Profiling the chemical composition of explosives

Cameron Johns, Joseph P. Hutchinson, Michael C. Breadmore, Rosanne M. Guijt, Emily

F. Hilder, Greg W. Dicinoski and Paul R. Haddad

Australian Centre for Research on Separation Science (ACROSS), School of Chemistry,

Faculty of Science, Engineering and Technology, University of Tasmania, Private Bag

75, Hobart, Tasmania, 7001, Australia

2

Introduction

The analysis of post-blast explosive residues has become an essential tool in counter-

terrorism initiatives.

Explosives based on inorganic salts and peroxides can be constructed using readily

available, low cost, and legally purchased components. Examples of their recent use in

terrorist bombings include: the Unabomber (USA, 1985–1996), the bombing of the

World Trade Center (USA, 1993), the Murrah Federal Building (USA, 1995), several

terrorist attacks in Indonesia (2002, 2005) and attacks on public transport systems in

Madrid (Spain, 2004) and London (UK, 2005).

Identification of explosives is a topic of considerable interest to forensic scientists and

counter-terrorism authorities. The broad area of explosives analysis can be divided into

two sub-areas: detection and identification of explosives or their major ingredients prior

to detonation (pre-blast screening analysis), or identification of explosives by analysis of

debris and residues after detonation (post-blast detection analysis). Pre-blast analysis is a

challenging problem that requires the ability to rapidly screen for the presence of

inorganic and organic starting materials used to make explosive devices. Such screening

is currently being used at fixed locations, such as airport terminals, in efforts to

apprehend potential terrorists. On the other hand, post-blast analyses are required to

determine the type and composition of an explosive after detonation has occurred.

Explosive devices have a vast range of chemical compositions, hence a forensic scientist

must be equipped with tools that enable the quantitative analysis of the evidence

collected in these cases.

In our laboratory, we develop analytical methods which can be used for both pre-blast

and post-blast applications. However the main focus of this article will be on the post-

blast analysis of “home-made” devices (sometimes also referred to as “improvised

explosives” or “improvised explosive devices (IEDs)”) constructed from low explosives.

Inorganic home-made explosives employ vigorous oxidation/reduction chemical

reactions using strong inorganic oxidisers, such as nitrate, perchlorate or chlorate which

can be obtained or refined from commercial sources. High explosives, such as nitrated

organic compounds (eg.TNT, RDX, PETN), are much more difficult to obtain because

their sale, storage and use are highly regulated.

Some of the analytical methods previously used for the analysis of inorganic explosives

include ion chromatography (IC), capillary electrophoresis (CE), scanning electron

microscopy-energy dispersion X-ray, mass spectrometry, X-ray powder diffraction,

infrared, atomic absorption spectroscopy and various spot tests. However, many of these

techniques are not sensitive or specific enough for the analysis of inorganic anions or

cations in explosive residues. We have found that the analytical techniques of IC and CE

offer complementary, sensitive and selective technology for determining a large range of

inorganic ions of relevance to the chemical identification of these types of explosives.

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Inorganic improvised explosive devices

Non-stoichiometric quantities of reagents are normally used in making the IED, and the

detonation is frequently inefficient. As a result, inorganic species used as starting

materials are often found in the residues. Reduced forms of the oxidising species used in

the explosive mixture are also present in the residues, especially chloride formed from the

reduction of perchlorate and chlorate and small amounts of nitrite from nitrate. Metals

added to IEDs to increase the temperature of reaction (such as aluminium and

magnesium) produce ions which can be detected in post blast residues upon acidification

to convert insoluble oxides and unreacted starting material into ionic species. Table 1

lists the main chemical compositions used to prepare typical inorganic improvised

explosive devices and the characteristic anions and cations that can be detected in the

post-blast residues using analytical techniques such as IC and CE.

