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Chemical profiling of explosives

Brust, G.M.H.

Publication date 2014

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Citation for published version (APA): Brust, G. M. H. (2014). Chemical profiling of explosives.

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Chapter 5

Investigation of isotopic linkages between precursor materials and the improvised high-explosive product HMTD

This chapter has been published as:

C.M. Lock, H. Brust, M. van Breukelen, J. Dalmolen,Anal. M. Chem. Koeberg, D.A. Stoker 84 (2012) 4982-92 86 Chapter 5

Abstract

The results of isotope-ratio mass spectrometry (IRMS) on hexamethylene triperoxide diamine (HMTD) and its precursor hexamethylenetetramine (hexamine) are presented in this chapter. HMTD was prepared from hexamine using several different sources of hexamine under both controlled laboratory conditions and in field experiments that represent the less controlled conditions that are likely to be observed in forensic casework scenarios. Precursor and product carbon-isotope δ values consistently fit a linear relationship regardless of precursor or conditions. The magnitude of the isotope fractionation observed is affected by the efficiency of the reaction, with greater yielding reactions giving rise to HMTD with δ values more similar to the precursor material than lower yielding reactions. Nitrogen-isotope δ values comparing precursor with product show some linearity when the reaction conditions are carefully controlled however, results indicate a poor fit with linearity when synthesis conditions are more variable. Despite the greater variation, the HMTD product is consistently enriched in 15N compared with the hexamine precursor. Having prepared multiple HMTD samples from various precursors using a range of experimental conditions we have observed results, which may be useful in forensic investigations of improvised explosives materials. Chapter 5 Investigation of isotopic linkages between precursor materials and HMTD 87

5.1 Introduction

The Forensic Explosives Laboratory (FEL) analyses approximately 250 cases each year related to the criminal misuse of explosives, the Netherlands Forensic Institute (NFI) approximately 200 cases. In some instances there may be evidence such as recovered recipes, chemicals and apparatus suggesting explosives have been or were intended to be illegally manufactured. Continuing research programs at both laboratories aim to expand and improve the portfolio of analytical techniques utilized for the provision of high-quality forensic evidence. Reported here are results supporting the application of isotope-ratio mass spectrometry (IRMS) as an analytical tool which can provide links between precursor materials used to manufacture explosives. Previously, isotopic links between hexamethylenetetramine (hexamine) and the high explosive cyclomethylene trinitramine (RDX) were demonstrated [1]. While RDX is a powerful explosive commonly employed for military purposes, the synthesis is lengthy and requires a moderate level of skill to control, and for this reason it is considered unlikely to be encountered as home-made explosive in casework at either laboratory at present. Our initial research demonstrated the important concept that isotope analysis could provide additional information in forensic investigations where illegal manufacture of explosives is suspected. The systematic change observed in isotope signatures (fractionation) from Chapter 5 Chapter precursor to product provides a relationship which could be exploited to predict the isotope signatures of synthetically-related materials. Moreover the magnitudes and direction of fractionation were explained by bond cleavage/formation steps proposed for the synthesis.

The peroxide explosive hexamethylene triperoxide diamine (HMTD) has previously been encountered in UK, The Netherlands and international cases [2,3], and forms the focus of this investigation. HMTD is a sensitive primary explosive, which may be synthesized from hexamine and hydrogen peroxide under acidic conditions [4,5]. The purpose of this investigation was to assess whether or not systematic and predictable isotope fractionation occurs during the synthesis of HMTD and hence whether or not a relationship between precursor and product can be established. Further samples were 88 Chapter 5

prepared to test the linkage for more realistic casework samples manufactured under improvised conditions. The synthetic conditions were also varied to assess the impact on the precursor–product relationship.

IRMS has been broadly utilized to determine similarities or differences between otherwise indistinguishable samples [6]. Differences in isotope signatures result from natural or synthetic reactions. The isotopic signature of starting materials has a significant influence on the signature of the product, as does the synthetic pathway employed during the synthesis [7]. While HMTD is relatively simple to prepare from basic starting materials, mechanistic information suggesting potential bond cleavage/ formation was not found in a literature survey. As a result of this work we have proposed a mechanism, which will be the subject of future investigations.

5.2 Experimental

HMTD was synthesized using hexamine, hydrogen peroxide and citric acid. The hexamine and hydrogen peroxide samples used as precursors were varied to investigate the influence of each on the δ13C, δ15N, and δ18O values of the resulting HMTD. Initial laboratory-based experiments were conducted to observe effects of changing precursor materials under controlled conditions. Field synthesis experiments were then conducted to mimic a forensic-casework scenario, utilizing improvised starting materials. Chapter 5 δ2H and δ18O values for 34 hydrogen peroxide samples were analyzed, providing a background population which included samples from different manufacturers and different concentration solutions ranging from 2% to 60% (w/v). All but one batch of HMTD were synthesized using 30% (w/v) hydrogen peroxide (greater concentrations were diluted using ultrapure water). One sample of HMTD (HMTD06) was prepared using 18% (w/v) hydrogen peroxide (HP1) and in this instance a greater volume of hydrogen peroxide was used to maintain a consistent reaction stoichiometry. Seventeen hexamine samples were collected, of which nine were used to synthesize HMTD. δ13C and δ15N values were determined for a background population of hexamine. The Investigation of isotopic linkages between precursor materials and HMTD 89 majority of hexamine samples were sourced from chemical suppliers. However, the field experiments utilized hexamine extracted from fuel tablets to mimic improvised sources of hexamine that might be encountered in forensic casework. Where the same hexamine source was used in synthesis experiments at different locations the starting materials were labeled A and B. Three sources of citric acid were used in total. The citric acid source used for the initial controlled syntheses remained constant while later experiments explored effects of using three different sources of citric acid.

