INSTRUMENTAL METHODS IN ANALYTICAL CHEMISTRY
THE INTERACTION BETWEEN SUBSTANCE AND LIGHT
In order to study about the interaction between substance and light, we must have basic idea
about electromagnetic radiation.
1. Electromagnetic Radiation:
Electromagnetic radiation is a form of energy having both electrical component and
magnetic component, perpendicular to each other as shown in the Figure 1. It is having both particle
and wave nature and hence called as dual nature. Each quantum of radiation is called photon.
Figure 1: Both electrical and magnetic components of electromagnetic radiation.
There are different radiations like cosmic, γ- rays, X- rays, Ultra Violet, visible, Infrared,
microwaves, radio waves. When white light (visible radiation) is passed through a prism, it will
undergo dispersion and split into individual colors, violet, indigo, blue, green, yellow, orange, red
(Like a rainbow) as shown in the Figure 2:
Figure 2: Dispersion of white light.
2. Properties of Electromagnetic Radiation:
2.1 Wavelength (λ) of radiation:
THE UNIVERSITY OF DODOMA, COURSE CODE: CH 3103
INSTRUCTOR: PROF. VEGI MAHESWARA RAO
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INSTRUMENTAL METHODS IN ANALYTICAL CHEMISTRY
The distance between two successive similar points on a wave is called as wave length of
radiation. The units are either Angstrom units or Nanometers.
1 Å = 10-8 cm
1 nm = 10-9 cm, therefore 1 Å = 10 nm
2.2 Wave number ( ̅ ):
It is defined as the total number of waves which can pass through a space of 1 cm. units are
cm-1. This unit is mainly useful in IR spectroscopy.
It is reciprocal of wavelength, i. e., ̅
2.3 Frequency ( ):
The number of waves which can pass through a point in one second. The units are either
cycles per second (cps) or Hertz (Hz). 1 cps = 1 Hz. Frequency and wavelength are inversely
related.
(Where c is velocity of light, 3 X 1010 cm/s)
1. Numerical problem: What is the frequency of radiation having a wavelength of 3000 Å.
2.4 Energy: The energy is a property of radiation which can be transferred to other objects. The
energy of radiation is directly proportional to its frequency.
Where, h is called Planck’s constant, h = 6.626 x 10-27 erg s. or 6.626 x 10-34 Joule. S.
2. Numerical problem: Calculate the energy associated with radiation having wavelength 4000 Å.
3. Electromagnetic spectrum:
The arrangement of all the radiations in the increasing order of their wavelengths or in the
decreasing order of their frequencies is called as electromagnetic spectrum.
Cosmic rays,
-rays,
X- rays, Ultraviolet, Visible, Infrared,
Microwave energy,
Radio waves
Increasing wavelengths
Decreasing frequency and energy
4. Interaction of electromagnetic radiation with the matter:
Spectroscopy is the study of the interaction of radiant energy (light) with matter. We know
from quantum mechanics that energy is really just a form of matter, and that all matter exhibits the
properties of both waves and particles. However, matter composed of molecules, atoms, or ions,
which exists as solid or liquid or gas, exhibits primarily the properties of particles. Spectroscopy
THE UNIVERSITY OF DODOMA, COURSE CODE: CH 3103
INSTRUCTOR: PROF. VEGI MAHESWARA RAO
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INSTRUMENTAL METHODS IN ANALYTICAL CHEMISTRY
studies the interaction of light with matter defined as materials composed of molecules or atoms or
ions.
In a gas, atoms or molecules are widely separated from each other; in liquids and solids, the
atoms or molecules are closely associated. In solids, the atoms or molecules may be arranged in a
highly ordered array, called a crystal, as they are in many minerals, or they may be randomly
arranged, or amorphous, as they are in many plastics. Whatever may be their physical state, the
atoms, molecules, and ions are in constant motion. For molecules, many types of motion are
involved. Molecules can rotate, vibrate, and translate (Move from place to place in space).
Interaction with radiant energy can affect these molecular motions. Molecules that absorb IR
radiation vibrate with greater amplitude; interaction with UV or visible light can move bonding
electrons to higher energy levels in molecules.
A change in any form of motion or electron energy level involves a change in the energy of
the molecule. Such a change in energy is called a transition; we have the possibility of vibrational
transitions, rotational transitions, electronic transitions, and so on in molecules. We have some of
the same kinds of motion in atoms and ions; atoms can move in space, and their electrons can move
between energy levels, but atoms or monoatomic ions cannot rotate or vibrate. The chemical nature
of matter (its composition), its physical state, and the arrangement of the atoms or molecules in the
physical state with respect to each other affect the way in which any given material interacts with
electromagnetic radiation.
The interaction of electromagnetic radiation and matter confirms to well establish quantum
mechanical laws. Atoms, ions, and molecules exist only in certain discrete states with specific
energies. The same quantum mechanical laws dictate that a change in state requires the absorption
or emission of energy, ΔE, exactly equal to the difference in energy between the initial and final
states. We say that the energy states are quantized. A change in state (change in energy) can be
expressed as:
Since we know that c=v, then:
These equations tell us that matter can absorb or emit radiation when a transition between
two states occurs, but it can absorb or emit only the specific frequencies or wavelengths that
correspond to the exact difference in energy between two states in which the matter can exist.
THE UNIVERSITY OF DODOMA, COURSE CODE: CH 3103
INSTRUCTOR: PROF. VEGI MAHESWARA RAO
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INSTRUMENTAL METHODS IN ANALYTICAL CHEMISTRY
Absorption of radiation increases the energy of the absorbing species (i.e., Efinal > Einitial). Emission
of radiation decreases the energy of the emitting species (i.e., Efinal < Einitial).
The lowest energy state of a molecule or atom is called the ground state. All higher energy
states are called excited states. Generally at room temperature molecules and atoms exist in the
