Characterization and origin identification of 2,4,6-trinitrotoluene through its by-product isomers by liquid chromatography–atmospheric pressure chemical ionization mass spectrometry

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Abstract

The by-products of industrial 2,4,6-trinitrotoluene (TNT), including isomers of trinitrotoluene, dinitrotoluene, trinitrobenzene and dinitrobenzene were investigated using liquid chromatography–mass spectrometry (LC–MS), in order to build a profile for the characterization of TNT samples from various origins. LC–MS with atmospheric pressure chemical ionization, in the negative-ion mode, was found to be the most suitable method for this study. The characterization of TNT by the by-product profile was demonstrated on a variety of TNT samples.

Introduction

Identification and characterization of explosives is of major significance in both forensic analysis of post-explosion residues and in environmental analysis of explosive-contaminated areas [1]. In addition to the type of explosives, it is important to know their country of origin and preferentially their manufacturer. Each manufacturer will produce the explosives with characteristic differences in the type and amount of by-products, impurities and additives, depending on the purity of the raw materials and solvents used and the type of manufacturing processes, thus resulting in a typical profile of by-products, organic impurities and additives.

The production process of 2,4,6-trinitrotoluene (TNT) [2], [3], a widely used military explosive, includes toluene formation from benzene and methanol, followed by three steps of nitration: from toluene to mononitrotoluene (MNT), to dinitrotoluene (DNT) and then to 2,4,6-trinitrotoluene (TNT). Nitration is carried out in the presence of nitric and sulfuric acid, followed by crystallization in alcohol or water and washings with sodium sulfite. The three steps of nitration can be carried out as batch processes or as a continuous process.

A typical group of resulting by-products are the isomers of trinitrotoluene (TNT), dinitrotoluene (DNT), trinitrobenzene (TNB) and dinitrobenzene (DNB), whose profile depends on the manufacturing processes (batch or continuous and concentration of acids) as well as the extent of the purification (crystallization and sodium sulfite washing). Other groups are organic impurities originating from solvents and reagents and additives such as stabilizers, plasticizers and dyes.

Several attempts were made in the past to develop methods for characterization of TNT by determining the presence of various impurities. These included nuclear magnetic resonance [4], liquid chromatography [5], gas chromatography [6], gas chromatography–mass spectrometry (GC–MS) [7]. Another method suggested to differentiate between TNT samples by finding differences in their 13C/12C and/or 15N/14N isotope ratios [8], [9]. Most of these methods could not demonstrate differences in actual TNT samples or had problems of reproducibility.

The main focus of this study is to develop a method of characterization and origin identification of TNT through identification of the corresponding profile of by-products. As the amount of these by-products is very small a highly sensitive and selective analytical method is needed for their unambiguous identification. Liquid chromatography–mass spectrometry (LC–MS) has already been proved to be a suitable technique for the analysis and identification of explosives [10], [11]. LC–MS, with atmospheric pressure chemical ionization (APCI), in the negative-ion mode, was found to provide best sensitivity and selectivity for nitroaromatic compounds [12]. In addition, tandem mass spectrometry with collision-induced dissociation (MS–MS-CID) was used for further identification of some of the nitroaromatic isomers.

Section snippets

Chemicals and reagents

Standard 2,4,6-TNT (1000 μg/ml in acetonitrile, purity 99%) was purchased from Supelco (Bellefonte, PA, USA). All other five TNT isomers (2,3,4-, 2,3,5-, 2,3,6-, 2,4,5-, 3,4,5-TNT) were provided by Wehrwissenschaftliches Institut für Werk-, Explosiv- und Betriebsstoffe (WIWEB) (Swisttal-Heimerzheim, Germany). 1,2-DNB (purity 99+%), 1,4-DNB (purity 98%), 3,4-DNT (purity 99%), 2,3-DNT (purity 99%), 2,6-DNT (purity 98%) and 2,4-DNT (purity 97%) were purchased from Aldrich (Milwaukee, WI, USA). The

TNT, DNT, DNB and TNB standard mixtures

The full scan negative-ion APCI mass spectra of the TNT isomers provided limited information on their identification, due to lack of fragment ions, despite the different abundance ratios of m/z 226 ([M–H]) to m/z 227 (M), observed for different TNT isomers. Therefore, MS–MS-CID analyses were performed. The m/z 227 of 2,4,6-TNT produced major fragment ions at m/z 210 (–OH, ortho effect, loss of 17 mass units) and m/z 197 (–NO, loss of 30 mass units). While the MS–MS-CID of m/z 227 of 3,4,5-

Conclusion

The by-product profiles obtained demonstrate that LC–APCI-MS in the negative ion mode, with back-up of MS–MS-CID for positive identification, is a promising technique for the characterization and origin identification of TNT samples through their TNT, DNB, DNT and TNB isomer profile. These isomers are by-products originating from the manufacturing process of TNT, and their combined profile is different for samples from different manufacturing plants. The mass chromatograms of the 4-times

Acknowledgments

We would like to thank Dr Manfred Kaiser from the Wehrwissenschaftliches Institut für Werk-, Explosiv- und Betriebsstoffe (WIWEB), Swisttal-Heimersheim, Germany for the TNT isomer standards and Dr Paul Waggoner from Auburn University, Auburn, AL, USA, Philip Rodacy from Sandia National Laboratories, Albuquerque, NM, USA, Dr Jari Tervo from the Defence Forces Research Institute of Technology, Lakiala, Finland and the Analytical Laboratory of the Israeli Police Headquarters, Jerusalem, Israel,

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