NOVEL SOLID-PHASE MICROEXTRACTION ADSORBENT FOR THE
FORENSIC DETECTION OF ACCELERANTS IN ARSON SAMPLES
GEETHA SELVARAJU
UNIVERSITI TEKNOLOGI MALAYSIA
iii
Specially dedicated
to my
beloved parents and brother…
iv
ACKNOWLEDGEMENTS
This research could not have been completed successfully without the support
and assistance of numerous individuals. First and foremost, Assoc. Prof. Dr. Umi
Kalthom Ahmad, my inspiring and most respectable supervisor who assisted and guided
me through this entire study. Followed by my co-supervisor, Assoc. Prof. Dr. Abdul
Rahim Yacob. I would like to express my deepest gratitude to both of them for their
cooperation, tolerances and willingness to share their valuable ideas and comments.
My sincere appreciation goes to the laboratory assistants of the Department of
Chemistry, UTM especially Mr. Dinda Ahmad Hairol and Mr. Hamzah Basiron for their
technical assistance throughout the study. Besides that, I would like to thank all the lab
assistants of the Molecular Biology and Microbiology laboratory, Faculty of Science and
Materials Science Laboratory, Faculty of Mechanical Engineering for their help in
handling instruments. Special thanks goes to all the academic staffs of Chemistry
Department, UTM, especially Assoc. Prof. Dr. Zainab Ramli who had shared her
valuable knowledge.
The continuous encouragement and moral support from my beloved family
members is highly appreciated as it was also important in making this research a
possible success. I would also like to thank all my dear friends for their invaluable help
in completing this project. My deepest gratitude goes to the Ministry of Science,
Technology and Innovation, Malaysia (MOSTI) for the financial support under IRPA
vote No. 74090. Last but not least, I would like to thank God for giving me the will
power and strength to accomplish this project report. May my work glorify His Name.
v
ABSTRACT
Numerous adsorbents are available commercially as coatings for solid-phase
microextraction (SPME) technique. However, some analytical methodologies might
demand specific properties for the extraction of selected compounds. In this study, a
simple, fast, effective and environmental friendly methodology for the determination of
accelerants in arson samples using headspace-SPME coupled to gas-chromatographyflame ionization detector (GC-FID) is presented. A lab-made coated fiber prepared by
sol-gel method, containing 1:1 molar ratio of octyltriethoxysilane (C8-TEOS):
methyltrimethoxysilane (MTMOS) was employed in this technique. The fiber was tested
for the headspace extraction of n-alkane standard hydrocarbons and common
accelerants. Applicability of the fiber was demonstrated through the detection of
accelerants in simulated arson samples. The C8-coated fiber showed a good selectivity
for accelerants. Compared with commercial PDMS/DVB fiber, the lab-made coated
fiber exhibited higher extraction capability for accelerants, higher thermal stability (up
to 300 °C) and longer lifetime (~ 200 times usage). Electron microscopy experiment
revealed that the surface of the fiber coating was well-distributed. A porous structure
was suggested for the sol-gel derived C8 coating with an approximate thickness of (3-4)
µm. The underlying mechanisms of the coating process were discussed and confirmed
by infra-red (IR) spectrum. HS-SPME parameters, such as extraction time, extraction
temperature and desorption time were optimized. The developed headspace-SPME
method using C8-coated fiber showed satisfactory reproducibility (RSD < 6%), linearity
(r > 0.9869) and detection limits for accelerants (0.7-1.0) µL. The lab-made SPME
adsorbent was shown to be a good alternative to commercial SPME fiber for the
determination of accelerants in arson cases.
vi
ABSTRAK
Pelbagai jenis penjerap salutan bagi teknik pengekstrakan fasa pepejal mikro
(SPME) boleh didapati secara komersial. Bagaimanapun, beberapa kaedah analisis
memerlukan ciri-ciri khas bagi pengekstrakan sebatian yang terpilih. Kajian ini
membentangkan suatu kaedah yang mudah, pantas, efektif dan mesra alam sekitar untuk
menentukan bahan penggalak kebakaran dalam sampel kebakaran yang disengajakan
dengan menggunakan teknik SPME secara ruang kepala (HS) yang digabungkan dengan
kromatografi gas-pengesan pengionan nyala (GC-FID). Gentian salutan buatan sendiri
yang disediakan melalui kaedah “sol-gel”, dengan nisbah molar oktatrietoksisilana (C8TEOS): metiltrimetoksisilana (MTMOS) 1:1 telah digunakan dalam teknik ini. Gentian
tersebut telah diuji untuk pengekstrakan hidrokarbon n-alkana piawai dan bahan
penggalak kebakaran yang lazim. Pengunaan gentian tersebut telah ditunjukkan melalui
pengesanan bahan penggalak kebakaran dalam sampel simulasi kebakaran yang
disengajakan. Gentian salutan C8 ini menunjukkan kepilihan yang baik bagi bahan
penggalak kebakaran. Berbanding dengan gentian komersil (PDMS/DVB), gentian
salutan buatan sendiri menunjukkan kebolehan pengekstrakan yang lebih tinggi bagi
bahan penggalak kebakaran, kestabilan terma yang lebih tinggi (sehingga 300 °C) dan
jangka hayat yang lebih lama (~ 200 kali penggunaan). Eksperimen mikroskopi elektron
menunjukkan bahawa permukaan gentian telah disalut dengan sempurna. Struktur
berliang telah dicadangkan untuk salutan C8 terbitan sol-gel dengan ketebalan yang
beranggaran (3-4) µm. Mekanisme proses penyalutan telah dibincangkan dan ditentukan
melalui spektrum infra-merah (IR). Parameter HS-SPME, seperti masa pengekstrakan,
suhu pengekstrakan dan masa penyahjerapan telah dioptimumkan. Kaedah HS-SPME
yang dibangunkan menggunakan gentian salutan C8 menunjukkan kebolehulangan (RSD
< 6%) kelinearan(r > 0.9869) dan had pengesanan (0.7-1.0) µL yang memuaskan bagi
bahan penggalak kebakaran. Penjerap SPME buatan sendiri menunjukkan suatu
alternatif yang baik kepada gentian SPME komersial bagi penentuan bahan penggalak
kebakaran dalam kes kebakaran yang disengajakan.
vii
TABLE OF CONTENTS
CHAPTER
TITLE
PAGE
THESIS TITLE
i
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
xi
LIST OF FIGURES
xii
LIST OF SYMBOLS/ABBREVIATIONS/NOTATION/
1
TERMINOLOGY
xv
LIST OF APPENDICES
xvi
INTRODUCTION
1.1
Background
1
1.2
Arson Crime Scene Investigation
2
1.2.1
Arson Samples and Burn Patterns
2
1.2.2
Analysis of Fire-Scene Samples
3
1.2.3
Court Presentation and Arson Evidence
4
1.3
Nature of Accelerants
5
1.4
The Evolution of Accelerant Extraction Technique
7
1.5
Solid-phase Microextraction (SPME)
9
1.5.1
Principles of SPME
11
1.5.2
Extraction Modes With Coated SPME Fibers
13
viii
1.5.3
Recovery of Accelerants by Headspace-SPME
(HS-SPME) Technique
1.5.4
Identification of Volatile Accelerants by
SPME-Gas Chromatography Analysis
15
1.6
Commercial SPME Fiber Coatings
17
1.7
Novel SPME Adsorbents
18
1.8
Sol-gel Technology
28
1.8.1
Sol-gel Method as a New Tool for SPME Fiber
Coatings
2
14
30
1.9
Problem Statements and the Needs of Study
31
1.10
Objectives and Scope of Study
33
EXPERIMENTAL
2.1
Introduction
34
2.2
Chemicals and Materials
34
2.3
Apparatus
36
2.4
Instrumentation
37
2.5
Preparation of the Sol-gel Derived C8-coated Fiber
38
2.6
Preparation of Standard Stock Solution
39
2.7
Procedure for Extractions using Headspace SPME
(HS-SPME)
39
2.8
Preparation of Spiked Fire Debris Sample
40
2.9
General Procedures for Characterization of the
C8-coated Fiber
41
2.9.1
Lifetime of the Coating
41
2.9.2
Thermal Stability
42
2.9.3
Scanning Electron Microscopy (SEM)
42
2.9.4
Calculation of Fiber Coating Volume
42
2.9.5
FTIR Analysis
43
2.10
Procedure for Optimization of Sol-gel Process
43
2.11
HS-SPME Optimization and Evaluation Procedures
44
ix
2.12
3
2.11.1 Optimization of HS-SPME Operating Conditions
44
Method Validation
46
2.12.1 Determination of Limits of Detection (LOD)
46
2.12.2 Preparation of Calibration Graph
46
CHARACTERIZATION OF THE C8-COATED FIBER,
OPTIMIZATION AND UNDERLYING
MECHANISM OF SOL-GEL PROCESS
3.1
Introduction
47
3.2
Characterization of the C8-coated Fiber
47
3.2.1
Selectivity for Standard Hydrocarbons
47
3.2.2
Extraction Capability for Standard Hydrocarbons
48
3.2.3
Lifetime of the Coating
49
3.2.4
Thermal Stability
51
3.2.5
Scanning Electron Microscopy (SEM) Analysis
53
3.2.5.1
Surface Characteristics of the Coating
53
3.2.5.2
Estimation and Reproducibility of the
Film Thickness
3.2.5.3
Determination of Extracting Phase
Volume
3.2.6
3.3
Fourier Transform Infrared (FTIR) Analysis
57
57
Optimization and Possible Underlying Mechanism of
Sol-gel Process
59
3.3.1
Optimization of Sol-gel Process
59
3.3.2
Possible Underlying Mechanism of the Coating
Process
3.4
55
Conclusions
60
66
x
4
HEADSPACE SPME OPTIMIZATION, METHOD
VALIDATION AND PRACTICAL APPLICATION OF
THE C8-COATED FIBER
4.1
Introduction
68
4.2
Optimization of HS-SPME Operating Conditions
68
4.2.1
Optimization of Extraction Time
68
4.2.2
Optimization of Desorption Time
70
4.2.3
Optimization of Extraction Temperature
72
4.3
Validation of the Analytical Method
4.3.1
4.4
Accuracy of HS-SPME Method Using C8-coated
Fiber
74
4.3.2
Detection Limits of Accelerants
76
4.3.3
Calibration Graph of Target Compounds
77
Analysis of Simulated Arson Samples Using C8-coated
Fiber
4.5
5
74
78
4.4.1
Selectivity for Accelerants
78
4.4.2
Extraction Capability for Accelerants
83
Conclusions
85
CONCLUSIONS AND FUTURE DIRECTIONS
5.1
Conclusions
86
5.2
Future Directions
89
REFERENCES
91
APPENDICES
103
xi
LIST OF TABLES
TABLE NO.
TITLE
PAGE
1.1
Properties of common accelerants
6
2.1
Names, functions and chemical structures of the principal
compounds of sol-gel coating solutions
35
Optimization of HS-SPME experimental parameters
[(a) exposure time, (b) desorption time and (c) extraction
temperature]
45
3.1
The extraction precision of C8-coated fiber (200 operations)
50
3.2
Reproducibility of the peak area after conditioning at a) 270 ˚C
and b) 300 ˚C
52
3.3
The approximate film thickness of five different C8-fibers
56
4.1
Inter-day precision of HS-SPME method for the extraction
of standard hydrocarbon compounds using C8-coated fiber under
optimum conditions
75
LODs for accelerants determined using C8-coated fiber under
optimum HS-SPME conditions
76
2.2
4.2
4.3
Regression line and correlation coefficient of standard hydrocarbon
compounds
77
xii
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
1.1
A (a) photograph and (b) an illustration of the commercially
available SPME device attached with coated fused-silica fiber.
10
1.2
Basic steps in sol-gel coating technology
29
1.3
The stages of dip coating process
31
2.1
Experimental setup for headspace SPME accelerant extraction
37
2.2
Photographs showing (a) the burning of carpet sample and (b) the
burnt fire debris sample
41
3.1
SPME-GC analysis of hydrocarbon compounds using C8-coated fibers
Peaks: (1) C8, (2) C10, (3) C12, (4) C14 and (5) C16 n-alkanes.
48
3.2
Comparison of extraction capability between C8-coated fiber
and PDMS/DVB fiber.
49
The surface view of sol-gel C8 coated fiber at (a) 150 and (b) 1000
fold-magnification obtained by SEM
54
The cross-sectional view of sol-gel C8 coated fiber at 4000
fold-magnification obtained by SEM
55
3.5 (a)
IR spectra of pure C8-TEOS
58
3.5 (b)
IR spectra of sol-gel derived C8-coating
59
3.6
Extraction capability of hydrocarbon compounds using fibers
prepared from sol–gel solution containing C8-TEOS:MTMOS with
the ratios of 0.5:1, 1:1 and 2:1
60
3.3
3.4
xiii
4.1
Extraction equilibration profiles of n-alkane hydrocarbon compounds
(C8, C10, C12, C14 and C16) at varying extraction time. Other SPME
conditions : extraction temperature of 80 °C and desorption time
of 80 s
70
4.2
Desorption profiles of C8-fiber for standard n-alkane hydrocarbon
compounds (C8, C10, C12, C14 and C16) by HS-SPME. Extraction
conditions: extraction time, 15 min; extraction temperature, 80 °C
72
Influence of extraction temperature on the signal intensity of
standard n-alkane hydrocarbons (C8, C10, C12, C14 and C16).
Extraction conditions: extraction time, 15 min; desorption time, 80 s
74
4.3
4.4
Calibration graph of HS-SPME for n-alkane hydrocarbon compounds
77
using C8-coated fiber
4.5 (a)
GC profiles of direct injection of gasoline. Peak identities: (1)
methylbenzene, (2) ethylbenzene, (3) 1, 3-dimethylbenzene,
(4) 1, 2- dimethylbenzene, (5) 1-ethyl-2-methylbenzene,
(6) 1, 2, 4-trimethylbenzene, (7) 1, 2, 3- trimethylbenzene
80
4.5 (b)
GC profiles of gasoline spiked burnt carpet sample using C8-coated
fiber. Peak identities: (1) methylbenzene, (2) ethylbenzene, (3) 1, 3dimethylbenzene, (4) 1, 2- dimethylbenzene,
(5) 1-ethyl-2-methylbenzene, (6) 1, 2, 4-trimethylbenzene, (7) 1, 2, 3trimethylbenzene
80
4.5 (c)
GC profiles of the blank burnt carpet sample using C8-coated fiber
4.6 (a)
GC profiles of direct injection of kerosene. Peak identities: (1) C9,
(2) C10, (3) C11, (4) C12, (5) C13, (6) C14, (7) 2, 6-dimethylnapthalene,
81
(8) C15
4.6 (b)
GC profiles of kerosene spiked burnt carpet sample using C8-coated
fibers. Peak identities: (1) C9, (2) C10, (3) C11, (4) C12, (5) C13, (6) C14,
(7) 2, 6-dimethylnapthalene, (8) C15
82
4.7
GC profiles of diesel spiked burnt carpet sample using C8-coated
fibers. Peak identities: (1) C9, (2) C10, (3) C11, (4) C12, (5) C13, (6) C14,
82
(7) C15, (8) C16 hydrocarbons
4.8 (a)
Comparison of sol-gel derived C8-coated fiber and commercially
available PDMS/DVB fiber in extracting hydrocarbon compounds from
gasoline spiked burnt carpet sample. Label identities: (1) methylbenzene,
(2) ethylbenzene, (3) 1, 3-dimethylbenzene,(4) 1, 2-dimethylbenzene, (5)
1-ethyl-2-methylbenzene, (6) 1, 2, 4-trimethylbenzene, (7) 1, 2, 3trimethylbenzene.
83
81
xiv
4.8 (b)
Comparison of sol-gel derived C8-coated fiber and commercially
available PDMS/DVB fiber in extracting hydrocarbon compounds
from kerosene spiked burnt carpet sample. Label identities: (1) C9,
(2) C10, (3) C11, (4) C12, (5) C13, (6) C14, (7) 2, 6-dimethylnapthalene,
84
(8) C15
4.8 (c)
Comparison of sol-gel derived C8-coated fiber and commercially
available PDMS/DVB fiber in extracting hydrocarbon compounds
from diesel spiked burnt carpet sample. Label identities: (1) C9,
(2) C10, (3) C11, (4) C12, (5) C13, (6) C14, C15, (7) C16
84
xv
LIST OF SYMBOLS/ABBREVIATIONS/NOTATION/TERMINOLOGY
GC
-
Gas Chromatography
FID
-
Flame ionization detector
GC-FID
-
Gas Chromatography with flame ionization detector
SPME
-
Solid phase micro extraction
HS-SPME
-
Headspace solid phase micro extraction
PDMS/DVB
-
Polydimethylsiloxane/Divinylbenzene
C8-TEOS
-
n-Octyltriethoxysilane
MTMOS
-
Methyltrimethoxysilane
TMMOS
-
Trimethylmethoxysilane
LOD
-
Limit of Detection
RSD
-
Relative standard deviation
v/v
-
Volume per volume
SEM
-
Scanning Electron Microscopy
xvi
LIST OF APPENDICES
APPENDIX
A
B
C
TITLE
PAGE
An enlarged scanning electron micrograph of the fiber surface
view at 100 fold magnification
103
Scanning electron micrograph: cross-sectional view of the
fiber at 1820 fold magnification. (Measurement of the film
thickness at two different points)
104
Presentations and Publications
105
1
CHAPTER 1
INTRODUCTION
1.1
Background
Arson continues to be both an urgent national problem and truly a contemporary
crime. Often characterized as a clandestine tool for criminals, the true arson picture is
not clearly known. The Fire and Rescue Department Malaysia reported that it has
responded to 103,994 fire cases between the year 1997 to 2003 (www.bomba.gov.my,
2006). Loss of life and damage of properties are always a grave concern in fires.