To assess the isotopic linkage between hexamine and HMTD the hexamine used was varied while the hydrogen peroxide remained constant. Table 5.1 provides the details of which starting materials were used for each HMTD sample, additional information on the isotopic composition of the starting materials is provided in Table 5.2.

5.2.1 Synthesis of HMTD The synthetic process followed produces a sensitive primary explosive material. There are many safety considerations associated with this work that are described. HMTD was synthesized using hydrogen peroxide, hexamine and citric acid according to previously reported methods [4,5]. Synthesis of HMTD took place in three locations: FEL, part of the Defense Science Technology Laboratory (Dstl) in the UK, the Netherlands Forensic 5 Chapter Institute (NFI) and in the training grounds of the Dutch Police Academy in Ossendrecht.

FEL The synthesis of HMTD was performed at the FEL in an explosives licensed facility. For safety reasons this reaction was required to be carried out under humid (minimum 65% humidity), conducting conditions to avoid static build up, which might result in accidental initiation of the explosive product material. A 1 M sodium hydroxide “killer” solution was prepared prior to synthesis. Any items contaminated with starting materials or product, were submerged in the killer solution for no less than 12 h to prevent continued reaction or accidental mixing of starting materials. Items were then removed, rinsed with water and discarded or rewashed for reuse. 90 Chapter 5

Reagents were added to a reaction vessel with air-powered mechanical stirring, in an ice bath. The temperature of the mixture was monitored closely. Were any sudden increase in temperature to be observed the entire reaction vessel would be submerged in the sodium hydroxide “killer” solution and the laboratory evacuated. The solution was gradually brought to room temperature (20% humidity) and left for approximately 18.5 h (overnight). During this time the HMTD product crystallized out of the solution. The white crystalline product was collected by filtration and washed with approximately 150 mL Millipore ultrapure water. The product material was air dried for approximately 5–6 h before being transferred to a pre-cleaned anti-static pot, and sealed in an antistatic bag. Approximately 55–70% yields were obtained.

NFI The NFI followed the same procedure as the FEL with the following variations. The relative humidity of the NFI laboratory was at approximately 50%. The stirring of the reaction solution was done non-mechanically and ceased after all the starting materials were dissolved. The total reaction time was varied and is indicated in the remainder of this article. The field experiments were performed by attendees of a police course on improvised explosives. Plastic coffee beakers were used as reaction vessels and stirring was done using wooden spatulas. Filtering was done over a coffee filter and washing was done using local tap water until the ‘washing water’ appeared neutral (pH ~7 using a lakmus/litmus paper). Because the synthesis was performed under improvised

Chapter 5 conditions, the following parameters were variable for the different syntheses: exact amount of starting materials, ambient temperature and relative humidity, number of washes. The reaction times also varied and were recorded. In the laboratory and field syntheses, yields of approximately 35–70% were observed.

5.2.2 Analytical measurements Isotope measurements were performed using three different instruments as described below. HMTD was identified as the synthesized product using a range of analytical techniques including infrared spectroscopy, Raman spectroscopy, thin-layer chromatography and liquid chromatography–mass spectrometry. Investigation of isotopic linkages between precursor materials and HMTD 91

FEL At FEL isotope-ratio measurements of hexamine and HMTD were performed using a Deltaplus XP isotope-ratio mass spectrometer (Thermo Fisher, Hemel-Hempstead, UK). δ13C and δ15N measurements were performed by preparing six replicate samples in 6 × 4 mm tin capsules (Elemental Microanalysis, Exeter, UK). Combustion of samples took place using a Costech elemental analyzer (Pelican Scientific, Stockport, UK) with catalytic oxidation and reduction tubes held at 1030°C and 650°C respectively. Gaseous species were separated by gas chromatography (3m, Hay-Sep 60-80 mesh) and passed to the mass spectrometer using helium as carrier gas. Carbon and nitrogen measurements were made by detecting the ions with mass-to-charge ratios 44, 45 and 46 (CO2) and 28,

29 and 30 (N2) respectively.

RDX (BAE Systems), sugar (Tate and Lyle) and urea (Fluka Biochemika) laboratory standards were previously calibrated using international reference materials (International Atomic Energy Agency, Vienna, Austria): sucrose (IAEA-CH-6), polyethylene (IAEA-CH-7), limestone (NBS 19), and ammonium sulphate (IAEA-N-1, IAEA-N-2, USGS25). Standard values are given in Table 5.3 [8,9]. Six replicates of each laboratory standard were analyzed alongside the samples and used to normalize the results [10]. International reference materials (IAEA-CH-6, -CH-7, -N-1, and -N-2) 5 Chapter were analyzed in duplicate throughout the analytical sequence as quality control checks.

