UNIVERSITI PUTRA MALAYSIA METHOD DEVELOPMENT,...
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UNIVERSITI PUTRA MALAYSIA
METHOD DEVELOPMENT, SPECIATED MEASUREMENTS AND CALCULATED REACTIVITIES OF BENZENE, TOLUENE, ETHYLBENZENE AND XYLENES FROM VEIDCLE EXHAUST
NORASALWA ZAKARIA
FSAS 2001 10
METHOD DEVELOPMENT, SPECIATED MEASUREMENTS AND CALCULATED REACTIVITIES OF BENZENE, TOLUENE, ETHYLBENZENE AND XYLENES FROM VEIDCLE EXHAUST
By
NORASALWAZAKARIA <'
Thesis Submitted in FuIfUment of the Requirement for the Degree of.Master of Science in the Faculty of Science and Environmental Studies
Univeniti Putra Malaysia
December 2001
For my lovely little girl;
Qistina Saf.fWah Mohd. Qayyum
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Abstract of thesis presented to the Senate ofUniversiti Putra Malaysia in fulfilment of the requirement for the degree of Master of Science
METHOD DEVELOPMENT, SPECIATED MEASUREMENTS AND CALCULATED REACTIVITIES OF BENZENE, TOLUENE, ETHYLBENZENE AND XYLENES FROM VEmCLE EXHAUST
By
NORASALWA ZAKARIA
December 2001
Chairperson: Dr. Puziah Abd. Latif
Faculty: Science and Environmental Studies
Mathematical modeling of ambient air photochemistry requires comprehensive
speciation of hydrocarbons from mobile source emission. The objective of this study is
to develop a simple and reliable method for analyzing tailpipe emission focusing on
benzene, toluene, ethylbenzene and xylenes (BTEX). The method consists of sampling,
qualitative analysis and quantitative analysis. The samples were collected in Tedlar bags
at cold start and hot start conditions and were injected manually using a gas-tight syringe
into Gas Chromatograph-Mass Spectrometer (GC-MS) operated on Electron Impact
Ionization (EI) mode.
The method developed has demonstrated ability to produce rapid and reliable
separations of exhaust hydrocarbons. An approximate of 50 hydrocarbon compounds
were identified in the exhaust ranging from C4 to e}2. It was found difficult to analyze
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C 1 to some of the C4 hydrocarbons without the required accessories for volatile organic
gases analysis.
The emISSIOn rate of BTEX was emphasized because of its potential
carcinogenicity and toxicity. High concentration of BTEX was observed during cold
start and hot start. The mean concentrations of BTEX at cold start were as follows:
benzene (55.4 ppm), toluene (184.7 ppm), ethylbenzene (50.2 ppm), m-xylene (143.9
ppm), p-xylene (59.0 ppm) and o-xylene (65.4 ppm). The mean concentrations ofBTEX
at hot start were as follows: benzene (82.4 ppm), toluene (198.3 ppm), ethylbenzene
(40.0 ppm), m-xylene (184.0 ppm), p-xylene (62.1 ppm) and o-xylene (50.3 ppm).
The concentrations of BTEX in weighted percentages for all cars were fairly
constant. The weighted percentages concentration was used to estimate the
photochemical ozone reactivity by applying the Maximum Incremental Reactivity (MIR)
factors to the concentrations. The ozone forming potential as a result ofBTEX emission
from vehicle exhaust were estimated as follows: 3.07 g O-J g benzene, 74.64 g O-J g
toluene, 13.0 g 03/ g ethylbenzene, 145.8 g O-J g m- and p-xylene, and 50.97 g O-J g 0-
xylene. The results suggested that BTEX emitted from the vehicle exhaust increases the
formation of photochemical ozone in the atmosphere significantly.
