FORENSIC ANALYSIS OF EXPLOSIVES FROM WATER SAMPLES USING SOLID PHASE MICROEXTRACTION – GAS CHROMATOGRAPHY

KIU KEE HENG

UNIVERSITI TEKNOLOGI MALAYSIA 2006

�

UNIVERSITI TEKNOLOGI MALAYSIA

PSZ 19:16 (Pind. 1/97)

BORANG PENGESAHAN STATUS TESIS

JUDUL : FORENSIC ANALYSIS OF EXPLOSIVES FROM WATER SAMPLES USING SOLID PHASE MICROEXTRACTION – GAS CHROMATOGRAPHY

SESI PENGAJIAN : 2005/2006

Saya : KIU KEE HENG (HURUF BESAR) mengaku membenarkan tesis ini disimpan di Perpustakaan Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti berikut : 1. Hakmilik tesis adalah dibawah nama penulis melainkan penulisan sebagai projek bersama dan

dibiayai oleh UTM, hakmiliknya adalah kepunyaan UTM. 2. Naskah salinan di dalam bentuk kertas atau mikro hanya boleh dibuat dengan kebenaran bertulis

daripada penulis. 3. Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan pengajian

mereka. 4. Tesis hanya boleh diterbitkan dengan kebenaran penulis. Bayaran royalti adalah mengikut kadar

yang dipersetujui kelak. 5.*Saya membenarkan/tidak membenarkan Perpustakaan membuat salinan tesis ini sebagai bahan

pertukaran di antara institusi pengajian tinggi. 6. **Sila tandakan (� )

SULIT (Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia seperti yang termaktub di dalam AKTA RAHSIA RASMI 1972)

TERHAD (Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/

badan di mana penyelidikan dijalankan)

TIDAK TERHAD Disahkan oleh ____________________________________ _______________________________ (TANDATANGAN PENULIS) (TANDATANGAN PENYELIA) Alamat Tetap : 70, Jalan Meranti, P.M. DR. UMI KALTHOM BT AHMAD 96100 Sarikei,

Sarawak. Tarikh : Tarikh : CATATAN : * Potong yang tidak berkenaan.

** Jika Tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali tempoh tesis ini perlu dikelaskan sebagai SULIT atau TERHAD.

(NAMA PENYELIA)

“I hereby declare that I have read this project report and in my opinion

this project report is sufficient in terms of scope and quality

for the award of the degree of Master of Science (Chemistry).”

Signature : ______________________________________

Name of Supervisor : ASSOC. PROF. DR. UMI KALTHOM AHMAD

Date :

FORENSIC ANALYSIS OF EXPLOSIVES FROM WATER SAMPLES USING

SOLID PHASE MICROEXTRACTION – GAS CHROMATOGRAPHY

KIU KEE HENG

A project report submitted in partial fulfilment of the

requirements for the award of the degree of

Master of Science (Chemistry)

Faculty of Science

Universiti Teknologi Malaysia

OCTOBER 2006

ii

I declare that this project report entitled “Forensic Analysis of Explosives from Water

Samples Using Solid Phase Microextraction – Gas Chromatography” is the result of

my own research except as cited in the references. The project report has not been

accepted for any degree and is not concurrently submitted in candidature of any other

degree.

Signature : ______________

Name : KIU KEE HENG

Date :

iii

Specially dedicated for my beloved Almighty God, father, mother,

brothers and sisters in Christ and friends…

iv

ACKNOWLEDGEMENT

First, I would like to express my deepest gratitude to my God because of His

grace and blessings. My sincere thanks also go to my project supervisor, Associate

Professor Dr. Umi Kalthom Ahmad, for her continuous supervision, encouragement

and advice.

Besides, I would like to thank all the lecturers and seniors for their effort to

guide and help me in times of difficulty. I would like to thank the laboratory

assistants in the Department of Chemistry and all the staff who have always been

helpful and friendly in helping me to solve my problems and made my master year

project a successful one.

Special Thanks also goes to the officers of the PDRM forensic laboratory in

Cheras and Maktab Teknik PDRM, Muar for the sampling and collection of post

blast water samples.