Economically, arson impacts insurance premium rates, removes taxable property assets
and degrades communities. Historically, the inner areas of large cities often are most
hard-hit and the result is that much of the cost of this destructive crime is placed on
those who can least afford it (Dennett, 1980).
Arson can be defined as an intentional or attempted damaging or destruction, by
means of fire or explosion, of the property of another without the consent of the owner,
or of one’s own property or that of another with intent to defraud (Rush, 2000). The
motives to commit arson are numerous, including revenge, fraud or profit, sabotage,
pyromania, vandalism, concealment of another crime and terrorism. Arsons are
particularly difficult crime to investigate and prosecute. Its difficulty lies in these areas
(O’ Connor, 1987):
2
•
Every fire scene need to be treated as a potential arson scene (from the
standpoint of security, preservation and evidence) until clear proof of natural or
accidental cause is discovered.
•
The crime itself, if successful, destroys the physical evidence at its origin. In
some cases, the evidence is still there but it requires careful and methodical
analysis.
Arson is a crime that destroys evidence rather than creates it as it progresses and
normally there is not much first hand eyewitness evidence. Therefore the findings of
a scientific investigation are considered important in criminal and civil court actions.
1.2
Arson Crime Scene Investigation
An important aspect of an investigation of a suspected arson case involves the
chemical analysis of the debris remaining after the fire. The aim of the on-site
investigation is to establish beyond reasonable doubt the cause of the fire origin from an
investigation of the fire debris and remaining building structure and obtaining physical
evidence pertinent to the ignition source.
1.2.1 Arson Samples and Burn Patterns
The presence and distribution of a flammable liquid at a fire scene indicates a
deliberately lit fire unless it can be readily explained otherwise. “Sniffers” or portable
hydrocarbon detectors are used by many successful investigators (DeHaan, 1997).
Burn patterns are the basis of all fire investigation. It can be used to indicate the
direction of fire spread observed on structural elements or wall surfaces. The typical Vpattern associated with most fires is a result of several factors, one being that the natural
3
plume shape of a large fire is a cone. This cone’s shape depends on the size of fire with
respect to a ceiling that tends to flatten and spread it horizontally. If a fire is located
some distance away from the wall, a shallow U- or V-pattern may appear (O’Connor,
1987; DeHaan, 1997).
Arson samples are normally collected at the suspect crime scene and submitted
to the laboratory either in new aluminium cans or forensic evidence bags. In collecting
samples, investigators need to look for places where the ignitable liquids would be
protected from the heat because those volatile liquids are driven off by high temperature
and not likely to be found in the deepest char areas. Multiple samples should be taken so
that the spread of the accelerant is ascertained (Furton et al., 1995).
1.2.2 Analysis of Fire-Scene Samples
The majority of fire-scene evidence submitted to the laboratory for analysis
consist of various materials, such as wood, plastics, soil, carpet, cloth, metals, liquids
and other organic and inorganic substances. Laboratory examinations of the fire debris
includes (Almirall et al., 1996a; Harris and Wheeler, 2003; Almirall et al., 2004; Ma et
al., 2004):
•
Macroscopical and microscopical examination of the debris - (most fire debris
consists of burned or partially burnt substances, weathering has already occurred
before collection. Therefore brief visual examination of fire debris will not cause
any great loss of accelerants).
•
Recovery of the accelerants from the debris – [separation of analytes from the
matrix (fire debris, soil and water) and concentrating them].
•
Identification of the accelerants – (chemical analysis of the isolated substances
and the identification of the mixture).
4
The difficulties with the identification of accelerants traces isolated from fire
debris or environmental samples (water and soil) result mainly from the fact that the
isolated traces differ qualitatively and quantitatively from the initial mixtures. These
differences appear as a result of (Cavanagh et al., 2002; Sandercock and Pasquier,
2003):
•
Evaporation - in the isolated traces, there is a lack of volatile compounds in
comparison to the high boiling ones.
•
Pyrolysis - as a result of high temperature effect, the big molecules can
disintegrate and the disintegration products arise.
•
Microbiological decomposition.
•
In the isolated traces, the additional substances appear - for example matrix
components
or
matrix
decomposition
products,
extinguishing
medium
components.
1.2.3 Court Presentation and Arson Evidence
The investigator must use laboratory analysis to support arguments in court.
Opinion and theories can be readily challenged in court because of the complex and
varied nature of fires, but laboratory evidence is irrefutable proof of the presence of an
accelerant. These are the basic elements that need to be considered to prove the crime of
arson (Icove and Dehaan, 2004):
•
There has been a burning of property - (this must be shown to the court to be
actual destruction, not just scorching or soothing).
•
The burning is incendiary in origin – (proof of the existence of an effective
incendiary device accomplished by showing how all reasonable natural or
accidental cases have been considered and ruled out).
•
The burning is shown to be started with malice – (this act of burning requires a
specific intent of destroying property).
5
1.3
Nature of Accelerants
Accelerants are any type of material or substance added to the targeted materials
to enhance the combustion of those materials and to accelerate the burning. Accelerants
such as petrol, kerosene, diesel, mineral turpentine and methylated spirits are widely
utilized in everyday life. Owing to their physico-chemical properties such as volatility
and flammability, they are also used as ignitable liquids to commit arsons by offenders.
Exotic accelerants such as industrial solvents are rarely used and if so are readily
identified by chemical analysis because of their similar chemical characteristics to the
common accelerants (Furton et al., 1995; Harris and Wheeler, 2003).
Accelerants can be classified as either mixtures of compounds or pure
compounds. The chromatographic analysis of a pure compound will feature a single
peak while a mixture will give several peaks that contribute to a fingerprint which is
used to identify the accelerant. The more volatile components of an accelerant evaporate
at a faster rate than the heavier components so that the overall chemical profile of the
accelerant will change during the fire and before sampling (Jackowski, 1997).
The amount of accelerant remaining at the fire scene available for sampling is
governed by factors such as the initial loading, volatility and water solubility of the
accelerant, severity of the fire, porosity of the substrate material, dryness of the area
after the fire and elapsed time between the fire and sampling. (Almirall et al., 1996b;
Cavanagh et al., 2002; Almirall et al., 2004).
Samples of a suspected accelerant are often located at the scene in a container
and are supplied for analysis. Samples of this type are usually unaffected by evaporation.
It is difficult to conclusively determine if a sample of an accelerant was the same as that
to initiate or propagate a fire, because of the universal composition of the common
accelerants (Dennet, 1980).
6
The chemical components of the common accelerants are aliphatic and aromatic
hydrocarbons and oxygenated hydrocarbons such as alcohols. The oxygenated
hydrocarbons are somewhat water soluble and therefore washed away during the
extinguishing of the blaze, so that little trace remains (Cavanagh et al., 2002;
Sandercock and Pasquier, 2002).
The most common accelerants used by arsonists are generally low to medium
boiling hydrocarbon liquids. Some common accelerants and their properties are shown
in Table 1.1.
Table 1.1: Properties of common accelerants (Lee et al., 1986).
Accelerant Flash
Boiling
Point (°C) Point (°C)
Self-
Explosive Limit (%)
Ignition
Flammable
Range
Temperature Low
High
(%)
Gasoline
-50
102-230
495
1.3
6.0
1.3-6.0
Kerosene
110-165
400-572
490
1.16
6.0
1.2-6.0
Fuel oil
110-165
350-500
490
1.16
6.0
1.2-6.0
110-190
375-650
494
-
-
-
Turpentine 95
-
488
0.8
6.0
-
Petroleum
300-400
450-500
0.9
8.0
1.1-6.0
No.1
Fuel oil
No.2
20-45
benzin
Gasoline, the most important fuel of petroleum origin, is a mixture of volatile,
low-boiling and midrange hydrocarbons. The average molecular weight of gasoline is
usually taken to be close to that of n-octane, about 114. Modern automotive gasoline
contains more than 100 hydrocarbons in a complex mixture. Before being sold as a
motor fuel, however, a variety of additives is added by the refinery. These additives
7
often include compounds to improve the burning characteristics of the fuel and dyes to
identify one product from another (Almirall et al., 1996a; Almirall et al., 1996b).
Kerosene and the distillate fuels of higher boiling ranges were considered useful
only for illumination until diesel motor became a common device. Although diesel fuel
is similar to kerosene, it spans a wider range of less volatile components. Kerosene and
heavier petroleum distillates have long been of significance in the setting of deliberate
fire. Their lower volatility presents less hazard to the user than does gasoline. The liquid
is more persistent so that less haste is required in its ignition and there is much less
danger of explosion (Ren and Bertsch, 1999; Llyod and Edmiston, 2003).
Fuel oil No.1 (Jet Fuel A) generally contains paraffinic and olefinic
hydrocarbons in the C10-C16 range (this range denotes hydrocarbons having 10 to 16
carbon atoms linked together). It falls in the fraction which follows gasoline in the
simple distillation of crude oil. Diesel fuel contains low sulfur content and additives to
improve combustion. Diesel fuel and domestic heating oil may both be described as fuel
oil No. 2. The heavier stove oil used in room heaters contains hydrocarbons from a
slightly higher boiling point range, usually (375-650)°F. The product for home use is
carefully controlled to minimize sulfur and water content which could corrode heating
units having minimal service. Those fuels intended for industrial service have higher
sulfur and ash content and cover a range of products and applications (O’Connor, 1987;
DeHaan, 1997; Icove and Dehaan, 2004).
1.4
The Evolution of Accelerant Extraction Technique
The forensic discipline of ignitable liquid and fire debris analysis is rapidly
changing. Refinements in existing methods as well as development of new techniques
are changing the routine methods of analysis. Optimization of existing accelerant
extraction, analytical techniques and research into novel methods of extraction have
8
improved the recovery of ignitable liquids from debris samples, sample turnover and
reduced the number of inconclusive findings (Dolan, 2003).
The earliest methods of identification relied upon simple identification of the
headspace odor - often referred to as a “nasal appraisal”. With improvement in analytical
instruments, the extraction technique used was sampling a headspace of heated fire
debris with a syringe and then injecting the sample into a GC for analysis. Heated
headspace analysis is also used for sample screening because it is simple, rapid and easy
technique to apply. A sample that gives a negative result when screened by heated
headspace would then be subjected to a more sensitive extraction technique. The
technique however, discriminates against the less volatile components in the sample
which will give less data from the chromatographic analysis for interpretation (Eisert
and Pawliszyn, 1997; Lord and Pawliszyn, 2000).
Distillation technique are used with steam distillation being the most popular.
Distillation involves heating the sample with an extraction medium and condensing the
vapor to provide a sample of the accelerant in the extraction medium used. The various
medium that have been used are water, ethanol and ethylene glycol. Vacuum distillation
with sub ambient trapping of the volatiles can also be used. The accelerant may be
further concentrated by controlled evaporation of the medium or by solvent extraction
from the medium. Steam distillation can be a lengthy technique and extraction times of
up to 48 hours have been reported as being necessary for some samples. The technique
however, requires considerable clean up of the apparatus between samples and also
considerable operator attention, therefore the sample turnover is low (Eisert and
Pawliszyn, 1997; Lord and Pawliszyn, 2000).
Solvent extraction is also used and involves soaking the fire debris in a suitable
solvent and then filtering and evaporating the solvent to concentrate the sample. The
advantages of solvent extraction is that it readily extracts the less volatile components of
an accelerant and therefore does not discriminate between fire debris samples. The
9
technique however, requires the use of high purity solvents which are expensive and also
matrix components such as monomers, plasticizers, glues and resins are co-extracted
which may interfere with the subsequent analysis. Both distillation and solvent
extraction require further concentration of the raw extracts to increase the sensitivity of
the technique (Dolan, 2003).
Dynamic headspace sampling is widely used for the extraction of accelerants.
The debris is heated inside the headspace gas chromatograph and if any volatile
hydrocarbons are present, they will vaporize and be present in the air or “headspace”
directly above the debris. The transfer gas used to sweep the headspace to the adsorbent
is usually nitrogen or the headspace can be drawn through the absorbent with a vacuum.
Microwave ovens have also been used to heat the sample and the steam generated
sweeps the headspace to the absorbent. A major problem of dynamic headspace
extraction technique is the carry over of contaminants from previous samples through
the gas transfer lines. It was found that by removing the outward gas transfer line and
connecting the charcoal tube directly to the sample container, the contamination problem
was reduced (Eisert and Pawliszyn, 1997; Lord and Pawliszyn, 2000; Dolan, 2003).
1.5
Solid-phase Microextraction (SPME)
In addition to optimizing existing extraction techniques, the field of forensic
science has also introduced a new extraction technique for application to fire debris
analysis: solid-phase microextraction. Much interest has been shown recently in
replacing conventional accelerant residues extraction technique with headspace SPME
technique ever since the technique was invented in the early 1990’s by Prof. Janusz
Pawliszyn. SPME is a fast, simple, time-efficient, environmental friendly and sensitive
technique, which does not require the use of solvents either for sample preparation or
cleanup. Analyte extraction and pre-concentration are combined in one single step
10
(Mester et al., 2001; Theodoridis et al., 2000). Figure 1.1 shows the dimensions and
architecture of the commercially available SPME device attached with lab-made fiber.
(a)
(b)
Plunger
Septum
piercing
needle
Plunger
retaining
screw
Fiber
attachment
tubing
Barrel
19.5
cm
1.0
cm
Coated
fused
silica
fiber
Figure 1.1 : (a) A photograph and (b) an illustration of the commercially available
SPME device attached with coated fused-silica fiber.
In SPME, the outer surface of a solid fused-silica fiber is coated with a selective
stationary phase. Thermally stable polymeric materials that allow fast solute diffusion
are commonly used as stationary phases. The extraction is based on the enrichment of
components on a polymer or adsorbent coated fused silica fiber by exposing the fiber
either directly to the sample or to its headspace. The extraction time can be considered
complete when the analyte concentration has reached a distribution equilibrium between
the sample matrix and the fiber coating. When using this in experimental conditions, it
means that when the equilibrium is reached, the amount of the extracted analytes is
11
constant and independent of an increase in extraction time. This is followed by thermal
desorption in a gas chromatograph (Zhang et al., 1994a).
The major advantages of SPME technique are in terms of speed, sensitivity, cost
effectiveness and versatility (Zhang et al., 1994b). In SPME, the equilibration can be
reached in only 20-30 minutes which makes it ideal for quick screening. The parts per
trillion detection limits have been attained, with an ion-trap detector. The solvent
purchase and disposable costs are greatly reduced in this technique. Fibers are reusable
(lifetime depends on conditions of use, more than 100 uses have been achieved). SPME
can be used with any GC or GC-MS with split/splitless or on-column injection.
1.5.1 Principles of SPME
A traditional approach to SPME involves coated fibers. The transport of analytes
from the matrix into the coating begins as soon as the coated fiber has been placed in
contact with the sample. Typically SPME extraction is considered to be complete when
the analyte concentration has reached distribution equilibrium between the sample
matrix and the fiber coating. In practice, this means that once equilibrium is reached, the
extracted amount is constant within the limits of experimental error and it is dependent
of further increase of extraction time. The equilibrium can be described as (Lord and
Pawliszyn, 2000; Wang et al., 2005):
n=
KfsVfVsCo
KfsVf + Vs
(1.1)
where n is the number of moles extracted by the coating, Kfs is a fiber coating or sample
matrix distribution constant, Vf is the volume of the fiber coating, Vs is the sample
volume and Co is the initial concentration of a given analyte. Eq (1.1) indicates that,
12
after equilibrium has been reached, there is a direct proportional relationship between
sample concentration and the amount of analyte extracted.
Eq. (1.1), which assumes that the sample matrix can be represented as a single
homogeneous phase and that no headspace is present in the system, can be modified to
account for the existence of other components in the matrix by considering the
volumesof the individual phases and the appropriate distribution constants. The
extraction can be interrupted and the fiber analyzed prior to equilibrium. Constant
convection conditions and careful timing of the extraction are necessary to obtain
reproducible data (Lord and Pawliszyn, 2000).
When the sample volume is very large, Eq. (1.1) can be simplified as (Wang et
al., 2005) :
(1.2)
n = KfsVfCo
which points to the usefulness of the technique for field applications. In this equation,
the amount of extracted analyte is independent of the volume of the sample. In practice,
there is no need to collect a defined sample prior to analysis as the fiber can be exposed
directly to the ambient air, water or production stream. The amount of extracted analyte
will correspond directly to its concentration in the matrix, without being dependent on
the sample volume.
The equilibrium equation for analysis in a vial containing headspace should be
expressed as shown in Eq (1.3) (Zhang and Pawliszyn, 1995; Lord and Pawliszyn,
2000):
n=
KfsVfVs
KfsVf + KhsVh + Vs
Co
(1.3)
13
Where Khs and Vh represent the headspace/sample distribution coefficient and the
volume of the headspace, respectively. The fact that SPME is an equilibrium rather than
an exhaustive extraction technique means that even after the extraction process has been
completed, a substantial portion of the analytes usually remain in the matrix. This
presents an opportunity for quantification based on internal standardization, namely the
standard is loaded onto the fiber prior to extraction step, instead of spiked into the
sample.
1.5.2 Extraction Modes With Coated SPME Fibers
Generally, there are three types of extractions that can be performed using
SPME: direct extraction, headspace configuration and membrane protection approach.
In the direct extraction mode, the coated fiber is inserted or immersed into the
sample and the analytes are transported directly from the sample matrix to the extracting
phase. In order to facilitate rapid extraction, some level of agitation is required to
transport analytes from the bulk of the solution to the vicinity of the fiber. Natural
convection of air is sufficient for gaseous sample to facilitate rapid equilibration (Zhang
et al.,1994b).
