Related Information
Raman is a technique for identification and analysis of molecular species, it is similar to FT-IR spectroscopy, but it has a few advantages, for example Raman can be used to study, solids, liquids, powders, gels, slurries and aqueous solutions. Raman does not require any special sample preparation meaning that many studies may be performed in situ. An example of this is the analysis of mixed pharmaceutical tablets in a sealed blister pack. Tablets can be identified through a sealed blister pack, without destroying the sample and the relative concentration of the substances in the tablet can even be determined.
Raman spectroscopy is based on detection of scattered light i.e. the Raman effect. In general when light interacts with a substance it can do so in three main ways:-
» The light may be absorbed
» The light may be transmitted
» The light may be scattered
Raman spectroscopy is a result of the scattering of light. The radiation may be scattered elastically, that is without any change in its wavelength and this is known as Rayleigh scattering. Conversely the radiation may be scattered inelastically resulting in the Raman effect. There are two types of Raman transitions, upon collision with a molecule a photon may lose some of its energy, this is known as Stokes radiation or the photon may gain some energy and this is known as anti-Stokes radiation. When viewed with a spectrometer it can be seen that both the Stokes and anti-Stokes radiation are composed of lines which correspond to molecular vibrations of the substance under investigation.
Raman peaks on the Stokes side are intense, as they arise from molecular vibrations which arrise from energy transfer from the photon to the molecule. Anti-stokes peaks have the same wavelength shift as stokes but they involve an transfer of energy from an vibrating molecule into a photon, obviously as a vibrating molecule is requires this is a quite rare event, as a result of this Anti-stokes spectra are weak.
If stokes stectra are acquires, then there is little additional information to be acquired from collecting anti stokes data. Most Raman spectrometers do not collect anti-stokes data. One andvnatge of collecting anti-stokes spectra is that the ratio of stokes and anti-stokes offers an insight into the Bolzmann distribution and so can be used to tell the temperature of the samples at the point of laser irridiation.
Each compound has its own unique Raman spectrum which can be used as a finger print for identification. A complete Raman Spectrum of Cyclohexane is shown below.
Raman spectroscopy is similar to I.R. spectroscopy but has several distinct advantages.
Using IR spectroscopy on aqueous samples, results in a large proportion of the vibrational spectrum being masked by the intense water signals. With Raman spectroscopic techniques aqueous samples can be performed with ease as Raman signals from the water molecule are relatively weak.
Using Raman spectroscopy, spectra of samples in transparent containers such as glass or plastic, can be obtained.
Modern Raman Instruments can acquire spectra in seconds with no sample preparation and recent advances in software allow for real time sample identification of quantification.
Resonance Raman
Resonance Raman techniques require no extra equipment. Resonance Raman scattering occurs when the photon energy of the exciting laser beam matches that of an electronic transition of a chromophoric group within the system under study.
Under these conditions bands belonging to the chromophore are selectively enhanced by factors of about x1000.
In 1974 it was discovered that pyridine molecules that were absorbed unto a electrochemically roughen surface yield many times more intense Raman Signals than they normally would. This effect was later named Surface Enhanced Raman Spectroscopy (SERS).
The theoretical understanding of SERS is not clear but the technique has found application in many areas of physics, chemistry and biology, yielding information on how molecules interact with surfaces whilst allowing detection of very low concentrations of various analytes.