Last but not least, I would like to thank my beloved family for giving me

moral support. My sincere thanks also go to all my dearest brothers and sisters in

Christ for all their prayers, advice, encouragement and care for me.

v

ABSTRACT

The forensic identification of bulk explosive and postexplosion residues is

important in bombing investigation. The information can be used to determine the

type of explosive, to link the suspect to the bombing case and ultimately to provide

evidence in court. Many analytical techniques have been applied to the identification

of explosives and explosive residues. In this study, a solid-phase microextraction

(SPME) technique followed by gas chromatography (GC) was used to determine the

explosive residues. Gas chromatography with electron capture detection (GC-ECD)

is very sensitive towards post-blast residues and therefore useful for the

determination of explosive molecules. Several parameters including adsorption time,

desorption time and temperature, SPME fiber types, stirring effect and salting out

effect were optimized to obtain reproducible data with good accuracy. Addition of 10

% w/v of NaCl in the aqueous medium and use of a Carboxen/Polydimethylsiloxane

(CAR/PDMS) coating fiber led to optimal extraction efficiencies. LOD of studied

explosive compounds for the optimized SPME method was quite low (0.02-0.10

ppm) except for PETN (0.67 ppm). SPME/GC-ECD was optimized and applied to

the trace analysis of explosive residues in real pond water and water sample from

collection trays. 2,6-DNT, TNT and PETN were successfully detected from water

samples. For some sampling points, TNT and PETN were not detected due to the

amount of explosives used and low concentration of the explosives in water samples.

Direct Immersion SPME/GC-ECD was found to be applicable for qualitative and

quantitative analysis of explosives in post blast water samples.

vi

ABSTRAK

Pengenalpastian forensik tentang bahan letupan dan sisa letupan adalah

sangat penting dalam penyiasatan kes letupan. Maklumat yang berkenaan boleh

digunakan untuk menentukan jenis bahan letupan, mengaitkan suspek dengan kes

pengeboman dan akhirnya memberi bukti di dalam mahkamah. Banyak teknik

analisis telah digunakan untuk pengenalpastian bahan letupan dan sisa letupan.

Dalam kajian ini, teknik pengekstrakan mikro fasa pepejal (SPME) bersama dengan

kromatografi gas (GC) telah digunakan untuk menentukan sisa letupan.

Kromatografi gas bersama pengesanan tangkapan elektron adalah peka terhadap sisa

bahan letupan dan berguna untuk pengenalpastian molekul bahan letupan. Beberapa

parameter termasuk masa penjerapan, masa dan suhu nyahjerapan, jenis gentian

SPME, kesan pengadukan dan kesan garam telah dioptimumkan untuk mendapat

data yang tepat. Penambahan 10 % w/v NaCl dalam medium akues dan penggunaan

gentian Carboxen/Polydimethylsiloxane (CAR/PDMS) menghasilkan pengekstrakan

yang paling effisien. Had pengesanan bagi bahan letupan yang dikaji dengan

pengoptimuman SPME adalah rendah (0.02-0.10 ppm) kecuali bagi PETN (0.67

ppm). SPME/GC-ECD telah dioptimumkan dan digunakan untuk menganalisis sisa

letupan dalam air kolam sebenar dan sampel air daripada dulang tadahan. 2,6-DNT,

TNT dan PETN berjaya dikesan daripada sampel air. Bagi beberapa lokasi

pengsampelan, TNT dan PETN tidak dikesan disebabkan oleh amaun bahan letupan

yang digunakan dan kepekatan yang rendah dalam sampel air. SPME/GC-ECD

secara rendaman terus didapati sesuai digunakan untuk menganalisis bahan letupan

dalam sampel air selepas letupan secara kualitatif dan kuantitatif.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

TITLE

DECLARATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF ABBREVIATIONS/SYMBOLS/TERMS

