Raman - General Theory

Raman Spectrometry is based on inelastic light scattering by molecules. In the analytical technique, a laser is used to illuminate the process sample. The molecules in the sample vibrate or rotate at characteristic frequencies. The light scattered back from these molecules is shifted in frequency by amounts specific to each type of molecule.

The scattered light is collected from the process using a window probe or an insertion probe. The light is then analyzed in a spectrographic detector and computer to determine the concentration of the components in the process. The effect is both qualitative and quantitative.

Because Raman scattering obeys different rules than FTIR and NIR, certain molecules can be measured with higher sensitivity and greater ease. The Raman technique uses light scattering to identify and quantify molecular species in a process and often works where other spectroscopic techniques do not.

Advantages Gained

Here are some benefits of Raman analysis:

• Use with water---water is an ideal solvent used in many manufacturing operations. The Raman water spectrum is weak so other molecules can be measured in the presence of water more easily than with NIR and FTIR.
• Measure difficult gases---diatomic molecules such as hydrogen, oxygen, and nitrogen can only be detected spectroscopically using Raman.
• Spectra are very specific---Raman spectra are usually sharp so interference between molecules is minimized.
• Monitor remotely---Raman analyzers use low-cost fiber-optic cables to connect probes hundreds of meters from the instrument.
• Inherently safe---only low-power laser light is used and no electrical connection is needed.
• Simplify sampling---no product is extracted from the process. In some cases, the sample loop can be eliminated by inserting the probe directly into the unit. In processes where a sample loop is currently being used for other types of analyzers, a Raman analyzer enables direct analysis without sample extraction.
• No consumables---means no disposal costs for a waste stream. Only electric power is required to operate the analyzer.