Infrared and Raman Spectroscopy

Raman Spectroscopy

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

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.

History of IR Spectroscopy

The following events in the years have led to the development of modern day usage of IR spectroscopy.

• 1800 - Sir William Herschel discovered infrared radiation
• 1880 - Bolometer was introduced by Langley
• 1905 - Coblentz published first IR spectra
• 1937 - E. Lehrer discovered first fully-automated spectrometer
• 1940 - Developments in USA took place
• 1946 - First spectra catalogue published
• 1950 - Well-resolved spectra obtained
• 1960s - introduction of fourier transform technique, first use of Michelson interferometer and first commercial spectrometer
• 1965 - Technique found widespread usage

Classification of Spectral Division in Infrared Spectroscopy

Infrared region can be classified into three types, according to the wavelength:

• Near infrared (NIR) region - 12500 to 4000 cm^-1 - Combination of overtones and vibrations
• Mid infrared (MIR) region - 4000 to 400 cm^-1 - H₂O stretching and bending vibrations
• Far infrared (FIR) region - 400 to 0 cm^-1 - lattice vibrations, MO₄, MO₆

Instrumentation of Infrared 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.

Radiation Sources

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.

Microscope 

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.

Spectrometer

Three kinds of spectrometers can be used in infrared spectroscopy. They are as follows:

• Non-dispersive spectrometer - no possibility of variable wavelength selection
• Dispersive spectrometer - selection of variable wavelength through series of filters and gratings
• Fourier Transform spectrometer - wavelength dependent radiation modulation through spectra splitting by interferometer

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

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. 

Infrared Spectroscopy Detectors

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:

• LnSb detector - liquid-nitrogen cooled, photoconductive or photodiode - used in 10000 - 1500 cm^-1
• MCT detector - liquid-nitrogen cooled, photoconductive - 12500 - 400 cm^-1
• DTGS detector - room temperature, pyroelectric, MIR region 

Sample Preparation for Infrared Spectroscopy

• Liquids, gases and solids are used as samples for the measurement of IR spectrum.
• KBr pellet is the mainly used sample preparation method in which KBr is used for dilution sample materials through the pressure compression like tablet preparation.
• For insoluble materials, the ATR method is preferred.
• Oil suspensions can be applied on the film or sample holder.
• Single crystals should have a double-sided polish.

Applications of IR Spectroscopy

IR spectroscopy is a sensitive tool for identification of minerals in the field of geosciences.

More applications are as follows:

• Recognition of minerals
• Concentration and speciation
• Spatial orientation of dipoles
• Information about bonds, bond distances and atoms
• Quantitative and qualitative determination of defects in minerals and structural incorporated molecules. Examples are PO₄, CO₃, OH, H₂O, CO₂, SiO₆, SiO₄

Differences of Raman Spectroscopy over Infrared Spectroscopy

Raman spectroscopy differs from the infrared spectroscopy in the following ways:

• Some selection rules exhibiting partial complementary information
• As scattering effect is weaker, sensitivity is lower
• Appropriate for aqueous solutions
• Contaminations, fluorescence of the sample and visible excitation can overlap with the signal
• Sample preparation is not a requirement
• Bands, existing below 400 cm^-1 can be measured.

Raman Spectroscopy Applications

The main use of Raman spectroscopy is studying the chemical composition of materials.

• The other applications are as follows:
• Structural characterization and identification of minerals
• Analyses of archaeometric objects and gemstones
• Concentration and speciation
• Mineral inclusions
• Other characterizations like metamictization, OH content, impurities and thermal maturity.

Conclusion

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.