LIST OF APPENDICES

i

ii

iii

iv

v

vi

vii

xi

xii

xiv

xvi

1

INTRODUCTION

1.1 Preamble

1.2 Explosives

1.2.1 Types of Explosives

1.2.1.1 Organic Explosives

1.2.1.2 Inorganic Explosives

1.2.1.3 High Order Explosives, Propellants

and Pyrotechnics

1.2.2 Properties of Explosives

1.2.3 Extraction Techniques for Explosive

Residues

1

1

3

3

4

4

5

8

9

viii

1.2.3.1 Soxhlet

1.2.3.2 Ultrasonic

1.2.3.3 Centrifugation

1.2.3.4 Swabbing

1.2.3.5 Solid-phase Microextraction

1.3 Explosions

1.3.1 Physical Explosions

1.3.2 Chemical Explosions

1.3.3 Atomic Explosions

1.4 Detonation

1.4.1 Detonation Parameters

1.5 Separation Method for Explosives

1.5.1 Gas Chromatography-Electron Capture

Detection

1.5.2 High-performance Liquid Chromatography

1.5.3 Other Techniques

1.6 Principle of SPME

1.6.1 Extraction Mode of SPME

1.6.2 Parameters that Affect SPME

1.6.3 Advantages of SPME

1.6.4 SPME Fiber Types

1.6.5 Forensic Application of SPME

1.6.6 SPME Application in Explosive Analysis

1.7 Objective of Study

1.8 Scope of Study

9

10

10

11

12

12

13

13

14

14

15

16

16

18

19

20

22

23

23

24

26

28

30

30

2

EXPERIMENTAL

2.1 Chemicals and Explosive Standards

2.2 Apparatus

2.3 Instrumentation

2.4 Sampling of Post Blast Water Samples

2.4.1 Sampling Method

32

32

32

33

34

36

ix

2.4.2 Sample Storage

2.5 Procedures

2.5.1 Preparation of Standard Stock Solution

2.5.2 DI-SPME Procedure

2.5.3 SPME Fiber Conditioning

2.5.4 DI-SPME Optimizations

2.5.5 Preparation of Calibration Graph

2.5.6 Determination of LOD

2.6 Analysis of Water Samples

37

38

38

39

39

40

41

41

42

3

RESULTS AND DISCUSSION

3.1 GC Separation of Target Compounds

3.2 SPME Sampling Mode

3.3 DI-SPME Optimization

3.3.1 Selection of SPME Fiber

3.3.2 Effect of Extraction Time

3.3.3 Effect of Desorption Time

3.3.4 Effect of Desorption Temperature

3.3.5 Effect of NaCl on Analyte Extraction

3.3.6 Stirring Effect

3.4 Evaluation of SPME/GC-ECD Method

Performance

3.4.1 Preparation of SPME Calibration Graph

3.4.2 Reproducibility for SPME

3.4.3 Detection Limit for SPME

3.5 Application to Water Samples

3.5.1 Samples from Pond Water

3.5.2 Samples from Collection Tray on the

Ground

3.5.3 Comparison between Water Sample from

Collection Tray and Field Pond

43

43

44

45

45

47

48

49

51

52

53

54

55

56

57

57

60

67

x

4

CONCLUSION AND SUGGESTIONS

4.1 Conclusion

4.2 Suggestions

REFERENCES

APPENDICES

69

69

70

72

77

xi

LIST OF TABLES

TABLE

1.1

1.2

1.3

2.1

2.2

2.3

2.4

2.5

3.1

3.2

3.3

3.4

3.5

3.6

TITLE

Properties of Some Explosives

Commercially Available SPME Fibers

A Summary Review of Explosive Residues Analysis using

SPME

Information of Explosives Exploded at Different Sampling

Points

List of Sample Collected

Conditioning Temperature and Time of SPME Fibers

Optimization of DI-SPME Parameters

Volume of standard stock solution and blank pond water

needed for the preparation of working solutions

Analysis of calibration standards by SPME/GC-ECD using

CAR/PDMS fiber

Reproducibility for DI-SPME of each analyte for the

CAR/PDMS fiber after optimization

Detection limit data for optimized DI-SPME method of each

analyte using the CAR/PDMS fiber and comparison of LOD

with other study

SPME results for explosives in pond water samples using

GC-ECD

Concentration of explosives in water samples from collection

trays by using SPME-GC-ECD

Concentration of 2,6-DNT and TNT in water sample from

collection tray and real pond

PAGE

8

25

31

35

38

39

40

41

55

55

56

59

61

67

xii

LIST OF FIGURES

FIGURE

1.1

1.2

1.3

1.4

1.5

1.6

2.1

2.2

2.3

2.4

3.1

3.2

3.3

TITLE

Types of Explosives

Chemical structure of (a) TNT; (b) 2,4-DNT; (c) 2,6-DNT

and (d) PETN

Types of Explosives and Their Applications

Schematic Diagram of an ECD

Schematic Diagram of SPME

SPME Extraction Mode (a) Direct Immersion;

