introduction to spectroscopy

1. Lecture # 01
Dr. Abdul Rehman
Assistant Professor
Department of Microbiology and Biotechnology
Abasyn University, Peshawar
Introduction to Spectroscopy
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2. What is Spectroscopy..?
Spectroscopy is the study of the interaction between matter and
electromagnetic radiation as a function of wavelength (λ). Or
Spectroscopy is the use of the absorption, emission or scanning of
electromagnetic radiation by matter in order to qualitatively or
quantitatively study of matter. The matter can be atoms, molecules or
solids.
In fact, historically, spectroscopy referred to the use of visible light
dispersed accordingly to its wavelength e.g. by a prism.
Later the concept was expanded greatly to include any interaction with
radiative energy as a function of its wavelength or frequency.
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3. Cont…
Spectroscopic data is often represented by an emission spectrum, a plot of
the response of interest as a function of wavelength or frequency.
Spectral measurement devices are referred to as spectrometers,
spectrophotometers, spectrographs or spectral analyzers.
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4. History of Spectroscopy..
The history of spectroscopy began in the 17th century.
Advances in optics, specifically prisms, enabled systematic observations
of the solar spectrum.
Isaac Newton first applied the word spectrum to describe the rainbow
of colors that combine to form white light.
During the early 1800s, Joseph von Fraunhofer made experimental
advances with dispersive spectrometers that enabled spectroscopy to
become a more precise and quantitative scientific technique.
Since then, spectroscopy has played and continues to play a significant
role in chemistry, physics and astronomy.
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5. Electromagnetic Radiation..
Electromagnetic radiation—light—is a form of energy whose behavior is
described by the properties of both waves and particles.
Some properties of electromagnetic radiation, such as its refraction when
it passes from one medium to another are explained best in Fig. 1 by
describing light as a wave.
Other properties, such as absorption and emission, are better described by
treating light as a particle.
 The exact nature of electromagnetic radiation remains unclear, as it has
since the development of quantum mechanics in the first quarter of the
20th century.
Nevertheless, the dual models of wave and particle behavior provide a
useful description for electromagnetic radiation.
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6. Fig. 1 Dual Nature of Electromagnetic Radiation
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7. Wave Properties of Electromagnetic Radiation..
Electromagnetic radiation consists of oscillating electric and magnetic
fields that propagate through space along a linear path and with a
constant velocity.
In a vacuum electromagnetic radiation travels at the speed of light, c,
which is 2.997 92 × 10e8 m/s.
When electromagnetic radiation moves through a medium other than
a vacuum its velocity, v, is less than the speed of light in a vacuum.
The difference between v and c is .sufficiently small (<0.1%)
The oscillations in the electric and magnetic fields are perpendicular to
each other, and to the direction of the wave’s propagation.
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8. Particle Properties of Electromagnetic Radiation..
When matter absorbs electromagnetic radiation it undergoes a change in
energy.
The interaction between matter and electromagnetic radiation is easiest to
understand if we assume that radiation consists of a beam of energetic
particles called photons.
When a photon is absorbed by a sample it is “destroyed,” and its energy
acquired by the sample.
The energy of a photon, in joules, is related to its frequency by the following
equation.
E = hv
where h is Planck’s constant, which has a value of 6.626 × 10–34 J . s.
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9. What is Wavelength..?
Forms of electromagnetic radiation like radio waves, light waves or
infrared (heat) waves make characteristic patterns as they travel
through space.
Each wave has a certain shape and length. The distance between peaks
(high points) is called wavelength.
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10. Techniques in Spectroscopy..
1) Reflection of Light:
Reflection is a process whereby incoming EM radiations is
reflected after interaction with the surface of an object.
2) Scattering of Light:
Scattering is a random process whereby EM radiation is
absorbed and immediately re-emitted by particles or molecules.
3) Absorption of Light:
Absorption is a process whereby EM radiation is absorbed by
particles or molecules and converted to another form of energy.
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11. Cont…
4) Emission Spectrum:
The emission spectrum of a chemical element or chemical
compound is the spectrum of frequencies of EM radiation emitted due to an
atoms making a transition from a high energy state to a lower energy state.
The energy of the emitted photon is equal to the energy difference between
the two states.
5) Spectrum:
The data that is obtained from spectroscopy is called a
spectrum. A spectrum is a plot of the intensity of energy detected versus the
wavelength of the energy.
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13. Types of Spectroscopy..
1. ABSORPTION SPECTROSCOPY
Absorption spectroscopy uses the range of electromagnetic spectra into which
a substance can be absorbed.
“Absorption” is the phenomenon that occurs when a transition from a lower
level to a higher level takes place with transfer of energy from the radiation
field to the atom or molecule.