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Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia sebagai memenuhi keperluan untuk ijazah Master Sains
PEMBENTUKAN KAEDAH, PENll..AIAN SPESIS DAN PENGUKURAN KEREAKTIFAN BENZENA, TOLUENA, ETILBENZENA DAN XILENA DARI
EKZOS KENDERAAN
Oleh
NORASALWAZAKARIA
Disember 2001
Pengerusi: Dr. Puziab Abd. Latif
Fakulti: Sains Dan Pengajian Alam Sekitar
Pembentukan model matematik untuk fotokimia udara rnemerlukan penilaian
yang komprehensif terhadap spesis hidrokarbon dari sumber yang bergerak. Tujuan
kajian ini adalah untuk membentuk satu kaedah yang ringkas dan berkesan untuk
menjalankan analisis ke atas hidrokarbon dari ekzos kenderaan terutamanya benzena,
toluena, etilbenzena dan xilena (BTEX). Kaedah ini terdiri daripada teknik peyampelan,
analisis kualitatif dan anal isis kuantitatif. Semua sarnpel ekzos diperangkap di dalam beg
Tedlar dan disuntik secara manual menggunakan jarum kedap udara ke dalarn Gas Kromatografi-Spektrometer Jisim (GC-MS) yang beroperasi pada mod Pengionan
Impak Elektron.
Kaedah yang dibentuk telah berjaya mengasingkan konponen-komponen
hidrokarbon di dalam ekzos. Secara kasar, 50 spesis hidrokarbon telah dikesan iaitu dari
v
C.$ hingga e\2. Komponen hidrokarbon dari Cl hingga sebahagian dari C4 tidak berjaya
dikesan oleh alat GC-MS yang digunakan kerana ia memerlukan aksesori tambahan
untuk tujuan tersebut.
Kadar pelepasan BTEX perlu dititikberatkan kerana ia bersimt karsinogen dan
toksik. Hasil kajian menunjukkan kepekatan BTEX yang tinggi semasa kenderaan
dihidupkan pada permulaan sejuk dan pada permulaan panas. Kepekatan min BTEX
pada keadaan permulaan sejuk adalah seperti berikut: benzena (55.4 ppm), toluena
(184.7 ppm), etilbenzena (50.2 ppm), m-xilena (143.9 ppm), p-xilena (59.0 ppm) and 0-
xilena (65.4 ppm). Kepekatan min BTEX pada keadaan permulaan panas adalah seperti
berikut: benzena (82.4 ppm), toluena (198.3 ppm), etilbenzena (40.0 ppm), m-xilena
(184.0 ppm), p-xilena (62.1 ppm) and o-xilena (50.3 ppm).
Kesan BTEX terhadap pembentukan ozon fotokimia dianggar menggunakan
faktor 'Peningkatan Reaktiviti Maksimum' (MIR). Potensi pembentukan ozon akibat
pelepasan BTEX dari ekzos kenderaan adalah dianggarkan seperti berikut: 3.07 g OJ! g
benzena, 74.64 g OJ! g toluena, 13.0 g 031 g etilbenzena, 145.8 g OJ! g m- and p-xilena,
dan 50.97 g 031 g o-xilena. Ini menunjukkan bahawa kandungan BTEX di dalam ekzos
kenderaan mempunyai kesan yang ketara terhadap pembentukan ozon fotokimia di
atmosfera.
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ACKNOWLEDGEMENTS
My deepest appreciation goes to Dr. Puziah Abdul Latif who has given
generous advice and constructive comment throughout the study. Her understanding
and invaluable guidance is truly treasured.
I would like to thank En. Azman and Dr. Azizi for the ongoing feedback,
which helps to sharpening my idea. Thank you for your invaluable time.
A special acknowledgement goes to Pn. Nahariah and En. Sujak for their
competent assistance in the laboratories works.
Finally, to my lovely husband and parents, thank you for the immense
understanding and support.
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I certify that an Examination Committee met on 8th December 2001 to conduct the final examination of Norasalwa Zakaria on her Master of Science thesis entitled "Method Development, Speciated Measurements and Calculated Reactivities of Benzene, Toluene, Ethylbenzene and Xylenes from Vehicle Exhaust" in accordance with Universiti Pertanian Malaysia (Higher Degree) Act 1980 and Universiti Pertanian Malaysia (Higher Degree) Regulations 1981. The Committee recommends that the candidate be awarded the relevant degree. Members of the Examination Committee are as follows:
Anuar Kassim, Ph.D. Associate Professor Faculty of Science and Environmental Studies Universiti Putra Malaysia (Chairman)
Puziah Abd. Latif, Ph.D. Faculty of Science and Environmental Studies Universiti Putra Malaysia (Member)
Azman Zainal Abidin, M.Sc. Faculty of Science and Environmental Studies Uruversiti Putra Malaysia (Member)
Azizi Hj. Muda, Ph.D. Associate Professor Faculty of Science and Environmental Studies Universiti Putra Malaysia (Member)
AINI IDERIS, Ph.D Professor/ Dean of Graduate School Universiti Putra Malaysia
Date: - a JAN 2002
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The thesis submitted to the Senate of Universiti Putra Malaysia has been accepted as fulfilment of the requirement for the degree of Master of Science.
