Studying the trimethylsilylation of alkylcatechols in Asian lacquers by analytical pyrolysis coupled with online micro-reaction sampler

Abstract

Asian lacquers are natural polymers obtained from the sap of trees of the Anacardiaceae family. Their main chemical components are long-chain alkylcatechols that form a complex polymeric network upon curing. Analytical pyrolysis is the most powerful tool to study these polymers from a chemical point of view. However, derivatisation of polar pyrolysis products, especially alkylcatechols, is needed to ensure their detection by gas chromatographic techniques. In this study, in situ trimethylsilylation using hexamethyldisilazane (HMDS) applied to the pyrolysis of urushi (Toxicodendron vernicifluum) and thitsi (Gluta usitata) lacquers was investigated to address the issue of partial derivatisation of alkylcatechols in flash pyrolysis conditions. By using an online micro-reaction sampler (MRS) coupled to pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), the derivatisation reaction was studied under controlled conditions. The reaction time between HMDS and lacquer pyrolysis products generated at 400 °C was investigated, proving that 12 s is sufficient for the complete trimethylsilylation of alkylcatechols. This information is crucial to understand the factors affecting in situ derivatisation, showing that slightly extending the contact time between the derivatising agent and the pyrolysis products is a relatively easy solution to partial derivatisation deriving from steric hindrance. This optimised procedure yields simpler chromatograms compared to flash pyrolysis and has high potential for the enhanced characterisation of these materials and the detection of lacquer molecular markers in samples containing complex mixtures of organic materials.