Spectroscopy is a popular analytical technique that studies the interaction between electromagnetic radiation and matter. The interactions give rise to electronic excitations, molecular vibrations or nuclear spin orientations, which can then be analyzed with different spectroscopic instrumentation. There is a vast amount of spectroscopy techniques available, including infrared, Raman, nuclear magnetic resonance, ultraviolet and visible spectroscopy. Two of the most popular spectroscopic techniques that are often compared are infrared spectroscopy (IR) and Raman spectroscopy. In this article, we discuss the benefits and disadvantages of both techniques.
What They Are
Raman spectroscopy provides a molecular fingerprint of the chemical composition and molecular structure of a sample through non-destructive analysis. It has been around since the discovery of Raman Scattering in 1928, but over the past 20 years, the amount of Raman techniques available has greatly increased. These have included Fourier Transform (FT)-Raman, dispersive visible Raman, chiral Raman, UV resonance enhanced Raman, surface enhanced Raman, confocal Raman microscopy, as well as the development of handheld instrumentation.
Infrared Spectroscopy is also a non-destructive spectroscopic technique that provides a molecular fingerprint but was discovered a long time before the principles of Raman spectroscopy. The theory of infrared spectroscopy had been around since the F.W. Herschel discovered infrared light in 1800. Whilst there are multiple IR systems, including near infrared, research using IR tends to focus on the Fourier Transform infrared spectrometer.
How They Work
Both IR and Raman spectroscopy are types of vibrational spectroscopy. IR works with the infrared region of the electromagnetic spectrum. It works by measuring how much light is absorbed by the bonds of a vibrating molecule. Raman spectroscopy works by the detection of inelastic scattering, also known as Raman scattering, of monochromatic light from a laser, usually in visible, near infrared or near ultraviolet range.
To make a transition Raman active, the polarizability of the molecule during the vibration and the electron cloud of the molecule must change positionally. For an IR detectable transition, the molecule must have a dipole moment change during vibration. The spectra also differ, with IR showing irregular absorbance lines and Raman showing a scattered Rayleigh line and the Stoke/anti-Stoke lines.
Both Raman and IR spectroscopy are used across a wide range of areas, including the pharmaceutical, academic, polymer, bioprocessing and biomedical analysis industries. The techniques can be used both quantitatively and qualitatively to identify functional groups, monitor reaction processes and detect impurities.
A big advantage of using Raman over IR is that the sample preparation is much easier and less time-consuming. Speed is crucial in the analysis because runtimes need to be as short as possible so that more samples can be analyzed. The molecules analyzed do not need to possess a permanent dipole moment like molecules analyzed with IR.
A major problem that comes up with IR analysis is interference. Water cannot be used in IR due to its intense absorption of IR, whereas it can be used as a solvent in Raman spectroscopy. The fact that water is a weak Raman scatterer means that samples can be analyzed in their aqueous form, which is highly beneficial to the pharmaceutical industry.
Raman spectroscopy does not need a reference light path as it is a scattering technique, meaning that fiber optics and remote sampling can be used. The portability allows for remote analysis through glass containers, well plates and aqueous samples. Unlike IR, Raman can also be used to analyze gases, but this requires further specialized equipment.
Because of a high spatial resolution, caused by an excitation wavelength in the visible and near-infrared range, combining Raman spectroscopy with microscopy is incredibly powerful for taking images of biological samples.
A major advantage of IR over Raman is the cost. Raman spectroscopy is a much more expensive technique to use than IR since high powered lasers and amplification sources are needed to get sensitive results. The heating of samples through the intense laser radiation can also destroy the sample or cover the Raman spectrum.
Raman spectroscopy alone is not a very sensitive technique compared with IR, so methods such as surface enhanced Raman spectroscopy have had to be developed to fix this problem. IR spectroscopy has been an understood established technique for much longer then Raman, so the techniques provide a greater sensitivity and reliability compared to Raman techniques such as surface-enhanced Raman spectroscopy. Some portability of IR is also available via handheld FTIR systems
Whilst IR and Raman have both advantages and disadvantages, many scientists believe that they should still be used to complement each other when trying to identify unknown substances. IR can be used to give an indication of ionic character, whereas Raman will give an indication of covalent character. Whilst both instruments can be used for the analysis of light colored samples, IR is used alone for fluorescent and colored samples, but Raman is used for aqueous samples and translucent containers. The use of both techniques gives scientists a fuller picture about their analytes of interest.
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