Development of Mass Spectrometric Method for Analysis of Cyclic

method for the identification of organic explosives and propellants. David DeTata , Peter Collins , Allan McKinley. Forensic Science International...
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Anal. Chem. 2005, 77, 7796-7800

Development of Mass Spectrometric Method for Analysis of Cyclic Nitramine Explosives DTIW and HNIW Chagit Denekamp* and Alexander Tsoglin

Department of Chemistry, TechnionsIsrael Institute of technology, Haifa 32000, Israel

Mass spectrometric (MS) methods are used for the analysis of two novel nitramine explosivesshexanitrohexaazaisowurzitane (HNIW) and 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazaisowurzitane (DTIW). The methods include electrospray (ESI) and atmospheric pressure chemical ionization techniques for liquid chromatography/MS (LC/MS), chemical ionization for direct introduction (DCI), and gas chromatography/MS (CI-GC/MS). It is found that HNIW (438 Da) is detectable using both positive and negative modes of DCI and in the negative mode ESI-MS. Several anions were found to complex with HNIW, e.g., CF3CO2-, Cl-, Br-, I-, NO3-, and NO2-. On the other hand, DTIW could only be detected using positive DCI and CI-GC/MS, where an MH+ ion (m/z 263) was formed. The fragmentation pathways of the two nitramines were further studied by MS2 experiments. Apparently, the main fragmentation pathway of the MH+ ion of DTIW involves the loss of nitrous acid. Several anion adducts of HNIW that were studied dissociate to afford neutral HNIW and the added anions. However, Cl-, Br-, I-, and NO2- afford a series of fragments that resulted from the dissociation of the isowurzitane structure. For these anions, limit of detection was also found. To understand some of the HNIW fragmentation pathways, DFT calculations were used.

sively studied the formation of anion adducts and their stability under negative ion ESI conditions;1-3 they reported the formation of anion adducts using ammonium halides (NH4X, where X is F, Cl, Br, and I) as attached anions. Vairamani et al. measured negative ion ESI mass spectra for a series of isomeric cis and trans dicarboxylic acids in the presence of halide ions. The acids under study showed a greater tendency to form adduct ions with Cl- under ESI conditions compared with the other halide ions used.8 Several research studies demonstrate the detection of explosives using negative ESI.9-16 The analysis of different explosives as adducts with various small anions is possible, depending on particular mobile-phase additives. Recently, Mathis and McCord proposed to use negative ESI for the detection of adducts of explosives with chloride, formate, acetate, and nitrate as a mixture, enabling the simultaneous detection of different explosives.17 Nitramine compounds have been known for about 100 years. Along with their product development, many analytical methods were introduced in order to determine their presence both qualitatively and quantitatively. One of these methods is MS with direct introduction or coupled with liquid or gas chromatography. All nitramines contain the same or similar side groups (N-NO2), which are connected through alkyl chains. Hence, their typical fragmentation involves the loss of this group regardless of the specific structure, and it is difficult to attribute MS spectra to a

Development of high-performance liquid chromatography/ mass spectrometry (HPLC/MS) and gas chromatography/mass spectrometry (GC/MS/MS) methods that enable the analysis of different classes of explosives is of great interest for forensic work. There are several advantages to combining chromatographic methods with mass spectrometric detection, among them are excellent sensitivity and selectivity, reproducibility, low sample consumption, and low detection limits. Electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) are the methods of choice for the combination with HPLC, while electron impact and chemical ionization (CI) are useful in combination with GC. Although ESI is more frequently used in the positive ion mode, studies using negative ion ESI have been slowly increasing in recent years.1-7 Recently, Cole et al. exten-