It is also important to be able to identify and quantify the “background” ions already

present in the environment. The identity of these background ions will depend on the

location of the explosion and on-site environmental blank analyses are therefore

imperative. For example, inorganic ions typically present in soil include chloride, nitrate,

sulfate, sodium, potassium, calcium, magnesium and ammonium. Zinc and iron oxides

are present on steel surfaces, and carbonate is readily produced in samples due to

absorption of atmospheric carbon dioxide. Analytical methods used for analysis of post-

blast residues must therefore be able to differentiate between indicator ions produced in

the explosion and background ions already present in the environment around the site of

the explosion.

Sample collection procedures

Post-blast residues become scattered widely around the vicinity of the detonation and

thus a suitable means for the collection of the residues is required prior to analysis. Real

world detonations can be simulated, whereby small amounts of explosive devices are

detonated under controlled conditions and residues are collected on “witness plates”;

which are suspended metal sheets used for residue collection purposes. The testing

arrangement for such controlled explosions is shown in Figure 1. Background levels of

ions present in the surrounding environment must be determined by sampling areas not

exposed to explosion residues. Some examples of surfaces that are suitable for sampling

around the site of detonation include aluminium or steel, wood, glass, plastic, bitumen

and concrete. To ensure all background ions are accounted for it is also advisable to

collect soil samples.

A recent case study has been provided by Royds et al. 1 documenting the 2002 terrorist

bombings in Indonesia. IEDs containing chlorate, sulfur and aluminium were detonated

near popular nightclubs and foreign embassies, killing 202 people. This case study noted

the need for field-portable instrumentation in mobile laboratories. Meticulous procedures

were required to ensure that sample integrity was maintained from the crime scene to a

court of law, including recording of the chain of custody of evidence and ensuring sample

integrity through non-contamination. It was noted that even with large amounts of

4

explosives, the amount of water-soluble residues that could be collected were reduced as

a result of water used to fight fires, from burst water mains or from precipitation or other

climatic conditions. This action left only trace amounts of residues remaining for

identification purposes. Preferred sampling sites included elevated surfaces such as street

signs, pieces of clothing from victims, or cracks in bitumen that were shielded from the

elements. Orthogonal testing procedures increased confidence in the results, and there

was a need for high-throughput analytical procedures as approximately 2400 samples

were analysed after this terrorist act. Soil samples from in and around the crater provided

no evidence above background levels of the marker ions from the explosive device.

Sample pre-concentration (such as the use of short ion-exchange column to enrich sample

components) can be performed prior to instrumental analysis when residue levels are low.

However, this process will also lead to pre-concentration of the background ions and this

may limit the utility of this approach.

Sample collection methods for post-blast analysis typically involve wiping selected areas

to be sampled with swabs wetted with an appropriate solvent. Our work has typically

used rayon swabs moistened with water to collect these inorganic residues. The swab is

then sonicated to aid extraction of the residues into water, and the solution filtered prior

to analysis. Soil samples have been analysed by extraction with water, and sonication

followed by filtration prior to instrumental determination.

Analytical methods for the analysis of explosive residues

As the location where a terrorist will detonate an explosive device is not known prior to

the event, analytical detection technologies must be capable of being transported to the

site of detonation and be employed as quickly as possible to aid in the investigation of

events. Hence, it is important to have portable instrumentation capable of performing

rapid analyses. Such instrumentation can be housed close to the detonation site in a

mobile laboratory. This type of mobile laboratory was used during range day exercises

conducted with the Australian Federal Police and is shown in Figure 2.

IC is a rugged and reliable analytical technique for the analysis of inorganic ions. By

their nature the majority of improvised explosives are thus amenable to analysis by IC,

which as an analytical technique is characterised by high sensitivity and selectivity, along

with excellent robustness and reliability. IC is well-suited to the analysis of inorganic

explosives, and methods for the analysis of slurry explosives and bombing debris were

developed as early as 1980.