In the headspace mode, the analytes need to be transported through the barrier of
air before they can reach the coating. This modification serves primarily to protect the
fiber coating from damage by high molecular mass and other non-volatile interferences
present in the sample matrix such as proteins. This mode also allows the modification of
the matrix, such as a change of the pH or temperature, without damaging the fiber
(Zhang and Pawliszyn, 1993).
14
Another indirect SPME extraction mode is through a membrane. The main
purpose of the membrane barrier is to protect the fiber against damage, similar to the use
of headspace SPME when very dirty samples are analyzed. However, membrane
protection is advantageous for determination of analytes having volatilities too low for
the headspace approach. In addition, a membrane made from appropriate material can
add a certain degree of selectivity to the extraction process. The use of thin membranes
and increased extraction temperatures will result in faster extraction times (Mester et al.,
2001; Theodoridis et al., 2000).
1.5.3 Recovery of Accelerants by Headspace-SPME (HS-SPME) Technique
The separation and recovery of accelerant residues from fire debris are essential
steps for the identification of the accelerants. HS-SPME is a simple and convenient
method for the recovery of accelerant vapors from fire debris. The objectives of
analyzing the debris is to determine the presence of any volatile hydrocarbon
compounds. Initially, therefore, the accelerant residues (volatile compounds) need to be
separated from the debris (nonvolatile matrix). The technique is employed by
introducing a coated SPME fiber into the headspace above the debris which is placed in
an air-tight container. The volatilized hydrocarbon compounds are extracted and
concentrated in the coating. After exposure to the sample headspace, the fiber is
transferred to the analytical instruments for desorption of the extracted analytes (Snow
and Slack, 2002.
Theoretically, any volatiles present will reach a vapor equilibrium in a sealed
container. Molecules of the accelerants actually occupy the entire interior space of the
container. Part of the interior is occupied by the liquid phase of the accelerant and the
remainder is occupied by its vapor phase. The continuous molecular motion in liquids
causes the molecules to constantly leave the liquid phase to enter the vapor phase. At the
same time, molecules also continuously leave the vapor phase to reenter the liquid
15
phase. The molecules of a volatile compound at any given temperature have a specific
average kinetic energy that is dependent upon the absolute temperature of the liquid.
Some molecules at the liquid surface have energies greater than the average energy,
therefore, they continuously leave and enter the gaseous phase (Snow and Slack, 2002b;
Zhang and Pawliszyn, 1993).
Heating the sample to an elevated temperature provides energy for analyte
molecules to overcome energy barriers which tie them to the matrix, enhances the mass
transfer process and increases the vapor pressure of the analytes. However, excessive
temperatures encourage the degradation of synthetics in the debris, such as carpet. These
degradation products add to the complexity of the chromatogram which is used for
accelerant identification. Excessive heating also lowers the level of real accelerants in
debris and this does not contribute to the sensitivity of the method (Zhang and
Pawliszyn, 1995).
1.5.4 Identification of Volatile Accelerants by SPME-Gas Chromatography
Analysis
The analytical instrument used most frequently with SPME has been the gas
chromatograph (GC). Standard GC injectors, such as split/splitless can be applied to
SPME as long as a narrow insert with an inside diameter close to the outside diameter of
the needle is used. The narrow inserts are required to increase the linear flow around the
fiber, resulting in efficient removal of desorbed analytes. The split should be turned off
during SPME injection. Under these conditions, the desorption of analytes from the fiber
is very rapid (Lord and Pawliszyn, 2000).
GC uses a stream of gas (nitrogen or helium) as a carrier to move a mixture of
gaseous materials along a column, tube filled or coated with a separating compound. The
analytes interact with the separating compound by alternatively dissolving in it and then
16
volatilizing to be swept further along the column by the carrier. The SPME fiber is
introduced onto the column at one end and at the other end is a detector (Eisert and
Pawliszyn, 1997). Columns for GC analysis of suspected arson residues are usually set
up to offer the best separation of components having volatilities between those of
pentane (C5H12) and triacontane (C30H62), since that range includes all of the commonly
encountered petroleum products. Packed tubular column (3 mm in diameter) were
commonly used but it has now been supplanted by a very narrow capillary columns (0.1
mm in diameter). This resulted in shorter analysis time and the usage of smaller sample
sizes (DeHaan, 1997).
Although the column can be selected to separate a mixture of compounds on the
basis of their chemical properties, most separations in volatile accelerants analysis are
made on the basis of differences in the boiling points of the various components. Almost
all the components of petroleum distillates of interest are either aliphatic (straight-chain)
or aromatic (ring) in structure, thus will have very similar chemical properties within
those two groups. The heavier a molecule is, the less volatile it is and it will appear last
on the chromatogram (Lee et al, 1986; Zeeuw and Luong, 2002).
Flame ionization detector (FID) has been the standard detector for analysis of
hydrocarbons but now specialized detectors can be used in arson work. Electron capture
detectors, which use a minute radioactive source as a monitor, are very sensitive to
particular chemical species such as organometallics and halogenated hydrocarbons.
They can be used to characterize a petroleum distillate more precisely than is possible
with (FID) alone. There are nitrogen-phosphorus detectors that are not sensitive to
common hydrocarbons but will select those compounds in a complex mixture that
contain nitrogen or phosphorus. Since the crude oil feed stocks for gasoline vary
considerably in their nitrogen, phosphorus and sulfur containing components, it is very
likely that such techniques may allow discrimination between gasoline from different
origins (DeHaan, 1997; Zeeuw and Luong, 2002).
17
Mass spectrometer (MS) allows each molecule to break apart into small sub
molecular pieces and by counting those pieces, the chemical structure of the original
molecule can be established. MS is an analytical technique that needs to be fed one
compound at a time while GC is a separation technique that is good at separating a
mixture of compounds. Since most of the volatile residues in fire debris involve complex
mixtures, coupling the two together gives the best tool. GC/MS has the capability of
displaying a mass spectrum for each peak, and scan selected ions, those that are
characteristic for a particular chemical species of interest (Borusiewicz et al., 2004;
Lennard et al., 1995).
1.6
Commercial SPME Fiber Coatings
To date, several different coating materials are available from Supelco. There are
three different polydimethylsiloxane (PDMS) films of different thickness (7, 30 and 100
µm), 85 µm polyacrylate (PA), 65 µm polydimethylsiloxane/divinylbenzene
(PDMS/DVB), 75 µm, Carboxen/PDMS, 65 µm Carbowax/DVB and 50 µm Carbowax
template resin fibers (Dolan, 2003).
For the right selection of the polymer coating, the general principles of “like
dissolves like” applies. PDMS coated fibers are typically the first choice. They are very
rugged and liquid coatings are used as GC stationary phases withstanding temperatures
up to 300 °C. This coating is non polar showing a high affinity for non-polar compounds.
Very thin fiber coatings should be used whenever the sensitivity is sufficient. The
extraction time is shorter for a thinner coating and smaller distribution constants of the
analytes (Lord and Pawliszyn, 2000; Dolan, 2003).
The PA fiber shows a high affinity for polar compounds such as phenols and
polar pesticides. PA is a solid polymer, thus the equilibration times are higher compared
to the liquid PDMS fibers. The mixed phase coatings are more suitable for volatile
18
compounds. The take up of these fibers is significantly higher compared to PDMS.
When changing from PDMS to carbowax using the mixed phase coatings, more polar
analytes can be extracted such as ketones and alcohols (Eisert and Pawliszyn, 1997;
Lord and Pawliszyn, 2000; Dolan, 2003).
All the commercially available coatings can be categorized according to two
types of extraction mechanism, absorption and adsorption. Both PDMS and PA extract
analytes via absorption. The absorbent type fibers extract the analytes by partitioning
them into a ‘liquid-like” phase in which the analytes would migrate in and out of the
coating. The remaining coatings, including PDMS-DVB, Carbowax-DVB, Carbowax
Template Resin and Carboxen are mixed coatings, in which the primary extracting phase
is a porous solid, extracts analytes via adsorption. The adsorbent type fibers extract
analytes by physically interacting with the analytes and the extraction can be
accomplished by trapping the analytes in the internal pores of the stationary phase
(Gorecki et al., 1999).
1.7
Novel SPME Adsorbents
Recently, researches had been carried out in developing a new SPME fiber which
is more effective than commercially available fibers. Works by Gbatu et al. (1999) have
shown that sol–gel procedure can be used to prepare SPME fibers having desired
selectivity with judicious choice of precursors. These fibers are stable in different types
of organic and acidic solvents and basic solutions unlike commercial SPME fibers.
Zuin et al. (2004) employed a novel SPME fiber coated with PDMS/PVA copolymer to determine organochlorine and organophosphorus. The resulting procedure
was shown to be a good alternative and environmentally friendly analytical method. Hou
et al. (2004) successfully used C16-MCM-41 mesoporous materials as a novel SPME
19
fiber coating. The results indicated that this fiber coating has very high extraction
efficiency and good selectivity towards aromatic hydrocarbons.
Yu et al. (2004) successfully used allyloxy bisbenzo 16-crown-5 trimethylsilane
to prepare the sol–gel derived bisbenzo crown ether/hydroxyl terminated silicone oil
(OH-TSO) SPME coating. The long lastingness of this novel fiber is longer than that of
commercial fibers. Works by Lopes et al. (2004) have shown that the PDMS/PVA
composite can be used as a material for SPME fiber coating. It can be synthesized by
sol-gel polycondensation route. The highly microporous coating gave faster extractions
and showed potentially higher analyte loadings. Several researches (Li et al., 2004a; Li
et al., 2004b) have successfully developed amide bridged-C4/OH-TSO sol-gel coated
novel fiber for analysis of aliphatic amines and chlorophenols. The fiber exhibits better
sensitivity, high thermal stability and satisfactory fiber-to-fiber reproducibility.
For the first time, sol-gel dendrimer coated capillaries were used for solventless
microextraction and preconcentration in chemical analysis by Kabir et al. (2004). Due to
the strong chemical bonding with capillary inner walls, sol-gel dendron coatings showed
excellent thermal and solvent stability. The fiber was capable of extracting both polar
and nonpolar analytes including PAHs, aldehydes, ketones, phenols and alcohols.
Araanda et al. (2000) used two polycrystalline graphites (pencil lead and glassy carbon)
as sorbents for solid phase microextraction of non-ionic alkyl phenol ethoxylate
surfactant. Preliminary results showed that pencil leads and glassy carbon performed
equally well as SPME fibers (PDMS/DVB and Carbowax-TPR) in terms of LOD, linear
dynamic range and precision.
Works by Yun (2003) have shown that sol–gel derived diallytriethylene
glycol/hydroxyl–terminated silicone oil (DATEG/ OH-TSO) coated SPME fibers have a
powerful stationary phase for extraction of polar and non-polar analytes from headspace
for the sample due to the selectivity of open crown and three–dimensional network
provided by sol–gel process. Owing to the strong chemical binding between the coating
20
and surface, these fibers have high thermal and chemical stability in extraction for high–
boiling point compounds.
Works by Zeng et al. (2001) have shown that sol–gel derived hydroxydibenzo14-crown-4 coated fiber has good affinity for several aromatic amine derivatives. Wang
et al. (2003) successfully developed a new sol–gel coated dihydoxy-terminated benzo15-crown-5 (DOH-B15C5) fiber which exhibits high solvent stability, longer application
lifetime, thermal stability (≥ 350 °C), improved selectivity and sensitivity towards
different aromatic compounds (non-polar or polar). The porous structure increases the
speed of extraction and desorption.
Yu et al. (2002) successfully developed a new porous sol–gel hydroxyfullerene
(fullerol) fiber which is high temperature and solvent resistance with long durability. It
shows excellent extraction characteristics for both less volatile organic compounds and
polar substances, especially for planar molecules. Basheer et al. (2005) successfully
synthesized amphiphilic and hydrophilic oligomers and coated on fused silica capillaries
using sol-gel technique. The sol-gel coated fibers were successfully used for the
determination of triazine herbicides in reservoir water samples. Both types of coating
were stable under high temperature (up to 280 ˚C). Compared to commercially available
SPME adsorbents such as PDMS/DVB and PA, the new materials showed comparable
selectivity and sensitivity towards both non-polar and polar analytes. Quantitative results
obtained from these capillaries exhibited precision at LOD values comparable with
commercial coating materials. The coated capillaries also exhibited longer application
lifetime.
Hun et al. (2005) successfully prepared a novel poly(dimethylsiloxane)/βcyclodextrin (PDMS/β-CD) coating for solid-phase microextraction (SPME) in the form
of membrane instead of fiber. The fibers were capable of extracting non-polar polycyclic
aromatic hydrocarbons (PAHs) and polar phenolic compounds. The membrane showed
high extraction efficiency without sacrificing the analytical speed. Ugur et al. (2004)
21
described the use of SPME method with new design of functionalized stable
overoxidized sulfonated polypyrrole (OSPPy) film electrode to extract metal ions (Ni
(II) and Cd (II)) without derivatization from water of high silane. The extremely fast
uptake and release were attained by applied potentials inducing changes in the solution
pH. Fundamental results showed that (OSPPy) film offers a wide potential range for
electroanalytical
exploration
from
selective
electrodes
to
separation
and
preconcentration sampling device.
Chong et al. (1997) and Malik et al. (2004) successfully developed sol-gel
PDMS fibers for the extraction of PAHs. The sol-gel fiber exhibited enhanced thermal
stability (up to 320 ˚C) and the coating possess a porous structure and reduced thickness
which provide enhance extraction and mass transfer rate in SPME. Works by Liu et al.
(2005) proved that sol-gel derived co-poly(butyl methacrylate/hydroxyl terminated
silicone oil) (BMA/OH-TSO) has the best affinity for 2-chroethyl ethyl sulfide (CEES)
in soil. The coatings proved to be stable at high temperature (350 ˚C) and have a longer
lifetime (150 times).
Works by Wang et al. (2000) showed that polyethylene glycol coated fibers have
a high thermal stability (300 ˚C). This work proves that the thickness of the polymeric
layer can be controlled by varying the dipping time and the concentration of the sol
solution. Zhou et al. (2005) successfully developed a lab-made calixarene fiber which
exhibited a high affinity for the phenolic compounds and a higher extraction capability
compared to commercial polyacrylate fiber. Liu et al. (2005b) developed sol-gel coated
hydroxyl-terminated silicone oil-butyl methacrylate-divinbylbenzene (OH-TSO-BMADVB) and validated for the determination of volatile compounds in wine. The novel
coating was found to be very effective in carrying out extraction of both polar alcohols
and fatty acids and non-polar esters.
Liu et al. (1997) successfully prepared porous layer–coated SPME fibers by
immobilizing the C-8 and C-18 bonded silica particles (5 µm diameter, typically used
22
for reversed-phase HPLC) onto the surface of metal wires using high-temperature
epoxy. The fiber was capable of extracting aromatic hydrocarbons. Porous silica
particles provide a large specific surface area, therefore, the capacity of the coating is
increased. It was found that the desorption time using these fibers was less than 10 s,
which provides high chromatographic efficiency without cryogenic trapping.
Xia and Leidy (2001) prepared C18-bonded silica-coated multifibers by applying
thin film of epoxy glue over the glass fiber and pressing onto a C18-bonded silica particle
bed. It was found that the absorption rate of this fiber was 10 times higher than that of a
commercial PDMS fiber. It was observed that there was a 50 °C difference of desorption
temperature between the PDMS polymer and the C18 bonded silica. The desorption
temperature indicated that the analyte interaction with the C18-bonded silica was stronger
than that with the PDMS polymer.
Rodrigues et al. (2002) coated SPME fibers with modified silica particles bonded
to methyl (C1), hexyl (C6), octyl (C8), and polymeric octadecyl (C18) groups. The fiber
showed good selectivity for PAHs. The fibers could be used for up to 100 extraction
cycles without a significant reduction of extraction performance. However, because the
fibers were very brittle the lifetime of the fibers was mainly limited by breakage. When
these porous layer fibers were subjected to GC at 300 °C, some extraneous peaks were
observed. This is probably due to the release of some monomers in the epoxy glue used
to bond the porous layer.
Cai et al. (2003) have successfully prepared sol-gel dibenzo-18 crown-6 SPME
fiber coating by sol-gel technology and applied for the extraction of aliphatic amines.
Compared to commercial PDMS and PA fiber, the sol-gel dibenzo-18 crown-6 is more
sensitive and selective for derivatized aliphatic amines. The performance of the coating
is stable at high temperature (< 350 °C) and it has a long lifetime (can be used < 150
times).
23
Djozan and Bahar (2004) have investigated the efficiency of polyaniline (PANI),
coated gold wire, for use as a fiber for SPME. Aniline monomers were
electropolymerized on gold wires by applying a constant current to an acetate buffer
containing NaClO4 as supporting electrolyte. These fibers were capable of extracting
some aliphatic alcohols from gaseous samples.
Mohammadi et al. (2004) used the electrochemical fiber coating (EFC) technique
for the preparation of dodecylsulfate-doped polypyrrole (PPy-DS), and applied it as a
new fiber for SPME. PPy-Ds film was directly electrodeposited on the surface of a
platinum wire from an aqueous solution containing pyrrole and sodium dodecylsulfate,
using cyclic voltammetry (CV). The fiber coating can be prepared easily in a
reproducible manner, it is inexpensive and has a stable performance at high temperatures
(up to 300 °C). The new fiber was evaluated for the extraction of PAHs from water
samples.
Bagheri
et
al.
(2005a)
prepared
three
different
coated
fibers,
polydimethylsiloxane (PDMS), polyethylenepropyleneglycol monobutyl ether (Ucon)
and polyethylene glycol (PEG) based on sol-gel technology. For the first time, these
fibers were evaluated for the SPME of semi-volatile drugs such as, dextromethorphan
(DM) and dextrorphan (DP) in plasma. High-temperature conditioning of these fibers
leads to consistent improvement in peak area reproducibility.
