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Submitted By: Guided By:
Arpit Modh 16BCH035 Dr. Ankur Dwivedi
Parth Kasodariya 16BCH028 Assistant Professor
CHEMICAL ENGINEERING DEPARTMENT
INSTITUTE OF TECHNOLOGY
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This is to certify that Mr. Arpit Modh (16BCH035) and Mr. Parth Kasodariya (16BCH028)
students of Chemical Engineering, 4th
semester, of Nirma University, has satisfactorily completed
the seminar “Spectroscopy” on as a partial fulfilment towards the degree of B. Tech. in Chemical
Dr. Ankur Dwivedi Dr. J. P. Ruparelia
Assistant Professor Head of Department
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3. List of Figures
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We take this opportunity to express our gratitude to our guide Mr. Ankur Dwivedi for his
exemplary guidance and monitoring throughout the course of Seminar. We would be grateful that
granted permission to perform practical in the sophisticated laboratory to understand the subject
practically. We would like to thank Nirma University to grant us this course which made us active
in this kind of research and making report. It helped us to improve our skills on the particular
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Our knowledge of spectroscopy is based on more than 20 years of experimentation in a wide
array of discipline ranging from art to applied physics. Spectroscopy is one the most important
analytical technique available to research. One of the great advantages of spectroscopy is that
virtually any sample may be studied in any state. Here find an explanation of the principles for a
range of spectroscopy techniques AAS, UV-Vis and IR. Spectroscopy is the study of interaction
of the electromagnetic radiation in all its forms with matter.
We are comprising the principle, theory, instrumentation, handling and application in various
kinds of spectrophotometer. This report shows the basic review of various kinds of spectroscopy
and the application in different and important fields. One can identify the structural part of any
compound. Ultraviolet and visible spectroscopy are the oldest methods for the quantitive analysis
and the structural analysis since years.
Which can help us to check purity of any substance. UV absorption spectroscopy can discriminate
those types of compounds which absorb or adsorb UV radiation. In UV absorption spectrum, the
main motive is to find the amount of radiation absorbed at various wavelengths. It mainly
characterises aromatic and conjugated olefins. Thus these are the most important and useful tool
to study atomic and molecular structure and is used in wide range of samples.
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List of figures
Figure No. Title Page No.
Figure 1 The electromagnetic spectrum 11
Figure 2 Spectroscopic Transition between molecular Energy Levels 12
Figure 3 Arrangement for atomic absorption spectroscopy 14
Figure 4 Hollow cathode lamp 15
Figure 5 Electrodeless Discharge Lamp 16
Figure 6 Illustration of the excitation of molecular vibrations in IR 20
Figure 7 Diagram of Infrared Spectrometer 21
Figure 8 Single-pass Monochromators 23
Figure 9 Magnified views of a grating monochromator 23
Figure 10 Path of IR Radiation diffracted by a grating monochromator 24
Figure 11 Electronic excitation energies 27
Figure 12 Absorption and intensity shift 29
Figure 13 Ultraviolet Visible Spectrophotometer diagram 33
Figure 14 Barrier layer cell 34
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Title Page No.
1 Introduction 9-12
1.1 Spectroscopy 9
1.2 The Electromagnetic Spectrum 9
1.3 Origin of Spectrum 12
2 Atomic Absorption Spectroscopy 13-18
2.1 Introduction 13
2.2 Principle and theory 13
2.3 Instrumentation 14
2.3.1 Source 15
2.3.2 Chopper 16
2.3.3 Atomiser 16
2.3.4 Nebulisation of the liquid sample 17
2.3.5 Monochromator 17
2.3.6 Detector 17
2.4 Analytical features of AAS 18
2.4.1 Sensitivity 18
2.4.2 Detection limit 18
2.4.3 Working range 18
2.4.4 Accuracy 18
2.5 Safety and Precautions 18
2.6 Advantages and Disadvantages 18
3 Infrared Absorption Spectroscopy 19-26
3.1 Introduction 19
3.2 Principle and Theory 19
3.3 Instrumentation 21
3.3.1 Source 21
3.3.2 Monochromator 22
3.3.3 Sample cell and sampling of substance 22
3.3.4 Detectors 24
3.4 Selection Rules 25
3.5 Application of IR Spectroscopy in Organic Compound 25
3.6 Application of IR Spectroscopy in general 26
4 Ultra Violet – Visible Spectroscopy 27-34
4.1 Introduction 27
4.2 Principle and Theory 27
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4.2.1 Types of Transitions in Organic Molecules 28
4.2.2 Chromophore and Related Terms 28
4.2.3 Woodward-Feiser Rules for calculating Absorption Maxima 29
4.2.4 Fieser-Kuhn Rule for conjugated polyenes 32
4.3 Instrumentation 33
4.3.1 Sources 33
4.3.2 Monochromator 34
4.3.3 Detector 34
4.4 Application 34
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1.1 Spectroscopy 
Spectroscopy is the study of interaction between matter (i.e. molecules, atoms, nuclei) and radiated energy (i.e.
electromagnetic radiation). It is precisely based on how each electromagnetic radiations interacts with molecules,
compounds, atoms or even the nucleus. The most important thing of such interaction is that energy is absorbed
or emitted by the matter in fixed amounts called “Quanta”.
The study of taking experimental measurements of radiation frequency (emitted or absorbed) and the energy
levels based on the observations is called “Spectroscopy”. It is the most important and useful tool to study atomic
and molecular structure and is used in wide range of samples. The types of spectroscopy further carried out with
1. Atomic Spectroscopy: This comprises the interaction of electromagnetic radiation with atoms which generally
remains in their lowest energy state called the ground state. In the gaseous state the atoms are monoatomic. And
absorbs electromagnetic radiation, resulting in transition of electron from one electronic energy to another one.
If a photon has the energy greater than or equal to the difference between two quantized energy levels then and
then the electromagnetic absorption can be done.
ΔE = hv Equation i
Where ΔE is the energy difference between two energy levels and v is the frequency of photon. Applications of
atomic spectroscopy in the field of chemistry are very few but though they are important. i.e., laser technology is
developing in this era.