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AINI IDERIS, Ph.D.
Professor/ Dean of Graduate School Universiti Putra Malaysia
Date: .1 4 MAR iwtj2
DECLARATION
I hereby declare that the thesis is based on my original work except quotations and citations, which have been duly acknowledged. I also declare that it has not been previously or concurrently submitted for any other degree at UPM or other institutions.
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TABLE OF CONTENTS
DEDICATION ABSTRACT ABSTRAK ACKNOWLEDGEMENTS APPROVAL DECLARATION LIST OF TABLES LIST OF FIGURES LIST OF ABBREVIATIONS
CHAPTER
1
2
3
INTRODUCTION 1.1 Introduction 1.2 Statement of the Problems 1.3 Objectives of the Study 1.4 Scope and Limitation of the Study 1.5 Significance of the Study
LITERATURE REVIEW 2.1 Introduction 2.2 Sources ofBTEX 2.3 BTEX as Air Toxics 2.4 Health Effects ofBTEX 2.5 Impact on Air Quality
2.5.1 BTEX as Photochemical Ozone Precursors 2.5.2 BTEX as Photochemical Smog Precursors 2.5.3 BTEX Acid precursors
2.6 Techniques ofBTEX Sampling 2.7 The Method ofBTEX Analysis
2.7.1 BTEX in Ambient Air 2.7.2 BTEX in Vehicle Exhaust
2.8 Analytical Instrument for BTEX Analysis 2.8.1 Introduction 2.8.2 Principal Operation of Gas Chromatograph 2.8.3 Principal Operation of Mass Spectrometer
2.9 GC-MS Conditions for VOC Analysis 2.10 Qualitative Analysis ofBTEX 2.11 Quantitative Analysis ofBTEX
METHODOLOGY 3.1 Development of Sampling Train 3.2 Sources of Vehicle Exhal.st and Ambient Air
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Page 11 III V V11 Vlll X Xl11 xv XVll
1 1 3 4 4 5
7 7 8 9 10 11 11 13 14 14 20 20 21 22 22 23 24 26 30 32
36 36 38
4
5
3.3 Specification of The Instrument 40 3.4 Chemicals and Reagents 41 3.5 Preparation of Working Standard Solutions 41
3.5.1 Preparation of Liquid Working Standard 41 3.5.2 Preparation of Gas Standard Mixture 42
3.6 Method Development for BTEX Analysis 43 3.7 Sample Analysis Procedure 48
3.7.1 Instrument Tuning 48 3.7.2 Blank Sample Run 49 3.7.3 Working Standard Run 50 3.7.4 Sample Run 50
3.8 Qualitative Analysis of the Exhaust 51 3.9 Quantitative Analysis of the Exhaust 52
3.9.1 External Standardization Method 53 3.9.2 Area Normalization Method 53 3.9.3 Other Calibrant Mixtures Used For Calibrating
Exhaust Hydrocarbons 54
RESULTS AND DISCUSSIONS 55 4.1 Method Development For BTEX Analysis 55 4.2 Method of Vehicle Exhaust Sampling 70 4.3 Method of Ambient Air Sampling 74 4.4 Qualitative Analysis of Vehicle Exhaust 75
4.4.1 Qualitative Analysis of Target Analyte 75 4.4.2 Qualitative Analysis of Hydrocarbons Other Than
BTEX in the Vehicle Exhaust 83 4.5 Quantitative Analysis of the Exhaust 92
4.5.1 Calibrant Mixture 92 4.5.2 Concentration ofBTEX in the Exhaust 93 4.5.3 Quantification Using Area Normalization Method 99
4.6 Photochemical Ozone Reactivity 103
CONCLUSIONS AND RECOMMENDATIONS 5.1 Findings of the Study 5.2 Recommendations For Improving the Results of
The Study 5.3 Recommendation For Future Studies
106 106
108 109
REFERENCES APPENDICES
Appendix A AppendixB
VITA
111 117 118 129 131
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LIST OF TABLES Table Page
1.1 Comparison Between Recommended Malaysian Guideline (RMG) and National Ambient Air Quality 2
2.1 Categories of Organic Compounds 7
2.2 Physical Properties of Benzene, Toluene, Ethylbenzene and Xylenes 8
