Raman spectroscopy

Introduction and implicit in Theory

Raman spectrometry is a spectroscopic technique named after and discovered by Sir Chandrasekhara Venkata Raman in 1928. Sir Raman published work in 1922 on the “The Molecular Diffraction of Light” which was the initial probe which finally led to the find of Raman Spectroscopy ( 1 ) . Along with associates Sir Raman investigated this freshly discovered spectrometry and was the first Asiatic individual to be awarded the Nobel Prize for Physics in 1930 ( 2 ) .

Raman Spectroscopy is a light sprinkling technique. It can be described as a procedure where a photon of light interacts with a sample to bring forth scattered radiation of different wavelengths. When monochromatic visible radiation is focused upon a sample, it will interact with the sample. The visible radiation may be deflected, absorbed or scattered in some manner ( 3 ) .

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On analysis of the frequence of the scattered radiation, a little sum of radiation is seen which is scattered at different wavelengths, and besides the incident radiation wavelength. This incident radiation wavelength is known as Rayleigh Scattering. The little sums of the radiation scattered at different wavelengths is known as Stokes and Anti-Stokes Raman Scattering ( 3 ) . Described by Lord Rayleigh, Rayleigh Scatter is the dispersing procedure without any alteration in frequence. Depending on the vibrational province of the molecule, Raman shifted photons of visible radiation can be of higher or lower energy.

Approximately merely 1 ten 10-7 of the scattered visible radiation is Raman. The alteration in wavelength of the scattered photon provides the chemical and structural information ( 3 ) .

As mentioned above, it has been established that visible radiation which is scattered from a molecule has assorted constituents – the Rayleigh spread and the Stokes and Anti-Stokes Raman spread. The Raman Effect being due to photons being scattered inelastically, losing or deriving energy as consequence of their interactions with the vibrational molecules of the sample ( 17 ) . These frequences are in the scope associated with rotational, vibrational and electronic degree passages. Due to the scattered radiation happening in all waies there may besides be noticeable alterations in the polarization and wavelength of the molecule ( 3 ) .

The stronger of the two procedures is Stokes Dispersing where photons are scattered at low energy. The population province of a molecule is by and large in its land vibrational province ; this is the larger Raman dispersing consequence. When a little figure of molecules are at a higher vibrational degree, photons can be scattered at higher energy. This is the weaker Anti-Stokes Raman sprinkling.

Incident photons will respond with the present molecule, and the energy lost or gained by a photon assistance designation of the type of bond nowadays ( 3 ) . Depending on the symmetricalness of the molecule non all quivers will be observed with Raman spectrometry. However adequate information is attained in most instances to set up some individuality of the molecular construction.

Changes in vibrational, rotational and electronic energies of a molecule can do Raman dispersing to happen. The energy of the quiver of the sprinkling molecule is equal to the difference in the energy between the incident molecule and the Raman scattered photon ( 4 ) .

The procedure of light soaking up requires the energy of the incident photon to be equal to the difference in energy between two provinces of the molecule, and the passage between the two provinces being accompanied by a dipole minute alteration in the molecule. The molecule will now be found in an electronic aroused province or a vibrational aroused province ( 5 ) .

Photons that interact with a molecule, but are non absorbed, will be scattered and the incident photons will non necessitate to be between the two provinces of the molecule for dispersing to happen. Here the photon polarizes the electronic cloud of the molecule and this causes the formation of a practical aroused province. This is an highly short lived aroused province and the energy of the photon will be re-radiated.

During re-radiation of the photon, the photon is said to be elastically scattered ( 5 ) .

Instrumentality

The Raman system typically consists of four major constituents:

I ) Excitation Source ( Laser )

two ) Sample light system and light aggregation optics

three ) Wavelength picker ( Filter or Spectrophotometer )

four ) Detector ( Photo rectifying tube array, Charged Coupled Devices or Photomultiplier Tubes )

The sample is illuminated with a optical maser beam in the Ultraviolet, Visible or near-infrared scope. The cheapest light beginning is the He/Ne optical maser at 638.2nm. A better beginning is the Argon ion optical maser at 488.0 nanometer. However the best instruments now use the close IR Nd-YAG ( Neodymium ions in Yttrium aluminum garnet ) optical maser at 1064nm ( 6 ) . A lens collects scattered light and sends it through an intervention filter or spectrophotometer to obtain the Raman spectrum of a sample ( 7 ) .

As random Raman sprinkling is weak, the chief trouble of Raman spectrometry is dividing the Raman dispersing from intense Rayleigh sprinkling. A point of concern is the fact the strength of isolated visible radiation from the Rayleigh sprinkling may transcend the strength of the Raman Effect. By cutting the spectral scope near to the optical maser line where stray visible radiation has most consequence, this job can be overcome. Commercially available intervention filters are used which cut off the spectral scope of ± 80-120 cm-1 from the optical maser line ( 7 ) . This method does non allow the sensing of low frequence Raman modes in the scope below 100 cm-1.