2. Molecular Spectroscopy: This comprises the interaction of electromagnetic radiation with molecules. This ends
up in transitions between rotational and vibrational energy levels. So that the spectra of molecules are more
complicated than the atoms revealing important knowledge about molecular structure. Rotational, Vibrational,
Raman, Electronic, NMR, ESR, Mossbauer are various types of spectra given by molecular species.
1.2 The Electromagnetic Spectrum
The human eye can sense the light intensity ranging from violet light (λ = 400 nm) through the rainbow colours
to red light (λ = 800 nm). The lights having shorter wavelengths than 400 nm and longer than 800 nm do exist,
but they are not be detected by the human eye. Ultraviolet light (λ < 400 nm) can be detected on photographic
film or in a photoelectric cell and infrared light (λ > 800 nm) can be detected either photographically or using a
heat detector such as a thermophile
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The electromagnetic spectrum is the range of all kinds of electromagnetic radiations. Radiation is the energy
which travels and spreads out. The visible light that comes from a lamp in our house and the radio waves that
come from a radio station are two types of EM radiation. The other types of electromagnetic radiation that makes
the electromagnetic spectrum are microwaves, infrared light, gamma-rays, ultraviolet light and X-rays. The
Figure 1 shows the electromagnetic spectrum. The electromagnetic spectrum from lowest energy or
longest wavelength (at the top) to highest energy or shortest wavelength (at the bottom). . 
Radio: Your radio captures radio waves emitted by radio stations, bringing your favourite music. Stars and gases
also emits radio waves in space. Radio waves can travel easily through air, but can also be conducted along a
Radio waves are received and emitted by antennas. Metal rod resonators are used as conductors. By artificial
generation of radio waves, an electronic device called a transmitter generates an alternate electric current which
is applied to an antenna. The oscillating electrons in the antenna generate oscillating magnetic and electric fields
that radiate from the antenna as radio waves. In reception of radio waves, the oscillating magnetic and electric
fields of a radio wave couple to the electrons in an antenna which pushes them back and forth, creating oscillating
currents which are applied to radio receiver. Earth's atmosphere is generally transparent to radio waves, except
for layers of charged particles in the ionosphere which can reflect certain frequencies. (λ = 10 -
² to 105
Microwave: Microwave radiation can cook your popcorn within a few minutes time but is also used
by astronomers to learn about the structure of galaxies around. They are quite similar to visible light and are
conducted through tubes or ‘waveguides’.
Microwaves are radio waves of short wavelength, from about 10 cm to 1 mm, in the SHF and EHF frequency
bands. Microwave energy is generated through klystron and magnetron tubes and with solid state devices such
as Gunn and IMPATT diodes. Even though they are emitted and absorbed by short antennas, they are also
absorbed by polar molecules, coupling to rotational and vibrational modes, resulting in bulk heating. (λ = 10-3
Infrared: Night vision goggles are manufactured for sensing the infrared light emitted by our skin and objects
with heat as heat sensors. Infrared light very useful to us to map the dust between stars. Infrared radiation is the
most useful and very much routine part of people’s life in the whole electromagnetic spectrum. Human eyes are
not capable to see the infrared light but we can feel it as heat. (λ = 7.5×10 -7
Ultraviolet: Ultraviolet radiation emitted by the Sun is the reason of skin tans and burns. "Hot" objects in space
also emit UV radiation. There are two regions of ultraviolet distinguishing “vacuum ultraviolet” and
“ultraviolet”. This is because air starts absorbing below 1.8×10-7
m starting from 1.8×10-7
m the region is the
beginning of analytical chemistry. (λ = 10-8
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Figure 1 The electromagnetic spectrum 
Visible: Among the all type of radiation our eyes detect only a small portion of light - visible light. Fireflies, light
bulbs, and stars all emit visible light. Cone-shaped cells in our eyes act as receivers tuned to the wavelengths in
this narrow band of the spectrum.
The spectrum does not contain all the colours that the human eyes and brain can recognize. Unsaturated colours
such as pink or purple variations such a magenta are absent because they could be made only by a mixture of
multiple wavelengths. Colours containing only single wavelength are called pure colours or spectral colours. (λ
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X-ray: Doctors use X-rays to image your teeth or for examine bones and airport security uses them to see inside
people’s luggage. Hot gases in the universe also emit X-rays. X-rays are able to pass through most of solid
X-ray photons have that energy that they are capable to ionize atoms and disrupt molecular bonds. These makes
it ionizing radiation and therefore they are harmful to living cell. But also they are used in the treatment of cancer
to kill malignant cells. (λ = 10-11
Gamma ray: Doctors use gamma-ray imaging to see inside your body. The biggest gamma-ray generator of all
is the Universe. They are having the smallest wavelengths with corresponding highest energy in the
They are generated from the hottest and most energetic objects in the universe as like neutron stars, pulsars,
supernova explosions and regions around the black holes. On the earth, gamma waves are generated by nuclear
explosions, lightning and the less dramatic activity of radioactive decay. (λ < 2 × 10-11
1.3 Origin of Spectrum 
According to Quantum mechanics every energy levels are quantized having appropriate quantum numbers.
Consider two molecular energy levels En and Em, as shown in Figure 2. If a photon of frequency v falls on a
molecule in the ground state and its energy hv is exactly the energy difference ΔE between the two molecular
energy levels, then the molecule undergoes a transition from lower energy level to higher one. With the
absorption of a photon of energy hv. The spectrum thus obtained is called absorption spectrum. If the molecule
falls from the excited state to the ground state with the emission of a photon of energy hv, the spectrum is called
the emission spectrum.
Figure 2 Spectroscopic Transition between molecular Energy Levels
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Atomic Absorption Spectroscopy
2.1 Introduction   
The atomic absorption was first observed in 1802 with the discovery of the Fraunhofer lines in the sun’s spectrum.
Later on, Alan Walsh demonstrated it in mid-1950. Atomic spectroscopy itself shows that how efficient and
useful it is. It is the most powerful technique for the quantitative determination of trace metals in liquids. This
method provides a total metal content of the sample and is almost independent of the molecular form of the liquid.