(b) Headspace

Setup of Apparatus for the DI-SPME

Position of Sampling Points A-D at Impact Area

Set up of Water Sampling Trays

Location of water collection at sampling point D.

X: Explosion Point; P: Water Sample Collecting Point

Chromatogram of explosive standard mixture by direct

injection, GC conditions using temperature programming at

100 °C held for 2 min and raised to 250 °C at the rate of 15

°C/ min and kept for 1 min at 250 °C. Injector temperature

was 230 °C and detector temperature was 250 °C

Comparison of extraction efficiency using different SPME

fibers; (a) CAR/PDMS; (b) PDMS and (c) Ficonsica in

extracting individual analytes

Extraction time profile for PETN, 2,-4-DNT, 2,6-DNT and

TNT by SPME/GC-ECD using CAR/PDMS fiber at room

temperature

PAGE

3

5

7

17

21

22

33

35

36

37

44

47

48

xiii

3.4

3.5

3.6

3.7

3.8

3.9

3.10

3.11

3.12

3.13

3.14

3.15

Desorption time profile for PETN, 2,-4-DNT, 2,6-DNT and

TNT by SPME/GC-ECD using CAR/PDMS fiber at room

temperature and 30 min extraction time

Effect of desorption temperature on the analysis of

explosives by SPME/GC-ECD. Fiber was immersed for 30

min in the solution and desorption time was 3 min in GC

injector port

Influence of varying percentage of NaCl on DI-SPME

extraction performed at room temperature, 40 min

extraction time and 3 min desorption time. Desorption

temperature was 230 °C

Influence of stirring effect on the SPME extract of

explosive mixtures

Calibration graph of DI-SPME for target compounds in

blank pond water

GC-ECD chromatograms of the extracts of (a) blank pond

water; (b) D-P1; (c) D-P2; (d) D-P3 and (e) a standard

solution of mixture of explosives

GC-ECD chromatograms of the SPME extracts of (a) 10 %

v/v NaCl; (b) A-1; (c) A-2 and (d) A-3 post blast samples

GC-ECD chromatograms of the SPME extracts of (a) B-1;

(b) B-2 and (c) B-3 post blast samples

GC-ECD chromatograms of the SPME extracts of (a) C-1;

(b) C-2 and (c) C-3 post blast samples

GC-ECD chromatograms of the SPME extracts of (a) D-1;

