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RETENTION BEHAVIOR RETENTION BEHAVIOR RETENTION BEHAVIOR RETENTION BEHAVIOR OF POLYMEROF POLYMEROF POLYMEROF POLYMER––––SOLUTE SOLUTE SOLUTE SOLUTE

SYSTEMS VIA INVERSE GAS SYSTEMS VIA INVERSE GAS SYSTEMS VIA INVERSE GAS SYSTEMS VIA INVERSE GAS

CHROMATOGRAPHY CHROMATOGRAPHY CHROMATOGRAPHY CHROMATOGRAPHY

By

Prof. Dr. Ashraf Yehia Z. El-Naggar

and

Dr. Adil A. Gobouri

and

Eid H. Al–osaimi

International E – Publication www.isca.me , www.isca.co.in

INTERACTION CHARACTERISTICS OF POLYMERINTERACTION CHARACTERISTICS OF POLYMERINTERACTION CHARACTERISTICS OF POLYMERINTERACTION CHARACTERISTICS OF POLYMER––––

SOLUTE SYSTEMS VIA INVERSE GAS SOLUTE SYSTEMS VIA INVERSE GAS SOLUTE SYSTEMS VIA INVERSE GAS SOLUTE SYSTEMS VIA INVERSE GAS

CHROMATOGRAPHY CHROMATOGRAPHY CHROMATOGRAPHY CHROMATOGRAPHY

By

Prof. Dr. Ashraf Yehia Zaki El-naggar

Prof. of Analytical and Petroleum Chemistry

Chemistry Department, Science Faculty, Taif University,

Taif-Al-Haweiah-P.O. Box 888 Zip Code 21974, KSA.

Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt.

Dr.Adil A. Gobouri

Assistant Professor of Organic Chemistry, Science Faculty, Taif University,

Kingdom of Saudi Arabia

.

Eid Hamed Al–osaimi

MSc in Analytical Chemistry from Chemistry Department, Science Faculty,

Taif University, Kingdom of Saudi Arabia.

2013

International E – Publication www.isca.me , www.isca.co.in

International E – Publication 427, Palhar Nagar, RAPTC, VIP-Road, Indore-452005 (MP) INDIA

Phone: +91-731-2616100, Mobile: +91-80570-83382, E-mail: [email protected], Website: www.isca.me , www.isca.co.in

© Copyright Reserved

2013 All rights reserved. No part of this publication may be reproduced, stored, in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, reordering or otherwise, without the prior permission of the publisher. ISBN: 978-93-83520-29-9

Publication – E Internatioanl in.co.isca.www ,me.isca.www

International Science Congress Association iii

PREFACE

Inverse gas chromatography is a relatively rabid and reliable method for the

determination of polymer- solute interaction coefficient.

Gas Chromatographic characterization of polymers as stationary phases was

studied through retention behavior, polarity through Rohrschneider scheme,

selectivity and resolution via inverse gas chromatography. This

characterization was shown to be very useful, leading to materials with

suitable properties for advanced applications, especially in the field of

mixture separation.

Thermodynamic parameters (ΔH, ΔG and ΔS) were determined using inverse

gas chromatography in order to help us to investigate the interaction

characteristics of the solutes of different polarities with the studied polymers

as stationary phases in gas chromatography.

Gas chromatographic evaluation of these polymers was done mainly through

their use in the analysis of selected mixtures of different polarities including

paraffinic hydrocarbons, aromatic hydrocarbons and alcohols. The

efficiency of separation for the studied mixtures was evaluated in terms of

resolution, and separation factor.

Dr. Ashraf Yehia El-Naggar Prof. of Analytical and Petroleum Chemistry


International Science Congress Association iv

TABLE OF CONTENT

Preface iii

Table of Content iv

Chapter 1: Introduction 1

(1-1) Retention parameters 3

(1-2) Types of Stationary Phases 7

(1-3) Characterization of Stationary Phases 21

(1-4) Efficiency of Gas Chromatographic Separation 25

Chapter 2: Solute- Stationary Phase Interaction 28

(2-1) Thermodynamic properties 28

(2-2) GC detectors 31

(2-3) Application of gas chromatography 38

Chapter 3: The common used materials and Methods 43

(3-1) Chemicals and symbols 43

(3-2) Optimum flow rate method 44

(3-3) Condition of gas chromatograph 44

(3-4) Stationary phase characterization 45

(3-4) Condition of thermodynamic parameters 46

Chapter 4: Characterizations of Stationary Phases 47

(4-1) Determination of optimum flow rate 47

(4-2) Rohrschneiders scheme 50

(4-3) Selectivity index 53

Chapter 5: Thermodynamic parameters 55

Chapter 6: Separation mechanism 63

Chapter 7: Chromatographic Application 65

(7-1) Separation of normal paraffinic hydrocarbons 67


International Science Congress Association v

(7-2) Separation of aromatic hydrocarbons 69

(7-3) Separation of alcohols 72

Conclusion 75

Acknowledgement 77

References 78

About Authors 89


International Science Congress Association

1

CHAPTER 1:Introduction

Chromatography is an analytical method for separating

components of mixtures. This process involves distribution of a solute

component between two phases: a mobile phase and a stationary phase. Gas

chromatography (GC) plays, nowadays, a vital role in many analytical

laboratories. This is due to the elegance and the versatility of such technique

which find so many applications in different areas such as petroleum industry

for assessing petroleum and derivatives quality, environment,

pharmaceuticals, chemicals, food science and many more.

The main driver for GC, as a separation technique, is the column and

the stationary phase i.e. the packing. Thus, using packing that is characterized

by high selectivity and effectiveness is a base requirement for gas

chromatographic separation of complex mixtures. So, the demand for even

more stable and efficient phases gave impetus to further developments. This

may be realized by discoveries of new selective stationary phases and by

modifying known stationary phases and column surfaces. Modifications of

stationary phases continue with advances in deactivating and coating

techniques such as coating process (surface modification of adsorbents). This

permits for many applications to be optimized and realized, which had been

earlier thought to be impossible by GC.

In gas liquid chromatography (GLC), the stationary phase is a liquid dispersed

on an inert support, while the mobile phase is an inert gas. The gas

chromatographic separation of a given mixture of components depends on

their distribution coefficient between the two phases. By using selective

stationary phases, gas liquid chromatography provides good separation of

components with nearly the same molecular weight and boiling points. The



2

separated components are detected with a suitable detector at the end of the

column. Thus, gas chromatography was used to identify and determine the

quantity of each component of the mixture to be analyzed.

On the other hand, although capillary columns have introduced new

methodologies in GC, Packed columns still offer some preferable

characteristics compared to capillaries. Primarily, allowing the entire samples

to be injected without the need of a splitter. This yields greater sample

sensitivity. Thus, the advantages of packed column are due to volume of

stationary phase per unit column length, in addition to the ease of column

preparation, resistance to contamination, short retention time.

Solid adsorbents such as molecular sieves, porous polymer beads,

graphitized carbon black, alumina, and silica gel are widely used as stationary

phases for packed column. This shows highly selective separation especially

for gases, volatile polar and non polar compounds, sulfur gases and

hydrocarbon impurities air pollutants e.g. CO, CO2 in air, solvents…..etc.

Moreover, silica-bonded phase chromatographic columns are the most widely

employed for the analysis of a great diversity of compounds by different

chromatographic techniques.

In gas solid chromatography (GSC), the stationary phase is a solid that

acts as adsorbent and packed in the column. The components of the mixture

distribute themselves between the gas phase and the adsorbed phase on the

surface of the solid (1).

Silica gel and alumina can be easily prepared and modified by

calcinations and special chemical treatments such as coating by a Carbowax

polymeric material (2, 3). These modifications offer the additional advantages

of improving separation and reducing peak asymmetry. The effect of different

concentrations of silica and alumina in aluminosilicates and the correlations

between the types of modification and the surface characteristics (e.g., surface

area, pore volume, pore radii, number of surface –OH groups) and gas



3

chromatographic separation are studied in order to select the adequate support

or stationary phase to elute the components of the mixture.

In order to evaluate the modified stationary phases, one should follow

some gas chromatographic parameters such as column efficiency, polarity of

stationary phases, thermodynamic parameters, etc. In this context, some

theoretical aspects will be presented.

(I-1) Retention parameters

Over the history of gas-liquid chromatography, thousands of substances

have been used as stationary phases. General considerations that influence the

choice of a particular liquid for use as a stationary phase in gas-liquid

chromatography include the liquid should be unreactive, have low vapor

pressure, good coating characteristics (wet the materials used in column

fabrication) and reasonable solubility in some common volatile organic

solvent.

The most popular liquid phases for gas chromatography are poly

siloxanes. The properties of these phases include a wide temperature

operating range, acceptable diffusion and solubility properties for different

solute types, chemical inertness, good film forming properties and ease of

synthesis with a wide range of chromatographic selectivity. The poly

siloxanes used in gas chromatography are generally linear polymers. In

addition, high molecular weight hydrocarbons such as hexadecane, squalane

(C30H62), Apolane-87 (C87H176) and Apiezon greases have long been used as

nonpolar stationary phases in gas chromatography. Moreover, they are used as

nonpolar reference phases in various schemes proposed to measure the

selectivity of polar phases. On the other hand ether and ester phases are

almost used in packed column gas chromatography for the separation of polar



4

compounds (e.g. succinate and adipate esters of ethylene glycol, diethylene

glycol and butanediol).

a) Retention time and retention volume

The chromatogram is the graphical representation of the elution process into

the chromatographic column. Retention times or retention volumes are the

most common parameters used to measure retention within the column.

Compounds are eluted from the column in the inverse order of the magnitude

of their distribution coefficients with respect to the stationary phase (4), thus,

peaks for less-retained solutes emerge before the peaks for highly-retained

solutes.

The mobile phase is assumed to be completely inert to the column, so no

interactions between mobile phase and stationary phase take place. The

required time for the mobile phase to be eluted from the column is the so-

called dead (hold-up) time (tm), which is the time for any solute not

interacting with the stationary phase to be eluted. As solutes interact with the

stationary phase, they are eluted after the mobile phase and the time between

solute injection and the emerging the solute from the column is called

retention time, the difference between mobile phase retention time and solute

retention time is so-called adjusted retention time (t*r ). The adjusted retention

time is defined by Eq. 1

t*r = tr – tm (1)

The volume of the mobile phase in the column is called Hold-up volume

(or dead volume) Vm and the volume of mobile phase necessary to enable the



5

solute migration throughout the column from the moment of entrance to the

moment in which it leaves is called retention volume Vr

the difference between mobile phase retention volume and solute retention

volume is so-called adjusted retention volume (V*r). The adjusted retention

volume is defined by Eq.

V*r = Vr – Vm (2)

Times and volumes in GC are related by the flow rate, F, at the pressure of the

column outlet. If the hold-up and retention time, tm and tr. are multiplied by F,

the gas hold-up volume Vm and the total retention volume of the solute, Vr

can be determined respectively.

Vm = tm . F (3)

Vr = tr . F (4)

The adjusted retention volume, V*r, can be obtained in the same way

V*r = t*

r . F (5)

b) Retention index system

It was not until the late 50’s when Kováts (5) proposed his well-known

retention index system in terms of isothermal retention indices. This system

uses n-alkanes as reference substances, which are non-polar, chemically inert

and soluble in most common stationary phases.

In the Kováts system, the retention index of a substance I is equivalent to 100

times the number of carbon atoms of a hypothetical n-alkane with the same



6

adjusted retention time, adjusted retention volume, specific retention volume,

etc. The retention index values for the n-alkanes, used as the fixed points on

the retention index scale, are defined as 100 times the carbon number of the n-

alkane on all phases. Consequently, the retention index of any substance “X”

is calculated from Eq. (6) by co-injection of X with two n-alkanes usually

differing by one carbon number,

Ix = 100 n + 100 Error! (6)

where t*n’ and t*

n+1 are adjusted retention times (or adjusted retention volume,

specific retention volume, etc.) of the reference n-alkane hydrocarbons eluting

immediately before and after substance “X” and t*x is the adjusted retention

time for substance “X”.

