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Raman spectroscopy is one of the kinds of non-destructive analytical techniques, which provides detailed information with regard to phase, chemical structure, molecular interactions and polymorphic crystallinity. It is an ideal technique for sample detection and quantification. Raman imaging, Rama shift, Raman analysis and Raman spectrum are the terms often used in Raman spectroscopy.
Infrared spectroscopy falls under the category of absorption spectroscopy, It deals with the infrared region of the electromagnetic spectrum. Infrared light has a lower frequency and longer wavelength than visible light. A commonly used laboratory technique involved in this category is FTIR spectroscopy and the instrument operated is the FTIR spectrometer. It is used for the identification and study of chemicals.
The article throws light on the basic concepts, features and sample preparation of infrared spectroscopy and deals with comparison to Raman spectroscopy.
The following events in the years have led to the development of modern day usage of IR spectroscopy.
A general setup of an IR spectrometer is inclusive of a sample, light source, spectral apparatus, microscope, computer and detector.
The light source emits polychromatic infrared light and is focused on the sample. It is absorbed partially by the sample when it is passed through the sample. Molecules present in the sample interact with the light, take up energy and use the energy to vibrate when the dipole moment changes. The amount of light transmitted through the sample is estimated by the detector. The result obtained is a characteristic spectrum, which shows the absorbance or transmittance of electromagnetic radiation as a function of wavenumber or wavelength.
Commonly used light source for IR spectrometers is globar, which has its emission in the FIR region. Below 100 cm-1 mercury high pressure lamps are used.
Nernst rod is also used, which emits the radiation in the MIR region. In addition, semiconductor lasers are used.
Air-cooled metallic helices are used in the NIR region. Tungsten-halogen lamps are also used for the NIR region. Gas lasers are also used.
Infrared microscopes utilize reflecting optics like Cassegrain objectives with 36x or 15x magnification in the place of quartz and glass. Usage of microscope aids in selecting a distinct area for measurement on the sample and eliminating contaminations from the sample.
Three kinds of spectrometers can be used in infrared spectroscopy. They are as follows:
IR spectrometer use mirror optics instead of lens as lens absorb most radiation below 5000 cm-1
The following are the pros of FT spectrometer over the dispersive spectrometer:
Simultaneous measurement of wavelengths
Lesser measurement time at same signal-to-noise ratio
At same spectral resolution, higher spectral throughput
Higher wavenumber stability
As sample is placed behind the interferometer, scattering is neglected
Spectral resolution is the concept of the distance between the two neighbouring absorption maxima having the same height, separated by absorption minimum, whose transmittance is 20% higher than the band maxima.
FT spectrometer provides the best resolution of 0.0001 cm-1. Dispersive spectrometer with a prism monochromator gives the resolution of 20 cm-1 . At the same time, a dispersive spectrometer with grating monochromator offers resolution of 0.2 cm-1.
Detectors aid in the conversion of optical signal into electrical signals. Though a plethora of collectors like pneumatic, thermal, photoelectric, pyroelectric types are there, photoelectric detectors are selected nowadays due to their greater sensitivity. The incident light changes the electrical conductivity in the irradiated semiconductor material and the photosignal is estimated as a change in voltage through the current or resistance.
Below are the detectors commonly used in infrared spectroscopy:
IR spectroscopy is a sensitive tool for identification of minerals in the field of geosciences.
More applications are as follows:
Raman spectroscopy differs from the infrared spectroscopy in the following ways:
Raman Spectroscopy Applications
The main use of Raman spectroscopy is studying the chemical composition of materials.
A precise elucidation to Raman spectroscopy and infrared spectroscopy has been given. History of infrared spectroscopy has been stated. Then, classification of spectrum in infrared radiation has been offered. Further, instrumentation for infrared spectrometers has been discussed in detail. In addition, sample preparations for infrared spectroscopy have been given. Applications of infrared spectroscopy have been listed. Differences of Raman spectroscopy over infrared spectroscopy have been provided. Finally, applications of Raman spectroscopy have been indexed.