Exploring analytical pyrolysis; Characterisation of organic binding media in Ethiopian Christian art using pyrolysis gas chromatography mass spectrometry

Abstract

Karakterisering av lipid- og protein-baserte bindemidler ble utforsket ved hjelp av pyrolyse-GC/MS (analytisk pyrolyse). Markører ble identifisert i seks malingsreplikaer av bindemiddelprøver uten pigmenter (linolje, bivoks, eggeplomme, eggehvite, animalsk lim og kasein) samt seks malerprøver fra etiopisk, kristen kunst, pyrolysert med en derivatiseringsagent tilstede (tetrametylammonium hydroksid, TMAH). I tillegg ble markører identifisert for tre replikaer av bindemiddelprøver (linolje, eggeplomme og animalsk lim) og en etiopisk, kristen maleprøve analysert uten derivatisering. De to analytiske tilnærmingene (med eller uten derivatisering) ble sammenlignet, og vurdert som komplementære: analyser med derivatisering passet bra for lipid-markører, mens analyser uten derivatisering passet bra for protein-markører. I tillegg til karakteriseringsprosessen, ble reproduserbarheten av det sensitive analytiske system evaluert. Karakteriseringsmetoden, som hadde blitt foreslått fra tidligere undersøkelser, ble justert basert på utfordringer knyttet til reproduserbarhet.

Analytical pyrolysis was explored using pyrolysis-GC/MS with a high precision resistively filament pyrolyser (Pyrola®2000, Pyrol AB) for characterisation of lipid-based and proteinaceous binding media. Markers were identified for six paint replicas (mock samples) of binding media without pigments (linseed oil, beeswax, egg yolk, egg white, animal glue and casein) as well as six paint samples of Ethiopian Christian art, which were pyrolysed in presence of a derivatising agent (tetramethylammonium hydroxide, TMAH). Additionally, markers were identified for three paint replicas (linseed oil, egg yolk and animal glue) and one Ethiopian Christian paint sample analysed without derivatisation. These two analytical approaches (with and without derivatisation) were compared and deemed complementary: Analyses with derivatisation were suited for lipid markers, while analyses without derivatisation were suited for proteinaceous markers. In addition to the characterisation process, the reproducibility of the powerful, but sensitive analytical system was evaluated. The characterisation method, suggested by previous investigators, was adjusted based on reproducibility-related challenges encountered.