Determination of Peroxide-Based Explosives Using Liquid

Colesniuc, Jeongwon Park, Ivan K. Schuller, Andrew C. Kummel, and William C. Trogler. Journal of the American Chemical Society 2008 130 (12), 3712...
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Anal. Chem. 2006, 78, 8150-8155

Technical Notes

Determination of Peroxide-Based Explosives Using Liquid Chromatography with On-Line Infrared Detection Rasmus Schulte-Ladbeck,†,‡ Andrea Edelmann,§ Guillermo Quinta´s,§ Bernhard Lendl,*,§ and Uwe Karst†,|

Chemical Analysis Group and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands, Institute for Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9-164, A-1060, Vienna, Austria, and Institute of Inorganic and Analytical Chemistry, University of Mu¨nster, Corrensstrasse 30, 48149 Mu¨nster, Germany

A nondestructive analytical method for peroxide-based explosives determination in solid samples is described. Reversed-phase high-performance liquid chromatography in combination with on-line Fourier transform infrared (FT-IR) detection is used for the analysis of triacetonetriperoxide (TATP) and hexamethylenetriperoxide diamine (HMTD). In contrast to other liquid chromatographic methods with optical detection, no derivatization or decomposition of the peroxides is required. The peroxides are identified and quantified via their characteristic absorption spectra in the mid-infrared range of the electromagnetic spectrum. The detection limit of 0.5 mmol L-1 for HMTD and 1 mmol L-1 for TATP allows the identification of the explosives in complex matrixes. In recent years, an increasing use of homemade explosives has been observed by legal authorities. Triacetonetriperoxide (TATP) and hexamethylenetriperoxide diamine (HMTD) are two substances that have found significant illegal use. They are easy to synthesize, and the starting materials are readily available. The occurrence of peroxide-based explosives is observed in different areas. The most important and alarming recent reports are situated in the field of terrorism,1,2 the latest being the bombing of London subway trains using TATP-based explosives.3 Nevertheless, the potential danger in accidents caused by amateur chemists4 and also the use of these substances in drug crimes5 is not to be overlooked. As the number of cases coming to court is increasing, * To whom correspondence should be addressed. E-mail: blendl@ mail.zserv.tuwien.ac.at. Tel: ++43 1 5880115140/Fax: ++43 1 5880115199. † University of Twente. ‡ Current address: Bundeskriminalamt, KT 16, 65173 Wiesbaden, Germany. § Vienna University of Technology. | University of Mu ¨ nster. (1) The Independent, London 1996, (Oct 8), 7. (2) Cooper, R. T. Los Angeles Times 2001, (Dec 29), A12. (3) Cotte-Rodrı´guez, I.; Chen, H.; Cooks, G. Chem. Commun. 2006, 953-955. (4) Evans, H. K.; Tulleners, F. A. J.; Sanchez, B. L.; Rasmussen, C. A. J. Forensic Sci. 1986, 31 (3), 1119-1125. (5) White G. M. J. Forensic Sci. 1992, 37, 652-656.

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it is especially important to have multiple methods for the unambiguous identification of these substances. TATP was synthesized already in the 19th century by Wolffenstein.5 He proposed a structure (Figure 1), which was proven later.6 TATP is one of the most sensitive explosives known. It is as sensitive to impact as nitroglycerin but has also a significant sensitivity toward friction, static electricity, and temperature changes. The explosive power equals TNT, which classifies it as a primary explosive with the potent characteristics of a secondary explosive. There are no military or commercial applications, as TATP tends to sublime rapidly.8 As with TATP, HMTD has been known for a long time.9 The impact sensitivity of HMTD is half that of TATP, still being considerable. The friction sensitivity is equally high for both substances. Bayer and Villiger first documented a structure of HMTD10 (Figure 1), and although Grisewald11 proposed an alternate possibility later, the first described structure is the most widely accepted today.12 For HMTD, there is also no known military or industrial application, the reason for this being the known temperature instability. These explosives have no significant absorption in the ultraviolet region and are not fluorescent as well. TATP can be identified by means of IR spectroscopy (KBr pellet) or chemical ionization mass spectrometry.4,5,13 Both methods are also available for the identification of HMTD.13,14 Desorption electrospray ionization mass spectrometry has recently been proposed for the detection of trace amounts of TATP on ambient surfaces by alkali metal complexation.3 Liquid chromatography/mass spectrometry (LC/MS) methods applying atmospheric pressure chemical (6) (7) (8) (9) (10) (11) (12)

