spectroscopy is a technique that measures the energy of photons generated by
the inelastic scattering of monochromatic excitation photons. During
excitation, photons interact with the electron cloud of an analyte and a small
percentage of these photons undergoes a change in energy as a result of the
interaction. This energy change corresponds with molecular vibrations,
rotations, and other low-frequency modes and is plotted by instrumental
operating software to produce a spectrum. Raman spectroscopy is often described
as a complimentary technique to FTIR because both techniques are used to probe
vibrational energy levels although different selection rules allow FTIR and
Raman to interrogate different sets of molecular vibrations. Raman scattering
is an inherently low probability phenomenon and as such, the Raman scattering
cross-section of a particular analyte will generally be several orders of
magnitude less than the corresponding FTIR absorption cross-section.
Practically, this means that the sensitivity of Raman spectroscopy is lower
than FTIR spectroscopy. There are several advanced Raman spectroscopy
techniques that generate a stronger Raman intensity when compared to
spontaneous Raman scattering such as Coherent Anti-Stokes Raman Spectroscopy (CARS),
Resonance Raman Spectroscopy, Surface Enhanced Raman Spectroscopy (SERS) and
Tip Enhanced Raman Spectroscopy (TERS).
Raman instruments are available in a variety of configurations. The most common instruments are bench top
models but a wide variety of portable and handheld models are also available commercially.
Instrument cost ranges from approximately $10,000 to approximately $100,000.