TLC and HPTLC-APCI-MS for the rapid discrimination of plant resins frequently used for lacquers and varnishes by artists and conservators

Abstract

Introduction:Depending on their terpenoid and phenolic constituents plant resinscan be classified as diterpenoid, triterpenoid or phenolic resins; thereby the profile ofditerpenes and triterpenes is considered as genus- or even species-specific.Objectives:We aimed to develop a simple, rapid, inexpensive, sensitive and specificmethod for the identification of resin-specific triterpenoid and phenolic compoundsin plant resins using (HP)TLC [(high-performance) thin-layer chromatography] com-bined with APCI-MS (atmospheric pressure chemical ionisation mass spectrometry)and post-chromatographic detection reactions.Methods:Twenty resin samples from different plant species were analysed. Differentextraction procedures, post-chromatographic detection reagents as well as varioussorbents and solvents for planar chromatography were tested. To evaluate thepotential of the optimised (HP)TLC-APCI-MS methods, parameter such as limit ofdetection (LOD) was determined for selected marker compounds.Results:Our protocol enabled qualitative analyses of chemotaxonomic molecularmarkers in natural resins such as dammar, mastic, olibanum and benzoin. For the firsttime, the application of thionyl chloride-stannic chloride reagent for a specificpost-chromatographic detection of triterpenes is reported, sometimes even allowingdiscrimination between isomers based on their characteristic colour sequences. Fortriterpene acids, triterpene alcohols and phenolic compounds, detection limits of2–20 ng/TLC zone and a system precision with a relative standard deviation (RSD) inthe range of 3.9%–7.0% were achieved by (HP)TLC-APCI-MS. The applicability ofthe method for the analysis of resin-based varnishes was successfully tested on amastic-based varnish. Thus, the method we propose is a helpful tool for the discrimi-nation of resins and resin-based varnishes with respect to their botanical origin.