Multivariate Analysis of Combined Fourier Transform Near-Infrared Spectrometry (FT-NIR) and Raman Datasets for Improved Discrimination of Drying Oils

Abstract

This work explores the application of chemometric techniques to the analysis of lipidic paint binders (i.e., drying oils) by means of Raman and near-infrared spectroscopy. These binders have been widely used by artists throughout history, both individually and in mixtures. We prepared various model samples of the pure binders (linseed, poppy seed, and walnut oils) obtained from different manufacturers. These model samples were left to dry and then characterized by Raman and reflectance near-infrared spectroscopy. Multivariate analysis was performed by applying principal component analysis (PCA) on the first derivative of the corresponding Raman spectra (1800-750 cm(-1)), near-infrared spectra (6000-3900 cm(-1)), and their combination to test whether spectral differences could enable samples to be distinguished on the basis of their composition. The vibrational bands we found most useful to discriminate between the different products we studied are the fundamental ν(C=C) stretching and methylenic stretching and bending combination bands. The results of the multivariate analysis demonstrated the potential of chemometric approaches for characterizing and identifying drying oils, and also for gaining a deeper insight into the aging process. Comparison with high-performance liquid chromatography data was conducted to check the PCA results.

Vibrational Spectroscopies and Chemometry for Nondestructive Identification and Differentiation of Painting Binders

“…This last band has a frequency close to the usual value at which the amide I band is observed (1665 cm −1 ) [18,22]. Egg yolk and whole egg have the lowest values for polyunsaturated fatty acids in comparison with drying oils [2,8]. Nevertheless, characteristic proteinaceous bands are present, as can be seen in Figure 2(b).…”

“…As regards drying oils, the main near infrared absorption bands are represented by the combination of methylenic C-H stretching and bending (between 4250 and 4350 cm −1 ) and first CH 2 stretching overtone bands (in the 5700-5800 cm −1 spectral range). The second overtone of ester carbonyl stretching mode is observed at 5176 cm −1 , whereas the combination of ester C=O and methylenic C-H stretching modes occurs at about 4690 cm −1 [6,8]. Raw FT-NIR spectra of drying oils do not show significant spectral differences between each other.…”

“…Recent authors have proposed the use of spectroscopic techniques, as vibrational and electronic spectroscopic methods, coupled with multivariate analysis for the discrimination of different painting binders (animal glue, whole egg, and linseed oil) [2,[4][5][6] and also of organic compounds of the same class, like lipidic binders [7][8][9][10], subjected to natural ageing.…”

“…Nevertheless, there are several different causes that can lead to random noise in Raman and FT-NIR spectra, thus making the identification of spectral bands more difficult, particularly in computational analyses, as in the present work. To identify small differences in the original spectra, first derivatives of the spectral data were used for PCs extraction [4,6,8,9,13]. As a result, an increase in sensitivity/specificity for differentiation capability and classification can be expected, due to the new dimensions added to the dataset [8,9].…”

“…To identify small differences in the original spectra, first derivatives of the spectral data were used for PCs extraction [4,6,8,9,13]. As a result, an increase in sensitivity/specificity for differentiation capability and classification can be expected, due to the new dimensions added to the dataset [8,9]. Principal Component Analysis (PCA) can be used to explain the variance-covariance structure of a set of data by the original variables' linear combination that form a new set of independent coordinates, called principal components (PC).…”