X-Ray Diffraction (XRD)

Rakesh Pani
 Introduction
 X-rays were discovered by Wilhelm Roentgen who called them x - rays
because the nature at first was unknown so, x-rays are also called Roentgen
rays. X-ray diffraction in crystals was discovered by Max von Laue. The
wavelength range is 0.01 to about 10 nm.
Wilhelm Röntgen

 X-rays are short wave length electromagnetic radiations produced by the deceleration
of high energy electrons or by electronic transitions of electrons in the inner orbital of
atoms

The penetrating power of x-rays depends on energy also, there are two types of x-rays.
i) Hard x-rays: which have high frequency and have more energy.
ii) soft x-rays: which have less penetrating and have low energy.
2
Max von Laue
 Introduction
X-ray Diffraction (XRD) is a non-contact and non-destructive technique used to understand the
crystalline phases, different polymeric forms and the structural properties of the materials

X – ray diffraction
“ Every crystalline substance gives a pattern; the same substance always gives the same
pattern; and in a mixture of substances each produces its pattern independently of the others”

The X-ray diffraction pattern of a pure substance is, therefore, like a fingerprint of the substance.
It is based on the scattering of x-rays by crystals.
Definition
The atomic planes of a crystal cause an incident beam of X- rays to interfere with one another
as they leave the crystal. The phenomenon is called X- ray diffraction.

3
 Timeline

 1665: Diffraction effects observed by Italian mathematician Francesco Maria Grimaldi

 1868: X-rays Discovered by German Scientist Röntgen

 1901: First Nobel prize in Physics to Röntgen

 1912: Discovery of X-ray Diffraction by Crystals: von Laue

 1912: Bragg’s Discovery

4
 Why XRD?

 Measure the average spacing's between layers or rows of atoms

 Determine the orientation of a singlecrystal

 Find the crystal structure of an unknown material

 Measure the size, shape and internal stress of small crystalline regions

5
 What is Diffraction ?
A diffracted beam may be defined as a beam composed of a large number of scattered rays
mutually reinforcing each other

Scattering
Interaction with a single
particle

Diffraction
Interaction with a
crystal

6
 What is X-ray Diffraction ?

X-ray diffraction is based on constructive interference of monochromatic x-rays and a crystalline

sample. These x-rays are generated by a cathode ray tube, filtered to produce monochromatic
radiation, collimated to concentrate and directed towards the sample. The interaction of incident
rays with the sample produces constructive interference when conditions satisfy Bragg’s law.
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 Bragg’s Law

 Constructive
interference
X-ray 2 occurs only
when Sir William
n λ = AB + BC Henry Bragg
AB=BC
n λ = 2AB Sin θ = AB/d
AB=d sin θ
n λ =2d sin θ
λ = 2 d hklsin θhkl
Sir Lawrence Bragg

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 Order of Diffraction
Rewrite Bragg’s law λ=2 sinθ d/n

A reflection of any order as a first order Diffraction from planes, real or fictitious,
spaced at a distance 1/n of the previous spacing.

Set d’= d/n

An nth order reflection from (hkl) planes of spacing d may be considered as a first
order Diffraction from the (nh nk nl) plane of spacing d’= d/n

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 Basics of Crystallography
 The atoms are arranged in a regular pattern, and there is as
smallest volume element that by repetition in three dimensions
describes the crystal. This smallest volume element is called a
unit cell.

 Crystals consist of planes of atoms that are spaced a distance d

apart, but can be resolved into many atomic planes, each with a
different d spacing.

Lattice  The dimensions of the unit cell is described by three axes : a, b, c

and the angles between them α, β , and γ are the lattice constants
which can be determined by XRD.

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 Miller Indices
 Crystal structures are made up of a series of planes of
atoms in which each plane is spaced with a distance d with
each other. But various atomic planes in a crystal can be
resolved with different d- spacing. For distinguishing
different planes there is a coordinate system introduced by
William Hallowes Miller called Miller indices (i.e., h, k, l).
William Hallowes
Miller
 Miller indices-the reciprocals of the fractional intercepts
which the plane makes with crystallographic axes.
 Reciprocals are taken to avoid ∞ in the indices.

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Instrumentation
 Production of x-rays (Source Of X-Rays)
 Collimator
 Monochromator
a. Filter
b. Crystal Monochromator
 Detectors
a.Photographic methods
b.Counter methods

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Production of x-rays
 X-rays are produced whenever a
charged particles are accelerated.
 In XRD, X-rays are generated
when high velocity electrons
impinge on a metal target.
 A source of electrons – hot W
filament, a high accelerating
voltage between the cathode (W)
and the anode and a metal target,
Cu, Al, Mo, Mg.
 The anode is a water-cooled
block of Cu containing desired
target metal.

13
Production of x-rays

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 Collimator

 In order to get a narrow, focused beam of x-rays, the x- rays generated

by the target material are allowed to pass through a closely packed
metal plates separated by a small gap.

 The collimator absorbs all the x-rays except the

narrow beam that passes between the gap.

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Monochromator
A monochromator is an
optical device that transmits
a mechanically selectable
narrow band of wavelengths
of light or other radiation
chosen from a wider range of
wavelengths available at the
input. The name is from the
Greek roots mono-, "single",
and chroma, "colour", and
the Latin suffix -ator,
denoting an agent.

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Types Of Monochromators
Monochromatization can be broadly divided into two,
1.Interference Filters
2.Crystal Monochromator

Crystal Monochromators can be again divided into two

i) Flat crystal Monochromator

ii)Curved crystal Monochromator

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 Interference Filters
 X-ray beam may be partly monochromatized by insertion of
a suitable filter.

 A filter is a window of material that absorbs undesirable

radiation but allows the radiation of required wavelength
to pass.

 Interference filters contain several optical layers

deposited on a glass substrate or transparent quartz. The
specific performance characteristics of the filter are
determined by the thickness of the optical layers.

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 Crystal monochromators
Crystal monochromators are made up of suitable crystalline material positioned in the x-ray beam so that the
angle of reflecting planes satisfied the Bragg’s equation for the required wavelength the beam is split up into
component wavelengths crystals used in monochromators are made up of materials like NaCl, lithium
fluoride , quartz etc. Pyrolytic graphite can be used for broad band and silicon for narrow band.

