Raman and infrared spectroscopy are widely used techniques in various industries due to their ability to recover quantitative and qualitative information on materials and molecular species. Both can be adapted to perform measurements in-situ and, therefore, for online process and optimization control. Infrared (IR) and Raman spectroscopy possess a key similarity: their ability to recover information on molecular vibrations. However, does that mean using both techniques together is redundant, or are there scenarios where this may be advantageous?
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Infrared Spectroscopy
IR spectroscopy takes its name from the wavelengths of light it uses. There are a range of different implementations of infrared spectroscopy, ranging from simple absorbance/transmission measurements to more complex attenuated total reflection (ATR) configurations. Still, all rely on the same principle.
When a molecule interacts with a photon, if the incident photon's energy is equal to the spacing between its energy levels, there is a probability of the photon being absorbed. It also results in the excitation of a spectroscopic transition in the molecule.
The more atoms a molecule has, the greater the number of vibrational modes and degrees of freedom. Each of those vibrational modes has a particular energy, or frequency, associated with it related to the bond strength between the associated atoms. This is, in turn, influenced by their masses and surrounding chemical environments.
In a simple scanning transmission experiment, an infrared beam is passed through the sample, and the beam's energy is stepped across a range. A detector records any depletion in the intensity of the light as it is absorbed by various vibrational modes in the sample to recover the infrared spectrum.
IR spectroscopy can be used for species identification because every molecule has its own 'fingerprint' spectrum. As the exact energies and intensities of the vibrational transitions vary with chemical structure, IR spectroscopy can provide specific structural information on a molecule.
Raman Spectroscopy
Raman spectroscopy involves a slightly more complex process than IR transmission spectroscopy but can also be used to recover information on the vibrational structure of molecules. In a Raman experiment, the sample is irradiated with an intense light source, typically a laser. Some photons will be inelastically scattered by the molecule, with energies greater or smaller than the incident photon energy. This energetic displacement, the 'Raman shift', can be used to investigate transitions such as vibrational transitions in the molecule.
While excitation wavelengths in the near-infrared to UV are the most common, it is possible to stimulate Raman processes with a range of photon energies to investigate different phenomena and other transitions such as electronic. Variations of Raman spectroscopy using multiple pulse sequences are also becoming increasingly popular, even in microscopy applications, due to their sensitivity to different physical phenomena.2
Particularly in microscopy, some of the developments of variations in Raman methodology, including coherent anti-Stokes Raman Scattering (CARS), are to enhance tissue penetration in cell samples, reduce issues from fluorescence, and signal interference from unwanted scattering.3 For specimens that fluoresce efficiently, recording the weak Raman signal on top of a strong fluorescence background can be nearly impossible.
How Do Infrared and Raman Spectroscopy Differ?
One of the crucial fundamental differences between Raman and infrared is the selection rules of the excitation process. A vibrational mode is considered to be infrared active if there is a change in the dipole moment across the bond during the vibration. Typically, the intensity of the observed transition is proportional to this dipole moment, which is why bonds to very electronegative elements like the oxygen in a carbonyl group show very high infrared activity.
A Raman process, however, involves a different type of interaction with the molecule. Here, a Raman active mode is not dependent on a change in the dipole moment during the vibration but instead relies on a change in the polarizability.
For homonuclear diatomic molecules, their vibrational modes are all Raman active but IR inactive. This difference in the selection rules that govern the spectroscopy technique makes IR and Raman highly complementary.
Raman scattering is an inherently weak effect. The need to use longer excitation wavelengths to avoid excess fluorescence further reduces the intensity of Raman signals. However, when it comes to microscopy applications, Raman measurements typically require less sample preparation and have fewer restrictions on sample thicknesses used.4
While infrared detectors, particularly in the mid-infrared, can be costly and often suffer from high noise levels5, standard IR spectrometers are generally much cheaper than their Raman equivalent. The potential need for multiple excitation wavelengths and laser sources for different sample types can also further drive up the Raman experiments' costs.
For most industrial applications and analytical challenges, Raman and IR as vibrational spectroscopy methods can be considered complementary techniques due to their inherent sensitivity to varying vibrational modes. Both approaches perform well when combined with chemometrics analysis and can provide independent complementary information that, when combined, can improve overall confidence in species identification.6
References and Further Reading
Bakeev, K., (2010) Process analytical technology. Chichester, West Sussex: Wiley.
Kang, D., Li, R., Cao, S., & Sun, M. (2021) Nonlinear optical microscopies: physical principle and applications. Applied Spectroscopy Reviews, 56(1), 52–66. https://doi.org/10.1080/05704928.2020.1728295
Petrov, G. I., Arora, R., Saha, A., Heathcote, R. D., Ravula, S., Brener, I., & Yakovlev, V. V. (2007) Raman versus CARS microscopy: when one is better than the other. In A. Periasamy & P. T. C. So, Multiphoton Microscopy in the Biomedical Sciences VII, 6442, 39–46. Available at: https://doi.org/10.1117/12.700007
Matthäus, C., Bird, B., Miljković, M., Chernenko, T., Romeo, M., & Diem, M. (2008) Infrared and Raman Microscopy in Cell Biology. Methods in Cell Biology, 89(8), 275–308. Available at: https://doi.org/10.1016/S0091-679X(08)00610-9
Razeghi, M., & Nguyen, B. M. (2014) Advances in mid-infrared detection and imaging: A key issues review. Reports on Progress in Physics, 77(8). Available at: https://doi.org/10.1088/0034-4885/77/8/082401
Rohman, A., Windarsih, A., Lukitaningsih, E., Rafi, M., Betania, K., & Fadzillah, N. A. (2019) The use of FTIR and Raman spectroscopy in combination with chemometrics for analysis of biomolecules in biomedical fluids: A review. Biomedical Spectroscopy and Imaging, 8(3–4), 55–71. Available at: https://doi.org/10.3233/bsi-200189
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