(b) D-2 and (c) D-3 post blast samples

Biological degradation of TNT in water under anaerobic

conditions to yield mono-, di- and triamino toluene

Comparison between (a) a standard solution of explosives

mixture; (b) water sample from collection tray and (c) water

sample from field pond

49

50

51

53

54

58

62

63

64

65

66

68

xiv

LIST OF ABBREVIATIONS/SYMBOLS/TERMS

2,4-DNT - 2,4-Dinitrotoluene

2,6-DNT - 2,6-Dinitrotoluene

ACN - Acentonitrile

CAR/PDMS - Carboxen/polydimethylsiloxane

CW/DVB - Carbowax/Divinylbenzene

DI-SPME - Direct immersion solid-phase microextraction

DNT - Dinitrotoluene

ECD - Electron capture detector

EGDN - Ethylene glycol dinitrate

FID - Flame-ionization detector

GC - Gas chromatography

GC-ECD - Gas chromatography with electron capture detection

GC-MS - Gas chromatography-mass spectrometry

HMX - 1,3,5,7-Tetranitro-1,3,5,7-tetrazacyclooctane

HPLC - High-performance liquid chromatography

HS-SPME - Headspace solid-phase microextraction

IC - Ion chromatography

IR - Infra-red

LC-MS - Liquid chromatography-mass spectrometry

LLE - Liquid-liquid extraction

LMNR - Lead mononitroresorcinate

LOD - Limit of detection

MECC - Micellar electrokinetic capillary chromatography

NaCl - Sodium chloride

NG - Nitroglycerine

NPD - Nitrogen-phosphorus detector

xv

PA - Polyacrylate

PDMS - Polydimethylsiloxane

PDMS/DVB - Polydimethylsiloxane/divinyl-benzene

PETN - Pentaerythritol tetranitrate

R2 - Correlation coefficients

RDX - 1,3,5-Trinitro-1,3,5-triazacyclohexane

RSD - Relative standard deviation

SDME - Single-drop microextraction

SFC - Supercritical fluid chromatography

SPE - Solid phase extraction

SPME - Solid-phase microextraction

TEA - Thermal energy analyzer

TLC - Thin layer chromatography

TNT - Trinitrotoluene

UV - Ultra-violet

UXO - Unexploded ordnance

xvi

LIST OF APPENDICES

APPENDIX

A

B

TITLE

Data of Optimized SPME/GC-ECD Method

Data for Evaluation of SPME/GC-ECD Method

Performance

PAGE

77

84

CHAPTER 1

INTRODUCTION

1.1 Preamble

The forensic identification of bulk explosive and post explosion residues is

important in bombing investigation. The information can be used to determine the

type of explosive, to link the suspect to the explosive and ultimately to provide

evidence in court. Many analytical techniques have been applied to the identification

of explosives and explosive residues (Grob and Barry, 2004).

The battles of World War 1 and 2 required huge amounts of explosives as

fillings of bombs, mines, shells, torpedoes etc. The main raw materials for

nitroaromatics benzene, phenol and toluene were not available in sufficient quantities

and it became necessary to search for substitutes. These were found among other

compounds in mononitronaphthalenes, dinitronaphthalenes and trinitronaphthalenes

which still today can represent a hazard on former explosives production sites, shell

filling stations and ammunition destruction facilities. Nitronaphthalenes were used

during World War 1 and to a minor extent in World War 2 (Bausinger et al., 2004).

After the end of the world war, many environmental problems were being discovered

at many of these locations. Surrounding lands were found to be laden with explosive

residues, the common being 2,4,6-trinitrotoluene (TNT) and nitroaromatic and

nitramine impurities. The highly toxic nature of many of these substances, coupled

with their persistence in the environment, requires thorough characterization of

contaminated areas (Kleiböhmer et al., 1993).

2

The bombings that occurred in Bali on the 12th October 2002 were the most

callous act of terrorism in Indonesian history. Two hundred and two people were

killed and many hundreds more were injured. Tiny fragments of tartan fabric were

recovered from sites surrounding the blast epicenter. Numerous pieces of metal were

also found amongst the debris and these were found to match fragments recovered

from living and deceased victims (Royds et al., 2005). Indonesian police investigated

the devastating bombing of a night club have found traces of plastic explosive at the

site of the attack. After investigation by police together with expert from the FBI,

they concluded that C4 explosive was used – a type manufactured mainly in the

United State and used widely by military forces around the world (BBC News,

2002). There was another bombing in Bali in October 2005. At least 25 people were

killed in this explosion and around 120 people were injured. Bali police chief

informed that there was evidence that the explosion materials were attached to the

body of the bombers and the composition of the bombs included TNT and metal

slugs (ABC News, 2005).

Terrorism bombings are an increasingly common form of terrorist violence

worldwide. Thousand of people have been killed or injured from such bombing in

several countries. Turkey has been one of the target countries frequently suffering

from terrorist action. From the study of terrorist bombing which related deaths in

Istanbul (Turkey) between 1976 and 2000, a total of 45,714 autopsy case reports

from the Morgue Department of Council of Forensic Medicine. There were 120

deaths due to terrorist bombing and terrorist bombing related deaths were

significantly higher during the years (Yavuz et al., 2004).

In spite of bombing cases that cause many environmental problem, wartime

activities including dumping of ammunition and sinking of warships have also

resulted in the undersea deposition of considerable amounts of unexploded ordnance

(UXO). Most explosives are toxic, and the leaching from UXO is considered to be a

potential source of contamination for surrounding water. Due to the lack of effective

tools to survey underwater areas, rapid and sensitive techniques for environmental

monitoring are thus needed (Monteil-Rivera et al., 2004).

