Characterization and identification of urushi using in situ pyrolysis/silylation–gas chromatography–mass spectrometry

Abstract

a b s t r a c tAnalytical pyrolysis coupled with gas chromatography and mass spectrometry is a very powerful tool to analyze polymers and macromolecules. In this paper this technique was applied to the characterization of the urushi oriental lacquer (qi-lacquer in Chinese) with the aim to set up an alternative analytical method to identify the lacquer.Urushi is mostly rich in alcoholic polar moieties, thus, a derivatization step is required, which is crucial for the detection and MS identification of all the compounds deriving from the thermal degradation. In this work hexamethyldisilazane (HMDS) was used for the first time as a derivatizing agent, thus, proving to be a valid alternative to more common methylating agents.Adopting this in situ Py/silylation-GC-MS procedure, we were able to characterize urushi lacquer and identify characteristic pyrolytic profiles of silylated alkylcatechols, silylated alkylphenols, aliphatic hydrocarbons, alkylbenzenes as well as characteristic molecular biomarkers.The method was thus used to analyze six archaeological samples from Sichuan (China) dating back from the 2nd century BC to the 12th century AD, in which the presence of urushi was suspected, and the lacquer was identified in all of them through the identification of all the characteristic pyrolytic profiles and molecular biomarkers.The method set up is also promising to be used for the characterization of the other oriental lacquers, such as laccol and thitsi.

Characterisation of oriental lacquers from Rhus succedanea and Melanorrhoea usitata using in situ pyrolysis/silylation-gas chromatography mass spectrometry

“…The formation of mono-TMS catechols can be attributed to steric hindrance effects and was also revealed in the analysis of urushi from Rhus Vernicifera [23]. Of the mono-TMS alkylcatechols (Figure 2), several saturated and unsaturated compounds, mostly appearing as pair peaks, were identified for both the Burmese and the Vietnamese lacquers.…”

“…Both pyrograms showed different classes of pyrolysis products, mainly mono-TMS alkylcatechols, bis-TMS alkylcatechols, TMS alkylphenols, alkylbenzenes and aliphatic hydrocarbons, arising from the pyrolytic cleavages of C-C and C-O bonds present in the polymeric network of the oriental lacquers. In a previous work [23] we showed that the extraction of specific m/z values enabled pyrolytic profiles of these characteristic pyrolysis products to be highlighted in urushi samples, which could be used for diagnostic purposes. In particular, m/z 179, 253, 180, 91 and 57 were used to extract the pyrolytic profiles of mono-TMS alkylcatechols, bis-TMS alkylcatechols, TMS alkylphenols, alkylbenzenes, and aliphatic hydrocarbons respectively.…”

“…The silylated derivatives of 3-heptadecenylphenol and 3-heptadecenylcatechol were the most abundant pyrolysis products for the Vietnamese lacquer (Figure 2b). These compounds were also detected in the profile of the Burmese lacquer, but not the lacquer from Rhus Vernicifera [23]. They can thus be considered as molecular biomarkers of the oriental lacquer from Rhus Succedanea [46], if no ω-phenylalkyl-catechols and phenols are detected.…”

“…These compounds can be considered as molecular biomarkers of the oriental lacquer from Melanorrhoea Usitata, since these ω-phenylalkyl-catechols and phenols are known to be present in the sap extracted from this tree and not in the sap extracted from Rhus Vernicifera nor Rhus Succedanea [43]. The mass spectra of the ω-phenylalkyl-catechols showed the typical fragmentation of TMS-alkylphenols and catechols, with the peak at m/z 179 ascribable to the trimethylsilyloxytropylium ion [23,44]. In the case of 2-(12-phenyldodecyl)phenol (TMS) and 2-(10-phenyldecyl)phenol (TMS), the most abundant peak in the mass spectrum was at m/z 180, which can be ascribed to the trimethylsilyloxytropylium ion after the addition of a H˙ and subsequent rearrangement into a cyclohexadienyl structure [45].…”

“…The use of thermally-assisted reactions with tetramethylammonium hydroxide (TMAH) as a derivatising agent has shown good results so far for the analysis of oriental lacquers [19][20][21][22]. In a previous work, Py-GC-MS with in situ silylation using hexamethyldisilazane (HMDS) was proven to be a suitable alternative tool for a complete characterisation of the urushi lacquer from Rhus vernicifera and its identification in archaeological objects [23]. Asian lacquered objects can present coatings containing, in addition to oriental lacquers, mixtures of drying oils, proteins, carbohydrates, resinous materials, waxes.…”