ion mobility spectrometry of peroxide explosives tatp and hmtd

ION MOBILITY SPECTROMETRY OF PEROXIDE EXPLOSIVES TATP AND
HMTD
Andrew J. Marr and David M. Groves
Defence Science and Technology Laboratory (Dstl), Fort Halstead, Sevenoaks, Kent, TN14 7BP,
United Kingdom.
Abstract
The ion mobility and mass spectra of
TATP (triacetone triperoxide) and HMTD
(hexamethylene triperoxide diamine) have
been recorded in both positive and negative
ion mode using a gas chromatograph-ion
mobility spectrometer-mass spectrometer
(GC-IMS-MS). The effects of temperature,
humidity, dopant concentration and analyte
concentration have been investigated. Dopant
concentration and temperature were found to
have significant effects on the spectra of both
compounds. TATP was very sensitive to
detection in positive ion mode employing a
signal correlating to a molecular ion species.
However, it was found to be effectively
undetectable at trace levels in negative ion
mode. HMTD spectra were observed in both
ion modes.
Introduction
The organic peroxides triacetone
triperoxide (TATP) and hexamethylene
triperoxide diamine (HMTD) are highly
sensitive primary explosives (structures are
shown in Figure 1). The ease of manufacture
of these compounds from readily obtainable
domestic and industrial products, coupled with
explosive power and
ease of initiation has
resulted in their usage in H
O O
3 C
improvised explosives C
devices. In 2001, H 3 C
Richard Reid, the socalled
‘Shoe Bomber’,
O
O
used TATP as an
C
improvised detonator in
his attack on an
American Airlines flight.
Ion mobility TATP
spectrometry (IMS) is
the core technology underpinning the majority
of trace explosive detection systems. Previous
work has indicated that TATP and HMTD can
be detected using IMS but the results were
limited [2,3]. The aim of this research was to
identify the nature of the product ion peaks
(PIPs) for TATP and HMTD using IMS/MS and
to study the behaviour of these ions as a
function of the sample concentration and the
internal temperature, humidity and dopant
concentration of the system.
Experimental
A GC-IMS-MS system was assembled by
attaching a Perkin Elmer Autoscan GC to a
PCP IMS-MS via a heated transfer line that
contained an extension of the capillary
column. A 5 m long, narrow bore (250 mm
inner diameter) column was used with a HP 5
(95 % methyl siloxane / 5% phenylsiloxane)
stationary phase with helium as the carrier
gas. The injector temperature was set at 175
×C. The oven temperature was set at 55 ×C
for TATP and 120 ×C for HMTD and remained
constant throughout sample elution
(isothermal conditions). The end of the
column was positioned so samples were
CH 3
C
CH 3
O
N
O
H 3 C CH 3
Figure 1: Structures of HMTD and TATP
CH 2
CH 2
O
O
O
O
CH 2
CH 2
CH 2 O O CH 2
HMTD
N
Received for review June 6, 2003, Accepted July 15, 2003

60 - Ion Mobility Spectrometry of Peroxide Explosives TATP and HMTD
directly injected into the ionisation source
region of the IMS.
Samples of TATP and HMTD in solution
(acetone and chloroform) were manually
injected into the GC and upon elution from
the column were entrained in a carrier gas
(air) flowing at 100 and 200 cm3/min. A
counter flow of air at 500 cm3/min entered
from the detector end of the drift cell. A
dopant could be added to the carrier flow
prior to it entering the IMS system. Ammonia
(NH3) and dichloromethane (CH2Cl2) were
used as dopants in positive and negative ion
mode, respectively. It was also possible to
minimise dopant concentrations by
preventing their addition to the carrier gas.
The humidity of the carrier flow was
monitored. Most of the studies were
performed with the air relatively dry (< 10
parts per million by volume (PPMV)) but
some measurements were made with the
humidity increased by adding water vapour
to the gas by passing it through a water
bubbler.
IMS spectra were recorded at series of
different IMS drift cell temperatures; 70,
100, 140, 170 and 220 ×C for TATP and 70,
100,150 and 200 ×C for HMTD. The electric
field strength within the drift cell was
maintained at 200 V/cm produced by a
maximum electrode voltage of 3 kV.
The collector electrode of the IMS has a
¼ inch diameter hole drilled through its
centre through which approximately 7% of
the total ion current passed. The sampling
orifice (diameter 25 mm), separating the IMS
from the MS regions, located behind the
collector surface allows only a very small
fraction of the total ion current (~10-5) to
pass through for analysis by the quadrupole
mass spectrometer. Mass spectra were
recorded with quadrupole region pressures
from 5 ¥ 10-5 to 8 ¥ 10-5 Torr. Mass spectra
were measured up to 500 atomic mass units
(amu) with the IMS gating grids fully open.
Single ion monitoring experiments were also
performed. In this mode, two spectra are
produced: an IMS spectrum and the selected
mass ion signal as a function of ion mobility.
In combination, these provide a means of
identifying the product ion peak (PIP) with
which the ions of the selected mass are
associated.
Results and discussion
TATP
Table 1: Optimum conditions for TATP and HMTD IMS detection
ion mode
drift cell
temperature (°C)
TATP could only be detected at trace
levels (sub mg) levels in positive ion mode.
humidity
(PPM V )
dopant
conditions
PIP identity
TATP positive 100 < 100 none (TATPNH 4 ) +
b)
a)
RIP
Acetone +
m/z = 58
((TATP)•NH 4 ) +
m/z = 240
12 18 24 30
drift tim e / m s
Figure 2: IMS spectra of TATP recorded at
100 oC at low humidity (< 10 PPMV) a) in
the presence of ammonia dopant and b)
with dopant levels minimised.
HMTD
negative 150