An aniline-based polymer was electrochemically prepared by Bagheri et al.
(2005b). This new fiber coating was applied for solid phase microextraction of some
priority phenols from water samples. The polyaniline (PANI) film was directly
electrodeposited on the platinum wire surface in sulfuric acid solution using cyclic
voltammetry (CV) technique. This new coating can be prepared easily in a reproducible
manner and it is rather inexpensive and stable against most of organic solvents. The
thickness can be precisely controlled by the number of CV cycles.
24
Sun et al. (2005) have successfully developed activated carbon fiber (ACF) for
use in SPME. The fiber was used to determine benzyl chloride, benzyl dichloride and
benzyl trichloride in water samples by headspace analysis. Experiments showed that
ACF has a higher adsorption capacity than the commercial coated fibers except for
CAR-PDMS. The compounds studied could be desorbed completely within 60 s. The
fairly well-distributed surface of ACF ensures complete desorption in a short time.
3-(Trimethoxysily) propyl methacrylate (TMSPMA) was first used as precursor
as well as selective stationary phase to prepare the sol-gel derived TMSPMA-hydroxylterminated silicone oil (TMSPMA-OH-TSO) SPME fibers for the analysis of aroma
compounds in beer by Liu et al. (2005c). The fiber was found to be very effective in
carrying out simultaneous extraction of both polar alcohols and fatty acids and non-polar
esters in beer. The fiber exhibited better sensitivity to most of the investigated analytes
compared to commercial PDMS, PDMS/DVB and PA fibers. High thermal stability and
long lifetime are also characteristics of the new fiber.
SPME fiber coated with a novel γ-Al2O3 has been prepared and used to screen
gaseous samples for traces of volatile organic compounds (VOC) by Wei et al. (2004).
Results showed that it has good thermal stability (up to 350 °C), a long life span (< 180
times), higher extraction capacity and good selectivity for alkanes and esters. Physical
characteristics showed that the surface of the fiber was stacked with fine particles of γAl2O3 and it was porous. It was reported that the diffusion into the porous layer might be
responsible for the low diffusion kinetics which prolonged the equilibration time.
Djozan et al. (2004) coated gold wire with polypyrrole (PPY) by
electropolymerization for use in SPME. The adsorptive property of the coating was
modified by doping with tetrasulfonated nickel phtalocyanine (NiPcTS). NiPcTS as a
dopant enhances the sensitivity of PPY. The fiber was found to be capable of extracting
BTEX compounds from water sample. Farajzadeh and Rahmani (2005) have
successfully developed a new SPME fiber by electrolysis. The anode was oxidized to
25
copper (I) to produce copper (I) chloride as a sorbent for the studied amines on the
copper wire. This fiber was capable of extracting the studied amine compounds and
showed excellent fiber-to-fiber repeatability. It also exhibited lower detection limits and
shorter analysis time.
A new kind of membrane used for SPME was prepared with amine compounds
by Yang et al. (2004). The self-prepared SPME membrane was proven to be effective
for analysis of forensic toxic such as dichlorvos, morphine, Phenobarbital, methylamphetamine, MDA, ephedrine and TNT. Tsuda et al. (2004) prepared a new type of
fiber adsorbent attached with silica microparticles. The silica microparticles were
formed by the polymerization of silica oligomers on glass fibers, which were woven into
a glass filter. The surface of the silica microparticles was chemically modified by
bonding C18-ligands.
Djozan and Assadii (2004) have conclusively demonstrated that pencil lead fiber
modified by heating with water vapour can be used for precise analysis of PAH in water
by SPME followed by GC-FID. Low cost and high temperature resistance are the main
advantages of this fiber. It has characteristics required for use in routine analytical
procedures. Djozan and Zehni (2003) investigated a simple and inexpensive method for
the preparation of new SPME fiber consisting of copper wire coated with copper
sulphide. The fibers seem very effective for extraction of aliphatic alcohols and amines
from aqueous samples.
Oliveira et al. (2005) successfully used a glass ceramic rod as a base for the
preparation of SPME fibers using sol-gel technology. The glass-ceramic rod was coated
with PDMS using sol-gel reaction. The film thickness for this new base was much
bigger than the fused silica base. The characteristics of this new base can be attributed
to its stronger exchange capacity. The glass ceramic fiber coated with PDMS by sol-gel
reaction shows good extraction characteristics for BTEX compounds.
26
Co-poly (hydroxy-terminated silicone divinylbenzene) coating was first prepared
by sol-gel and cross-linking methods and applied to the solid-phase microextraction of
phosphate and methylphosphonate from and water by Liu et al. (2003). The new fiber
possessed high thermal (to 380 °C) and solvent stability and a long lifetime. Compared
to commercial SPME fibers, the sol-gel coated OH-TSO/DVB fiber showed better
selectivity and sensitivity towards phosphate and methylphosphonate. Therefore, the
novel fiber may be a good choice for analyzing warfare agents with SPME.
Silveira et al. (2005) developed silica-based glass fibers and modified their
surfaces with Nb2O5. Fiber glass with a composition of 29 % Li2O, 1 % ZrO2, 5% BaO
and 65 % SiO2 was coated with Nb2O5 using the metallo-organic decomposition (MOD)
technique. The Nb2O5-coated fibers showed excellent results both in the adsorption of
phenols and in the adsorption and separation of alcohols using SPME.
A novel alumina-based hybrid organic-inorganic sol-gel coating was first
developed for SPME from a highly reactive alkoxide precursor, aluminium sec-butoxide
and a sol-gel active polymer hydroxyl-terminated polydimethylsiloxane (OH-TSO) by
Liu et al. (2006a). As compared to silica based hybrid materials, the ligand exchange
ability of alumina makes it structurally superior extraction sorbents for polar compounds
such as, fatty acids, phenols, alcohols, aldehydes and amines. The fiber was capable of
extracting volatile alcohols and fatty acids in beer.
Liu et al. (2006b) demonstrated a new coating technology for the preparation of
SPME coating by sol-gel method. The extraction phase of PDMS containing 3 % vinyl
group was physically incorporated into sol-gel network without chemical bonding. The
extraction phase itself was then crosslinked at 320 °C, forming an independent polymer
network and can withstand desorption temperature of 290 °C. Application to real life
sample extraction showed that the SPME fiber is suitable for the determination of both
volatile and semi-volatile compounds.
27
Gierak et al. (2006) successfully prepared new carbon fibers for SPME by the
direct pyrolysis of methylene chloride on the quartz core. Owing to the high partition
coefficients of the studied substances obtained on carbon fibers, it was possible to do the
analysis of organic substances occurring in trace amounts in different matrices. The fiber
was used for the analysis of benzene, toluene and xylene (BTX) contents in petrol.
Cai et al. (2006) prepared three new crown ether SPME coatings using sol-gel
method. In this work, three kinds of vinyl crown ethers with different cavity and benzyl
substitutions were crosslinked with vinyl triethoxysilane under radical initiator. These
new coatings showed higher extraction efficiency and sensitivity for organophosphorus
pesticides (OPPs) compared with commercial fibers-85 µm PA and 65 µm PDMS/DVB.
The benzo-15-crown-5 (B15C5) coating with larger polarity had the best selectivity for
OPPs and it was successfully used for the determination of OPPs in juice, fruit and
vegetables.
Panavaite et al. (2006) developed a very simple method for the preparation of a
new SPME fiber consisting of stainless steel wire coated with commercial silicone based
high temperature resistant glue. The construction of the fiber is very simple and can be
completed in a few minutes. The developed fiber was found to be mechanically stable
and exhibited a relatively high thermal stability (up to 260 °C). The proposed SPME
fiber offers and attractive alternative to commercially available PDMS based fibers for
the extraction of non-polar or less-polar compounds.
Anilinemethyltriethoxysilane (AMTEOS) was first used as precursor as well as
selective stationary phase to prepare the sol-gel derived AMTEOS/PDMS SPME fibers
by Hu et al. (2006). The novel SPME fibers exhibited higher extraction efficiency, good
thermal stability, high porosity and longer lifetime compared with commercial SPME
coatings. Owing to the introducing of the phenyl group in AMTEOS, the fiber offers
better sensitivity towards aromatic compounds, such as monocyclic aromatic
hydrocarbons (MAHs) and polycyclic aromatic hydrocarbons (PAHs).
28
1.8
Sol-gel Technology
Sol-gel technology provides efficient incorporation of organic components into
inorganic polymeric structures in solution under extraordinarily mild thermal conditions.
In general, the sol-gel process, involves the evolution of inorganic networks through the
formation of a colloidal suspension (sol) and gelation of the sol to form a network in a
continuous liquid phase (gel). Through this process, homogeneous inorganic oxide
materials with desirable properties of hardness, chemical and thermal resistance, polarity
and tailored porosity can be produced at room temperatures (Brinker and Scherer, 1990;
Klein, 1988).
The precursors for synthesizing these colloids consist of a metal or metalloid
element surrounded by various reactive ligands. A colloid is a suspension in which the
dispersed phase is so small (~ 1-1000 nm) that gravitational forces are negligible and
interactions are dominated by short-range forces, such as van der waals attraction and
surface charges. An alkoxy is a ligand formed by removing a proton from the hydroxyl
of an alcohol, as in methoxy (•OCH3) or ethoxy (•OC2H5). Metal alkoxides are members
of the family of metallorganic compounds, which have an organic ligand attached to a
metal or metalloid atom. Metal alkoxides are most popular because they react readily
with water. The most widely used metal alkoxides are the alkoxysilanes, such as
tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). However, other alkoxides
such as aluminates, titanates and borates are also commonly used in the sol-gel process,
often mixed with (TEOS) (Dietz and Camara, 2005; Hofacker et al., 2002).
At the functional group level, three reactions are generally used to describe the
sol-gel process as shown in Figure 1.2; hydrolysis (R1), alcohol condensation (R3) and
water condensation (R2). The hydrolysis reaction (R1) replaces alkoxide groups (OR)
with hydroxyl groups (OH) through the addition of water. Subsequent condensation
reactions (R2 and R3) involving the silanol groups (Si-OH) produce siloxane bonds (SiO-Si) plus the by-products water or alcohol. Under most conditions, condensation
29
commences before hydrolysis is complete. However, conditions such as pH, H2O/Si
molar ratio (R) and catalyst can force completion of hydrolysis before condensation
begins. Although hydrolysis can occur without addition of an external catalyst, it is most
rapid and complete when they are employed. Mineral acids, for example HCl or
ammonia are most generally used, however, other catalysts such as acetic acid, KOH,
amines, KF and HF are also employed. It has been observed that the rate and extent of
the hydrolysis reaction is most influenced by the strength and concentration of the acid
or base catalyst (Brinker and Scherer, 1990; Brinker et al., 1992; Klein, 1988).
Figure 1.2: Basic steps in sol-gel coating technology (Brinker and Scherer, 1990)
Additionally, because water and alkoxides are immiscible, a mutual solvent such
as an alcohol is utilized. With the presence of alcohol as the homogenizing agent,
hydrolysis is facilitated due to the miscibility of the alkoxide and water. As the number
of siloxane bond increases, the individual molecules are bridged and jointly aggregate in
the sol, which is a colloid suspension of solid particles of a diameter of hundred
nanometers in a liquid phase. When the sol particles aggregate, or cross-link into a
network, a micromolecular gel is formed. Upon drying, trapped volatiles are then driven
off (Brinker et al., 1992).
30
1.8.1 Sol-gel Method as a New Tool for SPME Fiber Coatings
Sol-gel technology offers a simple and convenient pathway for the synthesis of
advanced material systems and for applying them on as surface coatings. Many different
combinations of alkoxides and organoalkoxysilanes have been prepared to produce
coatings with improved mechanical properties such as abrasion resistance and high
chemical stability. However, the characteristics and properties of a particular sol-gel
coating are strongly related to a number of factors, such as, pH, H2O/Si molar ratio (R),
nature and concentration of catalyst, ageing temperature and time and drying during the
synthesis of the coating (Dietz and Camara, 2005; Arkles, 2001).
Dip coating technique is commonly used in preparing sol-gel coated fibers. The
substrate to be coated is immersed in sol-gel solution and withdrawn with a well defined
withdrawal speed at atmospheric condition. The stages of dip coating process includes
dipping of the substrate (fused silica fiber) into the coating solution, wet layer formation
by withdrawing the substrate and finally gelation of the layer by solvent evaporation as
shown in Figure 1.3 (Brinker and Scherer, 1990).
During the withdrawal of the substrate from the solution, the inner layer moves
upward with the substrate, while the outer layer is returned to the solution. The thickness
of the deposited film is related to the position of the streamline dividing the upward and
downward - moving layers. The faster the substrate is withdrawn, the thicker the
deposited film. Evaporation is generally relied upon to solidify the coating. The most
significant factor in the rate of evaporation is the rate of diffusion of the vapor away
from the film surface. The evaporation rate can be controlled by the deposition ambient.
Since condensation continues to occur during sol-gel coating formation, the relative
condensation and evaporation rates will dictate the extent of further cross-linking that
accompanies the deposition and drainage stages (Atanacio et al., 2005; Brinker et al.,
1992; Klein, 1988).
31
Syringe
holder
Fused
silica
fiber
Coated
layer
Coating
solution
Figure 1.3: The stages of dip coating process (Brinker and Scherer, 1990)
Among many inherent advantages of sol-gel technology are its unique ability to
achieve molecular level uniformity in the synthesis of organic-inorganic composites,
strong adhesion of the coating to the substrate due to chemical bonding and high thermal
stability which overcomes the sample carryover problem in SPME-GC analysis
(Hofacker et al., 2002; Kabir et al., 2004; Li et al., 2004a; Li et al., 2004b; Lopes and
Augusto, 2004; Yu et al., 2004).
1.9
Problem Statements and the Needs of Study
Interpretation of forensic evidence and in particular, the ability to uniquely detect
accelerant is a crucial challenge in the scientific investigation of arson. Limitations in
this area are posing a significant impact on police investigations and the successful
prosecution of arson cases. An improvement in accelerant extraction came with the
development of SPME technique as a viable standard method in arson analysis.
The qualities that enable an SPME adsorbent to be successfully used for
accelerant extraction and analysis are its selectivity towards accelerant components
32
which separates and concentrates the accelerant from the headspace to yield a sample
that is suitable for introduction to GC-FID. Commercial SPME adsorbent have generally
good extraction properties, however some aspects of their performance need to be
improved to overcome the limited use of SPME in gas chromatographic applications
(Dolan, 2003).
Although a number of adsorbents are commercially available for SPME but some
analytical methodologies might demand specific properties for extraction of selected
compounds, special coatings that have a particular volume and selectivity towards
particular analytes. Commercially available fibers are rather expensive and have a short
lifetime (40-100 times) (Wang et al., 2003; Hun et al., 2005; Zhou et al., 2005). These
fibers have relatively low recommended operating temperatures and generally remain in
the range of 200-270 °C (Chong et al., 1997; Wang et al., 2000; Huo et al., 2004; Ugur
et al., 2004; Yu et al., 2004; . Zuin et al., 2004; Hun et al., 2005; Liu et al., 2005a; Zhou
et al., 2005). Poor thermal stability leads to incomplete sample desorption and sample
carry-over problem (Liu et al., 2005b). Conventional fibers have poor solvent stability.
The equilibrium and speed of the extraction process are related to physico-chemical
characteristics of the sorbent, this limits the extent of selectivity obtainable using
commercial fibers (Yun, 2003).
Improved mechanical and chemical resistance is in an increasing demand in
coating applications such as SPME. Recently, many novel coatings have been developed
using different techniques and technology for use in SPME. However, up to now, none
of the novel fibers have been evaluated for the determination of accelerants in arson
analysis.
33
1.10
Objectives and Scope of Study
The objectives of this study are as follows:
i.
To investigate suitable SPME adsorbents for arson analysis.
ii.
To evaluate the home-made SPME fiber against commercially available fibers
iii.
To apply the developed fiber for detection of accelerants in fire debris.
In this study, SPME fiber coated with alternative materials was prepared by
using sol-gel technology. The scope of the research is limited for the extraction of
common accelerants in simulated arson samples. All the extractions in this study were
performed using headspace –SPME mode.
34
CHAPTER 2
EXPERIMENTAL
2.1
Introduction
This chapter covers the chemicals, materials and apparatus used in this study. It
also includes general experimental work in the development, characterization and
evaluation of the lab-made SPME fiber for the detection of accelerants in arson samples.
2.2
Chemicals and Materials
The fiber coating material, n-octyltriethoxysilane (C8-TEOS), the precursor,
methyltrimethoxysilane (MTMOS) and the end capping reagent, trimethylmethoxysilane
(TMMOS) were obtained from Fluka Chemika, Switzerland. Table 2.1 lists the names,
functions and chemical structures of the principal ingredients of the sol-gel coating
solutions. Analytical grade n-hexane was obtained from J. T. Baker, New Jersey, (U. S.
A). while hydrochloric acid (HCl) and HPLC grade methanol were obtained from BDH
Chemicals, Poole, England. Distilled deionized water was prepared in the laboratory,
Department of Chemistry, UTM. Individual standards of n-alkanes (C8, C9, C10, C12, C13,
C14, C15 and C16) were purchased from Fluka Chemika.
Samples of diesel and unleaded gasoline were purchased from a petrol station in
Skudai, Johor while kerosene was obtained from a grocery shop at Taman Universiti,
35
Skudai, Johor. Samples of carpet were purchased from a carpet retail shop in Taman
Ungku Tun Aminah, Skudai, Johor.