C). Retention Factor ( k )

The Retention Factor is another measure of retention. It is the ratio of the

amount of time a solute spends in the stationary phase and mobile phase

(carrier gas). It is calculated using Equation 7. The retention factor is also

known as the partition ratio or capacity factor. Since all solutes spend the

same amount of time in the mobile phase, the retention factor is a measure of

retention by the stationary phase. For example, a solute with a k value of 6 is

twice as retained by the stationary phase (but not the column) as a solute with

a k value of 3. The retention factor does not provide absolute retention

information; it provides relative retention information. An unretained

compound has k = 0.

k = Error! = Error! (7)



7

(I-2) Types of Stationary Phases

The number of stationary phases suitable for gas chromatography is

quite large, and the choice is dictated largely by the nature of the sample.

Stationary phases are described as non-polar or polar according to their

structure and separating abilities. In general, the most suitable stationary

phase for a given sample is that which is chemically similar to it. There are

three types of stationary phase: solid stationary phase, liquid stationary phase

and liquid-solid stationary phase.

a) Solid stationary phases (GSC):

Bare porous solids have been used as GC stationary phases in the gas–

solid chromatography (GSC) mode. These solid adsorbents are generally

more stable over a wider temperature range and less sensitive to oxygen than

their coated counterparts. GSC often affords much better selectivity for the

separation of geometric and isotopic isomers, and is also well suited for the

separation of permanent gases and small hydrocarbons. However, the

development of this promising technique has been slowed by its intrinsic

difficulties. For example, adsorption isotherms in GSC are often non-linear,

leading to retentions that vary with sample volume, to asymmetric peaks and

to incomplete resolutions. Additionally, the very large surface area typical of

some solids have lead to excessively long retention times, thus limiting the

broader use of GSC. Despite this drawback, the specific advantages of GSC

and its unique separation abilities in some applications have recently lead to

increased interest in the technique.



8

The most important adsorbents that can be used as solid stationary

phase are molecular sieves, carbosieves, porous polymers, silica gel and

alumina.

1) Molecular sieves

Molecular sieves have a pore size defined by the geometric structure

of the zeolite (2). Typical pore size is 4–10 Ǻ (0.4–1 nm) which accounts for

the unusually large specific surface area. It has been used mainly for the

separation of permanent gases. The separation on such adsorbent is based on

multiple-retention mechanisms. The first selection will be done on size.

Molecules that are smaller than the pore size will diffuse inside the pores.

Large molecules are difficult to enter the pore and will elute earlier. The

second mechanism of separation involves adsorption at active sites. This

adsorption mechanism is responsible for the high retention of permanent

gases. The retention of components with dipole interaction and hydrogen-

bonding formation is very high. As a result, carrier gas and samples should be

as dry as possible. Water is absorbed by the molecular sieve and will cause a

reduction of retention time. However, the adsorbed water can be removed by

heating the molecular sieve for a few hours at 300oC.

2) Porous polymers

The porous polymers, like the Porapaks, HayeSep, GasChrom,

Tenax-GC, and the Chromosorb Century Series, are used in special fields of

applications such as the analyses of solvents, acids, and alcohols in water.

Porous polymers are prepared by the co-polymerization of styrene and divinyl

benzene or other related monomers. The pore size and specific surface area

can be varied by the amount of monomer added to the polymer. Porous

polymers are also available with different selectivity. By incorporating vinyl

pyridine or methacrylate groups, the general selectivity can be changed and



9

the porous polymer can be made much more polar. Polar porous polymers

have been reported, such as the Pora PLOT S (medium polarity) and Pora

PLOT U (polar) type.

3) Deposited carbon adsorbents

Carbon materials are highly selective for hydrocarbons(3). Because of

the non-polar characteristics of its surface, carbon will retain unsaturated

hydrocarbons. Carbon materials show a high tolerance for acidic matrices so

this type of material is ideal for analyzing volatile compounds in a strongly

acidic matrix. Carbopack and Graphpac graphitized carbons are ideal for

analyzing many kinds of C1 and C10 compounds, including alcohols, free

acids, amines, ketones, phenols, and aliphatic hydrocarbons. Carbopack B

(Graphpac-GB) and Carbopack C (Graphpac-GC) carbons have surface areas

of about 100 and 12 m2/g, respectively. Therefore, Carbopack B packings will

have a larger sample capacity than Carbopack C packings. Carbopack B HT

or C HT is hydrogen treated for deactivation.

4) Silica gel

Silica gel is a common sorbent for the chromatographic separation of

organic compounds like C1–C8 hydrocarbons, volatile sulfur compounds, and

halogenated compounds in the C1–C6 range. The structure of silica gel is a

matrix of particles consists of silicon atoms joined together with oxygen

atoms by siloxane bonds (Silicon-oxygen-silicon bonds). On the surface of

silica, some residual, uncondensed hydroxyl groups from the original

polymeric silicic acid remain. These residual hydroxyl groups confer upon

silica gel its polar properties. The silica surface is quite complex and contains

more than one type of hydroxyl group, strongly bound or 'chemically'

adsorbed water, loosely bound or 'physically adsorbed' water and hydrogen

bonded silanol group. There are three types of hydroxyl group. The first is a



10

single hydroxyl group attached to a silicon atom which has three siloxane

bonds joining it to the gel matrix. The second is one of two hydroxyl groups

attached to the same silicon atom which, in turn, is joined to the matrix by

only two siloxane bonds. These twin hydroxyl groups are called Geminal

hydroxyl groups. The third is one of three hydroxyl groups attached to a

silicon atom which is now only joined to the silica matrix by only a single

siloxane bond.

The chemical and adsorption properties of silica gel depend on number

and reactivity of the surface silanol groups. Two forms of adsorption centers

may exist over the surface of silica gel, 1) Hydroxyl group, 2) Coordinated

unsaturated atoms of silicon and surfaced electronegativity atoms of oxygen

nascent at dehydration of silica gel. The surface of dehydrated silica gel

begins slightly to chemisorb oxygen, most likely on the centers of -Si type.

The main purpose of silica surface modification is shielding of the

active silanol groups and attachment to the accessible adsorbent surface

organic ligands which are responsible for specific surface interactions.

5) Alumina

Alumina (Al2O3) is known as a good adsorbent for gas-solid

chromatographic separation of all C1–C4 hydrocarbons and some geometric

isomers can be separated as baseline. But practical uses of alumina are

restricted because of its large adsorption capacity. However, this capacity can

be controlled by introducing an organic group into the alumina surface. It is

known that some organic groups can be easily fixed chemically on the

alumina surface.

Alumina exists in eight different polymorphs, seven metastable phases

(γ, δ, κ, ρ, η, θ, and χ) and the thermally stable α-phase. γ-alumina, called

active alumina which is formed by low temperature dehydration (< 4500C) of

boehmite (γ-AlOOH). Upon heating, γ-Al2O3 undergoes a series of



11

polymorphic phase transformations from a highly disordered cubic close-

packed lattice to the more ordered cubic close-packed θ-Al2O3. At higher

temperatures, e.g., 1100oC–1200oC, θ-Al2O3 undergoes a reconstructive

transformation to form the thermodynamically stable hexagonal close-packed

α-phase. Generally, this transformation sequence may be illustrated as

follows:

γ-Al-OOH450oC γ-Al2O3

900oC θ1100-1200oC α

b) Liquid stationary phases (GLC)

Over the history of gas-liquid chromatography, thousands of substances have

been used as stationary phases. General considerations that influence the

choice of a particular liquid for use as a stationary phase in gas-liquid

chromatography include the liquid should be unreactive, have low vapor

pressure, good coating characteristics (wet the materials used in column

fabrication) and reasonable solubility in some common volatile organic

solvent.

The most popular liquid phases for gas chromatography are poly siloxanes.

The properties of these phases include a wide temperature operating range,

acceptable diffusion and solubility properties for different solute types,

chemical inertness, good film forming properties and ease of synthesis with a

wide range of chromatographic selectivity. The poly siloxanes used in gas

chromatography are generally linear polymers. In addition, high molecular

weight hydrocarbons such as hexadecane, squalane (C30H62), Apolane-87

(C87H176) and Apiezon greases have long been used as nonpolar stationary

phases in gas chromatography. Moreover, they are used as nonpolar reference

phases in various schemes proposed to measure the selectivity of polar

phases. On the other hand ether and ester phases are almost used in packed

column gas chromatography for the separation of polar compounds (e.g.



12

succinate and adipate esters of ethylene glycol, diethylene glycol and

butanediol).

Advantage of Liquid Stationary Phase

For the great majority of analytical problems, gas liquid

chromatography has proved to be a very efficient technique.

The use of liquid phases has the following advantages (6):

1) Liquid phases are available in great variety. Thus, adequately selective

phases can be found for a particular separation.

2) The amount of liquid phase in the column can be varied easily.

Therefore, both preparative and high efficiency columns can be made

with the same liquid phase.

3) Liquid phases are available in great purity and in well-defined quality.

Thus, retention values are reproducible.

4) Both packed and open tubular column can be packed with liquid phases

in a simple manner.

In some cases, the volatility of stationary phase could lead to undesirable

bleeding which can disturb the proper function of the detector. The bleeding

of stationary phase causes baseline shift in programmed operation,

contaminates the trapped solutes, and changes the separation character of the

column until an unacceptable deterioration occurs.

In 1961 Martin (7) suggested that adsorption of solute at the gas-liquid

interface might make an important contribution to solute retention. He studied

hydrocarbon solutes (non-polar) on three stationary phases, n-hexadecane

(non-polar), 1-chloronaphthalene (moderately polar) and B,B`-thiodipropio-

nitrile (TDPN, strongly polar). He found that the strength of adsorption

relative to solution increased with increasing the polarity of the stationary

phase. Thus, with non-polar hydrocarbon solutes, liquid surface adsorption



13

occurred for polar but not non-polar stationary phases. This general

conclusion was reproduced and extended by Pecsok et al (8) for a wide range

of hydrocarbon solutes with the same stationary phases, and directly

substantiated by the static measurements of Martire et al (9) on benzene and

cyclohexane in TDPN. The results of Martin (7) and Pecsok (8) show that

saturated hydrocarbon solutes are more subjected to adsorption at the liquid

surface of TDPN than aromatic or polar molecules. Thus, saturated

hydrocarbons eluted rapidly with respect to aromatic since their mechanism

of retention is adsorption. Similar findings were observed by Pecsok and

Gump (10).

Choice of the particular liquid stationary phase depends on the column

temperature to be used and on the chemical nature of the compounds to be

separated. If tailing occurs it usually means that adsorption is taking place on

the solid support and it can be often controlled by the use of more polar

stationary phase or by the pretreatment of the support with such substances

such as dimethyldichlorosilane (DMCS) or hexamethyl-disilazane (HMDS).

Polymers as stationary phases

A wide variety of polymeric liquid-phases have been developed as the

stationary phases in gas chromatography (11, 12). Various deactivation and

polymer-coating processes have been also proposed for the preparation of GC

columns with good separation performance and stability.(13, 14). In contrast to

the successful applications of these polymer-coated columns, such as wall-

coated, support-coated and pellicular-coated . The polymer-packed stationary

phases, however, have been limited, except for characterizing the surface of

the stationary phase in inverse GC (IGC)(15, 16). In general, the surface

characteristics of the stationary phase have been analyzed with IGC technique

by injecting the standard samples into a polymeric liquid-phases stationary

phase -packed column and measuring the elution behavior.



14

This characterization method is based on the observation of the specific

interactions between the surface of the polymeric liquid-phases stationary

phase and the standard solutes injected as the probes. Consequently, it is quite

natural that a synthetic polymeric liquid-phases stationary phase can be

employed as a separation medium in GC, if the polymeric liquid-phases

stationary phase possesses both the thermal stability for the operation at an

elevated temperature and the resistance to the gaseous chemical species

through the column during the separation.

