Raman and FTIR (Fourier-transform infrared) spectroscopy offer molecular information about the structure and composition of chemical and biological samples. Because of the fundamental principles that govern each technology, both can yield complementary information. However, frequently one technology is a better choice, depending on the nature of the application.
a. 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 reflect crystal lattice structure and polymorph form.
b. Infrared spectroscopy provides a “fingerprint region” of the spectrum where intramolecular vibrations are well-defined and highly characteristic of the bonding of atoms.
A practical example of differentiation between these two technologies is in the investigation of a crystallization process, in which Raman analyzes solid crystal form(s) and IR concurrently measures the solution-phase characteristics such as supersaturation.