Raman Spectroscopy

Written by Tiziana Pasciuto

Translated by Sarah Fortunée Tabbakh

Italian Version here

SENTERRA Raman microscope of the Bruker Optics.

Of the many diagnostic techniques used to study and characterise cultural heritage, one of the most recent and innovative techniques is the Raman spectroscopy. 

It is a vibrational non-invasive and non-destructive technique that was developed by the Indian physicist C. V. Raman. His studies of the diffuse reflection and the discovery of the so-called Raman effect awarded him the Nobel Prize for physics in 1930. From that moment on, developing technologies have contributed to the optimization of the above-mentioned technique. 

When a group of molecules – in our case, the cultural good – is hit by a laser beam with a monochromatic wavelength between 300 and 1064 nm, three effects can be observed: 

- A big part of the incident radiation is absorbed by the sample and/or passes through;

- Another part is diffused with the same initial wavelength (Rayleigh effect);

- A miniscule part is diffused with a different wavelength than the initial one (Raman effect). This weak signal is the one that transmits an immense quantity of data. 

Raman spectroscopy is an exceptionally useful technique when it comes to identifying substances of crystalline nature, pigments and colourants, whether organic or inorganic, as well as to individualize restoration materials and degradation products. In other fields, it is used to characterize precious stones and to identify drugs and narcotics. 

 The measurements give punctual and superficial information; moreover, they can be carried out on various kinds of materials without damaging them, such as paper, wood, textile, papyrus, etc. The instrumentation can be stationary or portable, which makes the analysis in situ possible, in order to study and characterise goods located in environments that are not easily reachable (hypogeous, for example) or fragile goods that cannot or must not be moved (frescoes or paintings of big dimensions, for example). 

It is a perfect technique, then, isn’t it? Not exactly. 

Like every technique in our field, there are analytical limitations, among which the difficulty to individuate the binding agents when old and mixed with pigments; the possibility to create a microburst in the sample, if the power of the laser beam is too high; and the difficulty in identifying amorphous substances. 

One of the most important limitations of the technique is represented by the fluorescence phenomenon that is specifically observed in organic samples, such as colourants, glues and adhesives. It consists in the reemission of the received electromagnetic radiations by the analysed substance. The weak Raman signal is then easily masked by the fluorescence, and it becomes impossible to identify the analysed substance. 

The thermal effect is another phenomenon that masks the Raman signal, and is observed in presence of absorbent and dark-coloured materials (carbon black), and has a bigger probability to burn the sample. 

However, various methods have been studied to amplify the potentialities of the technique and bypass these limitations, specially the fluorescence effect. 

You would love to know what they are, wouldn’t you? We’ll talk about it soon. Stay tuned!

- R. L. McCreery, Raman Spectroscopy for Chemical Analysis, A John Wiley & Sons, Inc., Publication, 2000: https://www.thevespiary.org/library/Files_Uploaded_by_Users/no1uno/pdf/Textbooks/McReery.Raman.Spectroscopy.for.Chemical.Analysis.pdf 

- E. Smith, G. Dent, Modern Raman Spectroscopy: A Practical Approach, A John Wiley & Sons, Inc., Publication, 2005: http://www.chemistry.uoc.gr/lapkin/Modern_Raman_Spectroscopy__A_Practical_Approach.pdf

- Journal of Raman Spetroscopy, John Wiley & Sons Ltd, I.F. 2.395: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1097-4555