Analytical Protocol for Detecting Organic Materials

From a search of the available literature, the project team found that a number of analytical procedures have been published for characterizing either Asian or European lacquer. However, there are several reasons an ideal analytical procedure should be applicable to the widest possible range of both Asian and European materials.

Fundamentally, it is not necessarily obvious from visual observations alone which type of lacquer is present on a piece of furniture. Many japanning techniques produced excellent, high-quality imitations of Asian lacquer. The project team also found that in many of the published procedures only major components in the lacquers could be identified. Yet knowledge of minor components is also important, because they may affect the appearance and stability of lacquer. Finally, one must always take into consideration that unusual materials not described in published recipes and formulations may be present.

Application of a single test method would streamline the analytical process and ensure that a minimum of sample material would efficiently yield a maximum of information. The project team set out to develop such a testing method.

Pyrolysis gas chromatography/mass spectrometry using tetramethylammonium hydroxide for derivatization (TMAH-Py-GC/MS) proved to be the best overall method for characterizing lacquers of both types of origin. The test results from TMAH-Py-GC/MS were processed using a calibration table to automatically detect all known chemical compounds from the two types of lacquer. Presently, the table holds the names, retention-time data, and significant ions from the mass spectra of nearly 300 compounds. Systematic detection of all of the marker compounds in every sample tested is, without doubt, the single greatest advantage of using the calibration table to process the data.

The procedure works well on intractable materials. It possesses sufficient sensitivity to detect minor and trace components, allows for materials from conservation treatments to be detected, and can identify materials from Asian and European lacquer formulations even if they are aged. TMAH had the minor drawback of producing multiple derivatives of the substituted catechols in Asian tree saps. This limitation was overcome by tabulating the peak area results for all derivatives formed from each catechol compound.

Minimizing the amount of sample required for testing remains an active area of research. It is difficult to obtain an adequate amount of sample either from lacquered objects with pristine surfaces or from those that lack removable attachments (such as the gilded mounts on the Getty Museum furniture), because the sampled area would be unacceptably large.

Research continues on finding ways to modify the TMAH-Py-GC/MS procedure to accommodate smaller samples. Presently, small samples are best analyzed using Fourier-transform infrared spectrometry with reference spectra databases. This technique can identify major components in lacquers as well as nitrocellulose and other modern synthetic materials, but it is incapable of differentiating the three Anacard tree saps.

Samples were analyzed by Py-GC/MS with derivatization of the lacquer samples with tetramethylammonium hydroxide (TMAH) prior to analysis. The rationale for this approach is to convert carboxylic acids, alcohols and phenols to more volatile products prior to analysis. A Frontier Lab PY-2020D double-shot pyrolyzer system equipped was used for pyrolysis. The pyrolysis interface was maintained at 320°C.

The pyrolyzer was interfaced to an Agilent Technologies 5975C inert MSD/7890A gas chromatograph/mass spectrometer. A Frontier Ultra ALLOY15 capillary column was used for the separation (30 M x 0.25mm x 0.25 μm), with helium carrier gas set to 1 ml/minute. The split injector was set to 320°C with a split ratio of 50:1 and no solvent delay. The GC oven temperature program was 40°C for 2 minutes, then ramped to 320°C at 20°C /minute, followed by a 9-minute isothermal period. The MS transfer line was at 320°C, the source at 230°C, and the MS quad at 150°C. The mass spectrometer was scanned from 33-600 amu at a rate of 2.59 scans per second. The electron multiplier was set to the autotune value. Samples were placed into a 50 μl stainless steel Eco-cup fitted with an Eco-stick, and three microliters of a 25% methanolic solution of TMAH were introduced for derivatization.

After three minutes, the cup was placed into the pyrolysis interface where it was purged with helium for three minutes. Samples were pyrolyzed using a single-shot method at 550°C for 6 seconds.

The resulting total ion chromatograms and associated mass spectra are interpreted using a reference table of characteristic marker compounds for a very large selection of raw materials associated with the production of both Asian and European lacquers.

The table, now comprising nearly 300 compounds, was compiled from published and unpublished data as well as in-house analytical results. This reference table is available to interested researchers upon request by contacting Michael Schilling (mschilling@getty.edu).