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 Detectors
X-ray detectors are devices used to measure the
flux, spatial distribution, spectrum, and other
properties of X-rays.

Detectors can be divided into two major

categories: Imaging detectors, such
as photographic plates and X-ray film
(photographic film), now mostly replaced by
various digitizing devices like image plates or flat Schematic diagram of a diffractometer system
panel detectors) and dose measurement devices
called counter methods.

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 Types of detectors
The x-ray intensities can be measured and recorded either by

1)Photographic methods
2)Counter methods

a) Geiger - Muller tube counter

b) Proportional counter
c) Scintillation detector
d) Semi conductor detectors

Both these types of methods depends upon ability of x-rays to ionize matter and differ only in the
subsequent fate of electrons produced by the ionizing process.

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 Photographic method
 To record the position and intensity of x-ray beam a plane or cylindrical film is used.
 The film after exposing to x-ray is developed
 The blackening of the developed film is expressed in terms of density units D given by
D = log I₀/I, I₀-incident intensity
I - Transmitted intensity
D - Total energy that causes blackening of the film D is
measured by densitometer
The photographic method is mainly used in diffraction
studies since it reveals the entire diffraction pattern on a single film .
 Disadvantage : time consuming and uses exposure of several hours

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 A) Geiger - Muller tube counter
Geiger tube is filled with inert gas likeargon
Central wire anode is maintained at a positive potential of 800 to 1500V .

 The electron is accelerated by the potential gradient and causes the ionization of
large number of argon atoms, resulting in the production of avalanche of electrons
that are travelling towards central anode

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 B) Proportional counter
 Construction is similar to Geiger tubecounter

 Proportional counter is filled with heavier gas

like xenon and krypton

 Heavier gas is preferred because it is easilyionized

 Operated at a voltage below the Geigerplateau

 The dead time is very short (~ 0.2μs), it can beused

to count high rates without significant error.

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 C) Scintillation detector:

 In a scintillation detector there is large sodium iodide crystal activated with a

small amount of thallium
 When x-ray is incident upon crystal , the pulses of visible light are emitted which can be
detected by a photo multiplier tube
 Useful for measuring x-ray of short wavelength

Crystals used in scintillation detectors include sodium iodide ,anthracene, napthalene and
p-terphenol
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 D) Semi-conductor detectors
 When x-ray falls on silicon lithium drifted detector an electron (-e) and
a hole (+e)
 Pure silicon made up with thin film of lithium metal plated onto one end
 Under the influence of voltage electrons moves towards +ve charge and
holes towards –ve
 Voltage generated is measure of the x-ray intensity falling on crystal
 Upon arriving at lithium pulse is generated
 Voltage of pulse=q/c; q-tot charge collected on electrode, c-detector
capacity.

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 X-ray diffraction methods
 There are several XRD methods which are generally used for investigating the
internal structures and crystal structures of various solid compounds.

X-Ray Diffraction Method

1. Laue’s photographic method
Laue Rotating Crystal Powder a)Transmission method
b)Back reflection method
•Different
Lattice Parameters
Orientation Lattice constant Single 2.Rotating crystal method
Polycrystal (powdered)
• Single Crystal Crystal Monochromatic
Monochromatic Beam
• Polychromatic Beam Beam VariableAngle 3.Powder method
VariableAngle
• Fixed Angle

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 The Laue method
Laue in his very first experiments used white radiation of all possible wavelengths and
allowed this radiation to fall on a stationary crystal. The crystal diffracted the X-ray beam and
produced a very beautiful pattern of spots which conformed exactly with the internal
symmetry of the crystal. Let us analyze the experiment with the aid of the Bragg equation.
The crystal was fixed in position relative to the X-ray beam, thus not only was the value for d
fixed, but the value of was also fixed.

X-Ray Laue Method

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 A) Transmission Laue method
In the transmission Laue method, the film is placed behind the crystal
to record beams which are transmitted through the crystal.
One side of the cone of Laue reflections is defined by the transmitted
beam. The film intersects the cone, with the diffraction spots
generally lying on an ellipse.

 Can be used to orient crystals for solid state experiments.

 Most suitable for the investigation of preferred orientation sheet

particularly confined to lower diffraction angles.

 Also used in determination of symmetry of single crystals.

Transmission Laue method
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 B) Back-reflection Laue method

 In the back-reflection method, the film is placed between the x-

ray source and the crystal. The beams which are diffracted in a
backward direction are recorded.
 One side of the cone of Laue reflections is defined by the
transmitted beam. The film intersects with the cone in which the
diffraction spots generally lie on a hyperbola.

 This method is similar to Transmission method however, back-

reflection is the only method for the study of large and thick
specimens.
Back-reflection Laue method

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Disadvantages of Laue method
 Big crystals are required
 Crystal orientation is determined from the position
of the spots. Each spot can be indexed,
i.e. attributed to a particular plane, using
special charts.
 The Greninger chart is used for back-reflection
patterns and the Leonhardt chart for transmission
patterns.
 The Laue technique can also be used to assess
Back-reflection Laue method
crystal perfection from the size and shape

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 The Bragg’s x-ray spectrometer

Bragg’s x-ray spectrometer

 A - Cathode

 B-B’ – Adjustable slits

C - crystal
E - ionization chamber
 One plate of ionization chamber is connected to the positive terminal of a H.T Battery ,
while negative terminal is connected to quadrant electrometer(measures the strength of
ionization current) 32
 Working of Bragg’s x-ray spectrometer
 Crystal is mounted such that ѳ=0°and ionization chamber is adjusted to receive x-rays
 Crystal and ionization chamber are allowed to move in small steps
 The angle through which the chamber is moved is twice the angle through which the crystal is
rotated
 X-ray spectrum is obtained by plotting a graph between ionization current and the glancing angleѳ
 Peaks are obtained, corresponding to Bragg’s diffraction.
 Different order glancing angles are obtained with known values of λ and n and from the observed
value of ѳand d can be measured.

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Determination of crystal structure by bragg’s law
 X-Rays fall on crystal surface

 The crystal is rotated and x-rays are made to reflect from various lattice planes

 The intense reflections are measured by bragg’s spectrometer and the glancing

angles for each reflection is recorded

 Then on applying bragg’s equation ratio of lattice spacing for various groups of planes

can be obtained.