δ18O measurements were performed by preparing six replicate samples in 6 × 4 mm silver capsules (Sercon, Crewe, UK). Pyrolysis of samples was achieved using a Thermo Finnigan TC/EA elemental analyzer (Thermo Fisher, Hemel-Hempstead, UK). The pyrolysis reactor tube, packed with glassy carbon and held at 1375°C, converts the sample into gaseous products including nitrogen and carbon monoxide which are separated by gas chromatography (1.2 m, 5 Å molecular sieve, 80-100 mesh, custom- made column, Elemental Microanalysis, Exeter, UK) and passed in the helium carrier gas to the mass spectrometer. Oxygen-isotope ratios are determined from measurements of carbon monoxide ions with mass-to-charge ratios 28, 29 and 30. 92 Chapter 5 σ 0.17 0.14 0.16 0.18 0.71 0.19 0.19 0.73 0.31 0.61 0.51 0.81 0.51 0.47 0.55 0.43 0.55 0.55 0.53 0.33 0.63 0.52 0.93 0.57 0.49 0.37 0.59 0.59 0.39 0.87 0.56 0.50 0.46 0.46 0.46 0.40 0.23 0.44 0.24 VSMOW Oxygen O 37.10 35.16 27.71 31.42 31.57 31.69 38.17 31.68 27.43 31.28 37.29 37.87 33.43 32.41 29.93 32.91 32.31 34.70 27.25 27.34 26.61 33.07 39.84 35.66 34.01 34.01 33.25 34.21 32.32 36.62 32.89 38.56 30.29 40.58 28.89 30.40 28.82 34.46 40.28 18 δ 6 4 4 6 6 4 6 6 4 4 6 4 4 4 4 4 4 6 4 6 6 n 6 6 6 6 3 4 4 4 4 4 5 4 3 4 4 6 6 11 σ 0.11 0.14 0.16 0.16 0.18 0.16 0.10 0.19 0.15 0.70 0.12 0.31 0.21 0.03 0.05 0.05 0.38 0.07 0.08 0.08 0.08 0.06 0.09 0.06 0.06 0.06 0.29 0.34 0.06 0.06 0.34 0.34 0.26 0.23 0.34 0.25 0.25 0.25 0.25 AIR N 15 17.40 25.75 18.79 19.35 27.47 19.63 27.67 27.32 19.38 24.10 19.89 25.73 20.19 24.19 27.68 27.87 14.39 21.44 27.80 24.77 14.28 20.43 25.89 25.38 23.97 25.30 20.39 28.32 25.20 20.68 25.84 20.36 26.89 22.69 26.09 28.48 28.09 28.25 26.24 Nitrogen δ 6 5 4 6 4 4 5 6 6 6 6 6 6 6 6 6 6 4 4 6 4 7 6 3 4 4 n 6 6 6 4 4 3 6 6 4 5 4 4 18 σ 0.16 0.12 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.05 0.05 0.05 0.05 0.05 0.02 0.07 0.02 0.02 0.07 0.08 0.07 0.02 0.02 0.09 0.08 0.06 0.06 0.06 0.28 0.04 0.00 0.00 VPDB C Carbon -37.14 -39.14 -37.77 -37.51 -38.11 -36.14 -37.53 -39.35 13 -37.08 -37.23 -35.89 -55.96 -55.87 -55.60 -36.45 -35.20 -36.35 -36.65 -43.70 -54.51 -36.03 -54.45 -38.48 -38.27 -36.90 -54.42 -38.34 -38.44 -56.09 -36.44 -56.09 -56.44 -44.76 -48.72 -46.81 -44.32 -44.07 -40.23 -40.26 δ 5 6 6 6 6 5 5 6 4 3 6 4 6 5 5 6 6 6 6 5 n 3 4 4 6 4 4 3 7 6 3 4 6 4 5 4 4 4 6 18 ------σ 1.76 0.71 0.76 0.41 0.91 1.62 1.53 1.33 1.63 1.02 1.82 1.02 1.27 0.55 0.67 0.33 0.42 1.54 0.95 0.59 1.24 0.87 0.90 0.66 0.50 0.60 0.23 0.64 2.48 indicates field synthesis at NFI. ‡ ------VSMOW 8.31 71.01 70.18 21.62 47.37 31.56 63.41 38.76 24.76 40.71 49.95 78.01 15.97 79.86 27.00 18.50 22.79 50.62 23.56 26.63 62.69 