The versatility of analytical atomic absorption spectroscopy can be realized form the fact that 60-70 elements,
including most of the common rare earth metals, have been determined by this technique in concentration that
range from trace to macro quantities. Direct application of the technique in limited to metals, with the exceptions
of B, Si, As, Se and Te, several of the non-metals have been estimated by indirect methods. As atomic absorption
spectroscopy does not demand sample preparation, it is an ideal tool for no-chemist also, i.e. the engineer,
biologist or clinician are interested only in the significance of the results. As an analytical technique, it has become
increasingly important because of its high sensitivity and that only one element can be determined at a time. A
change in light source and a change of analytical wavelength are necessary to determine a second element.
2.2 Principle and theory  
The absorption of energy by ground state atoms in the gaseous state develops the atomic absorption spectroscopy.
AAS is a spectroscopic analysis technique that determine the concentration of particular elements and quantity
determination of chemical elements using the absorption of optical radiation by free atoms. The analytic
concentration is calculated through amount of absorption.
Principle: “When a beam of monochromatic radiation is passed through the atoms of an element, the rate of
decrease of intensity of radiation is directly proportional to the intensity of incident light and concentration of the
solution.” When a solution having metals in it is introduced into a flame, the vapour of metallic species will be
obtained. Some of the metals atoms may be raised to an energy level sufficiently high to emit the characteristic
radiation of the metal – a phenomenon that is utilised in the same kind of technique of the emission flame
photometry. Although there is a large number of metal atoms which remains at the ground state. These ground
state atoms are receptive of light radiation of their own specific resonance wave length.
Thus, when a light of this wavelength is allowed to pass through a flame having atoms of the metallic species,
part of that light will be absorbed and the absorption will be directly proportional to the density of the atoms in
the flame. This is how the amount of light absorbed is determined and further based on that the concentration of
the metallic species can be calculated.
𝐴𝑡 𝑣 𝑡ℎ𝑒 𝑡𝑜𝑡𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 =
𝑁𝑓 Equation ii
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Where e is the charge of the electron having mass of m, c the speed of the light, N the number of atoms that can
absorb at frequency v in the light path and f the oscillator strength or ability for each atom to absorb at frequency
v. As π, e, m and c are constants. The above Eq. 1 can be deduced to following expression:
𝑇𝑜𝑡𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 × 𝑁 × 𝑓 Equation iii
From above equation we can conclude that atomic absorption does not depend on wavelength and temperature.
These two makes atomic absorption spectroscopy distinct over flame absorption spectroscopy.
2.3 Instrumentation 
A schematic diagram of the atomic absorption spectroscopy is shown below in Figure 3. The principle of the
instrumentation is similar to other spectroscopic absorption methods.
Figure 3 Arrangement for atomic absorption spectroscopy 
✓ Light of a certain wavelength, which is able to emit the spectral lines corresponding to the energy required
for an electronic transition from the ground state to an excited state, is allowed to pass through the flame.
✓ The solution gets dispersed into a mist of very small droplets which evaporates in the flame to give salt
and then the vapour of the salt. At least a part of this vapour will be dissociated into atoms of the element
to be measured.
✓ Thus the flame possesses free unexcited atoms which are capable of absorbing radiation from an external
source when the radiation corresponds exactly to the energy required for a transition element from the
ground electronic state to upper one.
✓ Then the unabsorbed radiation from the flame is allowed to pass through a monochromator the unabsorbed
radiation is led into the detector which is then registered by a photodetector, the output of which is
amplified and measured on recorder. Absorption is measured by the difference in transmitted signal in the
presence and absence of test element.
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For all types of atomic absorption spectrometer, the following components are required:
2.3.1 Radiation Source
There are mainly two types of sources: (1) Line source and (2) Continuous source. In the line source, the radiation
source for atomic absorption spectrometer should emit stable, intense radiation of the element with lower
wavelength, usually a resonance line of the element. The resonance spectral lines should be narrow as compared
with the width of the absorption lines to be measured. These lines should be no general background or other
extraneous lines emitting within the band pass of the monochromator. The problem of using such narrow spectral
lines has been solved by adopting a hollow cathode lamp as the radiation source.
(a) Hollow Cathode Lamp: The hollow cathode lamp is a bright line source for most of the elements determinable
by atomic absorption. The construction of hollow cathode lamp is shown in Figure 4. The cathode of the lamp
is a hollow cylinder of the metal whose spectrum is to be produced. The anode and cathode are sealed in a
glass cylinder normally filled with either neon or helium at low pressure. At the end of the glass cylinder is a
window transparent to the emitted radiation.
Figure 4 Hollow cathode lamp  
Hollow cathode lamps have a finite lifetime. Adsorption of fill gas atoms onto the inner surfaces of the lamp
is the primary cause for lamp failure. As fill gas pressure decreases, the efficiency of sputtering and the
excitation of sputtered metal atoms also decreases, reducing the intensity of the lamp emission. To prolong
hollow cathode lamp life, some manufacturers produce lamps with larger internal volumes so that a greater
supply of fill gas at optimum pressure is available.
(b) Electrodeless Discharge Lamp: For most elements, the hollow cathode lamp is a completely satisfactory
source for atomic absorption. In a few cases, the quality of the analysis is impaired by limitations of the
hollow cathode lamp. The primary cases involve the more volatile elements such as As, Sb, Sn, Pb, Cd, etc.,
where low intensity and short lamp life are a problem. The atomic absorption determination of these elements
can often be dramatically improved with the use of brighter, more stable sources such as the ‘‘electrodeless
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Figure 5 Electrodeless Discharge Lamp 
Figure 5 shows electrodeless discharge lamp (EDL). A small amount of the metal or salt of the element for which
the source is to be used is sealed inside a quartz bulb. This bulb is placed inside a small, self-contained RF
generator or ‘‘driver’’. When power is applied to the driver, an RF field is created. The coupled energy will
vaporize and excite the atoms inside the bulb, causing them to emit their characteristic spectrum. An accessory
power supply is required to operate an EDL.