(2) Zhu, J.; Cole, R. B. J. Am. Soc. Mass Spectrom. 2000, 11, 932-941. (3) Cai, Y.; Cole, R. B. Anal. Chem. 2002, 74, 985-991. (4) Khairallah, G.; Peel, J. B. J. Phys. Chem. A 1997, 101, 6770-6774. (5) Khairallah, G.; Peel, J. B. Int. J. Mass Spectrom. 2000, 194, 115-120. (6) Zhu, J.; Cole, R. B. J. Am. Soc. Mass Spectrom. 2001, 12, 1193-1204. (7) Aplin, R. T.; Moloney, M. G.; Newby, R.; Wright, E. J. Mass. Spectrom. 1999, 34, 60-61. (8) Kumar, M. R.; Prabhakar, S.; Kumar, M. K.; Reddy, T. J.; Vairamani, M. Rapid Commun. Mass Spectrom. 2004, 18, 1109-1115. (9) Casetta, B.; Garofolo, F. Org. Mass Spectrom. 1994, 29, 517-525. (10) Garofolo, F.; Longo, A.; Migliozzi, V.; Tallarico, C. Rapid Commun. Mass Spectrom. 1996, 10, 1273-1277. (11) Yinon, J.; McClellan, J. E.; Yost, R. A. Rapid Commun. Mass Spectrom. 1997, 11, 1961-1970. (12) Zhao, X. M.; Yinon, J. J. Chromatogr., A 2002, 977, 59-68. (13) Wu, Z. G.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2002, 74, 1879-1883. (14) Zhao, X. M.; Yinon, J. J. Chromatogr., A 2002, 946, 125-132. (15) Gapeev, A.; Sigman, M.; Yinon, J. Rapid Commun. Mass Spectrom. 2003, 17, 943-948. (16) Sanchez, C.; Carlsson, H.; Colmsjo, A.; Crescenzi, C.; Batlle, R. Anal. Chem. 2003, 75, 4639-4645. (17) Mathis, J. A.; McCord, B. R. Rapid Commun. Mass Spectrom. 2005, 19, 99-104.

* Corresponding author. Phone: ++972-4-8293736. Fax: ++972-4-8293736. E-mail: [email protected]. (1) Cai, Y.; Concha, M. C.; Murray, J. S.; Cole, R. B. J. Am. Soc. Mass Spectrom. 2002, 13, 1360-1369.

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© 2005 American Chemical Society Published on Web 11/03/2005

particular compound when dealing with a mixture of several explosives. As a result, suitable types of ionization methods should be those that enable mostly parent ion determination. Furthermore, as the nitro group is electron-rich, negative mode is frequently used.9-16 In the past few decades, two novel nitramine compounds, 4,10dinitro-2,6,8,12-tetraoxa-4,10-diazaisowurzitane (DTIW, 1) and

hexanitrohexaazaisowurzitane (HNIW, 2), were synthesized. However, both are not incorporated in the armament industry yet. Moreover, some of their properties are still to be determined, as well as analytical methods for qualitative and quantitative determination. While there are several studies considering HPLC analysis of 2, only a few of them are concerned with either MS detection or the routes of its decomposition by MSn of this explosive. Hawari and co-workers studied the degradation of 2 promoted by iron powder18 and alkaline hydrolysis.19 Liquid chromatography and ESI-MS analysis was used to determine the decomposition products. They reported that while iron powder causes an initial cleavage of the C-C bridge and a reduction of two of the nitramine groups, leading to an imine and its dicarbinol intermediates, the alkaline hydrolysis proceeded via initial denitration. The presence of the intermediates was verified using 15N-labeled and 15NO3labeled 2. Reich and Yost used LC/APCI-MS for trace detection and quantitation of several explosives in a mixture,19 2 among them. Chlorinated solvents were found to be useful as Cl- source in production of chloride adducts ([M + Cl]-) of the explosives under study.19 Analysis of 2 using positive CI mass spectrometry with dimethyl ether as reagent gas was reported by Yinon and co-workers.20 Several ions were observed, the ions of m/z 392 being the most abundant and attributed to a loss of NO2. Other peaks correspond to the molecular ion at m/z 438 and an [M + 15]+ adduct of m/z 453. A cyclodextrin-assisted capillary electrophoresis coupled with a quadrupole ion trap was used to resolve and detect several explosives, in the negative mode,21 including 2. Thus, a method for trace detection of the explosives and their degradation products in soil and aquatic environments was developed. A weak signal at m/z 500 ion was attributed to a [2 + NO3]- adduct ion.14 As mentioned above, many studies have been dedicated to the determination of known nitramine compounds such as RDX, HMX, etc. However, very little is known about the quantitative determination of 2 by mass spectrometry, while nothing is reported (18) Hawari, J.; Balakrishnan, V. K.; Mpnteil-Rivera, F.; Halasz, A.; Corbeanu, A. Environ. Sci. Technol. 2004, 38, 6861-6866. (19) Hawari, J.; Balakrishnan, V. K.; Halasz, A. Environ. Sci. Technol. 2003, 37, 1838-1843. (20) Yinon, J. Advances in Analysis and Detection of Explosives: Proceedings of the 4th International Symposium on Analysis and Detection of Explosives; Sep 1993; pp 299-307. (21) Hawari, J.; Groom, C. A.; Halasz, A.; Paquet, L.; Thiboutot, S.; Ampleman, G. J. Chromatogr., A 2005, 1072, 73-82.