23.34 48.32 68.39 24.97 32.22 22.58 60.25 64.60 H Hydrogen 2 δ ------4 6 6 6 4 6 4 6 n 9 4 4 4 4 6 4 4 4 4 4 4 4 4 5 6 5 6 3 4 4 Chapter 5 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 1 3 3 3 3 3 3 3 3 1 1 1 2 2 1 1 1 1 2 indicates synthesis at NFI, † Citric acid 2 O 2 HP1 HP6 HP2 HP2 HP2 HP2 HP2 HP2 H HP14 HP13 HP31 HP31 HP33 HP32 HP32 HP32 HP32 HP32 HP32 HP32 HP32 HP32 HP32 HP32 HP32 HP32 HP30 HP30 HP29 HP26 HP25 HP25 HP25 HP28 HP24 HP24 HP24 HP24 HP24 Precursors HEX4 HEX7 HEX5 HEX2 HEX17 HEX17 HEX17 HEX17 HEX17 HEX17 HEX17 HEX17 HEX17 HEX17 HEX17 HEX17 HEX16 HEX16 HEX15 HEX15 HEX15 HEX15 HEX15 HEX15 HEX15 HEX15 HEX15 HEX12 HEX10B HEX10B HEX10B HEX10B HEX10B HEX10A HEX10A HEX10A HEX10A HEX10A HEX10A Hexamine * * * * * * ‡ † * † † ‡ * ‡ † † † ‡ † † † † † † † ‡ ‡ ‡ * † * ‡ ‡ ‡ ‡ ‡ ‡ † † HMTD06 HMTD30 HMTD33 Sample ID HMTD20 HMTD02 HMTD03 HMTD12 HMTD37 HMTD15 HMTD24 HMTD25 HMTD32 HMTD22 HMTD28 HMTD10 HMTD17 HMTD19 HMTD11 HMTD13 HMTD08 HMTD04 HMTD07 HMTD01 HMTD09 HMTD38 HMTD39 HMTD21 HMTD27 HMTD29 HMTD31 HMTD16 HMTD35 HMTD36 HMTD23 HMTD05 HMTD26 HMTD18 HMTD14 HMTD34 Indicates laboratory synthesis at FEL, Table 5.1. Precursor 5.1. Table materials used to synthesize batches HMTD of and resulting isotope values HMTD. of * Investigation of isotopic linkages between precursor materials and HMTD 93 σ 0.11 0.18 0.18 0.16 0.18 0.10 0.10 0.05 0.26 0.20 0.28 0.22 0.26 0.44 0.22 VSMOW 1.82 1.06 1.34 0.05 0.08 6.38 2.32 3.44 4.66 2.38 2.87 2.66 2.20 Oxygen -1.90 -0.45 O 18 δ 8 4 8 4 3 3 3 2 3 n 11 16 14 16 16 16 σ 0.2 0.12 0.01 0.01 0.03 0.03 0.05 0.38 0.09 0.06 AIR N 0.4 1.06 15 -2.45 -0.12 -0.41 -0.67 -0.37 -0.83 -0.02 -0.24 Nitrogen δ 5 7 6 6 6 6 6 6 6 6 n σ 0.16 0.78 0.01 0.01 0.03 0.05 0.02 0.02 0.02 0.06 VPDB C Carbon 13 -34.75 -52.13 -35.93 -35.20 -42.17 -34.87 -41.90 -43.20 -46.98 -42.29 δ 5 7 6 6 6 6 6 6 6 6 n σ 1.13 0.75 0.51 0.81 0.53 4.55 0.32 0.83 0.40 0.46 0.46 0.34 0.80 0.84 0.64 Chapter 5 Chapter VSMOW H -87.18 -76.72 -51.89 -52.73 -91.07 -32.30 -69.58 -82.01 -85.85 -54.48 -82.25 -48.57 -107.15 Hydrogen 2 -112.29 -103.88 δ 8 8 4 4 3 3 3 3 2 n 11 14 14 16 16 16 -values hexamine for (HEX) and hydrogen peroxide (HP) precursor materials. 30% Applichem (diluted from 50%) 30% Solvay (diluted from 60%) 30%Fisher (diluted from 50%) 30% Fisher 30% Fisher 18% Teknique Basics18% 30% Fisher Esbit K. F. Meyer K. F. Fisher Aldrich Esbbit, GmbH & Co. Esbit Description Riedel–deHäen Sigma Aldrich Fluka Biochemika Aldrich HP 26 HP 28 HP 29 HP 31 HP 25 HP 30 HP 32 HP 33 HP 24 HP 14 HP 13 HP 6 HP 2B HP 1 HP 2A HEX 17 HEX 15 HEX 4 HEX 10A HEX 12 HEX 16 Precursor HEX 5 HEX 2 HEX 7 HEX 10B Table 5.2.Table Summary measured of δ 94 Chapter 5