2.3.2 Chopper 
A rotating wheel is introduced between the flame and hollow cathode lamp. This rotating wheel is called chopper
and is introduced to break the continuous light coming out from the lamp into an intermittent or pulsating light.
This will give a pulsating current in the photocell. There is also a continuous current emitted by light which is
coming out a flame. But only the pulsating current is amplified and recorded and this is how the absorption of
light will be measured without interference from the light emitted by the flame itself.
Fore effective atomic absorption of the sample it is needed to convert these into small atoms. Atomisation is the
element which breaks the liquid molecules and converges into small vapour atoms through high temperature and
graphite furnace. There are two kinds of atomisers: (1) Flame atomiser and (2) Non-Flame atomiser
(1) Flame Atomiser:
There are two types: (a) Total consumption burner and (b) Pre-Mixed burner
(a) Total consumption burner: In this sample, this sample is atomised into the flame. The sample, the fuel
and oxidizing agents are passed through separate passes and mixed at the open tip of the flame. Then
flame breaks the sample liquid into the droplets which will be evaporated and later on burned. Acetylene
and hydrogen are used as fuel gases while as an oxidant oxygen is used. This method makes so much of
noise and difficult to use.
(b) Pre-Mixed burner: This method is widely use because of uniformity of the flame intensity. The sample
solution, the fuel and the oxidizing agents are mixed before they reach to the tip. By heating the radicals
are formed of the mixture which initiates combustion. This method is very suitable for the study of metals
like Ga, In, Ti, Pb, Mn, Ni, Pd etc.
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(2) Non-flame Atomiser: The electro thermal atomiser system is used in this method. Carbon atomiser is one the
important instrument in this category as it can achieve temperature up to 3000 °C. The heated carbon furnace
thermal to brae chemical bonds. Within the sample held in carbon tube and produce free ground state atoms.
These atoms are capable to absorb energy in the form of light and are elevated to excited state. The absorbance
of the energy depends on the concentration of the particular elements.
2.3.4 Nebulization of the Liquid Sample
The sample is converted into small droplets before entering into the burner. This method to form small droplets
from the liquid sample is called nebulisation. The use of a gas moving at high velocity by nebulization is called
pneumatic nebulisation. The liquid sample needs to be maintained at a controlled rate in order to create a fine
aerosol spray for introduction into the flame.
It mixes fuel and oxidant for the introduction into the flame. In the burner, a back pressure of about 250 torr
occurs at the tip of the burner due to the high velocity of the aspirating gas. As the liquid is drawn up in the
capillary, it is broken into droplets by the high velocity gas stream. The process of extremely fast and must be
controlled as high temperature is dangerous to the instrument.
In the atomic absorption spectroscopy, Monochromators are the important part of the instrument. It is used for
the separating all the lines emitted from the hollow cathode and select a given absorbing line. It can resolute till
0.5 Å. The selection of the specific wavelength light absorbed by the sample excluding others allows the
determination of the selected elements. Prism and gratings are the most common Monochromators.
Photodetectors are used to sense the light or electromagnetic energy. The most preferable detector is
photomultiplier tube as it can efficiently compares all the lines. In this photomultiplier tube there is evacuated
envelope containing a series of electrodes called dynodes and anode. As soon as photon falls on the photocathode,
an electron is dislodged and the proton is accelerated to dynode 1 with the liberation of two or more electrons
from this dynode.
Same thing happens to dynode 1 to dynode 2 with liberation of more electron. Thus, the current multiplies at each
dynode and the final current is received by the anode and EMF is produced for the further actions. Then the
results will be sent to the control computer and analyses it and the read-out will be provided by it.
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2.4 Analytical features of AAS 
Sensitivity: It is simply termed as the slop of the line in the absorption graph. It is indirectly expressed in atomic
absorption spectroscopy as a characteristic concentration in terms of g/ml for 1% of absorbance. The sensitivity
for 1% of the absorbance is determined by below equation:
𝐶1 % =
C1% is the concentration that gives rise to 1% absorption C0.1 is the concentration that gives rise to 0.1 absorbance.
Detection Limit: The detection limit is defined as the concentration of an element which results in the shifting
and absorbance signal to an amount that equal to peak-to-peak noise of the base line. In atomic absorption
spectroscopy approximately 2-5 times lower than characteristic concentration.
Working range: It can work under approximately 2-3 times of the magnitude of the wavelength of the light.
Accuracy: Especially in GF atomic absorption spectroscopy accuracy can be affected by sample matrix. E.g. Via
analyte transport, formation of thermally stable compound and non-atomic absorption.
2.5 Safety and Precautions 
● Check the integrity of the gas system that cylinders are secured to immovable objects and that tubing and
connecters do not have gas leaks and the integrity of the burner
● Check drain bottle regularly and make sure the keep it empty before the run
● Don’t stare at the flame while atomisation without protection. Hollow cathode lamps are under negative
pressure and should be handle with care and dispose it properly to avoid implosion
2.6 Advantages 
● The analysis is too much faster for the sample as 10-15 second each.
● The instrument has very precise analysis
● Till a moderate interference can be rectified easily
● The run for measurement of any sample is quite based on automation
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Infrared Absorption Spectroscopy
3.1 Introduction  
In this look at instrumentation for IR spectroscopy, we will be limiting our attention to instrumentation concerned
with spectroscopy in the middle region (4000 – 200 cm-1
). It is absorption in this region which gives structural
information about a compound. IR absorption takes place in a region of longer and therefore lower energy
wavelengths than do ultraviolet and visible absorption. The wavelength comprises the energy of vibrational,
stretching and bending atomic bonds within the molecules.
The IR spectrum of an organic compound, some of the frequencies are absorbed, while other frequencies are
transmitted through the sample without being absorbed. On plotting the graph of absorption or transmittance
against frequency, we get the IR spectrum. The alkane molecules will only absorb infrared light of a particular
frequency if there is an energy transition within the molecule such that ΔE = hv.
The absorption band near 3000 cm-1
correspond to the frequency of the bending vibrations of C-H bonds and are
called the C-H (bend) absorptions. Infrared spectroscopy is therefore basically vibrational spectroscopy.