2.3 Health Effects on Human and The Exposure Limit ofBTEX 10
2.4 Column Flow and Total Flowrate Correlated with Column Diameter 29
3.1 Profile of the Test Vehicle 38
3.2 Sampling Locations of Ambient Air 39
3.3 Conditions That Were Varied During Developing Method For BTEX Analysis 44
3.4 Sequence of the Method Development Process 45
3.5 GC-MS Operating Parameters For Speciating BTEX and Other Hydrocarbons In The Vehicle Exhaust 51
4.1 Results of Varying GC-MS Conditions in Developing BTEX Analysis Method 56
4.2 Name of Compounds And Retention Time of the Peaks Labeled in Chromatogram in Figure 4.19 Above 82
4.3 Peak Report for Figure 4.20 84
4.4 Retention Time, Relative Retention Time And Compounds Identified In the Exhaust 86
4.5 Concentration Of Benzene and Toluene Calculated Using Three Different Cali brant Mixtures 92
4.6 Concentration OfBTEX (ppm) For Car 1, Car 2, Car 3 and Car 4
Tested At Cold Start 94
4.7 Concentration OfBTEX (ppm) For Car 1, Car 2, Car 3 and Car 4 Tested At Hot Start 94
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4.8 Statistical Analysis Of The BTEX Concentration In the Exhaust Samples Tested At Cold Start And Hot Start 97
4.9 Concentration OfBTEX (ppm) For Car 5, Car 6 and Car 7 Tested At 3000rpm 98
4.10 Normalized Area For BTEX In the Exhaust Sample For Car 1, Car 2, Car 3 And Car 4, Tested At Cold Start 99
4.11 Normalized Area ForBTEX In the Exhaust Sample For Car 1, Car 2, And Car 4, Tested At Hot Start 99
4.12 Normalized Area For BTEX In the Exhaust Sample For Car 5, Car 6, And Car 7, Tested At 3000rpm 100
4.13 Calculated Reactivities of Benzene, Toluene, Ethylbenzene and Xylenes From Vehicle Exhaust 103
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LIST OF FIGURES
Figures Page
2.1 Schematic Diagram Of Gas Chromatograph-Mass Spectrometer 23
2.2 Total Ion Chromatogram of a BTEX Mixture 24
2.3 Mass Spectrum ofEthylbenzene 25
3.1 Block Diagram of The Sampling Train 37
4.1 Sample Mix TOC 1A Analyzed Using BTXSCN7.Met 66
4.2 The Effect of Insufficient Solvent Cut 67
4.3 Circled Areas of The Chromatogram Illustrate Split Peak, Tailing and Broaden Peaks 70
4.4 A Chromatogram Showing No Split Peaks, Tailing and Broaden Peaks 70
4.5 Sample TLF16, Motor Exhaust Collected in Hexane 73
4.6 Air Sample Collected at The Outside Corridor of labatan A1am Sekitar, UPM 74
4.7 Mass Spectrum of Benzene From The Exhaust Samples Run Under BTXSCN12.Met 76
4.8 Mass Spectrum of Benzene Acquired From the System's Public Library 76
4.9 Mass Spectrum of Toluene From The Exhaust Samples Run Under BTXSCN12.Met 77
4.10 Mass Spectrum of Toluene Acquired From the System's Public Library 77
4.11 Mass Spectrum of Ethylbenzene From The Exhaust Samples Run Under BTXSCN12.Met 78
4.12 Mass Spectrum of Ethyl benzene Acquired From the System's Public Library 78
4.13 Mass Spectrum of m-xylene From The Exhaust Samples Run Under BTXSCN12.Met 79
4.14 Mass Spectrum ofm-xylene Acquired From the System's Public Library 79
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4.15 Mass Spectrum ofp-xylene From The Exhaust Samples Run Under BTXSCN12.Met 80
4 . 16 Mass Spectrum ofp-xylene Acquired From the System's Public Library 80
4. 17 Mass Spectrum of o-xylene From The Exhaust Samples Run Under BTXSCN12.Met 81
4.18 Mass Spectrum of o-xylene Acquired From the System's Public Library 81
4.19 Chromatogram Of Working Gas Standard Analyzed Using BTXSCN12.Met 82
