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Raman Spectroscopy is a non-destructive chemical analysis technique which provides detailed information about chemical structure, phase and polymorphy, crystallinity and molecular interactions. It is based upon the interaction of light with the chemical bonds within a material.
Raman is a light scattering technique, whereby a molecule scatters incident light from a high intensity laser light source. Most of the scattered light is at the same wavelength (or color) as the laser source and does not provide useful information – this is called Rayleigh Scatter. However a small amount of light (typically 0.0000001%) is scattered at different wavelengths (or colors), which depend on the chemical structure of the analyte – this is called Raman Scatter.
A Raman spectrum features a number of peaks, showing the intensity and wavelength position of the Raman scattered light. Each peak corresponds to a specific molecular bond vibration, including individual bonds such as C-C, C=C, N-O, C-H etc., and groups of bonds such as benzene ring breathing mode, polymer chain vibrations, lattice modes, etc.
Fig. 2: A typical Raman spectrum, in this case, of aspirin (4-acetylsalicylic acid). The inset image shows the detail which is present in the spectrum
Information provided by Raman spectroscopy
Fig. 3: Raman spectra of ethanol and methanol, showing the significant spectral differences which allow the two liquids to be distinguished.
Raman spectroscopy probes the chemical structure of a material and provides information about:
- Chemical structure and identity
- Phase and polymorphism
- Intrinsic stress/strain
- Contamination and impurity
Typically a Raman spectrum is a distinct chemical fingerprint for a particular molecule or material, and can be used to very quickly identify the material, or distinguish it from others. Raman spectral libraries are often used for identification of a material based on its Raman spectrum – libraries containing thousands of spectra are rapidly searched to find a match with the spectrum of the analyte.
Fig. 4: Mineral distribution
In combination with mapping (or imaging) Raman systems, it is possible to generate images based on the sample’s Raman spectrum. These images show distribution of individual chemical components, polymorphs and phases, and variation in crystallinity.
Raman spectroscopy is both qualitative and quantitative.
The general spectrum profile (peak position and relative peak intensity) provides a unique chemical fingerprint which can be used to identify a material, and distinguish it from others. Often the actual spectrum is quite complex, so comprehensive Raman spectral libraries can be searched to find a match, and thus provide a chemical identification.
The intensity of a spectrum is directly proportional to concentration. Typically, a calibration procedure will be used to determine the relationship between peak intensity and concentration, and then routine measurements can be made to analyze for concentration. With mixtures, relative peak intensities provide information about the relative concentration of the components, while absolute peak intensities can be used for absolute concentration information.
Raman is used for microscopic analysis
Fig. 5: A modern Raman microscope system
Raman spectroscopy can be used for microscopic analysis, with a spatial resolution in the order of 0.5-1 µm. Such analysis is possible using a Raman microscope.
A Raman microscope couples a Raman spectrometer to a standard optical microscope, allowing high magnification visualization of a sample and Raman analysis with a microscopic laser spot. Raman micro-analysis is easy: simply place the sample under the microscope, focus, and make a measurement.
A true confocal Raman microscope can be used for the analysis of micron size particles or volumes. It can even be used for the analysis of different layers in a multilayered sample (e.g., polymer coatings), and of contaminants and features beneath the surface of a transparent sample (e.g., impurities within glass, and fluid/gas inclusions in minerals).
Motorized mapping stages allow Raman spectral images to be generated, which contain many thousands of Raman spectra acquired from different positions on the sample. False color images can be created based on the Raman spectrum – these show the distribution of individual chemical components, and variation in other effects such as phase, polymorphism, stress/strain, and crystallinity.
Type of samples analyzed with Raman
Raman can be used to analyze many different samples. In general it is suitable for analysis of:
- Solids, powders, liquids, gels, slurries and gases
- Inorganic, organic and biological materials
- Pure chemicals, mixtures and solutions
- Metallic oxides and corrosion
In general it is not suitable for analysis of:
- Metals and their alloys
Typical examples of where Raman is used today include:
- Art and archaeology – characterization of pigments, ceramics and gemstones
- Carbon materials – structure and purity of nano-tubes, defect/disorder characterization
- Chemistry – structure, purity, and reaction monitoring
- Geology – mineral identification and distribution, fluid inclusions and phase transitions
- Life sciences – single cells and tissue, drug interactions, disease diagnosis
- Pharmaceutics – content uniformity and component distribution
- Semiconductors – purity, alloy composition, intrinsic stress/strain microscope.
Analysis of solids, liquids and gases
Raman spectra can be acquired from nearly all samples which contain true molecular bonding. This means that solids, powders, slurries, liquids, gels and gases can be analyzed using Raman spectroscopy.
Although gases can be analyzed using Raman spectroscopy, the concentration of molecules in a gas is typically very low, so the measurement is often more challenging. Usually specialized equipment such as higher powered lasers and long path length sample cells are necessary. In some cases where gas pressures are high (such as gas inclusions in minerals) standard Raman instrumentation can easily be used.
Analysis from a mixture of materials
The Raman spectrum from a material will contain Raman information about all of the molecules which are within the analysis volume of the system. Thus, if there is a mixture of molecules, the Raman spectrum will contain peaks representing all of the different molecules. If the components are known, the relative peak intensities can be used to generate quantitative information about the mixture’s composition. In case of complex matrixes, chemometrics methods might also be employed to build quantitative methods.