Spectroscopic Measurment

8/3/2019 Spectroscopic Measurment

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MANUFACTURING ENGINEERING

Sem- 1 (B.Tech)

SHAILENDRA OJHA


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Most spectroscopic methodsare differentiated as either atomic or molecularbased on whether or not they apply to atoms ormolecules. Along with that distinction, they can beclassified on the nature of their interaction:

Absorption spectroscopy uses the range of theelectromagnetic spectra in which a substanceabsorbs. This includes atomic absorptionspectroscopy and various molecular techniques,

such as infrared

, ultraviolet-visible


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Absorption spectroscopy refers to spectroscopic techniques that

measure the absorption of radiation, as a function of frequencyor wavelength, due to its interaction with a sample. The sampleabsorbs energy, i.e., photons, from the radiating field. Theintensity of the absorption varies as a function of frequency, andthis variation is the absorption spectrum.

Absorption spectrum -A material's absorption spectrum is thefraction of incident radiation absorbed by the material over arange of frequencies. The absorption spectrum is primarily

determined by the atomic and molecular composition of thematerial. Radiation is more likely to be absorbed at frequenciesthat match the energy difference between two quantummechanical states of the molecules. The absorption that occursdue to a transition between two states is referred to as

an absorption line and a spectrum is typically composed of many


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1. Analytical Chemistry- Absorption spectroscopy isuseful in chemical analysis because of its specificity andits quantitative nature. The specificity of absorptionspectra allows compounds to be distinguished from oneanother in a mixture.

2. Atomic and molecular physics-Theoretical models, principally quantummechanical models, allow for the absorptionspectra of atoms and molecules to be related to

other physical properties such as electronicstructure, atomic or molecular mass,and molecular geometry. Therefore,measurements of the absorption spectrum are

used to determine these other properties.


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Infrared spectroscopy (IR spectroscopy) is

the spectroscopy that deals with the infrared region of the electromagneticspectrum, that is light with a longer wavelength andlower frequency than visible light. It covers a range of techniques, mostlybased on absorption spectroscopy. As with all spectroscopic techniques, itcan be used to identify and study chemicals

Infrared spectroscopy exploits the fact

that molecules absorb specific frequencies that are characteristic of theirstructure. These absorptions are resonant frequencies, i.e. the frequency ofthe absorbed radiation matches the frequency of the bond or group thatvibrates. The energies are determined by the shape of themolecular potential energy surfaces, the masses of the atoms, and the

associated vibronic coupling.Number of vibrational modes- 1.Symmetric

2.antisymmetric stretching,3.Scissoring4.rocking5.wagging6.twisting:


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Fourier transform infrared (FTIR) spectroscopy is a measurement technique thatallows one to record infrared spectra. Infrared light is guided through aninterferometer and then through the sample (or vice versa). A moving mirror insidethe apparatus alters the distribution of infrared light that passes through theinterferometer. The signal directly recorded, called an "interferogram", representslight output as a function of mirror position. A data-processing technique

called Fourier transform turns this raw data into the desired result (the sample'sspectrum): Light output as a function of infrared wavelength (or equivalently,wavenumber). As described above, the sample's spectrum is always compared to areference.


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1. In establishing the identity of the compounds

2. In detecting impurities in a sample .

3. To ascertain hydrogen bonding in a molecule.

4. Provides valuable information of molecular

symmetry, dipole moments , bond lengths, etc.

5. Distinguishing positional isomers of a

compound.


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Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry (UV-Vis or UV/Vis) refersto absorption spectroscopy in the ultraviolet-visible spectralregion. This means it uses light in the visible and adjacent (near-UV and near-infrared (NIR)) ranges. The absorption in the

visible range directly affects the perceived color of thechemicals involved. In this region of the electromagneticspectrum, molecules undergo electronic transitions. Thistechnique is complementary to fluorescence spectroscopy, inthat fluorescence deals with transitions from the excited state tothe ground state, while absorption measures transitions fromthe ground state to the excited state.


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The instrument used in ultraviolet-visible spectroscopy iscalled a UV/Vis spectrophotometer. It measures the intensityof light passing through a sample I, and compares it to theintensity of light before it passes through the sample Io.

The basic parts of aspectrophotometer are a light source, a holder for the sample,a diffraction grating or monochromator to separate thedifferent wavelengths of light, and a detector. The radiationsource is often a Tungsten filament (300-2500 nm), a deuterium

arc lamp, which is continuous over the ultraviolet region (190-400 nm) or more recently, light emitting diodes (LED)and Xenon arc lamps for the visible wavelengths. The detectoris typically a photodiode or a CCD. Photodiodes are used with

monochromators, which filter the light so that only light of asin le wavelen th reaches the detector.


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1. Determination of dissociation of energy of a molecule with greataccuracy from the wavelength , which separates thediscontinuous part of the spectrum from the continuous partof the spectrum.

2. In providing information regarding moment of inertia , vibrational frequency and interatomic distances in case of diatomic molecules.

3. In identification of cis or trans isomers of a compound .

4. Determination of presence of impurity in compound .

5. Determination of structure of organic of organic compounds.

Spectroscopic Measurment

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