Characterisation of Siccative Oils, Resins and Pigments in Art Works by Thermochemolysis Coupled to Thermal Desorption and Pyrolysis GC and GC-MS S. Prati 1, & , S. Smith 2 , G. Chiavari 1 1 Laboratorio di Chimica Ambientale, Universita ` di Bologna, via Marconi 2, 48100, Ravenna, Italy; E-Mail: prati@ciam.unibo.it 2 School of Health & Life Science, King’s College London, Franklin-Wilkins Building, 150 Stamford St, London SE1 9NN, UK Received: 31 July 2003 / Revised: 9 September 2003 / Accepted: 9 October 2003 Online publication: 3 February 2004 Abstract Thermal desorption in the presence of tetramethylammonium hydroxide (TMAH) has been applied to materials belonging to the artistic field to verify the effectiveness of the derivatisation reaction. Thermal desorption of siccative oils and abietane resins give consistent results with traditional pyrolysis in methylation conditions, while as regard antraquinonic dyes the milder thermal treatment leads to the formation of more diagnostic compounds than pyrolysis. Keywords Gas Chromatography-mass spectrometry Thermal desorption Pyrolysis Oil and resins Tetramethylammonium hydroxide Introduction Siccative oils, natural resins and pig- ments in works of art and objects of antiquity present a challenge for the analyst, especially where ageing pro- cesses lead to changes in the chemical characteristics of these materials. Many constituents in these materials are poorly characterised because they are complex matrices that include sparingly soluble and non-volatile entities that may be difficult to extract and analyse by traditional techniques. Pyrolysis, thermal degradation at temperatures of the order of 600–800 °C, and in particular flash pyrolysis, is the most commonly used technique for characterising such refractory fractions from a host of matrices [1–11]. In con- junction with derivatisation reagents, it has been shown that the more polar fractions can be recovered quantitatively [12–13]. However, determination of the actual molecular composition is depen- dent on an accurate re-formulation of the pyrolytic fingerprint, which can be problematic for complex matrices. Thermal desorption, is a milder ther- mal technique (usually involving temper- atures of about 300 °C) that aims to evolve molecules intact from complex matrices. Using this method, quantitative recovery of PAHs and alkanes from het- erogeneous matrices such as airborne particulate matter has been achieved [14–15]. The technique has also been used for the analysis of PAHs or PCBs in samples of soils, sediments and for the determination of flavours in food [14–18]. Thermal desorption in tandem with pyrolysis is a less frequently used tech- nique, however it has the advantage of separating the semi-volatile fraction from the non-volatile fraction and as a result the pyrolytic fragments obtained are not complicated by thermal degradation products of the semi-volatile fraction as in the case of straight pyrolysis [14–15]. Thermally assisted derivatisation using tetramethyl ammonium hydroxide (TMAH) is an efficient means of achiev- ing methylation of acidic hydrogens in polar organic entities[12–13]. It has been most commonly used in pyrolysis based methods, however, it has been demon- strated that hydrolysis and trans-methyl- ation of certain types of polar groups can occur at lower temperatures (300–400 °C) than the much higher temperatures associated with pyrolysis [19–25]. Thermally assisted derivatisation or thermochemolysis using TMAH of humic acids at 300 °C and 700 °C, has been shown to yield the same product distribu- tion at both temperatures, and it would appear that TMAH promotes a saponifi- cation-esterification reaction [20]; similar results have been obtained with lignins [21], the aliphatic biopolymer cutan [22] and for protein characterisation in geo- polymers [23]. However, some studies have suggested that quantitative recovery can DOI: 10.1365/s10337-003-0141-4 2004, 59, 227–231 0009-5893/04/02 Ó 2004 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH Original Chromatographia 2004, 59, February (No. 3/4) 227