Identification of synthetic organic pigments: the role of a comprehensive digital Raman spectral library

Abstract

Identification of pigments in modern and contemporary arts is indispensable to determine correct conservation strategies, to study degradation processes and to answer authenticity‐related questions. Since the early 20th century, the introduction of synthetic organic pigments (SOPs) has enormously increased the number of available pigments. Micro‐Raman spectroscopy has proven to be the first line technique for the identification of these pigments, which often only show small variations on the same basic chemical structures. To ensure a correct identification, however, an as complete as possible library of reference spectra is needed. Although reference spectra of SOPs have been published before, they have always been limited to a certain number of pigments. Some publications discuss only one or a few chemical classes, and others are limited to a certain number of pigments belonging to different classes; none, however, have attempted to create an extensive library of commercially available pigments. Moreover, most of the reference spectra published so far are only available as small imprints or as peak lists and, as such, hardly usable for spectral matching algorithms. Often, flow charts have been developed on the basis of the pigments studied, to help in identifying unknown SOPs. In this paper, a digital spectral library that comprehends almost 300 spectra of different SOPs is presented. The library was tested by means of non‐invasive analysis of four contemporary paintings from the collections of the Stedelijk Museum voor Actuele Kunst (Ghent, Belgium). Published flow charts have been evaluated in relation to digital search algorithms by using the extensive library of SOPs. To enable fellow research and conservation institutes to make use of these data, the nearly 300 reference spectra of synthetic organic pigments are available in digital format on http://modern.kikirpa.be. Copyright © 2012 John Wiley & Sons, Ltd.

HPLC-DAD and HPLC-ESI-Q-ToF characterisation of early 20th century lake and organic pigments from Lefranc archives

“…Due to the poor solubility of organic lake pigments and synthetic pigments in any medium (including water or organic solvents), the most commonly used approaches for their detection and characterisation exploit Raman spectroscopy or, whenever possible, surface-enhanced Raman spectroscopy (SERS) [18][19][20][21][22][23][24][25][26][27][28], thanks to the availability of dedicated spectral libraries. Whenever sampling has been permitted, analytical pyrolysis coupled with gas chromatography-mass spectrometry (Py-GC/MS) [1,[29][30][31][32] has been successfully applied.…”

“…Whenever sampling has been permitted, analytical pyrolysis coupled with gas chromatography-mass spectrometry (Py-GC/MS) [1,[29][30][31][32] has been successfully applied. Direct temperature mass spectrometry (DTMS) [25,33,34] and laser desorption mass spectrometry (LDMS) [34,35] have also been used, but have not yielded positive identification when isomers are present, which is often the case when organic pigments are concerned.…”

Investigation of tattoo pigments by Raman spectroscopy

“…4, and detailed spectral analyses of many of these pigments can also be found in Fremout and Saverwyns. [22] These pigments can have complex spectra due to the variety of functional groups they contain. Also, those that are in the same pigment class can have only small spectral variations due to their highly similar structures (e.g.…”

“…[21] Additional studies have focused on spectral library development, including that by Fremout and Saverwyns using micro-Raman spectroscopy at 785 nm to evaluate 736 approximately 300 synthetic organic pigments [22] and that by Burgio and Clark with a 1064-nm Fourier transform Raman instrument to develop a database of pigments, minerals, and media. [23] A study by Poon et al focused on in situ pigment analysis of a model system of tattoos in pig skin, and the authors were able to identify most of the expected six pigments; however, the authors noted that simple evaluation after smearing tattoo inks onto glass slides led to high fluorescent backgrounds when interrogating with a 632.8-nm He-Ne laser.…”

Optical resolution from Fisher information

“…In practice, to identify the chemical composition of a sample, its corresponding spectrum does not need to be fully resolved. Characteristic information about the sample can alternatively be retrieved by digitally matching its spectrum to a set of previously recorded spectra [15][16][17][18]. In this section, the Fisher information will be employed to discuss spectral resolution in detail.…”