ground state. Excited states are energetically unfavorable; the molecule or atom wants to return to
the lowest energy ground state by giving up energy, often by emitting light. Because they are
energetically unfavorable, excited states are usually short-lived, on the order of 10-9–10-6 s.
Emission of light therefore occurs rapidly following excitation.
Absorption spectra are obtained when a molecule or atom absorbs radiant energy that
satisfies the equation ΔE = hv = hc/. The absorption spectrum for a substance shows us the
energies (frequencies or wavelengths) of light absorbed as well as how much light is absorbed at
each frequency or wavelength. The absorption spectrum is having dark lines or bands on the bright
background. The nature of the molecule or atom dictates the amount of light absorbed at a given
energy. The complete spectrum of a substance consists of the set of energies absorbed and the
corresponding intensity of light absorbed. A graph of the intensity of light amplitude changes on the
y-axis vs. the frequency or wavelength on the x-axis is constructed. It is this graph of intensity vs.
energy that we call a spectrum.
Emission spectra are obtained when an atom or molecule in an excited state returns to the
ground state by emitting radiant energy. An emission spectrum can result from many different ways
of forming an excited state. Atoms and molecules can be excited not only by absorption of
electromagnetic radiation, but also by transfer of energy due to collisions between atoms and
molecules, by addition of thermal energy, and by addition of energy from electrical discharges.
3. Atomic Absorption and Emission of radiation
Every atom is capable of absorbing its own characteristic frequencies or wavelength of
electromagnetic radiation. This process transfers energy to the atom and results in a decrease in the
intensity of the incident radiation. The atomic absorption involves the absorption of electromagnetic
radiation by free atomic species. The wavelength that is most strongly absorbed usually corresponds
to an electronic transition from the ground state to the lowest excited state known as the resonance
line. The atomic absorption spectrum is dark well defined lines on the bright background. Atomic
spectrum is very simple because it is line spectrum.
THE UNIVERSITY OF DODOMA, COURSE CODE: CH 3103
INSTRUCTOR: PROF. VEGI MAHESWARA RAO
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INSTRUMENTAL METHODS IN ANALYTICAL CHEMISTRY
When an atom absorbs electromagnetic radiation, the electrons present in an atom will
undergo transition from lower to higher energy level. Such atom is said to be in an excited or
unstable state. The excited atom loses all or part of this energy as radiation, photons of energy will
be emitted which correspond to the difference between the energy levels involved. The radiation
emitted is of specific frequencies and will show up as bright lines on dark background if dispersed
as a spectrum. Atomic emission is displayed by many elements; particularly the metals which after
being excited either thermally or electrically emit a discontinuous lines spectrum due to transition
ending in the ground state. The atomic absorption and emission spectra’s are characteristic of the
atoms (like finger print), therefore these are useful for the qualitative identification of the atoms.
The atomic absorption and emission mechanisms are shown below:
4. Molecular Absorption and Emission of radiation
When a molecule absorbs electromagnetic radiation, it will result in either electronic or
vibrational or rotational transition from lower to higher energy level. Such a molecule is said to be
in an excited or unstable state. Molecular absorption involves the absorption of radiation by
molecules in solution. This type of absorption usually takes place in ultraviolet and visible region of
the electromagnetic spectrum. Absorption of radiation in this region causes transition of electrons
from molecular bonding orbital to the higher energy molecular antibonding orbital. The molecular
absorption spectrum is dark band of wavelengths on bright background.
If a molecule loses all or part of this energy as radiation, photons of energy will be emitted
which correspond to the difference between the energy levels involved. The radiation emitted is of
specific bands of frequencies and will show up as bright bands on dark background. Molecular
emissions are more complex than atomic emissions. The radiation emitted consists of broad bands
of radiation rather than the narrow lines associated with atomic emission. Also the resulting
spectrum is approximately the mirror image of the absorption spectrum of the compound. The
molecular absorption and emission spectra’s are characteristic of the molecule, therefore these are
useful for qualitative identification of the molecules.
THE UNIVERSITY OF DODOMA, COURSE CODE: CH 3103
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INSTRUMENTAL METHODS IN ANALYTICAL CHEMISTRY
For molecules, however, the UV absorption usually occurs over a wide range of
wavelengths because molecules normally have many excited modes of vibration and rotation at
room temperature. In fact, the vibration of molecules cannot be completely "frozen out" even at
absolute zero. Consequently, a collection of molecules generally has its members in many states of
vibrational and rotational excitation. The energy levels for these states are quite closely spaced,
corresponding to energy differences considerably smaller than those of electronic levels. The
rotational and vibrational levels are thus "superimposed" on the electronic levels. Hence in a
molecule the electronic transitions are accompanied by vibrational transitions and the vibrational
transitions are accompanied by rotational energy transitions. Therefore a molecule undergoes the
electronic, vibrational and rotational energy transitions simultaneously as shown in the following
figure.
The schematic energy level diagram for the molecules given below:
The total energy of the molecule is sum of the electronic, vibrational and rotational energies:
Etotal = E electronic + E vibrational + E rotational
E electronic> E vibrational > E rotational
and
5. Types of spectroscopy
Based on the nature of the radiation that is being absorbed or emitted, there are different types of
spectroscopy. These could be:

