Mineral Oil Residues in Food Part 4 - Removing Natural Occurring Alkanes

Importance of the Topic

Mineral oil residues in food, comprising saturated (MOSH) and aromatic (MOAH) hydrocarbons, pose safety and regulatory challenges. Natural odd-numbered alkanes in food matrices such as rice or chocolate interfere with MOSH quantification, leading to false positives and elevated detection limits. A reliable cleanup strategy is essential to selectively remove these interferences and ensure accurate, routine monitoring.

Study Objectives and Overview

The study aims to integrate a flash chromatography step using activated aluminum oxide (Alox) and silica gel into an automated LC-GC-FID workflow. Objectives include:

• Selective retention of long-chain natural alkanes (>C20) on Alox.
• Automated transfer of MOSH and MOAH fractions from LC to GC.
• Reduction of background interferences to improve quantitation accuracy.

Methodology and Instrumentation

Sample preparation involves hexane extraction of homogenized foods spiked with internal standards, followed by centrifugation and concentration. Cleanup utilizes glass flash columns packed with 10 g Alox (basic, 0.063–0.2 mm) topped with 3 g silica gel to remove polar impurities. The MOSH fraction elutes in n-hexane; MOAH is analyzed directly after LC.

LC parameters include a silica column with a n-hexane/dichloromethane gradient, a D₂ UV detector at 230 nm, and an Axel Semrau Chronect LC-GC interface. GC analysis employs a Shimadzu GC-2010 dual FID with Restek MXT columns under hydrogen carrier gas and temperature programming from 60 °C to 400 °C.

Used Instrumentation

• Shimadzu LC-20AD with Allure Silica column (250 × 2.1 mm, 5 µm).
• Axel Semrau LC-GC Chronect interface.
• Shimadzu GC-2010 dual FID.
• Restek MXT Siltek guard column (10 m × 0.53 mm) and MXT-1 analytical column (15 m × 0.25 mm × 0.1 µm).

Main Results and Discussion

Online LC-GC-FID chromatograms demonstrate clear separation of MOSH and MOAH. In a rice sample spiked with 5 mg/kg mineral oil, natural n-alkanes (C25–C35) produced a MOSH signal of 10.8 mg/kg before cleanup. After Alox flash chromatography, these interferences were fully removed, yielding a true MOSH level of 4.31 mg/kg. MOAH quantitation remained robust, contributing to a total oil recovery of 4.95 mg/kg.

Benefits and Practical Applications

The added flash chromatography cleanup:

• Eliminates odd-numbered plant alkanes, preventing false positives.
• Enhances sensitivity and lowers detection limits.
• Integrates seamlessly with automated LC-GC-FID for routine quality control.

Future Trends and Potential Applications

Emerging directions include coupling with high-resolution mass spectrometry for structural identification, miniaturized and green chromatography approaches, and AI-driven data analysis for faster interpretation. Expanded use in a wider range of food and environmental samples is expected.

Conclusion

Incorporating activated aluminum oxide flash chromatography into LC-GC-FID workflows offers a robust solution for removing natural alkane interferences in MOSH analysis. This strategy improves accuracy, reproducibility, and throughput for routine monitoring of mineral oil residues in food.

Reference

1. EFSA Panel on Contaminants in the Food Chain. Scientific Opinion on Mineral Oil Hydrocarbons in Food. EFSA Journal (2012) 2704.
2. Fiselier K., Fiorini D., Grob K. Activated aluminium oxide selectively retaining long chain n-alkanes. Analytica Chimica Acta 634 (2008) 96–101.
3. Fiselier K., Fiorini D., Grob K. Integration into on-line HPLC-LC-GC-FID to remove plant paraffins. Analytica Chimica Acta 634 (2008) 102–109.
4. Biedermann M., Grob K. On-line HPLC-GC analysis of mineral oil contamination. J. Chromatogr. A 1255 (2011) 76–99.
5. Biedermann M., Grob K. On-line HPLC-GC method of analysis. J. Chromatogr. A 1255 (2011) 56–75.