Components of a Raman spectrometer:
Within Raman spectroscopy, an intense radiation, preferably a laser, in the visible region interacts along with the molecules and gets scattered. A scattered radiation is measured perpendicular to the sample. Most of the scattered light is of the similar frequency as in which of the incident radiation and is known as Rayleigh scattering. A small fraction (1 in 107) of the scattered radiation has frequencies variant from that of the incident radiation. This is known as Raman scattering. These scatterings could be explained in terms of elastic and inelastic collisions of the photons constituting the radiation along with the molecules. The wave theory instead explains it in terms of the induced polarisation of the molecule on interaction along with the radiation.
Like IR spectra, the origin of Raman spectra also includes transitions amongst the vibrational stages of the molecules. Therefore because of the differences in the way these signals originate the two spectra might differ significantly. For a vibrational mode to be Raman active it must have an associated change in polarisability. The IR and Raman activities of different modes of vibration of molecules could provide useful structural information. The rule of mutual exclusion states which, "for a molecule having a centre of symmetry the Raman active vibrations are IR inactive and vice versa".
A few of the scattered light in Raman spectra is found to be polarised. This depends on the symmetry of the molecular vibration causing the scattering and is a result of the nature of the polarisability modifications during the vibration. The nature of the scattered light is expressed in terms of degree of depolarisation. A symmetric vibration provides a polarised or partially polarised Raman line while the non-symmetric vibrations provide depolarised signals.
Such as the IR instruments, there are five primary components of a Raman spectrometer. The common set up of the Raman spectrometer is same to that of the corresponding IR instruments. The difference being in which the scattered radiation is collected at right angles to the radiation and it is passed by a filter before sending it to a transducer for detection. Within terms of the components, the Raman spectrometers differ from the IR instruments in terms of the sources, a sample handling devices and the transducers.
Raman lines are intrinsically extremely weak. The spectral intensities within Raman spectroscopy could be enhanced through using some special techniques such as Resonance Raman spectroscopy, Coherent Anti-Stokes Raman Spectroscopy (CARS) and Surface Enhanced Raman Scattering (SERS) etc.
Raman spectroscopy could be used for quantitative and qualitative analysis of inorganic, organic and biological samples. The quantitative analysis is based on the intensity of the Raman scattered light being proportional to concentration although the qualitative determinations depend on the wavelength shifts being variant for a huge range of different molecular vibrations.