Wolffenstein R. Chem. Ber. 1895, 28, 2265-2269. Groth, P. Acta Chem. Scand. 1969, 23 (4), 1311-1329. Bellamy A. J. J. Forensic Sci. 1999, 44 (3), 603-608. Legler L. Chem. Ber. 1881, 14, 602-604. Bayer, A.; Villiger, V. Chem. Ber. 1900, 33, 2479. von Grisewald, C.; Siegens, H. Chem. Ber. 1921, 54, 490. Schaefer, W. P.; Fourkas J. T.; Tiemann, B. G. J. Am. Chem. 1985, 107, 2461-263. (13) Zitrin, S.; Kraus, S.; Glattstein, B. Proceedings of the International Symposium on the Analysis and Detection of Explosives; U.S. Government Printing Office: Washington DC, 1984; pp 137-141. (14) Suelzle, D.; Klaeboe, P. Acta Chem. Scand. 1988, A24, 165-170. 10.1021/ac0609834 CCC: $33.50

© 2006 American Chemical Society Published on Web 11/03/2006

Figure 1. FT-IR-ATR spectrum of triacetonetriperoxide (TATP) (top) and hexamethylenetriperoxide diamine (HMTD) in the 1550 - 750-cm-1 spectral region.

ionization and electrospray ionization for the identification of HMTD15,16 and TATP16,17 were reported too. These LC/MS methods use a number of low-mass fragments to identify and quantify HMTD and TATP; this may render the identification rather difficult in more complex matrixes. The use of ion mobility spectrometry has been also recently reported for the analysis of TATP in ion positive mode,18 as well as for TATP and HMTD.19 The use of gas chromatography/mass spectrometry has also been employed to analyze HMTD20 and TATP residues in explosion sites.21,22 Another option to detect HMTD and TATP is by postcolumn UV irradiation followed by a reaction of the formed hydrogen (15) Crowson, A.; Beardah, M. S. Analyst 2001, 126, 1689-1693. (16) Xu, X. M.; van-de-Craats, A. M.; Kok, E. M.; de-Bruyn, P. C. A. M. J. Forensic Sci. 2004, 49 (6), 1230-1236. (17) Widmer, L.; Watson, S.; Schlatter, K.; Crowson, A. Analyst 2002, 127, 16271632. (18) Buttigieg, G. A.; Knight, A. K.; Denson, S.; Pommier, C.; Denton, M. B. Forensic Sci. Int. 2003, 135, 53-59. (19) Marr, A. J.; Groves, D. M. Int. J. Ion Mobility Spectrom. 2003, 6 (2), 5962. (20) Gielsdorf, W. Fresenius’ J. Anal. Chem. 1981, 308, 123. (21) Stambouli, A.; El-Bouri, A.; Bouayoun, T.; Bellimam, M. A. J. Forensic Sci. Int. 2004, 146 (1), S191-S194. (22) Muller, D.; Levy, A.; Shelef, R.; Abramovich-Bar, S.; Sonenfeld, D; Tamiri, T. J. Forensic Sci. 2004, 49 (5), 935-938.

peroxide with p-hydroxyphenylacetic acid under catalysis of peroxidase.23 The drawbacks of this method are the use of a complex setup comprising four HPLC pumps and the need to prepare fresh amounts of the derivatizing agents daily. An additional method is based on postcolumn irradiation with electrochemical detection.24 This approach solves the need for freshly made chemicals and has a simpler setup, but still is a destructive and indirect method. An additional option is a fast qualitative test on peroxide-based explosives,25 which is based as well on the photochemical decomposition of TATP and HMTD, but without analytical separation. The formed hydrogen peroxide oxidizes, in the presence of peroxidase, 2,2′-azinobis(3-ethylbenzothiazoline)-6-sulfonate to a colored radical cation, which is determined photometrically.25 Similar approaches have been applied as well for the analysis of TATP in the gas phase.26 Recently, new approaches based on sensors27 or mid-infrared cavity ringdown spectroscopy28 have been described as well. Legal authorities strongly need analytical methods, which are able to unambiguous identify and quantify TATP and HMTD in (23) (24) (25) (26)

Schulte-Ladbeck R.; Kolla, P.; Karst U. Anal. Chem. 2003, 75, 731-735. Schulte-Ladbeck R.; Karst U. Chromatographia 2003, 57, S61-S65. Schulte-Ladbeck, R.; Kolla, P.; Karst, U. Analyst 2002, 127, 1152-1154. Schulte-Ladbeck, R.; Karst, U. Anal. Chim. Acta 2003, 482, 183-188.

