Journal of Chromatography A, 1101 (2006) 254–260 Characterization of waxes used in pictorial artworks according to their relative amount of fatty acids and hydrocarbons by gas chromatography J. Peris-Vicente a , J.V. Gimeno Adelantado a,∗ , M.T. Dom´ enech Carb ´ o b , R. Mateo Castro a , F. Bosch Reig a a Department of Analytical Chemistry, University of Valencia, c/Dr. Moliner 50, 46100 Burjasot, Valencia, Spain b Department of Conservation and Restoration of Cultural Heritage, Polytechnic University of Valencia, Camino de Vera 14, 46022 Valencia, Spain Received 19 April 2005; received in revised form 29 September 2005; accepted 30 September 2005 Available online 24 October 2005 Abstract A study attempted to characterize natural waxes used in pictorial works of art was carried out by means of gas chromatography. The analytical treatment requires prior hydrolysis of the waxes to release the fatty acids (FA) (myristic (myr), palmitic (pal), oleic (ole), stearic (ste), araquidic (ara), behenic (beh), lignoceric (lig), cerotic (cer)) from the main esters of the waxes. The formation of volatile derivatives of the fatty acids was carried out by derivatization with ethyl choroformate (ECF). This derivatization reagent was chosen due to the speed, safety and quantitativity of the reaction. The analyzed hydrocarbons were n-eicosane, n-heneicosane, n-docosane, n-tricosane, n-tetracosane, n-pentacosane, n-hexacosane, n- heptacosane, n-octacosane, n-nonacosane, n-tricontane n-hentriacontane, n-dotriacontane, n-tritriacontane, n-tetratriacontane, n-pentatriacontane, main constituents of the waxes. No derivatization is needed to analyze the hydrocarbons. Ethyl ester derivatives and hydrocarbons are adequately separated by gas chromatography, identified by flame ionization detection and confirmed by mass spectrometry. To characterize natural waxes, peak area ratios of each fatty acids with respect to the palmitic acid and peak area ratios of each hydrocarbons with respect to n-heptacosane were calculated. The proposed method provides a good characterization of different waxes most frequently used in artworks, such as beeswax, carnauba wax and ceresin, and has been successfully applied to real samples. This is the first report on the application of ECF to the analysis of fatty acids in wax. © 2005 Elsevier B.V. All rights reserved. Keywords: GC; Natural waxes; Fatty acids; Hydrocarbons; Ethyl chloroformate derivatization; Art analysis 1. Introduction Waxes have been used since ancient times in decoration, preservation, as colouring matter and as binder [1]. They have also been found in Neolithic burials and in Egyptians mummies [2–4]. Their hydrophobic properties were used to protect the building surface to water corrosion in ancient Egypt [5] and in Rome [2,4]. They were also used in ancient pottery as sealers [6,7]. Waxes have been employed in pictorial works of art made by the encaustic technique [2,8]. In this technique, wax is molten by heating, mixed with pigments, and applied liquid on the dye surface. In some cases, the wax is solved with drying oils or yolk egg, then mixed with the selected pigment and extended on the picture. ∗ Corresponding author. Tel.: +34 963864533; fax: +34 96864436. E-mail address: jose.v.gimeno@uv.es (J.V. Gimeno Adelantado). Chemically, waxes are mixtures of organic substances, usu- ally long chain molecules. They are composed by hydrocar- bons [5], tri-, di- or mono-esters of medium length fatty acids, long chain alcohols, free long alcohols, aldehydes, ketones, - diketones, sterols, triterpenols and triterpenic acids [5,9]. Their chemical compositions depend on their animal, vegetal or min- eral origin [5,10]. Spectroscopic techniques, such as FT-Raman [11], FTIR [12], horizontal attenuated total reflectance (HATR)-FTIR [13] have been used to characterize waxes, but this kind of method only permits to distinguish among characteristic groups. Sep- aration methods such as supercritical gas chromatography [9], liquid chromatography [14,15,16], gas chromatography [1,2,10,17] or pyrolysis gas chromatography [10] shall be used. Some wax components are not directly analyzable by GC, due to their low volatility. Ketones have to be analyzed after deriva- tization with 2,4-dinitrophenylhydrazine (2,4-DNPH) [18] and long-chained alcohols after silylation [1,10,17]. Aldehydes [19] 0021-9673/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2005.09.083