Resins, archaeological

The survival of a-boswellic acid, p-boswellic acid and their O-acetates, which have been isolated only from frankincense, has been demonstrated in archaeological samples [99,107,113]. These compounds are considered as very useful specific chemical markers for the identification of frankincense in resinous archaeological materials. 

Another interesting application of DE-MS was in the examination of a resinous archaeological substance sampled from the material contained in a Roman amphora (2nd 4th... 

Most ancient wooden objects recovered in archaeological excavations are usually in a decayed, weak, and friable condition that requires stabilization before the objects can be safely handled and studied. Stabilization of wood and decayed wooden objects, generally includes the use of consolidants, liquid solutions of a resin that impregnates and fills gaps in the wood and on drying solidifies, strengthening its fragile, deteriorated structure (Thompson 1991 Rowell and Barbour 1990). 

However, in many archaeological samples pimarane diterpenoids are often absent, and of the abietane compounds only dehydroabietic acid remains. In fact, dehydroabietic acid is present as a minor component in the fresh resins, but its abundance increases on ageing at the expense of the abietadienic acids since the latter undergo oxidative dehydrogenation to the more stable aromatic triene, dehydroabietic acid [2,18]. If oxygen is available, dehydroabietic acid can be oxidized to 7-oxodehydroabietic acid and 15-hydroxy-7-oxodehydroabietic acid. Since these diterpenoid compounds are often the dominant components in archaeological samples [95,97], they are considered characteristic for the presence of Pinaceae resins. 

In archaeological findings the occurrence of a high abundance of 28-norolean-17-en-3-one has been correlated to smouldering or burning processes undergone by Pistacia resins [94,123,124]. Mastic also contains a polymeric fraction (15 20%) identified as cw-1,4-poly-p-myrcene [128]. 

M.P. Colombini, F. Modugno, E. Ribechini, Chemical study of triterpenoid resinous materials in archaeological findings by means of direct exposure electron ionisation mass spectrometry and gas chromatography/mass spectrometry, Rapid Communications in Mass Spectrometry, 20, 1787 1800 (2006). 

S. Hamm, J. Bleton, A. Tchapla, Headspace solid phase microextraction for screening for the presence of resins in Egyptian archaeological samples, Journal of Separation Science, 27,235 243 (2004). 

P.F. van Bergen, T.M. Peakman, E.C. Leigh Firbank, R.P. Evershed, Chemical evidence for archaeological frankincense boswellic acids and their derivatives in solvent soluble and insoluble fractions of resin like materials, Tetrahedron Letters, 38, 8409 8412 (1997). 

E. Ribechini, F. Modugno, M.P. Colombini, Direct exposure (chemical ionization) mass spectrometry for a rapid characterization of raw and archaeological diterpenoid resinous substances, Microchimica Acta, 162, 405 413 (2008). 

In the following sections, the instrumental features of direct mass spectrometry based techniques (DI-MS, DE-MS and DTMS) are presented, followed by a discussion of some mass spectra of standard compounds and reference materials. Finally, a series of case studies related to the presence of resinous materials in archaeological findings and works of art are reported and discussed. 

The results show that DE-MS alone provides evidence of the presence of the most abundant components in samples. On account of the relatively greater difficulty in the interpretation of DE-MS mass spectra, the use of multivariate analysis by principal component analysis (PCA) of DE-MS mass spectral data was used to rapidly differentiate triterpene resinous materials and to compare reference samples with archaeological ones. This method classifies the spectra and indicates the level of similarity of the samples. The output is a two- or three-dimensional scatter plot in which the geometric distances among the various points, representing the samples, reflect the differences in the distribution of ion peaks in the mass spectra, which in turn point to differences in chemical composition of... 

Figure 3.17 DE mass spectrum of the resinous material collected from the Roman amphora recovered from a waterlogged archaeological site in Liguria (Italy) [18]...

A. M. Pollard and C. Heron, The chemistry and use of resinous substances, in Archaeological Chemistry, RSC Paperbacks, Cambridge, 1996. 

Headspace solid phase microextraction (HS-SPME). With this extraction technique, it is possible to concentrate volatile compounds thus allowing their detection even at trace levels, as in the case of volatile and semi-volatile terpenes in archaeological findings [7,31]. Chapter 10 outlines how resinous materials are investigated using HS-SPME-GC/MS. 

GC/MS has provided the first chemical evidence of the presence of benzoe resin in an archaeological material from the Mediterranean area [18]. The procedure was... 

F. Modugno, E. Ribechini, M. P. Colombini, Aromatic resin characterisation by gas chromato graphy mass spectrometry raw and archaeological materials, J. Chromatogr., A, 1134, 298 304(2006). 

J. S. Mills, R. White, Natural resins of art and archaeology their sources, chemistry and identification, Stud. Conserv., 22, 12 31 (1977). 

Few papers deal with the use of SPME in the characterisation of terpenic resins in archaeological or museum objects [23 26]. 

Table 10.1 Compounds trapped by headspace SPME from different resins, gum resins and archaeological samples, presented by Increasing retention indices, with the corresponding relative peak areas (%) for each substance... 

SPME/GC/MS is an efficient technique to reveal the presence of resinic substances in archaeological samples. Indeed, volatile terpenes are still present in very old archaeological samples (4000 years old), particularly in the case of compact matrixes, and can be trapped by the SPME fibre. In comparison with methylene chloride extraction, SPME is very specific and allows the direct analysis of the volatile terpenes content in complex mixtures including oils, fats or waxes. For this reason, headspace SPME is the first method to use when analysing an archaeological sample it will either allow the identification of the resin or indicate further sample treatment in order to detect characteristic triterpenes. The method is not really nondestructive because it uses a little of the sample but the same sample can be used for several SPME extractions and then for other chemical treatments. 

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