Table 2.1: Names, functions and chemical structures of the principal compounds of solgel coating solutions
Ingredient
Methyltrimethoxysilane
(MTMOS)
Function
Sol gel precursor
Chemical structure
CH3
H3CO
Si
OCH3
OCH3
n-Octyltriethoxysilane
(C8-TEOS)
Coating stationary
phase
[CH3(CH2)6CH2]n
CH3CH2O
Si
OCH2CH3
OCH2CH3
Trimethylmethoxysilane
(TMMOS)
End-capping reagent
CH3
H3C
Si
CH3
OCH3
Hydrochloric Acid (0.1 M)
Acid catalyst
Methanol
Solvent
HCl
CH3OH
36
2.3
Apparatus
Two glass apparatus (400 cm3 and 125 cm3 ) for sample preparation step of
headspace SPME was specially designed (Yong, 2004). SPME holder and commercially
available PDMS/DVB fibers purchased from Supelco, Bellefonte, Pennsylvania, U.S.A
was used for the headspace technique. A 10 µL syringe used for direct injection method
was obtained from Hamilton Co, Reno, Nevada, U.S.A. Used commercial SPME fibers
were employed for the coating process.
Eppendorf polypropylene micro centrifuge tubes (2 mL) used to place the sol-gel
solution prior to centrifuge were obtained from Hamburg, Germany. Aluminium stubs
used to place fiber samples prior to electron microscopy experiments were provided by
Materials Science Lab, Faculty of Mechanical Engineering, UTM. A hotplate/stirrer
Model HS 0707V2 from Favorit, United Kingdom was used to heat the samples. An
SGT capillary column cutter with a rotating diamond blade from Shortix, Holland was
used to cut the coated fibers for SEM analyses. Other apparatus used in this study
included micropipettes Model 5000DG Nichipet (20-200 µL) and (0.5-10 µL) from
Japan, vials, teflon magnetic stirring heads, dessicator, aluminium foil, double edged
carbon tape (for placing SPME coated fibre on aluminium stubs) and a cigarette lighter
(used to remove polyimide protective layer from the fiber).
An illustration of the experimental setup for headspace SPME is shown in Figure
2.1. Prior to experiments, the glass apparatus were silanized to prevent the polar analytes
from being adsorbed onto the glass wall. The sample apparatus was sealed with a septum
to prevent sample evaporation. Headspace was created by partially immersing the
sample apparatus containing analyte into a hot water bath. Subsequently, the SPME
needle was pushed through the sub seal rubber septum (1.5 cm diameter) and the fiber
was exposed in the gas phase (headspace). The hot water bath was stirred in order to
homogenize the heat transfer and a thermometer was used to monitor the temperature
during the extraction. After the extraction, the fiber was withdrawn into the needle and
37
removed from the sample matrix. The fiber was then immediately inserted into the
heated GC injector port for subsequent analyte desorption.
Thermometer
SPME Holder
Sub seal septum
Sample preparation bottle
Exposed fiber
Spiked carpet sample
Magnetic bar
Hotplate/stirrer
Figure 2.1: Experimental setup for headspace SPME accelerant extraction
2.4
Instrumentation
Gas chromatography analyses were conducted using a Hewlett-Packard 6890 GC
(Wilmington, Delaware, U.S.A.). The HP 6890 gas chromatograph was equipped with
FID and a HP ChemStation for data processing. An Ultra-1 capillary column (Agilent)
of dimensions 25 m x 0.20 mm x 0.11 µm film thickness was used. Helium was used as
the carrier gas at a flow rate of 1.2 mL/min. The injection port temperature was set at
250 °C and FID temperature at 310 °C. SPME injections were performed using a split
mode injection (5:1). Figure 2.2 shows the oven temperature program applied for Ultra 1
column.
Studies of fiber coating structure and thickness were made by means of Philips
Scanning Electron Microscope Model XL 30 SEM (Philips Electronic Instruments
Company, Mahwah, New Jersey) equipped with a ThermoNoran energy dispersive Xray detection system (EDX). The fourier transform infrared (FTIR) analysis was
38
performed using a FTIR-8300 spectrometer (Shimadzu Corporation, Kyoto, Japan). A
Hettich Zentrifugen, Centrifuge Universal 32R was used to separate the sol solution
from the precipitate (if any).
2.5
Preparation of the Sol-gel Derived C8-coated Fiber
Generally, the preparation of the solid phase microextraction fiber by sol-gel
coating method includes the steps of providing the fiber structure, providing a sol-gel
solution comprising a sol-gel precursor, an organic material with at least one sol-gel
active functional group, a sol-gel catalyst, an end-capping reagent and a solvent system.
Preferably, the sol-gel precursor includes an alkoxy compound. The organic material
includes a monomeric or polymeric material with at least one sol-gel active functional
group. The sol-gel catalyst is taken from the group consisting of an acid, a base or a
fluoride compound (Wu et al., 2001).
In this study, the sol-gel solution was prepared by mixing an appropriate amount
of C8-TEOS, MTMOS, methanol, 0.1M HCl and distilled de-ionized water. The sol-gel
solution was stirred for 4 hours at room temperature. The mixture was then transferred
into an Eppendorf microcentrifuge tube and centrifuged at 13,000 rpm for 5 min. The
precipitate at the bottom of the tube, if any, was removed and the top clear sol solution
was used for fiber coating. Prior to sol-gel coating, the protective polymeric layer was
removed from a 1 cm segment of the used conventional fiber at its end. This was
accomplished by burning off the polyimide protective layer using a cigarette lighter. The
exposed fused silica surface (burnt section of the fiber) was pretreated by washing it
with excess methanol and water. It was then air dried at room temperature.
The pretreated portion of the fiber (bare outer surface of the fiber end) was then
reacted with sol-gel solution under controlled conditions to produce a surface bonded
sol-gel coating on the portion of the fiber. This was done by dipping the bare tip of fused
39
silica vertically into the sol solution. It was held inside the sol solution for ~20 min. The
coating process was repeated three times, using a freshly prepared sol solution each
time. This was followed by the end-capping process. The coated fiber was end-capped
by dipping it into the mixture of TMMOS/methanol solution. After 1 min, the fiber was
removed from the end-capping solution and placed in a dessicator at room temperature.
Before use, the fiber was conditioned at 250 °C under helium for approximately
1-2 hours in the GC injection port. After removal from the injector, the fiber was cooled
to room temperature. The conditioning cycle was repeated a few more times until a
stable GC baseline was obtained. The fiber was then ready for SPME-GC experiments.
2.6
Preparation of Standard Stock Solution
A concentration of 20 % (v/v) standard stock solution of n-alkanes was
preparaed by mixing C8, C10, C12, C14, and C16 hydrocarbons in equal volume into a 25
mL volumetric flask. Another standard solution of n-alkanes was prepared by mixing C9,
C13, and C15 hydrocarbons in equal volume into a 25 mL volumetric flask. This resulted
in a 33.3 % (v/v) concentration stock solution. Both prepared stock solution represented
the hydrocarbon compounds commonly found in gasoline, kerosene and diesel. A 1.0 µL
aliquot of both the standard mixture was then introduced into the GC injection port by
direct injection.
2.7
Procedure for Extractions using Headspace SPME (HS-SPME)
In order to extract standard hydrocarbon compounds using the lab-made C8-
coated fiber, 30 µL from the prepared standard solution of n-alkanes (C8, C10, C12, C14,
and C16) was placed in the sample preparation apparatus which was immersed in a hot
water bath and heated for 20 min at 100 °C (this initial experimental conditions were
40
chosen according to the previous study done in this lab by Yong, 2004). The C8-coated
fiber was exposed in the headspace for an appropriate time. Finally the exposed fiber
was retracted and the fiber extracts were analyzed using GC-FID. The oven temperature
was initially set at 40 °C, programmed at a rate of 10 °C/min until a final temperature of
270 °C. Blank runs of the fiber were performed prior to each extraction to ensure that
there were no carryover of analytes from previous extraction. The HS-SPME procedure
was repeated using commercially available PDMS/DVB fiber for comparison.
For the extraction of accelerants using C8-coated fiber, 30 µL of neat accelerant
(petrol, kerosene or diesel) was individually placed in the sample preparation apparatus.
The extraction steps mentioned above were employed. The same HS-SPME procedure
was carried out again using spiked fire debris sample for the determination of
accelerants in simulated arson samples. Blank analysis of the fire debris sample was
carried out before the recovery experiments.
2.8
Preparation of Spiked Fire Debris Sample
A sample of carpet (20 cm × 13 cm) placed on a sheet of aluminium foil was
ignited with a fire starter and left to burn until about one-third remained on the
aluminium foil as shown in Figure 2.2 (a). Fire was extinguished by cutting off the
oxygen supply. The partially burnt carpet was then exposed to the surrounding air for 30
minutes to let it cool down [Figure 2.2 (b)]. 100 µL of neat accelerant (petrol, kerosene
or diesel) was individually spiked onto the burnt carpet fire debris sample. The spiked
sample was placed in the glass apparatus prior to extraction using HS-SPME. The
extracted analytes were then analyzed by GC-FID. Blank samples of unspiked burnt
carpet samples were prepared in a similar way.
41
(a)
(b)
Figure 2.2: Photographs showing (a) the burning of carpet sample and (b) the burnt fire
debris sample
2.9
General Procedures for Characterization of the C8-coated Fiber
2.9.1 Lifetime of the Coating
30 µL from the prepared standard solution of n-alkanes (C9, C13, and C15) was
placed in the HS-SPME sample preparation apparatus. The extraction was conducted
and the analytes were desorbed in GC-FID as described in Section 2.7. In order to
determine the life span of the coated fiber, the extraction and desorption cycle was
repeated 200 times under the same HS-SPME and GC operating conditions. The fiber’s
life span was studied by monitoring the change of its extraction ability after it had been
used for 50, 100, 150 and 200 times.
42
2.9.2 Thermal Stability
The effect of conditioning temperature on the stability of the C8-coated fiber was
evaluated by subjecting the fiber to successively higher conditioning temperatures, 270
°
C and 300 °C respectively for 1 hour prior to extraction. The same HS-SPME extracting
conditions described in Section 2.7 were applied. Triplicate GC analyses were
performed for each conditioning temperature.
2.9.3 Scanning Electron Microscopy
Scanning Electron Microscopy analysis was carried out to investigate the
structure of the coated surface and estimate the thickness of the coating. The 1 cm
segment of the coated fiber end was cut using a diamond cutter. The fiber was then
placed on an aluminium stub with the help of a double edged carbon tape. The stubs
were placed in a dessicator prior to SEM analysis. The SEM was carried out by the
bombardment of electrons of 30 KeV on the target sample (C8-fiber). (100-4000) times
magnification of the fiber was observed using the scanning electron microscope. In order
to estimate the film thickness and determine the reproducibility of the coating thickness,
five C8-coated fibers were prepared in a similar way as described in Section 2.5. All the
fibers were subjected to SEM analysis under the same condition. The film thickness was
estimated from the cross-sectional view of the fiber.
2.9.4 Calculation of Fiber Coating Volume
The coating volume of both commercial PDMS/DVB fiber and lab–made C8coated fiber were determined. The extracting phase volume (Vf) can be calculated by the
following equation (Perez et al., 2001):
43
Vf = π L (e2 + ae)
Where
L:
The fiber coating length (1.0 cm)
e:
The film thickness of the coating polymer
a:
The diameter of the fused silica base (110 µm)
(2.1)
2.9.5 FTIR Analysis
The sol-gel derived C8-coated fibers were conditioned at 300 °C under helium
protection for 2 hr and then dipped in methylene chloride for 2 hr prior to IR analysis. A
section of the coating was harvested with a razor blade from the pretreated fiber, then
ground and blended with potassium bromide (KBr). The KBr pellet spectra of the
coatings were acquired with air as background at a resolution of 4 cm-1 over the full midIR range (4000-400 cm-1). IR spectrum of pure C8-TEOS (fiber coating material) was
recorded by liquid film method over the same frequency region and at the same
resolution time for comparison.
2.10
Procedure for Optimization of Sol-gel Process
Optimization of the sol-gel process was affected by varying the molar ratios of
the silane precursors. Optimization of the sol-gel process was done by preparing three
different fibers from sol-gel solutions containing C8-TEOS to MTMOS in the ratio of
0.5:1, 1:1 and 2:1. The fiber preparation method described in Section 2.5 was employed.
HS-SPME extractions of hydrocarbons (Section 2.7) was done in triplicate for each
fiber. The extracted analytes were then thermally desorbed at the GC injection port.
44
2.11
HS-SPME Optimization and Evaluation Procedures
2.11.1 Optimization of HS-SPME Operating Conditions
30 µL from the prepared standard solution of n-alkanes (C8, C10, C12, C14, and
C16) was placed in the sample preparation apparatus and followed by HS-SPME
extraction. When the desired fiber exposure time was reached, the fiber was retracted
and the analytes were thermally desorbed in GC injection port.
Several variables such as fiber exposure time, desorption time and extraction
temperature have to be taken into account for the optimization of the proposed HSSPME-GC method using C8-coated fiber. A set of experiments herein performed in a
stepwise fashion served to optimize the experimental parameters. The variables were
optimized by employing the procedures mentioned above. Triplicate analyses were
carried out for each parameter.
The fiber exposure time was preliminary optimized by keeping the extraction
temperature (80 °C) and desorption time (80 sec) constant. The analyte desorption time
at the GC injection port was then optimized by keeping the extraction temperature (80
°
C) and extraction time (15 min) constant. Finally the extraction temperature was
optimized by keeping the fiber exposure time (15 min) and desorption time (80 sec)
constant. The variables optimized were in accordance to Table 2.2.
45
Table 2.2: Optimization of HS-SPME experimental parameters [(a) exposure time, (b)
desorption time and (c) extraction temperature]
(a)
(b)
(c)
Exposure
Time (min)
5
10
15
20
25
30
35
Desorption
Time (sec)
80
80
80
80
80
80
80
Extraction
Temperature ( °C)
80
80
80
80
80
80
80
Desorption
Time (sec)
10
20
30
40
50
60
70
80
90
Exposure
Time (min)
15
15
15
15
15
15
15
15
15
Extraction
Temperature ( °C)
80
80
80
80
80
80
80
80
80
Extraction
Temperature ( °C)
40
50
60
70
80
90
100
Exposure
Time (min)
15
15
15
15
15
15
15
Desorption
Time (sec)
80
80
80
80
80
80
80
46
2.12
Method Validation
2.12.1 Determination of Limits of Detection (LOD)
The detection limits of fire accelerants were investigated using the lab-made C8coated fiber. Individual accelerants (petrol, kerosene and diesel) were spiked onto a
blank burnt carpet sample in decreasing amount (initially 1 µL). Similar fire debris
sample preparation as described in Section 2.8 was employed. The HS-SPME
extractions were carried out with the optimized conditions of the C8-fiber. Blank
samples were also extracted in order to differentiate the burnt matrix profile in the
presence of accelerants.
2.12.2 Preparation of Calibration Graph
A series of working solutions were prepared in individual volumetric flask (5mL)
by diluting the standard stock solution with n-hexane. The resultant concentrations were
a series of 2.00, 4.00, 6.00, 8.00 and 10.00 % (v/v) standard mixture. The HS-SPME
extractions were carried out with the optimized conditions of the C8-fiber. Triplicate
analyses were performed at each concentration level. The calibration graph was
constructed by comparing average peak area counts against analyte concentrations.
47
CHAPTER 3
CHARACTERIZATION OF THE C8-COATED FIBER, OPTIMIZATION AND
UNDERLYING MECHANISM OF SOL-GEL PROCESS
3.1
Introduction
In this chapter, the characterization of the lab-made sol-gel derived C8-coated
fiber is described. The optimization of sol-gel process and the chemistry behind the solgel coating technique involved in this study were also discussed.
3.2
Characterization of the C8-coated Fiber
3.2.1 Selectivity for Standard Hydrocarbons
In order to examine selectivity of the lab-made SPME fiber towards hydrocarbon
compounds, a mixture of n-alkanes (C8, C10, C12, C14 and C16 ) were subjected to
headspace SPME using sol-gel derived C8-coated fiber and the GC profiles were
compared with that from direct injection. The GC profiles obtained from headspace
SPME using C8-coated fiber was comparable with the profiles of hydrocarbon standards
from direct injection. There was a significant comparison of carbon compounds in the
similar retention time ranging from 4-18 min. There was no significant differences of
hydrocarbon peaks and all the hydrocarbon components were well separated as shown in
Figure 3.1.
48
Figure 3.1: SPME-GC analysis of hydrocarbon compounds using C8-coated fibers
Peaks: (1) C8, (2) C10, (3) C12, (4) C14 and (5) C16 n-alkanes.
3.2.2 Extraction Capability for Standard Hydrocarbons
The extraction capability of the sol-gel derived C8-coated fiber for hydrocarbons
was determined by comparing it with the extraction capability of commercially available
PDMS/DVB fiber. PDMS/DVB fiber was selected for comparison because previous
work done in this lab (Yong, 2004) proved that the fiber has the highest sensitivity
towards hydrocarbon compounds. Figure 3.2 shows the comparison of extraction
capability between C8-coated fiber and commercially available PDMS/DVB fiber. The
C8-coated fiber exhibited a slightly higher extraction capability for all the hydrocarbon
compounds by contrast with conventional PDMS/DVB fiber. A higher extraction
capability yielded by C8-coated fiber could be due to the existence of higher surface area
for the C8-coated fibers. The result obtained is in good agreement with the previous
work done by Gbatu et al. (1999).
49
Based on the previous studies, almost all the fibers prepared by sol-gel
technology gave superior extraction capability when compared to commercially
available fibers. Zuin et al. (2004) have stated that the extraction capacity of sol-gel
PDMS/PVA fiber for pesticides was higher than that of commercially available PDMS
fiber. Liu et al. (2005a) reported that sol-gel derived acrylate/silicone fiber yielded
higher extraction efficiency to 2-chloroethyl ethyl sulfide in soil compared with
conventional polyacrylate (PA) fiber. Works done by Cai et al. (2003) showed that the
selectivity and extraction capability of the sol-gel polymethylsiloxane fiber for
organochlorine pesticides was higher than that of conventional PDMS fiber.
