Raman Spectroscopy is a crucial instrumental technique to get the information about vibration mode of substances. The chemical structure, crystalline properties, and molecular interactions of matter can be identified by this advanced technique. This analytical process is completely non-destructive and very efficient. The property of a substance is measured after the interaction of light with the chemical bonds within a sample.
The light of the same wavelengths is scattered from the laser source and does not provide useful information about something. These wavelengths of light are generally known as Rayleigh Scatter. Rayleigh Scatter hits the sample molecule and a small amount of light is scattered at different wavelengths or colors based on the chemical structure of the sample analyte. The scattering phenomenon at this moment is generally called Raman Scatter.
The spectrum obtained by this analysis contains a number of peaks that show the intensity and wavelength position. Each peak relates to a definite molecular bond vibration and individual bonds like C-C, C=C, N-O, C-H, etc.
This light scattering technique was discovered and named by the Indian physicist C. V. Raman in 1928. The chemical structure as well as their characteristics can easily be justified by this technique.
It is becoming an increasingly essential instrumental qualitative and quantitative tool to find out the properties of polymeric materials.
Principle of Raman Spectroscopy
The working principle of Raman spectroscopy relates to the passing of monochromatic radiation through the sample analyte. The light of the same wavelengths from the source strikes the sample and light is then reflected, absorbed, or scattered. The scattered light wavelength has different frequencies as the vibration and rotational properties vary. The changes of wavelength in the IR spectra can be studied to evaluate the sample analyte property.
There are some differences between the scattered light and incident light energy which is known as the Raman shift. Stokes scattering happens when the energy of the scattered lights is less than the energy of an incident light. The anti-Stokes scattering phenomenon happens when the energy of the scattered lights is more than the energy of an incident light.
Instrumentation of Raman spectroscopy
Light source: This spectroscopy consists of a laser light source. The radiation of the spectrum obtained from the sample analyte depends upon the light from the laser source. It should be used a shorter wavelength to produce stronger Raman scattering.
Sample holder: A sample is placed in a sample holder where the laser light hits and it undergoes scattering. Then the scattered light passes through the filter.
Filter media: A filter is generally used in this spectroscopy for separating the Raman scattered light from the Rayleigh scattered light. This phenomenon produces high-quality Raman spectra. There are various filters used in this separation purposes like notch, long pass, and volume halogen filters.
LCD detector: This part helps to identify the scattered light signal from the sample. LCD array detector is very popular to use as a detector in this instrument. The LCD detector has the ability to detect signals of different wavelengths efficiently.
Computer: A computer is attached to this spectroscopy to draw a final spectrum for the unknown sample.
Types of Raman Spectroscopy
There are four types of Raman Spectroscopy namely Resonance Raman Spectroscopy (RRS), Micro-Raman Spectroscopy, Surface-enhanced Raman Spectroscopy (SERS), and Non-linear Raman Spectroscopic Techniques.
Application of Raman Spectroscopy
This spectroscopic technique is used to know the structure as well as bonding of CO₂, NH3, mercurous salts, halogen compounds, and chloro complexes of mercury.
It is very important to know the proper electrolytic dissociation, hydrolysis process, and transition from crystalline state to amorphous state of polymeric substances.
This technique is suitable for identifying about the presence or absence of various linkages in a molecule, the structure of compounds, and the study of isomeric compounds.
This versatile technique is essential for evaluating the polymeric compound characterization. The polymer crystallinity, tacticity, and amorphous character can be identified through this technique.
This process can be completed easily and quickly to the proper analysis of mixtures that are difficult with any other method.
This technique is used in the field of telecommunication to measure the various frequency regimes with a surplus amount of energy
This analytical tool is used to identify the structure of nanowires.
It is an essential tool to understand the structure and properties of DNAs and proteins.
It is used to identify the presence of minerals on Mars.
Light scattering types from Raman Spectroscopy:
There are two possible light scattering states from this spectroscopy which are given below:
Elastic scattering: This scattering is called Rayleigh scattering which contains the same energy as the light that initially struck the sample. It expresses the scattered light property contains the same frequency, wavelength, and color as the initial state of light.
Inelastic scattering: It is called Raman scattering which contains different energy than the light that initially struck the sample. It expresses the scattered light property contains different frequencies, wavelengths, and colors as the initial state of light
Explanation of Raman spectra
The Raman spectra can be explained by the following approaches for substance identification:
By identifying functional groups within the sample molecules
The functional group of every molecule creates a Raman spectrum at characteristic Raman shifts. The spectrum signal provides information about the progress of various types of reactions such as oxidation, polymerization, etc.
The aldehyde functional group creates a spectrum in the range of 1730 cm–1 to 1700 cm–1 due to the stretching vibration of the carbonyl group.
By using the characteristic of “fingerprint region”
The characteristic vibration of the molecular scaffolding known as skeletal vibration can be detected by the Raman spectrum. This type of vibration is generally found at Raman shifts below 1500 cm–1 and has specific characteristics for identification purposes. The region below 1500 cm–1 is a very important part of the spectrum known as the “fingerprint” region.
By computer-aided interpretation software
At present, the components from the mixture sample can be identified easily by using computer-aided interpretation software containing a comparison algorithm and a spectral database. The obtained result from this software is expressed as the Hit Quality Index (HQI) which is a matching factor for analyses.
The HQI range lies between 0 (for “no match”) to 100 (for “exact match”). This identification by the computer-aided interpretation software is very effective in identifying the substance within seconds and non-technical users can easily explain the produced results.
The difference between IR spectroscopy technique and the Raman spectroscopy technique
IR spectroscopy relates the absorption of light energy corresponding to the vibrational energy of molecules. Raman spectroscopy relates the scattering of incident light radiation from the laser source at an energy shifted by the vibrational energy of the molecule
Infrared spectroscopy is based on the absorption of light energy at specific vibrational frequencies due to the changes in the dipole moment.
Raman spectroscopy depends on the change of polarizability in a molecule at the Raman shift. The infrared spectroscopic technique is essential to the analyses of hetero-nuclear functional group vibrations as well as polar bonds in molecules. Raman spectroscopic technique is also essential to the analyses of homo-nuclear molecular bonds such as C-C, C=C, and C≡C bonds.
References:
Lyon, L. A., Keating, C. D., Fox, A. P., Baker, B. E., He, L., Nicewarner, S. R., … & Natan, M. J. (1998). Raman spectroscopy. Analytical Chemistry, 70(12), 341-362.
Mulvaney, S. P., & Keating, C. D. (2000). Raman spectroscopy. Analytical Chemistry, 72(12), 145-158.
Frequently Asked Questions (FAQ’s)
What is Raman spectroscopy?
This technique is used to identify the vibrational, rotational, and low-frequency modes of the molecules as well as get information about chemical structure, crystalline properties, and molecular interactions of sample matter quickly.
What is Raman scattering?
It is defined as the scattering of photons by the excited molecules after absorption of light radiation from the laser source.
What are the types of Raman’s Spectroscopy?
There exist four types of Raman Spectroscopy nowadays namely Resonance Raman Spectroscopy (RRS), Micro-Raman Spectroscopy, Surface-enhanced Raman Spectroscopy (SERS), and Non-linear Raman Spectroscopic Techniques.
What are the uses of Raman Spectroscopy?
It is used to know the polymer crystallinity, tacticity, and amorphous character. This technique has a crucial need in the field of pharmaceutical, petrochemical, and food industries to evaluate the rotational, vibrational, and lower-frequency modes of the molecules.
