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The study of the interaction between radiation and the matter is called spectroscopy.

Molecules in an organic compound have the tendency to absorb specific frequencies according to their structural characteristics. These frequencies can be in the range of visible light, infrared or ultraviolet radiations.


What is Spectroscopy?

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When a beam of polychromatic light is passed through a slit, it falls on a prism and gets divided into its constituent colors. This image of color is known as spectrum.
We can see only the visible region ranging from 4300 to 6900Å. The complete spectrum extends over a wide range from radio waves to gamma rays. As we move form radio waves to gamma rays, the wavelength decreases while energy and frequency increases.

This is because of energy-frequency relation.
E = h$\nu$ = $\frac{hc}{\lambda}$

E = energy
h = Planks’ constant
C = velocity of light
$\lambda$= Wave length
$\nu$= Frequency
This spectrum is also known as electromagnetic spectrum and this technique to study the interaction between matter and radiated energy is called as spectroscopy.

The determination of elemental composition by its electromagnetic or mass spectrum is called as Atomic spectroscopy which is closely related to other forms of spectroscopy. Atomic spectroscopy involves visible and ultraviolet light and shows two types of spectrum.
  1. Emission spectrum
  2. Adsorption spectrum
All atoms of different elements show distinct spectra, hence used for identification and quantization of a given sample. Since every atom has certain quantized energy levels for electrons, these electrons get excited by supplying some amount of energy.

To produce this atomic state several methods can be used but flames are the most common source of excitation because of their high temperature. Light from the hollow cathode lamp source is passed through the flame and then through a mono chromator which isolates the particular wavelength selected for analysis.

The change in intensity of the light detected by the detector and generates a voltage signal which is read out on a digital-panel-meter or recorder. Flame supplied enough amount of energy for excitation.

Since elements are not stable in excited state, they will quickly return to the ground state and emit the energy in the form of a photon of light. The energy of photon is exactly same as the energy difference between the excited state and the ground state.

There are two possible types of atomic spectra namely emission spectrum and adsorption spectrum.

Atomic Emission Spectroscopy

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When a beam of polychromatic light is passed through a prism and followed with a spectroscope, emission spectrum is obtained. Hence the light beam is examined directly by the spectroscope after passing through the prism.
For emission spectrum, the substance has to be excited, so, it emits radiation. This excitation process can be done in different ways:
  • By heating substance at a high temperature
  • By passing the electricity in a gas discharge tube
  • By passing electric discharge in metallic filament
Emission spectrum can be of two types:
  1. Continues spectrum
  2. Line spectrum
1. Continuous Emission Spectrum
  • When a dense gas or solid substances is subjected to the heat, they radiate light.
  • This emitted light shows a broad range of wavelengths hence the apparent spectrum seems smooth and continuous.
  • The sun light, tungsten filament in light bulb, electric cooking stove burners, flames, cooling fire embers form continue spectrum.
Continuous emission spectrum
2. Line Emission Spectrum
  • When atoms of any substance excited by supplying some amount of energy, they emit radiations which form bright lines in spectrum.
  • These lines are not continuous but separated by dark line. Such type of spectrum is termed as line spectrum.
  • Lines in line spectrum can be of different colors which depends on substance and temperature.
  • Generally the light from neon sign lamps, sodium vapor lamp and mercury vapor lamp gives line spectrum.
  • For example, sodium shows two yellow lines at the wave length 589.0nm and 589.6nm which are clearly visible. Hence when an atom drops from higher energy level to lower energy level, it emits some amount of energy in the form of radiation of exact energy.
  • For example, when a hydrogen atom drops from the second energy level to the first energy level, it gives off a radiation whose energy is equal to the difference of energy of both energy levels. The bright line at the specific wavelength corresponds to the energy difference resulting in a specific color giving off an emission spectrum.
Band Emission Spectrum
  • This type of spectrum is observed when the substance is in the molecular state and emits radiation in excited state.
  • Each molecule forms a certain band hence also called as molecular spectrum.
  • The best example of band spectrum is carbon arc with a metallic salt or vacuum tube.
  • In band spectrum, the luminous bands are separated by dark space and each band is composed of several fine lines.
  • Each line in band corresponds to one excitation of electron.

When a light beam passes through a cold and dilute gas, the atoms of gas absorb some certain amount of light. Due to this absorption there are some dark lines observed in spectrum. Atomic absorption process is the measurement of the amount of light which is absorbed by the cloud of atoms.

The number of atoms are directly related to the amount of absorbed light, hence as the number of atoms increase, the amount of light absorbed increases.
The excited atom cloud, required for atomic absorption spectrum, is created by supplying enough heat to the sample.

This energy is used to dissociate the chemical compounds into free atoms. Under normal flame conditions maximum number of atoms will remain in the ground state and easily absorb light from a source lamp. Because of this absorption, the dark line is observed in spectrum.

Like emission spectrum, absorption spectrum can also be of three types.

1. Continuous absorption spectrum

When a substance absorbs a continuous range of wave length, it forms continuous absorption spectrum. For example, red glass absorbs all other colors apart from red.

2. Line absorption spectrum

When absorbing substance is in vapor or gaseous state, this type of spectrum is observed which contains sharp dark lines.

3. Band absorption spectrum

This type of spectrum is observed by aqueous solution of potassium permanganate (KMnO4), which gives five absorption bands in green region. Hence the overall picture of atomic spectrum is as follows.

Atomic Spectrum

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Organic Spectroscopy

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  1. Molecules in an organic compound have the tendency to absorb specific frequencies according to their structural characteristics.
  2. These frequencies can be in the range of visible light, infrared or ultraviolet radiations.
  3. The electrons in the molecules of an organic compound undergo transition when they absorb or emit light.
  4. This is the reason that the color perceived by the organic compounds depends on the absorption of light radiations in the visible range.
  5. Infrared spectroscopy is very successful in organic chemistry.
  6. The types of bonds present in a compound as well as their lattice arrangements can be found by the absorption of IR radiations when they emit thermal radiations. The frequency at which the absorption of the radiation takes place matches the frequency of the vibrating bond.
  7. Nuclear magnetic resonance spectroscopy analyzes the magnetic properties of certain atomic nuclei like hydrogen and carbon which determines their local environments in an organic compound through which the structure of the compound can be determined.
  8. UV spectroscopy is used in highly conjugated organic compounds which absorbs UV light or light in the visible regions. When electrons within the atoms are excited from one electronic state to another, their solutions show change in color based on changes in the wavelength due to absorption of visible light by the d electrons.
  9. Organic compounds with solvents may either have significant or weak UV absorptions because the pH value and polarity of the solvent do affect the absorption capability of the organic compound.
  10. By passing a beam of IR light through a sample of organic compound the infrared spectrum of the sample can be recorded.
  11. On examining the light rays that are transmitted, we get to measure the quantity of energy absorbed at each wavelength.
  12. Absorption takes place when the IR frequency is equal to the frequency of the bond. Analysis of these absorption characteristics reveals details about the molecular structure of the sample.

More topics in Spectroscopy
Atomic Spectra Chromophore
Visible Spectroscopy HPLC Detectors
Reverse Phase HPLC UV Vis Spectroscopy
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