Klaas Jan van den Berg

Analysis of fresh triterpenoid resins and aged triterpenoid varnishes by high-performance liquid chromatography–atmospheric pressure chemical ionisation (tandem) mass spectrometry

Fresh triterpenoid (dammar and mastic) resins and aged triterpenoid varnishes were analysed by on-line HPLC–MS using atmospheric pressure chemical ionisation (APCI). Twenty components were identified in the fresh resins [dammarenolic acid, ursonic acid, oleanonic acid, hydroxydammarenone (I and II), dammaradienol, oleanonic aldehyde, dammarenediol, ursonic aldehyde, oleanolic aldehyde, ursolic aldehyde, hydroxyhopanone, dammaradienone, moronic acid, (3l,8R)-3,8-dihydroxypolypoda-13E,17E,21-triene, (8R)-3-oxo-8-hydroxypolypoda-13E,17E,21-triene, masticadienonic acid, isomasticadienonic acid, 3-O-acetyl-3-epimasticadienolic acid and 3-O-acetyl-3-epiisomasticadienolic acid]. Analysis of the aged varnishes revealed the presence of some oxidised triterpenoid components [11-oxo-oleanonic acid, 11-oxo-oleanonic acid, 3-oxo-25,26,27-trinordammarano-24,20-lactone, 20,24-epoxy-25-hydroxy-3,4-seco-4(28)dammaren-3-oic acid and some ocotillone type molecules]. Most constituents of these complex samples were well resolved by reversed-phase HPLC. APCI-MS provides useful information about the molecular mass and the presence of certain functional groups. Specific marker compounds were found, which enable the discrimination between aged dammar and mastic varnishes by HPLC–APCI-MS. The fragmentation behaviour of triterpenoids under APCI conditions was compared to that under electron impact conditions. Mass spectrometric fragmentation of triterpenoids with a saturated ring system is straightforward. Triterpenoids with an unsaturation in the ring system probably give rise to double bond migration. Subsequent low energy MS–MS analysis does not provide more structural information, since mainly non-specific ring fragment ions are formed.

Mass spectrometric methodology for the analysis of highly oxidized diterpenoid acids in Old Master paintings

Diterpenoid resins from larch and pine trees and the corresponding fractions in a >100-year-old wax-resin adhesive and varnish and a 200-year-old resin/oil paint sample were analysed with by gas chromatography/mass spectrometry (GC/MS) using several off-line and on-line derivatization methods. The main resin compounds were highly oxidized abietic acids. Important products found are hydroxydehydroabietic acids (OH-DHAs), 7-oxoDHA, di-OH-DHAs and 15-OH-7-oxoDHA. The last two compounds have not been reported to occur in artworks before. Larixyl acetate, an important marker from larch resins, was found to be still present in high amounts in the adhesive. A large number of mass spectra of the different oxidation products and larixol and larixyl acetate are presented and their fragmentation behaviour under electron impact conditions is discussed. An index for the degree of oxidation (IDOX) of the abietic acids is presented as an indicator of the degree of oxidation of the matrix in which the resin is present. The IDOX was 0.10, 0.67, 0.81 and 0.76 for the fresh resins, the dark-aged adhesive, the aged varnish and the resin/oil paint, respectively (measured with pyrolysis (Py)-tetramethylammonium hydroxide (TMAH)-GC/MS). Py-TMAH-GC/MS and direct temperature-resolved mass spectrometry are reliable, valuable and fast techniques for the assessment of the presence and degree of oxidation of diterpenoid resins. Copyright 2000 John Wiley & Sons, Ltd.

The use of microspectrofluorimetry for the characterization of lake pigments

In this paper, the potential of confocal microfluorescence spectroscopy is explored for the characterization of selected red lake pigments and paints based on alizarin, purpurin and eosin (weak, medium and strong emitters). The anthraquinone pigments have been used since ancient times by artists, and eosin lakes were used by impressionist painters. Reconstructions of artists paints based on 19th century recipes are examined. The paints were made using the lake pigments bound in a range of binding media including gum arabic, collagen, a vinyl emulsion and linseed oil. The acquisition of the spectra is rapid, with high spatial resolution and the data reliable and reproducible. Together with full emission spectra, it was possible to acquire well-resolved excitation spectra for purpurin, alizarin and eosin based colors. The present investigation suggests that micro-emission fluorescence can also be used as a semi-quantitative method for madder lake pigments, enabling the determination of purpurin lake ratio in a mixture of purpurin and alizarin, which is important for provenance studies. The data obtained with microfluorescence emission with those acquired with fiber-optic fluorimetry are compared. The spatial resolution used, 8microm, is appropriate for the analysis of individual pigments particles or aggregates in a paint film. Micro-emission molecular fluorescence proved to be a promising analytical tool to identify the presence of selected red lake pigments combined with a range of binding media.