 Ratio’s will be different for different crystals

 Experimentally observed ratio’s are compared with the calculated ratio’s ,particular

structure may be identified.

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 Rotating Crystal Method

 Single crystal mounted with one axis normal to a

monochromatic x-ray beam
 Cylindrical film placed around the sample
 As sample rotates, some sets of planes momentarily satisfy
Bragg condition
 When film is laid flat, a series of horizontal lines appears
 Because crystal rotates about a single axis, possible Bragg
angles are limited - not every plane is able to produce a
diffracted spot
 Sometimes used to determine unknown crystal structures
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 Powder crystal method:
X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase
identification of a crystalline material and can provide information on unit cell dimensions. The
analyzed material is finely ground, homogenized, and average bulk composition is determined.

Fine powder is struck on a hair with a gum ,it is suspended vertically in the axis of a cylindrical camera
 When monochromatic beam is allowed to pass different possibilities may happen
1.There will be some particles out of random orientation of small crystals in the fine powder
2.Another fraction of grains will have another set of planes in the correct positions for the
reflections to occur
3. Reflections are possible in different orders for each set
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 Powder crystal method:

 If the angle of incidence is ѳthen the angle of reflection

will be 2θ.
 If the radius is r the circumference 2πr corresponds to a

scattering angle of 360°.

θ =360*1/πr
 From the above equation the value of θ can be calculated

and substituted in Bragg’s equation to get the value of d.

POWDER CRYSTALDIFFRACTION

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 Applications
• The electron density and accordingly, the position of the atoms in complex structures, such as
penicillin may be determined from a comprehensive mathematical study of the x-ray
diffraction pattern.
• The elucidation of structure of penicillin by xrd paved the way for the later synthesis of penicillin.
• The powder xrd pattern may be thought of as finger print of the single crystal structure, and it may
be used conduct qualitative and quantitative analysis.
• XRD can also be used to determine whether the compound is solvated or not.
• Particle size determination by applying the relation.
v= V. δθ. cos θ / 2n

Where v = the volume or size of an individual crystalline

V = the total volume of the specimen irradiated
n = the number of spots in a deffraction ring at a Bragg angle θ

It is used to assess the weathering and degradation of natural and synthetic , minerals.
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 Limitations
Homogeneous and single phase material is best for identification of an unknown

Must have access to a standard reference file of inorganic compounds (d-spacings, hkls)

Requires tenths of a gram of material which must be ground into a powder

For mixed materials, detection limit is ~ 2% of sample

For unit cell determinations, indexing of patterns for non-isometric crystal systems is
complicated

Peak overlay may occur and worsens for high angle 'reflections'

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 Conclusions
For materials including metals, minerals, plastics, pharmaceuticals and semiconductors XRD
apparatus provide highly accurate tools for non-destructive analysis.

The diffraction systems are also supported by an extensive range of application software.

X-ray Diffraction is a very useful to characterize materials for following information.

• Phase analysis
• Lattice parameter determination
• Strain determination
• Texture and orientation analysis
• Order-disorder
transformation and many
more things.
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X-RAY FLUORESECENCE

Rakesh Pani
 X-ray fluorescence

• An X-ray fluorescence (XRF) spectroscopy is an x-ray instrument used

for routine, relatively non-destructive chemical analyses of rocks,
minerals, sediments and fluids.
• It is typically used for bulk analyses of larger fractions of geological
materials.
• One of the most widely used methods due to its relative ease, low cost
of sample preparation and the stability and use of X- ray spectrometers.
• One of the best analytical techniques to perform elemental analysis in
all kinds of samples, no matter if liquids, solids or loose powders.
2D & 3D MODELS OF XRF
What can be analysed by X-Ray Fluorescence?

Polymers

Ores and Raw Materials

Oils
Metals, Slags

Chemicals
Ceramic

Glass
Food Products
Typical Samples for X-Ray Fluorescence
 Fundamental Principles of XRF
• XRF works on methods involving interactions between electron
beams and x-rays with samples.
• Made possible by the behavior of atoms when they interact with radiation.
• When materials are excited with high-energy, short wavelength radiation
(e.g., X-rays), they can become ionized.
• If the energy of the radiation is sufficient to dislodge a tightly- held inner
electron, the atom becomes unstable and an outer electron replaces the
missing inner electron.
• When this happens, energy is released due to the decreased binding energy
of the inner electron orbital compared with an outer one.
• The emitted radiation is of lower energy than the primary incident X-rays
and is termed fluorescent radiation.
• Because the energy of the emitted photon is characteristic of a transition
between specific electron orbitals in a particular element, the resulting
fluorescent X-rays can be used to detect the abundances of elements that are
present in the sample.
Excitation of the sample

Sample
Secondary X-Rays or X-
Ray Fluorescence which is
characteristic for the
elemental composition of
the sample

Spectrometer
Principle of the excitation by X-Rays

 An Incoming X-Ray photon strikes

an electron, the electron breaks
free and leaves the atom.
Principle of the excitation by X-Rays

X-Ray Photon

 This leaves a void that must be filled by

an electron from an outer shell.
 The excess energy from the new electron
is released (fluorescence) in the form of
an x-ray photon.
Excitation and Interactions between shells

Layer L

Layer K

Layer N

Layer M
 XRF - HOW IT WORKS
• An XRF spectrometer works because if a sample is illuminated by an
intense X-ray beam, known as the incident beam, some of the energy is
scattered, but some is also absorbed within the sample in a manner that
depends on its chemistry.
• The incident X-ray beam is typically produced from a Rh target, although
W, Mo, Cr and others can also be used, depending on the application.
• When x-ray hits sample, the sample emits x-rays along a spectrum of
wavelengths characteristic of the type of atoms present.
• If a sample has many elements present, the use of a Wavelength Dispersive
Spectrometer allows the separation of a complex emitted X-ray spectrum
into characteristic wavelengths for each element present.
• Various types of detectors used to measure intensity of emitted radiation.
• Examples of detector used the flow counter and the scintillation detector.
• Flow counters measure long wavelength(>0.15nm) x-rays typical of
elements lighter than zinc.
• The scintillation detector is commonly used to analyze shorter
wavelengths in the X-ray spectrum(K spectra of element from Nb to I; L
spectra of Th and U).
 Benefits of XRF analysis