Ion Mobility Spectrometry of Peroxide Explosives TATP and HMTD - 61
TATP spectra is characterised by the
presence of two PIPs;
an adduct of the molecular ion, m/z
240, K 0 = 1.36 tentatively identified as
(TATPNH4)+, and
an ion with m/z 58, K 0 = 2.32,
tentatively identified as the acetone
molecular ion.
In the presence of ammonia dopant the
m/z 58 ion is the most dominant PIP but
when the dopant concentration is minimised
the larger ion becomes dominant as is shown
in Figure 2. This suggests the TATP reacts
with the ammonia causing molecular
fragmentation but the mechanism for this
reaction has not been established. Increasing
the temperature caused a similar but less
pronounced effect reducing the intensity of
the molecular adduct ion. In this case it was
possible to observe fragmentation of the
molecular adduct ion within the drift cell.
At higher temperatures (> 130 ×C) the
molecular adduct ion PIP is not present but
two additional peaks, m/z 73, K 0 = 2.28 and
m/z 89, K 0 = 2.18, are observed. The effect
of the dopant is less significant but its
presence still increases the intensity of the
lowest mass ion (m/z 58) at the expense of
the larger ions.
Increasing the humidity of the carrier
gas had some effect on the sensitivity of the
system. This was most noticeable at lower
temperatures when ammonia dopant
concentration was higher.
Optimum sensitivity for detection of
TATP was obtained at an IMS temperature of
100 ×C, with dopant concentration minimised
and humidity low (< 100 PPMV) employing
the m/z 240 PIP. However, relatively
sensitive detection was achievable over a
range of temperatures and dopant
conditions.
HMTD
HMTD produced complex IMS spectra in
both positive and negative ion modes. In
positive ion mode at lower temperatures (<
130 ×C) molecular adduct ions, m/z 209, K 0
= 1.50 and m/z 226, K 0 = 1.49, were
observed. At temperatures above 100 ×C
PIPs associated with fragment ions were also
observed. The two most significant fragment
PIPs were associated with m/z 90, K 0 = 2.10
and m/z 145, K 0 = 1.73. As was observed for
TATP, increasing the temperature and the
presence of ammonia dopant caused a
reduction in the intensity of the molecular
ions and an increase in the intensity of the
fragment ion PIPs. In addition to fragment
ions a HMTD PIP associated with a m/z 224,
K 0 = 1.49 ion was also observed at higher
temperatures (130 ×C). This ion was
tentatively identified as an adduct of the
dialdehyde form of HMTD produced by the
cleavage of one of the O-O bonds. This ion
was more thermally stable than the
molecular ion adducts and had significant
intensity at 220 ×C.
In negative ion mode all HMTD PIPs
were associated with fragment ions. From the
GC elution times it could be shown that these
were produced from products of thermal
degradation of HMTD within the GC. The main
ions observed were associated with the m/z
138/140/142 which were identified as the
chloride adducts of the protonated and
oxidised forms of radicals produced by
cleavage of the three O-O bonds,
N(CH2O)3H²Cl- and N(CH2O)2(CHO)²Cl-
(both K 0 = 1.88), which agrees with results
from a previous study [1]. When the chloride
dopant concentration was minimised
additional non-chloride adduct PIPs were also
observed, m/z 72, K 0 = 2.42 and m/z 150, K 0
= 1.82.
Sensitive detection of HMTD was
achieved in both ion modes. The optimum
conditions were in negative mode at an IMS
temperature of 150 ×C, in the presence of
chloride dopant at low humidity levels (< 100
PPMV). In positive mode, detection was best
achieved with ammonia dopant concentration
minimised at low humdity levels (< 100
PPMV) at temperatures of 100 to 130 ×C.
However, without a full assessment of the
thermal degradation effects that occurred in
the GC these results are only preliminary.
Conclusions
An examination of the IMS spectra of
the peroxide explosives TATP and HMTD over
a range of parameters has produced a series
of complex spectra. The optimum conditions
determined for their detection are
summarised in Table 1.
The TATP results provide clear evidence
that trace detection of TATP is only feasible in
positive ion mode, contradicting previous
Copyright © 2003 by International Society for Ion Mobility Spectrometry

62 - Ion Mobility Spectrometry of Peroxide Explosives TATP and HMTD
studies. Sensitive HMTD detection was
possible in both modes.
References
[1] ‘Investigation of the Thermal Decomposition
of TATP and HMTD by Ion Mobility
Spectrometry/Mass Spectrometry’, Cho I.,
Chamberlain T. Brunk S., Gill R., and
LaMonica D., Proceedings of the 3rd
International Aviation Security Technology
Symposium, Atlantic City, Nov 2001.
[2] Kuja F; Grigoriev A. Debono R and Nacson S,
Proceedings of the 7th International
Symposium on the analysis and detection of
explosives, Edinburgh, June 2001.
[3] McGann W J, Goedecke K, Becotte-Haigh, P
Neves J, and Jenkins A, ‘Simultaneous dualmode
IMS detection system for contraband
detection and identification’ Int. J. IMS 4,
144 – 147 (2001).
Acknowledgements
This work has been funded by the UK
Department for Transport.
British crown copyright 2003. Published
with the permission of the Defence Science
and Technology Laboratory on behalf of the
Controller of HMSO.
Copyright © 2003 by International Society for Ion Mobility Spectrometry