Identification of non-cross-linked compounds in methanolic extracts of cured and aged linseed oil-based paint films using gas chromatography-mass spectrometry.

Methanolic extracts of paint samples of different composition and age were qualitatively investigated by GC-MS using an on-column injector after off-line methylation or trimethylsilyl derivatisation, and on-line thermally assisted (trans)methylation with tetramethylammonium hydroxide using Curie-point pyrolysis-GC-MS. The combination of these three analytical strategies led to the identification of typical oxidation products of unsaturated fatty acids by interpretation of their mass spectrum. Some of the identified compounds have not been reported before. Both the off-line and on-line GC-MS strategy show series of short-chain fatty (di)acids and C16 and (oxidised) C18 fatty acids. The major advantage of the on-line pyrolysis-GC-MS approach is that chemical work-up is minimal and very quick. With this technique both the carboxylic acid functionalities, and hydroxyl groups are methylated. Young paint films are shown to contain relatively more oxidised C18 fatty acids and less diacids compared to older paints, which is indicative for the on-going oxidation processes within the paint. After trimethylsilylation, monoacylglycerols are detected indicative for hydrolytic processes, which reflect the relative distribution of the most prominent silylated fatty acids present. Relatively more C16 and C18 monoacylglycerols are found in young paints, whereas older paints contain higher amounts of monoacylglycerols of diacids.

Thermally assisted transmethylation gas chromatography–mass spectrometry of suberin components in cork from Quercus suber L.

Thermally assisted transmethylation gas chromatography - mass spectrometry with tetramethylammonium hydroxide has been applied for the qualitative and quantitative analysis of the suberin fraction of wax-free cork. With this method, hydrolysis of esters together with methylation of carboxylic and hydroxylic groups was observed. Mostly long chain aliphatic components, such as alkanols, alkanoic acids, ω-hydroxyacids, α,ω-alkanoic diacids, and several 9,10-epoxy and 9,10-dihydroxy forms have been found in keeping with results of other workers in the field. Two other components that may give a more complete view of the architecture of suberin were also found, namely the phenolic compound ferulic acid, and glycerol which is probably esterified to carboxylic groups of the aliphatic building units of suberin.

Investigation of crimson-dyed fibres for a new approach on the characterization of cochineal and kermes dyes in historical textiles

The colorant behaviour of cochineal and kermes insect dyes in 141 experimentally-dyed and 28 artificially-aged samples of silk and wool was investigated using ultra-high performance liquid chromatography coupled to photodiode array detector (UHPLC-PDA), liquid chromatography electrospray ionisation mass spectrometry (LC-ESI-MS) and image scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDX). Partial-least squares discriminant analysis (PLS-DA) was then used to model the acquired UHPLC-PDA data and assess the possibility of discriminating cochineal insect species, as well as their correspondent dyed and aged reference fibres. The resulting models helped to characterize a set of 117 red samples from 95 historical textiles, in which UHPLC-PDA analyses have reported the presence of cochineal and kermes insect dyes. Analytical investigation of the experimentally-dyed and artificially-aged fibres has demonstrated that the ratio of compounds in the insects dye composition can change, depending on the dyeing conditions applied and the type of fibres used. Similarities were observed when comparing the UHPLC-MS and SEM-EDX results from the dyed and aged references with the historical samples. This was verified with PLS-DA models of the chromatographic data, facilitating the classification of the cochineal species present in the historical samples. The majority of these samples were identified to contain American cochineal, which is in agreement with historical and dye identification literature that describe the impact of this dyestuff into European and Asian dyeing practices, after the Iberian Expansion in the 16th century. The analytical results emphasize the importance of using statistical data interpretation for the discrimination of cochineal dyes, besides qualitative and quantitative evaluation of chromatograms. Hence, the combination of UHPLC-PDA with a statistical classification method, such as PLS-DA, has been demonstrated to be an advisable approach in future investigations to assess closely related species of natural dyes in historical textile samples. This is particularly important when aiming to achieve more accurate interpretations about the history of works of art, or the application of natural dyes in old textile production.

