Investigation of a yellow lake pigment using 2D-LC and EC-SERS

Abstract

The identification of natural organic pigments is fundamental for the conservation, preservation, and historical interpretation of artwork. Natural pigments, derived from plant and insect sources, can experience fading and degradation over time due to light, air, and humidity exposure. As such, these naturally-derived pigments are referred to as fugitive pigments. Although these pigments are derived from simple sources, it can be a significant challenge to characterize these pigments by a single method since they are found in complex matrices and are comprised of many components varying in their chemical structure and properties. Many spectroscopic techniques have been used successfully for the identification of fugitive pigments, such as Raman and infrared spectroscopy. Although these techniques are quite useful, they have their disadvantages. Raman spectroscopy is inherently weak, with only one in a million photons undergoing inelastic scattering. In addition, molecules which are also fluorescent, such as many natural organic pigments, will be difficult to detect using Raman spectroscopy due to the competing fluorescence. An enhanced variant of Raman spectroscopy, called surface-enhanced Raman spectroscopy or SERS, has shown much promise in this area over the past 15 years, with many natural organic pigments being identified in precious artworks as a result. However, a significant limitation of SERS is the ability to identify organic pigments present in a complex mixture and this limitation is due to the fact that SERS by itself offers no separation capabilities. While coupling of separation techniques such as thin layer chromatography and liquid chromatography have been attempted and have been successful, even for artist colourants, the combination of multidimensional chromatography and SERS has not yet been attempted and is the focus of this thesis work.1,2 Multidimensional liquid chromatography (2D-LC) is appropriate for addressing the increasing demand for decoding complex samples and has recently been found to be useful for the separation of artist colourants.3 However, it remains challenging to identify the components in such complex mixtures in a rapid and confirmatory way. Consequently, this thesis work has explored a 2D-LC-EC-SERS method, wherein electrochemical SERS (EC SERS) is used as the offline detection modality for multidimensional chromatography. This thesis work hypothesizes that 2D-LC-EC-SERS can be used for the identification of components in pigment samples, with a future goal of elucidation of the pigment degradation pathways for yellow lake pigments in particular.