Raman spectroscopy

What is Raman spectroscopy and what is it used for?

Raman spectroscopy is a non-destructive analytical technique with applications in chemical testing, pharmaceuticals, geology, polymer characterization (including microplastics analysis), and the semiconductor industry, among others. It is based on the inelastic scattering of photons, known as Raman scattering. The Raman spectrum provides information about chemical structure and distribution, phase and polymorphism, crystallinity, and molecular interactions.

How does Raman spectrometry work?

In Raman spectroscopy, a monochromatic light source is directed onto a sample. The photons interact with the molecules of the sample and are scattered inelastically. The scattered light is detected by an optical spectrometer. Raman spectra occur when light interacts with molecular vibrations and rotational energies in the sample. The energy of the photons can shift up or down, and these changes in wavelength are recorded from the incident light.

Chemical bonds have characteristic vibration frequencies, which can be seen as peaks in the Raman spectrum. Once the chemical bonds in the sample have been identified by the characteristic peaks, conclusions can be drawn about the chemical structure of the compound(s) the sample contains.

Advantages and disadvantages of Raman spectroscopy analysis

Raman spectroscopy is a versatile technique suitable for most sample matrices. Compared to other methods like FTIR and TED-GC-MS, Raman also tends to be more cost-effective, which makes microplastic quantification with Raman the recommended method for identifying microplastic particles in relatively simple sample matrices, such as water.

The biggest weakness of Raman spectroscopy is the fluorescence present in some substances, which can obscure underlying spectral information. Fluorescence can emerge from the sample itself or from the impurities it contains. In addition, Raman cannot detect very small microplastic and nanoplastic particles that have a diameter of less than 1 µm. The py-GC-MS method is an alternative in such cases, although it does not provide the particle size distribution, but instead reports concentrations in micrograms per liter.

What's the difference between Raman and FTIR?

Raman spectroscopy gives data complementary to infrared (IR) spectroscopy. Where IR spectroscopy is sensitive to polar bonds and hetero-nuclear group vibrations, such as OH stretching in water, Raman spectroscopy is sensitive to homo-nuclear molecular bonds, such as C - C, C = C, and C ≡ C bonds. Because of this, Raman spectroscopy can be used to analyze samples in aqueous solutions, which would be hard to analyze with FTIR. Raman spectroscopy is also more sensitive to inorganic materials.

If you're interested in the differences between Raman and FTIR in microplastic analysis more specifically, you can explore the topic further in our blog on ISO 16094 standards. ISO 16094-2 outlines the procedure for analyzing microplastics with vibrational spectroscopy methods (i.e. Raman and FTIR) and specifies some of the key differences between them.