An Investigation of Parameters for the Use of Citrate Solutions for Surface Cleaning Unvarnished Paintings

Abstract This study investigates the surface cleaning of an unvarnished painting using citrate solutions at a range of pH. Cleaning tests followed a similar method to that used for cleaning paintings. The effects of the reagents were examined with light microscopy (LM) and scanning electron microscopy (SEM), and an assay for assessing clearance of residual citrate was carried out using gas chromatography–mass spectrometry (GC-MS). The results are used to evaluate the effects that might be observed when using these cleaning reagents, with the aim of improving risk assessment for surface dirt removal from unvarnished paintings. At low acceleration voltages, SEM allowed detailed examination of paint from the same location before and after sequential cleaning treatments, and the results are interpreted in combination with LM and visual assessment. Paint samples treated with solutions at selected concentrations were assessed by semi-quantitative GC-MS analysis. Clearance of the paint surface with water removed the reagent from the surface, and a detection limit of 1 ng is postulated for the method used.

Modern Oil Paints – Formulations, Organic Additives and Degradation: Some Case Studies

This study examines some of the degradation phenomena exhibited by modern oil paintings and twentieth century oil paint formulations and possible correlations with the various organic additives typically introduced by manufacturers. The research takes into consideration historical and modern tube oil paints produced by different European manufacturers and selected twentieth century oil paintings which showed degradation problems such as efflorescence, softening, cracks and sensitivity towards water and polar solvents. The composition of oil paints and paintings samples was studied with XRF, SEM-EDX, ATR-FTIR and GCMS. The results showed that both artists’ oil paints and paintings samples contain a complex mixture of additives to the oil binders and pigments. The presence of aluminium and zinc stearates, added as dispersion agent gelling agents, was detected in most paint formulations of HKS, W&N, Talens and Maimeri (c. 1940-present). Several paint films showed an unusually high content of fatty diacids, suggesting that, in drying, an oxidative reaction was favoured above polymerisation. This occurrence, together with the diversity of lipidic media, additives, pigments and driers present in the industrial formulations could play a crucial role in paint failure and instability, as showed by the analysed case studies. The obtained results may help understanding the behaviour of modern oil paints and painting surfaces and contribute to improved conservation methods.

A cause of water-sensitivity in modern oil paint films: The formation of magnesium sulphate

Abstract Unvarnished twentieth-century oil paintings are often sensitive to aqueous swabbing, a method routinely employed by conservators for surface cleaning. This study proposes a connection between sensitivity and the presence of magnesium sulphate heptahydrate which has been identified on the surface of some of water-sensitive paintings. The probable source of magnesium is magnesium carbonate, an additive in some twentieth-century oil paints, which has reacted with atmospheric sulphur dioxide (SO2). Films made using modern manufactured paints and formulations made in the laboratory were exposed to gaseous SO2 and raised relative humidity and examined using scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction to characterize the crystalline entities. Films containing magnesium carbonate formed magnesium sulphite and sulphate hydrates. Films containing zinc oxide were also investigated. These formed zinc and sulphur containing salts. Sensitivity to swabbing with water before and after exposure was evaluated. Films that developed salts, demonstrated increased sensitivity to aqueous swabbing after exposure to SO2. Findings suggest that increased water sensitivity may be due to a combination of the formation of hygroscopic degradation products and to weakening of the paint film due to salt-induced disruption of the surface.

Water Sensitive Oil Paints in the Twentieth Century: A Study of the Distribution of Water-Soluble Degradation Products in Modern Oil Paint Films

Water sensitivity has been observed on unvarnished, twentieth century oil paintings, which presents issues for surface cleaning. Previous studies have shown that manufacturers’ formulations are contributing to one of the known causes of water sensitivity: magnesium sulphate heptahydrate formation. It has been proposed that the formation of this water soluble degradation product is a result of interactions between the paint additive magnesium carbonate with environmental sulphur dioxide at elevated relative humidity. The present study examined the presence of this water soluble degradation product on the surface and in the bulk paint films of naturally aged Winsor & Newton paint swatches made from the 1940s to the 1990s. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) was the primary method for detection. Heptahydrate crystalline entities on the surface were found to be below the detection limit for X-ray diffraction (XRD). Water immersion tests suggested that water sensitivity due to magnesium sulphate heptahydrate formation is largely a surface phenomenon. Painting case studies helped to contextualise the problem in light of future conservation treatments for water sensitive paintings.