Analytical Chemistry
Article
18% (w/v) hydrogen peroxide (HP1), and in this instance a
greater volume of hydrogen peroxide was used to maintain a
consistent reaction stoichiometry.
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 performed using
wooden spatulas. Filtering was performed over a coffee filter,
and washing was performed using local tap water until the
“washing water” appeared neutral (pH ∼ 7 using a lakmus/
litmus paper). Because the synthesis was performed under
improvised 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.
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
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.
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 chromatog-
raphy mass spectrometry.
FEL. At FEL, isotope ratio measurements of hexamine and
HMTD were performed using a Deltaplus XP isotope ratio mass
spectrometer (Thermo Fisher, Hemel-Hempstead, U.K.).
δ13C and δ15N measurements were performed by preparing
six replicate samples in 6 × 4 mm tin capsules (Elemental
Microanalysis, Exeter, U.K.). Combustion of samples took place
using a Costech elemental analyzer (Pelican Scientific,
Stockport, U.K.) with catalytic oxidation and reduction tubes
held at 1030 °C and 650 °C, respectively. Gaseous species were
separated by gas chromatography (3 m, Hay-Sep 60−80 mesh)
and passed to the mass spectrometer in a helium carrier gas.
Carbon and nitrogen measurements were made by detecting
the ions with mass/charge ratios 44, 45, and 46 and 28, 29, and
30, 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-CH6),
polyethylene (IAEA-CH7), limestone (NBS19), and ammo-
nium sulfate (IAEA-N1, IAEA-N2, USGS 25). Standard values
are given in Table 2.8,9 Six replicates of each laboratory
standard were analyzed alongside the samples and used to
normalize the results.10
International reference materials (IAEA-CH6, -CH7, -N1,
and -N2) 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, U.K.). Pyrolysis of samples was
achieved using a Thermo Finnigan TC/EA elemental analyzer
(Thermo Fisher, Hemel-Hempstead, U.K.). 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 chromatog-
raphy (1.2 m, 5 Å molecular sieve, 80−100 mesh, custom-made
column, Elemental Microanalysis, Exeter, U.K.) and pass in the
helium carrier gas to the mass spectrometer. Oxygen isotope
ratios are determined from measurements of carbon monoxide
ions with mass/charge ratios 28, 29, and 30.
To assess the isotopic linkage between hexamine and
HMTD, the hexamine used was varied while the hydrogen
peroxide remained constant. Table 1 provides the details of
which starting materials were used for each HMTD sample.
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 U.K., The Netherlands Forensic 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.
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 of Millipore UHQ water. The product material was air-
dried for approximately 5−6 h before being transferred to a
precleaned antistatic 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 performed nonmechanically and ceased
after all the starting materials were dissolved. The total reaction
time was varied and is indicated in the remainder of this article.
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 (NBS19),
potassium nitrate (USGS 34; IAEA-NO3), and benzoic acid
(IAEA-602). Six replicates of each laboratory standard were
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dx.doi.org/10.1021/ac300642c | Anal. Chem. 2012, 84, 4984−4992