3.2 Principle and Theory  
The energy differences occurring in molecules due to vibrations and rotations creates the absorption which makes
the Infrared spectroscopy. Thus for absorption, a molecule must have a change in dipole moment as molecule
and consequence of mode of vibrations. The relative positions of atoms in a molecules are not fixed but fluctuate
steadily due to different kinds of vibrations.
Molecular vibrations can be excited via two physical mechanisms: the absorption of light quanta and the inelastic
scattering of photons Figure 6 (Herzberg 1945). Direct absorption of photons is achieved by irradiation of
molecules with polychromatic light that includes photons of energy matching the energy difference hvk between
two vibrational energy levels, the initial (e.g., ground state) and the final (e.g., first excited state) vibrational state.
As these energy differences are in the order of 0.5 and 0.005 eV, light with wavelengths longer than 2.5 mm, that
is infrared (IR) light, is sufficient to induce the vibrational transitions. Thus, vibrational spectroscopy that is based
on the direct absorption of light quanta is denoted as IR absorption or IR spectroscopy. In IR spectroscopy, the
vibrational transitions are induced by absorption of light quanta from a continuous light source in the IR spectral
region.  
Each normal mode has definite vibrational frequencies. If we a bond between a pair of atoms obeys Hook’s law,
on displacing one atom a distance x from its equilibrium position with respect to the other atom, then the force
operating is 𝐹 = −𝑘𝑥 if k = force constant.
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Figure 6 Illustration of the excitation of molecular vibrations in IR 
The simple harmonic motion of atoms is given by the fundamental vibration frequency V:
m1 and m2 are masses of atoms,
k = force constant and µ =
c = velocity of light
µ = reduced mass
m1 and m2 = masses of atoms
This equation is the reduced mass of a diatomic oscillator. Usually k for single bond is 2 × 105
to 8 ×
while for a double bond it is 8 × 105
to 12 × 105
and for a triple bond it is
12 × 105
to 18 × 105
. K is larger due to higher vibrational frequencies.
Motion involving H-atoms have higher frequencies than motions with behaviour atoms. Bending motions exhibit
lower frequencies of absorption than fundamental stretching patterns. Radiation can be expressed in terms of the
wave number (σ) as:
Types of Molecular Vibrations 
There are two types: (1) Stretching and (2) Bending
(1) Stretching: In stretching the bond length varies with constant bond angle. There are two types of it:
(a) Symmetric, where two or more bonds varies together.
(b) Asymmetric, where bonds varies there length independently.
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(2) Bending: In bending the bond angle varies with constant bond length. There four types of it:
(a) Rocking, where the atoms move like a pendulum on a clock.
(b) Scissoring, where the atoms move away and towards from each other
(c) Wagging, where the atoms move towards (+) or away (-) the observer out-of-plane together.
(d) Twisting, where the atoms move independently out-of-plane.
3.3 Instrumentation  
The instrumentation for IR spectroscopy relates in the middle region (4000 – 200 cm-1
) due to absorption by
molecules in this region, which given structural information about a compound. Here below the schematic
diagram is presented in Figure 7:
Figure 7 Diagram of Infrared Spectrometer
IR instruments require energy which provides a means for isolating narrow frequency bands. The radiation source
must emit IR radiation which must be
(a) Intense enough for detection
(c) Extend over the desired wavelength
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Although these are examples of continuous light frequencies, only few will be absorbed by the samples. These
are the common sources of IR radiations:
(a) Incandescent Lamp: It is a tightly wound coil of nichrome wire, electrically heated to 1100 K. It produces
a lower intensity with lower spectral emissivity.
(b) Nernst glower: It consists a hollow rod which is 2 mm in diameter and 30 mm in length in dimension of
rare earth oxides like zirconia, yttria and thoria. Platinum wires are sealed to the ends and a current passed
through the hollow rod. It can reach up to 2200 K. The main disadvantage of Nernst glower is that it emits
a wide range of IR radiation, the intensity of radiation remains steady and constant over long periods of
time. The biggest disadvantage of it is it fails frequently mechanically and energy concentrates in the
visible and near IR region.
(c) Globar Source: It is a rod of sintered silicon carbide which is about 50 mm in length and 4 mm in
diameter. When it is heated to a temperature between 1300 and 1700 ºC, it strongly emits radiation in the
IR region. It emits maximum radiation at 5200 cm-1
. Water cooling of the electrical contacts is needed to
prevent arcing. The spectral output is comparable with Nernst glower, except at short wavelength where
its output becomes larger.
(d) Mercury Arc: In the far infrared region (wavelength < 200 cm-1
) the sources described above lose their
effectiveness and special high pressure Mercury lamp is used. Beckman devised the quartz envelope emits
the radiation whereas at longer wavelength the mercury plasma provides radiation through the quartz.
The radiation source emits radiations of various frequencies. As the sample in IR spectroscopy absorbs only at
certain frequencies, it therefore becomes necessary to select desired frequencies from the radiation source and
reject the radiations of other frequencies. These selection is based on mainly two types of monochromators:
(a) Prism Monochromators: Any prism used as a dispersive element must be constructed of materials which
transmit in the infrared. While glass and quartz were utilized in the visible and ultraviolet, they absorb
and are unsatisfactorily in the infrared as of its high dispersion in the region of 4 to 15 µm, a region which
in the IR. A single pass monochromator has been illustration in Figure 8. The sample is kept at or near
the focus of the beam before the slit ‘S1’.
The radiation from the source after passing through the sample and entrance slit, strikes the off-axis
parabolic Littrow mirror. Which renders the radiation parallel and sends it to prism P. Mirror L returns
through the prism a second time and focuses into the exist slit of the monochromator, through which it
finally passes into the detector section. In double pass monochromators, there occurs a total of four passes
of radiation through the prism as shown. The double pass monochromator produces more resolution than
the monochromator in the radiation, before it finally passes on the detector. Prism of lithium fluoride or
calcium fluoride give more resolution in the region where the significant stretching vibrations are located.
23. Page 23 of 38
Figure 8 Single-pass Monochromators 
(b) Grating Monochromator: If a prism is replaced by a grating, higher dispersion can be achieved.
