Raman spectroscopy Principles, Theory, Instrumentation,Application

1. Presentation on Advanced spectral analysis
Topic-Raman Spectroscopy
Submitted to
Mr. Rajat Ghosh
Asst. Professor ,Dept. of Pharmacy
Tripura University(a central University)
Submitted by
Tapas Majumder
Enrollment no-2106240007
Dept. of Pharmacy
M.Pharmacy(2nd sem)

2. CONTENTS
1. Introduction
2. Principle
3. Instrumentation
4. Application
5. Advantages
6. Limitations
7. References
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Sir ChandrasekharaVenkata Raman(1888-1970)

3. INTRODUCTION:-
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o In 1928 the Indian physicist C. V. Raman discovered that the visible wavelength of a small fraction of the
radiation scattered by certain moleculesdiffers from that of the incident beam and furthermore that the shifts
in wavelength depend on the chemical structureof the molecules responsiblefor the scattering. Raman was
awarded the 1931 Nobel prize in physics for this discovery and for his systematic explorationof it. National
science day is celebratedday is celebratedon 28th February each year to mark the discovery of the Raman
effect by Indian physicist Sir C.V. Raman on 28th February 1928.
o Raman spectroscopyis a scattering technique.
o It can be easily understoodif the incident ray considered as consistingof particlesor photons that strikethe
moleculeof the sample. Most of the encounters are elastic i.e the photons are scatteredwith unchanged
energy and frequency. However, sometimes the moleculetakes up energy from the photons or give up
energy too the photons. They are thereby scatteredwith diminishedor increasedenergy with hence with
lower or higher frequency. The frequency shifts are thus measures of the amounts of energy involvedin
the transitionbetween the initialand final states of the scatteringmolecule, this shift is known as
Raman shift.
o Raman spectroscopyprovides a molecularfingerprint of the sample molecule.

4. Principle
o Raman spectroscopymainly based on the difference in frequency and wavelength of scattered photon from
incident photon. Difference in frequency and wavelength because of interaction of incident photonwith the
sample which their transition from ground energy levels to ‘Virtual’excited states. These excited states are
highly unstableand particles decay instantaneouslyto the ground state by one of the following 3 different
processes. They are-
o Rayleighscatteringor Elasticscattering- In this scattering the photon energy and the stateof the
moleculeafter the scatteringevents are unchanged. Hence ,Rayleighscatteredlight does not contain
much information about the structureof molecularstates.
o Inelasticscatteringor Raman scattering-In this scattering the frequency of photons of monochromatic
light changes upon interactionwith the vibrationalmodes of a molecule. This effect was first discovered
experimentally by C.V. Raman in 1928in an experiment using the sun as a light source.
o 2 types –a) STOKES SCATTERING
o b) ANTI-STOKESSCATTERING 4

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o Stokes scattering-When the moleculegains energy ΔE, the photon will be scatteredwith energy hν − hνvib
and the scattering is known as Stokes’scattering and hence they possess lowerfrequency or energy and
higher wavelength compare to incident photon.
o Anti-stokes scattering-Conversely, if the moleculeloses energy ΔE, the scatteredphoton will have energy
hν + hνvib, and this type of scattering is known as anti-Stokes’scattering. Generally, Stokes’radiation is stronger
than the anti-Stokes’radiation and hence they possess higher frequency or energy and shorterwavelength.

6. INSTRUMENTATION
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7. A Raman spectrometerrequires a light source, a sampleholderor cell, collectinglenses ,Notch filters,
wavelengthselectiondevices and detector, along with the usual Signal Processingand displayequipment.
Since Raman spectroscopymeasures scattered radiation, the light source and simple cell are usuallyplaced at 90°
to the wavelength selector. The Radiation is being measured in Raman spectroscopyis either visibleor NIR.
LIGHT SOURCES:-Monochromaticlight sources are required for Raman spectroscopy. The Raman signal is
directlyproportionalto the power of the light sourcewhich makes the laseris both monochromaticand very
intensea desirablelight source. The use of these intense light sources has greatly expanded the application of
Raman spectroscopybecause of the dramaticallyincreased intensity of the signal and the simultaneous
improvement of the signal to noise ratio.
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8. FILTER:-
o It is used to eliminate unwanted radiation and getting monochromaticradiations
o They may be made of nickel oxide glass or Quartz glass.
o Holographic interferencefilters Edge filter and HolographicNotch filter, have been developed that
dramaticallyreduce the amount of Rayleigh scattering reaching the detector.
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SAMPLE- ILLUMINATION SYSTEM:-
o Sample handlingeasier than IR because glass can be used for windows, lenses and otheroptical
componentsinstead of the more fragile atmosphericallyless stablecrystalline halides.
o In addition , the laser source is easily focused on a small sample area and the emitted radiation
efficiently transportedto the slit or entranceapertureof a spectrometer. As a result , very small
samples can be investigated.

