of 1000 individual PETN particles at laser fluences of 0.57, 1.33, 2.88, 5.39, and 7.95 nJ/µm2 are shown in Figure 5. Analytical Chemistry, Vol. 79, No. 5, March 1, 2007

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The average particle’s aerodynamic diameter was 1.10 µm (RSD, 10.6%) for all laser powers. A small but significant signal from the parent ion is seen at m/z -317 at a fluence of 0.57, 5.39, and 7.95 nJ/µm2. The most prominent peak at all laser fluences is attributed to [NO3]- at m/z -62, due to the presence of nitrate esters in PETN.19 The [PETN + NO3]- complex can be seen at m/z -378, and its intensity increases with laser energy, allowing more facile identification of PETN when using higher D/I laser energies. A [PETN + NO2]- complex is also seen at m/z -362, as well as [PETN + OH]- (m/z -333), both of which follow the same trend toward higher laser fluences. Few fragments appear in the spectrum due to the symmetry of the molecule, and they are more prevalent at higher laser fluences. The main fragments may be [C3H3N2O5]- and [C3H3N2O4]- occurring at m/z -147 and -131, respectively. Several plastic explosive compositions were also analyzed by SPAMS to test the ability of the instrument to identify individual explosives within a particle of mixed composition. Composition B (Comp B) is approximately 63% RDX, 35% TNT, and 1% wax or binder to make the mixture safer to handle by preventing friction detonation.46 Mass spectra were obtained at 0.50, 1.47, 2.77, 5.34, and 8.01 nJ/ µm2. The average particle’s aerodynamic diameter was 1.00 µm (RSD, 5.0%) for all laser powers. Figure 6 shows the average spectra of Comp B taken at a laser fluence of 0.50 nJ/ µm2 compared to the average spectra of TNT at 0.70 nJ/µm2 and RDX at 0.57 nJ/µm2. Dashed lines identify mass peaks present in the Comp B spectrum that are attributed to each component (TNT or RDX). The relative sensitivity of the detector to each explosive was beyond the scope of this work. Multiple peaks in the Comp B spectrum match peaks present in TNT; in fact, the [TNT-H]ion is the most prevalent peak in the Comp B spectrum. The TNT dimer compounds, [2(TNT-H) + Na]- (m/z -475), and [2TNTNO2-H + K]- (m/z -446) are also visible in the Comp B spectrum. Table 1 summarizes the mass peaks present in the Comp B spectrum as well as their possible origins. There are several peaks in the Comp B spectrum that cannot be identified as either TNT or RDX, such as m/z +92, +124, +164, and +180. These peaks may be due to an unknown complex between TNT and RDX fragments or the plasticizers present in the compound. Figure 7 displays an average mass spectrum of Semtex 1A taken with 0.51 nJ/µm2 laser fluence. Semtex 1A is a common plastic explosive developed in the 1960s in the former Czechoslovakia that is made from a combination of PETN (83.5%) and plasticizers (16.5%). Mass spectra were obtained at fluences of 0.51, 1.46, 2.70, 5.19, and 7.78 nJ/µm2. The average particle’s aerodynamic diameter was 1.07 µm (RSD, 4.7%) for all laser fluences. Figure 7 also shows the average mass spectra of PETN at 0.57 nJ/µm2, with the dashed lines identifying mass peaks present in the Semtex 1A sample and the pure PETN sample. Table 1 summarizes the origins of the peaks present in the Semtex 1A mass spectra. The peak in the Semtex 1A spectrum at m/z +117 is not present in the pure PETN sample. One possibility for the origin of this peak is the presence of various plasticizers in the Semtex 1A sample which may cause additional (46) Central Forensic Science Laboratory, Ministry of Home Affairs, Government of India, 2004.

