Structural Elucidation of an Unknown Compound in the Fatty Acid Methyl Esters (FAMEs) Extract of Avocado Using APGC-HRMS

Significance of the Topic

Avocado is valued for its high lipid content rich in omega fatty acids and other bioactive lipids. Monitoring the fatty acid methyl ester (FAME) profile ensures both nutritional quality and food safety. Unanticipated compounds in complex lipid extracts can pose unknown risks, making their rapid and reliable identification essential in food analysis and quality control.

Aims and Overview of the Study

This application note describes the analysis of a FAME extract from avocado. Objectives were twofold: to confirm the identity of known FAMEs and to structurally elucidate an unexpected chromatographic peak. The workflow combined gas chromatography with atmospheric pressure chemical ionization high-resolution mass spectrometry (APGC-HRMS) and advanced data processing tools.

Methodology and Instrumentation

The study employed a GC-APGC-Xevo G2-XS QTof platform with the following conditions:

• Column: DB-5MS (30 m × 0.25 mm, 0.25 µm)
• Injection: splitless, injector at 280 °C
• Oven program: 100 °C (4 min), 5 °C/min to 240 °C (15 min)
• Carrier gas flow: 1.2 mL/min
• Ionization: APGC in positive mode with water vapor for protonation
• Acquisition: data-independent MSE with low (6 eV) and high (20–50 eV) energies over 50–1000 m/z

Data were processed in the UNIFI Scientific Information System, using the Discovery Toolset for elemental composition, database searching (ChemSpider, FooDB), and fragment matching.

Main Results and Discussion

Five expected FAMEs (C16:0, C16:1, C18:1 cis/trans, C18:2) were confirmed by accurate mass (<±3 ppm) and characteristic fragment patterns. A novel peak at 31.29 min (m/z 303.2683) was not in the target list. Structural elucidation in UNIFI proposed 3-pentadecenylphenol, supported by:

• High-energy fragments showing alkyl chain cleavages and hydride abstractions
• Loss of H₂O from the phenolic ring (m/z 285.2582)
• Aromatic ring fragment consistent with a benzene core

Database searches revealed this compound in cashew nut and Ginkgo biloba fruit lipids, supporting a tentative identification.

Benefits and Practical Applications

The APGC source provided soft ionization preserving molecular ions, while alternating collision energies yielded comprehensive fragmentation. The UNIFI platform accelerated unknown screening and reduced manual interpretation. This approach is directly applicable to routine food quality, authenticity testing, and contaminant surveillance.

Future Trends and Potential Uses

Integration of GC-APGC-HRMS with expanded spectral libraries will enable broader detection of unexpected constituents. Coupling machine learning algorithms for pattern recognition may further streamline unknown identification. Adapting this workflow to other food matrices can enhance safety and compliance across the food industry.

Conclusion

A combined GC-APGC-HRMS and UNIFI workflow successfully identified known FAMEs in avocado and elucidated a previously unknown phenolic lipid. The methodology demonstrates robust capabilities for comprehensive lipid profiling and rapid unknown characterization in complex food extracts.

References

1. Duarte PF et al. Avocado: characteristics, health benefits and uses. Food Tech. 2016;46(4):747–754.
2. Analytical chromatography of FAMEs. Sigma-Aldrich Application Note.
3. Hydride abstraction mechanisms in MS. Mass Spectrom Textbook Chapter 7.
4. FooDB. Compound FDB002351: 3-Pentadecenylphenol.