• Minimal or no sample preparation

• Non-destructive analysis
• Na11 to U92 analysis, ppm to high % concentration range
• No wet chemistry – no acids, no reagents
• Analysis of solids, liquids, powders, films, granules etc.
• Rapid analysis – results in minutes
• Qualitative, semi-quantitative, to full quantitative analysis
• For routine quality control analysis instrument can be ‘used by anyone’
 Applications
X-Ray fluorescence is used in a wide range of applications,
including

• research in igneous, sedimentary, and metamorphic petrology

• soil surveys
• mining (e.g., measuring the grade of ore)
• cement production
• ceramic and glass manufacturing
• metallurgy (e.g., quality control)
• environmental studies (e.g., analyses of particulate matter on
air filters)
 Applications
• petroleum industry (e.g., sulfur content of crude oils and petroleum
products)
• field analysis in geological and environmental studies (using portable, hand-
held XRF spectrometers)
X-ray fluorescence is limited to analysis of :

• relatively large samples, typically > 1 gram

• materials that can be prepared in powder form and effectively
homogenized

• materials for which compositionally similar, well-characterized

standards are available
• materials containing high abundances of elements for which
absorption and fluorescence effects are reasonably well understood
 Strengths & limitations of XRF
Strengths

X-Ray fluorescence is particularly well-suited for investigations that

involve:

• bulk chemical analyses of major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K,
P) in rock and sediment

• bulk chemical analyses of trace elements (>1 ppm; Ba, Ce, Co, Cr, Cu, Ga,
La, Nb, Ni, Rb, Sc, Sr, Rh, U, V,Y, Zr, Zn) in rock and sediment
 Limitations
In theory the XRF has the ability to detect X-ray emission from virtually all
elements, depending on the wavelength and intensity of incident x-rays.
However...

• In practice, most commercially available instruments are very limited in

their ability to precisely and accurately measure the abundances of elements
with Z<11 in most natural earth materials.
• XRF analyses cannot distinguish variations among isotopes of an element,
so these analyses are routinely done with other instruments.
• XRF analyses cannot distinguish ions of the same element in different
valence states, so these analyses of rocks and minerals are done with
techniques such as wet chemical analysis or Mossbauer spectroscopy.
 XRF detection system
No mater how the secondary X-ray radiation (X-Ray fluorescence) is produced
in XRF machines there are TWO WAYS to detect this radiation:

◦ Wavelength Dispersive System (WDS) and

◦ Energy Dispersive System (EDS).
EDS spectrometer

ED-XRF

SEM-EDX
Handheld XRF

Specifically designed for

the rigorous demands of
nondestructive elemental
analysis in the field.
Spectroscopic techniques
3.1 INTRODUCTION
Infrared, Raman and UV spectroscopic techniques are the most inspiring

research tools for the physicists and chemists since these techniques provide structural

information of chemical compounds. In recent years, Fourier transform (FT)

spectroscopy techniques enjoy the advantages over the conventional IR and Raman

spectroscopy by offering some unique features. The ultimate performance of any IR

spectrometer depends on the signal to noise ratio (SNR) value. The improved SNR

value is attained by incorporating FT-techniques in infrared and Raman

spectrometers. Frequency precision, good spectral subtractions and high resolution are

difficult to achieve with conventional Raman spectroscopy. Hence a new technique,

in which the Raman module is an accessory to FT-IR spectrometer, called FT-Raman

spectroscopy. FT-IR, FT-Raman and UV techniques have been found to be useful in

increasing the spectral sensitivity and thus yielding new information about the atomic

or molecular species or the functional group molecular bonds that exist in the sample.

Modern spectrometers are generally attached with sophisticated computers and

high energy sources like lasers which allow spectrum storage and retrieval, scale

expansion, repetitive scanning, spectral comparison, spectral simulation, automatic

control of slit, etc. Often these are under the control of a microprocessor and

microcomputers. Accessories such as beam condensers, reflectance units, polarizers

and microcells can usually be added to extend versatility or accuracy. In this chapter

the instrumentation and sample handling techniques of FT-IR, FT-Raman and UV

spectrometers are presented.

The IR and Raman spectroscopic techniques are based on different principles.

Change in electric dipole moment is necessary for IR activity, whereas the Raman

criterion depends on the change in polarizability during vibration. The intensity of the

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IR and Raman spectra mainly depends on the magnitude of the change in dipole

moment and the polarizability respectively. The IR and Raman methods are used to

study the materials existing in various states. The molecular vibrational frequencies

obtained by these methods have been used in molecular mechanics and they yield

considerable information about the interatomic forces in various molecules [1]. An

ultraviolet spectrum of the molecule is based on the transitions of electrons between

the electronic energy levels by absorbing UV radiation. UV spectroscopic techniques

used to examine bonding and charge transfer interactions in the sample.

3.2 INFRARED SPECTROSCOPY

Infrared (IR) spectroscopy is one of the most common and widely used

spectroscopic techniques. Absorbing groups in the infrared region absorb within a

certain wavelength region. The absorption peaks within this region are usually sharper

when compared with absorption peaks from the ultraviolet and visible regions. In this

way, IR spectroscopy can be very sensitive to determination of functional groups

within a sample since different functional group absorbs different particular frequency

of IR radiation. Also, each molecule has a characteristic spectrum often referred to as

the fingerprint. A molecule can be identified by comparing its absorption peak to a

data bank of spectra. IR spectroscopy is very useful in the identification and structure

analysis of a variety of substances, including both organic and inorganic compounds.

It can also be used for both qualitative and quantitative analysis of complex mixtures

of similar compounds.

3.3 INSTRUMENTATION
3.3.1 Introduction
The basic components of an infrared spectrophotometer are shown in Fig.3.1.

A source provides radiation over the entire range of the infrared spectrum. The

monochromator disperses the light and then selects a narrow wave number range. The
39
detector measures the energy and transforms it into an electric signal. This signal is

further amplified and registered by the recorder. With recent improvements in

instrumentation, the infrared region of the electromagnetic spectrum is now

considered to cover the range from approximately 12500-10 cm-1 (0.8-1000 m). It is

generally subdivided into three sections: near infrared (12500-4000 cm-1), mid

infrared region (4000 - 400 cm-1) and the far infrared (400-10 cm-1). The mid infrared

region is one of the most commonly used standard laboratory investigations as it

covers almost all the vibrational and rotational transitions. The FT-IR spectra of most

of the samples were recorded in mid and far IR regions and are used for the present

investigation [2-7].