4.20 Total Ion Chromatogram Of An Exhaust Sample Tested At Cold Start 83
4.21 Chromatogram of a Liquid Fuel Analysis 91
4.22 Concentration ofBTEX in The Exhaust Sample Tested at Cold Start 95
4.23 Concentration ofBTEX in The Exhaust Sample Tested at Hot Start 95
4.24 Concentration of BTEX In The Exhaust Sample Tested At 3000rpm 98
4.25 Concentration ofBTEX in The Exhaust of Motorcycle (Yamaha 7Occ) 101
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LIST OF ABBREVIATIONS
arnu atomic mass unit
BTEX benzene, toluene, ethylbenzene and xylenes
CS cold start
EI Positive Electron Ionization
eV electro Volt
FID Flame Ionization Detector
FTP Federal Test Procedure
GC Gas Chromatograph
GC-MS Gas Chromatograph-Mass Spectrometer
HPLC High Performance Liquid Chromatograph
HS Hot Start
ID Internal Diameter
K Kelvin
nun minute
MIR Maximum Incremental Reactivity
MS Mass Spectrometer
mlz mass to charge ratio
NMHC Non-Methane Hydrocarbon
ppm part per million
ppb part per billion
RI Relative Intensities
rpm rotation per minute
xvu
r.t retention time
r.r.t relative retention time
T Temperature
TIC Total Ion Chromatogram
V Volt
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1.1 Introduction
CHAPTER 1
INTRODUCTION
Motor vehicles are recognized as a major contributor of hydrocarbons to the
atmosphere in urban area and have been associated with serious environmental problems
such as photochemical smog, photochemical ozone and acid rain in addition to adverse
effects on human health. Although emissions from individual vehicle are generally low,
the combined emissions from the total number of vehicle and the increasing mileage
traveled, proved them to be the principal source of anthropogenic pollutant. In Malaysia,
an increase in the number of vehicles on the road was reported from 8,510976 in 1997,
to 9,141 ;357 at the end of 1998 (JPJ, 2000).
Traditionally, measurements of auto-vehicle emiSSion were limited to total
hydrocarbon (THC), carbon monoxide (CO) and oxides of nitrogen. Other primary
pollutants in the exhaust such as hydrocarbons were not emphasized. Hydrocarbons in
the exhaust like benzene, toluene, ethylbenzene and xylenes (BTEX) should be
emphasized since they react with hydroxyl radicals in the atmosphere to form secondary
pollutants that propagate the formation of photochemical ozone. Hence, detailed
composition and characterization of vehicular emission is required simply because the
individual hydrocarbons has different inherent toxicities and ozone-forming reactivity.
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Vehicle exhaust comprises of many hydrocarbons ranging from C1-CI2. The
identification of the full spectrum of the compounds present in vehicle exhaust and air is
only possible using chromatographic techniques. However, due to instrumentation
limitation and costing, the study was limited to C, to Cl2 hydrocarbons with particular
interest on light aromatic hydrocarbons like benzene, toluene, ethylbenzene and xylene
(BTEX). BTEX was chosen because it constitutes a significant fraction of the organic
compounds in the exhaust and fuel as well as in the troposphere. Furthennore, benzene
is known as carcinogen and others are considered as air toxicants. Table 1.1 shows the
comparison between the Recommended Malaysian Guideline and the United States
National Ambient Air Quality Standards (NAAQS) of common air pollutants.
