Analytical Pyrolysis with In-Situ Silylation, Py(HMDS)-GC/MS, for the Chemical Characterization of Archaeological and Historical Amber Objects

Article excerpt

Authors: Maria Perla Colombini (corresponding author) [1]; Erika Ribechini [1]; Marco Rocchi [1]; Paola Selleri [1]

Introduction

Since ancient times the unique physical properties of amber have been exploited in the production of jewels, tools, ornaments, and works of art. Amber belongs to the class of fossil resins and is formed from plant deposits by the evaporation of volatile components, maturation, and polymerization reactions over a geological timescale. In Europe, the largest known deposit of amber deriving from conifers is found in northern Europe, mostly, in the Baltic area. This fossil resin is thus called Baltic amber and represents a specific subset of amber containing up to 8% by weight of succinic acid [1, 2, 3]. However, deposits of ambers of different geological eras and botanical origins can also be found all over Europe including Italy, Spain, Germany and Romania, and in the other part of the World, such us Canada, Alaska, Brasil. Amber objects are an important part of our cultural and ethnographic heritage, and studying them reveals the history, technology, rituals and trade routes of the past. The determination of its chemical composition can be used for an easy identification of amber among other fossil resins and copals and to distinguish it from imitations, widely diffused on the market.

However, the study of ambers represents a real challenge for analytical chemistry. This is mainly due to the relatively small amounts of samples available from artworks, to the high degree of polymerization, to the complex mixture of very similar organic compounds, which are characterized by a wide variety of molecular weights, which make up the fossil resins, and to the presence of possible degradation products.

A variety of physical and chemical techniques have been employed in the identification and characterisation of ambers [1, 2, 4]. In particular, spectroscopic analyses including FT-IR, FT-Raman and NMR have been widely exploited [2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. Such analytical approaches use bulk analysis to examine ambers and to obtain information on the botanical origins of archaeological findings. In addition, due to the high molecular weight/polymeric nature of ambers, some researchers have used analytical pyrolysis coupled with GC/MS systems (Py-GC/MS) [1, 2, 5, 20, 21, 22, 23]. When analytical pyrolysis is used, the chemical composition of the sample is reconstructed on the basis of an interpretation of the molecular profile of the thermal degradation products of the original components, and on the recognition of specific molecular markers or molecular patterns, which give characteristic fingerprints of the pyrolysed material. In fact, the classification [1] of ambers used in almost all of the scientific literature, is based on the structure of the resinite polymeric matrix, as determined by pyrolysis-gas chromatography-mass spectrometry [20, 24]. As with many organic natural substances, under pyrolysis ambers produce low volatile molecules that contain polar functionalities, which are not efficiently separated by GC.

The most common approach to overcome this kind of problem is to use thermally assisted derivatisation reactions. The use of derivatisation reagents that transform the polar pyrolysis products into less polar and more volatile compounds, improves the analytical performance and the detection limits of the technique. Tetramethyl ammonium hydroxide (TMAH) allows hydrolysis and methylation to be obtained simultaneously and is thus widely used derivatisation reagent [1, 20]. Recently, silylating reactions using hexamethyldisilazane (HMDS) have been proposed as an alternative in the analysis of ambers [21, 23, 25]. HMDS proved its potential with respect to the strongly alkaline TMAH reagent. Actually, the main limitations of this derivatizing reaction are related to the occurring of secondary reactions: particularly, decarboxylation reactions undergone by carboxylic acids and the formation of dehydration products and other by-products produce pyrograms of difficult interpretation. …