Identification of diterpenes in canvas painting varnishes by gas chromatography-mass spectrometry with combined derivatisation.

A derivatisation method that combines the formation of ethyl esters from the carboxylic groups and trimethylsilyl ethers from hydroxyl groups of the components of diterpenic resins is presented in this paper. This methodology involves two experimental steps: (1) formation of ethyl esters using ethyl chloroformate; and (2) the esterified compounds are lead to react with trimethylsilylimidazole to form the corresponding trimethylsilyl ethers. The main advantage of the proposed method is the possibility of performing simultaneously the analysis of amino acids from proteins, fatty acids from drying oils, and diterpenic compounds from natural resins usually found in works of art. This methodology is of considerable interest due to the requirements of minimum sampling that usually involves the analysis of works of art. A chemometric study has been developed to adjust the optimal working conditions of the proposed derivatisation method in which chromatographic peak areas of the larixyl acetate derivative and the abietic acid derivative referred to n-hexadecane as internal standard have been compared. Samples of Venetian turpentine naturally aged have been used in this study. Finally, the efficiency of the proposed derivatisation method has been tested on other diterpenic resins and pigments commonly used in fine arts such as Strasbourg turpentine, Canada balsam, colophony, copper resinate and a sample from a Renaissance Altarpiece.

Electrochemical identification of metal ions in archaeological ceramic glazes by stripping voltammetry at graphite/polyester composite electrodes.

The electrochemical response of metal ions in different samples of coloured ceramic tin-lead glazes attached to graphite/polyester composite electrodes is described. In addition to the ubiquous signals for lead, reductive dissolution processes are followed by anodic stripping peaks for Co, Cu, Sb, Mn, Sn and Fe, enabling the direct identification of such elements in microsamples proceeding from archaeological glazed tiles from Valencia (Spain) workshops (16th-18th century). Additional anodic and cathodic peaks corresponding to redox processes involving metal species in solution generated during stripping processes are also used. Peak potentials, Tafel plots and shape parameters are used for characterising the different species.

Identification of lipid binders in paintings by gas chromatography. Influence of the pigments.

The influence of the presence and the type of pigments in the lipid binding media of paintings were studied by gas chromatography with flame ionization detector. The drying oils were linseed stand oil, poppy oil and sunflower oil, and the pigments studied were cadmium red, cobalt blue, tin white, lead white, chalk and plaster of Paris, commonly used in paintings. The results indicate that the stearic/palmitic ratio and the presence of pigments are quite stable during ageing. However, some differences in the oleic acid/palmitic acid ratio were found, depending on the type of pigment present in the lipid binding media. These variations are related to the drying effect of the pigments. The proposed method has been applied to the identification of drying oils in two samples from baroque paintings in the "Basilica de la Virgen de los Desamparados" of Valencia, Spain.

Identification by GC-FID and GC-MS of amino acids, fatty and bile acids in binding media used in works of art.

GC-FID was used as single methodology for the identification and differentiation of proteins, lipids and ox bile from binders used in artistic paintings. The samples were hydrolyzed by HCl. Subsequently, the simultaneous formation of volatile derivatives of the amino, fatty and bile acids with ethyl chloroformate was performed quickly and safely in an aqueous medium. The derivatives were separated by capillary GC and characterized by GC-MS. The ageing of drying oils was studied, identifying pelargonic acid among other degradation products. Proteinaceous and lipoid binding media were characterized by means of the quotients between the areas of the peaks for each amino or fatty acid with respect to the area of the peak for alanine or palmitic acid. Fatty acids from ox bile were easily identified by their retention times characteristic for eicosanoic, docosanoic and pentadecanoic acids. The suggested method was applied to the analysis of binders in baroque paintings by Palomino in Valencia (Spain). Animal gelatine and linseed oil were found.