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fragmentation of the PETN or may fragment themselves. An additional composition of Semtex, Semtex 1H, was also analyzed with the SPAMS instrument. Semtex 1H contains PETN (25%) and RDX (60.5%), along with oil and plasticizers (14.4%) for stability. Mass spectra were obtained at 0.57, 1.43, 2.66, 5.34, and 7.83 nJ/µm2. The average particle’s aerodynamic diameter was 1.08 µm (RSD, 12.8%) for all laser fluences. The average mass spectrum of Semtex 1H obtained with a laser fluence of 0.57 nJ/ µm2 is shown in Figure 8 compared to the spectra of PETN and RDX obtained at 0.57 nJ/µm2. Although the relative sensitivity of SPAMS to each explosive was beyond the scope of this work, several peaks were qualitatively attributed to each component (dashed lines), and the data are summarized in Table 1. The analysis of the composite explosive samples highlights a benefit of the SPAMS single-particle analysis system. Information on the specific components of an individual particle can be obtained. In comparing the spectra from Semtex 1A and Semtex 1H, the presence of the peaks due to RDX corroborates the information that the 1H variety also contains a percentage of RDX, while the 1A does not. As composite explosives such as Semtex are commonly used in terrorist activities, the ability to identify the composition of these materials may provide information to aid in the investigation and attribution of terrorist attacks. CONCLUSIONS This study demonstrates the application of the SPAMS to the detection of high explosives. To the authors’ knowledge, this study marks the first single-particle detection of micrometer-sized high explosives particles via dual-polarity time-of-flight mass spectrometry. Parent ions were detected in individual samples of TNT (m/z -226) and PETN (m/z -317), as well as explosive mixtures containing as little as 50% of each compound. While no parent ion was detected for RDX, several adducts were seen, such as [RDX + NO3]- which were also detected in explosive mixtures. Analyses of several composite explosives highlighted the potential of the SPAMS system to identify the individual explosive components in a single particle of mixed composition. Although the peak intensities varied, mass spectral peaks that provide information on the explosive components of a particle were seen for all explosives studied and over a range of laser fluences. This demonstrates the ability of SPAMS to function as a common detector for the six explosives queried in this study without manipulating the operating conditions (e.g., laser fluence), as is necessary in IMS.1 At a laser fluence of approximately 0.6 nJ/ µm2, all six explosives could be identified by mass peaks due to the parent ion or an adduct. Once a positive signal was obtained for an explosive at this low fluence, the D/I laser energy could then be tuned to provide more information on the nature of the subsequent particles of that explosive via fragmentation. SPAMS is sensitive, specific, reliable, and reagent-free and may provide a viable option for airport passenger and baggage screening. The ability of the SPAMS system to determine the identity of a single particle is a valuable asset when the target analyte is dangerous in small quantities or has no legal reason for being present in an environment. Since the presence of a particular explosive compound can be identified from just one particle (∼1 pg), the SPAMS system is an excellent system for differentiating explosive particles from nonrisk particles. Also, the unique information in the mass spectra of the explosives studied

(e.g., parent ion peaks) demonstrates the high specificity of the technique; it is not limited by threshold settings such as the IMS detectors currently used in airport screening.1 The simultaneous acquisition of positive and negative ion spectra provides complementary information that is useful for differentiating target compounds. This study, combined with the previous work of our group, describes a method of detecting biological, chemical, and explosive threats with one instrument with no modification, making it a prototype universal point detection system. ACKNOWLEDGMENT We would like to thank Raul Garza of LLNL for providing the explosive samples. We would also like to thank Paul Steele of LLNL for software development. The development of the advanced BAMS system at LLNL was funded through the LLNL LaboratoryDirected Research and Development Grant 02-ERD-002 and through DARPA and TSWG in the Department of Defense. This

work was performed under the auspices of the U.S. Department of Energy (DOE) by University of California, Lawrence Livermore National Laboratory under Contract W-7405-ENG-48. A.N.M. performed this research while on appointment as a U.S. Department of Homeland Security (DHS) Fellow under the DHS Scholarship and Fellowship Program, administered by the Oak Ridge Institute for Science and Education (ORISE) for DHS through an interagency agreement with DOE. ORISE is managed by Oak Ridge Associated Universities under DOE Contract No. DE-AC05-06OR23100. All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of DHS, DOE, or ORISE.

Received for review August 23, 2006. Accepted December 20, 2006. AC061581Z

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