A. Y. El-naggar and G. Turky (17) evaluated three types of polyethylene

glycols of different molecular weights namely, 600, 4000 and 20000 as liquid

stationary phases in gas liquid chromatography. They investigated the

retention mechanism for the studied polymer stationary phases of 15% by

weight on chromosorb PAW and their thermodynamic parameters via inverse

gas chromatography. They found that the separation of n-alcohols is obtained

toward polyethylene glycol (PEG20000) of relatively higher molecular

weight. Saturated hydrocarbons can be separated very efficiently using low

molecular weight polyethylene glycol (PEG600).

A. Y. El-naggar and G. Turky (18) studied the effect of polymer-layer

thickness on the polarity dielectrically. The dependence of specific retention

volume, enthalpy, and entropy upon the loading of polymer on support was

also studied. In all investigated polymers, the loads 5 and 10% deactivate the

support surface, resulting in a decrease in its capacitances, which reflects the

polarity. The loads 15% PEG20,000 and 25% PEG4000 can elute n-alcohols

and exhibit high efficiency of separation; however, in the case of PEG600,

loading has to exceed 25% to be sufficient for eluting n-alcohols. The lower

thickness of the coated polymers was preferred for good separation of

saturated hydrocarbons and also for cyclohexane and aromatic hydrocarbons.



15

J.K.Suh.etal(19) prepared Poly(p-tert-butyltrimethoxymonopropyl -

oxycalix[4larene-methylsiloxane) as a stationary phase in isothermal capillary

gas chromatography exhibiting good separation of some positional isomers

which can not easy to separated by other technique.

R.P.Corent. et al(20) synthesed Highly crosslinked functional polymer particles

with narrow size distribution by precipitation copolymerization of

divinylbenzene, ethylene glycol dimethacrylate and vinylbenzyl chloride

using a simple reflux protocol. These particles were tested as stationary phase

in high-performance liquid chromatography for the separation of polycyclic

aromatic hydrocarbons in reversed-phase mode.

X.Sun . et al (21) synthesed a novel ionic liquid (IL) bonded polysiloxane and

another one with chloride anion ([PSOMIMI[Cl1) was also prepared and

applied them in the separation of fatty acid methyl esters exhibiting good

selectivity.

C.Y.Liu. et al (22) Compared and evaluated of copper complex-containing

siloxane polymers as stationary phases for capillary gas chromatography, and

applied these columns to the analysis of phthalate esters.

Solid Support

It is known that solids are more thermally stable than liquids, and

possess lower vapor pressures. Therefore, columns packed with active solids

are not subjected to bleeding at high temperatures. The most obvious

disadvantage of solid adsorbents is that they frequently give rise at low

concentrations of adsorbent to strongly curved isotherm; this leads to tailing

with consequent loss of efficiency in elution analysis. In addition, they are

difficult to prepare with consistent properties from batch to batch, so that

retention data may have to be continually checked if the column packing is



16

frequently damaged. Such disadvantages could be prevented by appropriate

pretreatment.

Many active solids are efficient catalysts, whereas catalytic activity is

rare in gas–liquid chromatography. Catalysis may result in the irreversible

adsorption of a substance on the column, or the complete or partial

conversion of substances giving rise to the artifacts in the chromatogram.

The function of a solid support is to hold the liquid phase used for packed

column in gas liquid chromatography immobile during the separation process.

It should be inert, easily packed and has a large surface area. The surface must

have sufficient energy to hold the stationary phase in place and to cause it to

wet the surface in the form of a thin film. In addition, it should be a good

conductor of heat and mechanically and thermally stable to avoid changes in

properties on handling and while in use.

In general, the support influences both the retention time and the width

of the peak. the nature of the support is a main factor controlling peak width,

because it determines both the liquid distribution and gas phase flow path. On

the other hand, the presence of active sites on its surface leads to undesirable

adsorption effects which cause tailing and may result in catalyzed

decomposition or rearrangements of the solutes passing through the column.

The adsorption phenomena have the greatest effect when the solutes and the

stationary phase greatly differ in their polarities, this illustrated as following,

adsorption of analytes on the support is most pronounced for polar substances

chromatographed on non polar stationary phases. In contrast, if the stationary

phase is polar, the effect of the support is smaller because the support active

sites are blocked by the functional group of the stationary phase.

There are different materials suitable for used as solid support in gas

chromatography such as silica, alumina, and other new materials. In the

following a brief review of the supports:



17

1- Silica Support

One of the great attractions of the use of silica as support for

chromatographic columns is the ease with its surface can be modified with

different groups or polymers by reaction with the silanol groups.

Diatomite

Diatomite (diatomaceous earth) is composed of the skeletons of

diatoms; the skeletal material is essentially amorphous silica with small

amounts of alumina and metallic oxide impurities. There are two type of

diatomite supports prepared and used in gas liquid chromatography. The first

one is a Chromosorb P (pink supports) which contains complex iron oxides

give this material its characteristic pink color, calcination at temperature

exceed of 900°C is used to agglomerate and strengthen this natural material.

The second type is Chromosorb W (white support) which prepared by

calcining diatomite in the presence of a small amount of sodium carbonate

results in the formation of a white material in which metal impurities are

converted to colorless sodium silicates. The bulk chemical composition of the

two types of diatomaceous earth is nearly similar but there is a great different

in their surface area (18). The Chromosorb W has a smaller surface area and

slightly basic character when compared to the Chromosorb P. Therefore, pink

support gives higher column efficiency but white support has a lower

adsorpitivity (19, 20). Chromosorb G is a specially prepared white support that is

harder, more robust, and inert and has a low loading capacity but higher

density than the regular white supports.

The existences of a wetting transition are closely associated phenomena

on silanised diatomaceous earth supports on gas liquid chromatography. these

phenomena occur only when the surface tension of the non-wetting liquid

exceeds the critical surface tension of the support by an amount small enough

to give a contact angle which, though not zero, is still low. However, Liquids



18

such as PEG-400 with a much higher surface tension have a large contact

angle and small surface area. PEG-400 is not distributed throughout the

interior and exterior of each support but is largely restricted to the outer

surface of the particle as a thick non-uniform layer. The existence of a sharp

wetting transition depends not only on surface tension relationship but also on

the pore structure of the silanised support. The pink support with the high

density possess the necessary combination of porosities: a small porosity in

pores sufficiently fine to tie up a quantity of liquid by capillary action, and a

large porosity in larger pores, characteristic of the diatomite structure having a

narrow spread of pore size but still small enough to exhibit capillary action. In

contrast, the pores of the white support are too large for a wetting transition to

exist.

In gas liquid chromatography, to hold the stationary phase in the

column as a liquid of high surface area, a solid support is required. Therefore,

the support must be a porous material of high surface area and low surface

activity. In fact the nature of the support is a main factor controlling peak

width because it determines both the liquid distribution and gas phase paths (23).

Adsorption of a solute by the solid support contributes to retention in

the majority of GLC systems, but to very different extent, depending on the

nature of the solute, solvent and support. This adsorption may be due to

incomplete coverage which can occur either because the liquid does not wet

the surface or because it is coated at too low a liquid loading to form a

monolayer on a wetted surface (24).

In the case of wettability of the support by the stationary phase, the

way in which the liquid is distributed on the support depends on the structure

of the solid and on the relative surface tensions of liquid and solid surfaces

and of the solid-liquid interface. In this context, one should point out the



19

classification of the solid supports into unwetted supports and wetted

supports.

Coating of solid support

This treatment involves the coverage of the support with polymers or

metals to forms a stable layer on the surface, with the result that the adsorbent

becomes more homogenous and the chromatographic peaks more

symmetrical. In the following a brief review of the coating technique:

Coating with polymers

Supports are usually coated with a small proportion of non-volatile

polymers as stationary liquid phase such as PEG, SE-30...and etc using one of

several evaporation methods. In outline, the stationary phase in a suitable

solvent is mixed with the support, the solvent removed, and the dried packing

added to the empty column with the aid of pressure or suction (24-27).

Coating of alumina support with moderately polar stationary phase

(polypropylene glycol 2000) system was studied (28). It was found that, the

solid support surface is assumed to be successively covered with three

different types of liquid phase layers, first with a monolayer, second with a

double layer and finally with a bulk liquid layer, as the liquid loading

increases.

Hayrapetyan et al(29), studied the influence of the coating with

several polymers polyoctadecyl-methacrylate-methylmethacrylate co-

polymer, polyacrylamide gel, polyvinyl alcohol, poly-2-

hydroxyethylmethacrylate on the surface characteristic of silica gel

having different pore diameter. It was found that:



20

i) A large decrease in specific surface area was observed in the case of

polyoctadecyl-methacrylate-methylmethacrylate co-polymer.

ii) It is more appropriate to use wide-pore silica with a higher pore

volume for the modification with a polymer.

iii) In the silica gel with small pores and a lower pore volume, a

significant decrease of the specific surface are observed.

Coating with metals

Several studies were done dealing with coating of silica and

alumina with various inorganic salts in order to obtain interesting

chromatographic characteristics. For example, Kopecni et al(30),

proved that modification of silica with the alkali metal ions resulted in

decreased partition coefficients than that of unmodified silica. On the

other hand great selectivity for alumina metal modifications is

proposed e.g. Alkali-metal fluorides to give good separations of high

boiling point hydrocarbons solutes and geometric isomers (31), trialkali

metal phosphate and heat treatment before and after coating to give

good resolution for geometric isomers of aromatic hydrocarbons and

their halogenated derivatives (32), also sodium chloride modified

alumina showed good peak resolution for separation of geometric

isomers of 2-olefins and polychloro derivatives of benzene (33).

In another study, alumina surface was modified with various

alkali salts to improve the separation of alkenes/alkanes pair (34). The

study examined the change in retention properties on modified

alumina and the specific effects of the halide anions and metal

cations. It is postulated that the modifier anion may have a role in

reduction of active sites with the consequently elimination of the peak

tailing of unsaturated compounds. However, it is believed that this



21

process occurs readily with no predominant group trend. The metal

cations have a group trend for increasing alkenes/alkanes separation

selectivity with decreasing cation size is in the following order Li+ >

Na+ > K+ > Cs+. Strgelczyk et al(35), modified alumina by

impregnation with palladium as small particles to improve the

selectivity of hydrogen isotopes separation. It was found that, at the

same palladium loading alumina is the most efficient because of the

higher Pd mass content per unit volume of the column. Also, Lee et

al(36), found that column packed with palladium–platinum alloy on

alumina (PPA) exhibits good separation efficiency of hydrogen

isotopes separation.

Recently, Khan et al(37), found a good column packing material

for separation of light hydrocarbon gases by coating alumina with

Fe2O3. The selectivity for alkenes/ alkanes pair has been improved

due to the small ionic radius (0.64Å) and high electronegativity (1.8)

of Fe3+, which increased the interaction of π electrons of alkenes with

dipole Lewis acid site of the aluminum ions and Fe3+ on the modified

alumina surface(38).

(I-3) Characterization of Stationary Phases

The chromatographic retention is based on the interactions between the

solute and the stationary phase, which include directional force, induction

force, dispersion force, hydrogen bond and others. The difference of the

retention is due to the molecular structure of the solute and the properties of

the stationary phase. Thus, the characterization of stationary phases fall into

two types: the first is estimated by chromatographic tests for selected

compounds, these tests include polarity, selectivity and thermodynamic

parameters. The second is the surface characterization for understanding



22

adsorption and desorption phenomena including surface texture (surface area,

average pore size and pore size distribution) and surface chemistry (functional

groups and their distribution) of porous solids. Generally, the characterization

of stationary phases is useful to elucidate retention mechanisms and predict

the retention parameters (39-46).

(a) Polarity of Stationary Phase

Polarity of stationary phase is determined by stationary phase structure,

polarity of functional groups and amount of each group. In order to compare

stationary phase polarities, a systematic method must be used to normalize

instrumental variables; only then can retention data be effectively compared

for different chromatographic system.