about 1. A full characterization of the compounds of interest consists of chromatography, mass spectrometry, and tandem mass spectrometry. In the present study, gas chromatography, electron impact or chemical ionization mass spectrometry, and collisioninduced dissociation (CID) are used for the full characterization of 1, while anion attachment is used in the case of 2. The effect of the anion attached to 2 on the CID spectra was also investigated. EXPERIMENTAL SECTION Materials. Hexanitrohexaazaisowurzitane (2) and 4, 10-Dinitro-2,6,8,12-Tetraoxa-4,10-Diazaisowurzitane (1) were provided by Rafael Ltd. (P.O. Box 2250, Haifa 31021, Israel).22 HPLC grade solvents were used without purification. KCl, LiCl, Ca(NO3)23H2O, NaBr, NaI, KI, and NaNO2 were purchased from Aldrich Chemical Co., Inc. Mass Spectrometry. Positive chemical ionization (PCI) mass spectrometric analysis and CID measurements were carried out on a Finnigan TSQ-70B triple-stage quadrupole mass spectrometer. Compound 1 (1 mL, of a g mL-1 solution in dichloromethane) was introduced on a RTX-TNT1 (Restek) capillary column at 250 °C. The temperature was programmed from 100 to 220 °C at 10 °C min-1. The scan rate was 1 scan s-1. PCI measurements were performed at 150 °C ion source temperature and 0.4 Torr (indicated) reagent gas pressure (isobutane). CID measurements were performed with argon as target gas (0.3 mTorr, indicated) at 30-eV collision energy (indicated). Negative mode electrospray ionization MS spectra were recorded using an LCQDUO ion trap instrument (Thermo-Finnigan, San Jose, CA) with ESI and APCI interfaces. The samples were dissolved in acetonitrile (1.0 mg in 100 mL) and introduced into the ESI source at a flow rate of 0.5 mL h-1. Isolation of precursor ions for MS/MS was accomplished with a window of 3 amu. Helium was used as collision gas at 17-19% energy, so that parent ions could still be detected. Postblast experiments were carried out using a falling hummer test instrument. Calculations. All the calculations were carried out using Gaussian 98 package of programs.23 Molecules under study were optimized at the B3LYP/6-31++G(d), hybrid density functional level of theory, and structures were analyzed using analytical frequencies calculations. RESULTS AND DISCUSSION Determination of 1, Using GC/MS and MS/MS. Nitranime 1 is slightly soluble in acetonitrile and dichloromethane. However, (22) We thank Dr. L. Gottlieb and Dr. G. Korogodsky for supplying us with samples of HNIW and DTIW. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robbins, D. J.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.; Burant, J. C.; Millam, J. M.; Iyenger, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Salvador, A.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian, Inc.: Pittsburgh, PA, 2003.

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Scheme 1. Elimination of a Bisnitrimine Moiety of 146 Mass Units

Figure 1. GC chromatogram (a) and CH4-PCI mass spectrum (b) of nitramine 1. (c) CID spectrum measured for the CH4-PCI produced MH+ ion of 1.

