High-Resolution Mass Spectrometry and Nontraditional Mass Defect Analysis of Brominated Historical Pigments

Abstract

The implementation of high-resolution mass spectrometry systems offers new possibilities for the analysis of complex art samples such as historical oil paintings. However, these multicomponent systems generate large and complex data sets that require advanced visualization tools to aid interpretation, especially when no chromatographic separation is performed. In the context of this research, it was crucial to propose a data analysis tool to identify the products generated during the synthesis, drying, and aging of historical pigments. This study reports for the first time a nontraditional mass defect analysis of oil paint samples containing a fugitive brominated-organic pigment, eosin or geranium lake, by using direct infusion electrospray ionization in combination with a high-resolution Orbitrap mass spectrometer. The use of nontraditional Kendrick mass defect plots is presented in this study as a processing and visualization tool to recognize brominated species based on their specific mass defect and isotope pattern. The results demonstrate that this approach could provide valuable molecular compositional information on the degradation pathways of this pigment. We anticipate that mass defect analysis will become highly relevant in future degradation studies of many more historical organic pigments.

Chemical Mapping of the Degradation of Geranium Lake in Paint Cross Sections by MALDI-MSI

“…This confirms the low stability of geranium lake since its degradation already begins during the drying process, and it is catalyzed by light irradiation and lead white. This result is consistent with our previous studies, where we already detected the generation of fluoresceine, via the debromination mechanism of eosin species, during drying of oil paint samples by DART-HRMS and DI-ESI-HRMS …”

“…MALDI-MSI spectral data of all samples presented the characteristic isotopic pattern of the tetrabrominated eosin monomer at m / z 648.37 ([C 20 H 9 O 5 Br 4 ] + ), accompanied by the peaks associated with the tribrominated and nonbrominated eosin forms at m / z 568.56 ([C 20 H 10 O 5 Br 3 ] + ) and 332.63 ([C 20 H 13 O 5 ] + , aka fluorescein), respectively (Figure A), together with their K and Na adducts. These signals were assigned by comparison with data reported previously, , analyzing their isotopic patterns and distributions, and evaluating the possible additions to the most intense and monoisotopic ions based on mass differences. As GL was widely used by Van Gogh, its degradation pathway has been extensively investigated in solution and mock-up samples. ,,− These studies have revealed that an intermediate mechanism can be associated with the debromination process of the eosin molecule before leading to the breakdown of the xanthene moiety that causes total discoloration. ,, The ability to perform distribution analysis of the intact eosin monomer and its known degradation products on paint cross sections was evaluated next.…”

“…These signals were assigned by comparison with data reported previously, , analyzing their isotopic patterns and distributions, and evaluating the possible additions to the most intense and monoisotopic ions based on mass differences. As GL was widely used by Van Gogh, its degradation pathway has been extensively investigated in solution and mock-up samples. ,,− These studies have revealed that an intermediate mechanism can be associated with the debromination process of the eosin molecule before leading to the breakdown of the xanthene moiety that causes total discoloration. ,, The ability to perform distribution analysis of the intact eosin monomer and its known degradation products on paint cross sections was evaluated next. 10 μm spatial resolution ion images of these characteristic ions in samples GL100 and GL50 are presented in Figure B,C.…”

“…34,36,52−55 These studies have revealed that an intermediate mechanism can be associated with the debromination process of the eosin molecule before leading to the breakdown of the xanthene moiety that causes total discoloration. 35,51,56 The ability to perform distribution analysis of the intact eosin monomer and its known degradation products on paint cross sections was evaluated aging time in all studied samples. It is observed that the patterns of the parent compound (containing four Br) and its tribrominated form are significantly different than the completely debrominated secondary product.…”

Constellation: An Open-Source Web Application for Unsupervised Systematic Trend Detection in High-Resolution Mass Spectrometry Data

“…Selecting the “CH 2 ” base for data transformation is useful for visualization of hydrocarbons, but any custom base can be implemented to reveal common structural themes in particular analytes of interest. Ideally, “a base for renormalization is a molecular fragment that can be added or removed from a chemical formula without creating radicals.” For example, Alvarez-Martin et al used the most abundant isotope of a single tetra-brominated eosin molecule ([C 20 H 9 O 5 79 Br 2 81 Br 2 ] + , abbreviated as EoBr 4 ), as a base unit ( R = 648.7132) in their KMD analysis of degraded brominated historical pigments, allowing them to easily detect brominated species in complex HRMS data . Indeed, both axes of a KMD plot can be modified in this way, leading to “2D” mass defect plots .…”