Raman spectroscopy
Raman spectroscopy is a method for the analysis of the inelastic scattering of light at molecules or solids and is used for the analysis of material characteristics, among other aspects.
Table of contents
How does raman spectroscopy work?
The basic prerequisite for Raman spectroscopy is a monochromatic light source. Because the light scattered during Raman scattering is of relatively low intensity, the light source must also have a very high radiation intensity. Lasers have both characteristics, lasers are available on the market with different fixed frequencies or as tunable devices.
For the analysis of the reflected, scattered light, first all the light at the excitation wavelength (that is the Rayleigh scattering) must be removed using an optical filter. The remaining scattered light (the Raman scattering) is guided to an optical grid and split into its individual wavelengths. A CCD sensor produces a spectrum from this light.
Raman spectroscopy is suitable for the analysis of a large number of substances. It is possible to analyse liquids, gases and solids.
Raman spectroscopy at Quality Analysis
At Quality Analysis we offer a broad spectrum of measuring and analytical services. These services also include the use of Raman spectroscopy to identify inorganic and organic samples as well as their composition and crystal orientation. Having spectroscopic analysis undertaken by us offers you a whole string of technical and commercial advantages.
In our modern laboratory, we use Raman spectroscopy for residual dirt analysis for the assessment of fibres, plastics or salts, for the verification and the identification of filmic contamination as well as particulate contamination and in chemical analytics, in particular in plastics analytics. There the method is used, e.g. for the identification of deposits, residues, inclusions, media, substances, additives and materials (plastics).
For years we have used Raman spectroscopy reliably and routinely to analyse samples for our customers. For this reason, we are also able to obtain exact measurement results in challenging conditions. The spectrometers and databases we use are from renowned brand-name manufacturers and as such guarantee not only precise results, but also maximum protection for the material.
Raman spectroscopy: physical principles
Light incident on a non-transparent medium is predominantly scattered without changing its wavelength. This effect is termed Rayleigh scattering. A small part of this visible light is, however, scattered in a different wavelength. This phenomenon is called Raman scattering or the Raman effect, after the Indian physicist and Nobel laureate C. V. Raman. But what exactly is Raman scattering?
Raman scattering describes the interaction of the photons (particles of light) with the molecules of the medium. The photons create molecular vibration in the sample. During this process the photons lose energy. Because the wavelength of the light is dependent on its energy, the wavelength is reduced by the loss of energy, in other words: the frequency changes compared to that of the incident light. The frequencies produced by Raman scattering are dependent on the material on which the light is incident. The frequency differences are dependent on various energies in the material such as the rotation, spin-flip and vibration processes. Part of this energy is transferred from the material to the light and changes the frequency of the light. This is the so-called Raman effect.
The Raman spectrum of each substance has certain areas with higher and lower areas of Raman intensity (so-called bands); these areas produce a characteristic image. This image can be compared to known patterns in a spectral library and the type of sample and its characteristics determined beyond doubt.
What does a raman spectrum look like?
The Raman spectrum is characterised by the bands mentioned. These areas of higher Raman intensity are characteristic for every substance. In this way the spectrogram of an unknown substance can be compared to samples from a spectral database. If the bands are in the same places, the classification is unambiguous, as in our example for the comparison of a sample to the spectrum for polypropylene.
This is the Raman spectrum for a particle of polypropylene (red) compared to a reference from a spectral database (blue). The identification is unambiguous.
Using Raman spectroscopy, it is possible to draw conclusions about the following material characteristics, among others:
- Chemical composition
- Crystallinity/crystal orientation
- Temperature
- Doping
- Compression/relaxation
What are the advantages and disadvantages of raman spectroscopy?
Compared to other spectroscopic methods, for example FTIR spectroscopy, Raman spectroscopy offers a few advantages that result above all from the usage of different lasers in the visible to near IR range for a very wide range of materials. Specifically, these include:
- In situ measurement without sample preparation
- Spectral resolution <1-2 cm-1
- Analysis of solids, liquids, gases, powders and granulates
- Analysis of biological systems possible
- Detection of homonuclear molecules (e.g. O2)
- Measurement of polar groups
- Guiding of light using optical fibres over long distances and for parallelisation
- Integration of Raman spectrometer in light microscope possible without problems
Disadvantages of raman spectroscopy
Depending on the specific characteristics of the material to be analysed (e.g. the area of the excitation wavelength), Raman spectroscopy also has disadvantages. In particular, these include:
- Low scattering cross-section (requires large number of molecules and high-intensity laser)
- Possible heating of the sample by the laser beam
- Not possible to analyse fluorescing substances
However, these disadvantages are minimised to a large extent by using modern lasers such that they are increasingly irrelevant for the practical use of Raman spectroscopy.
Where is raman spectroscopy used?
Since the invention of more powerful lasers that are at the same time less aggressive on the material, Raman spectroscopy has become established in almost all areas of chemical analytics. Thanks to the high information density, chemicals can not only be reliably identified, pure material concentrations can also be assessed in complex mixtures. Raman spectroscopy offers many other possible applications.
Structural analysis
During the analysis of the structure of chemical substances, the process determines the structure of the molecules in a chemical substance. This process, important in chemistry and pharmaceuticals, determines the polarisation of the Raman scattered light. If this light is completely polarised, the molecules are isotropically polarised; on the other hand if the polarisation of the scattered light is incomplete, the molecules are anisotropically polarised. The exact degree of depolarisation is determined by placing various polarisation filters in the beam path.
Material research
If a material is treated thermally or mechanically, its internal stress can change. If you now compare the Raman spectra of a sample of the treated and untreated material, the changes in the stress can be detected in the form of frequency shifts. Higher frequencies are indicative of a compressive stress, while lower frequencies indicate an increase in the tensile stress.
Medical diagnostics
The applications of Raman spectroscopy in medicine are very varied. In this way, for example, the chemical composition of kidney stones can be analysed immediately after their removal. Then the patient can be given tailored recommendations for the prevention of new stones without complex analyses in specialist laboratories. It is also possible to analyse a living biological sample using its Raman spectrum. Neither of these methods has become established as standard yet.