SOURCE SAMPLE MONO- DETECTOR

CHROMATOR

RECORDER AMPLIFIER

Fig.3.1 Block diagram of the major components of

an infrared spectrophotometer.
3.3.2 Source
The source of radiation for FT-IR spectrometer is a black body radiator. The

ideal infrared source gives a continuous and high radiant energy output over the entire

infrared region. The total amount of energy radiated and the spectral distribution of

this energy are dependent upon the temperature of the source. The usual sources of

infrared radiation are Nernst glower and Globar. The Nernst glower is composed of

oxides of thorium, zirconium and yttrium. It is operated at a temperature of 1000 to

1800ºC. The Globar is a small rod of silicon carbide usually 5 cm in length and 0.5

cm in diameter. It is operated at temperatures between 1300 and 1700ºC. The

maximum radiation for the Globar occurs in the 5500-5000 cm-1 region. Nichrome

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wire, carbon arc, rhodium wire and tungsten filament lamp are also used as light

source. In a commercial infrared spectrometer either a nichrome wire or a platinum

filament contained in a ceramic tube is commonly used as infrared source for the

range 4000-400 cm-1 [8].

3.3.3 Monochromator

A monochromator is a means of separating wavelengths of the source

radiation. The monochromator is used to separate polychromatic radiation into a

suitable monochromatic form. This is achieved by means of prisms or diffraction

gratings. A grating or a prism disperses the radiation from the source into its spectral

elements according to the equation nλ=2dsinθ. The most common prism material is

NaCl and it has a lower frequency cut-off at about 650 cm-1. It has a good resolution

in the range 1000 to 650 cm-1 and moderate resolution throughout 4000 to 1000 cm-1.

A monochromator thus carries out three functions: (i) it disperses the

radiation according to its wave number components (ii) it restricts the radiation falling

on the detector into a narrow wave number range, and (iii) it maintains the energy

incident on the detector to an approximately constant level when no sample is present

throughout the wave number range of the instrument [9]. Some instruments use a

double monochromator. That is, the exit slit of the first monochromator serves as the

entrance slit for the second monochromator. As a result, the spectra obtained with

spectrophotometers having double monochromator have higher resolution [2].

3.3.4 Detector

Detectors used in infrared spectrophotometers usually convert the thermal

radiant energy into electrical energy. The infrared detectors may be selective or non-

selective. The selective detectors are those whose response is markedly dependent

upon the wavelength of the incident radiation. Examples of this type are photocells,

41
photographic plates, photoconductive cells and infrared phosphors. Photoconductive

cell has a rapid response and high sensitivity. These cells are made of materials such

as PbS, lead selenide or selenium. When illuminated by infrared light these cells show

an increase in conductivity. These cells are useful in the range 0.5 and 3.5 microns.

The non-selective detectors are those whose response is directly proportional

to incident energy but relatively independent of wavelength. Common examples

include thermocouples, bolometers and pneumatic cell. Recent detectors, fabricated

from crystals are known as pyroelectrics and take less time than other thermal

detector. Hence with these crystals, radiation can be chopped at a higher rate.

However, these are more expensive and not widely employed.

3.3.5 Amplifiers and recorders

The radiant energy received by the detector is converted into measurable

electrical signal and is amplified by the amplifiers. The amplified signal is registered

by a recorder or a plotter. The recorder is driven with a speed which is synchronized

with that of a monochromator, so that, the pen moving across the chart, records the

transmittance of the sample as a function of the wavenumber.

3.4 SAMPLE HANDLING TECHNIQUES

Sample handling is considered as an important technique in infrared

spectroscopy. There are various methods of sample preparation to enable almost any

type of sample to be examined. Some significant problems arise when trying to

construct sample containers for vibrational spectrometry, because every material has

some vibrational absorption. The material that has a minimum interference in the

regions of interest is used as sample. The material of choice for IR spectroscopy is a

solid potassium bromide plate. Such plates are used in a number of ways.

Polyethylene pellets are used for recording the far IR spectra.

42
(a) Solids
Solids are sampled in a wide variety of ways. If the sample is soluble, it may be

dissolved and handled as for a liquid. Solid samples for which no solvent is suitable

can be prepared for analysis by incorporating them into a pressed pellet of alkali

halide, usually potassium bromide. The sample is mixed with a weighed amount of

powdered potassium bromide and the mixture is admitted to a pressure of several

tones in a die, to produce a highly transparent plate or disc which can be inserted into

the spectrophotometer. The use of KBr eliminates the problems of additional bands

due to mulling agent. KBr does not absorb infrared light in the region 2.5–15 m and a

complete spectrum of the sample is obtained. Solid samples have also been examined

in the form of a thin layer deposited by sublimation or solvent evaporation on the

surface of a salt plate. Another method, called mulling has also been developed, in

which the powdered sample is mixed to form a paste with little heavy paraffin oil. The

mull is sandwiched between salt plates for measurement. Mulls are formed by

grinding 2 to 5 mg of finely powered sample in the presence of one or two drops of a

heavy hydrocarbon oil called Nujol [2, 10-12].

(b) Liquids

The spectra of a pure liquid can be measured as very thin films squeezed

between two alkali halide windows of a demountable cell. This technique can produce

a film of thickness 0.01 mm or less. The cells are then taken apart and cleaned. This

method is most useful for qualitative work only because the sample thickness cannot

be controlled. For quantitative work sealed liquid cells of fixed path length in the

range 0.01 to 0.1mm are used. Liquid cells consist of two alkali halide windows

usually NaCl or KBr, separated by a spacer of suitable thickness made of teflon or

lead which limits the volume of the cell [2].

43
(c) Gases

The vapour is introduced into a special cell, usually about 10cm long that can

be placed directly in the path of one of the infrared beams. The end walls of the cell

are usually made of sodium chloride, which is transparent to infrared. Most organic

compounds have too low a vapour pressure for this phase to be useful. The low

frequency vibrational changes in the gaseous phase often split the high frequency

vibrational bands [13].

