Combining Fourier transform infrared and Raman spectroscopies with Gaussian deconvolution: An improved approach for the characterization of emeralds

In the present work, five natural emeralds from various localities and one hydrothermal synthetic emerald were investigated by scanning electron microscopy–energy-dispersive X-ray technique (SEM–EDX), Raman and Fourier transform infrared (FTIR) attenuated total reflection spectroscopy. A two-step procedure was used for this study. In the first step, the samples were analyzed by SEM–EDX to evidence differences in the chemical composition and, in particular, to estimate their different alkali contents. In the second step, a detailed vibrational analysis was made, paying a special attention to three specific vibrational bands generally related to the content of alkalis: the infrared OH (3,800–3,400 cm−1) and Si─O (1,000–1,300 cm−1) stretching bands and the Raman band around 1,066 cm−1. In particular, in order to resolve overlapped bands present in these latter two ranges, a peak finding was done by curve fitting with Gaussian functions following a minimizing procedure. This analysis was accomplished by the second and fourth derivative of the FTIR and Raman spectra. These processing procedures allowed us to capture unambiguously spectral vibrational differences related to differences in the chemical composition of the investigated emeralds and, in particular, to their alkali content.