When atoms or molecules absorb light, the incoming energy excites a structure
(in energy quanta) to a higher energy level. The type of excitation depends on
the light wavelength. Electrons are promoted to higher orbits by ultraviolet or
visible light. Vibrations are excited by infrared light and microwaves excite the
rotations.
An absorption spectrum is a way to represent the absorption of light as a
function of wavelength. The spectrum of an atom or molecule depends on its
energy-level structure, and absorption spectra are useful for identifying
compounds.
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15. 2. EMISSION SPECTROSCOPY
“Emission” occurs during transition from a higher level to a lower level if
energy is transferred to the radiation field.
When no radiation is emitted the phenomenon is called “nonradiative
decay.”
This type of spectroscopy relies on the range of electromagnetic spectra
in which a particular substance radiates.
The substance first absorbs energy and then radiates (that is, emits) this
energy as light.
The excitement energy that is absorbed first can come from a number of
different sources, including collision (from high temperatures or other
means), chemical reactions or light.
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16. Cont…
It also should be noted that atoms or molecules once excited to high-
energy levels then can decay to lower levels by emitting radiation.
This is called emission or luminescence.
When atoms are excited by a high-temperature energy source this light
emission commonly is called atomic or optical emission, and for atoms
excited with light, it is called atomic fluorescence.
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18. 2. SCATTERING SPECTROSCOPY
“Scattering” refers to light that is changed in direction (called redirection)
from its interaction with matter. It may or may not occur with energy
transfer.
This spectroscopy form measures certain physical properties by
determining the amount of light that a particular substance scatters at
different wavelengths.
It differs from other spectroscopy types primarily because of speed. The
scattering process is much faster than absorption or emission.
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20. Spectrophotometry..
Spectrophotometry is the quantitative measurement of the reflection
or transmission properties of a material as a function of wavelength.
Spectrophotometry uses photometers that can measure a light beam's
intensity as a function of its wavelength known as
spectrophotometers.
Important features of spectrophotometers are spectral bandwidth, the
percentage of sample-transmission, the logarithmic range of sample-
absorption, and sometimes a percentage of reflectance measurement.
A spectrophotometer is commonly used for the measurement of
transmittance or reflectance of solutions, transparent or opaque
solids, such as polished glass, or gases.
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21. Types of Spectrophotometr..
There are two major classes of devices: single beam and double beam.
A double beam spectrophotometer compares the light intensity
between two light paths, one path containing a reference sample and
the other the test sample.
A single-beam spectrophotometer measures the relative light intensity
of the beam before and after a test sample is inserted.
Although comparison measurements from double-beam instruments
are easier and more stable, while single-beam instruments can have a
larger dynamic range and are optically simpler and more compact.
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22. Double Beam Spectrophotometer
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23. Single Beam Spectrophotometer
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25. UV-visible spectrophotometry..
Ultraviolet–visible spectroscopy or
ultraviolet-visible spectrophotometry (UV-
Vis or UV/Vis) refers to absorption
spectroscopy in the ultraviolet-visible
spectral region.
This means it uses light in the visible and
adjacent (near-UV and near-infrared [NIR])
ranges.
The absorption or reflectance in the visible
range directly affects the perceived color of
the chemicals involved.
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26. Applications of UV/Visible Spectrophotometry..
UV/Vis spectroscopy is routinely used in analytical chemistry for the
quantitative determination of different analytes, such as
1. Transition Metal Ions
2. Highly Conjugated Organic Compounds
3. Biological Macromolecules.
Organic compounds, also absorb light in the UV or visible regions of the
electromagnetic spectrum. The solvents for these determinations are often
water for water-soluble compounds, or ethanol for organic-soluble
compounds.
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27. IR spectrophotometry..
Infrared spectroscopy (IR spectroscopy or Vibrational Spectroscopy) is the
spectroscopy that deals with the infrared region of the electromagnetic
spectrum, that is light with a longer wavelength and lower frequency than
visible light.
It covers a range of techniques, mostly based on absorption spectroscopy.
As with all spectroscopic techniques, it can be used to identify and study
chemicals.
For a given sample which may be solid, liquid, or gaseous, the method or
technique of infrared spectroscopy uses an instrument called an infrared
spectrometer (or spectrophotometer) to produce an infrared spectrum.
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28. Applications of IR Spectrophotometry..
Infrared spectroscopy is a simple and reliable technique widely used in both
organic and inorganic chemistry, in research and industry.
It is used in quality control, dynamic measurement, and monitoring
applications such as the long-term unattended measurement of CO2
concentrations in greenhouses and growth chambers by infrared gas
analyzers.
It is also used in forensic analysis in both criminal and civil cases, for
example in identifying polymer degradation. It can be used in determining
the blood alcohol content of a suspected drunk driver.
IR-spectroscopy has been successfully used in analysis and identification of
pigments in paintings and other art objects such as illuminated
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