Raman or FTIR Spectroscopy

author: Annie
2023-01-12

Although Raman and FTIR Spectroscopy give complimentary information and are often interchangeable, there are some practical differences that influence which one will be optimal for a given experiment. Most molecular symmetry will allow for both Raman and IR activity. One special case is if the molecule contains a center of inversion. In a molecule that contains a center of inversion, Raman bands and IR bands are mutually exclusive, i.e. the bond will either be Raman active or it will be IR active but it will not be both. One general rule is that functional groups that have large changes in dipoles are strong in the IR, whereas functional groups that have weak dipole changes or have a high degree of symmetry will be better seen in Raman spectra.
Choose Raman Spectroscopy when:
  1. Investigating carbon bonds in aliphatic and aromatic rings are of primary interest
  2. Bonds that are difficult to see in FTIR (i.e., 0-0, S-H, C=S, N=N, C=C etc.)
  3. Examination of particles in solution is important, e.g. polymorphism
  4. Lower frequency modes are important (e.g. Inorganic-Oxides) 
  5. Reactions in aqueous media are investigated
  6. Reactions in which observation through a reaction window is easier and safer (e.g. high pressure catalytic reactions, polymerizations)
  7. Investigating lower frequency lattice modes is of interest
  8. Investigation of reaction initiation, endpoint, and product stability of biphasic and colloidal reactions
Choose FTIR Spectroscopy when:
  1. Studying liquid-phase reactions
  2. Reactions in which reactants, reagents, solvents and reaction species fluoresce
  3. Bonds with strong dipole changes are important (e.g. C=O, O-H, N=O)
  4. Reactions in which reagents and reactants are at low concentration
  5. Reactions in which solvent bands are strong in Raman and can swamp key species signal
  6. Reactions in which intermediates that form are IR active

Advantages of Inline Raman Spectroscopy

Raman Spectroscopy offers numerous advantages. Since Raman instruments use lasers in the visible region, flexible silica fiber optic cables can be used to excite the sample and collect the scattered radiation, and these cables can be quite long if necessary. Since visible light is used, glass or quartz can be used to hold samples. In the study of chemical reactions, this means that the Raman probe can be inserted into a reaction or can collect Raman spectra though a window, for example in an external reaction sample loop or flow cell. The latter approach eliminates the possibility of sample stream contamination. The ability to use quartz or Hi-grade Sapphire as a window material means that high pressure cells can be used to acquire Raman spectra of catalytic reactions. In the study of catalysts, operando spectroscopy using the Raman effect is quite useful for studying in situ, real-time reactions on catalytic surfaces. Another advantage of Raman is that hydroxyl bonds are not particularly Raman active, making Raman spectroscopy in aqueous media straightforward. Raman spectroscopy is considered non-destructive, though some samples may be effected by the laser radiation. One consideration that needs to be made when choosing this technique is how fluorescent a particular sample may be. Raman scattering is a weak phenomena and fluorescence can swamp the signal making it difficult to collect quality data. This issue often can be alleviated by using a longer wavelength excitation source.
With respect to reaction analysis, Raman spectroscopy is sensitive to many functional groups but is exceptional when obtaining molecular backbone information, providing its own unique molecular fingerprint. Because Raman utilizes a bonds polarizability and has the potential to measure lower frequency, it is sensitive to crystal lattice vibrations giving the user polymorphic information that can be challenging to obtain by FTIR. This allows Raman to be used very effectively to study crystallization and other complex processes. 

Raman Spectroscopy Instrumentation

A modern, compact Raman spectrometer consists of several basic components, including a laser that serves as the excitation source to induce the Raman scattering. Typically, solid state lasers are used in modern Raman instruments with popular wavelengths of 532 nm, 785 nm, 830 nm and 1064 nm. The shorter wavelength lasers have higher Raman scattering cross-sections so the resulting signal is greater, however the incidence of fluorescence also increases at shorter wavelength. For this reason, many Raman systems feature the 785 nm laser. The laser energy is transmitted to and collected from the sample by fiber optics cables. A notch or edge filter is used to eliminate Rayleigh and anti-Stokes scattering and the remaining Stokes scattered light is passed on to a dispersion element, typically a holographic grating. A CCD detector captures the light, resulting in the Raman spectrum. Since Raman scattering yields a weak signal, it is most important that high-quality, optically well-matched components are used in the Raman spectrometer. 

Raman Spectroscopy Analytical Software

When spectrum is collected consistently over the course of an experiment, it can reveal a 'molecular video' that provides key information regarding the kinetics, mechanisms, and form changes during a reaction.
Traditionally, this analysis has been performed by spectroscopists with expert knowledge in finding key areas of interest and trending these wavenumbers over time. However, advances in software have enabled this expertise to be automated in a way that experts and non-experts alike can easily extract key information quickly for fast, confident decision making.




 

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