We have recently published IC methods for the analysis of anionic and cationic residues

relevant to post-blast inorganic explosives 2. These methods allow separation of 18

anions and 12 cations in a single simultaneous analysis, with suppressed (for anions) or

non-suppressed (for cations) conductivity detection. The analyte sets include all marker

and background ions likely to be present in the commonly used IEDs and have been

applied successfully to the analysis of post-blast residues and the subsequent chemical

identification of the type of explosive used in blind test samples. Examples of this

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efficacy are provided in Figure 3, which shows anion and cation separations of residues

from perchlorate/sugar, chlorate/sugar, and chlorate/perchlorate/sugar IEDs. These

residues were found to contain chlorate and/or perchlorate present as the oxidising

material. The reduction of these species is responsible for the large chloride peaks.

Additionally, their counter-ion can be identified from the cation analysis. Sodium is used

in the chlorate/sugar device, and potassium is used in the perchlorate/sugar and

chlorate/perchlorate/sugar devices. These methods allow the type and chemical

composition of these inorganic IEDs to be readily determined.

CE is an alternative technique to IC and has been growing in popularity for the

identification of explosive residues. CE is characterised by high separation efficiencies

which permits ions to be rapidly separated. The instrumentation is also relatively simple

and is amenable to miniaturisation leading to portability for field applications. Universal

detection techniques such as indirect spectrophotometric or conductivity detection can be

used in conjunction with CE since many of the inorganic ions present in post-blast

residues show low absorptivities in the UV-visible region of the electromagnetic

spectrum.

Recently we have reported highly sensitive CE methods capable of separating the 15

target anions and 12 target cations present as either background or marker ions in the

post-blast residues of typical IEDs. These methods were implemented on a commercially

available portable CE instrument which was modified to operate in the indirect

spectrophotometric detection mode using a sensitive, miniature light-emitting diode

(LED) photometric detector 3 or with a commercial contactless conductivity detector

4.

Contactless conductivity detection combined with the development of suitable

electrolytes allowed a 10-fold improvement in the limits of detection compared to

indirect LED detection while offering a reduction in the total analysis times.

Anion and cation separations by CE of residues collected following detonation of

ammonium nitrate-fuel oil (ANFO), black powder, and sodium chlorate devices are

shown in Figure 4. The dominant species were found to be ammonium and nitrate ions

from the ANFO device and potassium and nitrate ions from the black powder device.

The presence of the chlorate ion in the post-blast residues of the sodium chlorate device

is indicative of this type of explosive since this ion is not generally present in natural or

urban environments. In addition, chlorate is typically not the dominant ion present in

residues due to its consumption during the explosion. Acidification of the residue from

the chlorate device prior to cation analysis was required to convert insoluble metals and

their oxides (such as magnesium) into water-soluble ions for detection. Corrosion by-

products of zinc are present on the surface of the galvanised witness plates. These were

subsequently collected by the swabbing procedure and converted to zinc ions upon

acidification.

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Conclusions and future directions

Electrophoretic and chromatographic techniques have been successfully developed over a

relatively short period of time for the chemical identification of IEDs. The two

techniques provide complementary selectivities, with CE offering highest efficiency and

shortest run times and IC offering highest reproducibility and detection sensitivity. These

methods allow the separation and identification of a wide range of ions, which in turn can

be used to establish the chemical composition and type of inorganic explosive used. This

information will be vital in providing leads towards tracking the sources of chemicals

used to create these IEDs.

The future direction of our research is to develop analytical methods for the analysis of

organic high explosives. This work will concentrate on developing portable instruments

capable of analysing multiple types of explosives, such as organic and peroxide-based

explosives in addition to inorganic explosives, on a single platform. Our work has, and

continues to be performed, with funding and co-operation from the National Security

Science and Technology Unit (Department of Prime Minister and Cabinet), the

Australian Federal Police, the National Institute for Forensic Science, the ChemCentre

WA and Forensic Science South Australia.