Reflection gratings are more common than prisms and are preferred over transmittance gratings. The
gratings offer linear dispersion and maybe constructed from a wide variety of materials. The grating is
essentially a series of parallel straight lines cut into a plane surface in the Figure 9. Dispersion by a grating
follows the law of diffraction in Figure 10. It follows the following mathematical relation.
𝑛𝜆 = 𝑑(𝑠𝑖𝑛 𝛼 ± 𝑠𝑖𝑛 𝛽 )
where n is the order (a whole number), λ the wavelength of the radiation, d the distance between grooves,
α the angle of incidence of beam of IR radiation and the β the angle of dispersion of light of a particular
Figure 9 Magnified views of a grating monochromator 
Grating Monochromator possesses the following advantages over prism:
(a) It can be made with Aluminium which is moisture free while prism made from metal salts is affected by
(b) It can be used over considerable wavelength ranges.
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Figure 10 Path of IR Radiation diffracted by a grating monochromator 
3.3.3. Sample cell and sampling of substance
As IR spectroscopy has been used for the characterization of solid, liquid and gas samples. It is evident that
samples of different phases have to be handled. But these samples have to be treated differently. However, the
only common point to the sampling of different phases is that the material containing the sample must be
transparent to IR radiation. This condition restricts our selection to only certain salts like NaCl and KBr depending
on wavelength range to be studied.
(a) IR spectra of liquids: IR spectra of liquid compound maybe obtained either from the neat liquid or from
a solution of the liquid in an appropriate solvent. It is desirable to obtain the spectrum from the neat liquid,
if possible, since interference by solvent absorption is thereby avoided. Samples can be held using a liquid
sample cell made of alkali halides. Aqueous solvents cannot be used as they will dissolve alkali halides.
Only organic solvents like chloroform can be used.
(b) IR spectra of solids: Various techniques are used for preparing solid samples such as pressed pellet
technique, solid run in solution, solid films, mull technique etc.
There are three categories of detectors:
Thermocouples consists of a pair of junctions if different metals. For example, two pieces of bismuth fused to
either end if a piece antimony. The potential difference between the junctions changes accordingly to the
differences in the temperature between the junctions.
Pyroelectric detector are made from a single crystalline water of a pyroelectric material, such as triglycerine
sulphate. The properties of pyroelectric material are such that when an electric field is applied across it, electric
polarization occurs. While when the field is removed, the polarization persists. The degree of polarization is
25. Page 25 of 38
Photoconducting detectors such as the mercury cadmium telluride detector comprises a film of semiconducting
material deposited on a glass surface, sealed in an evacuated. Absorption of I-R promotes nonconducting valence
electrons to a higher, conducting state. The electrical resistance of the semiconductor decreases. These detectors
are better than pyroelectric detectors and are used in GC-FTIR.
3.4 Selection Rules  
Most of the molecules possess dipole moment due to unequal sharing of electrons with in the bonds linking the
constituent atoms. When a polar bond undergoes stretching vibrations along the internuclear axis, the electron
distribution changes. As a result dipole moment also undergoes a change. In other words, a vibration produces a
fluctuating dipole moment.
There are different rules known as selection rules, which determine whether a vibration would be effective for a
particular type of spectrum or not. For IR spectrum, the rule is that only those vibrations are effective in causing
absorption which are not centro-symmetric (i.e., the vibrations are not symmetrical about the centre of the
molecule.) Since most of the functional groups in organic chemistry are not centro-symmetric, they respond very
well to infrared spectroscopy.
3.5 Application of IR Spectroscopy in Organic Compound  
Organic functional groups differ from on another both in the strength of the bond(s) involved and in the masses
of the atoms involved. For instances, the O-H and C=O functional groups each contains atoms of different masses
connected by bonds of different strengths. The important applications of infrared spectroscopy are given as
follows for identification of components:
● Alcohols and phenols produce characteristic infrared bands due to O–H stretching and C–O stretching,
which are both sensitive to hydrogen bonding. For alcohols, the broad O–H stretching band is centred at
, while for phenols this band appears 50–100 cm−1
lower than the alcohol wavenumber.
● In alkanes, C–H stretching bands in aliphatic hydrocarbons appear in the 3000–2800 cm−1
range and the
C–H stretching bands of methyl groups and methylene groups are readily differentiated.
● Aromatic compounds show useful characteristic infrared bands in five regions of the mid-infrared
● Carboxylic acids (RCOOH) exist as dimers, except in dilute solution, due to strong intermolecular
hydrogen bonding. Carboxylic acids show a strong broad O–H stretching band in the 3300–2500 cm−1
range. The C=O stretching band of the dimer is observed near 1700 cm−1
, while the free acid band is
observed at higher wavenumbers (1760 cm−1
● Primary, Secondary and Tertiary amines are differentiated by using infrared spectra.
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3.6 Application of IR Spectroscopy in general  
The following are some important applications which give important clues to the structures of coordination
● Determination of Purity
● Shape of symmetry of a molecule
● Presence of water in a sample
● Measurement of paints and varnishes
● Examination of old paintings and artifacts
● In industry for quality check and identification of materials made in research labs.
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Ultraviolet and Visible Spectroscopy
4.1 Introduction   
These both Ultraviolet and visible spectroscopy are the oldest methods for the quantitive analysis and the
structural analysis since years. The other name of UV spectroscopy is Electronic spectroscopy as it involves the
promotion of the electrons from the electrons from the ground state to higher energy or excited state. The
wavelengths of ultraviolet and visible 2000 Å – 4000 Å and 4000 Å - 8000 Å respectively. Ultraviolet wavelength
region is also subdivided into two parts: (1) Near ultraviolet comprising 2000 Å to 4000 Å while (2) Vacuum
ultraviolet comprises the region below 2000 Å.
4.2 Principle and theory  
Ultraviolet absorption and emission are simply based on the absorption and emission of electron from lower
energy state to higher one and vice versa. UV spectroscopy comprises the theory around the electrons only which
makes it easy but though it is complicated in the Fourier series. The distinct type of electrons involved in the
organic molecules to produce ultraviolet spectra region are as follow:
1) σ-electrons The electrons involved in the saturated bonds like C-C and C-H bonds in paraffin. These
electrons are having the highest energy among all the types such as these electrons are having much more
energy than UV radiation produces. Thus through UV radiation we cannot identify them.