Polarity of stationary phases has been assessed by considering the

retention time data for selective solutes of different polarities with the aim of

estimating what so called a retention index. The retention index of an

arbitrary substance X is found by logarithmic interpolation between two

neighbouring solutes: equation (6) .

If the alkanes are chromatographed simultaneously with substance X or, at

least, are chromatographed under identical condition, then Vg in equation (6)

can be replaced by any adjusted retention quantity, i.e. the adjusted retention

time or volume, capacity ratio k, etc.

The idea of the retention index was firstly introduced by Kovats (5).

The Retention Index (I), called Kovat's retention index (5), indicates where

compounds will appear on a chromatogram with respect to straight-chain

alkanes injected with the compound. Kovat's retention index of an arbitrary

sample component X is found by logarithmic interpolation between two

neighbouring solutes as given in formula 6.



23

Each alkane assigned an arbitrary I value 100 time its number of carbon

atoms. The advantages of retention index system attributed to, the constant of

I values for n-alkanes on each stationary phase which determined by actual

injection of the n-alkanes. Accordingly, I values provide a way of comparing

the retention characteristics of various stationary phases and also to compare

relative elution orders of various compounds for a given column and

conditions or for two different columns.

Kovats considered the retention data of n-alkanes. The retention index is a

constant characterizing the intensity of interaction between a solute and a

stationary phase at a given temperature (47). If the less polar stationary phase is

chosen as the reference phase, (the phase that gives the smallest index for the

chosen solute), the polarity is measured by the difference,

ΔI = I b – I

a (8)

Where, a is the less polar stationary phase, b is the more polar stationary

phase, and the solute used is solute other than an alkane.

On the basis of Kovàts retention indices, Rohrschneider(48) originally

proposed that the differences among the indices should be-expressed in terms

of three parameters. Later, it was found that, to take into account the basic

types of interaction, ΔI must be expressed by the sum of fine products of

factors characterizing the polarity of the substance (a to e) and that of

stationary phase (x to s) (49-52).

ΔI = ax + by + cz + du + es (9)

Five standard compounds were selected, that would reflect the main

types of interactions. The orientation forces are represented by factor c (ethyl

methyl ketone), the charge transfer forces by factor a (donor, benzene) and d



24

(acceptor, nitro methane) and hydrogen bonding by factor b (hydrogen donor,

ethanol) and e (hydrogen acceptor, pyridine). In order to characterize the

polarity of a particular stationary phase, the differences between the retention

indices for these five standard solutes obtained on this phase and on non polar

phase must thus be determined.

(b) Selectivity of Stationary Phase:

The selectivity of a given stationary phase is the ability of the

stationary phase to separate a pair of solutes having very similar properties

such as boiling points, molecular weights, number of carbon atom in the

molecule and vapor pressures. The selectivity is not an intrinsic property of

stationary phase: it depends on the choice of solutes and can be expressed in

different terms such as the value of separation factor for estimating the degree

of separation of two component, and uniformity criterion for estimating the

degree of separation of multi component mixture. Early attempts to define

selectivity scales were based on the systems of characteristic phase constants

introduced by Rohrschneider (48) and subsequently modified by McReynolds

(53), Synder ̉’s(54) and Kersten(55) studied solvent selectivity triangle, selectivity

indices (56) Hawakes polarity indices (57) , solubility parameters (58-60), and the

partial molar Gibbs free energy of solution for functional groups or specified

solutes (61-67). The selectivity of the stationary phase is defined as its relative

capacity to enter into specific intermolecular interactions such as dispersion,

induction, orientation, hydrogen-bond formation, and charge transfer

complexation (68).

Selectivity can be expressed in terms of uniformity criterion which

introduced by T. G. Andronikiashvili et.al (69). He used the uniformity

criterion for estimating the degree of separation of multi component mixture;

the uniformity criterion Δwas given according to the following equation:



25

t

τKn

Δeffk

= (10)

where,k

n is the number of peaks on the chromatogram,τ is the base width of

the narrowest peak,eff

K is the separation factor for the worst

separated pair of components, and t is the duration of analysis.

The values of Δ varied from 0 to 1; Δ=1 corresponds to the best separation.

(I-4) Efficiency of Gas Chromatographic Separation:

(a) Column Efficiency

After conditioning the column a few preliminary tests are performed to

ensure that the performance of the column is adequate. A measure of

column efficiency is made with a test sample. In this section, a brief

overview of relevant relationships and equations is presented.

Number and height equivalent of theoretical plates:

The number of theoretical plates (N) is a measure of column efficiency.

A theoretical plate is defined as the average distance traveled in one

distribution step or partition of the analyte from the mobile phase into the

stationary phase and vice versa(70). The plate number can be directly

calculated from the retention time

N = 16 (tr /wb)2 (11)

N = number of theoretical plates (efficiency of a column)

tr = retention time of the peak

wb = is peak width at the base



26

Both retention time and plate number are directly proportional to the

length of the column. The longer the column, the longer the retention time

and the higher the plate count. Since the total number of plates for a particular

column is based on its length, it is useful to express column efficiency in

terms of height equivalent to a theoretical plate (HETP or H). This height,

expressed in micrometers, is the height of one theoretical plate. A small

HETP allows for more plates per column and greater efficiency. HETP can be

calculated as follow:

H = L / N (12)

H = height equivalent to a theoretical plate.

L = length of the column.

N = number of theoretical plates.

(b) Stationary phase efficiency

(1) Separation Factor

The degree to which two components are separated is a function of the

ratio of their retention times and the sharpness of the peaks. The ratio of the

adjusted retention times of two components, 1 and 2 is termed the separation

factor α

αααα = Error! (13)

The separation factor can also be expressed as,

(14)αααα = Error!

Where, k is the retention factor, which is equal to Error!, tm is the gas hold up

time or the retention time of an unretained solute.



27

With co-eluting solutes k2 = k1 and α = 1.0. By convention α is never less

than 1.0.

Solute pairs with large α values can be separated at high resolution

columns, but at low resolution α approaches unity, columns with increasingly

larger numbers of theoretical plates are required to achieve separation(71,72).

(2) Resolution Factor between Two Peaks (R)

The degree to which two components are resolved is termed resolution

R (73-75).

R = 2 (tr(2) – tr(1)) / (Wb(1) + Wb(2) ) (15)

When R = 1, then there is a considerable degree of overlap, and when

R = 1.5, the base line separation usually achieved but asymmetry, tailing, or

gross discrepancies between the sizes of the two peaks can cause

compilations.



28

CHAPTER 2. Solute-Stationary Phase Interaction

In gas chromatography, solutes elute in an order of their net vapor

pressures. This order depends on the interaction between the solute and the

stationary phases under a given set of conditions. For liquid stationary phase,

such interactions are a function of the solubility of the solute in the stationary

phase (76). For solid stationary phase, the adsorption of molecules on the

surface of the solid is influenced principally by the chemistry and textural

properties of the adsorbent surface.

Three types of solute – stationary phase interactions are of concern in gas

chromatography. These interactions include dispersion, dipole and proton

interactions (77, 78). The dispersion interactions constitute the major part of the

total attractive force between a hydrocarbon solute and the stationary phase,

and they are significant for any dissolved solute molecules and the

surrounding stationary phase solvent. Dipole interactions arise from the

alignment of the dipoles of the solute and the stationary phase. The base-acid

interaction (hydrogen bonding) could be attributed to hydroxyl groups, which

may be present in the solutes leading to the formation of hydrogen bonds with

the stationary phase. The ability of a stationary phase to participate in

hydrogen bonding is usually a measure of its basicity. The hydroxyl group

can however also behave as a base and is retained by Lewis and Brønsted

acids.

(2.1) Thermodynamic properties

Gas chromatography (GC) is a powerful and accurate method for measuring

the thermodynamic properties and investigating the properties of solvents and

the behavior of solutes in stationary phases. Polymers as stationary phases

demonstrate excellent chromatographic properties, such as a wide range of



29

operating temperatures, high column efficiency, good thermal stability, and

inertness resulting in low bleed at high temperatures and low detection limits.

Also, it is considered to be of unique selectivity in the separation of nonpolar,

moderate and polar compounds.

In order to study the retention and separation mechanism of this

stationary phases, the relation between retention volume and carbon

number(79, 80) and the thermodynamic parameters of solution, the enthalpy of

solution (ΔH), the free energy of solution (ΔG), and the entropy of solution

(ΔS) were measured.

Many others (81-85) studied the feasibility of GLC for the thermodynamic

investigation of different stationary phases and hydrocarbon solutes. Seifert

and Kraus (86) investigated the solution behavior of liquid crystal phases in

capillary GC throw their thermodynamic parameters.

The aim of this investigation is to study the thermodynamics of solution in the

three studied stationary phases using n-hexane , benzene and ethanol as model

solutes representing their families paraffinic and aromatic hydrocarbons and

to correlate the measured thermodynamic quantities and structures of studied

the stationary phases and solutes.

Specific retention volumes, Vg, were calculated from the corrected peak

retention times and the column operating condition by using the well-known

equation derived by Littlewood et al (87). The specific retention volume Vg is

given as:

Vg = (Vr − Vm) /w (16)

Where Vr is the elution volume of the probe under conditions of instantaneous

equilibrium between the stationary and mobile phases; Vm is the void volume

of the column (which is equal to the elution volume of an ideal marker; i.e., of



30

a probe that does not interact with the column at all); w is the mass of the

polymer on the column.

The elution volumes, Vr, were calculated from the flow rate, F, measured

at the column outlet and the elution time te as (James and Martin,(88)):

Vr = j tr F (17)

j is a correction factor to account for the compressibility of the gas.

The partial molar excess enthalpies ΔH of mixing at infinite dilution

were calculated from an equation formulated by Greene and Pust (89).

for the direct calculation of ΔH from the relation between the logarithm

of retention time and reciprocal of temperature. Accordingly, ΔH can be

determined from the slope of this relation:

tr = LAB/F exp(−ΔH/RT ) (18)

where tr = retention time, R = linear velocity of zone, A = intestinal area

of column, B = constant, F = gas velocity, and ΔH = heat of solution.

The excess partial molar free energy of solution can be expressed as

(Kessaissa et al.) (90):

ΔG = -2.3 RT ln Vg (19)

where T is the column temperature and Vg is the retention volume.

The entropy of solution can be calculated by knowing ΔH and ΔG from

the relation:

ΔS = (ΔH − ΔG)/T (20)



31

(2.2) GC detectors (91)

(a) Detector characteristics

The detector produces an electrical signal which is proportional to either the

concentration or the mass of the analyte molecules in the effluent stream. The

signal is displayed as a chromatogram on a chart recorder, or on the screen of

a desktop computer data system.

The general characteristics of GC detectors which need to be considered are

the following:

(1) Universality vs. selectivity: If a detector responds with similar sensitivity

to a very wide variety of analytes in the effluent it is said to be universal

In the other extreme, a selective detector may give a significant response

to only a limited class of compounds or possessing certain types of

functional groups or substituents which posses certain affinitie or

reactivities.

(2). Destructive vs. nondestructive: Some detectors destroy the analyte as part of

the process of their operation (e.g., by burning it in a flame or fragmenting it

in the vacuum of a mass spectrometer). Others leave it intact and in a state

where it may be passed on to another type of detector for additional

characterization.

(3) Mass flow vs. concentration response: In general, destructive detectors are

mass flow detectors. If the flow of analyte in effluent gas stops, the signal

drops to zero. A nondestructive detector does not affect the analyte, and the

concentration measurement depends on the volume of the effluent



32

(4) Requirement for auxiliary gases: Some detectors do not function well with the

carrier gas composition or flow rates from a capillary column effluent. Makeup

gas, sometimes the same as the carrier gas, may be required to increase flow

rates through the detector to levels at which it responds better and/or to

suppress detector dead volume degradation of resolution achieved on the

column. Some detectors require both air and hydrogen supplied at different

flow rates than the carrier to support an optimized flame for their operation.

