Editorial Feature

Probe Based Raman Spectroscopy - A Guide

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When chemists want to find out what molecules are in a sample and the bonds between them, they might use Raman spectroscopy. The technique utilises laser light to observe changes in the vibrational, rotational or electronic energy of a system to determine which functional groups there are thus providing a structural fingerprint by which the molecule can be recognized.

Raman spectroscopy studies the scattering of photons upon interaction with molecules in a sample. Most photons scatter elastically meaning the photons have the same wavelength as the incident light. However, approximately one in a million photons are inelastically scattered and its wavelength is shifted – usually lower – with respect to the incident light; this is known as the Raman effect.

The technique is good for investigating organic and inorganic chemistry, observing in particular the unique covalent double bonds found in many molecules, to provide a detailed analysis of solids, powders, liquids and gases. It is fast and non-destructive making it ideal for a wide range of applications. It also provides quantitative results; the intensity of the Raman band is directly proportional to the number of molecules in the band, therefore the band gives the concentration of the molecules.

Many advanced types of Raman spectroscopy have been developed over the years aimed at improving the sensitivity or spatial resolution of the technique including surface-enhanced Raman, tip-enhanced Raman and polarized Raman. Probe based Raman spectroscopy is an enhancement of the technique that allows for optimal signal collection using fiber optics not only in the laboratory but also in restricted spaces or hostile environments. It is a perfect illustration of how fiber optics can be joined with other optical components to obtain a simple and flexible measurement.

The purpose of a Raman probe is to carry the laser beam to a sample material, and to collect and filter the returning Raman signal. The probes are specific, non-invasive and can be used in water or aqueous systems. They offer effective signal collection and a complete reduction of the excitation laser line using precise beam steering, optical filtering and signal collection. Filtering reduces the signal below the intensity normally expected of typical Raman bands to yield a clean spectrum.

General purpose probes allow for universal analysis of solids and surfaces in the laboratory. These probes – often made of stainless steel - offer high signal collection and laser line filtering. As well as being used in teaching labs, general purpose probes might be utilised in biofuel analysis, biotechnology and polymer analysis. They could also be used in medical diagnostics as a clinical tool; it can provide rapid, non-invasive, real-time molecular analysis of disease specific changes in tissues.

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Immersion probes offer the chance to analyse a clear or turbid sample in situ: they can be immersed directly into solutions offering real-time measurements. Some feature temperature or pressure-resistant sleeves, meaning they can be used in process environments as well as in routine lab work. Such probes can be used in teaching laboratories, in medical diagnostics and polymer analysis, and in food and drink quality control.

Process probes are used in industrial environments where they are likely to be subjected to extremes in temperature and pressure. They have a specific design to protect the probe optics meaning they can be immersed in the process stream or aggressive surroundings without it affecting the filtering and focussing optical design. They have a high collection efficiency and laser line filtering and feature stainless steel armour for protection. Process probes can be employed in agricultural measurement and monitoring, biofuel analysis, food and beverage quality control, medical diagnostics and polymer analysis.

References and Further Reading

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Kerry Taylor-Smith

Written by

Kerry Taylor-Smith

Kerry has been a freelance writer, editor, and proofreader since 2016, specializing in science and health-related subjects. She has a degree in Natural Sciences at the University of Bath and is based in the UK.

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