Figure 3.2: Comparison of extraction capability between C8-coated fiber and
PDMS/DVB fiber.
3.2.3 Lifetime of the Coating
A coating’s lifetime is important for practical application (decline of efficiency
with the number of analysis). The long lastingness of the coated fiber was determined by
200 continued operations carried out with the same C8-coated fiber and oven
temperature program. The C8-coated fiber have been used for hydrocarbon extractions
50
(C9, C13, and C15) and subjected to GC for more than 200 times. There was no significant
differences of hydrocarbon peaks obtained in each operation. Referring to Table 3.1, all
the hydrocarbon compounds gave a low relative standard deviation (RSD) value ranging
from (3.8-5.4) % which shows an acceptable reproducibility. This proves that the coated
surface of the fiber was not partially depleted during the continued operation. It was still
stable and reusable. Such a long service life are possibly due to the strong chemical
bonding between the sol-gel generated C8-coated composite coating and the silica
surface (Chong et al., 1997; Zeng et al., 2001; Liu et al., 2003).
Table 3.1: The extraction precision of C8-coated fiber (200 operations)
Hydrocarbon
compounds
C9
Extraction Period
50th
Average RSD
Peak
(%)
Area
(n=3)
176.5
4.9
100th
Average RSD
Peak
(%)
Area
(n=3)
153.4 5.1
150th
Average RSD
Peak
(%)
Area
(n=3)
238.1 5.3
200th
Average RSD
Peak
(%)
Area
(n=3)
204.9 5.4
C13
340.7
3.8
433.5
4.2
488.2
3.9
404.8
4.3
C15
418.2
4.6
501.3
4.9
490.7
5.0
523.6
4.8
The result obtained is comparable with those obtained by other researches. A
single sol-gel PDMS/PVA fiber was recorded to be used for more than 300 extractions
during a time range of more than five months before degradation of the coating (Zuin et
al., 2004). Liu et al. (2006) reported that the extraction efficiency of sol-gel aluminahydroxyl-terminated polydimethylsiloxane coating had no obvious decline after the fiber
was run at least 160 sorption/desorption processes. Zeng et al. (2001) reported that the
extraction ability of sol-gel hydroxydibenzo-14-crown-4 fiber showed no significant
decline after it has been used 150 times. Works by Wang et al. (2000), proved that sol-
51
gel polyethylene glycol coated fiber was still reusable although it has been used for more
than 150 extractions.
3.2.4 Thermal Stability
Thermal stability is a crucial property of SPME coatings in practical
applications. Good thermal stability will reduce the possibility of polluting the detector
and expand the scope of applications of SPME. The effect of conditioning temperature
on the stability of the C8-coated fiber was determined by conditioning the fiber at high
temperatures (270 and 300) °C for 1 hour prior to extraction. It is apparent from Table
3.2 that the extraction capacity of the C8-coated fiber was not significantly affected by
the temperature used for thermal conditioning. High temperature conditioning lead to
consistent improvement in peak area repeatability for SPME-GC analysis. The RSD
value of < 4.6 % can be routinely obtained for hydrocarbons on C8-coated fibers
conditioned at 270 °C and 300 °C. There was a slight improvement of the peak area
reproducibility achieved by using higher fiber conditioning temperature (300 °C).
The C8-coated fiber can be routinely used at 300 °C without any sign of bleeding,
whereas for commercial PDMS/DVB fiber, the highest temperature the coating layer can
endure is less than 280 °C, and thus the range of analyte molecular weights that can be
handled by SPME-GC is limited. Enhanced thermal stability of sol-gel coated fiber
allows one to overcome the sample carryover problem often encountered in SPME and
expand the SPME application range towards higher boiling point. Such a high thermal
stability might be due to the strong chemical bonding between the sol-gel generated
composite coating and the silica surface (Chong et al., 1997; Yu et al., 2002; Yun, 2003;
Cai et al., 2003; Yu et al., 2004). Therefore sol-gel C8-fiber provided efficient release of
extracted analytes without negative consequences from the use of high temperatures.
52
The high thermal stability of C8-fiber determined in this study is similar to those
obtained by other researchers using different sol-gel fibers. Chong et al. (1997) reported
that sol-gel PDMS fiber can be routinely used at 320 ˚C without any sign of bleeding.
Liu et al. (2003) reported that sol-gel silicone divinyl benzene fibers can be used at 380
˚C. Works done by Zeng et al. (2001) proved that sol-gel hydroxydibenzo-14-crown-4
fiber exhibited high thermal stability (up to 340 ˚C).
Table 3.2: Reproducibility of the peak area after conditioning at a) 270 ˚C and
b) 300 ˚C.
a) C8-coated fiber conditioned at 270 ˚C
Hydrocarbon
Compounds
C8
Average Peak
Area (n = 3)
151.7
RSD (%)
C10
248.1
4.5
C12
377.3
4.1
C14
470.3
3.9
C16
366.6
4.4
3.7
b) C8-coated fiber conditioned at 300 ˚C
Hydrocarbon
Compounds
C8
Average Peak
Area (n = 3)
174.9
RSD (%)
C10
280.3
3.8
C12
417.4
2.9
C14
620.3
3.3
C16
514.7
3.6
2.9
53
3.2.5 Scanning Electron Microscopy (SEM) Analysis
3.2.5.1 Surface Characteristics of the Coating
The morphology of the lab-made fiber was investigated by the scanning electron
microscope. Figure 3.3 (a) and (b) represents the surface view of sol-gel C8 coated fiber
at 150 and 1000 fold-magnification respectively. An enlarged scanning electron
micrograph of the fiber surface viewed at 100 fold magnification is shown in Appendix
A. Figure 3.4 represents the cross-sectional view of the chemically bonded polymeric
layer of C8 coating at 4000 fold-magnification. As can be seen from Figure 3.3 (a), it is
obvious the surface of the fiber coating was well-distributed. A homogeneous porous
structure was suggested for sol-gel C8-coating [Figure 3.3 (b)]. However, the SEM
technique does not possess enough resolution to show clearly both micro- and
mesopores, since they fall in the (2-500) Å range. Similar unsatisfactory resolution has
been repoted by Azenha et al. (2005) for sol-gel PTMOS fiber.
Chong et al. (1997) and Yu et al. (2004) have reported that porous structure of
the sol-gel coating should significantly increase the surface area on the fiber.
Consequently, with such a porous coating structure, even an apparently thinner coating
will be able to provide enhanced stationary-phase loadings and therefore, high fiber
sample capacity. Wang et al. (2003) and Zeng et al. (2001) have stated the porous
structure of sol-gel coating helps faster mass transfer during extraction and therefore the
equilibration time is shorter. Figure 3.4 reveals that the fiber was not uniformly coated.
This suggestion is agreeable to those reported by Gbatu et al. (1999).
Examination of the scanning electron micrograph of the fiber prepared from 1:1
molar ratio of C8-TEOS:MTMOS showed no apparent cracks (Figure 3.3 (a) and
Appendix A). This is in good agreement with the previous work done by Gbatu. Works
done by Gbatu et al. (1999), showed that no apparent crack was visible for the fiber
prepared from 1:1 molar ratio of C8-TEOS:MTMOS but the cracks were visible in the
fiber coated with a higher molar ratio (2:1). This could be due to the presence of the
54
bulky C8-TEOS group in a very large amount. Therefore all the analyses in this study
were performed using fibers prepared from a 1:1 molar ratio.
(a)
(b)
Figure 3.3: The surface view of sol-gel C8 coated fiber at (a) 150 and (b) 1000 foldmagnification obtained by SEM.
55
Sol-gel
derived
C8-coating
Fused
silica
base
Figure 3.4: The cross-sectional view of sol-gel C8 coated fiber at 4000 fold
magnification obtained by SEM.
3.2.5.2 Estimation and Reproducibility of the Film Thickness
One of the most attractive features of sol-gel coatings for SPME is the thickness
of the coatings. It is possible to obtain extremely thin films of sorbents with large
extractive power. Such thinner coatings allows faster extractions and easier thermal
desorption when compared to conventionally coated fibers (Zuin et al., 2004; Oliveira et
al., 2005). The film thickness of five different lab-made C8 fibers were measured in
order to estimate the thickness of C8 layer and determine the reproducibility of the
coating thickness.
As can be seen from Appendix B, the thickness of the coating varied along the
length of the fiber. This result was expected and it was reported by Gbatu et al. (1999).
56
It has been reported that the varied thickness obtained might be due to the rate at which
the fused silica tip is removed from sol-gel solution after coating is not controlled and
the portions of activated fused silica fibers that are exposed to the sol-gel solution for
coating are not uniform. Therefore the film thickness of each fiber was measured at two
different points and the average coating thickness were calculated.
Referring to Table 3.3, the average thickness of the C8 coating was estimated as
being (~ 3-4) µm, which is considerably thinner than most conventional SPME fibers
(65 µm for PDMS/DVB). The film thickness obtained in this study is comparable with
previous works done by Azenha et al. (2005) which stated (~ 0.2-1) µm thickness for
sol-gel derived phenyltrimethoxysilene(PTMOS) fiber, Lopes et al. (2004) reported ~ 5
µm thickness for sol-gel derived PDMS/PVA fiber and Basheer et al. (2005) obtained (~
5-7) µm thickness for sol-gel amphilic and hydrophilic oligomers coated fibers. The
reproducibility of the coating thickness obtained in this study is rather promising. Works
done by Azenha et al. (2005) revealed that the film thickness reproducibility was
unsatisfactory for sol-gel PTMOS fiber.
Table 3.3: The approximate film thickness of five different C8-fibers
Fiber
Number
Thickness Measured at Two Different
Points (µm)
Average Coating
Thickness (µm)
1
3.895
4.063
~4
2
3.498
3.921
~4
3
3.287
4.652
~4
4
2.576
2.824
~3
5
2.528
3.247
~3
57
3.2.5.3 Determination of Extracting Phase Volume
The fiber coating volume need to be determined as it is one of the factor that
effects the diffusion rate. Thicker coatings result in longer extraction time because
diffusion is slow within the polymer extraction phase. Larger volume of extracting phase
require longer equilibration times, thus the coating of choice is the thinnest one which
will provide sufficient sensitivity for the determination (Mester et al., 2001).
The extracting phase volume of both commercial PDMS/DVB fiber and labmade C8-coated fiber were calculated as described in equation 2.1 of Section 2.9.4. As
the diameter of the fused silica base is 110 µm, the coating length is of 1.0 cm and the
film thickness is of (~ 3-4) µm, the volume of extracting phase on the lab-made C8coated fiber is (~ 0.011-0.014) mm3. A 65 µm PDMS/DVB commercial fiber leads to an
extracting phase volume of 0.357 mm3. It is apparent that the volume of extracting phase
on the lab-made C8-coated fiber is considerably less than that of commercial
PDMS/DVB fiber. Therefore the C8 coating has a larger affinity towards the analytes
than regular PDMS/DVB.
Similar result has been reported by Lopes et al. (2004) for sol-gel PDMS/PVA
fiber. The PDMS/PVA has a coating thickness of approximately 5 µm, which leads to a
volume of extracting phase of ~ 0.046 mm3. The extracting phase volume of the
PDMS/PVA fiber is almost 1/3 of the coating volume of the 30 µm PDMS commercial
fiber, 0.132 mm3. It has been reported that the PDMS/PVA allows faster extractions and
easier thermal desorption as compared to conventional PDMS fiber.
3.2.6 Fourier Transform Infrared (FTIR) Analysis
The IR spectra of sol-gel derived C8-coating and pure C8-TEOS is shown in
Figure 3.5 (a) and (b) respectively. The Si-O group peak of C8-TEOS at 1390 cm-1 also
58
appeared in the sol-gel derived C8-coating. It most probably prove the successful binding
of C8-TEOS (fiber coating material) to the stationary phase. The decreased intensity of
the peak in sol-gel derived C8-coating indicated that it has been successfully bonded
with other compounds.
Figure 3.5 (a) : IR spectrum of pure C8-TEOS
59
Figure 3.5 (b) : IR spectrum of sol-gel derived C8-coating
3.3 Optimization and Possible Underlying Mechanism of Sol-gel Process
3.3.1 Optimization of Sol-gel Process
In order to optimize the sol-gel process, fibers prepared from different molar
ratios (0.5:1, 1:1; and 2:1) of C8-TEOS to MTMOS were used for extraction. A higher
ratio of C8-TEOS:MTMOS resulted in higher extraction capability as shown in Figure
3.5. This is due to the content of C8-TEOS co–precursor, fiber coating material which
serves as a hydrophobic extracting phase. The fiber prepared from 2:1 molar ratio of C8TEOS:MTMOS, displayed the highest extraction capability due to the presence of C8
groups in a very large amount. Whereas, the fibers prepared from 0.5:1 molar ratio,
60
yielded the lowest extraction capability possibly due to the low content of C8 groups.
Fiber prepared from 2:1 molar ratio appeared to give higher extraction yields compared
to that of the other molar ratios tested.
However, fiber prepared from 2:1 molar ratio was not chosen for analysis
because the scanning electron micrographs of the fiber were not good. Moreover, a
higher amount of coating material (C8-TEOS) will be needed for the preparation of fiber
with 2:1 molar ratio of C8-TEOS:MTMOS compared to fiber prepared with l:1 molar
ratio. This may not be cost-effective.
Average Peak Area
600
500
400
300
200
100
0
C8
C10
C12
C14
C16
Hydrocarbon Compounds
0.5:1 ; C8-TEOS:MTMOS
1:1 ; C8-TEOS:MTMOS
2:1 ; C8-TEOS:MTMOS
Figure 3.6: Extraction capability of hydrocarbon compounds using fibers prepared from
sol–gel solution containing C8-TEOS:MTMOS with the ratios of 0.5:1, 1:1
and 2:1.
3.3.2 Possible Underlying Mechanism of the Coating Process
The in situ creation of the sol-gel C8 coating involved two major sets of reactions
(Chong et al., 1997; Gbatu et al., 1999; Wang et al., 2000; Zeng et al., 2001; Liu et al.,
61
2003; Li et al., 2004b; Lopes et al., 2004; Liu et al., 2005a): (1) hydrolysis of the
precursor and (2) polycondensation of the hydrolyzed products. These reactions are
catalyzed by acids or bases and lead to the formation of a polymeric network. In this
study, MTMOS was used as a precursor and HCl as the catalyst. There are several steps
during the sol-gel processing.
The first step is the hydrolysis reaction of the precursor and co-precursor under
the acid catalyst. The hydrolysis reaction of MTMOS precursor can be presented by the
following equation:
CH3
H3CO
CH3
Si
OCH3
OCH3
HCl
HO
+ H 2O
Si
OH
Catalyst
MTMOS
OH + CH3OH
(3.1)
In the sol gel solution, C8-TEOS is the coating stationary phase. However it also reacts
as a co-precursor which contains hydrophobic alkyl chain (octyl group). The hydrolysis
reaction of C8-TEOS co-precursor can be presented by the following equation:
[CH3(CH2)6CH2]n
CH3CH2O
Si
OCH2CH3
+
H 2O
OCH2CH3
C8-TEOS
HCl
Catalyst
(3.2)
[CH3(CH2)6CH2]n
HO
Si
OH
+
CH3CH2OH
OH
The second step is that the hydrolyzed products undergo polycondensation
reactions to produce a three-dimensional polymer network. The octyl groups of the C8-
62
TEOS co-precursor do not participate in the poly-condensation reactions, they are
exposed and serve as the hydrophobic extracting phase. Polycondensation reactions
involving the hydrolyzed MTMOS and C8-TEOS can be depicted by the following
scheme:
CH3
Si
HO
CH3
+
OH
m HO
Si
OH
CH3
OH
O
Si ( O Si )m O
HO
OH
O
CH3
(3.3)
[CH3(CH2)6CH2]n
Si
HO
[CH3(CH2)6CH2]n
OH
p
+
HO
OH
OH
Si
OH
[CH3(CH2 )6CH2]n
HO
(3.4)
O
Three dimensional
network
Si ( O Si )p O
O
[CH2(CH2)6CH3]n
These reactions are followed by chemical binding of the coating stationary phase
to the growing silica network. The chemical reaction involved can be schematically
represented by the following equation.
