Optimizing Forensic Detection of Explosive Substances: Extended Column Analysis of TNT

Abstract

Trinitrotoluene (TNT) is a secondary high explosive substance which is found in many explosive compositions. It is used for military and industrial purposes; however, it is also misused for illegal and terrorist activities. Forensic analysis for detection of TNT in pre and post blast explosion samples is important for identifying the antisocial elements and tracking the source of their illegal activities. Traditional Gas Chromatography mass spectrometer instruments use short column having length of 15-meter for analysis of explosive substances due to their thermolabile nature. Our work presents multiple methods using longer column of 30-meter length for analysis of TNT. The same instrument can be used for multiple applications thus precluding the need of changing the GC column every time for analysis work of different types of the samples leading cost effective of the organisation.

Introduction

Trinitrotoluene (TNT) is a widely used explosive in military and industrial applications, but it is also involved in illegal activities like bombings. Forensic analysis of explosion debris to detect TNT is crucial for investigations. Common detection methods include color tests, Thin Layer Chromatography (TLC), FTIR, and chromatographic techniques combined with Mass Spectrometry (MS).

Mass Spectrometry (MS), especially when combined with chromatography (GC-MS or LC-MS), offers precise identification by analyzing mass-to-charge ratios. LC-MS is preferred for thermolabile explosives but is not always available; thus, GC-MS with a standard 30m column remains widely used in forensic labs due to cost and versatility, despite requiring some trade-offs.

This work focuses on developing and validating a GC-MS method using a 30m column for TNT detection in forensic samples, including pre- and post-blast debris. The method adapts and modifies six existing GC-MS protocols from environmental studies to suit forensic needs.

Experimental procedures involved preparing TNT solutions, extracting TNT from soil samples at blast sites, performing color tests with alcoholic KOH (which yields a purple-red color in presence of TNT), and characterizing samples by FTIR spectroscopy to confirm TNT presence. GC-MS analysis of known TNT and forensic extracts showed matching retention times and mass spectral data across all six adapted methods, demonstrating the reliability of the method for forensic TNT detection.

Conclusion

The known TNT sample was characterised using FTIR spectroscopy and it was confirmed to contain TNT. This TNT sample and unknown Forensic Case sample were simultaneously analysed with colour test and GC-MS methods. The results of colour test indicated presence of TNT in the forensic case sample. The GC retention times obtained using the six methods for known TNT sample and the unknown extract of forensic case sample matched thus identifying the presence of TNT in forensic sample. Further the library match of the mass spectra of the forensic sample resulted in hit list of compounds, first of which was Trinitrotoluene hence confirming the presence of TNT in forensic sample in all the GC-MS methods (HE-1, HE-2, HE-3, HE-4, HE-5 and HE-6). The main aim of this study was to perform qualitative analysis of TNT using an extended GC column of 30 m length and optimization of GC-MS methods previously used in environmental and bio-remediation sample analysis to the forensic scenario. The detector employed was MS which itself is an identification technique. Thus we used a hyphenated technique for identification of the detected analyte TNT in the forensic case sample. In case of qualitative analysis employing GC, identification of analyte (here TNT) is based on retention time (RT) and if the GC is coupled with MS, it is based on the spectral data. While using a hyphenated technique like GC-MS along with TNT known sample and comparing it with the unknown such as the forensic case sample, we actually confirmed the presence of TNT by using two analytical methods, not just one. Here we have not only employed GC for separation, but also for identification on the basis matching of RT values using a known TNT sample. Further the mass spectral data at this RT value was subjected to library spectral matching using the NIST Library. Library search also resulted in identification of trinitrotoluene with excellent match as all the values of SI and RSI obtained were above 900. This comparison of RT values and the library match and reverse match values in table 2 shows that TNT could be detected in forensic case sample by using all the six GC-MS methods (HE-1, HE-2, HE-3, HE-4, HE-5 and HE-6) studied in this work. Although various methods were used in previous works for detection of TNT in case of environmental and bio-remediation samples, our work has optimized these six methods for detection of TNT in forensic post blast samples. Thus, qualitative identification of TNT could be achieved on the basis of studying the gas chromatography RT values and further confirmed on the basis of interpretation of the mass spectra library search results using all the six instrumental methods.