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real samples. In spite of that, there is no selective method available that may be used for nondestructive analysis of TATP and HMTD in complex samples, e.g., in solid substances confiscated by legal authorities. Infrared spectrometry is a nondestructive, powerful, and versatile analytical tool for qualitative and quantitative determinations of analytes in a broad range of matrixes. It is also a very suitable method for the identification of TATP and HMTD because, even in complex matrixes, the molecular fingerprint of the analytes can still be recognized clearly. Moreover, the coupling of Fourier transform infrared spectrometry (FT-IR) with high-performance liquid chromatography (HPLC) increases both the applicability and the accuracy of the whole procedures by reducing the spectral interferences. Solvent elimination prior to IR detection is normally preferred over on-line detection when coupling LC and IR because of better sensitivity and access to the whole spectrum due to the absence of eluent infrared absorption. Nevertheless, by using a flow cell, detection becomes much less complicated from a technical viewpoint. Furthermore, in contrast to solvent elimination techniques, nonvolatile buffer systems can easily be handled.29,30 In addition, in the case of on-line IR measurement, there is the possibility of further hyphenation, for example, with MS detectors or postcolumn derivatization and fluorescence detection. This way of coupling also allows the measurement of infrared spectra of analytes without any orientation, crystallization, or oxidative degradation effects that might occur when solvent removal interfaces are used. Using on-line coupling between isocratic HPLC and FT-IR, a number of analytical procedures have been recently developed for the determination of a variety of analytes including different sugars and acids presents in wines31,32,33 with concentrations between 0.5 and 10 mg mL-1, and sugars in nonalcoholic beverages34 in the range of 5 and 100 mg mL-1. Various applications have been reported using the off-line coupling: the determination of phenolic acids35 using a developed flow-through microdispenser system, protein conformational studies,36,37 isomer analysis of organic mixtures,38 polymer additive analysis,39 or herbicide residues in water samples.40 The minimal concentrations detected by most of the aforementioned HPLC(27) Dubnikova, F.; Kosloff, R.; Zeiri, Y.; Karpas, Z. J. Phys. Chem. A 2002, 106, 4951-4956. (28) Todd, M. W.; Provencal, R. A.; Owano, T. G.; Paldus, B. A.; Kachanov, A.; Vodopyanov, K. L.; Hunter, M.; Coy, S. L.; Steinfeld, J. I.; Arnold, J. T. Appl. Phys. B 2002, 75, 367-376. (29) Kok, S. J.; Wold, C. A.; Hankemeier, Th.; Schoenmakers, P. J. J. Chromatogr., A 2003, 1017, 83. (30) Somsen, G. W.; Gooijer, C.; Brinkman, U. A. Th. J. Chromatogr., A 1999, 856, 213-242. (31) Vonach, R.; Lendl, B.; Kellner, R. J. Chromatogr., A 1998, 824, 159. (32) Edelmann, A.; Diewok, J.; Rodriguez Baena, J.; Lendl, B. Anal. Bioanal. Chem. 2003, 376, 92. (33) Edelmann, A.; Ruzicka, I.; Frank, J.; Lendl, B.; Schrenk, W.; Gornik, E.; Strasser, G. J. Chromatogr., A 2001, 934, 123. (34) Vonach, R.; Lendl, B.; Kellner, R. Anal. Chem. 1997, 69, 4286. (35) Surowiec, I.; Baena, J. R.; Frank, J.; Laurell, T.; Nilsson, J.; Trojanowicz, M.; Lendl, B. J. Chromatogr., A 2005, 1080, 132. (36) Turula, V. E.; de Haseth. Appl. Spectrosc. 1994, 48, 1255. (37) Turula, V. E.; de Haseth, J. A. Anal. Chem. 1996, 68. (38) Somsen, G. W.; van Stee, L.; Gooijer, C.; Brinkman, U. A. Th.; Velthorst, N. H.; Visser, T. Anal. Chim. Acta 1994, 290, 269. (39) Somsen, G. W.; Rozendom, E. J. E.; Gooijer, C.; Brinkman, U. A. Th.; Velthorst, N. H. Analyst 1996, 121, 1069. (40) Somsen, G. W.; Jagt, I.; Gooijer, C.; Velthorst, N. H.; Brinkman, U. A. Th. Symp. Hyphenated Techn. Chromatogr. Bruges, 1996.