9. o LIQUID SAMPLES:- Water is weak Raman scattered but a strong absorberof IR radiation. Thus, aqueous
solutionscan be studied by Raman but not by IR.
o SOLID SAMPLES:- Raman spectra of solid samples are often acquired by filling a small cavity with the
sample after it has been ground to a fine powder. Polymers can usuallybe examined directly with no simple
treatment.
o GAS SAMPLES:- Gas are normally contain glass tubes ,1-2 cm in diameter and 1mm thick . Gases can also
be sealed in small capillary tubes.
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10. WAVE LENGTH- SELECTION DEVICE(Monochromator)
o The Wavelength selector is the most critical componentin a Raman spectrometerthrough which the intensity
information of individual frequencies is extracted. There are basically two types of wavelength selection
mechanisms, dispersive and non dispersive. A dispersivespectrometerrelieson a dispersivecomponents to
separatelight spatiallyaccordingto the wavelengthof the diffractiongrating and prism. For the non
dispersivespectrometerlight can be selectedeither by an opticalfilteror by an interferometer, such as
the Ft- Ramanspectrometer.
o A high qualitywavelength selection device is required in Raman to separate the relatively weak Raman lines
from the intense Rayleigh-scattered radiation.
o Traditional dispersive Raman spectrometersused doubleor even triple grating monochromators.
DETECTOR OR TRANSDUCER:-
o Instruments with monochromatorinvisible use photomultiplier tube as transducer or detector because of the weak signals being
measured. Many spectrometers also used Photon counting systems to measure the Raman intensity because Photon counting is
inherently a digital technique such systems are readily interfaced to modern computersystem.
o The photodiode array was the first array detector to be used. It allows the simultaneous collection of entire Raman spectra.
photodiode arrays are typically used in conjunction with an image intensifier to amplify the Weak Raman signal.
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11. o Most recently charge transfer devices ,such as CCDs(chargecoupleddevice) and charge injectiondevices
(CID),have been used in Raman spectrometers. This instrument contains high qualitybandpassand band
rejection filters to provide good straylight rejection the CCD array can be a two dimensional array or in some
cases a Linear array.
o PHOTOMULITIPLER TUBE:- consists of a photocathode,series of dynodesand an anode.
o Photonincident on the cathodegenerate electronsdue to photoelectriceffect. These electronsare accelerated in
a high electric field between cathodeand adjacent dynode. The acceleratedelectron impinges on the dynode
generating additionalelectronsdue to secondary emission. A cascade of such dynodestructuresquickly
amplifies the number of electronswhich generates a large current pulse when it impinges on the anode.
o In comparison with detectors, the advantages of PMT are their high gain ,low noise level and short transit time.
o The operation voltage in typical commercial PMTs is above 1000V.
o Because of their large active region PMT are mainly used for single channel detection.
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12. CHARGE COUPLEDDEVICE:-
o A CCD or Charge Coupled Device is a highly sensitive photon detector. It is divided up into a large number of
light-sensitive small areas known as pixels, which can be used to assemble an image of the area of interest.
o A CCD is a silicon-based multi-channel array detectorof UV, visible and near-infra light. These are used for
spectroscopy, since they are extremely sensitiveto light.That makes these detectorssuitable for analysis of the
inherentlyweak Raman signal. It also allows multi-channeloperation, meaning the entire spectrum can be
detected in a single acquisition.
o CCD detectorsare typically one dimensional, referred to as linear, or two dimensional, referred to as area arrays
of thousandsor millions of individual detectorelements. Those elements are known as pixels. Each element
interacts with light to build up a charge. The brighter the light, and/orthe longer the interaction, the more charge
is registered. At the end of the measurement, readoutelectronicspull the charge from the elements, and each
individual charge reading is measured.
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13. Application
Raman Spectroscopyis used in many various fields for both qualitative and quantitative analyses of the molecule
because it provides information very easily and rapidly. Let discuss some of the major fields where Raman
spectroscopyis widely used.
1. Life sciences;-
• Identification and Classification of Cells
• Stem Cell Analysis
• Monitoringthe contentsof Proteins, Lipids, and Nucleic acid
2. Pharmaceuticalsand Cosmetics:-
• To Determine the Distribution ofAPI
• Early Drug Development
• Cosmetics Analysis
• Quality control for the Final Product
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14. Advantages
o Can be used with solids and liquids.
o No sample preparationneeded.
o It is highly non-destructive.
o Not interfered by water.
o Raman spectra are acquired quickly within a seconds.
o Samples can be measured or analyzed through glass or a polymer packaging.
o Laser light and Raman scattered light can be transmitted by optical fibers over long distance for
remote analysis.
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15. Limitation
o Fluorescencecan often contaminateRaman Spectra.
o More expansive.
o Sample heating through the intense laser radiation can destroy the sample or cover the Raman
spectra.
o It is not suitablefor metal alloys.
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16. References
o https://www.intechopen.com/chapters/72277
o https://www.jasco-global.com/principle/1-what-is-raman-spectroscopy/
o https://studiousguy.com/raman-spectroscopy-uses/
o https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Molecular_and_Atomic_Spectroscopy_(Wenzel
)/5%3A_Raman_Spectroscopy
o http://www.science4heritage.org/COSTG7/booklet/chapters/raman.htm
o https://www.bruker.com/en/products-and-solutions/infrared-and-raman/raman-spectrometers/what-is-raman-
spectroscopy.html
o https://www.edinst.com/blog/spectral-resolution-in-raman-spectroscopy/
o https://en.wikipedia.org/wiki/Photomultiplier_tube
o https://en.wikipedia.org/wiki/Charge-coupled_device
o https://youtu.be/1_IqMY6t6w0
o https://youtu.be/cXCl9k757cc
o https://youtu.be/m8TnYWwgUTE
o https://youtu.be/50HyHc6tHJI
o Principles of Instrumental analysis by Skoog, Holler,Crouch7th edition, chapter18th ,page no-437-451
o Undergraduateinstrumentalanalysis by James w. Robinson 7th edition , page no-321-348 16

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