Getting lacquered: Pyrolysis with lithium attachment mass spectrometry for gentle analysis of Japanese lacquers

Japanese lacquer from the urushi tree

 

Japanese lacquer has been used in the country for more than 6000 years and is recognised as one of the best ancient coating techniques. It has been used principally to provide protective coatings on wooden objects but other types of surface like porcelain, paper and bottles have also been treated.

The lacquer is a natural substance, originating from the clear sap of the urushi tree which oozes from the trunk after the bark is cut, like gum from a rubber tree. It is sometimes referred to simply as urushi. After a long period of aging it can be applied neat, or can be mixed with other materials like charcoal or cinnabar to colour it black or red, or with metallic powder like gold, silver or copper to create lavish designs.

Once dried, the lacquer is highly resistant to acid, alkali and alcohol exposure and can resist temperatures up to 300°C. One of the few weaknesses is its vulnerability to UV light, like other natural finishes.

When ancient and modern lacquers are tested to establish their origins and authenticity, pyrolysis techniques are to the fore. The most common is pyrolysis-GC/MS, in which samples are heated in the absence of air and the products that are evolved are analysed in the mass spectrometer. Although it can be effective, the entities are often the products of secondary reactions like rearrangements and intermolecular reactions, which can complicate spectral interpretation.

Recently, a team of Japanese scientists developed an alternative procedure which is gentler, allowing the primary pyrolysis products to be detected. Toshihiro Fujii, Masamichi Tsukagoshi, Yuki Kitahara and Seiji Takahashi from Meisei University, Toyko, applied this technique, known as used lithium ion (Li⁺) attachment mass spectrometry (IAMS), to analyse the pyrolysis products released from lacquers as they were heated on a special probe.

Hot lacquers

Modern Japanese lacquer was coated onto glass plates and allowed to dry for at least one month before analysis. The films were loaded onto the probe of a commercial ion-attachment mass spectrometer and positioned in the ionisation chamber under vacuum. An infrared lamp heated the films rapidly from 50 to 500°C and the pyrolysis products that were emitted were ionised by irradiation with Li⁺ before being drawn into the quadrupole mass spectrometer.

The close proximity of the sample to the ionisation source ensured that primary pyrolysis products would be detected, rather than the secondary products observed with conventional pyrolysis GC/MS. They were detected as lithium ion adducts from which it was easy to deduce the molecular formulae of the various species.

Many pyrolytic components corresponding to different compound classes were assigned, although the use of a low-resolution quadrupole mass spectrometer meant that some of these were not unambiguous. These assignments could be made with more confidence by employing a high-resolution mass spectrometer and/or tandem mass spectrometry.

Pyrolytic differences for Japanese lacquers

There were some qualitative and quantitative differences between the lithium ion attachment mass spectra and those measured by pyrolysis GC/MS with electron ionisation, due to the different timescales involved: primary versus secondary products, respectively.

Hydrocarbons were common species observed by both techniques. The peak patterns were very similar, with C5-C9 alkanes the major alkanes found. For higher alkanes from C10-C20, those of lower mass were more abundant in IAMS, with C₁₄H₃₀ having the highest relative peak intensity. In pyrolysis GC/MS, the higher alkanes were more dominant.

Unsaturated carboxylic acids were also found by both methods, acetic acid being dominant in IAMS. Species of higher molecular mass were observed in pyrolysis GC/MS, with heptadecanoic acid the most abundant.

There were also marked differences for the catechols. The higher catechols found in the IAMS spectra were missing from the pyrolysis GC/MS spectra, which Fujii ascribed to decomposition within the longer timescale before detection.

One key advantage of the gentler nature of IAMS was illustrated by the detection of a series of peaks at m/z 315-345, which were ascribed to urushiol monomers, a type of catechol. These are signals which can be used to distinguish Japanese and Chinese lacquers from Vietnamese and Burmese lacquers, which are characterised by the catechols laccol and thitsiol, respectively. This difference will be useful for archaeological studies on ancient lacquerware to help determine its origin.

While the implementation of a high resolution mass spectrometer will help improve the interpretation of the mass spectra, the combination of pyrolysis with ion attachment mass spectrometry gives contrasting and complementary results from pyrolysis GC/MS for Japanese lacquers.