Makeup flow dilutes the effluent but does not change the detection mechanism

from concentration to mass-flow detection.

(5) Sensitivity and linear dynamic range: Detectors (both universal, and selective

ones) vary in their sensitivity to analytes. Sensitivity refers to the lowest

concentration of a particular analyte that can be measured with a specified

signal-to-noise ratio. The more sensitive the detector, the lower this

concentration. The range over which the detector’s signal response is linearly

proportional to the analyte’s concentration is called the linear dynamic range.

(b) Types of detectors

1- The Thermal Conductivity Detector (TCD) (91)

TCD Characteristics

Universal (except for H2 and He); non-destructive; concentration detector; no

auxiliary (aux.) gas; works better with a parallel column; insensitive; limited

dynamic range.

The TCD was the first widely commercially available GC detector. It

measured differences in the thermal conductivity and/or specific heat of

highly thermally conductive (either H2 or He) carrier gas when diluted by

small concentrations of much less conductive analyte vapor (anything else). A



33

schematic of a typical TCD is shown in Figure 1, A current through a thin

resistive wire heated the wire in the detector flow cell. The thermal

conductivity of the flowing carrier gas cooled the wire. When analytes were

in the stream, their lower thermal conductivity produced less cooling, which

caused the wire’s temperature to rise and its resistance to increase. This wire

resistor was in a “Wheatstone bridge” circuit (an arrangement of four resistors

on the sides of a square). One of the other resistors was in a matching TCD

cell connected to a matching column and flow with no analyte passing

through. In isothermal GC, carrier flows and temperatures would remain

constant, but with temperature programming of the column the temperature

would increase and the flow would decrease, independently affecting the

conductivity.

Fig. 1. Thermal Conductivity Detector



34

2- Flame Ionization Detector (FID) (91)

FID Characteristics :

Nearly universal (all carbon compounds except CO, CO2, HCN, but not many

inorganic gases); destructive; mass flow detector; needs H2 and air or O2 aux.

gas; sensitive with wide dynamic range.

The FID is the most commonly employed detector, as it gives a response to

almost all organic compounds. A schematic of a typical FID is shown in

Figure 2, on a molecular basis the signal is roughly proportional to the

number of carbon atoms in the molecule. Hydrogen and oxygen (or air) must

be separately provided to fuel a flame in the detector cell. The H2 is

introduced and mixed with the carrier effluent from the GC column. Even if

H2 is used as capillary carrier gas, an additional separately controlled H2

supply is necessary to adjust the appropriate fuel supply for the flame. The

mixed gas enters the cell through a jet, where air or O2 flows past to serve as

the flame oxidizer supply. An electrical glow plug in the cell (not illustrated)

can be pulsed to ignite the flame. The fuel and oxidizer flows are adjusted

with needle valves to achieve a stable flame with optimal FID response, often

by bleeding a volatile unretained hydrocarbon into the carrier stream to

provide a reference signal. The jet tip is charged by several hundred volts

positive relative to several “collector electrodes” or a “collector ring”

surrounding the flame. In the absence of eluting organic analytes, no current

flows in the jet collector circuit. When a carbon-containing analyte elutes into

the flame, the molecule breaks up into smaller fragments during the cascade

of oxidation reactions. Some of these are positively charged ions, and they

can carry current across the flame in the circuit. Although the ionization

efficiency of the FID is low, its base



35

current is also very low. Hence its signal-to-noise ratio is very high. Against

such a low background, even very small ionization currents can be accurately

measured using modern electronics which draw very low currents (high input

impedance, voltage measurement circuits). Hence the good sensitivity and

extraordinary dynamic range of this detector, often exceeding six orders of

magnitude.

FID exhibits high efficiency of separation of petroleum crude oil especially

paraffinic hydrocarbons and aromatic hydrocarbons in addition to the polar

compounds such as alcohols, ketons, esters, etc. depending on the used

selective column.

Fig. 2. Flame Ionization Detector



36

3- The Electron Capture Detector (ECD) (91)

ECD Characteristics

Very selective (for organic compounds with halogen substituents like

pesticides, nitro and some other oxygen-containing functional groups); non-

destructive; concentration detector; needs to use N2 or argon/CH4 as carrier,

or as makeup gas if used with H2 or He capillary carrier flow; extreme but

highly variable sensitivity but with limited dynamic range.

The invention of the ECD is generally attributed to Lovelock, based on his

publication in 1961(92). It is a selective detector that provides very high

sensitivity for those compounds that “capture electrons.” It is an ionization-

type detector, but unlike most detectors of this class, samples are detected by

causing a decrease in the level of ionization. When no analytes are present,

the radioactive 63Ni emits beta particles as follow .

ß- 63Ni

These negatively charged particles collide with the nitrogen carrier gas and

produce more electrons as follow

ß- +N2—,2e+N2*

The electrons formed by this combined process result in a high standing

current (about 10-8 a) when collected by a positive electrode. When an

electronegative analyte is eluted from the column and enters the detector, it

captures some of the free electrons and the standing current is decreased

giving a negative peak: as follow .



37

A+e- —A* (7)

The negative ions formed have slower mobilities than the free electrons and

are not collected by the anode.

The mathematical relationship for this process is similar to Beers Law (used

to describe the absorption process for electromagnetic radiation).

Thus, the extent of the absorption or capture is proportional to the

concentration of the analyte.

The carrier gas used for the ECD can be very pure nitrogen (as indicated

in the mechanism presented) or a mixture of 5% methane in argon. When

used with a capillary column some make-up gas is usually needed, and it is

convenient to use inexpensive nitrogen as make-up and helium as the carrier

gas.

A schematic of a typical ECD is shown in Figure 3. 63Ni is shown as the beta

emitter although tritium has also been used; nickel is usually preferred

because it can be used at a higher temperature (up to 400°C) and it has a

lower activity (and is safer).

The ECD is one of the most easily contaminated detectors and is adversely

affected by oxygen and water. Ultrapure, dry gases, freedom from leaks, and

clean samples are necessary. Evidence of contamination is usually a noisy

baseline or peaks that have small negative dips before and after each peak.

Cleaning can sometimes be accomplished by operation with hydrogen carrier

gas at a high temperature to burn off impurities, but dismantling is often

required. ECD is a sensitive and selective detector for halogenated materials

but one which is easily contaminated and more prone to problems.



38

.Fig. 3. Electron Capture Detector



39

(2.3).Application of gas chromatography

Gas chromatography has been applied successfully to numerous compounds

in variety of fields . it is applied to both organic and inorganic compounds .

Even metals as volatile chelates has been successfully handled . The only

requirement is reasonable vapor pressure at the temperature of operation . we

have chosen only a few examples from the major fields

1- : Clinical

Gas chromatography now routinely handles many compounds of Clinical

interest. Methods; for amino acid (93,94); carbohydrate(95); CO2 and O2 in blood (96,97); fatty acid and derivatives (98) ; plasma triglycerides (99) steroids(100);

barbiturates(101) ; and vitamins (102) , have been described in recent reviews or

articles.

2- pharmaceuticals and Drugs :

Gas chromatography plays an essential role in the analysis of

pharmaceutical products and drugs . It is used in quality control, analysis of

new products and in monitoring metabolites in biological fluids.H. A. Lioyd

et al(103) first demonstrated the G.C. analysis of alkaloids. Gudzinowicsa and

S. J. Clark (104) provided dat for sensitivity of barbituratates by ECD and FID.

K,Langel .et al (105) described fully validated GC-MS method 50 different

drug compounds can be identified and quantitated simultaneously from a

single oral fluid sample.



40

.

3- Food The analytical tool gas chromatography was applied in food in order to

evaluate an preferred minor amount in the food to check for food quality. Gas

chromatography coupled with FID or MS or ECD or ASD is the more

efficient tool used food maintaining The high purity and high quality of food

.T. Nishimoto and M. Uyeta(106) described a GC., procedure for antioxidants

and preservatives. GC usually in combination with TLC and column

chromatography, is used to study adulteration, contamination and

decomposition of foods : olive oil (107) lard(108) dairy products (109) ; and

plasticizers in food(110). L. D. Metcalfe (111) describes a rabid esterification for

fatty acid and J. G. Niklly (112) uses glas beads as the solid support to separate

them. H.Kataoka .et al (113) determine of the contents of isophorone in food

samples which is cancer causing possibility using headspace solid-phase

microextraction (HS-SPME) coupled with gas chromatography- mass

spectrometry (GC- MS). T.Cajka .et al (114) analyze the pesticide in fruit-

based baby food which are registered for use in agriculture to meet food

supply demands using GC-MS. Others (115-118) Determine trace food-derived

hazardous compounds in with selective detectors.

4- Petroleum

It was in the petroleum industry that gas chromatography initially saw its

widest application. It has been successfully used to separate the entire rang of

products from light hydrocarbon gases up to waxes, asphalts and crude oil .

The composition and heat value of natural gas has been determined by GC(119-

120). E. M. Krupp, et al(121) investigate the determination of trimethylarsine in

natural gas and its partitioning into gas and condensate phases using

(cryotrapping)/gas chromatography coupled to inductively coupled plasma

mass spectrometry and liquid/solid sorption techniques. R.Gras,et al(122)



41

investegate Practical method for the measurement of Alkyl mercaptans in

natural gas by multi-dimensional gas chromatography, capillary flow

technology, and flame ionization detection. Nagy. E Moustafa And Jan.

Andersson(123)analyze the polycyclic aromatic sulfur heterocycles in Egyptian

petroleum condensate and volatile oils by gas chromatography with atomic

emission detection. C. T. Tran et al(124) Using the comprehensive two-

dimensional gas chromatography/time-of-flight mass spectrometry for the

characterization of biodegradation and unresolved complex mixtures in

petroleum .T. Dutries , et al (125) Improved hydrocarbons analysis of heavy

petroleum fractions by high temperature comprehensive two-dimensional gas

chromatography. E. Saari. et al(126)evaluate the occurrence of a matrix effect

in the gas chromatographic determination of petroleum hydrocarbons in soil.

The results indicate that solid phase extraction does not appear to be effective

enough in removing interfering matrix components from the extract. As a

result the effect of the matrix-induced chromatographic response prevents the

accurate determination of total petroleum hydrocarbon concentrations in soil

by GC- FID. D. Mao, et al(127) assessment of aqueous solubility (leaching

potential) of soil contaminations with petroleum hydrocarbons (PH) is

important in the context of the evaluation of (migration) risks and

soil/groundwater remediation. Field measurements using monitoring wells

often overestimate real PH concentrations in case of presence of pure oil in

the screened interval of the well.

5- Environment

The environmental pollution has been, and will continue to be of great public

concern. Environmental pollution is factor contributing to many chronic

diseases – lung cancer, emphysema, bronchitis and asthma. Many analytical

methods are used to measure environmental pollution, but gas

chromatography is of ever increasing importance. The separating or



42

concentrating power of column, combined with the sensitivity and selectivity

of detector, make GC an ideal technique to determine many compounds of

interest in pollution work.

G.C. has been used to concentrate trace impurities (128) ; to detect and size

organic aerosols (129,130) ; and to separate and determine polycyclic

hydrocarbons (131) . A sulfur specific microcoulometric detector capable of

seeing 5 ppm of SO2 and 1 ppm of H2S is describe (132) . Total organics in air

can be determined by a portable gas chromatograph (133) .



43

CHAPTER 3. The common used Materials and Methods

3.1. Chemicals and symbols

All chemicals were of reagent grade and were used without further

purification.

• Analytical grade paraffinic hydrocarbons pentane, n-hexane,

and n-heptane were purchased from BDH (99%), n-octane and

n-nonane from BDH (98%) and n-decane from Aldresh

Company (98%).

• Analytical grade aromatic hydrocarbons, benzene, toluene,

ethyl benzene, propyl benzene and butyl benzene were

purchased from BDH company of purities 95%, 98.5%, 98%,

98%, 98.5% respectively.

• The series of alcohols, methanol, ethanol, propanol , butanol ,

pentanol and hexanol. from BDH grade of purities ranged from

98% to 99.5%.