RDX (BAE Systems), and polyethylene glycol (PEG, Fisher Bioreagents), laboratory standards were previously calibrated using international reference materials (International Atomic Energy Agency, Vienna, Austria): limestone (NBS 19), potassium nitrate (USGS34, IAEA-NO-3) and benzoic acid (IAEA-602). Six replicates of each laboratory standard were analyzed alongside the samples and used to normalize the results. International reference materials (IAEA-NO-3, USGS34) were analyzed in duplicate throughout the analytical sequence as quality-control checks.

Oxygen and hydrogen measurements of hydrogen peroxide starting materials were determined at Queen’s University Belfast using a Thermo Finnigan TC/EA elemental analyzer coupled in sequence to the Deltaplus XP isotope-ratio mass spectrometer.

Burn tests were conducted by igniting small aliquots of the isolated products with a micro torch flame in an armored fume cupboard. A very rapid burn rate provides a crude indication of the presence of an energetic material, as would be expected were HMTD successfully synthesized.

Infrared spectra were obtained using a Bio-Rad Digilab Division FTS 3000MX Excalibur Series spectrometer fitted with a Specac Mark II Golden Gate single reflection ATR accessory.

Chapter 5 Portions of each of the solid products obtained were dissolved in acetone and spotted onto a thin-layer chromatography plate adjacent to HMTD reference standards and blank

solvent samples. The plates (Macherey-Nagel Polygram SilG/UV254 polyester plates with 25 mm silica gel containing a fluorescent indicator, 200 × 200 mm, CAMLAB, Cambridge, UK) were run over 10 cm using a 90% toluene 10% ethyl acetate eluent, then allowed to air dry. The plate was observed under UV light (254 nm) before being sprayed with 1% diphenylamine solution and allowed to stand for 5–10 min. The plate was then heated. Presence of HMTD is indicated by the development of dark blue spots which fade with heating. Investigation of isotopic linkages between precursor materials and HMTD 95

Table 5.3. Standard values used for isotope δ value determinations [8,9].

2 13 15 18 Standard δ HVSMOW (‰) δ CVPDB (‰) δ NAIR (‰) δ OVSMOW (‰) RDX (BAE Systems) -33.42 -9.93 23.91 Sugar (Tate and Lyle) -11.36 Urea (Fluka Biochemika) -0.96 IAEA-CH-6 -10.45 IAEA-CH-7 -100.3 -32.15 NBS 19 -1.95 28.65 IAEA-N-1 0.4 IAEA-N-2 20.3 USGS25 -30.4 PEG (Fisher Bioreagents) -17.05 USGS34 -27.9 IAEA-NO-3 25.6 IAEA-601 23.14 IAEA-602 71.28 SLAP -428.0 -55.5 VSMOW 0.0 0.0 NFI Tap Water -43.3 -6.6 GISP -189.5 -24.76 NBS 22 -119.6 -30.03 USGS24 -16.05 Candle wax – K46-1-i-1 -127.6 -30.4 Chapter 5 Chapter

NFI Carbon-, nitrogen-, oxygen- and hydrogen-isotope ratios were measured at the NFI by continuous-flow isotope-ratio mass spectrometry (CF-IRMS) on a Delta V Advantage (Thermo Fisher Scientific, Bremen, Germany). Online sample preparation was done on either an elemental analyzer (FlashEA 1112; carbon and nitrogen) or a high-temperature conversion elemental analyzer (TC/EA; oxygen and hydrogen), both coupled to the IRMS by a ConFlo IV open-split interface (all Thermo Fisher Scientific).

Samples for carbon and nitrogen analysis were prepared in tin capsules as described for FEL. The oxidation tube temperature was set at 1020°C and reduction reactor at

650°C. N2 and CO2 gases were separated using a 3-m stainless-steel packed GC column 96 Chapter 5

(Porapak QS, ID 5 mm) before entering the ConFlo IV open-split interface. Here the

sample was diluted to 95% for CO2 so that the signal intensity of the sample was similar to the intensity of the reference gases.

Oil (NBS 22), graphite (USGS24), polyethylene (IAEA-CH-7), ammonium sulphate (IAEA-N-1, IAEA-N-2) were measured to normalize the data. Candle wax (K46-1-i-1), urea (Fluka Biochemika) and sugar (Tate and Lyle) were measured as quality-control checks (Table 5.3). Offline corrections were based on the method by Sharp [11].

Hydrogen- and oxygen-isotope measurements were made by preparing samples in silver capsules and dropping them into the TC/EA reactor (1400°C) by a MAS 200R autosampler. The TC/EA system at the NFI is modified with a reversed-flow adapter according to Gehre et al. [12]. Hydrogen peroxide samples were analyzed using the same reversed-flow adapter system and similar reactor conditions but with an AS3000 injector autosampler. Benzoic acid (IAEA-602 and IAEA-601), NBS 22, polyethylene (IAEA-CH-7), SLAP, GISP and VSMOW were measured to normalize the data against the VSMOW scale. Candle wax (K46-1-i-1) and NFI tap water, were measured as quality control checks (Table 5.3). Offline corrections are based on the method by Sharp [11].

Raman spectra were obtained using a portable Smiths Detection Responder RCI spectrometer using the internal sample compartment and using a total integration time

Chapter 5 of 30 seconds per sample.

5.3 Results

5.3.1 Isotope analysis of precursor materials Mean δ13C and δ 15N values for 17 different hexamine samples were determined from

13 15 replicate measurements (Table 5.2). Observed δ CVPDB values and δ NAIR values for the hexamine samples ranged from -52.13‰ to 34.75‰ and 2.45‰ to 1.06‰, respectively. Mean δ2H and δ18O values for 34 different hydrogen peroxide samples (including the water used for dilutions) were determined from replicate measurements. Observed Investigation of isotopic linkages between precursor materials and HMTD 97

2 18 δ HVSMOW values and δ OVSMOW values for the hydrogen peroxide samples ranged from -147.04‰ to -18.78‰ and 6.57‰ to 13.43‰, respectively. Some difficulties were encountered with hydrogen peroxide sample analyses, meaning that it was not possible to obtain isotope values for all of the hydrogen peroxide starting materials. Samples where values could not be measured were not used for the synthesis of HMTD.

