Vincent van Gogh's still lifes Irises and Roses were investigated to shed light onto the degree to which the paintings had changed, both individually and in relation to each other since they were painted, particularly in regard to the fading of the red lakes. Non-invasive techniques, including macroscopic X-ray fluorescence mapping, reflectance imaging spectroscopy, and X-radiography, were combined with microanalytical techniques in a select number of samples. The in-depth microchemical analysis was necessary to overcome the complications that arise when evaluating by non-invasive methods alone the compositions of passages with complex layering and mixing of paints. The results obtained by these two approaches were complemented by color measurements performed on paint cross-sections and on protected edges, and with historical information provided by the artist's own descriptions, early reviews and reproductions, and the data was used to carry out digital color simulations that provided, to a certain extent, a visualization of how the paintings may have originally appeared.
Surface-enhanced Raman spectroscopy (SERS) has found increasing acceptance in art conservation and forensic science for its great potential in detecting trace amounts of material. However, SERS is not a separation technique, and, therefore, it is not always suitable for distinguishing different components in a mixture. Coupling of thin layer chromatography (TLC) and SERS has been investigated in this article as a tool for the separation and identification of four alkaloids, namely harmalol, harmaline, harmane and harmine, from the seed extract of Syrian rue (Peganum harmala). The alkaloids contained in this plant were historically used as a dye and for medicinal purposes and have recently drawn attention due to their antitumor activity. The use of TLC over high-performance liquid chromatography (HPLC) is a convenient way to reduce the amount of material, equipment and time needed for the analysis, and coupling of TLC with SERS provides vibrational information on each compound in the mixture. HPLC analyses with diode array detection were also carried out as a test of our technique, to ascertain the composition of Syrian rue extract and validate the results obtained from TLC-SERS investigations. In addition, Syrian rue extract and its commercial alkaloid components were characterized by SERS and normal Raman spectroscopy here for the first time, in order to provide valuable reference data to be used for identification purposes.
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