Heating and Mixing
Electrophoresis and Blotting
Polyacrylamide Gel Electrophoresis
Agarose Gel Electrophoresis
PCR & qPCR Thermal Cycler
Thermal Cycler (PCR)
Real-time Thermal Cycler (qPCR)
PCR Workstations & Cabinets
UVP BioImaging Systems
UVP Benchtop Transilluminators
Electrophoresis & Blotting
Shaker & Mixer
Orbital Shaking Incubator
Water Purification System
Aermax - Air Purification
Medical Oxygen Concetrators
-150°C Cryogenic Freezer
-86°C Ultra Low Temp Freezer
-40°C Low Temp Freezer
-18 ~ -25°C Biomedical Freezer
-20°C Biomedical Freezer
4° ± 1°C Blood Bank Refrigerators
2~8°C Pharma Refrigerators
2~8°C ICE Lined Refrigerators
-25°C ~ + 4°C Mobile Freezer/Collers
20~24°C Blood Platelet Incubators
Raman spectroscopy is one of the most widespread spectroscopic techniques and is used in sophisticated research laboratories. It is complementary to infrared absorption spectroscopy. It involves radiating a monochromatic light source or laser on the sample and detection of the scattered light. Raman imaging and Raman spectroscopy imaging are the other terms often used for the technique.
Without interaction, the majority of the scattered light will pass through the sample. So, the detector receives energy, which is of the same frequency as the excitation source. It is known as elastic scattering. It is otherwise called Rayleigh scattering. A very meager amount of the scattered light (1 in 107) is shifted in energy from the frequency of the laser. This phenomenon is called the Stokes shift or Raman shift.
The anti-Stokes shifted Raman energy is feeble than Stokes shifted Raman energy at room temperature. So they are ignored and removed by filters.
The scattering happens because of the interactions between the incident electromagnetic waves and the vibrational energy levels of the molecular samples. The interaction is perceived as a disturbance to the electric field of the molecule.
Raman spectroscopy is not only limited to intramolecular vibrations, but also to crystal lattice vibrations and other motions of extended solids. The Raman spectra analysis is much significant in the fields of mineralogy and geochemistry.
Raman spectroscopy is explained by the interaction of the electromagnetic field with the bonds of the molecule. The dipole moment induced by the external electric field in a molecule is directly proportional to the electric field.
P = alpha*E
Where alpha is proportionality constant, represents the polarizability of the molecule. The ease with which the electron cloud around the molecule can be distorted is measured by the polarizability of the molecule. The induced dipole scatters or emits light at the frequency of the incident light. Within the bond of the molecules, there is a change in polarizability, which gives rise to Raman scattering. The intensity of the scattering is directly proportional to the square of the induced dipole moment.
When the vibration is not able to change the polarizability much, the value of the polarizability equals near-zero, leading to the lower intensity of the Raman spectrum. The vibrations of the highly polar groups like O-H bonds are generally weak. So, when an external electric field is incapable to induce a greater change in the dipole moment, and bending or stretching of bonds will not happen, leading to a weak Raman signal.
Strong Raman scatterers are groups with distributed electron clouds like carbon-carbon double bonds. In an external electric field, the pi-electron cloud of the double bond is easily distorted. Stretching or bending the bond alters the distribution of electron density consequently and generates a greater change in induced dipole moment.
For the molecules to be polarized, the incident photon energy excites vibrational modes of the molecules, providing scattered photons that are diminished in energy equal to the number of vibrational transition energies. This gives rise to the peaks in a Raman spectrum. The number of peaks is linked to the number of degrees of freedom a molecule consists of.
To be Raman active, a molecule should possess a change in polarizability. Polarizability, in other words, is the relative tendency of electron clouds to be distorted from the normal shape.
The plot of the intensity of the shifted light against the frequency defines the Raman spectrum of the sample. Usually, Raman spectra are plotted against laser frequency in such a way that the Rayleigh band exists at 0 cm-1. On this scale, the positions of the band exist at frequencies. These correspond to the levels of energy of different functional group vibrations. Thus, the Raman spectrum can be interpreted as equivalent to the infrared absorption spectrum.
The following points list the advantages in the use of Raman spectroscopy:
Raman spectroscopy suffers from a few limitations and they are mentioned as follows:
A precise introduction to Raman spectroscopy has been given initially. Then, the Stokes shift and anti-Stokes shift have been briefed. Raman scattering has been explained. In addition, molecular polarizability and its relation to the Raman spectrum have been discussed elaborately The Raman spectrum has been defined. Further, the pros and cons of Raman spectroscopy have been listed.