Scope of applications of Raman Spectroscopy

Scope of applications of Raman Spectroscopy
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Raman Spectra of Minerals (Image Courtesy : https://eps.wustl.edu)

Raman Spectroscopy offers several advantages over IR spectroscopy which can be summarized as:

  • Non – destructive and requiring no sample preparation
  • Convenience of handling aqueous samples
  • Mid-IR results in complex overlapping bands which can make interpretation difficult. In contrast Raman spectroscopy offers clean, narrow bands with little or no overtone or combination bands
  • Up-gradation with microscope or fiber optic probes extends the applications to difficult to access sampling points and samples varying from microscopic to large sizes.

Pharmaceutical applications have already been covered earlier. In this article scope of applications in other areas is briefly explored

Organic Compounds

Both IR and Raman spectroscopy complement each other for providing information on presence of functional groups and structural arrangements. The stretching vibrations of functional groups provide the difficult to detect IR signals whereas in the Raman spectra these are strong and give useful information on the surroundings of the group in the molecule. Raman spectroscopy also provides valuable information for measuring symmetric vibrations from C – C bonds and diatomic molecules such as O2 and N2.

Inorganic Compounds

Raman spectroscopy finds application in analysis of inorganic samples for several reasons

  • Laser light sources cover a broad spectrum covering regions from visible to IR to near- IR
  • Glass containers can be used for the analysis of aqueous inorganic samples
  • Spectra can be recorded for all three phases – solids, liquids or gases

Applications include studies on metal – ligand bonding and homonuclear diatomic molecules

Biological and life sciences

Studies have been successfully conducted on biological specimens including body fluids, tissues, cellular analysis and microbial identification. In combination with microscope it is possible to trace the adsorption behaviour of creams and other medical applications on the skin.

Material Characterization

Raman spectroscopy is a sound approach in fingerprinting of solids, liquids and gases. Due to coherence of laser light sources and addition of microscope it is possible to characterize extremely small sample volumes and sizes leading to characterization of nano materials for structural and conformational characteristics.

Process Monitoring

Real-time monitoring of gases, liquids or even solid manufacturing lines is possible using Raman spectroscopy. Sampling options such as fiber optic probes extends measurement of samples remotely even in tanks and reaction vessels.

Semiconductors

Raman spectroscopy finds applications in quality control of semiconductor materials such as GaAs, InGaAs,Si,etc.In combination with a microscope it is possible to determine microstructure of thin semiconductor films and monitor the wafer structure for identification of surface defects

Geological and Gemstone Analysis

Raman bands of minerals are generally sharp with little overlap so provide a rapid analysis for characterization of complex mineral mixtures.

Due to its nondestructive and non-invasive sample handling Raman spectroscopy has been successfully adapted to

  • Authenticate and grade gemstones
  • Identification of crystal types
  • Identify solid, liquid or gaseous inclusions in gemstones

Forensic Applications

Raman spectroscopy has proved to be a powerful tool in crime investigations. The reasons justifying its application in this area are:

  • Ability to handle both organic and inorganic compounds
  • Small sample sizes with concentrations down to picomoles.
  • Examination of samples in-situ, inside glass or plastic containers
  • Non-destructive and non-invasive testing

These features are being exploited for investigation of illicit drugs counterfeit materials, fraudulent documents, explosives, gunshot residues, poisons, inks, paints, fibers and hair residues recovered from scene of crime

The technique has been applied in diverse application areas but has some inherent limitations:

  • Raman activitiy is largely affected by interfering Rayleigh scattering.This limitation can be overcome by adopting signal is enhancing techniques
  • Some samples are intrinsically fluorescent and some when observed in glass containers having rare earth element impurities upon irradiation can fluorescence. This signal can obscure the Raman signal
  • Resolution of the Raman spectrometer is limited by the resolution of the monochromator

The benefits far outweigh the limitations and as a result new applications are evolving at a rapid pace.

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