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.