2) π-electrons These electrons are involved in the unsaturated bonds in hydrocarbons such as alkene and
aromatic rings. These electrons do absorb and emit the UV radiation sometimes.
3) n-electrons These electrons do not take part in any type of bonding so they are having much lesser amount
of energy. N-electrons can be excited by UV radiation comprising components like oxygen, nitrogen,
sulphur and halogens.
Figure 11 Electronic excitation energies
28. Page 28 of 38
4.2.1 Types of Transitions in Organic Molecules  
The Figure 11 shows the energy level for different types of transitions in the molecules due to change in energy
values. The energy values for transitions are in following order:
< < σ→ σ*
These energy bands are divided into these types of energy levels:
Transition. This energy band comprises the region between 2700-3000Å including unsaturated
molecules like oxygen, nitrogen and sulphur
Transition. This energy band uses the electron resting on π orbital.
Transition. This type of transition happens in the saturated compound like (CH3)3N with lone pair
electrons. These transition ranges in the near ultraviolet region (1800-2000Å)
d) σ→ σ*
Transition. This transitions occurs in the compounds having only single bonds and no lone pair
electrons like saturated compounds.
4.2.2 Chromophore and Related Terms 
1) Chromophore- It is defined as isolated covalently bonded group that shows a specific absorption in the
ultraviolet or visible region (200-800 nm). This can be divided into two groups:
a) Chromophores which contain π electrons and which undergo π to π*
transitions. Ethylene and acetylene
are the example of such chromophores.
b) Chromophores which contain both π and nonbonding electrons. They undergo two types of transitions; π
and n to π*
. Carbonyl, nitriles, azo compounds, nitro compounds etc. are the example of such
2) Auxochrome- An auxochrome can be defined as any group which does not itself act as a chromophore but
whose presence brings about a shift of the absorption band towards the longer wavelength of the spectrum. –OH,-
OR,-NH2,-NHR, -SH etc. are the examples of auxochromic groups.
3) Changes in position and intensity of absorption- Absorption of some of the chromophores like alkene and
alkyne are in the near ultraviolet region. So they are able to study well. But we can study the position and the
intensity of absorption through some changes in structure and solvent. The changes are given below:
(i) Bathochromic shift or red shift: We can shift the region towards higher wavelength with the use of some
auxochrome like –OH and –NH2 or the by decreasing the polarity of solvent.
(ii) Hypsochromic shift or blue shift: We can shift the region towards shorter wavelength by removal of
conjugation in the system by changing the solvent.
(iii) Hyperchromimc shift: We can shift the region towards higher intensity of absorption by addition of
29. Page 29 of 38
(iv)Hypochromic shift: We can shift the region towards lesser intensity if absorption by groups which are
able to distort the geometry of the molecule.
Figure 12 Absorption and intensity shift
4.2.3 Woodward-Feiser Rules for calculating Absorption Maxima  
Woodward gave the rules for relating the maximum wavelength λmax with the given structure. He had done many
experiments regarding finding the wavelengths of conjugated dienes, trines and polyenes with up to 4 or less
double bonds. Feiser modifies the rule giving the structure by relating the position of λmax with the degree of
substitution of chromophore. Some of the important terms are discussed below:
Rules for calculating λmax for conjugated dienes, trines and polyenes:
1) Homoannular diene. A cyclic diene having conjugated double bonds in the same ring.
2) Heteroannular dienes. A cyclic diene having conjugated double bonds in the different ring.
3) Endocyclic double bond. Double bond present in a ring
4) Exocyclic double bonds. Double bonds present in the different rings attached to each other but that should
not be any aromatic compound.
30. Page 30 of 38
Parent values and contributions of different substituents are below:
(a) Parent Values
(i) Acyclic conjugated diene and heteroannular conjugated diene 215 nm
(ii) Homoannular conjugated diene 253 nm
(iii) Acyclic triene 245 nm
(i) Each alkyl substituents or ring residue 5 nm
(ii) Exocyclic double bond 5 nm
(iii) Double bond extending conjugation 30 nm
-OR 6 nm
-SR 30 nm
-Cl, -Br 5 nm
-NR2 60 nm
-OCOCH3 0 nm
E.g. λmax for 1, 4-dimethylcyclohex-1, 3-diene is 265 nm
Rules for calculating λmax for α, β – unsaturated carbonyl compounds:
(a) Parent values
(i) α, β – unsaturated acyclic or six membered ring ketone 215 nm
(ii) α, β – unsaturated five membered ring ketone 202 nm
(iii) α, β – unsaturated aldehyde 207 nm
(i) Each alkyl substituents or ring residue
At α position 10 nm
At β position 12 nm
At γ position 18 nm
(ii) Each exocyclic double bond 5 nm
(iii) Double bond extending conjugation 30 nm
(iv) Homoannular conjugated dienes 39 nm
31. Page 31 of 38
α β γ
-OH 35 30 50
-OR 35 30 17
-SR - 85 -
-OCOCH3 6 6 6
-Cl 15 12 -
-Br 25 30 -
-NR2 - 95 -
E.g. Find the λmax for the compound given below: 259 nm
Rules for calculating λmax for aromatic compounds
1) Base value:
a) ArCOR 246 nm
b) ArCHO2 50 nm
c) ArCO2H 230 nm
d) ArCO2R 230 nm
2) Alkyl group or ring residue in ortho and meta position 3 nm
3) Alkyl group or ring residue in para position 10 nm
4) Polar groups:
a) –OH, –OCH3, –OAlkyl in o, m position 7 nm
b) –OH, –OCH3, –OAlkyl p position 25 nm
c) –O (oxonium) in o position 11 nm
d) –O (oxonium) in m position 20 nm
e) –O (oxonium) in p position 78 nm
f) –Cl in o, m position 0 nm
g) –Cl in p position 10 nm
h) –Br in o, m position 2 nm
i) –Br in p position 15 nm
j) –NH2 in o, m position 13 nm
32. Page 32 of 38
k) –NH2 in p position 58 nm
l) –NHCOCH3 in o, m position 20 nm
m) –NHCOCH3 in p position 45 nm
n) –NHCH3 in p position 73 nm
o) –N(CH3)2 in o, m position 20 nm
p) –N(CH3)2 in p position 85 nm
E.g. Find the λmax for the compound given below: 274 nm
4.2.4 Fieser-Kuhn Rule for conjugated polyenes 
We can using the Woodward Fieser rule find out the maximum wavelength up to 4 double bond in polyenes. For
more than 4 double bond in polyenes we can use Fieser-Kuhn rule. In this rule there is an equation which can
give us the λmax:
𝜆 𝑚𝑎𝑥 = 114 + 5𝑀 + 𝑛(48 − 1.7𝑛) − 16.5𝑅 𝑒𝑛𝑑𝑜 − 10𝑅 𝑒𝑥𝑜 Equation vii
λmax is the wavelength of maximum absorption
M is the number of the alkyl substitute / Ring residue
n number of the conjugated double bond
Rendo is the number of rings with endocyclic double bond in the conjugated double bond
Rexo is the number of rings with exocyclic double bond in the conjugated double bond
We can also find εmax with under equation;
𝜀 𝑚𝑎𝑥 = 1.74 × 104
× 𝑛 Equation viii
εmax is the maximum absorptivity while n is the number of conjugated double bond.
