The Analysis of Mineral Oil in Sunflower Oil

Importance of the Topic

The presence of mineral oil residues in edible oils poses health risks and regulatory challenges. Sunflower oil, with its viscous nature and high molecular weight constituents, tends to foul gas chromatographic systems, making low-level detection of mineral oil particularly demanding. Developing a selective sample introduction strategy is essential to enhance sensitivity, maintain instrument performance, and ensure reliable food safety monitoring.

Objectives and Study Overview

This application note aimed to establish a gas chromatography–flame ionization detection (GC-FID) method capable of detecting and quantifying trace levels of food-grade mineral oil in sunflower oil. The study was divided into three key stages:

1. Obtain reference chromatograms for pure sunflower oil and mineral oil under splitless injection to optimize baseline conditions.
2. Select a suitable mineral oil peak for accurate identification and quantification within the sunflower oil matrix.
3. Implement and refine a selective exclusion protocol to transfer the target peak to the column while excluding involatile matrix components.

Methodology and Instrumentation

Sample introduction relied on controlled injection temperature programming to exclude high-boiling constituents after target peak elution. Two strategies were compared: manual high-temperature venting of involatiles and automated Difficult Matrix Introduction (DMI) using the Focus-DTD.

Key instrumentation and operating conditions included:

• Injector: ATAS Optic 2-200 programmable injector with fritted liner.
• GC System: HP 5890 gas chromatograph coupled to an FID.
• Carrier Gas Flows: Split mode at 350 ml/min, vent at 50 ml/min.
• Temperature Program: Initial 40 °C; ramp 4 °C/s to 300 °C; isothermal hold for 5 minutes to ensure complete transfer of the selected hydrocarbon peak.

Main Results and Discussion

Selective exclusion successfully isolated a large mineral oil hydrocarbon peak eluting at ~19.6 minutes, corresponding to an approximate boiling point of 441 °C (C29 surrogate). By holding the injector at 300 °C under split flow for 5 minutes, over 98% of involatile components remained in the liner, preventing column and detector fouling. Chromatograms showed:

• Sharper, well-resolved mineral oil peaks compared to conventional splitless injections.
• Reduced baseline noise and minimal carry-over of heavy sunflower oil constituents.
• Detection limits down to 5 ppm with a 1 µL injection volume.

Benefits and Practical Applications

Implementing selective exclusion offers several advantages:

• Prolonged column and detector lifespan by minimizing contamination.
• Enhanced sensitivity and peak resolution, enabling low-level quantification.
• Reduced maintenance and downtime by trapping involatile materials within disposable microvials.

Future Trends and Potential Applications

Automation of DMI workflows can further streamline high-throughput analysis, reducing operator intervention. Coupling selective exclusion with mass spectrometric detection may expand applicability to complex food matrices or environmental samples. Ongoing developments in injector technology and software-controlled thermal profiles are expected to refine chromatographic selectivity and throughput.

Conclusion

The optimized selective exclusion technique demonstrates a reliable approach for trace-level mineral oil analysis in sunflower oil. By isolating a characteristic hydrocarbon peak and preventing involatile matrix transfer, the method achieves low detection limits, maintains instrument integrity, and supports robust food quality control protocols.

References

• GL Sciences Technical Note No. 18: Principles of Selective Exclusion.
• GL Sciences Technical Note No. 18A: Selective Exclusion Procedures.