Spectroscopic Studies of Baltic Amber-Critical Analysis.

Using optical spectroscopy methods including absorption in the UV-VIS, FTIR, Raman, and fluorescence, the spectra of 25 different Baltic amber samples were measured, and the ability of each method to distinguish between thermally modified and naturally aged material was analyzed. The natural ambers studied are characterized by a wide range of spectral properties: the position of the transmission edge in the UV-VIS spectra, the absorbance ratios of the C-H and C=O groups in the IR spectra, a difference of approximately 20% in the fluorescence intensity level, and differences in the band ratios in the C=C and C-H bonds in the Raman spectrum. Spectral studies were carried out on samples of natural and thermally modified amber at temperatures of 100, 150, and 200 °C for 2-8 h. Drastic changes occur at temperatures above 150 °C: the color changes to dark brown, the UV-VIS transmission edge shifts, the absorbance of the C=O group increases, the absorbance intensity of the C=C bond decreases, and fluorescence disappears. In some special cases, fluorescence methods allow for the unambiguous distinction of amber from different geographical regions (e.g., Baltic and Dominican). Spectroscopic methods can distinguish natural amber from thermally modified amber only for large changes in the spectrum at temperatures of 150-200; for smaller changes, the differences between individual samples of natural amber may be greater than in the case of thermal modification.