Characterization of paint from artwork with automated THM pyrolysis and OPTIC Injector

Importance of the Topic

Thermally assisted hydrolysis and methylation (THM) combined with GC-MS provides a powerful approach for characterizing complex organic materials in artwork. By converting polar macromolecules into volatile methylated fragments, THM-GC allows identification of binders, resins and coatings with minimal sample consumption and high reproducibility.

Objectives and Study Overview

The primary goal was to evaluate automated THM inside an OPTIC injector for quantitative analysis of a complex paint model. A Round-Robin sample composed of dried linseed oil with ultramarine blue, Paraloid B82, sandarac, succinic acid, mastic, gum arabic and egg white was investigated to demonstrate method performance.

Methodology and Instrumentation

The sample was finely crushed, treated with 2 µl of tetramethylammonium hydroxide in methanol and introduced into a fritted liner within the OPTIC 3 Multi Mode Inlet operating in pyrolysis mode. Key instrumentation and conditions:

• Autosampler: CombiPAL Robotic Sample Processor
• GC-MS: Shimadzu 2010
• Column: TC-5 MS, 30 m × 0.25 mm × 0.25 µm
• Oven program: 40 °C (4 min); 7 °C/min to 240 °C; 10 °C/min to 320 °C (5 min)
• Hydrolysis: 100 °C ramped at 10 °C/s, 120 s
• Pyrolysis/methylation: 550 °C ramped at 30 °C/s

Main Results and Discussion

All constituents of the model paint were identified: methyl methacrylate and ethyl acrylate from Paraloid B82; glycerol, fatty acids and di-acids from linseed oil; dimethyl succinate as amber marker; hydroxylated sandaracopimaric acid derivative for sandarac; and various triterpenoid markers for mastic. Ten replicate analyses yielded peak area RSDs between 2.1 % and 8.9 %, demonstrating excellent repeatability.

Benefits and Practical Applications

• Quantitative and reproducible analysis of natural and synthetic binders
• Minimal sample preparation and consumption
• Rapid, automated workflow suitable for cultural heritage laboratories

Future Trends and Potential Applications

Advances may include coupling THM-GC with high-resolution mass spectrometry for untargeted screening, expansion of spectral libraries for heritage materials, and integration of chemometric tools to enhance interpretation of complex chromatograms.

Conclusion

Automated THM within the OPTIC injector delivers fast, reliable and quantitative characterization of complex paint samples, enabling detailed identification of binders and additives with high reproducibility.

References

Erwin Kaal and Geert Alkema, Application Note No. 108, GL Sciences, March 2009.