we were unable to produce ions from these solutions using ESI or APCI both in the positive and negative ion modes. We therefore applied ammonia, isobutane, and methane chemical ionization methods. Since 1 is stable under GC separation both GC/CI and MS/MS spectra could be measured (Figures 1). The GC column chosen for this experiment is specifically designed for energetic materials (Restek RTX-TNT1). The isobutane positive chemical ionization (iBu-PCI) mass spectrum of 1 (not shown) contains three major peaks, a highly abundant MH+ ion (relative abundance, RA 100%), a less abundant [MH - NO2]+ ion at m/z 171 (RA 12%), and an [MH - 16]+ ion (RA 26%) that is probably generated by the fragmentation of [M + C3H7]+ or [M + C3H5]+ attachment ions. The CID spectrum of the MH+ ion of 1 is quite complicated, indicating the initial loss of 47 u (HNO2) and several consecutive fragmentation product ions (not shown). Better sensitivity for the detection of 1 was encountered with methane positive chemical ionization (CH4-PCI, Figure 1) while ammonia PCI was found to be inefficient for either protonation or ammonium cationization of 1. The CH4-PCI mass spectrum of 1 shows characteristic nitramine behavior. Along with a highly abundant MH+ ion of m/z 263 (RA 100%) and an ion at m/z 188 that may correspond to the loss of HNO2 along with CO (RA 19%), there is an [MH - HNO2]+ ion at m/z 216 (RA 16%). The elimination of nitrous acid is general in the PCI and CID spectra 7798

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of nitramines. Limit of detection for 1 was found to be 1.7 µg mL-1 using GC/CH4-CI MS. The MH+ ion of 1 dissociates to afford an abundant peak of m/z 200 (RA 18%, Figure 1) upon CID, corresponding to the loss of nitric acid. Determination of 2 Using Negative ESI-MS and MS/MS. A positive iBu-desorption chemical ionization mass spectrum has also been recorded for 2 (not shown). Even though the sensitivity is low, an abundant [M + C3H7]+ ion is observed (RA 25%), accompanied by a low-abundance MH+ ion of m/z 439 (RA 10%) that undergoes a simultaneous loss of two NO2 moieties, affording an [MH - 2NO2]+ base peak at m/z 347. It was not possible to run a GC chromatogram for 2 as it decomposes during evaporation. Unlike 1, nitramine 2 forms attachment anions easily, in the ESI source.24 As described for other systems in the introduction, [M + X]- ions are produced, according to the chosen additives. Limit of detection was estimated for 2 with Cl-, Br-, I-, NO3-, and NO2- as 40, 20, 20, 2, and 2000 µg mL-1, respectively. Therefore, the best ionization conditions are in the presence of Ca(NO3)‚3H2O, in an acetonitrile-water mixture. We further studied the CID spectra of the ESI-produced [M + X]- ions. It is found that fragmentation characteristic of 2 depends on the identity of the attached anion X- and the experimental conditions. For example, the most abundant dissociation products in the CID spectra of [M + X]- ions (e.g., X ) NO3-, I-, etc.) that are measured with an FT-ICR (not shown) are X- and neutral 2, while corresponding CID measurements that were carried out with the aid of an ion trap give rise to fragments that result from the dissociation of the isowurzitane structure. Figure 2 exhibits CID spectra that were measure for NO3-, Cl-, Br-, and I- attachment ions. Although each CID spectrum indicates elimination of different neutrals, all [M + X]ions, except [M + I]-, lose 46, 47, or 146 u moieties. These masses correspond to NO2, HNO2, or C2H2N4O4, which is a bisnitrimine moiety (Scheme 1). Most interesting is the comparison between the three halogen attachment anions. The [M + Br]- ion dissociates in a nearly exclusive manner with the loss of 146 u (Scheme 1). This pyrolytic cleavage is the retrosynthesis of the isowurzitane skeleton that is generated in a one-pot condensation of three bisimine groups. Unlike the loss of HNO2 that initiates the dissociation of [M + Cl]- and probably involves interaction between the dissociating hydrogen atom and the chloride, the loss of the bisnitrimine group does not necessarily require charge. Evidently however, the presence of Br- does catalyze this pyrolytic process in some way. (24) Both [M + X]- and [2M + X]- ions are formed, and the ratio depends both on X and on source parameters.

Figure 2. CID spectra that were measure for [M + X]- ions of 2, where X is Br, Cl, I, and NO3.

Figure 3. Gas-phase geometries for two types of complexes between 2 and either Br- (left) or Cl- (right) calculated at B3LYP/6-31++G(d).