Upon light scattering on grates, isolated visible radiation is generated in the spectrometer. The quality of visible radiation produced depends on the status and quality of the grate. Holographic grates are by and large used in Raman spectrometers. These have less fabrication defects than the ruled grates. Stray light produced from the ruled grates is more intense than isolated visible radiation produced from the holographic grate. Reducing isolated visible radiation can besides be done utilizing multiple scattering phases. Using multiple scattering phases without the usage of notch filters, Raman frequencies every bit low as 3 – 5 cm-1 can be detected ( 7 ) .

Over the old ages analysts have used single-point sensors such as photon-counting Photomultiplier Tubes ( PMT ) . To obtain a Raman spectrum of nice quality, longer exposure times are frequently required. This is due to the failing of a typical Raman signal. In recent times, there has been an addition in research labs worldwide where research workers are utilizing multi-channel sensors like Photo rectifying tube Arrays ( PDA ) , or, Charge-Coupled Devices ( CCD ) for observing Raman scattered visible radiation. Sensitivity and public presentation of modern CCD detectorsare bettering and so are going the sensors of pick for Raman spectrometry ( 7 ) .

Comparison to Infra-red Spectroscopy

Raman spectrometry combines the advantages of Near-IR spectrometry with the advantages of Infrared spectrometry. Both Raman and Infrared spectroscopy excite cardinal molecular quivers even though their fingerprinting techniques are based on different physical procedures ( 8 ) . In infrared a vibrational manner of the molecule is required to hold a alteration in dipole minute. Radiation of the same frequence can now interact with the molecule and promote it to an aroused vibrational province. In Raman spectrometry, dispersing involves the deformation of negatrons around a bond, with remittal of the radiation as the bond returns to its normal province ( 16 ) . Water does non interfere with Raman spectrometry and so Raman is more utile than infrared when working with aqueous samples. Raman is besides rather utile for symmetrical molecules which have zero dipole minute. These molecules are non suited for Infrared ( 8 ) .

Raman spectrometry besides requires small sample readying in comparing to Infrared spectrometry. In line procedure control and distant analysis is besides possible with Raman. Infrared is merely used for qualitative analysis, whereas with Raman quantitative and qualitative analysis is possible. Glass containers are besides used in Raman. Infrared Spectroscopy does n’t hold jobs with background fluorescence, where in Raman ; fluorescence is ill-famed and can even dissemble the spectra. The sample can besides be damaged by optical maser visible radiation in Raman ( 5 ) .

Advantages & A ; Disadvantages

With the increasing technological progresss in computing machine and optical maser design, Raman spectrometry has going a more feasible option for everyday analysis in research labs worldwide ( 11 ) .

Raman spectrometry has a figure of advantages over other analysis techniques. Because Raman spectrometry is a scattering procedure, samples of any size or form can be used.

Very little sums of stuff can be studied down to microscopic degrees in the scope of 10 micrometers. The part from 80-500 cm-1 can be studied with no alterations on the same instrument ( 9 ) . Raman spectrometry can be used with solids, liquids or gasses. As mentioned above no sample readying is needed and it is a non destructive technique. There is no concern with sample thickness, size or form.

Furthermore samples can be analysed straight in bottles, bags or blisters ( 11 ) .

Besides no vacuity is needed which saves on assorted expensive pieces of vacuity equipment. In footings of clip, the spectra are produced comparatively rapidly. Further advantages include usage of aqueous solutions, usage of glass and the usage of down fibers ocular overseas telegrams for distant sampling ( 10 ) .

Disadvantages include that metals or metals can non be used. The Raman Effect is really weak, which leads to low sensitiveness, doing it hard to mensurate low concentrations of a substance ( 10 ) . Sample heating through the intense optical maser radiation can destruct the sample or dissemble the Raman spectrum. Serious jobs in Raman occur when big background signals from fluorescence from drosss or the sample itself arise ( 8 ) .

Other disadvantages of Raman spectrometry include equipment cost and the sensitiveness of the technique. Raman spectrophotometers can be rather dearly-won, depending on their applications, and the technique by and large can non vie with chromatography for analytical sensitiveness in quantitative analyses ( 11 ) .

Applications & A ; the Future of Raman Spectroscopy

Raman spectrometry is an of import analytical and research tool, being used for applications as broad runing as pharmaceutical, forensic scientific discipline, thin movies, polymers, geology and planetal scientific discipline, humanistic disciplines, and semiconducting materials.

