Raman spectroscopy

A spectroscopic fingerprint identifies chemical substances

As a vibrational spectroscopy technique, Raman spectroscopy provides detailed spectra of chemical substances similar to infrared spectroscopy. As opposed to an IR spectrometer, however, a Raman spectrometer captures this information in the visible spectral range and thus avoids the particular challenges of the infrared spectral region. Figure 1 shows a Raman spectrum of acetaminophen and illustrates the high information density of those spectra, which is essential for the reliable identification of chemical substances or the determination of pure substance concentrations in complex mixtures.

In comparison to other common techniques, Raman spectroscopy offers various advantages, in particular when used in process technology:

• High information density when compared to NIR and UV/Vis spectroscopy
• The Raman effect is linked to the polarizability of molecules and not to their dipole moment (as in IR absorption), enabling the detection of homonuclear molecules such as N_2, H₂, O₂
• Good detection of non-polar groups (-S-S-, -C-S-, -C=C-)
• Trouble-free measurements in aqueous media (as opposed to IR spectroscopy)
• Measuring in the visible spectral range, no need for elaborate optical components
• Inline measurement: only one access point required, measurement in backscattering geometry (as opposed to transmission spectroscopy)
• Possibility of using optical fibres for light transmission over long distances or for multi-plexing
  
• Spectrometers do not have moving part
• Extended spectral range well below 1000 1/cm, possible e.g. for the detection of weak bonds, all the way down to hydrogen bonds

At the same time, the use of Raman spectroscopy implies numerous challenges. First and foremost, the small cross section in the interaction with molecules increases the technical effort, so that costs and benefits must be weighed up depending on the intended use.

Raman spectroscopy on gases

Raman spectroscopy is a comparatively complex method, as it requires high-quality components such as powerful lasers and cooled cameras. The key issue: The Raman effect is extremely unlikely – only a very small number of photons are scattered inelastically and can be detected. This is particularly challenging in the case of gases, whose particle density is typically 1000 times lower than that of liquids or solids. Nevertheless, especially when analyzing gases, Raman spectroscopy offers decisive advantages over other methods, such as infrared spectroscopy. As shown in Figure 2, it enables the detection of homonuclear gases such as nitrogen (N₂), hydrogen (H₂), and oxygen (O₂), which are often difficult or impossible to detect using other spectroscopic methods. In principle, Raman spectroscopy can detect all of the gases present in a gas mixture simultaneously – without the need for additional auxiliary sensors for nitrogen or hydrogen, for example.

At Fraunhofer IPM, we have developed a simple and low-cost Raman sensor for hydrogen. Find out more about it here.