Density functional theory calculations were used in order to study the effect of attached anions on the energy and structure of 2. A polyfunctional large molecule such as 2 can give rise to several isomeric [M + X]- complexes, and it was therefore necessary to determine the preferable interaction site. As a first step 1,2-ethanediamine, N,N′-dimethyl-N,N′-dinitro (CH3N(NO2)CH2CH2N(NO2)CH3) was used as a model for the calculation of the

site of interaction with anions. The geometries of 1,2-ethanediamine, N,N′-dimethyl-N,N′-dinitro in the presence of Cl-, Br-, and NO3- has been calculated at the B3LYP/6-31++G(d) level of theory. The calculated results indicate that the only possible stabilizing interaction is between the acidic hydrogen atoms and the anion. However, the geometry of 1,2-ethanediamine, N,N′dimethyl-N,N′-dinitro (CH3N(NO2)CH2CH2N(NO2)CH3) is almost Analytical Chemistry, Vol. 77, No. 23, December 1, 2005

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Scheme 2. Fragmentation Pathways for Attachment Anions of Nitramine 2 under CID

identical in the presence of Cl-, Br-, or NO3-. The most significant difference are the X-H distances, where X is the anion and H is a hydrogen atom in 1,2-ethanediamine, N,N′-dimethyl-N,N′-dinitro. The shortest distances are 2.5274, 2.4219, and 2.1890 Å for H-Br, H-Cl, and H-NO3, respectively. Since the calculated model complexes could not explain the effect of a specific anion on the dissociation behavior of 2, the full structures of Cl- and Br- complexes with 2 were also calculated at the same level. It has already been established that complexation occurs between the anion and the acidic hydrogen atoms, therefore two possible complexes were considered (Figure 3). Due to the symmetry of 2, there are two identical interaction sites of one type and another single possible site for complexation. The geometries of the four [M + X]- complexes between 2 and either Br- or Cl- anions are shown in Figure 3. The first possible site of interaction gives rise to the upper structures in Figure 3, referred to as complex 1. These [M + X]- complexes indicate no significant effect of the anion on the geometry of 2. Calculation also show that the chloride and bromide affinities of complex 1 are 27.6 and 34.1 kcal mol-1, respectively, while the chloride and bromide affinities of complex 2 are 31.2 and 41.0 kcal mol-1, respectively. As mentioned above, the symmetry of 2 allows one interaction described by complex 1 and two identical complex 2 structures. Most interesting is the observation that while the geometry of 2 is somewhat changed by the presence of Cl- in complex 2, it is severely twisted in the presence of Br-, despite the higher bromine affinity. For example, the dihedral angle around the nitrogen atom adjacent to the halogen atom is reduced from 28.8° in neutral 2 to 21.8° in the chloride and 10.6° in the bromide complex. This might explain the tendency of [M + Br]- ions to undergo efficient specific dissociation that involves the isowurzitane structure. Scheme 2 summarizes the main fragmentation pathways of the four [2 + X]- ions (Figure 2). Iodide and nitrate adducts form

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products that eliminate the external anions. In the case of [2 + I]-, an abundant ion or m/z 207 is also present in the CID spectrum that could not be accounted for. To test our detection protocol for 2, postblast experiments were carried out using a falling hammer test instrument. A 40mg sample of 2 was placed between two cylinders after which a weight of 5 kg was dropped from a height that causes detonation. The explosion residue was extracted with acetonitrile that was further evaporated to a minimal volume for injection. ESI-MS/ MS analyses of [M + Br]- ions of 2 (m/z 517/519, after addition of KBr) was successful in 8 out of 10 repetitions. CONCLUSION Two new nitramine-based explosives were analyzed using MS techniques. It is shown that 1 is best detected by GC/MS with CH4-PCI. It provides an abundant MH+ ion that undergoes HNO2 elimination upon collisional activation. On the other hand, 2 is best detected by HPLC/MS with anions additives. The CID spectra of different [M + X]- attachment anions indicate specific behavior, depending on the anion attached. A unique dissociation is observed in the presence of Br- that involves a one-step decomposition of the isowurzitane structure and is therefore indicative of 2, unlike the typical elimination of HNO2 in nitramines. This provides an advantage to the detection of 2 in the presence of KBr, in combination with MS/MS measurements. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review July 26, 2005. Accepted October 6, 2005. AC051326Z