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FT-IR methods are in the milligram per milliliter range. In short, the use of flow cell HPLC-FT-IR has lower sensitivity but it is not a significant drawback for specific and quantitative determinations of major constituents of samples. The aim of this study was to develop a fast, accurate, and precise on-line HPLC-FT-IR nondestructive method for the determination of TATP and HMTD in solid samples, under isocratic HPLC separation conditions previously developed,23 as an alternative to existing procedures developed for trace explosives detection. EXPERIMENTAL SECTION Safety Note. TATP and its homologues as well as HMTD are extremely sensitive materials, which may lead to severe and spontaneous explosions under impact, friction, or temperature changes. The synthesis of these substances is only to be carried out by highly qualified and experienced personnel, under the use of appropriate safety measures (reinforced goggles and gloves, splinter-proof vessels, protective shield, etc.) and in small quantities. For this work, the substances were produced according to literature procedures (see below) in quantities not exceeding 100 mg. Working with larger amounts of the substance strongly increases the danger associated with spontaneous explosions. Synthesis of Standards. The synthesis of TATP and HMTD was performed according to literature procedures.6,12 For this work, the synthesis was carried out to obtain 100 mg of the explosives in the case of quantitative yield of the product. For safety precautions, refer to the Safety Note above. Excess amounts of the explosives can be destroyed according to ref 8. Reagents. All chemicals were purchased from Aldrich Chemie (Steinheim, Germany), Merck (Darmstadt, Germany), Sigma (Deisenhofen, Germany), and Fluka (Neu-Ulm, Germany) in the highest quality available. Acetonitrile for HPLC was Merck gradient grade. HPLC-FT-IR Separation of Analytes. Reference FT-IR spectra of standards in CHCl3 were recorded on a 3 bounce diamond attenuated total reflection (ATR) unit attached to a Bruker (Ettlingen, Germany) Matrix-F FT-IR spectrometer. ATR-FT-IR reference spectra were recorded by coaddition of 200 scans at a resolution of 4 cm-1. The HPLC system consisted of a Waters (Waters, Milford, MA) 7100 quaternary pump with a six-port injection valve (Rheodyne, Bensheim, Germany) equipped with a 20-µL injection loop. Chromatographic separation was achieved by a C18 reversed-phase column (LiChroSpher RP18 column, 250 × 3 mm, 5 µm, Merck) using an isocratic mobile-phase composition of acetonitrile/H2O (72:25) at 25 °C and using a flow rate of 0.6 mL min-1. A Bruker IFS 88 spectrometer (Bruker Optics GmbH, Ettlingen, Germany) equipped with a liquid nitrogen-cooled mercury cadmium telluride detector, a globar source, and a KBr beamsplitter was employed for HPLC-FT-IR measurements, using a 25µm CaF2 flow cell. The spectra were recorded by coaddition of 50 scans with a spectral resolution of 4 cm-1 at a mirror velocity of 100 kHz HeNe frequency for the IR beam. Prior to data acquisition and after equilibration of the HPLC system, a background spectrum was acquired averaging 200 scans. The FT-IR equipments employ the 5.0 version of the OPUS software developed by Bruker Optics, for the acquisition and processing of the FT-IR data.

Figure 2. Single-channel spectra of the flow cell filled with mobile phase (top) and the absorbance spectrum (bottom) of the same solution using a background of the flow cell with mobile phase.

RESULTS AND DISCUSSION FT-IR Spectra of Standards. Figure 1 shows the ATR-FT-IR absorbance spectra in the wavenumber region from 1550 to 750 cm-1 of pure solutions of TATP (750 mg g-1) and HMTD (800 mg g-1) in acetonitrile. The most intense bands are located at 1225 and 1175 cm-1 for HMTD and TATP, respectively, corresponding to C-O stretching vibrations. On the other hand, the characteristic IR spectra of both analytes in this region can be used to confirm the quantitative results obtained by HPLC-FTIR. The single-channel infrared spectra of the HPLC flow cell filled with the mobile phase (CH3CN/H2O; 75:25) together with a 100% line are shown in Figure 2 given in absorbance units and calculated from two consecutive recorded single-channel spectra. The region between 1243 and 1156 cm-1, which includes the most prominent analyte bands, provides adequate transmission resulting in a noise level of absorbance spectra, of only 3.4 × 10-5 (measured as root-mean-square). Therefore, qualitative and quantitative analysis of the target explosives can be carried out with the selected experimental conditions. HPLC-FT-IR. A reversed-phase (RP) C18 stationary phase is employed in combination with a mobile phase consisting of acetonitrile and water. The use of gradient elution is an important drawback when using flow cells in the mid-IR because the solvents

