Development and application of a Raman mapping instrument for the study of cultural heritage

Raman spectroscopy is a consolidated technique for the analysis of materials: it is based on the identification of the frequency shifts undergone by light scattered by a molecule; this behavior is due to an interaction of the incident photons with the vibrational quanta of the involved material. The possibility to obtain information on a molecular scale about the chemical identity of a sample in a non-destructive way, makes this method one of the favourite for the analysis of valuable objects, as it is in case of Cultural Heritage. In this field of application it is particularly urgent the necessity to monitor wide surfaces of heterogeneous objects often with an irregular shape. Compared to a laboratory measurement, moreover, an in situ analysis implies further requirements due to the dimensions and the portability of the employed instrumentation, which must guarantee an adequate sensitivity to Raman signal while keeping the necessarily limit of a non-invasive procedure. To answer to these needs, a portable Raman spectroscopy system which works with a non contact approach, has been conceived, designed and assembled. The device is able to analyze a surface 5 cm wide placed at a distance of 20 cm with a flexible mapping approach: a pair of galvanometric mirrors allows the deflection of the laser beam, which is focused on the point of interest by a dedicated optical system. This element was tailored to collect and guide the light scattered by the sample towards the instruments of analysis (spectrograph and charged coupled device) which, like the laser source, are remotely positioned and connected by optical fibers. This solutions permits to keep the measuring probe light and small, and to mount it on a tripod in order to ensure maximum flexibility and stability at the same time; the instruments of analysis can be, alternatively, connected to a more conventional Raman microscope when a deeper punctual analysis is needed. The chance of performing remote measurements not only considerably widens the range of object to study, but it also makes the device less affected by the vibrations which perturb contact analysis when performed out of the laboratory environment; moreover, the particular combination of the chosen optic elements ensures a depth of field ideal for the analysis of three-dimensional objects. The thesis treats the theoretical foundations of Raman spectroscopy to highlight its potentialities and stress the common problematiques (in primis the low efficiency with respect to competitive phenomena such as fluorescence); then, the phases of the project of the Remote Raman device are reported together with the methods to test its performances and features. One chapter is dedicated to the results of punctual analysis and of wide areas in samples of artistic or cultural interest; whenever it is possible, these are compared to the information obtained by other non-invasive techniques on the same objects. Last, surface-enhanced Raman measurements on amino acids in low concentrations are reported as a preliminary study for the analysis of organic traces in archaeological remains.  

Sviluppo e applicazione di un sistema di mappatura Raman per lo studio dei Beni Culturali