References

[1] A. Talaat, & A. Elbeih, & H. Tantawy, & A. Hussein, (2022). Thermal behavior and explosive properties of TNT/RGO. Journal of Physics: Conference Series. 2305. 012022. 10.1088/1742-6596/2305/1/012022. [2] J. Yinon, Mass spectrometry of explosives: Nitro compounds, nitrate esters, and nitramines, Mass Spectrometry Reviews 1982, 1, 257-307. [3] T. P. Forbes and E. Sisco, Recent advances in ambient mass spectrometry of trace explosives, Analyst, 143(9), 1948 - 1969, 2018. [4] J. Yinon, Explosives in Handbook of Analytical Separations, Vol. 2, 2000 Elsevier Science B.V. [5] J.Yinon, Analysis and detection of explosives by mass spectrometry in Aspects of Explosives Detection, 147-169, 2009 Elsevier B.V. [6] M. R. Lee, S. C. Chang, T. S. Kao, and C. P. Tang, Studies of Limit of Detection on 2,4,6-Trinitrotoluene (TNT) by Mass Spectrometry, Journal of Research of the National Bureau of Standards, Volume 93, Number 3, May-June 1988 [7] J. Yinon, Trace analysis of explosives in water by gas chromatography-mass spectrometry with a temperature-programmed injector, Journal of Chromatography A, 742 (1996) 205-209 [8] T. Tamiri, S. Zitrin, S. Abramovich-Bar, Y. Bamberger, GC/MS Analysis of PETN and NG in v vAdvances in Analysis and Detection of Explosives, 323-334, 1993 Kluwer Academic Publishers. [9] A.L. Lubrano, C. R. Field, G. A. Newsome, D. A. Rogers, B. C. Giordano, K. J. Johnson, Minimizing thermal degradation in gas chromatographic quantitation of pentaerythritol tetranitrate, Journal of Chromatography A,Volume 1394, 2015,154-158, https://doi.org/10.1016/j.chroma.2015.03.006. [10] Minimizing thermal degradation in gas chromatographic quantitation of pentaerythritol tetranitrate, Journal of Chromatography A, Volume 1394,2015,Pages 154-158. [11] T.H. Bünning, J.S. Strehse, A.C. Hollmann, T. Bötticher, E. Maser, A Toolbox for the Determination of Nitroaromatic Explosives in Marine Water, Sediment, and Biota Samples on Femtogram Levels by GC-MS/MS. Toxics 2021, 9, 60. https://doi.org/ 10.3390/toxics9030060. [12] Teh-Liang Chang, Identification of impurities in crude TNT by tandem GC-MS technique, Analytica Chimica Acta, Volume 53, Issue 2, 1971, Pages 445-448, ISSN 0003-2670, https://doi.org/10.1016/S0003-2670(01)82106-1. [13] Z. A.Alothman, A. H.Bahkali, A. M. Elgorban, M. S. Al-Otaibi, A. A. Ghfar, S. A. Gabr, S. M. Wabaidur, M. A.Habila, & A. Y. B. H. Ahmed, (2020). Bioremediation of Explosive TNT by Trichoderma viride. Molecules (Basel, Switzerland), 25(6), 1393. https://doi.org/10.3390/molecules25061393 [14] S.Siddiquee, , B. E.Cheong, , K.Taslima, , H.Kausar, , & M. M. Hasan, (2012). Separation and identification of volatile compounds from liquid cultures of Trichoderma harzianum by GC-MS using three different capillary columns. Journal of chromatographic science, 50(4), 358–367. https://doi.org/10.1093/chromsci/bms012 [15] N. Stoppacher, B. Kluger, S. Zeilinger, R. Krska, & R. Schuhmacher, (2010). Identification and profiling of volatile metabolites of the biocontrol fungus Trichoderma atroviride by HS-SPME-GC-MS. Journal of microbiological methods, 81(2), 187–193. https://doi.org/10.1016/j.mimet.2010.03.011 [16] B. Dawidziuk, J. Nawa?a, D. Dziedzic, D. Gordona and S. Popiel, Development, validation and comparison of three methods of sample preparation used for identification and quantification of 2,4,6-trinitrotoluene and products of its degradation in sediments by GC-MS/MS, Analytical Methods, 2018, 10(43), 5188 – 5196, The Royal Society of Chemistry. [17] T. Junk, and W. J. Catallo, (1998) ‘Environmental transformation products of 2,4,6-trinitrotoluene’, Chemical Speciation & Bioavailability, 10(2), pp. 47–52. doi: 10.3184/095422998782775844. [18] W. E. Pereira, D. L. Short, D. B. Manigold, and P. K. Roscio, Isolation and Characterization of TNT and Its Metabolites in Groundwater by Gas Chromatograph-Mass Spectrometer- Computer Techniques, Bull. Environm. Contain. Toxicol. 21,554-562 (1979). [19] E. Psillakis, G. Naxakis, N. Kalogerakis, Detection of TNT Contamination in Spiked-Soil Samples Using Spme and GC / MS, Global Nest: the Int. J. Vol 2, No 3, pp 227-236, 2000 [20] J.Almog and S.Zitrin, Colorimetric Detection of Explosives in: Aspects of Explosives Detection, 41-58 Elsevier B. V., 2009. [21] J. Yinon and S. Zitrin, The Analysis of Explosives, Pergamon Press, Oxford, Chapter 2, 1981, pp. 29–46. [22] A. Gujar, T. Anderson, D. Cavagnino, and A. Patel, Comparative analysis of mass spectral matching for confident compound identification using the Advanced Electron Ionization source for GC-MS, TECHNICAL NOTE 10598 [23] NIST/EPA/NIH Mass Spectral Library Compound Scoring: Match Factor, Reverse Match Factor, and Probability, www.jordilabs.com

Copyright

Copyright © 2025 D Kumar, Prajakta Uday Kotawadekar. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.