usually employed in RP-HPLC absorb in this region, thus providing a changing background, which has to be corrected to obtain the analytical signal corresponding to the analytes. On the other hand, for most applications, isocratic conditions are sufficient to obtain an appropriate chromatographic resolution of TATP and HMTD from each other and from possible matrix constituents. Therefore, although the use of a gradient is in principle possible to analyze highly complex samples, it was not necessary within this work. The method was applied to solid samples. The 500 mg of solid samples were spiked with HMTD and TATP. Ten milliliters of acetonitrile was added to the samples, carefully stirred, and the suspension filtered through a 0.22-µm nylon filter. The filtrate was directly injected in the HPLC system and analyzed. A chromatogram of a soil extract spiked with both TATP and HMTD is presented in Figure 3. The absorbance in the range from 1156 to 1243 cm-1 is plotted depending on the retention time. It is obvious that the separation of the two peroxides is easy. More information is gathered from two-dimensional data, as demonstrated in Figure 4. The wavenumber is plotted versus retention time of a sample containing TATP and HMTD. To identify the peroxides, the IR spectra extracted at a selected retention time may be compared with spectra from a database or Analytical Chemistry, Vol. 78, No. 23, December 1, 2006

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Figure 3. HPLC-FT-IR chromatogram of a soil sample spiked with TATP and HMTD. Measurement conditions as indicated in text. The trace shows absorbance changes in the region from 1156 to 1243 cm-1.

The HPLC-FT-IR procedure provided limits of detection of 1 × 10-3 mol L-1 (222 µg mL-1) for TATP and 5 × 10-4 mol L-1 (104 µg mL-1) for HMTD. The relative standard deviation (RSD) (n ) 4) for HMTD is 0.7% for a concentration of 1 × 10-2 mol L-1 (2.08 mg mL-1) and 7% for a concentration of 1 × 10-3 mol L-1 (208 µg mL-1). The RSD (n ) 4) for TATP is 2.9% for a concentration of 1 × 10-2 mol L-1 (2.22 mg mL-1) to 18.7% for a concentration of 1 × 10-3 mol L-1 (222 µg mL-1). An important advantage of the proposed technique as compared to “destructive” methods using photolysis of TATP and HMTD and subsequent detection of the formed hydrogen peroxide is direct measurement of the separated analytes. Therefore, instead of determination of the peroxides “indirectly” via their retention times, the IR detection allows direct identification of the analytes. This approach will therefore even be superior to the photolysis/LC methods with respect to selectivity. In future experiments, it is planned to use quantum cascade lasers (QCLs) to improve quantification of the analytes. The relative low sensitivity of the HPLC-mid-IR may be partially overcome by using QCLs as radiation sources, thus allowing increased pathlengths for absorbance measurements.33 On the other hand, it may be possible to produce an array of QCLs emitting at different wavenumbers41 or recently available external cavity QCLs with an increased tuning range42 and to integrate these in a single optical setup. This type of setup would also improve the assessment of the identity of the analytes using their relative absorption intensities at different wavenumbers.

Figure 4. Surface plot of an HPLC separation of TATP and HMTD with FT-IR detection.

with ATR spectra as discussed above. It is obvious that the extracted spectra correlate well with the reference ATR spectra in Figure 1, thus allowing the identification of the peroxides even out of mixtures of related compounds. 8154

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CONCLUSIONS An on-line HPLC-IR method has been developed as a nondestructive approach for the analysis of peroxide-based explosives. TATP and HMTD could be separated and identified on-line with limits of detection at or slightly below 1 mmol L-1. To obtain data from two independent detectors, the method may be combined on-line with other detection techniques such as mass spectrometry (41) Gmachl, C.; Tredicucci, A.; Sivco, D. L.; Hutchinson, A. L.; Capasso, F.; Cho, A. Y. Science 1999, 286, 749. (42) www.daylightsolutions.net.

or photochemical decomposition/fluorescence spectroscopy after enzymatic conversion. The newly developed method complements well the established methods in this field.

postdoctoral grant from the Ministerio de Educacio´n y Ciencia, Secretarı´a de Estado de Universidades e Investigacio´n (Spain) (EX2004-1245).

ACKNOWLEDGMENT B.L. is grateful for financial support received within project 15531 of the Austrian Science Fund. G.Q. is grateful for a

Received for review May 29, 2006. Accepted September 24, 2006. AC0609834

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