3

1.2 Explosives

An explosive can be liquid, gaseous or solid. A chemical explosive is a

compound of mixture or alone which upon initiation undergoes a rapid chemical

reaction without participation of external reactants such as atmospheric oxygen,

resulting in large amount of gaseous pressure. An explosive must be sensitive to

initiation. This reaction can be initiated by mechanical means (impact or friction), by

the action of heat (sparks) or by detonating shock (Blasting Cap) (Köhler and Meyer,

1993). Any explosive must contain its own oxygen to be defined as an explosive. It

cannot draw its oxygen from the air, as wood does when it burns (Brodie, 1973).

1.2.1 Types of Explosives

Explosives are either made of pure compounds or mixture of compounds.

Basically, explosives which are pure compounds can also be divided to two

categories that are organic and inorganic. Figure 1.1 shows the types of explosives

classified as either organic or inorganic (Cooper and Kurowski, 1996).

Figure 1.1: Types of Explosives

Explosives

Pure Compounds

Mixture of Compounds

Aliphatic Aromatic

Inorganic Organic

4

1.2.1.1 Organic Explosives

Almost all the explosive materials are organic compounds. An organic

compound is built on a skeleton of carbon atoms. These skeletons fall into two major

categories: aromatic and aliphatic. Explosives fall into both these categories (Cooper

and Kurowski, 1996).

Aromatic explosives contain a benzene ring and in all these explosives, the

benzene is nitrated. They are all solid crystalline materials. Trinitrotoluene (TNT) is

the most common aromatic explosive (Figure 1.2a). TNT is stable, insensitive to

shock and nontoxic. It is made from toluene, a petroleum byproduct. The nitro

groups are added to it in steps, reacting the toluene in a mixture of concentrated nitric

acid and with some sulfuric acid added. Disubstitution of toluene yields 2,4-

dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT) (Figure 1.2b and 1.2c).

2,4-DNT and 2,6-DNT are priority pollutants generated during the synthesis of the

explosive TNT, polyurethane foams, toluene, diiscocyanate and dyestuffs (Keenan et

al., 2005).

For aliphatic explosives, their compounds do not contain a benzene ring.

These explosives were the first modern explosives to be mass produced. The most

common aliphatic explosives are the nitrate esters. The nitrate esters comprise a large

number of short-chain explosives that contain the nitrate group, -ONO2. Some of

them are oily liquids, insoluble in water, with moderate to high vapor pressures.

They are very sensitive to impact. Usually, they are used in dynamites and in double-

base propellants. Pentaerythritol tetranitrate (PETN) is a major aliphatic nitrate ester

(Figure 1.2d). This white and powdered explosive is the major constituent in

detonating cords. It is also used in many mixed and cast explosives.

1.2.1.2 Inorganic Explosives

Inorganic materials are not based upon a carbon skeleton. Most inorganic

explosives are mixtures of fuels and oxidizers. Some examples of inorganic

5

explosives are ammonium nitrate, fulminates, silver fulminate and others.

Ammonium nitrate, the most readily available and cheapest salt of HNO3 is an

odorless, white compound used as a solid oxidizer in explosive mixtures.

Ammonium nitrate is also widely used as a fertilizer. It has a good chemical stability

and a low sensitivity to friction and shock (Yinon and Zitrin, 1996).

H3C NO2

NO2

O2N

(a)

H3C

NO2

O2N

(b)

H3C NO2

O2N

(c)

H2C

CCH2

CH2

H2C

OO2N

ONO2

O

O2N

O

NO2

(d) Figure 1.2: Chemical structure of (a) TNT; (b) 2,4-DNT; (c) 2,6-DNT and (d)

PETN

1.2.1.3 High Order Explosives, Propellants and Pyrotechnics

Explosives can also be classified by their applications: high-order explosives,

low-order explosives (propellants) and pyrotechnics, which are primarily

differentiated by the burning rate and the manner of initiation.