5.3.2 HMTD identification Infrared spectroscopy The spectra for each sample demonstrated several characteristic peaks corresponding to

(CH2)3N, CH2O and CO stretches, matching the library reference standard for HMTD with hit quality index (HQI) figures between 746 and 920. An HQI figure of 1000 reflects a perfect match.

Raman spectroscopy Raman spectra were collected for the samples prepared at the NFI and in the field experiments, and matched to an HMTD reference in the internal library. Hit qualities for all the HMTD samples were between 0.64 and 0.78. A hit quality of 1 represents a perfect match. Chapter 5 Chapter

Thin-layer chromatography A small quantity of each of the samples HMTD1–HMTD10 was dissolved in acetone and spotted onto a thin-layer chromatography plate alongside reference standards. Comparison of the retention factor for the HMTD standard with each of the products supported the observation that HMTD was produced.

Isotope analysis In total, 39 HMTD samples were prepared from the precursor materials available. Different combinations of precursor materials were applied under a variety of conditions to investigate the influence of these factors on the isotope signature of product HMTD. Hydrogen-, carbon-, nitrogen- and oxygen-isotope values (Table 5.1) ranged as follows: 98 Chapter 5

2 13 14 δ HVSMOW 8.31‰ to 79.86‰, δ CVPDB -56.44‰ to -35.20‰, δ NAIR 14.28‰ to 28.48‰

18 and δ OVSMOW 26.61‰ to 40.58‰.

5.4 Discussion

The peroxide explosive HMTD has been synthesized from precursor materials from a range of sources. HMTD was identified as the product using a number of analytical techniques and the stable-light-isotope ratios for precursors and products were measured.

5.4.1 Oxygen- and hydrogen-isotope ratios For the synthesis of samples in this study, a similar concentration of hydrogen peroxide was generally used. The precursor hydrogen and oxygen δ values were determined for the bulk hydrogen peroxide material. Under laboratory conditions, a linear relationship between precursor and product was initially observed, suggesting a useful link to the hydrogen peroxide precursor. When the concentration of hydrogen peroxide was changed (more diluted), the result did not fit the previously observed relationship. This may be due to the isotope signature of the aqueous matrix affecting the overall precursor isotope signature, because the water and any additives present in the hydrogen peroxide will contribute to the bulk isotope δ value. However, these materials will not participate in the reaction, invalidating the comparison of the two values. Kinetic effects of the reaction will differ and it is likely that this also contributes to the different

Chapter 5 relationship observed. The hydrogen precursor–product relationship is also confused by the presence of water in one of the precursor materials as described above, but because of the exchangeable nature of hydrogen, the product may incorporate hydrogen atoms from a range of sources. It was not possible to elucidate the cause of variations for δ18O and δ2H or to establish a robust precursor–product relationship using the data from this study; therefore, we do not discuss this further in this chapter.

5.4.2 Controlled laboratory conditions Comparison of the precursor and product isotope signatures obtained for initial batches of HMTD synthesized, under controlled laboratory conditions, revealed relationships Investigation of isotopic linkages between precursor materials and HMTD 99 between precursor and product, which may be useful during forensic investigations. Further synthesis experiments were subsequently conducted by the NFI (see section 5.4.3). The latter experiments conducted by the NFI explored effects of changing synthesis conditions, operator and use of improvised starting materials. Fig. 5.1 shows the carbon and nitrogen isotopic fractionation observed for HMTD synthesized in the laboratories at FEL and NFI where the source of precursor used was the only factor varied.

35

25

15 Hexamine (NFI) N vs AIR (‰)

15 Hexamine (FEL) δ 5 HMTD (NFI) HMTD (FEL) HMTD NFI (FEL protocol) -5 -64 -54 -44 -34

δ13C vs VPDB (‰) 5 Chapter Fig. 5.1. Carbon- and nitrogen-isotope ratios of the precursor hexamine and the synthesized HTMD. Closed symbols are used for the experiments performed at the NFI.

The slope of the carbon- and nitrogen-isotope shift is comparable. However, the amount of fractionation is different. In general, the NFI laboratory experiments show an isotope shift larger than the FEL experiments. A different time step in the procedure was found and two more experiments (black symbols) were performed at the NFI to synthesize HMTD following the exact same procedure as FEL. The isotope shift was closer to the FEL results but not equal. This offset in the isotope shift probably is related with subtle changes in laboratory conditions which can vary the reaction rate and ultimately result in variation in carbon- and nitrogen-isotope ratios. 100 Chapter 5

H+ C6H12N4 + 3H2O2 C6H12N2O6 + 2NH3 (5.1)

From the reaction equation (5.1) it is clear that hexamine provides the source of carbon and nitrogen while hydrogen peroxide provides the source of oxygen during the production of HMTD. Carbon- and nitrogen-isotope linkages were explored between hexamine as the precursor and HMTD as the product. Oxygen isotopic linkages might be expected between hydrogen peroxide and the product HMTD. This relationship is difficult to explore since the hydrogen peroxide starting material is an aqueous solution containing hydrogen peroxide, water and potentially a number of stabilizers or additives. All of these contribute to the bulk isotope signature of this precursor, but these are not all incorporated into the HMTD product making a direct precursor–product comparison unfeasible [13]. Under controlled conditions a close linear relationship is observed (R2 = 0.99) when carbon δ values for the hexamine precursor and HMTD product are plotted against one another. A poorer fit to linearity is observed R( 2 = 0.77) when the same relationship is explored using nitrogen δ values. This is not surprising considering there are two nitrogen-containing products resulting from this reaction.