(d) Solvents

Solvents of good infrared transparency over a convenient frequency range are

available and the spectra of the sample dissolved in carbon tetrachloride and carbon

disulphide provide the complete range. Chloroform is considered to be an important

solvent and is frequently used because it shows absorptions though it has less

symmetric molecule than carbon tetrachloride and carbon disulphide.

3.5 FOURIER TRANSFORM INFRARED SPECTROMETER

Fourier transformation technique is now of great importance in nuclear

resonance, microwave, infrared and Raman spectroscopy. The absorption spectrum in

a Fourier transform infrared spectrometer is obtained through interference technique

[14]. Interferometry is therefore known as Fourier transform spectroscopy. The

structural information from the observed diffraction patterns is obtained through a

mathematical manipulation known as Fourier transformation. Fourier transformation

is accomplished using a digital computer.

FT infrared spectrometer consists of two parts: (a) an optical system which

uses an interferometer and (b) a dedicated computer which stores data performs

computations on data and plots the spectra. A schematic diagram of the essential

components of an FT spectrometer based on Michelson interferometer is shown in

44
Fig.3.2. It consists of two perpendicular mirrors; one of which is a stationary mirror

and the other a movable mirror which can be displaced perpendicularly to the fixed

mirror at a constant velocity. Between these two mirrors the beam splitter is set at 450

from the initial position of the movable mirror. A parallel beam of radiation from an

infrared source is passed to the mirrors through the beam splitter. The beam splitter

reflects about half of the beam to the fixed mirror which reflects it back to the beam

splitter and transmits the other half to the movable mirror which reflects it back to the

beam splitter. The returning beams are again split and mixed about half going back to

source and half passing through the sample compartment. The composition of the

beam splitter depends on the spectral region of interest. For example, in the mid-

infrared region (4000-400 cm-1), a beam splitter of germanium coated on KBr plate

(substrate) is often used. Germanium reflects the radiation while KBr transmits most

of the desirable radiation. In the far infrared region, germanium coated on CsI

(800–200 cm-1) or germanium coated on Mylar (polyethylene terephthalete)

(650–10 cm-1) is used as beam splitter. A thin film of the beam splitter material is

coated on an optically flat substrate.

The return beams from both the mirrors along the same path length as their

incident path are recombined into a single beam at the beam splitter. The path length

of one of the return beams is changed in order to create phase difference to cause an

interference pattern. The recombined radiation is then directed through the sample and

focused on to the detector. The detector measures the amount of energy at discrete

intervals of mirror movement.

The movable mirror can be moved in a range of say ±5 cm. The mirror

velocities from 0.05 to 5 cm s-1 are used. Interferometer instruments need detectors

with response times short enough to detect and transmit rapid changes to the recorder.

45
The detector used in conjunction with rapid scanning interferometers in the mid-

infrared region at room temperature is triglycine sulfate with KBr windows as

pyroelectric bolometer. It has a high response time. Other most common detectors

used such as thermocouples, bolometers and Golay detectors have short response

time. The design of the Michelson interferometer is such as to make measurement in

any infrared region possible by simply changing the beam splitter and the detector [2].

3.6 ADVANTAGES OF FOURIER TRANSFORM TECHNIQUE

The main advantages of FT spectroscopy are the greater ease and speed of

measurement. The entire spectrum can be recorded within few seconds using

sophisticated computers. Recent developments in FT infrared spectrometers have thus

led to higher resolution, total wavelength coverage, higher accuracy in frequency and

intensity measurements. It can also be used in the characterization of all kinds of

samples. In FT method, all the source energy passes through the instrument and the

resolving power is constant over the entire spectrum. The signal to noise ratio is also

improved [15]. The smoothening of peaks and the vertical and horizontal expansion

of selective region is also possible.

3.7 RAMAN SPECTROSCOPY

The Raman spectroscopy is made sophisticated with the advent of gas lasers

and computers. The advantages of lasers are their high intensity, high

monochromaticity, narrow band width, high resolution and coherence. From the time

of invention of Raman Effect, both infrared and Raman spectra of chemical

compounds have been effectively used for the determination of molecular structure

and also for the quick identification of the presence of the characteristic group

frequencies in the compound as discussed in the literature [16].

46
Fig.3.2 Simplified diagram of Fourier transform infrared spectrometer.

47
 In order that for a molecular vibration to be Raman active there must be a

change in the polarizability of the molecule. An important advantage of Raman

spectra over infrared lies in the fact that water does not cause interference; indeed,

Raman spectra can be obtained from aqueous solutions. In addition, glass or quartz

cells can be employed, thus avoiding the inconvenience of working with sodium

chloride or other atmospherically unstable window materials. Raman spectra are

acquired by irradiating a sample with a powerful laser source of visible or near-

infrared monochromatic radiation. A major advantage of FT-Raman over

conventional dispersive Raman spectroscopy is its ability to render spectra that are

generally free of fluorescence interference. FT-Raman enjoys the wavelength

precision of FT-IR so that spectra may be co-added, resulting in a rapid improvement

in signal-no-noise ratio (SNR) performance.

3.8 INSTRUMENTATION
3.8.1 Introduction
In Raman spectrometer the sample is irradiated with monochromatic light and

the scattered light is observed at right angles to the incident radiation. Raman

spectrometer consists of source, interferometer and detector. Fig.3.3 represents the

basic diagram of an FT-Raman spectrometer.

3.8.2 Source

The sources used in modern Raman spectroscopy are nearly always lasers

because their high intensity is necessary to produce Raman scattering of sufficient

intensity to be measured with a reasonable signal to-noise ratio. Five of the most

common lasers along with their wavelength (nm) used for Raman spectroscopy are

Argon ion (488 or 514.5 nm), Krypton ion (530.9 or 647.1 nm), Helium / Neon (632.8

nm), Diode laser (782 or 830 nm) and Nd: YAG (1064 nm). Because the intensity of

Raman scattering varies as the fourth power of the frequency, argon and krypton ion
48
48
sources that emit in the blue and green region of the spectrum have an advantage over

the other sources. Diode and Nd:YAG laser which emit near-infrared radiation are

used as powerful excitation sources. Near-infrared sources have two major advantages

over shorter wavelength lasers. The first is that they can be operated at much higher

power (upto 50W) without causing photodecomposition of the sample. The second is

that they are not energetic enough to populate a significant number of fluorescence

producing excited electronic energy states in most molecules. Consequently,

fluorescence is generally much less intense or non-existent with these lasers. The

Nd/YAG line at 1064 nm is particularly effective in eliminating fluorescence. The

two lines of the diode array laser at 782 and 830 nm also markedly reduce

fluorescence in most cases.