6

High-order explosives are initiated by shock, usually through a booster

charge or another explosive such as a blasting cap (Grob and Barry, 2004). It can be

divided into two classes: primary explosives and secondary explosives. Primary

explosives differ from secondary explosives in that they undergo a vary rapid

transition from burning to detonation and have the ability to transmit the detonation

to less sensitive explosives. Primary explosives will detonate when they are

subjected to heat or shock. Primary explosives differ considerably in their sensitivity

to the heat and in the amount of heat they produce on detonation. Their detonation

velocities are in the range of 3500-5500 m/s (Akhavan, 1998). Typical primary

explosives which are widely used are lead azide, lead styphnate, lead

mononitroresorcinate (LMNR), potassium dinitrobenzofurozan and others.

For secondary explosives, they differ from primary explosives in that they

cannot be detonated readily by heat or shock and are generally more powerful than

primary explosives. They are less sensitive than primary explosives and can only be

initiated to detonation by the shock produced by the explosion of primary explosives.

Some secondary explosives are so stable that rifle bullets can be fired through them

or they can be set on fire without detonating. The more stable explosives which

detonate at very high velocities exert a much greater force during their detonation.

Values of their detonation velocities are in the range of 5500-9000 m/s (Akhavan,

1998). Examples of high-order explosives are trinitrotoluene (TNT), tetryl, picric

acid, nitrocellulose, nitroglycerine, 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX),

1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), pentaerythritol tetranitrate

(PETN) and others.

Propellant is explosive material with low rate of combustion. It may be either

solid or liquid (Köhler and Meyer, 1993). It is an explosive material which undergoes

rapid and predictable combustion (without detonation) resulting in a large volume of

hot gas. Propellants can be initiated by a flame or sparks and change from a solid to a

gaseous state relatively slowly. A homogeneous propellant is where the fuel and

oxidizer are in the same molecule, whereas a heterogeneous propellant has the fuel

and oxidizer in separate compounds (Akhavan, 1998). Gun propellants are

traditionally known to be homogeneous, whereas rocket propellants are

heterogeneous. The compositions of gun propellants have traditionally been

7

fabricated from nitrocellulose-based material. Meanwhile for the rocket propellant,

the composition is like gun propellant. The solid rocket propellant is manufactured in

the form of geometrical shapes known as grains. Other examples of propellants are

black powder, pyrodex, single-based and double-based smokeless powders and flash

powders.

The name pyrotechnic is derived from the Greek words ‘pyr’ (fire) and

‘techne’ (an art), which describes the effect observed from a burning pyrotechnic

composition (Akhavan, 1998). Pyrotechnics usually consist of a mixture of two

ingredients: a fuel and an oxidizer, which is specifically formulated to produce a lot

of energy. This energy is then used to produce a flame, or combined with other

volatile substances to produce smoke and light like fireworks, or to produce large

quantities of gas (firework rockets). Generally, the fuels are metals and the oxidizers

are either salts or metal oxides. They differ from explosives in that the reaction is

less than the bulk sound speed and the reaction rates are very low. There are five

major productions of pyrotechnics: sound, color light, heat, smoke and delay.

Usually different colors of smokes are produced due to the different material such as

zinc, sulfur, phosphorous acid and other compounds used in pyrotechnic (Cooper and

Kurowski, 1996). Figure 1.3 shows types of explosives and their applications

(Köhler and Meyer, 1993).

Figure 1.3: Types of Explosives and Their Applications

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1.2.2 Properties of Explosives

A systematic approach to the relationship between the explosive properties of

a molecule and its structure was proposed by van’t Hoff in 1909 and Plets in 1953.

According to Plets, the explosive properties of any substance depend upon the

presence of definite structural groupings (Akhavan, 1998).

Hence, every explosive has its own properties such as melting point, energy

of formation, color and others due to the structure and the functional groups present.

For isomer of explosives (such as 2,4-DNT and 2,6-DNT), although they have the

same empirical formula, they still have their own different properties that usually are

physical properties such as melting point and density. Table 1.1 shows the properties

of some explosives (Köhler and Meyer, 1993; Akhavan, 1998).

