Abstract
Several analytical techniques have shown use for the identification of historical synthetic organic pigments (SOPs) in samples. However, as the techniques differ—for example, in the extent of sample intervention, modes of detection, and limits of detection—so do they differ in results and insights into the colorants and colorant derivatives in a sample. Liquid chromatography (LC) is infrequently applied to the task of SOP identification, despite its ability to differentiate similar molecules, detect at low concentrations, and separate of complex samples and its established usage for organic dyes. This research compared results obtained with micro-Raman spectroscopy and pyrolysis gas chromatography with results from ultra-high performance (UP)LC coupled with photodiode array detection (UPLC-PDA), and high resolution mass spectrometry (UPLC-HRMS) to judge the quantity and quality of SOP information obtained by each technique.In this study, 67 historical samples were analyzed from historical samples and sample books (1918–1950), consisting of 10 oil paint tubes, 45 oil-bound paint-outs, and 13 gum-bound pigment washes using sample sizes less than a milligram. As few studies have used LC for SOP identification, special attention is also paid to issues around sample preparation and interpretation of results from UPLC-PDA and UPLC-PDA-HRMS results. Finally, archival sources and other contemporary documents are used to contextualize and evaluate the plausibility of the analytical results. The results indicate that specific combinations of analytical techniques are required for confident SOP identifications. While Raman results were accurate and independent of sample solubility, they relied heavily on database completeness and were not sensitive to mixtures or differences in relative amounts of SOPs. UPLC-PDA was an effective complement for these shortcomings, except for a few samples that were insoluble. The use of HRMS was critical for the elucidation of unknown SOPs.
Highlights
The use of newly-developed synthetic organic pigments (SOPs) in artists’ paint began in the latter half of the nineteenth century [1]
Reference pigments were obtained from the reference collection of the RCE, whose identities were substantiated by UPLC-PDA and HRMS analyses across several references of different origins with the same reported composition
SOP analysis by micro-Raman spectroscopy is advantageous in that sample preparation does not require chemical modification of the sample, only physical preparation to expose its contents
Summary
The use of newly-developed synthetic organic pigments (SOPs) in artists’ paint began in the latter half of the nineteenth century [1]. For manufacturers such as Lefranc, Schmincke, Winsor & Newton, Rober son’s and (Royal) Talens, SOPs brought attractive new colors to the market at lower prices which required less pigment than traditional inorganic pigments or natural organic lakes [2,3]. Since the most standard and traditional analytical methods in conser vation science focus on inorganic pigments, identification of the SOPs in art objects occurs less readily. More discerning approaches in organic analysis must become common in order to understand the spread of SOPs in the art world
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