Suppression of pigment interference in the gas chromatographic analysis of proteinaceous binding media in paintings with EDTA.

A method to suppress the interference of pigments in the analysis of proteinaceous media used in paintings is presented in this paper. This method is based on the formation of metallic ion-ethylenediaminetetraacetic acid (EDTA) complexes previous to the derivatisation process, using ethyl chloroformate (ECF), to transform the amino acids in N(O,S)-ethoxycarbonyl (EOC) ethyl esters. Test specimens, containing different proteinaceous media such as albumin, porcine gelatine and casein mixed with lead white, chalk, verdigris and raw Sienna have been prepared for carrying out this study. Different pH conditions have been probed for the different pigments studied. Values of peak area ratio of amino acids relative to the alanine, obtained using the proposed method on a series of protein-pigment test specimens, have been compared to those from specimens of pure protein in which direct method of derivatisation was applied. Finally, the method has been successfully applied to the analysis of 18th century wall paintings in which animal glue was used as binding medium.

Analytical study of proteinaceous binding media in works of art by gas chromatography using alkyl chloroformates as derivatising agents.

In this work, we present the results obtained in an analytical study of the different types of proteinaceous binding media most commonly used in paintings, using GC-FID as the technique of analysis and GC-MS as a confirmatory technique. The application of this methodology requires prior hydrolysis of the proteins in the binding media to obtain free amino acids and then volatile derivatives, in this case by reaction with chloroformates due to advantages of speed, safety and the aqueous medium in which the reaction occurs. The method proposed for the proteinaceous binding media study is to calculate the proportions of the different amino acids with respect to alanine. This method provided good characterisation of different binding media, such as pork gelatine, beef gelatine, albumin, egg white and casein. The proposed method is used for the identification of binding media (including mixtures of binders) present in real samples from paintings in the city 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.