Raman and Fourier Transform Infrared (FTIR) are vibrational spectroscopies that detect changes in a chemical bond’s polarisability (ie. The electron cloud around bond) and dipole moment, respectively. Both techniques are complimentary, and many vibrational modes are both Raman and IR active. In some cases, when there is a center of inversion in the crystal symmetry of the material, some vibrational bands will be only Raman active or IR active (not both – rule of mutual exclusivity). The Renishaw Invia Reflex Raman microscope (Renishaw, Gloucestershire, UK) located within the SSSC is its only kind in Canada, as it also equipped with an IlluminatIR II FTIR microscope accessory (Smith’s Detection, Danbury, CT). In addition, the Invia Reflex microscope supports multiple laser wavelengths, along with sophisticated software that allows automatic switching of lasers. This allows multiple excitation wavelength Raman and FTIR spectroscopies to be obtained on the same spot with relative ease for the general user!

Raman or FTIR…..Which one should I use?

This is a common question we often get asked, and the choice of using Raman or FTIR spectroscopy will depend on what kind of desired information and the sample composition. Both techniques are complimentary and provide information from molecular bond vibrations. The table below describes some of the differences:

FTIR spectroscopy measures changes in the dipole moment of a chemical bond. It is well suited for identifying functional groups for organic materials, and fluorescence material is not a problem. The diamond-ATR objective is well suited for powders and other solid materials. The back-aperture can go as small as 10 µm spot size. The ARO objective is well suited for small particles and thin sections. Some of the sample limitations for FTIR include:

• Wet materials – H₂O is a major contaminant in FTIR spectroscopy
• Thermal degradation – organic materials that have been degraded to amorphous carb on do not provide a good FTIR spectrum

Raman spectroscopy is a light scattering process in which inelastically scattered photons are measured from a high powered laser focussed on a sample. Raman spectroscopy measures the vibrational and lattice vibrations when there are changes in the polarizability of a molecule (ie. A distortion of the molecule’s electron cloud during vibration must cause the molecule to interact differently with the electric field of the incoming photon). Raman spectroscopy is ideally suited for:

• Graphitic materials (C=C stretch not seen in IR spectrum, but large change in polarizability)
• Various minerals and metal oxides
• Pharmaceutical products
• Polymorphisms, crystallinity distortions
• Raman confocal microscopes ideally suited for measuring embedded materials
• Raman maps with sub-mm resolution
• No interference from water – live cell imaging possible

Some of the limitations include:

Sample fluorescence. 

Low S/NThe S/N can be low for some materials.

Figure 1. Infrared spectroscopy measures vibrational transitions by changes in the dipole moment of the different molecular bonds.  Raman spectroscopy typically uses a high powered laser source (hν_ex) which excites electrons to a “virtual state” located between the electronic ground state and excited state. The majority of the photons will scatter elastically on the sample surface, and is commonly known as Rayleigh scattering. A small portion of the photons will be scattered inelastically (approx.1 part in 10 million) due to its interaction with the vibrations in the material. The Stokes Raman scattered photons are slightly absorbed by the material and will be at a slightly lower energy (red-shifted) compared to the incident photons. The antistokes Raman scattered photons gain a slight amount of energy from the material (blue-shifted). Resonant Raman spectroscopy is observed when the hν_ex source is near the same energy as an electronic transition. Electrons are excited into the higher electronic state and leads to an increase in Raman intensity (as much as a factor of 10⁶). Fluorescence is the main interference of Raman spectroscopy, and generally occurs when the hν_ex source is near the electronic transition energy, or it could originate from the sample matrix. Because fluorescence is much longer lived excited state lifetime (τ) compared to Raman transitions, the fluorescence emission will generally mask the Raman peaks.