FT-NIR microscopy: an advanced spectroscopic approach for the characterisation of paint cross sections

The present research was aimed at exploiting and evaluating the potentialities of FT-NIR microscopy, as a complementary approach to analysis in the MIR region, for the chemical characterisation of paint cross sections. Even if FT-NIR technique is still underutilised in the field of cultural heritage investigations, the integrated use of information recorded in the NIR and in the MIR regions proved to be extremely useful in the molecular investigation of organic and inorganic substances. In fact, combination and overtone bands present in the NIR region, even if weaker and less selective than those in the MIR region, are not distorted by reflection phenomena. Furthermore, NIR spectra can be efficiently used as a spectral fingerprint for the stratigraphic characterisation of paint cross sections. The proposed analytical protocol was applied on two historical samples, presenting different stratigraphic structures. Suitable chemometric methods were applied for the elaboration of multivariate chemical maps recorded in the range 700–7500 cm− 1. In particular, a comprehensive and efficient procedure based on an interactive brushing approach, which combines the structural information of the score scatter plots with the spatial information of the PC score maps, was used. Interestingly, NIR data allowed a thorough characterisation of paint structures, providing information for the identification of components and suggesting the differentiation among different types of proteins. Moreover, NIR spectra permitted to achieve an efficient distinction of different classes of natural resins, demonstrating that, even working at a microscopic level, the NIR region may support the identification of different terpenoid materials. Multivariate analysis performed on MIR data did not provide satisfactory results, probably due to the distortion of the spectra and overlapping of bands. Nevertheless, MIR outcomes were investigated to support the interpretation of NIR spectra and in attempt to define an integrated protocol for the characterisation of complex paint mixtures.