Ultraviolet/visible spectroscopy
THE UNIVERSITY OF DODOMA, COURSE CODE: CH 3103
INSTRUCTOR: PROF. VEGI MAHESWARA RAO
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INSTRUMENTAL METHODS IN ANALYTICAL CHEMISTRY

Infra-red (I R) spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy

Mass spectroscopy

X- ray spectroscopy
Apart from the above classification, spectroscopy can also be broadly classified into three
categories:

Electronic spectroscopy

Vibrational spectroscopy

Rotational spectroscopy
The electronic spectroscopy involves the movement of electrons between their various energy
levels, as a result of absorption or emission of electromagnetic radiation. The energy required for
electronic transition comes from ultraviolet or visible region.
Vibrational spectroscopy is concerned with the vibrations of molecules when photons of radiation
are absorbed. Molecules can vibrate with particular amounts of energy whose values are determined
by a quantum number. The energy needed for vibration of molecule comes from the infra-red
region.
Rotational spectroscopy is concerned with the energy that causes the rotation of a molecule. The
energy of the rotational level depends on the rotational quantum numbers. The energy required for
the molecular rotation is provided by the micro wave region of the spectrum.
THE UNIVERSITY OF DODOMA, COURSE CODE: CH 3103
INSTRUCTOR: PROF. VEGI MAHESWARA RAO
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