Chromogenic dyes in motion picture films at the turn of the 20th and 21st centuries: a multi-technique comparative study

Abstract

The cyan, magenta and yellow dyes in a series of four chromogenic motion picture films, dating from the late 20th and early 21st centuries, were characterized based on the use of Sequentially-Shifted Excitation (SSE™) Raman spectroscopy and/or (according to the amount of sample available) chromatographic techniques such as thin-layer chromatography (TLC) and high performance liquid chromatography (HPLC), coupled with surface-enhanced Raman spectroscopy (SERS) and mass spectrometry (MS), respectively. All methods were simply applied to samples of the emulsion layers removed from the films. The aim of this study was to gather information about the dyes that could be useful for the digital restoration of color motion picture films. In particular, the molecular classes of the couplers from which the dyes are derived, and to which different degradation processes can be associated, could be hypothesized thanks to the Raman and SERS spectra (the latter if obtainable) and the comparison with the corresponding spectra of reference dyes. Furthermore, an excellent match was found between the differences in Raman spectral patterns and mass spectrometric data for the examined dyes, thus proposing SSE™ Raman spectroscopy applied to emulsion samples as an easy-to-use method to discriminate the dyes themselves. Finally, the further correspondence between the analytical data and the spectral density curves of the films, when available, was verified, in order to constitute the first nucleus of a database that could provide those involved in digital restoration with valuable knowledge for reconstruction of the original colors.