Table 1.1: Properties of Some Explosives

2,4-DNT 2,6-DNT TNT PETN

Color Yellow

needle

Yellow

needle Pale yellow

crystalline

solid

Colorless

crystalline

solid

Empirical formula C7H6N2O4 C7H6N2O4 C7H5N3O6 C5H8N4O12

Molecular weight 182.1 182.1 227.1 316.1

Melting point (oC) 70.5 65.0 80.8 141.3

Density (g/cm3) 1.521 1.538 1.654 1.76

Energy of

formation (kJ/kg) -298.8 -159.5 -184.8 -1509

Enthalpy of

formation (kJ/kg) -374.7 -241.2 -261.5 -1683

Solubility in water

(mg/L) 270 @ 22°C 206 @ 25°C 130 @ 20°C 0.99

9

The thermodynamic concepts denote the energy which is bound during the

formation of a given compound from its constituent elements at constant volume

(energy of formation) or at constant pressure (enthalpy of formation, which includes

the mechanical work preformed at the standard state, 25 °C = 77 °F and a pressure of

1 bar).

1.2.3 Extraction Techniques for Explosive Residues

1.2.3.1 Soxhlet

A Soxhlet extractor can be used to extract solutes from solids contained in a

porous thimble, using any desired volatile solvent, which can be water-miscible or

water-immiscible (Patnail, 2004). As the solvent is heated, the solvent will be

vaporized. The distilled solvent drips into the porous thimble, immersing the solid

sample. When the solvent is full, solvent is siphoned back into the solvent reservoir

and redistilled (Grob and Barry, 2004). This process continues repeatedly as the

solvent in the flask is vaporized and condensed.

If sample particles are hydrophilic, they may repel with organic solvent,

reducing efficiency. Drying agents are often used to reduce this problem. A further

disadvantage is that Soxhlet extraction is usually very slow, often requiring several

hours. This is often pointed out when Soxhlet extraction is compared to other

instrumental techniques such as supercritical fluid extraction or accelerated solvent

extraction.

Soxhlet extraction of soil sample was performed in the study of Deuster et al.

(1997). A 5 g soil was extracted for 8 h with 150 mL n-hexane. The concentrated

extract was analyzed by high-performance liquid chromatography (HPLC). From the

result, soxhlet extraction could be used to extract explosive residues from soil

samples like other conventional extraction techniques.

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1.2.3.2 Ultrasonic

During the sonication extraction, ultrasonic vibration energy is used to

enhance the liquid-solid extraction. A converter, whose main component is a lead

zirconate titanate piezoelectric crystal, translates this electrical energy into high

frequency mechanical energy. The ultrasonic wave travelling through the solution

consists of alternate compressions and rarefactions (Grob and Barry, 2004). This

ultrasonic energy was found beneficial for the liquid-soil extraction of soils. The

major benefit of applying ultrasonic to soil extractions is the disaggregation of soil

particles, which occurs during the extraction procedure as the result of the sonic

energy. This increases the surface area of the soil and allows the extractant to further

penetrate the soil matrix.

In a study by Walsh (2001), soils were collected for traces of nitroaromatic,

nitramine and nitrate ester explosives and analyzed by HPLC. The analytes were

extracted from soils with acetonitrile by 18 hours of sonication in a cooled ultrasonic

bath. This method gave detection limits around 1 µg kg-1 to 10 µg kg-1 for the di- and

trinitroaromatics respectively. For RDX, HMX and nitrate esters (nitroglycerine, NG

and PETN), detection limit were 3 µg kg-1, 25 µg kg-1 and between 10-40 µg kg-1

respectively. Spike recovery studies revealed artifacts introduced when the mass of

the soil spiked was large (25.0 g) in proportion to the volume of spike solution added

(1.00 mL). Recoveries were excellent (around 100%) when 2 g soil samples were

spiked with 1.00 mL of solution (Walsh, 2001).

1.2.3.3 Centrifugation

A centrifuge is a device that uses centrifugal force to separate two or more

substances of different density such as two liquids or a liquid and a solid. Centrifuge

devices are used in a variety of scientific and technical application by spinning

carrier vessels (centrifuge tubes) at high rotation speeds and very high centrifugal

force. The theoretical basis of this technique is the effect of gravity on particles in