• nitromethane was purchased from Aldresh grade of purity

98.5%.

• pyridine and methyl ethyle ketone were purchased from BDH

grade of purities 97% and 98% respectively.



44

Symbols of the studied stationary phases

1. Dimethyl poly siloxane OV-101

2. Polyethylene glycol 20,000 Carbowax-20M

3. Caynopropyl poly siloxane OV-275

3.2.Optimum flow rate method

The optimum flow rat depends on the column length, column diameter and

column packing. The studied packing columns have the same length, same

diameter, same solid support and thicknessfilm of stationary phase. So,

they would have the same optimum flow rat, two organic solvents n-

hexane and benzene were injected in the split splitles injector under oven

temperature 120 oC at five different flow rates, 15, 20, 25, 30 and 40 ml

min-1. The height equivalent to theoretical plates (H) was determined for

each injected sample from the results obtained from the chromatogram

(retention time tr and peak width (W). From the correlation between the

height equivalent to theoretical plates and carrier gas flow rats we can

obtain the required optimum flow rate at the minimum H value.

3.3. Condition of gas chromatograph

The gas chromatograph used is shimmadzu 2014 equipped with flame

ionization detector (FID) and split/splitless injector. Helium was used as the

mobile phase at optimum flow rate 30 ml min-1, the carriers hydrogen and air

were used for FID at flow rate 45 and 450 ml min-1 respectively. The



45

chromatographic data were recorded using HP Chemstation software, which

was controlled by Microsoft Windows NT.

The gas chromatographic analysis was achieved on three studied

polymers of different polarities as stationary phases inside stainless steel tubes

of 4m in length and 1/8inch internal diameter. The thickness film of the

studied polymer as stationary phases is 3% on chromosorb P.A.W 60-80

mish.

The solutes used for chromatographic characterization were selected to cover

the wide range of polarity such as n-parrafins, aromatics, and alcohols. The

polarity indices were assessed with respect to the reference non-polar column

squalane (20% squalane on chromosorb WAW, 60-80 mesh).

3.4. Stationary phase characterization

Polarity index (Rohrschnieder)

The Roschnieders components, nitro methane, benzene, pyridine,

methyle ethyle ketone and ethanol in addition to the paraffinic series from

pentane to decane were injected at oven temperature 120oC in order to

obtain the retention index of each Roschnieders component. The helium

flow rat was maintained at 30 ml min-1.

Selectivity and applications

The paraffinic hydrocarbons from pentane to decane, the alcohols from

methanol to hexanol and the aromatics from benzene to butyl benzene,

were separated at different oven temperature programming starting from

50oC to 150 oC at a rate of 10 oC min-1. The helium flow rat was maintained

at 30 ml min-1.



46

3.5. Conditions of thermodynamic parameters

The determination of thermodynamic parameters (ΔH, ΔG and ΔS)

were achieved by injecting normal hexane, benzene and ethanol at different

oven temperatures: 70oC, 80oC, 90oC, 100oC and 110oC. these solutes were

selected as representative compounds of their families, paraffines, aromatics

and alcohols respectively. The helium flow rat was maintained at 30 ml min-

1.



47

CHAPTER 4. Characterizations of Stationary Phases

4.1. Determination of optimum flow rate

The present study should be occurred at the optimum flow rat of helium

carrier gas in order obtain high efficiency of gas chromatographic separation.

The studied three columns have the same column dimensions (column length

and column diameter), same support (Type & diameter) and the same

thickness film, so the experiment of optimum flow rat can be done on one

column representing the other columns. The probes n-hexane and benzene

were selected for this test at five different flow rates, 15, 20, 25, 30 and 40 ml

min-1. From the results of the gas chromatographic analysis given in Tables

(4&5), the height equivalent to theoretical plates (H) can be calculated froh

the last equation (11) , (12) .

Table 1. Retention data for determining the optimum flow rate using n-hexane

as a prob.

Flow

rates

tr W N H

15 7.35 1.35 474.272 0.843

20 5.65 1 510.760 0.783

25 4.75 0.8 564.063 0.709

30 3.85 0.6 658.778 0.607

40 3 0.5 576.000 0.694



48

Table 2. Retention data for determining the optimum flow rate using benzene

as a prob.

Flow

rates

tr W N H

15 9.88 1.80 482.046 0.830

20 8.18 1.35 587.372 0.681

25 6.87 1.05 684.932 0.584

30 5.54 0.8 767.754 0.521

40 4.19 0.65 665.557 0.601

15 20 25 30 35 40

0.60

0.65

0.70

0.75

0.80

0.85

H (

mm

)

Flow Rate (ml min-1)

n-Hexane

Fig. 4 The optimum flow rate using of the studied column dimensions using

n-hexane as a prob.



49

15 20 25 30 35 40

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

H (

mm

)

Flow Rate (ml min-1)

Benzene

Fig. 5 The optimum flow rate using of the studied column dimensions using

benzene as a prob.

From the results given in Tables 1and 2 and shown the Figures 4 and 5, it has

been found that the optimum flow rate at the minimum height equivalent to

theoretical plates was 30 ml min-1 for the probes n-hexane and benzene at all

the studied chromatographic columns.

S/R), but in ordinary gas chromatography the first term is negligible

and can be ignored (138).

The free energies of adsorption were calculated by the formula (139)

:

ΔG = -2.3 RT ln Vg (19)

where, R is the gas constant, T is the absolute temperature (K) and Vg is the

specific retention volume.

Figures (9-11) show the relationships between lnVg and 1000/T for the

studied polymers as stationary phases in gas chro



50

4.2.Rohrschneiders scheme

Characterization and evaluation the behavior of the studied polymer

stationary phases according to Rohrshneiders method is based on the

determination of retention indices for five solutes namely benzene, methyl

ethylketone, nitromethane and pyridine (X, Y, Z, U and S respectively)

belonging to different structural classes on the studied phases as well as on

squalane, chosen as the reference standard phase for this calculation.

The five Rohrschneider constants for the studied stationary phases are

obtained by calculating the differences observed for each of the substances

tested between their retention index on sqalane (Isqalane) and that corresponding

to the stationary phase being studied (Iphase)

Rohrschneider constants (∆I) = Iphase - Isqalane

The polarity of the studied chromatographic columns depends according to

Rohrschneider conceptions, not only on the stationary phase but also on the

solute to be analyzed. Thus each solute from the five selected solutes for this

characterization scheme may refer to a certain type of interaction between the

polymer stationary phase and solutes.

Generally, The Rohrschneider constants which are related to molecular

structures, allow an appreciation of the interactive forces between stationary

phase and solute as a function of compound class.

The orderly classification of polymer stationary phases with numerical data is

very valuable for comparison and selection of columns for particular type of

analysis. The sum of the five calculated values has been used to define the

overall polarity of the phase under test study.



51

In the present work, the polymers studied were characterized using

Rohrschneiders method for more investigation concerning the correlation

between the molecular structure and the polarities of these polymers towards

each type of solutes. Rohrschneider constants of the studied three polymers

are given in Table 3. .

The data obtained were determined at the same conditions (oven temperature

and mobile phase flow rate) on the studied polymers as stationary phases.

From the obtained data the following observations can be outlined:

The OV-101 has the lowest values of the Rohrschneider constants among the

studied polymers. These low values suggest that the dimethyl polysiloxane

stationary phase poorly retains the compounds that contain the corresponding

organic functions. This leads to low selectivity for this type of compound.

The overall polarity of the OV-101 stationary phase has the lowest value

reflecting low interactive forces between it and Rohrschneider solutes as a

function of compound class.

In case of OV-275 all its Rohrschneider constants have highest values than

those of the rest studied polymers. Among the Rohrschneider standards,

nitromethane and Pyridine exhibit the highest values compared with the other

five standards, followed by methyl ethyl ketone and benzene. Reflecting the

good ability of the OV-275 stationary phase to separate nitro compounds, the

aromatics including nitrogen compounds, ketones and aromatic hydrocarbons.

The high value of the overall polarity of OV-275 stationary phase indicating

the high interactive forces between it and Rohrschneider solutes as a function

of their families. Also, suggesting that the caynoprpyle stationary phase

strongly retains the compounds that contain the corresponding organic

functions. This leads to an improve selectivity for this type of compound.

The Rohrschneider constants of the poly ethylene glycol 20,000 (Carbowax-

20M) exhibit moderate values below OV-275 and above OV-101 indicating

their moderate polarity. These values decrease in the following order:



52

Pyridine, nitromethane, benzene, methyl ethyl ketone and Ethanol reflecting

the order of its efficiency of gas chromatographic separation toward the

corresponding families of theses constants.

Table 3 The difference in retention index of Rohrschneider constants of the

studied polymers

OV-275 Carbowax-

20M

OV-101 Squalane Symbole Compounds

819 722 16 0 X Benzene

1806 917 55 0 Y nitromethane

1709 1019 52 0 Z Pyridine

1011 624 39 0 U MEK

807 623 54 0 S Ethanol

6152 3905 216 0 Sum ∆I Summation

Table 4 The retention index for the five reference compounds given above (X,

Y, Z, U and S) on squalane are:-

I X Y Z U S

squalane 648 550 869 550 483



53

4.3.Selectivity index

Selectivity can be expressed in terms of uniformity criterion Δ (78) it is used

for estimating the degree of separation of multi component mixture; the

uniformity criterion was given according to the following equation:

t

τKnΔ

effk= (10)

where,k

n is the number of peaks on the chromatogram,τ is the base width of

the narrowest peak,eff

K is the separation factor for the worst separated pair of

components, and t is the duration of analysis.

Table 5 shows the calculated criterion for model systems comprising saturated

hydrocarbons, aromatic hydrocarbons and normal alcohols. It has been found

generally that all studied polymer stationary phases differ in their efficiencies

of separation according to the separated solutes. Saturated hydrocarbons can

be separated by the three studied polymers but in different degree of

efficiency, the dimethyl poly siloxane is the most efficient stationary phase as

illustrated from its criterion may be due to their low polarity which is

compatible with the polarity of saturated hydrocarbons.

For separation of aromatic hydrocarbons, the most polar caynopropyl poly

siloxane has the highest criterion number(Δ = 802 ). So it is the most

efficient stationary phase for these purpose followed by the polyethylene

glycol 20,000 (Δ = 702 ).

For separation of normal alcohols, only caynopropyl poly siloxane stationary

phase is success in separation of them giving peak sharp and prevent tailing



54

and broadening may be due to its high polarity is compatible with that of

normal alcohols.

This result agrees well and confirms the previous data obtained on the polarity

measurements of the studied polymers.

Therefore, the higher the polarity of the polymer, the higher the value of its

selectivity index. This means that the most polar stationary phase exhibits

high uniformity criterion number reflecting its high efficiency of separation of

polar compounds.

Table 5 Uniformity criterion Δ for different mixtures depending upon the

studied polymers as stationary phases

Families OV-101 Carbowax-20M OV-275

Paraffines 0.959 0.693 0.574

Aromatics 0.475 0.702 0.802

Alcohols 0.409 0.518 0.890



55

CHAPTER 5. Thermodynamic parameters

Gas chromatography affords a general technique for studying the

interactions of gases with solids or liquids and determination of the

thermodynamic parameters of the interaction: enthalpy (∆H), free energy

(∆G) and entropy (∆S).

Thermodynamic parameters of normal hexane, benzene and ethanol as

representative paraffin, aromatic hydrocarbons and alcohols respectively,

were calculated and listed in Table (6). The retention, selectivity and

interaction of the studied stationary phases are discussed in view of these

various thermodynamic parameters.