CH3
HO
Si ( O Si )m O
O
[CH3(CH2)6CH2]n
O
CH3
Growing silica network
+
HO
Si ( O
O
O
Si )p O
[CH2(CH2)6CH3]n
63
[CH3(CH2)6CH2]n
CH3
O
Si ( O
Si )m
O
CH3
HO
O
Si ( O
O
(3.5)
Si )p O
[CH2(CH2)6CH3]n
O
The silanol groups on the fused-silica fiber surface can also join in the condensation
reactions and provide chemical anchorage to the polymeric network in the immediate
vicinity of the fiber surface (evolving sol-gel coatings). Schematically this reaction can
be represented as follows:
Si OH
Si OH
+
HO
Si OH
CH3
O
Si ( O
Si )m
O
CH3
[CH3(CH2)6CH2]n
O
O
Si ( O Si )p O
O
[CH2(CH2)6CH3]n
Fused silica fiber surface
Si OH
CH3
O
Si HO
O
Si ( O
Si )m
Si OH
O
CH3
[CH3(CH2)6CH2]n
O
O
Si ( O
Si )p O
O
[CH2(CH2)6CH3]n
Sol-gel derived C8-polymeric coating
(3.6)
64
Thus, a surface-bonded polymeric coating is created by dipping the exposed end of the
fused silica fiber into the sol solution and allowing it to be there for a certain amount of
time. The coated fibers were treated with a solution of TMMOS to reduce the silanol
contents of the coating. Possessing a small molecular size, TMMOS molecules should
have greater access to the porous structure of the coating. In parallel with C8-TEOS, it
should be able to provide added deactivation to sol-gel coating. Deactivation of surfacebonded sol-gel C8-coating with TMMOS can be represented by the following scheme:
Si OH
OH
Si HO
O
Si ( O Si )m
Si OH
O
O
O
O
OH
Si ( O Si )p O
O
[CH2(CH2)6CH3]n
+
CH3
H3C
Si
CH3
TMMOS
OCH3
CH3
H 3C
Si
CH3
CH2
Si OH
O
Si HO
O
Si ( O Si )m
Si OH
OH
O
O
[CH3(CH2)6 CH2]n
O
O
Si ( O
Si )p O
O
[CH2(CH2)6CH3]n
Deactivated, sol-gel derived C8 Coating
(3.7)
65
C8-TEOS was added into this system not only to lengthen the silica network
leading to the increased surface area of the fiber and impart hydrophobic character to the
coating but also to help to spread the stationary phase on the fiber uniformly. Unlike the
commonly used sol-gel precursors (tetraalkoxysilanes), MTMOS (a methyl derivative of
the commonly used precursor tetramethoxysilane) is used. Such a choice aims at
overcoming some inherent problems that are faced at the drying step of sol-gel
processing. A sol-gel material, formed as a result of hydrolysis of alkoxysilane precursor
and subsequent condensation of the hydrolyzed products, may undergo cracking and
shrinkage during the drying step. They originate from the capillary thrust due to solvent
evaporation from the gel pores (Chong et al., 1997; Gbatu et al., 1999; Wang et al.,
2000; Hofacker et al., 2002).
For the use in SPME, preservation of the structural integrity is necessary for the
sol-gel coating to be able to provide desired material properties. Works by Mackenzie
(1995) have proved that the sol-gel network originating from an alkyl derivative of a
tetraalkoxysilane precursor, such as MTMOS, possess a more open structure and can
more effectively release stresses during drying, minimizing the cracking tendency.
Generally, sol-gel reactions in a silica-based system are rather slow and often require the
use of catalysts to accelerate the process (Liu et al., 2006), therefore HCl was included
in the preparation of sol sol-gel solution.
It should be pointed out that the purpose of deactivation in SPME fiber
technology is not identical with that in GC. In the preparation of a capillary GC column,
the column surface deactivation is an important step. It is because that there are some
silanol groups on the fused-silica capillary inner walls and the polar compounds are
prone to undergo adsorption interactions with silanol groups (column surface
interactions that are responsible for peak tailings and efficiency losses). Therefore in
conventional column technology, deactivation is usually carried out as a separate step to
derive the surface silanol groups. In the preparation of SPME fiber with sol-gel
technology, solute interactions can be even advantageous. The silanol groups can be
used as the chemical anchorage for the evolving sol-gel coatings. Such interactions will
66
provide enhanced selectivity for polar compounds in SPME. TMMOS can chemically
bind with the silanol groups of the coating surface (Chong et al., 1997; Wang et al.,
2000).
Sol-gel coating technology is characterized by a higher degree flexibility in
coating composition and selectivity. It should be noted that the surface coating obtained
from sol-gel chemistry is not purely organic, it is composite in nature. The composition
of this organic-inorganic coating can be controlled by varying the proportions of the sol
solution ingredients. This gives the possibility of selectivity tuning in SPME fiber
technology. In principle, by varying the composition of the sol solution, one should be
able to prepare widely diverse surface coatings, ranging from purely inorganic to purely
organic. Inorganic coatings pose great promise for SPME of volatiles (Chong et al.,
1997; Yu et al., 2002; Liu et al, 2005b).
3.4
Conclusions
A lab-made SPME fiber (sol-gel derived C8-coating) has been successfully
developed, characterized and evaluated for the HS-SPME of standard hydrocarbon
compounds. The lab-made fiber was capable of effectively extracting volatile
hydrocarbon compounds. The lab-made fiber gave superior characteristics when
compared to commercially available PDMS/DVB fiber. Compared with commercial
PDMS/DVB fiber, the new lab-made fiber exhibited higher extraction capability for
standard hydrocarbons, higher thermal stability (up to 300 °C) and longer lifetime (~ 200
times). Electron microscopy experiments revealed that the surface of the fiber coating
was well-distributed but not uniformly coated and a porous structure was suggested for
the sol-gel derived C8 coating with an approximate thickness of (3-4) µm. The volume
of extracting phase on the lab-made C8-coated fiber is between 0.011-0.014 mm3. The
reproducibility of the film thickness was found to be rather promising. The results
obtained from the optimization of sol-gel process indicated that fiber prepared from a
67
higher ratio of C8-TEOS to MTMOS resulted in higher extraction capability for standard
hydrocarbons. The possible underlying mechanisms of the sol-gel coating process were
discussed and the successful binding of C8-TEOS to the coating stationary phase was
confirmed by IR spectra.
68
CHAPTER 4
HEADSPACE SPME OPTIMIZATION, METHOD VALIDATION AND
PRACTICAL APPLICATION OF THE C8-COATED FIBER
4.1
Introduction
This chapter covers the optimization of headspace SPME experimental
parameters (extraction time, desorption time and extraction temperature). The precision
of the proposed method, linearity of the calibration graph for n-alkane hydrocarbon
compounds and the detection limits of accelerants obtained by using C8-coated fiber are
discussed. It also includes the discussion for the extraction of common accelerants in
simulated arson samples.
4.2
Optimization of HS-SPME Operating Conditions
4.2.1 Optimization of Extraction Time
SPME is based on an equilibration distribution process, the maximum amount of
analyte will be extracted at the equilibration time. Therefore the extraction equilibration
time (optimum fiber exposure time) is being considered as a decisive factor in improving
extraction efficiency (Cai et al., 2003). The longer extraction time leads to the higher
extraction efficiency and sensitivity at pre-equilibrium (Hou et al., 2004). In general,
69
equilibrium is compound dependent and can vary significantly among the different
compounds (Yu et al., 2004). It also depends on the mass transfer of the analytes
through the three-phase system: polymeric coating, headspace and sample matrix (Liu et
al., 2005). The extraction equilibration time was initially investigated by exposing the
fiber to the headspace of an apparatus containing target analytes for a variety of time,
from 5 to 35 minutes, until the amounts extracted remains constant.
The extraction time profiles were constructed by plotting the mass adsorbed,
measured as chromatographic peak area, against extraction time as shown in Figure 4.1.
The optimum fiber exposure time estimated from the profile is 25 minutes. Within the
duration of 25 minutes, the analytes would have migrated in and out of the coating until
the equilibration was established between the phases. Referring to Figure 4.1, the
extraction efficiency reaches a plateau after 25 minutes exposure of the fiber to the
headspace. Therefore it can be suggested that after 25 minutes, an equilibration has been
established.
According to a previous work done in this lab, Yong (2004) suggested 35
minutes equilibration for components commonly found in accelerants using commercial
PDMS fiber (cooling temperature ramp approach). A slightly shorter equilibration time
(25 minutes) was obtained in this study by using the lab-made C8-coated fiber as
compared to those reported by Yong (2004) for commercial PDMS fiber. This might be
due to the porous structure and thinner coating of the lab-made C8-coated fiber. Lopes et
al. (2004) reported that coatings provided by sol-gel process are thinner and therefore
saturation with the extracted analytes takes less time than with conventional fibers, that
are coated with relatively thicker films of sorbents.
The extraction equilibration time obtained in this study is comparable with
previous works done by Gbatu et al. (1999) which stated a 20 minutes equilibration for
organo-tin compounds, 25 minutes for organo-mercury compounds and 30 minutes for
organo-arsenic compounds using sol-gel derived C8-coated fiber. It has been reported
70
that the equilibration time obtained using C8-fiber is much shorter than those obtained
using commercial PDMS/DVB fiber (even after an hour the equilibration has not
completely reached equilibration). The difference in equilibration times of the
compounds in the two types of fibers can be attributed to difference in porosity.
Analytes will diffuse faster through a thinner, porous coating than a thicker, non-porous
coating. Chong et al. (1997) obtained 10 minutes equilibration for nonpolar analyte
using sol-gel PDMS fiber. A short equilibration time of 15 minutes has been achieved
for organic compounds using sol-gel PDMS/PVA fiber (Lopes et al., 2004).
Average Peak Area
600
500
400
300
200
100
0
0
5
10
15
20
25
30
35
40
Time (min)
C8
C10
C12
C14
C16
Figure 4.1: Extraction equilibration profiles of n-alkane hydrocarbon compounds (C8,
C10, C12, C14 and C16) at varying extraction time. Other SPME conditions:
extraction temperature of 80 °C and desorption time of 80 s.
4.2.2 Optimization of Desorption Time
The time needed for complete desorption of analytes from the C8-coated fiber
has to be carefully determined to achieve quantitative desorption and avoid carry-over
71
effects that occur among subsequent SPME analyses. The amounts of analyte desorbed
from the fiber will influence detection sensitivity. Nine desorption times ranging from
10 to 90 s in 10s increments were tested. The analytes were thermally desorbed at the
GC injection port set at a temperature of 250 °C, at which the volatile hydrocarbon
compounds can be desorbed immediately.
As can be seen from Figure 4.2 (relationship between response and desorption
time), the chromatographic signal increased gradually with the desorption time and
stabilized at about 50 s, which indicated that a state of equilibrium had been reached.
This result revealed that the desorption process is fast and all the hydrocarbon
compounds could be desorbed in 50 s. The optimized desorption time (50 s) can be
considered as the minimum time for quantitative transfer of the analytes from the C8fiber to the GC column. However to ensure complete desorption and to avoid possible
fiber carryover, the desorption time was set at 1 minute.
The desorption equilibration time exhibited by the lab-made C8-fiber is slightly
shorter than those reported by Yong (2004) using commercial PDMS fiber (cooling
temperature ramp approach). A short desorption time obtained in this study may be
attributed to much larger surface area of C8-coating which is favorable to rapid mass
transfer from the porous and thinner coating. Wang et al. (2000), Li et al. (2004a), Silva
et al. (2005), Liu et al. (2005a) and Cai et al. (2006) reported that sol-gel coatings
possess a porous structure that provides faster mass transfer during extraction as well as
analyte desorption processes during sample introduction, which shortens the desorption
time of analytes.
A short desorption time determined in this study is comparable with previous
works done by Gbatu et al. (1999), a static desorption time of 5 minutes was sufficient
for complete desorption of organotin and organomercury compounds from sol-gel C8coated fiber. The result is also comparable to those obtained by other researches using
different sol-gel fibers. Wang et al. (2000) obtained a 20 s desorption equilibration for
72
BTEX compounds using sol-gel derived polyethylene glycol coated fiber. An optimum
desorption time of 3 minutes
was suggested for aliphatic amines using sol-gel
calix[4]arene fiber (Li et al., 2004b). Dong et al. (2005) concluded that a 2 minutes
desorption time are sufficient to allow the complete desorption of organochlorine
pesticides from sol-gel calix[4]arene/hydroxyl-terminated silicone oil coated fiber.
Average Peak Area
600
500
400
300
200
100
0
0
10
20
30
40
50
60
70
80
90
100
Time (Sec)
C8
C10
C12
C14
C16
Figure 4.2: Desorption profiles of C8-fiber for standard n-alkane hydrocarbon
compounds (C8, C10, C12, C14 and C16) by HS-SPME. Extraction conditions:
extraction time, 15 min; extraction temperature, 80 °C.
4.2.3 Optimization of Extraction Temperature
The effect of extraction temperature on the amounts of analytes absorbed was
studied by performing HS-SPME extractions of hydrocarbons at temperatures ranging
from 40 to 100 °C. The average peak area versus extraction temperature profiles for HSSPME extractions of standard hydrocarbons were determined as shown in Figure 4.3.
73
Referring to Figure 4.3, it is apparent that peak areas increase as the extraction
temperature is increased. The extraction efficiency of the fiber increased with
temperature until 90 °C. Increasing the extraction temperature to 90 °C might have
helped transfer of analytes to the headspace and enhanced diffusion of analytes towards
the fiber. However, extraction yields tended to decrease slightly after 90 °C. This might
be because the distribution constant decreased with increasing temperature. In order to
control the stable temperature efficiently, 90 °C was selected as the extraction
temperature for analysis of volatile hydrocarbon compounds.
The extraction efficiency of sol-gel fiber is mainly controlled by two factors,
distribution velocity and partition coefficient. The distribution velocity, which helps
analytes to move near the solid-phase coating, increases with increased temperature.
Extraction is an exothermic process and the partition coefficient, which determines the
ratio of analytes extracted, is inversely related to temperature (Yu et al., 2002; Li et al.,
2004; Lopes et al., 2004; Basheer et al., 2005). This means that at a higher temperature,
diffusion coefficients are higher and therefore a faster distribution equilibrium is
achieved, but partition coefficients are lower and unavoidably result in a poorer
extraction efficiency. Zeng et al. (2001) and Zhou et al. (2005) reported that the first
factor (distribution velocity) is dominant in the temperature range less than the optimum
extraction temperature, while the second factor (partition coefficient) becomes dominant
at temperatures higher than the optimum extraction temperature.
74
Average Peak Area
600
500
400
300
200
100
0
0
5
10
15
20
25
30
35
40
Temperature (Degree Celcius)
C8
C10
C12
C14
C16
Figure 4.3: Influence of extraction temperature on the signal intensity of standard nalkane hydrocarbons (C8, C10, C12, C14 and C16). Extraction conditions:
extraction time, 15 min; desorption time, 80 s.
4.3
Validation of the Analytical Method
The investigation of the proposed method’s precision, linearity of calibration
graphs for n-alkane hydrocarbons and detection limits of accelerants was carried out by
applying the optimum HS-SPME experimental conditions.
4.3.1 Accuracy of HS-SPME Method using C8-coated Fiber
The accuracy of the developed method for three replicate analyses was
determined by calculating the relative standard deviations (RSD) of the peak areas for all
standard hydrocarbons (C8, C10, C12, C14 and C16) in burnt carpet sample spiked at the
same concentration level. In order to determine the reproducibility of the analytes’s peak
75
areas, the three consecutive analyses were performed on the same day. Reproducibility
of an analyte’s peak area is important in order to produce a comparable chromatographic
profile for visual pattern comparison.
Referring to Table 4.1, all the hydrocarbon compounds gave a low relative
standard deviation (RSD) value ranging from 3.9 %-5.5 % which shows an acceptable
reproducibility. Therefore the optimized HS-SPME using lab-made C8-coated fiber is
expected to produce good qualitative chromatographic profiles since comparable peak
area range was observed. These results indicated that quantitative analysis of
hydrocarbons was possible using the external standard method and the applicability of
lab-made C8-coated fiber for routine analysis. The result obtained is in good agreement
with those reported by Gbatu et al. (1999). It has been reported that the C8-coated fiber
exhibited good reproducibility for the analysis of organometals.
Table 4.1: Inter-day precision of HS-SPME method for the extraction of standard nalkane hydrocarbon compounds using C8-coated fiber under optimum
conditions.
Hydrocarbon
Average Peak Area
RSD (%)
Compounds
(n=3)
C8
211.3
4.8
C10
352.6
4.3
C12
487.5
5.1
C14
579.7
5.5
C16
391.4
3.9
76
4.3.2 Detection Limits of Accelerants
Detection limits of accelerants (lowest detectable amount determined based on
three times the baseline noise) were evaluated in order to access the performance of the
proposed method. The lab-made C8-coated fiber was employed for extractions of
gasoline, kerosene and diesel in spiked burnt carpet sample under optimum HS-SPME
conditions.
Referring to Table 4.2, for all the common accelerants, the LODs were between
0.7 to 1.0 µL. A minimum detectable amount of 0.7 µL were obtained for the C8 fiber
extraction of gasoline in burnt carpet sample. A slightly higher LODs (1.0 µL) were
detected for kerosene and diesel. These values are indicative of good performance of the
lab-made C8-coated fiber.
The detection limits of common accelerants obtained is this study using C8-fiber
is comparable to those reported by Yong (2004) using commercial PDMS/DVB fiber. In
the previous work done in this lab (Yong, 2004), it has been reported that LOD as low as
0.5 µL was obtained for gasoline. Detection limits of 1.0 µL were determined for the
PDMS/DVB fiber extraction of kerosene and diesel in burnt carpet sample.
Table 4.2: LODs for accelerants determined using C8-coated fiber under optimum HSSPME conditions
Common accelerant
Minimum detectable amount (µL)
This work
Previous work *
Gasoline
0.7
0.5
Kerosene
1.0
1.0
Diesel
1.0
1.0
* Yong 2004
77
4.3.3 Calibration Graph of Target Compounds
Calibration curves consisting of five different concentration of n-alkane
hydrocarbons were generated as regression lines and the correlation coefficients were
obtained. The analytes were extracted using lab-made C8-coated fiber under the
Average Peak Area
optimum HS-SPME conditions.
180
160
140
120
100
80
60
40
20
0
0
2
4
6
8
10
Analyte Concentration (% v/v)
C8
C10
C12
C14
C16
Figure 4.4: Calibration graph of HS-SPME for n-alkane standard hydrocarbon
compounds using C8-coated fiber
Table 4.3: Regression line and correlation coefficient of standard hydrocarbon
compounds.
Hydrocarbon
Equation
Correlation
Factor (R2)
Compounds
C8
y = 3.85x + 2.1
0.9869
C10
y = 8.31x + 2.0
0.9889
C12
y = 12.7x + 2.4
0.9958
C14
y = 16.7x + 2.8
0.9948
C16
y = 11.4x + 2.0
0.9937
12
78
As can be seen from Figure 4.4, the HS-SPME procedure with lab-made C8coated fiber yielded an excellent linear relationship between the peak area counts and
analyte concentration in the range of (2.00-10.00) % v/v. Good correlations with R2
value between 0.9869-0.9958 (R2 ~ 1) were obtained as shown in Table 4.3. It is
apparent that there were almost no factors in the HS-SPME extractions of n-alkane
hydrocarbons using lab-made C8-coated fiber that might cause the concentration
characteristics to deviate from linearity.