Table 1.1: Comparison Between Recommended Malaysian Guideline (RMG) And National Ambient Air Quality Standards (NAAQS)
Pollutant Averaging Time
Ozone 1 br 8br
CO· Ibr 8br
N02 Ibr 24br
S02 1 0 min 1 hr 2 4 br
Particle 24br TSP lyr PM 1 0 24br
lyr 3 ... measurement In mg/m
Source:�an, 2000
(ppm) 0.10 0. 1 1 30 9 0. 1 7
0. 1 9 0. 13 0.04
RMG NAAQS (,ug/m3) (pg/m)
200 235 1 20
35 x 101 1 0 Xl01
1 0 x 1 01
320 1 1 3 500 350 1 05 365
260 90 260 1 50 1 50 50
1.2 Statement of the Problems
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The analysis of BTEX emitted from motor vehicle exhaust is required for two
reasons. Firstly, under the Clean Air Act (CAA) of the United States, promulgated in
1990, benzene, toluene, and the xylenes have been listed as air toxicants. Furthennore,
benzene is carcinogenic. Secondly, benzene and its derivatives have been associated
with serious environmental problems such as photochemical ozone and photochemical
smog. Each of the compounds has a different ozone reactivity or ozone fonning
potential. Therefore, the concentration of benzene, toluene, ethyl benzene and xylenes
from the motor vehicles should be detennined accurately and must be monitored
continuously since the main anthropogenic source of benzene and its derivatives comes
from motor vehicles. For these to be accomplished, the analytical method for BTEX
analysis, using the available instrument, must be developed.
Accurate measurements and the information acquired from the analysis is
beneficial as the baseline for considering the requirement for regulating benzene
released from the motor vehicles as well as in the fuel. In addition, the ozone fonning
potential, as a result ofBTEX emission from the vehicle exhaust, can also be estimated.
1.3 Objectives of the Study
The objectives of the study are as follows:
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1. To develop a rapid and reliable method for trace level analysis for routine
monitoring of the volatile organic compounds, originated from motor vehicle
exhaust and ambient air�
2. To identify and quantify BTEX in motor vehicle exhaust;
3. To identify and quantify BTEX in fuel formulations and compare them to the
emission from various motor vehicles;
4. To estimate photochemical ozone reactivity ofBTEX as a result of vehicle
exhaust emission.
1 .4 Scope and Limitation ofTbe Study
1. The main scope of the study is to develop an analytical method for BTEX
analysis using Gas Chromatograph-Mass Spectrometer. The gaseous samples
will be introduced manually into the GC, using a gas tight syringe, but the liquid
sample will be injected into the GC using an auto-injector. Other accessories
required for gaseous analysis like automatic gas sampling valve, cryogenic
concentrator and a purge and trap unit are not available at the moment.
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2. The exhaust samples collected in this study were limited to the exhaust from the
cars only. Other gaseous samples that were collected for instance the ambient air
samples and a motorcycle's exhaust, were only used for comparison purposes
only.
1.S Significance of the Study
To the author's best knowledge, no studies have yet been conducted or published
in Malaysia involving a detailed speciation of hydrocarbons in vehicle exhaust, although
it is recognized as major anthropogenic pollutants. Thus this study will provide
infonnation on the strategy to develop an alternative method for analysis of volatile
compounds especially BTEX by direct injection of gaseous sample using gas tight
syringe into the GC-MS instead of using gas sampling valve or a purge and trap unit.
In environmental context, characterization of vehicle exhaust is needed for
several reasons. A complete speciation enable prediction of the quantity of ozone created
as a result of vehicle emission. Also, the technique and the method developed can be
used to collect data on the amount ofBTEX emitted in enclosed area like underground
car park or tunnel with no or poor ventilation so that by using these infonnation, proper
regulation can be enforced to regulate BTEX emission and proper control measure can
be installed where appropriate.
2.1 Introduction
CHAPTER 2
LITERATURE REVIEW
Organic compounds cover a wide range of groups including aliphatic
hydrocarbons, aromatics, halogenated hydrocarbons and oxygenated hydrocarbons.
Table 2. 1 below shows the categoeries for organic compounds, which are based on
their boiling point. Light aromatic hydrocarbons like benzene, toluene, ethylbenzene
and xylenes or BTEX, which have boiling points ranging from 80°C to 144°C are
categorized as volatile organic hydrocarbons (VOC).
Table 2.1 : Categories of Organic Compounds
Categories Bolling Point rC)
Very Volatile Organic Compound (WOC) < Oto 50-100
Volatile Organic Compound (VOC) 50-100 to 240-260
Semi Volatile Organic Compound (SVOC) 240-260 to 380-400
Source: Rafson, H.J. 1 988
Various aspects of BTEX were studied due to its adverse effects towards
human and to the environment. To human, benzene is a carcinogen and its
derivatives are considered as air toxicants. In air photochemistry, BTEX plays an
important role in the formation of photochemical ozone. Physical properties of
BTEX are tabulated in Table 2.2.
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