5.4.3 Variation in reaction conditions To explore the effects of changing synthesis conditions and operator and the use of improvised starting materials a series of syntheses were performed under improvised conditions (‘field experiments’). In addition, control experiments were performed in the

Chapter 5 laboratory to investigate the effect of particular reaction conditions. The results of these experiments are plotted in Fig. 5.2, together with the data of the laboratory experiments performed at FEL. The spread in the field experiments is larger than the variation in the laboratory experiments, even though the reaction time and conditions during the field experiment were kept as constant as possible. The field experiments provided data under more realistic forensic circumstances. It shows that although under laboratory conditions the product and precursor can be linked, this is not necessarily possible for casework. Investigation of isotopic linkages between precursor materials and HMTD 101

35

25

Hexamine (NFI) 15 Hexamine (Esbit)

N vs AIR (‰) Hexamine (FEL) 15 δ HMTD (NFI) 5 HMTD (FEL) HMTD field HMTD (NFI time) -5 -64 -54 -44 -34 δ13C vs VPDB (‰)

Fig. 5.2. Plotted are all hexamine and HMTD carbon- and nitrogen-isotope values. Circled, black diamonds are the data of the field experiments plotted. Results of laboratory-based experiments are also presented for comparison.

A time-controlled experiment illustrated the influence of the reaction time on the synthesized HMTD. The results (Fig. 5.3) indicated that generally the longer the reaction time, the closer the carbon-isotope ratio of the product HMTD to the carbon- isotope value of the precursor hexamine, which corresponds with increasing yields over time (24% after 3 h, 40% after 6 h 53% after 24 h to 57% after two days). A synthesis 5 Chapter experiment using reduced citric acid concentration (0.3 g instead of the 1.4 g used in all the other laboratory syntheses), produced a poor yield (5%) and had a marked effect on the isotope δ values observed for the HMTD product (Fig. 5.3). Overall we assume that incomplete reaction, due to a short reaction time or insufficient acid and resulting in a lower yield, will cause the carbon-isotope ratio of the HMTD to shift to more negative values than its precursor hexamine. All of the results in this study indicate that HMTD products are consistently enriched in δ15N and depleted in δ13C compared to the hexamine precursor material. We therefore discuss two features of this precursor– product isotope fractionation: (1) the direction on the internationally recognized scales and (2) the magnitude of the shift. 102 Chapter 5

35

6h 6h 3h 24h 25 48h 12d less citric acid 15 Hexamine (NFI)

Nvs AIR (‰) Hexamine (FEL) 15 δ HMTD NFI (time) 5 HMTD (time 12d) HMTD NFI HMTD (less citric acid) -5 -48 -44 -40 -36 -32 δ13C vs VPDB (‰)

Fig. 5.3. Results of the reaction-time controlled experiment and the synthesis with a reduced citric acid concentration. The slope of the solid arrows is comparable to the slope of the arrows in Fig. 5.1. The shaded diamond is the HMTD product synthesized with a lower concentration of citric acid.

5.4.4 Carbon-isotope ratios Carbon-isotope values of the HMTD are consistently shifted to more negative δ13C compared to its precursor. However, the magnitude of this change is relatively small, as

13 illustrated by a small fractionation factor εaverage = -2.6, standard deviation (SD) = 1.3 (Eq. 5.2). The light isotope is expected to be preferentially incorporated into the product during a reaction process. This is due to the weaker bond energies of the light isotope compared with the heavy isotope, meaning that the light-isotope bonds are more readily Chapter 5 cleaved and formed. Initially 12C isotopes are preferentially incorporated into the HMTD and the relative 13C-isotope concentration in the available reactant increases until the 13C is also incorporated into the product as this reaction proceeds. The small difference in δ values is attributed to incomplete reactions; the closer a reaction is to completion (the higher the yield), the closer the HMTD carbon value is to that of the hexamine precursor. This is illustrated by the trend observed in the reaction-time experiment (Fig. 5.3). The fractionation factor observed for different yields of HMTD is in line with this observation; lower yields resulted in a more negative δ13C value. This relationship may therefore provide some indication of the efficiency and yield of the reaction. The carbon Investigation of isotopic linkages between precursor materials and HMTD 103

δ value plot of the HMTD product and the hexamine precursor closely fit linearity and are relatively similar in magnitude even when reaction conditions are changed, because all of the carbon in the precursor is theoretically incorporated into the product (assuming a 1:1 stoichiometry of precursor to product). This assumption is supported by yields in excess of 50%.