The thermodynamic functions (∆H and ∆S) can be estimated by

measuring the specific retention volume, Vg of probes at different

temperatures using the following equation (134-136):

lnVg = ln(R trs) + ∆S/R – ∆H/R . 1/T (21)

where, R is the gas constant (8.314 Jmol-1K-1), ns (mole) is the total amount of

solute in the adsorbed state and T is the absolute temperature (K). Vg can be

calculated from the following relation (137):

Vg= (tr-tm) FaT/Ta j (22)

where, tr is the probe retention time, tm dead time (min) is the retention time

of unretained solute (methane), Fa is the flow-rate (ml/min) measured at

column outlet and at ambient temperature Ta, and j is the compressibility

correction factor.

By determining Vg at different temperatures, one can calculate ΔH from

the slope and ΔS value from the intercept of the plot of lnVg versus 1/T. The

intercept is equal to ln(Rtrs) + (Δ matography using n-hexane, benzene and

ethanol. These plots give linear relationship. ∆H & ∆S could be derived from



56

the slope and intercept of the straight line, The thermodynamic data of various

stationary phases are summarized in Table (6). ∆G was calculated at different

temperature (from 343K to 393K) and given in Table 7.

The advantages of the gas chromatographic technique compared to classical

static equilibrium methods for obtaining thermodynamic data for mixing

polymers and solutes lies mainly in the speed of obtaining data. The GC

separation of the studied probes could be illustrated by considering the

thermodynamic parameters of these solutes.

It is evident from the data that negative ∆H and ∆S values for all

studied polymers using normal hexane increase in the sequence

Dimethylpolysiloxane > polyethylene glycol > cyanopropylpolysiloxane. The

more negative the ∆H and ∆S values the greater the interaction between the

probe and stationary phase. A similar study was also done by Inel et al(140),

they evaluated the thermodynamic parameters (∆H, ∆S and ∆G) of some

probes, each representing a class of organic compounds (n-hexane,

cyclohexane and benzene) on 4A and 13X Zeolites, it was found that

thermodynamic parameter increase in the sequence cyclo-hexane < n-hexane

< Benzene. Also, Bilgic and Askin(141), obtained the same result for activated

alumina stationary phase. Although, n-heptane and methylcyclohexane

interact non-specifically, but n-heptane interacts more intensively than

methylcyclohexane, this result indicates a better contact of an open chain

structure molecules with the surface of stationary phases (142).

The entropy values of normal hexane at different positions using dimethyl

polysiloxane show relatively minute differences in negative values. This is

reflected on the GC separation. Since the polymethylsiloxane is considered to

be non-polar or of relatively negligible polarity, the separation appears to be

depending on probes’ boiling points rather than their polarity.



57

The negative ∆H and ∆S values for all studied stationary phases using

benzene as a probe increase in the sequence cyanopropylpolysiloxane >

polyethylene glycol > Dimethylpolysiloxane. This reflects the greater

interaction of aromatics with the cyano and hydroxyl groups of

cyanopropylpolysiloxane and polyethylene glycol respectively exhibiting

good separation. The stronger elution of benzene on cyanopropylpolysiloxane

than n-hexane most probably attributed to the contribution of the specific

interaction between the cyano group and the π-electrons of the benzene ring.

So, cyanopropyl polysiloxane can interact specifically with the three π-

electrons of benzene molecules. Therefore, polyethylene glycol interacts with

aromatics but with less degree compared with the high polar cyanopropyl

polysiloxane stationary phase giving good separation of aromatics but with

long duration time of analysis. This may be due to the nearly matched polarity

of the polymer and benzene molecule.

The negative ΔH and ∆S values for all studied polymers using ethanol as a

probe increase in the sequence cyanopropylpolysiloxane > polyethylene

glycol > Dimethylpolysiloxane. The high negative ΔH value of

cyanopropylpolysiloxane with ethanol reflecting the higher energies required

for the interaction between them, reflecting on the high efficiency of

separation of alcohols as shown in Figure 12. On the opposite,

dimethylpolysiloxane has bad separation of alcohols due to its low negative

∆H value and low polarity, this behavior agrees well with the gas

chromatographic separation.

The free energy ΔG of the substances is the most important thermodynamic

characteristic of the reaction. Table 7 shows the values of the partial molar

excess free energy ΔG of the selected solutes in the studied stationary phases

at different temperatures from 70oC to 120oC. Generally, the free energy

reveals that free energy differences exist in minute values, and the free energy



58

shows an increase as a function of temperature for all solutes on the studied

stationary phases.

Table 6. Thermodynamic parameters of the studied polymers as stationary

phases using n-hexane, benzene and ethanol as probs

Columns Solutes -ΔH KJmol-1

-ΔS Jmol-1

degree-1

OV-101

n-Hexane 319.498 822.735 Benzene 135.246 318.347 Ethanol 35.407 47.793

Carbowax-

20M


OV-275




59

Figure 6. Plot of Ln retention volume (Vg) of n-hexane versus reciprocal of temperature for dimethylpolysiloxane (OV-101) as stationary phase

2.80 2.85 2.90 2.95 3.00 3.05 3.10 3.15 3.20

4.25

4.30

4.35

4.40

4.45

4.50

4.55

4.60

1000/TK

n-Hexane on OV-101

Ln

vg

2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95

2.6

2.8

3.0

3.2

3.4

3.6

1000/TK

n-Hexane on Carbowax-20M

Ln

Vg

2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

1000/TK

n-Hexane on OV-275

Ln

Vg



60

Figure 7. Plots of lnVg versus 1000/T of n-hexane, benzene and ethanol

for polyehylene glycol (carbowax-20M) as stationary phase.

2.80 2.85 2.90 2.95 3.00 3.05 3.10 3.15 3.20

3.3

3.4

3.5

3.6

3.7

3.8

3.9

1000/TK

Benzene on OV-101

Ln

Vg

2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95

5.16

5.18

5.20

5.22

5.24

5.26

5.28

5.30

5.32

5.34

1000/TK

Benzene on Carbowax-20M

Ln

Vg

2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95 3.6

3.7

3.8

3.9

4.0

4.1

4.2

4.3

4.4

1000/TK

Benzene on OV-275

Ln

Vg



61

Figure 8. Plots of lnVg versus 1000/T of n-hexane, benzene and ethanol for

cyanopropyle polysiloxane (OV-275) as stationary phase.

2.80 2.85 2.90 2.95 3.00 3.05 3.10 3.15 3.20

2.5

2.6

2.7

2.8

2.9

3.0

3.1

3.2

3.3

3.4

1000/TK

Etnanol on OV-101

Ln

Vg

2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95

4.50

4.55

4.60

4.65

4.70

4.75

4.80

4.85

4.90

1000/TK

Ethanol on Carbowax-20M

Ln

Vg

2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95 5.12

5.14

5.16

5.18

5.20

5.22

5.24

5.26

5.28

5.30

5.32

1000/TK

Ethanol on OV-275

Ln

Vg



62

Table 7. ΔG (KJmol-1) for n-hexane, benzene and ethanol on the studied

polymer stationary phases

Columns

Solutes 70 oC 80 oC

90 oC

100 oC

110 oC 120

oC

OV-101

n-Hexane

28.790 29.071

29.490

29.641

29.687 29.780

Benzene

22.772 22.868

22.980

23.016

23.090 23.161

Ethanol

18.521 18.536

18.638

18.679

18.717 18.774

Carbowax-

20M

n-Hexane

23.580 23.964

24.048

24.293

24.326 24.397

Benzene

34.964 35.651

36.500

37.314

38.115 38.907

Ethanol

31.836 32.417

32.885

33.206

33.543 33.854

OV-275

n-Hexane

21.460 21.701

21.860

21.910

21.925 22.024

Benzene

28.692 28.776

28.961

29.013

29.163 29.284

Ethanol

34.768 35.508

36.325

37.079

37.850 38.640



63

CHAPTER 6. Separation mechanism

In order to elucidate the mechanism of gas chromatographic separation,

via either adsorption or partition mechanisms, ∆H of the studied polymer

stationary phases has been plotted versus ∆S (143) as shown in Fig. (9). This

plot was done using three solutes of different polarities, namely n-hexane;

benzene and ethanol. In all cases, the solutes exhibit the phenomenon of

enthalpy-entropy compensation and a linear dependence between ∆H and ∆S

is fulfilled. This indicates the predominance of one mechanism, named

partition mechanism of the selected solutes on the nonpolar, moderate polar

and high polar stationary phases.

In order to clarify the mechanism of gas chromatographic separation of non-

polar molecules on the studied stationary phases, continuing the ideas

presented by Gurevich and Roshchina (144), test molecules which have the

same carbon number n-hexanee and benzene were chosen. It should be

recalled here that the aromatic hydrocarbons adsorb more weakly on the

surface of dimethyl polysiloxane stationary phase than their linear analogues

because of the smaller number of units directly interacting with the surface of

polymer.



64

Figure (9): Plots of ∆S Vs ∆H for indicated solutes of different

polarity on the studied polymers as stationary phasesin gas

chromatography

60 80 100 120 140 160 180 200 220 240 260

100

200

300

400

500

600

700

-Δ H KJmol -1

-1

OV-275

---- Δ S Jmol S Jmol S Jmol S Jmol ----1 1 1 1 degredegredegredegre----1111

50 100 150 200 250 300 100

200

300

400

500

600

700

-Δ H KJmol-1

-1

Carbowax-20M


0 50 100 150 200 250 300 350 0

100 200 300 400 500 600 700 800 900

-Δ H KJmol -

1

OV-101




65

CHAPTER 7. Chromatographic Application

Study of the physico-chemical characteristics of the paraent and

modified stationary phases gives information about the behavior towards their

applications. The chromatographic evaluation of the phases was done mainly

through their employment in the analysis of selected mixtures of different

polarities. The substances used for this purpose are, n-Alkanets, Aromatic,

Alcohols, Ketones, Nitro compounds, Ethers, Esters and Poly aromatics.

Efficiency of polymers as stationary phases for gas

chromatographic separation

As can be seen from the literature survey, polymers are commonly used as

stationary phases for gas chromatographic analysis of hydrocarbons families

such as paraffinic hydrocarbons, aromatics hydrocarbons, alcohols and esters.

This is due to the differentiation degrees of their polarities and the high value

of their constants on Rohrschneider,s scale.

The efficiency of gas chromatographic separation for the studied polymer

samples can be evaluated in terms of resolution and separation factor.

Resolution could be considered as an important chromatographic parameter, it

is used as a quantitative measure of the degree of separation between two

adjacent peaks.

R = (2Δt / W1+W2) (15)

R = degree of separation between two adjacent peaks

Δt = the time interval between peaks

W1 = width of the first peak at the base

W2 = width of the second peak at the base



66

Separation factor is the ratio of the adjusted retention times of two

components; it measures only the separation of the centers of mass of the two

peaks:

α α α α = tr(2) / tr(1) (13)

Results of these two parameters for the studied polymer stationary phases

using the selective solutes are summarized in Tables (8-10) and portrayed in

Figures (10-12). It has been found that gas chromatographic separations using

polymers as stationary phases have different separation efficiencies.

7.1. Separation of normal paraffinic hydrocarbons

The separation of normal paraffinic mixture (C6- C10) is capable of testing

specific interaction between solute and studied stationary phases. The

resolution and separation factor of paraffinic mixture were given in Table (8),

and shown in Fig. (10). By definition the separation factor is greater than

unity because species 1 elutes faster than species 2. it has been found that αααα

values of paraffines on all studied polymers can separate paraffines but with

different degrees of separation efficiencies.

They depict the separation of n-Alkanes mixture on the selected

columns. It has been found that; dimethyl polysiloxane stationary phase gives

suitable surface for eluting paraffines as the previous works (145, 146). The

resolution values of normal paraffines on OV 101 is the highest followed by

carbowax-20M and OV-275 stationary phases as given in Table (8). So, the

mixture of n-alkanes can be separated with high efficiency by the order: OV-

101, carbowax-20M and OV-275 columns which compatible with their

unformaty criterion values (Δ = 0.959, 0.693 and 0.574) respectively. The

dimethyl polysiloxane (OV-101) exhibits the most efficient separation of n-

alkanes with sharp and symmetrical peaks. Unfortunately, OV-275 column



67

has bad separation of paraffin, this may be related to its high polarity and its

low uniformity criterion value toward paraffines. With respect to polyethylene

glycol (carbowax-20M), paraffins can separated, but with slightly lower

efficiency of separation (Δ = 0.693) as compared with the OV-101, this is due

to its moderate polarity and selectivity values toward paraffinic hydrocarbons.