This result is in good agreement with most of the research works done with
different sol-gel fibers as it shows a positive correlation between the peak area counts
and concentration of target compounds. Cai et al. (2003) reported that linear calibration
curves were obtained for all the tested amines using sol-gel derived dibenzo-18-crown-6
fiber. Liu et al. (2005) reported that the HS-SPME procedure with sol-gel derived
BMA/OH-TSO fiber showed excellent linearity in concentrations ranging from 0.1-10
µg/g. Yu et al. (2004) reported that the calibration curves were found to have good
linearity by correlation coefficients (R2) of more than 0.99 for organophosphorus
pesticides using sol-gel derived bisbenzo crown ether/(OH-TSO) fiber.
4.4
Analysis of Simulated Arson Samples Using C8-coated Fiber
The feasibility of the lab-made C8-coated fiber for the extractions of common
accelerants was evaluated using simulated arson samples (Section 2.8).
4.4.1 Selectivity for Accelerants
In order to examine selectivity of the lab-made SPME fiber towards petroleum
based accelerants, burnt carpet sample were individually spiked with known amount of
gasoline, kerosene and diesel. The samples were subjected to headspace SPME using
79
sol-gel derived C8-coated fiber and the GC profiles were compared with that from direct
injection. As can be seen in Figure 4.5 (b), all the hydrocarbon components in gasoline
spiked burnt carpet sample were recovered by using C8-coated fiber and the
chromatogram was comparable with the profile of gasoline from direct injection [Figure
4.5 (a)].
The hydrocarbon components in kerosene (Figure 4.6) and diesel (Figure 4.7)
spiked burnt carpet samples were also effectively extracted using the lab-made C8 fiber
and similar comparisons were obtained with direct injection of the respective
accelerants. This indicated that the lab-made SPME fiber favored the extraction of
hydrocarbons, thus providing a good selectivity towards hydrocarbons in petroleum
based accelerants. The GC profiles of gasoline, kerosene and diesel spiked samples
obtained in this study were in good agreement with those obtained by Borusiewicz et al.
(2004) using Tenax as adsorbent and Yong (2004) by using commercially available
fibers.
In the previous work done in this lab (Yong, 2004) a complete study on
identifying the hydrocarbon components in gasoline, kerosene and diesel had been
carried out. The same samples were used in this study . Therefore, the chromatograms
obtained in this study are comparable to those obtained by Yong (2004).
80
Figure 4.5 (a): GC profiles of direct injection of gasoline. Peak identities: (1)
methylbenzene, (2) ethylbenzene, (3) 1, 3-dimethylbenzene, (4) 1, 2dimethylbenzene, (5) 1-ethyl-2-methylbenzene, (6) 1, 2, 4-trimethylbenzene,
(7) 1, 2, 3- trimethylbenzene.
Figure 4.5 (b): GC profiles of gasoline spiked burnt carpet sample using C8-coated
fibers. Peak identities: (1) methylbenzene, (2) ethylbenzene, (3) 1, 3-
81
dimethylbenzene, (4) 1, 2- dimethylbenzene, (5) 1-ethyl-2-methylbenzene,
(6) 1, 2, 4-trimethylbenzene, (7) 1, 2, 3- trimethylbenzene.
Figure 4.5 (c): GC profiles of the blank burnt carpet sample using C8-coated fibers.
Figure 4.6 (a): GC profiles of direct injection of kerosene.Peak identities: (1) C9, (2)
C10, (3) C11, (4) C12, (5) C13, (6) C14, (7) 2, 6-dimethylnapthalene, (8) C15
82
(b)
Figure 4.6 (b): GC profiles of kerosene spiked burnt carpet sample using C8-coated
fibers. Peak identities: (1) C9, (2) C10, (3) C11, (4) C12, (5) C13, (6) C14, (7)
2, 6-dimethylnapthalene, (8) C15
Figure 4.7: GC profiles of diesel spiked burnt carpet sample using C8-coated fibers.
Peak identities: (1) C9, (2) C10, (3) C11, (4) C12, (5) C13, (6) C14, (7) C15, (8) C16
hydrocarbons.
83
4.4.2 Extraction Capability for Accelerants
The extraction capability of the sol-gel derived C8-coated fiber was determined
by comparing it with the commercially available PDMS/DVB fiber. PDMS/DVB fiber
was selected for comparison because previous work done in this lab, Yong (2004)
proved that the fiber has the highest sensitivity towards hydrocarbon compounds and
most suitable for accelerants identification. Both C8-coated fiber and PDMS/DVB fiber
were capable of extracting early, middle and late eluting hydrocarbon compounds
sufficiently. However, the C8-coated fiber showed a slightly higher extraction capability
by contrast with conventional PDMS/DVB fiber for all the accelerants as shown in
Figure 4.8. This result is comparable to those reported by Gbatu et al. (1999). A higher
extraction capability could be due to the existence of higher surface area for the C8coated fibers (Gbatu et al., 1999; Cai et al., 2003; Zuin et al., 2004; Liu et al., 2005a).
Average Peak area
Spiked with Gasoline
30
25
20
15
10
5
0
1
2
3
C8-coated fiber
4
5
6
7
PDMS/DVB
Figure 4.8 (a): Comparison of sol-gel derived C8-coated fiber and commercially
available PDMS/DVB fiber in extracting hydrocarbon compounds from gasoline spiked
burnt carpet sample. Label identities: (1) methylbenzene, (2) ethylbenzene, (3) 1, 3dimethylbenzene,(4) 1, 2-dimethylbenzene, (5) 1-ethyl-2-methylbenzene, (6) 1, 2, 4trimethylbenzene, (7) 1, 2, 3-trimethylbenzene.
84
A verag e P eak area
Spiked with Kerosene
20
15
10
5
0
1
2
3
4
5
C8-coated fiber
6
7
8
PDMS/DVB
Figure 4.8 (b): Comparison of sol-gel derived C8-coated fiber and commercially
available PDMS/DVB fiber in extracting hydrocarbon compounds from kerosene spiked
burnt carpet sample. Label identities: (1) C9, (2) C10, (3) C11, (4) C12, (5) C13, (6) C14, (7)
2, 6-dimethylnapthalene, (8) C15
A verage P eak A rea
Spiked with diesel
8
6
4
2
0
1
2
3
4
C8-Coated fiber
5
6
7
8
PDMS/DVB
Figure 4.8 (c): Comparison of sol-gel derived C8-coated fiber and commercially
available PDMS/DVB fiber in extracting hydrocarbon compounds from
85
diesel spiked burnt carpet sample. Label identities: (1) C9, (2) C10, (3)
C11, (4) C12, (5) C13, (6) C14, C15, (7) C16
4.5
Conclusions
The headspace SPME method using lab-made C8-coated fiber has been
successfully optimized. A short extraction (25 minutes) and desorption (50 s)
equilibration times arised from the porous structure of the sol-gel C8-coated fiber and
resulted in short analysis time. The optimum extraction temperature for standard
hydrocarbons was found to be 90 °C. The developed HS-SPME method using C8-coated
fiber exhibited a high degree of inter-day accuracy under the optimum experimental
conditions. The limits of detection for the common accelerants studied under the
optimum HS-SPME conditions were in the range of 0.7 to 1.0 µL. The linearity of the
calibration graphs for all the tested n-alkane hydrocarbons was excellent (r > 0.9869).
The C8 fiber was capable of effectively extracting components commonly found in
accelerants and exhibited a good selectivity for accelerants. Comparison of the lab-made
sol-gel derived C8-coated fiber with the commercially available PDMS/DVB fiber
showed that the lab-made fiber has a higher extraction capability for fire accelerants
under study.
86
CHAPTER 5
CONCLUSIONS AND FUTURE DIRECTIONS
5.1
Conclusions
In this study a lab-made sol-gel derived C8-coated fiber has been developed for
the headspace SPME-GC analysis of n-alkane standard hydrocarbons and common
accelerants. The developed fiber has been characterized in terms of selectivity and
extraction capability for standard hydrocarbons, lifetime, thermal stability and surface
morphology of the coating. The reproducibility of the C8 coating has been studied in
terms of coating thickness. Optimization of the sol-gel process was effected by varying
the molar ratios of the silane precursors. The successful binding of the coating material
(C8-TEOS) to the coating stationary phase during the formation of sol-gel C8-coating
was studied through the IR analysis. The chemistry behind the sol-gel coating technique
involved in this study were discussed. The HS-SPME method using lab-made C8-coated
fiber was preliminary optimized using n-alkane standard hydrocarbon mixture. The
experimental parameters were investigated in terms of fiber exposure time, analyte
desorption time and extraction temperature. The accuracy of the proposed method,
linearity of calibration curves and detection limits of accelerants were investigated.
Finally, the practical applicability of the lab-made fiber was studied via the HS-SPME of
common accelerants in simulated arson samples.
87
All the tested n-alkane standard hydrocarbons and hydrocarbon components in
gasoline, kerosene and diesel spiked burnt carpet samples were effectively extracted
using the lab-made fiber. This indicated that the lab made fiber has a good selectivity
towards n-alkane hydrocarbon compounds in petroleum based accelerants. Comparison
of the lab-made sol-gel derived C8-coated fiber with the commercially available
PDMS/DVB fiber showed that the lab-made fiber has a higher extraction capability for
standard hydrocarbons and accelerants.
It has to be high-lighted that the lifetime of the sol-gel C8-coated fiber was found
to be remarkably longer than that of commercially available fibers. The extraction ability
of the fiber showed no significant decline even after it has been used for 200 times [the
tested hydrocarbon compounds gave a low RSD value (< 5.5 %)]. This proves that the
C8 coated fiber surface was not depleted after it has been used for 200 extractions. In
contrast, the commercially available fibers can only be used for 40 to 100 extractions.
The sol-gel C8-coated fiber exhibited higher thermal stability compared to
conventionally coated PDMS/DVB fiber due to the strong chemical bonding between
sol-gel generated C8 composite and the fused silica surface. The fiber can be routinely
used at 300 °C without any sign of bleeding (RSD value of < 3.7 % was routinely
obtained for hydrocarbons on the C8-coated conditioned at 300
°
C), whereas
conventionally coated PDMS/DVB fiber begins to bleed above 270 °C [limitations listed
by the manufacturer (Supelco)].
Scanning electron micrographs revealed that the developed C8-coating possesses
a porous structure. It increases the surface area on the fiber, therefore, even a thin
coating will be able to provide enhanced stationary phase loading and high sample
capacity. The surface of the fiber coating was well-distributed but not uniformly coated.
No apparent cracks were observed on the surface of the fiber coated with 1:1, C8TEOS:MTMOS which indicated a good coating material for SPME. The film thickness
of the C8 coating was estimated to be between (3-4) µm, which leads to an extracting
88
phase volume of (0.011-0.014) mm3. The simplicity of the sol-gel coating procedure
provided a rather promising coating thickness reproducibility.
IR spectrum showed that the Si-O group peak identified in pure C8-TEOS also
appeared in the sol-gel derived C8-coating but in decreased intensity. This most probably
prove the successful binding of the coating material to the stationary phase. The results
obtained from the optimization of sol-gel process indicated that fiber prepared from a
higher ratio of C8-TEOS to MTMOS resulted in higher extraction capability for n-alkane
standard hydrocarbons.
The optimization of the HS-SPME method using C8 fiber revealed that the
porous structure and thinner coating of the sol-gel derived C8-coated fiber curtailed the
equilibration times, resulting in short analysis time. The highest amount of analytes were
extracted with a 25 minutes extraction time. A minimum desorption time of 50 s was
sufficient to allow complete desorption of hydrocarbon compounds from the C8 fiber.
The optimum extraction temperature for analysis of volatile hydrocarbon compounds
was found to be 90 °C.
The proposed HS-SPME method using C8-coated fiber showed satisfactory
precision, linearity and detection limits under the optimum experimental conditions. The
three consecutive analyses of standard hydrocarbons (C8, C10, C12, C14 and C16) spiked in
burnt carpet sample showed a good reproducibility with RSD below 6 %. The detection
limits of common accelerants (gasoline, kerosene and diesel) were determined to be in
the range of (0.7 -1.0) µL. HS-SPME procedure with C8-coated fiber yielded excellent
linearity in concentrations ranging from (2.00-10.00) % v/v with good correlations (r >
0.9869).
The sol-gel C8 composite proposed in this study as a material for SPME fiber
coating can be generated in situ by a simple, straightforward sol-gel polycondensation
route. The presented experimental results conclusively demonstrated the potential of solgel derived C8-coating in SPME and chromatographic separations. The resulting
89
procedure was shown to be a good alternative methodology for qualitative and
quantitative analysis of accelerants in arson samples, being a simple, fast, reproducible
and also an environmental friendly analytical method.
5.2
Future Directions
Further studies need to be carried out for a complete assessment of the lab-made
adsorbent and to improve the results obtained in this research. For future work on
characterizing the lab-made fiber, thermo gravimetric analysis of the sol-gel C8
composite should be performed in order to determine the mass loss and degradation of
the coating at extremely high temperatures (up to 700 °C). This study is essential as
SPME fibers with extended working temperature ranges reduces the amount of bleeding
from the fiber coating during thermal desorption. Nitrogen adsorption analysis need to
be carried out using a bulk quantity of the C8-coating in order to determine the surface
area. The values of surface area obtained can be used to analyze the porous appearance
of the coating (micro- or mesopores).
A study on solvent stability of the lab-made fiber can be carried out by exposing
the fiber to different organic and inorganic solvents (methylene chloride, acetonitrile,
acetone, hexane, xylene and distilled water) for various lengths of time prior to
extraction. The pH stability of the fiber can also be estimated by comparing the
extraction efficiencies of the sol-gel derived C8 coating in extracting hydrocarbons
before and after rinsing with acidic and alkaline solutions of various pH values.
The coating preparation reproducibility can be investigated in many aspects. In
this preliminary work, the coating preparation reproducibility of the C8 fiber has been
studied in terms of coating thickness. A different aspect, such as fiber-to-fiber and batchto-batch reproducibility should be investigated in future work. At least three C8 fibers
prepared in the same batch and three identically prepared C8 fibers in three different
90
batches need to be subjected to HS-SPME of n-alkane hydrocarbons. The coating
preparation reproducibility of the C8 fibers within batch and between batches should be
determined by calculating the RSD of the peak areas for all the hydrocarbons.
Although the indirect headspace-SPME extraction technique (HS-SPME) using
the lab-made C8-coated fiber has been successfully employed for the extraction of nalkane hydrocarbons, a different SPME extraction mode (direct fiber immersion
extraction) is suggested to be used in further study in order to investigate the
applicability of the lab-made fiber for both the extraction modes. For future work, an
alternative extraction approach (cooling temperature ramp) utilized by Yong (2004) can
be employed for HS-SPME of accelerants. The effects of different extraction methods
towards the recovery of accelerants can be studied by comparing the extraction at
constant temperature (done in this study) and cooling temperature ramp.
Since most volatile residues in fire debris involve complex mixtures, coupling
HS-SPME with GC/MS in future work gives the best tool for arson analysis. GC/MS has
the capability of displaying a mass spectrum for each peak and scan selected ions, those
that are characteristic for a particular chemical species of interest.
The development of the sol-gel derived C8-coated fiber is one of the latest
improvement in the field of forensic science. The objectives of this work have been
achieved with great success and promising results. The lab-made fiber has a high
potential of improving the quality of accelerant extraction, therefore it will be beneficial
for arson crime scene investigations. It is hoped that the positive attributes of the coating
will widen the application range of the fiber. The proposed ideas and suggestions would
hopefully pave the way for future directions in the improvement of the lab-made fiber’s
characteristics, accuracy of the coating preparation method and applicability of the fiber
for different SPME extraction approaches.
91
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103
Appendix A
An enlarged scanning electron micrograph of the fiber surface view at 100 fold
magnification
104
Appendix B
Scanning electron micrograph: cross-sectional view of the fiber at 1820 fold
magnification (Measurement of the film thickness at two different points).
Sol-gel
derived
C8-coating
Fused
silica
base
105
Appendix C
Presentation and Publications
Parts of this work have been presented at the following symposia and exhibition,
1. Umi K. Ahmad, Abdul Rahim Yacob and Geetha Selvaraju. “A New SPME
Adsorbent for the Forensic Analysis of Accelerant Residue”. Paper presented at
the Annual Fundamental Science Seminar (AFSS) 2005. Ibnu Sina Institute,
UTM, Johor Bahru (4th-5th July 2005).
2. Umi K. Ahmad, Abdul Rahim Yacob and Geetha Selvaraju. “A Home-made
SPME Fiber Coating for Arson Analysis”. Abstracts of the Simposium Kimia
Analisis Malaysia (SKAM 18), Hyatt Hotel Regency, Johor Bahru (12th-14th
September 2005).
3. Umi K. Ahmad, Geetha Selvaraju and Yong Teik Yau. “FICONSICA: A
Durable Forensic Fiber for Fire Debris Analysis”, Abstracts of the 17th
International, Invention, Innovation, Industrial Design and Technology
Exhibition (ITEX), Kuala Lumpur Convention Centre (KLCC), (19th-21st May
2006).
4. Umi K. Ahmad and Geetha Selvaraju. “Optimization of Headspace-SPME using
Lab-made C8-coated fiber for the Forensic Analysis of Arson Accelerants in Fire
Debris”. Paper presented at the 1st Regional Postgraduate Conference on
Engineering and Science (RPCES), Universiti Teknologi Malaysia, Skudai,
Johor (26th-27th July 2006).