5.4.5 Nitrogen-isotope ratios The magnitude of the nitrogen fractionation is much greater than that observed for carbon. From the stoichiometric reaction (Eq. 5.1), it can be seen that the single nitrogen- donating precursor results in two nitrogen-containing products, HMTD and ammonia. The δ15N value of the HMTD product is consistently more positive compared to the hexamine precursor, suggesting that the heavy isotope is preferentially incorporated into the HMTD. Alternatively, one might consider that the byproduct ammonia involves the greatest bond cleavage of the two products and therefore favors the light isotope. The isotope-enrichment factor (Eq. 5.2 and 5.3) illustrates the average change in δ values between precursor and product:

α = (1000 + δProduct) / (1000 + δReactant) (5.2)

ε = (α - 1) × 1000 (5.3) 5 Chapter

With α representing the fractionation factor and ε representing the enrichment factor. Table 5.4 indicates the carbon and nitrogen enrichment factors observed for different experiments. A greater magnitude of enrichment is observed for nitrogen compared with carbon (13ε ~ -2‰, 15ε ~24‰). This is considered to be resultant of multiple nitrogen- containing products. Although variation was observed in the controlled experiments (R2 = 0.77), much greater variation was observed using differing reaction conditions (R2 = 0.29), suggesting that the conditions of the reaction (reaction temperature and time) have a greater influence on the nitrogen-isotope ratio compared with the carbon-isotope ratio. 104 Chapter 5

The variation observed in the nitrogen precursor–product relationship adds complexity to interpretation of the information. The δ15N values consistently shifted to be more positive in the HMTD product compared with the hexamine precursor, but further work is required to elucidate influences of temperatures and reaction time.

Table 5.4. The enrichment factors (ε) of different experiments performed. Carbon Nitrogen ε SD ε SD FEL lab experiments -1.45 0.25 20.40 0.47 NFI lab experiments -2.39 0.44 26.36 1.99 Field and reaction-time experiments -3.01 1.48 24.46 3.59

5.4.6 Reaction mechanism Without a strongly established reaction mechanism a scenario was considered where two mol of hexamine might react to give one mol of HMTD. In this case, greater than 50% yields would not be expected. The small fractionation factor and yields in excess of 50% thus support a 1:1 ratio of reactant to product. A 2:1 reaction would result in greater fractionation than was observed, as approximately 50% of the carbon would remain unused. We propose here a mechanism for the formation of HMTD from hexamine (Fig. 5.4). We postulate that the hexamine will undergo an acid-catalyzed decomposition, in the presence of citric acid and hydrogen peroxide, to give a triperoxy tertiary amine (I) and protonated methyleneimine (II). Both of these species are in equilibrium Chapter 5 with formaldehyde and ammonia and possibly the hexamine completely decomposes to formaldehyde and ammonia before reforming I and II as transient ‘metastable’ intermediates. The nucleophilic addition of the peroxy groups (I) into the imine (II) leads to intermediate III. We postulate that these primary amines would readily dissociate under the acidic conditions yielding ammonia and HMTD. The requirement for a greater than catalytic amount of citric acid for optimum yields, as observed in this work, is thought to be due to the citric acid combining with the ammonia produced to form ammonium citrate. Consequently, the concentration of the product ammonia will be reduced and the equilibrium will be shifted in favor of the HMTD. Increasing the Investigation of isotopic linkages between precursor materials and HMTD 105 starting quantity of citric acid to over two equivalents may, therefore, result in increased yields. Further work is required to confirm this, and to lend evidence for our proposed mechanism.

HO OH HOO N N N N H H2O2 -H N N HN N HN N HN N N N N N Hexamine O

x3 N HN NH H H2O H H + NH NH + OOH OOH 2 NH3 OOH II I N N N x3 OOHOOOH H OOHOOOH O OO I OH O O OO

H N H2N NH2 2 H2N NH2 II III

N N N H O O O O O OO -NH3 O -H O x2 Chapter 5 Chapter O OO O OO O OO

H2N H2N HN H2N NH3 H2N H2N N

O OO O OO

N

HMTD Fig. 5.4. Postulated mechanism for the acid-catalyzed formation of HMTD from hexamine and hydrogen peroxide. 106 Chapter 5

5.5 Conclusions

Having prepared multiple HMTD samples from various precursors using a range of experimental conditions, we have observed results that may be useful in forensic investigations of improvised explosives materials. Precursor and product carbon-isotope δ values consistently fit a linear relationship regardless of precursor or conditions. The HMTD product has a δ13C value similar to that of the starting material. The magnitude of the isotope fractionation observed is affected by the efficiency of the reaction. Greater yielding reactions give rise to HMTD with δ13C values more similar to the precursor material than lower yielding reactions. Additionally the product is consistently more depleted in 13C compared with the precursor. Nitrogen-isotope δ values comparing precursor with product show some linearity when the reaction conditions are carefully controlled. However, results indicate a poor fit with linearity when synthesis conditions are changed. Despite the greater variation, the direction of isotope shift is consistent, with HMTD product having a more positive δ15N value compared with the hexamine precursor.

We consider hexamine reacts to form HMTD in a 1:1 ratio according to the proposed mechanism. This suggestion is supported by the yields and small carbon-isotope- enrichment factor. The trends observed in this study may provide useful information to forensic investigators linking precursor materials with improvised explosive devices,

Chapter 5 especially in the exclusion process. Further information may be accessed using oxygen- and hydrogen-isotope data, but analogous precursor–product comparisons with these elements are complicated by the aqueous matrix of hydrogen peroxide solutions. Investigation of isotopic linkages between precursor materials and HMTD 107

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