What is the difference between Raman and FTIR spectroscopy?

Raman spectroscopy yields information about intra- and intermolecular vibrations. The former provides a spectrum characteristic of the specific vibrations of atoms in a molecule and is valuable for identifying a substance, form and molecular backbone configuration to name a few. The latter yields information about lower frequency modes, which reflects crystal lattice structure and polymorph form.

Infrared spectroscopy's greatest value lies in its ability to probe the “fingerprint region” of the spectrum where intramolecular vibrations are well-defined and highly characteristic of the bonding of atoms.

A practical example of differentiation for these two technologies is in the investigation of a crystallization process, in which Raman analyzes solid crystal form(s) and IR measures the solution-phase characteristics such as supersaturation.

Read more on Raman vs IR.

What are the differences between Raman and FTIR Instruments?

The instrumentation and interface to the sample for these two techniques are similar in approach, but different in the details.

Raman spectrometers utilize a laser as the source (typically visible or near-IR), whereas IR spectrometers typically employ a black body radiator (such as a glow bar) to provide energy in the mid-IR region.

Read more on the differences between Raman and FTIR instruments.

How do I choose between Raman and IR spectrometers?

Although FTIR and Raman spectrometers are often interchangeable and give complementary information, there are practical differences that influence which one is optimal. Most molecular symmetry will allow for both FTIR and Raman activity. In a molecule that contains a center of inversion, IR bands and Raman bands are mutually exclusive (i.e., the bond will either be Raman active or 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 and no net dipole change, will be better seen in Raman spectra.

Choose ReactIR when:

  • Reactions in which reactants, reagents, solvents, and reaction species fluoresce
  • Bonds with strong dipole changes are important, e.g., C=O, O–H, N=O
  • Reactions in which reagents and reactants are at low concentration
  • Reactions in which solvent bands are strong in Raman and can swamp key species signal
  • Reactions in which intermediates that form are IR active
  • Learn more about ReactIR

Choose ReactRaman when:

  • Investigating carbon bonds in aliphatic and aromatic rings is of primary interest
  • Bonds that are difficult to see in FTIR (e.g., O–O, S–H, C=S, N=N, C=C, etc.)
  • Examination of particles in solution is important (e.g., polymorphism)
  • Lower frequency modes are important (e.g., metal-oxygen)
  • Reactions in which observation through a reaction window is easier and safer (e.g., high-pressure catalytic reactions, polymerizations)
  • Investigating lower-frequency lattice modes is of interest
  • Investigation of reaction initiation, endpoint, and product stability of biphasic and colloidal reactions
  • Learn more about ReactRaman