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Table 1 Typical inorganic home-made explosives and the major inorganic species

likely to be present in their post-blast residues.

Explosive type Composition Characteristic

anions

Characteristic

cations

ANFO Ammonium nitrate,

fuel oil

NO3-, NO2

- NH4

+, MeNH3

+

Black powder Nitrate salts, sulfur,

charcoal

NO3-, SO4

2-, S2O3

2- Na

+, K

+

Chlorate/sugar Chlorate salts, sugar ClO3-, Cl

- Na

+, K

+

Perchlorate/sugar Perchlorate salts,

sugar

ClO4-, Cl

- Na

+, K

+

Chlorate/perchlorate/

sugar

Chlorate salts,

perchlorate salts,

sugar

ClO3-, ClO4

-, Cl

- Na

+, K

+

8

Figure 1 Photographs of an explosive device prior to detonation; surrounded by

metallic “witness plates” to collect explosive residues followed by

detonation; and a resultant witness plate.

9

10

Figure 2 Photograph of a field deployable mobile laboratory capable of being

transported to the site of an explosive event.

11

Figure 3 Ion chromatographic separations of anionic and cationic post blast

residues from chlorate/perchlorate devices.

(Reprinted from: J. Chromatogr. A, 1182, 205 (2008) with permission

from Elsevier Science)

6 8 10 12 14 16

ca

rbon

ate

ca

lciu

m

mag

ne

siu

m

pota

ssiu

m

am

mon

ium

so

diu

mzin

c

perc

hlo

rate

su

lfa

te

nitra

te

ch

lora

te

Time (min)Time (min)

ch

lorid

e

Chlorate/perchlorate/sugar

Chlorate/sugar

Perchlorate/sugar

6 8 10 12 14

5 S5 S

12

Figure 4 Analysis of (a) anions and (b) cations extracted from post-blast residues

resulting from the detonation of three inorganic home-made explosive

devices using a portable capillary electrophoresis instrument.

(Reprinted from: Electrophoresis, 29, 4593 (2008) with permission from

Wiley-VCH)

5 6 7 8 9

ch

lori

de

nit

rate

su

lfa

te ca

rbo

na

tec

arb

on

ate

nit

rate

su

lfa

te

ca

rbo

na

te

I.S.

I.S.

I.S.

su

lfa

ten

itra

tec

hlo

rid

e

ANFO

black powder

(including barium)

sodium chlorate device

(including nitrate

and magnesium)

Migration Time (min)

ch

lora

te

(a)

0.5 mV

4 5 6 7 8

ca

lciu

m

ma

gn

es

ium

so

diu

mam

mo

niu

mp

ota

ss

ium

po

tas

siu

m

so

diu

m

I.S

I.S

I.S

zin

cma

gn

es

ium

am

mo

niu

m

po

tas

siu

m

ca

lciu

m

so

diu

m

ba

riu

m

Migration Time (min)

(b)

0.5 mV

13

References

1 D. Royds, S.W. Lewis, A.M. Taylor, Talanta, 67, 262 (2005).

2 C. Johns, R.A. Shellie, O.G. Potter, J.W. O’Reilly, J.P. Hutchinson, R.M. Guijt,

M.C. Breadmore, E.F. Hilder, G.W. Dicinoski, P.R. Haddad, J. Chromatogr. A,

1182, 205 (2008).

3 J.P. Hutchinson, C.J. Evenhuis, C. Johns, A.A. Kazarian, M.C. Breadmore, M.

Macka, E.F. Hilder, R.M. Guijt, G.W. Dicinoski, P.R. Haddad, Anal. Chem., 79,

7005 (2007).

4 J.P. Hutchinson, C. Johns, M.C. Breadmore, E.F. Hilder, R.M. Guijt, C. Lennard,

G. Dicinoski, P.R. Haddad, Electrophoresis, 29, 4593 (2008).