Analysis of organic colouring and binding components in colour layer of art works

Abstract

Two methods of analysis of organic components of colour layers of art works have been tested: IR microspectroscopy of indigo, Cu-phthalocyanine, and Prussian blue, and MALDI-TOF-MS of proteinaceous binders and a protein-containing red dye. The IR spectra distortion common for smooth outer surfaces and polished cross sections of colour layer of art works is suppressed by reflectance measurement of microtome slices. The detection limit of the three blue pigments examined is approximately 0.3 wt% in reference colour layers in linseed oil binder with calcite as extender and lead white as a drying agent. The sensitivity has been sufficient to identify Prussian blue in repaints on a Gothic painting. MALDI-TOF-MS has been used to identify proteinaceous binders in two historical paintings, namely isinglass (fish glue) and rabbit glue. MALDI-TOF-MS has also been proposed for identification of an insect red dye, cochineal carmine, according to its specific protein component. The enzymatic cleavage with trypsin before MALDI-TOF-MS seems to be a very gentle and specific way of dissolution of the colour layers highly polymerised due to very long aging of old, e.g. medieval, samples.

Tryptic peptide analysis of protein binders in works of art by liquid chromatography–tandem mass spectrometry

“…More recently, proteomics approaches [14][15][16] have been adapted for use on protein binders [17][18][19][20][21][22]. Herein, proteins are digested into peptides by use of a protease, usually trypsin, which predominantly cleaves the protein at the carboxyl side of lysine and arginine (except when followed by proline) [23].…”

“…HPLC coupled to a diode array detector (DAD) allows for an easy binder identification using chromatogram fingerprinting [17], but is unsuitable for the identification of complex mixtures since individual peptides cannot be identified due to the limited specificity of the DAD spectra. (Single) mass spectroscopy, as used by Kuckova et al [18][19][20][21] with MALDI-TOF-MS allows for peptide mass fingerprinting: mass peaks are compared with a library of known peptide masses from reference samples. Since a peptide is only characterised by its mass, unambiguous peak attributions are not possible.…”

A Novel Approach to Protein Analysis in Hard Tissues

“…The resulting soluble specific peptide fragments can be easily extracted and subsequently analyzed by mass spectrometry. This method was originally developed for the identification of protein binders in art works [3] and subsequently successfully used for analysis of proteins in insoluble materials like color layers or historical mortars [4][5][6][7][8]. We anticipated that the above-mentioned principle could also be applied to the analysis of proteins in hard tissues.…”

An evaluation of GC‐MS and HPLC‐FD methods for analysis of protein binders in paintings

“…Direct analysis of protein binders by the MALDI-TOF-MS method is very promising [41,42]. Direct infusion MS as a fingerprint of protein-binding media used in works of art is described in ref.…”