It was postulated that, the elution was directly correlated not only to

stationary phases polarity but also to polarity of solutes. Accordingly, the

polarity and selectivity are the main factors affecting the separation.

Table 8. Resolution and Separation factor of paraffins on the studied polymers

as stationary phases

No. Carbon Number

OV-101 Carbowax-20M OV-275

α R α R α R 1 C6-C7 1.3086 1.450 1.1204 1.05 1.549 0.284 2 C7-C8 1.3091 1.521 1.1951 1.12 1.593 0.329 3 C8-C9 1.2567 1.568 1.2689 1.06 1.452 0.293 4 C9-C10 1.2112 1.506 1.3101 1.16 1.342 0.267



68

OV-275

Carbwax-20M

OV-101

Fig.10. Gas chromatographic Separation of normal paraffins mixture from

n-pentane to n- hexane on the studied polymer stationary phases at 50OC

to 150OC at rate 10

oC min

-1anf flow rate 30 ml min

1.



69

7.2.Separation of aromatic hydrocarbons

The aromatic hydrocarbons (Benzene, toluene, ethylbenzene,

propylbenzene and butylbenzene) are important industrial chemicals. They

generally coexist in the catalytic reforming process in aromatic production.

Furthermore they were used as probes to investigateπ-complex formations.

The resolution and separation factor of aromatic hydrocarbons were given in

Table (9), and the gas chromatographic separation was shown in Fig. (11).

They depict the performance of the studied polymers on the separation of

aromatic mixture. Cyanoprpyl polysiloxane is the best studied stationary

phase for separating aromatic hydrocarbons obtaining good separation with

high resolution value. This could be directly linked with its polarity and

selectivity which elute specifically the molecules containing π-electrons.

Figure (11) illustrates the chromatographic separation of aromatic

hydrocarbons mixture on OV-101, Carbowax-20M and OV-275. The low

polarity and low selectivity of the polymer OV-101 as liquid stationary phase

make its surface unsuitable for separating aromatics, this bad separation was

evidenced with its lower uniformity criterion value (Δ = 0.475).

Consequently, the separation of aromatic hydrocarbons were improved on the

polymer OV-275 which exhibit high resolution values along each two

consecutive aromatic components in the mixture. This result was evidenced

with its high selectivity value towards aromatics (Δ = 0.890). Moreover, the

separation of aromatics on the carbowax-20M was achieved giving good

separation but accompanied by long duration time of analysis. this was

evidenced with the moderate uniformity criterion values (Δ= 0.702).

Also, good separation of aromatics was achieved on cyanoprpyle

polysiloxane having high separation factor, high resolution and high

uniformity criterion toward aromatics. this suggested that the polarity,



70

selectivity, resolution and unformaty criterion are the main factors affecting

the efficiency of separation.

On the other hand, reasonable separation of aromatic hydrocarbons was

achieved using polyethyleneglycol- 20M which has moderate polarity,

moderate resolution values and moderate uniformity criterion value. This was

reflected on the chromatogram shown in fig 11. having long duration time of

analysis.

Table 9 Resolution and Separation factor of aromatic hydrocarbons (benzene,

toluene, ethylbenzene, propylbenzene and butylbenzene) on the

studied polymers as stationary phases.

No. Carbon

Number OV-101 Carbowax-20M OV-275

α R α R α R 1 C6-C7 1.642 0.440 1.240 0.950 1.266 1.221 2 C7-C8 1.516 0.415 1.178 0.733 1.204 1.236 3 C8-C9 1.333 0.339 1.154 0.623 1.157 1.352 4 C9-C10 1.308 0.374 1.229 0.804 1.186 1.245



71

0V-275

Carbowax-20M

0V-101

Fig.11 Gas chromatographic Separation of aromatic hydrocarbons (1-

benzene, 2-toluene, 3-ethylbenzene, 4-propylbenzene and 5-

butylbenzene) on the studied polymer stationary phases at optimum

conditions (50oC; 10

oC/min to 150

oC)



72

Separation of alcohols .3.7

Alcohols (methanol, ethanol, propanol, butanol, pentanol and hexanol)

are the most polar organic compounds which interact with electron-donor

groups of the surface with a hydrogen bond evolving, reflecting on their bad

resolution and their highly tailing peaks.

The resolution and separation factor of alcohols on the studied polymer

stationary phases were given in Table (10). The gas chromatographic

separation of alcohols was shown in Fig. (12). The resolution of dimethyl

polysiloxane exhibits the lowest value compared with other studied polymers.

So, OV-101 can elute alcohols with unsymmetrical peaks and much skewed

peaks with a long tail (Fig. 12 ), due to its low polarity as discussed above.

This data was matched with its low criterion value and its low negativity of

enthalpy toward alcohols.

The resolution of OV-275 using alcohols as props exhibits the highest

value than the other polymers. Also, it has the highest selectivity and highest

negativity of enthalpy than the other studied polymer stationary phases.

These data reflecting on their separation of alcohols as shown in Fig 12.

Generally, the high polar cyanopropyl polysiloxane stationary phase

successes for eluting alcohols because of the similarity of their high polarities

coming from their hydroxyl groups. Thus, the better separation of light

alcohols till hexaneol was achieved on cyanopropyl polysiloxane with sharp,

symmetrical peaks and through an acceptable duration time of analysis.

The Resolution of alcohols on carbowax-20M column has values

lower than OV-275 and higher than OV-101 (Table 10 ). This reflected on the

separation of alcohols giving peaks having some unsymmetrical and some

tailing. This result was matched with polarity, selectivity and negativity of

enthalpy of carbowax-20M stationary phase using alcohols as probes.



73

From Fig. (11), the OV-275 was preferred than Carbowax-20M for

separating alcohols may be due to the high polarity of the polymer OV-275

resulting from cyanopropyle groups. This was evidenced by their differences

in uniformity criterion values.

Also alcohols are severely tailed on dimethylpolysiloxane, this

potentially due to the interaction of all oxygen groups on the surface of

stationary phase with the hydroxyl groups of alcohols producing hydrogen

bonds which is the reason of tailing peaks and bad separation. This result was

evidenced by its lower polarity value and lower negativity of enthalpy.

Table 10: Resolution and Separation factor of alcohols (1-methanol, 2-

ethanol, 3-propanol, 4-butanol, 5-pentanol) on the studied polymers

as stationary phases.

No. Carbon Number

OV-101 Carbowax-20M OV-275

α R α R α R 1 C1-C2 1.108 0.019 0.000 0.000 1.084 1.320 2 C2-C3 1.348 0.056 1.195 0.511 1.209 1.520 3 C3-C4 1.562 0.101 1.208 0.622 1.190 1.621 4 C4-C5 1.599 0.124 1.167 0.649 1.192 1.652 5 C5-C6 1.464 0.125 1.131 0.566 1.216 1.596



74

OV-275

Carbowax-20M

OV-101

Fig.12 Gas chromatographic Separation of alcohols (1-methanol, 2-

ethanol, 3-propanol, 4-butanol, 5-pentanol) on the studied polymer

stationary phases at optimum conditions (100oC; 5

oC/min to 220

oC)



75

Conclusion

In the present study, polymers gained a great attention since such

material can be easily prepared and their different polarities offer the

additional advantages of improving separation and reducing peak asymmetry.

Gas chromatographic characterization of the studied polymer was

followed by polarity assessment, selectivity and determination of the

thermodynamic parameters of the interaction of compounds as representatives

of their corresponding families with the investigated stationary phases.

Rohrschneider scheme of polarity helps to identified and arranged

polymer stationary phases according to their polarities. Paraffinic

hydrocarbons show higher selectivity values on the dimethyl polysiloxane

stationary phase than the cyanopropyl polysiloxane and polyethyleneglycol.

The opposite occurs using alcohols and aromatic hydrocarbons, OV 275

followed by carbowax- 20M exhibit higher selectivity than OV-101, these

data compatible with their polarities and reflecting on their gas

chromatographic separations.

Thermodynamic parameters assisted to study interaction between the

stationary phases and solutes of different polarities and elucidate the

separation mechanism. The high negative enthalpy and entropy values of

normal hexane in case of dimethyl polysiloxane reflect the high interaction

between normal paraffines and the nonpolar polysiloxane stationary phase.

The negative values of enthalpy and entropy on dimethyl polysiloxane

increase in the order normal paraffinic hydrocarbons > aromatic hydrocarbons

> alcohols. This indicates that the interactions on the non polar stationary

phase depends firstly on its polarity rather than on polarity of solutes. Also,

the GC separation of the studied probes on the dimethyl polysiloxane depends

on their boiling point rather than their polarity.



76

There is a slight difference in the free energy of the mentioned probes on

the studied stationary phases and ΔG is a function of temperature.

Using the three studied polymer stationary phases, the relation between

ΔH and ΔS of all studied probes is a linear relationship. This indicates the

predominance of the partition mechanism.

Chromatographic evaluation of the studied polymers as stationary phases

in gas chromatography was done mainly through their use in the analysis of

selected mixtures of different polarities including n-alkanes, aromatics and

alcohols. The efficiency of separation for the studied samples was evaluated

in terms of resolution, separation factor and uniformity criterion.



77

Acknowledgment

I would like to express deepest sincere gratitude to my Egyptian Petroleum

Research Institute (EPRI) the largest scientific organization and thanks to the

director of EPRI: Prof. Dr. Ahmed El-Sabagh and the vice director Prof.

Dr. Yasser Mostafa for their addition administratively and scientifically in

our institute.

Many thanks to my Prof. Dr. Abd-Elaziz Mostafa El-Fadly the more

human has a good impact in my life and for his continuous guidance, advice

and support, My prayers to God that keeps him and give him long old with

good health.

The author expresses his sincere thanks to Prof. Dr. Matar Mesehal Al-

Esaimi prof. of Physical Chemistry, Chemistry Department, Faculty of

Science, Umm AL-Qura University, KSA for his guidance advice and

invaluable help to Provide a gas chromatographic device throughout the work.

Also, It is a pleasant duty of the author to express his gratitude to all of his

colleagues at the chemical division in Umm Elkora University



78

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ABOUT AUTHORS

Prof. Dr. Ashraf Yehia El-naggar

Prof. of Analytical and Petroleum Chemistry. Dr. Ashraf Yehia El-naggar is erudite scholar and seasoned chemist scientist. He obtained his M. Sc degree and Ph. D. degree in analytical chemistry from Cairo university and Mansoura university, Egypt respectively. He obtained his Ass. Prof. and Prof. degrees in analytical and petroleum chemistry from Egyptian Petroleum Research Institute (EPRI). He was a head of chromatographic Department and vice director of Central Analytical Lab., EPRI. His area of research interests include petroleum chemistry, chromatographic science, biochemical degradation of wastes. fuels and recycle of wastes to useful compounds. Dr A. Y. El-naggar has several teaching and research experience. Now he is vice director of chemistry Department, Science Faculty, Taif University, KSA. He has published over ninety research articles and reviews in peer-reviewed journals.

Dr.Adil A. Gobouri Dr. Adil A. Gobouri has PhD in Organic Chemistry, The University of Manchester, United Kingdom. Assistant Professor of Organic Chemistry and Head of Chemistry Department, Science Faculty, Taif University, Kingdom of Saudi Arabia. His research interests in Physical Organic Chemistry, Dendrimers, Heterocyclic and Click Chemistry.

Eid Hamed Al–osaimi

Eid Hamed Al–osaimi has MSc in analytical chemistry from Chemistry Department Science Faculty, Taif /University, KSA. He is a teacher in Ministry of Education, Makkah, KSA. He worked now in Ph. D. in the same field.

retention behavior of polymer solute systems via … · b) retention index system it was not until...

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