33. Page 33 of 38
4.3 Instrumentation   
Figure 13 Ultraviolet Visible Spectrophotometer diagram 
It is important that the power of the radiation source does not change abruptly over its wavelength range. The
electrical excitation of deuterium or hydrogen at low pressure produces a continuous UV spectrum. The
mechanism for this involves formation of an excited molecular species, which breaks up to give two atomic
species and an ultraviolet photon both Deuterium and Hydrogen lamps emit radiation in the range 160 - 375 nm.
Quartz windows must be used in these lamps, and quartz cuvettes must be used because glass absorbs radiation
of wavelengths less than 350 nm.
Various UV radiation sources are as follows:
a) Deuterium lamp
b) Hydrogen lamp
c) Tungsten lamp
d) Xenon discharge lamp
e) Mercury arc lamp
Various Visible radiation sources are as follows:
a) Tungsten lamp
b) Mercury vapour lamp
c) Carbon lamp
34. Page 34 of 38
Mainly there are 2 monochromators like fused silica prism gratings. The essential elements of the
monochromators are an entrance slit, collimating lens, dispersing device (a prism or a grating), focusing lens, exit
slit. Polychromatic radiation (radiation of more than one wavelength) enters the monochromator through the
entrance slit. The beam is collimated, and then strikes the dispersing element at an angle. The beam is split into
its component wavelengths by the grating or prism. By moving the dispersing element or the exit slit, radiation
of only a particular wavelength leaves the monochromator through the exit slit.
i) Barrier layer cell
Barrier layer cell is also known as photovoltaic cell. In this cell the main working principle is the generation of
voltage between the two layers of silver and iron separated by semiconductor like selenium. This cell allows the
receiving power of wide range of spectrum. It can be deteriorated with the time due to transformation in selenium.
Figure 14 Barrier layer cell
4.4 Application   
1) Detection of impurities: It is one the best methods for the detection of impurities in the raw material. The
additional peaks shows the concentration of the impurities in the solution. It measures the absorption at
the particular wavelengths also. e.g. Impurity of Benzene in the Cyclohexane.
2) Structure elucidation of organic compound: It analyses the presence of different compounds having
different saturations. viz. it analyses the presence of alkane and alkene in the mixture.
3) Quantitative analysis: There are some methods by which we can find the absorbance of the different
compounds present in the solution. There is a widely used method called Simultaneous Equation Method
(Vierordt's Method). If a sample contains two absorbing drugs, each of which absorbs at the wavelength
35. Page 35 of 38
maximum of other, it may be possible to determine both drugs by the technique of simultaneous equations
(Vierordt’s method). Concentrations of several compounds present in the same mixture can be determined
by solving a set of simultaneous equations even if their spectra overlap.
4) Qualitative analysis: UV absorption spectroscopy can discriminate those types of compounds which
absorb or adsorb UV radiation. In UV absorption spectrum, the main motive is to find the amount of
radiation absorbed at various wavelengths. It mainly characterises aromatic and conjugated olefins.
5) Dissociation constants of bases and acids: Using the below equation if we know the ratio of
concentration of reacted and unreacted salts in the solution we can find the value of pKa. This ratio can
be found using the spectrophotometrically by plotting the graph of absorbance and wavelength that
different pH values.
𝑝𝐻 = 𝑝𝐾𝑎 + 𝑙𝑜𝑔 ([𝐴−] / [𝐻𝐴]) Equation ix
6) Chemical kinetics: Kinetics of reaction can also be studied by passing the UV radiation through the
reaction cell. Thus we can find the concentration in the reaction cell of the products and reactants.
36. Page 36 of 38
After completing the seminar on the topic of “Spectroscopy” we have covered the 4 kinds of spectroscopy. Which
mainly contain atomic and molecular spectroscopies. We had seen atomic absorption spectroscopy, which
works on the plank’s law. Where an electron of the atom gathers the energy by outsource and
Trans lifts to the excited stage. As same as then electron removes the energy and get back to the
base state. During this exchange of the energy electron emits the energy having some range of
wavelength. Same as ultraviolet and visible spectroscopy in which the electron emits the energy
in the range ultraviolet radiation.
We had learnt about the principles and theories of different spectrometry and spectrophotometers.
We had seen the instrumentation part where mostly all the given spectroscopy contains sample
preparation, source of radiation, interaction of radiation with the compound, identification of
emitted wavelength and detector. After getting the graph of the result we can calculate the specific
quantity or quantity of the molecule in the compound.
We have seen wide range of the applications of various spectrometers with some examples. Thus
we have completed the discussion of various kinds of spectrometry.
37. Page 37 of 38
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