The laser radiation is directed to the sample by means of a lens and a parabolic

mirror and the scattered light from the sample is collected and passed to a beam

splitter and to the moving mirrors. It is then passed through a series of dielectric filters

and focused onto a liquid nitrogen cooled detector.

3.8.3 Detectors

Raman spectrum can be photographed with an ordinary spectrograph.

Basically there are two different ways to detect and record Raman lines. The easiest

way is to gather the scattered light emerging through a glass window at the end of the

Raman sample tube. It is passed through a prism or grating and then focused on a

photographic plate. The plate is then developed and both the line frequencies and

intensities can be measured using external equipment. Modern spectrometers which

have photo multiplier tubes are direct measurements and facilitate automatic scanning

of a spectrum. The spectrum produced by the monochromatic is passed through a slit

which allows a narrow wavelength region to pass through which is focused on to a

49
photo multiplier type detector. This detector employs an amplifier and a recorder. It

directly provides the Raman Spectrum.

3.9 SAMPLE HANDLING TECHNIQUES

Sample handling techniques for Raman spectroscopic measurements are simpler

than for infrared spectroscopy because glass can be used for windows, lenses and

other optical components instead of the more fragile and atmospherically less stable

crystalline halides. In addition, the laser source is easily focused on a small sample

area and the emitted radiation efficiently focused on slit. Consequently, very small

samples can be investigated. In fact, a common sample holder for non-absorbing

liquid sample is a glass melting-point capillary.

(a) Liquid samples

The spectrum of a liquid can be recorded as neat or in solution. Ordinarily

about 0.3 ml of a liquid may be required. The sample could be taken in glass or silica

containers or capillaries. The spectra can be measured directly from the reaction

vessel. Water is a good solvent for recording the Raman spectra. Water absorbs

strongly in the infrared but it gives a poor Raman scattering. Raman spectroscopy is

thus a valuable tool for studying water soluble biological materials.

(b) Solid samples

The Raman spectra of solids as polycrystalline material or as a single crystal

can be recorded. No medium such as null, KBr or solvent is needed. A few milligrams

of the solid samples are required. Solid can be packed into a capillary tube as a

powder. The crystal can be mounted in a goniometer on a glass or silica fibre. The

spectra can be measured for different orientation of the crystal. For a single crystal,

the Raman spectrum varies depending on the direction of the crystal axis, when

50
polarized light is used as incident radiation. Raman spectra of adsorbed species can be

recorded at different temperatures and pressures [2].

(c) Gas samples

The Raman spectra of gases are generally weaker than those of liquids or solids

and hence may require cells of larger path length. The gas may be filled in a glass or

silica tube of 1 to 2 cm diameter. If the resolving power of the instrument is good and

if the molecule has sufficiently low moment of inertia, the rotational fine structure

may be observed on either side of the Rayleigh line. Generally a broad band contour

may be observed. The main advantage of Raman spectroscopy is that it may be used

for a wide variety of sizes and forms of the sample. Samples in gas, liquid and solid

states can be examined easily. In this study, the spectral data are recorded on

BRUCKER IFS 66V, FT-IR with FRA 1064 nm FT-Raman spectrophotometer.

3.10 FT-RAMAN SPECTROMETER

The FT-Raman method consists of measurement of spectra using a near

infrared laser, the collection of the scattered light and its analysis using appropriately

designed Michelson interferometers and Fourier transform processors. Fourier

transform Raman spectra are almost exclusively obtained with neodymium yttrium

aluminum garnet (Nd: YAG) lasers. The laser radiation is filtered to perfect its

monochromaticity and is then focused on to the sample. Light reflected and scattered

off the sample in a direction the reverse of illumination the so-called back scattering

arrangement is then filtered to remove the Rayleigh scattered light and allow only the

Raman scattering to pass. The noise associated with the intense Rayleigh scattering is

distributed over the entire spectrum in the Fourier transformation step and it seriously

degrades the desired Raman spectrum. Therefore, strong suppression of Rayleigh

scattering by means of a laser line rejection filter must be achieved before good

51
quality FT-Raman spectra can be obtained. Then the scattered light passes through a

Michelson interferometer. The incorporated beam splitter may be a wide range

component capable of operating from 1nm and through the mid infrared or it may be a

limited range device. The interferogram is then collected and detected on a near

infrared detector. The germanium photo resistor operating at the liquid nitrogen

temperature or indium doped gallium arsenide photo detector which operates at room

temperature is employed. The detector signal is digitized and then Fourier

transformed to generate a spectrogram. The spectrum is recorded as intensity of

scattering versus frequency shift using software [2].

3.11 APPLICATIONS OF RAMAN SPECTROSCOPY

Raman spectroscopy is an important tool for solving the intricate research

problems concerning the constitution of compounds. The technique can be applied to

investigate bond angles, structure, ionic-equilibria, nature of bonding, degree of

dissociation of strong electrolytes and the corresponding activity coefficients. Raman

spectra can be interpreted in terms of unit cells which may contain a few elements of

different polymer chains. Since peak overlap is low in Raman spectroscopy, mixtures

can easily be analyzed [9].

3.12 INTERPRETATION OF SPECTRA

The assignments of fundamental modes of vibrations and interpretations of the

spectra have been carried out by following empirical correlation of group frequencies,

infrared and Raman selection rules, the magnitude and relative intensities of the

spectra [17, 18]. In the case of polyatomic molecules, the interpretation of spectra is

highly complex due to the appearance of combination and overtone bands.

52
Fig.3.3 Basic diagram of an FT-Raman spectrometer.

53
3.13 ULTRAVIOLET SPECTROSCOPY

Ultraviolet spectroscopy employs ultraviolet and visible radiations for making

the transitions between the electronic energy levels. It helps to identify the functional

groups and nature of the bond in the organic compounds. It also provides the

information about molecular structure and oxidation state.