Presently there are many applications of Raman Spectroscopy. Some illustrations of the many in pattern include, Surface Enhanced Raman Spectroscopy, Resonance Raman Spectroscopy, Surface Enhanced Resonance Raman spectrometry, Hyper Raman, Spontaneous Raman, Coherent Anti-Stokes Raman, and Transmission Raman.

Surface Enhanced Raman spectrometry is a technique in which the sample is adsorbed onto a colloidal metal atom surface. Silver or gold is normally used. The adsorbed molecule produces Raman lines on spectra which are enhanced by 103 to 106 ( 17 ) .

Presently a few utilizations of Raman spectroscopy are on the border of being groundbreaking in the fact Raman had n’t been used in this manner antecedently. An illustration is Raman ‘s usage in Urology. Here it has been used to observe alterations at the molecular degree during the pathological transmutation of biological tissue. Raman spectroscopy has shown some encouraging consequences in the in vitro diagnosis of malignant neoplastic diseases of the vesica and prostrate. Raman showed itself to be an exciting tool for existent clip diagnosing and in-vivo rating of life tissue ( 12 ) .

Another similar survey showed that Raman can be used to accurately place benign prostate hyperplasia at three different classs of prostate glandular cancer in vitro ( 13 ) .

Raman spectrometry has been utile in dental medicine to analyze dental difficult tissue and concretion. The involvement here is the mineral constituents in enamel, dentin and concretion, and to calcium fluoride formed on/in enamel ( 14 ) . Raman spectrometry late has been used in coronary arteria disease with delighting consequences which encourage its usage further in that peculiar field. A 1.5mm Raman catheter capable of roll uping Raman spectra in the fingerprint and the high-wave figure parts is used to mensurate the chemical and molecular composing of coronary atherosclerotic lesions. Consequences showed that distinguishable spectral differences can be identified by intracoronary Raman spectrometry in vivo ( 15 ) .

Overall it is just to state that Raman spectrometry is non ever regarded as the first pick for analysis. However, it is doubtless a critical analytical tool available, which has some clear advantages over other methods of analysis. If these advantages and Raman ‘s other good properties are exposed more, Raman spectrometry could one time once more be back in the heads of analytical chemists worldwide.

Mentions

1 hypertext transfer protocol: //en.wikipedia.org/wiki/Raman_scattering

2 hypertext transfer protocol: //en.wikipedia.org/wiki/C.V._Raman

3 hypertext transfer protocol: //www.horiba.com/us/en/scientific/products/raman-spectroscopy

4 hypertext transfer protocol: //www.kosi.com/Raman_Spectroscopy

5 hypertext transfer protocol: //www.scienceofspectroscopy.info/edit/index.title=Raman_Spectroscopy

6 Dr. Ken Rutt, Raman Spectroscopy, 22/10/09, Slide figure 4

7 hypertext transfer protocol: //www.princetoninstruments.com/spectroscopy/raman

8 hypertext transfer protocol: //www.raman.de/htmlEN/home/advantageEn

9 hypertext transfer protocol: //www.uwo.ca/ssw/services/raman

10 hypertext transfer protocol: //www.doitpoms.ac.uk/tlplib/raman/advantages_disadvantages.php

11 hypertext transfer protocol: //americanpharmaceuticalreview.com

Raman Spectroscopy — A Powerful Tool for Non-Routine Analysis of

Pharmaceuticals, Adina L. Enculescu and Jeffrey R. Steiginga

12 Raman spectrometry and its urological applications Vishwanath S Hanchanale, Amrith R Rao, Sakti Das Indian diary or Urology, Year: 2008 | Volume: 24 | Issue: 4 |Page: 444-450

13 The usage of Raman spectrometry to place and rate prostate glandular cancer in vitro P Crow,1* N Stone,1 C A Kendall,1 J S Uff,1 J A M Farmer,1 H Barr,1, MP J Wright2 British Journal Cancer. 2003 July 7 ; 89 ( 1 ) : 106–108 Published online 2003 July 1

14 Adv Dent Res. 1997 Nov ; 11 ( 4 ) :539-47 Raman spectrometry in dental research: a short reappraisal of recent surveies Tsuda H, Arends J.State University of Groningen, The Netherlands

15 Development of an intracoronary Raman spectrometry, Chau, Alexandra H. ( Alexandra Hung ) , 1980 Massachusetts Institute of Technology, Dept. of Mechanical Engineering

16 Principles of Instrumental Analysis, Fifth Edition, Douglas A. Skoog, F. James Holler, Timothy A. Nieman, ISBN 0-03-002078-6

17 University of Massachusetts Boston, Chemistry Department, Physical Chemical Structure Laboratory, Experiment 7, The Raman Spectrum of CCl4

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