Thermochemolysis – A Simple and Rapid Methylation Method Based on TMAH for Gas Chromatographic Analysis of Linseed Oil and Amber

Significance of the Topic

Thermochemolysis with tetramethylammonium hydroxide (TMAH) enables in situ methylation of polar functional groups during pyrolysis-GC-MS.
This enhances volatility and chromatographic resolution of carboxylic acids, alcohols and phenolics in complex matrices such as oils and fossil resins.

Objectives and Study Overview

The study aimed to develop a fast, simple thermochemolysis procedure using TMAH for automated gas chromatographic analysis of linseed oil triglycerides and Eocene amber.
Key goals included optimizing pyrolysis temperature, evaluating reagent solvents, and determining botanical origin of fossilized resins.

Methodology and Instrumentation

Linseed oil and ground amber samples were pyrolyzed in a GERSTEL Thermal Desorption Unit with integrated pyrolysis module (TDU-PYRO) at temperatures between 350°C–1000°C. An Agilent 6890N GC with 5795B MSD and a GERSTEL MultiPurpose Sampler with Cooled Injection System (CIS 4) were used. TMAH solutions (10% in methanol, 25% in water) were added for in situ methylation.

• Pyrolysis: 0.2 µL sample aliquots, 20 s at optimized 500°C.
• Thermochemolysis: 1:1 sample-to-reagent ratio, solvent venting, split injection (1:100).
• GC-MS conditions: 30 m ZB-5MS column, He carrier, 40°C–320°C oven program, EI mode (70 eV, m/z 50–550).

Key Results and Discussion

Optimization identified 500°C as ideal for linseed oil to avoid secondary degradation. Methanolic TMAH provided higher fatty acid methyl ester (FAME) yields than aqueous reagent, confirming the importance of homogeneous sample–reagent mixing. Thermochemolysis converted all major fatty acids—including α-linolenic acid—into sharp, well-resolved FAME peaks and suppressed alkene artifacts. In amber analyses, TMAH treatment enabled detection of FAMEs corresponding to free acids, while pyrolysis-GC-MS revealed sesquiterpenoids and labdane-type diterpenoids, indicating a gymnosperm (conifer) origin.

Benefits and Practical Applications of the Method

• Automated in situ derivatization reduces manual sample preparation.
• Quantitative methylation of acidic compounds preserves structural information.
• Enhanced chromatographic separation on non-polar columns.
• Applicable to diverse matrices: oils, waxes, biological tissues, sediments and fossil resins.

Future Trends and Potential Applications

• Adaptation to high-throughput and miniaturized systems.
• Integration with tandem and high-resolution mass spectrometry.
• Expanded use in environmental monitoring and quality control.
• Application to emerging biopolymers and archaeological samples.

Conclusion

Thermochemolysis-GC-MS using TMAH in a TDU-PYRO system offers a rapid, robust route for the methylation and analysis of polar analytes in complex samples. The method delivers reproducible, high-resolution chromatograms with minimal sample handling and has been successfully applied to both linseed oil and fossil amber characterization.

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