3.13.1 Instrumentation

An UV-vis spectrophotometer consists of source, dispersive system (combined

in a monochromator), and detection system. In a double beam spectrometer, the

radiation coming from the monochromator is split into beams with the help of a beam

splitter. These are passed simultaneously through the reference and the sample cell.

The transmitted radiations are detected by the detectors and the difference in the

signal at all the wavelengths is suitably amplified and sent for the output. UV-visible

double beam spectrophotometer is shown in Fig.3.4.

3.13.2 Radiation Source

In ultraviolet spectrometers, the most commonly used radiation sources are

hydrogen or deuterium lamps, the xenon discharge lamps and mercury arcs. The

sources should provide stable output over the entire UV- visible range (190 nm to 780

nm). For measurements in the UV region, electric discharge sources like hydrogen or

a deuterium lamp are used. In these, the excitation of the gaseous molecules is

brought about by the passage of electrons through the gas at low pressures. A

hydrogen lamp is commonly used in spectrophotometers and it gives light in the

wavelength region of 160-375 nm. The radiant power of the hydrogen lamp is low

and it is replaced by deuterium lamps. The modern instruments use a tungsten

filament lamp as the radiation source. This consists of a thin, coiled tungsten wire that

is sealed in an evacuated glass bulb. This gives radiations in the range of

54
350-2200 nm. As the output depends on the voltage, the tungsten lamp is energized by

the output of a constant voltage transformer [19].

3.13.3 Monochromators
The monochromator is used to disperse the radiation according to the

wavelength. The essential elements of a monochromator are an entrance slit, a

dispersing element and an exit slit. The entrance slit sharply defines the incoming

beam of heterochromatic radiation. The dispersing element disperses the

heterochromatic radiation into its component wavelengths, whereas exit slit allows the

nominal wavelength together with a band of wavelengths on either side of it. The

position of the dispersing element is always adjusted by rotating it to vary the nominal

wavelength passing through the exit slit. The dispersing element may be a prism or

grating. Quartz and fused silica prisms which are transparent throughout the entire

UV range are widely used in UV spectrophotometers

3.13.4 Detectors

The detectors are used to convert a light signal into an electric signal which

can be suitably measured and transformed into an output. The detectors used in most

of the instruments generate a signal, which is linear in transmittance i.e. they respond

linearly to the radiant power falling on them. The transmittance values can be changed

logarithmically into absorbance units by an electrical or mechanical arrangement in

the signal read out device. Phototube, photomultiplier tube and diode array detectors

are three types of detectors which are widely used in UV spectrophotometers.

3.14 SAMPLE HANDLING TECHNIQUES

The UV-vis absorption spectra are usually determined either in vapour phase

or in solution. In order to take the UV spectrum of the sample, it is taken in a cell

called a cuvette which is transparent to the wavelength of light passing through it.
55
Fig.3.4 UV-visible double beam spectrophotometer.

56
Most of the spectrophotometers employ quartz cuvettes for both visible and UV

region. The sample whose spectrum is to be measured is dissolved in a solvent that is

transparent in the UV region. Hexane, ethanol and methanol are commonly employed

as solvents. In a typical measurement of a UV spectrum, the solution of the sample is

taken in a suitable cuvette and the spectrum is run in the desired range of the

wavelengths. The absorption by the solvent, if any, is compensated by running the

spectrum for the solvent alone in the same or identical cuvette and subtracting it from

the spectrum of the solution. This gives the spectrum only due to the absorption

species under investigation. In double beam spectrometers, the sample and the

solvents are scanned simultaneously.

57
REFERENCES
1. P.S. Sindhu, Molecular Spectroscopy, Mc Graw Hill, New Delhi, 1985.

2. D.N. Sathyanarayana, Vibrational Spectroscopy Theory and Applications,

New Age International publishers, New Delhi, 2004.

3. B.K. Sharma, Spectroscopy, GOEL Publishing House, Meerut, Eleventh

Edition, 1995.

4. H.H. Willard, L.L. Merritt, J.A. Dean, F.A. Settle, Instrumental Methods of

Analysis, Sixth Edition, CBS Publishers and Distributers, Delhi, 1986.

5. R.S. Khandpur, Handbook of Analytical Instruments, Tata McGraw-Hill

Publishing Company Limited, New Delhi, 2002.

6. K.A. Rubinson, J.F. Rubinson, Contemporary Instrumental Analysis, Prentice

Hall International, New Jersey, 2000.

7. D.A. Skoog, F.J. Holler, T.A. Nieman, Principles of Instrumental Analysis,

Fifth Edition Harcourt Asia PTE Ltd., Singapore, 1998.

8. N.B. Colthup, L.H. Daly, S.E. Wiberly, Introduction to Infrared and Raman

Spectroscopy, Academic Press, New York, 1964.

9. H. Kaur, Instrumental Methods of Chemical Analysis, Second Edition,

Pragathi Prakashan, India, 2003.

10. R.A. Nyquist, C.L. Pulzig, M.A. Leugers, The Hand Book of Infrared and

Raman Spectra Of Inorganic Compounds and Organic Salts, Academic Press,

San Diege, 1997.

11. William Kemp, Organic Spectroscopy, ELBS, Macmillan, UK, 1987.

12. R. John, Dyer, Applications of Absorption Spectroscopy of Organic

Compounds, Prentice Hall, 1971.

13. G. Rahut, P. Pulay, J. Phys. Chem., 99 (1995) 3093.

58
14. P.R. Griffiths, J.A. de Haseth, Fourier Transform Infrared Spectroscopy,

Wiley, New York, 1986.

15. C. N. Banwell, Fundamentals of Molecular Spectroscopy, Tata McGraw-Hill

Publishing Company Limited Company Limited, New Delhi, 1972.

16. T. Sundius, Vib. Spectroscopy, 29 (2002) 89.

17. S. Mohan, N. Sundaraganesan, Mink, Spectrochim. Acta A, 47 (1991) 1111.

18. R.M. Silverstein, G. Clayton Bassler, C. Morrill, Spectrometric Identification

of Organic Compounds, John Wiley and Sons, New York, 1981.

19. Gurdeep R. Chatwal, Sham K. Anand, Instrumental Methods of Chemical

Analysis, Himalaya Publishing House Pvt. Ltd., Mumbai.

59