Effect of pigments on the analysis of fatty acids in siccative oils by pyrolysis methylation and silylation

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Abstract

The relative abundance of the dicarboxylic acids azelaic (nonanedioic) and suberic (octanedioic) with respect to monocarboxylic fatty acids is diagnostic for the identification of siccative oils in painting layers. Fatty acids can be produced from the sample by thermal (pyrolysis) and/or chemical (hydrolysis) degradation, and then determined by gas chromatography–mass spectrometry (GC–MS) in the form of methyl or trimethylsilyl (TMS) ester derivatives. However, the presence of matrix components in the layer (e.g. pigments) might interfere with the analytical procedure. In this study, the presence of three inorganic pigments (cinnabar (HgS), lead white (2PbCO3·Pb(OH)2) and zinc white (ZnO)) in linseed oil layers was investigated in relation to the determination of fatty acids by analytical pyrolysis with in situ derivatisation. Two methods were applied: pyrolysis/methylation with tetramethylammonium hydroxide (TMAH) and pyrolysis/silylation with hexamethyldisilazane (HMDS), both in combination with online GC–MS. Pyrolysis/methylation and, to a lesser extent, pyrolysis/silylation were influenced by the presence of the pigments which caused a significant decrease in the relative content of azelaic acid. Discriminating siccative oil from egg tempera might be problematic in the presence of lead white and zinc white. Reactive pyrolysis–GC–MS was compared with the classical wet method (alkaline hydrolysis, silylation, GC–MS analysis) and the effect of sample preparation was considered.

Introduction

Reactive pyrolysis in the presence of a methylating [1], [2], [3], [4], [5], [6], [7] or a silylating agent [7], [8], [9] is a valid analytical technique for the characterisation of the organic matrix in paint layers. Among organic binders, siccative oils can be recognised by the presence of a higher content of dicarboxylic acids, in particular azelaic acid (nonanedioic acid), with respect to egg tempera and other materials. Azelaic acid is detected by gas chromatography–mass spectrometry (GC–MS) as dimethyl or bis-trimethylsilyl (TMS) ester by pyrolysing paint layers in the presence of tetramethylammonium hydroxide (TMAH) or hexamethyldisilazane (HMDS), respectively. Pyrograms obtained from egg tempera are characterised by an extremely low content of dicarboxylic acids, while monocarboxylic acids display a pattern similar to that of siccative oils [1], [3]. Proteinaceous markers and cholesterol derivatives could be used as distinctive markers for egg tempera, but they are scarcely reproducible and generally of low intensity [3]. Thus, fatty diacids are important diagnostic markers for the identification of siccative oils, but their analysis might be influenced by matrix components occurring in the paint layer.

It is known that basic pigments such as lead white (2PbCO3·Pb(OH)2) and zinc white (ZnO) can lead to the formation of metal carboxylates (soaps) [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Vignola showed that the acicular or fibrous crystals of the soaps contributed to reinforce the film, improving its toughness, flexibility, adhesion and durability [10]. A study on red lead oil paintings stated that the hardening and setting of paints could be attributed to the formation of a gel or lead soap between free PbO and free fatty acids [11]. For their properties, long-chain carboxylated soaps have acquired a considerable commercial importance as lubricants, driers in paints and inks, catalyst polymer stabilisers, fuel additives, germicides and corrosion inhibitors. It was shown that while their formation may reinforce the physical properties of the film, they can also have negative effects on paint films, such as the formation of protrusions [12], [13], [14], [15], [16], [17]. The term “lead white phenomenon” has been known since 1940 and is referred to as the slightly cracked and mottled appearance of paint oil films with lead-based pigments [18].

Due to the peculiar properties of metal carboxylates, it is expected that inorganic pigments may affect the results of analytical determinations of dicarboxylic acids. For instance, it has been found that the amount of extractable fatty diacids from oil paint layers is influenced by the nature of the pigment [21].

The aim of this study was to investigate the effect of pigments on the determination of azelaic acid in paint layers by reactive pyrolysis. To this purpose, linseed oil painted layers without pigments and in the presence of lead white, zinc white and cinnabar (HgS) were analysed under methylating and silylating conditions and the obtained results were compared with gaschromatographic analyses after saponification and silylation.

Section snippets

Materials

Hexamethyldisilazane 99% and tetramethylammonium hydroxide (25% in water) were from Aldrich. Standard painting layers were prepared by the Opificio delle Pietre Dure (Florence, Italy) in 1998. Layers of pure linseed oil without pigments and with cinnabar, lead white, zinc white were spread into glass support and dried in air at room temperature.

Reactive pyrolysis–GC–MS

Samples (0.1–0.5 mg) of layers were scratched from the support, inserted into a quartz capillary tube and added with 5 μL of the derivatisation agent

Pyrolysis/methylation–GC–MS

Samples of dried layers of linseed oil prepared without and with inorganic pigments, namely cinnabar, lead white and zinc white, were subjected to pyrolysis–GC–MS in the presence of tetramethylammonium hydroxide. Examples of the resulting total ion chromatograms are depicted in Fig. 1a–d. A series of mono and dicarboxylic fatty acid methyl esters are identified in all the chromatograms as characteristic products in accordance to the chemical nature of the binder. The relative distribution of

Conclusions

This study shows that both pyrolysis/methylation and pyrolysis/silylation are rapid and effective analytical techniques for the detection of diagnostic azelaic and suberic acid in pure siccative oil layers. However, results from pyrolysis/methylation are strongly affected by matrix effects caused by the presence of inorganic pigments. Cinnabar reduces the relative content of azelaic acid with resepct to neat oil in pyrolysis/methylation, but not in pyrolysis/silylation. In the presence of the

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    Paper presented at the 16th International Symposium on Analytical and Applied Pyrolysis, Alicante, May 2004.

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