Determination of Mineral Oil Saturated and Aromatic Hydrocarbons in Edible Oil by Liquid-liquid-gas Chromatography with Dual Detection

Significance of the Topic

Mineral oil hydrocarbons (MOH) contamination in edible oils has emerged as a critical food safety issue due to widespread environmental pollution and potential fraudulent use of technical-grade oils. Accurate differentiation and quantification of mineral oil saturated hydrocarbons (MOSH) and mineral oil aromatic hydrocarbons (MOAH) are essential for consumer protection and regulatory compliance.

Objectives and Study Overview

This study aimed to develop a robust analytical method that:

• Enhances the separation and quantification of MOSH and MOAH in edible oils
• Eliminates interferences from naturally occurring olefins such as squalene and its isomers
• Confirms the petrogenic origin of MOH contamination through hopane marker detection

The approach integrates liquid chromatography (LC) with gas chromatography (GC) using dual detection (flame ionization detector, FID, and triple quadrupole mass spectrometry, QqQ MS) in a single comprehensive workflow.

Methodology and Instrumentation

Sample Preparation:

• Eleven commercial edible oil samples (6 extra virgin olive oils, 2 olive oils, 2 pomace olive oils, 1 sunflower oil)
• Dilution at 1:5 (w/v) in n-hexane prior to injection

Liquid Chromatography:

• Two silica columns (150×3 mm ID and 250×2.1 mm ID, 5 μm dp) to retain triglycerides and separate MOSH/MOAH
• Silver-ion column (150×1 mm ID, 5 μm dp) to selectively retain olefins via Ag+–π complexation
• Gradient elution: 100% n-hexane to 50% dichloromethane over 7 min, flow reduced from 300 to 150 μL/min
• Six-port valve for isolation of column segments during fraction transfer

Gas Chromatography and Detection:

• Shimadzu 5D Ultra-e system coupling LC effluent to GC
• Programmable temperature vaporizer: 35 °C to 360 °C at 20 °C/s
• SLB-5ms capillary column (30 m×0.25 mm ID×0.25 μm df), helium carrier gas
• FID at 360 °C for quantification; QqQ MS (EI 70 eV) in full scan/MRM for confirmation
• MRM transitions targeting hopane markers C27–C35 with m/z 370>191 to 482>191

Main Results and Discussion

Improved LC Separation:

• The addition of the silver-ion column effectively delayed squalene and other olefins beyond the MOAH fraction, reducing integration uncertainty
• Comparative FID chromatograms demonstrated a clear reduction of olefin peaks and a more defined MOAH hump

Contamination Levels:

• Sunflower oil exhibited highest MOH levels (MOSH 2540 mg/kg; MOAH 355 mg/kg)
• Pomace olive oils: MOSH 230–445 mg/kg; MOAH 32–66 mg/kg
• Olive oils varied: one sample had MOSH 206 mg/kg; MOAH
• Extra virgin olive oils were least contaminated (MOSH 4–22 mg/kg; MOAH always

Hopane Detection:

• QqQ MS confirmed petrogenic origin of MOH via hopane MRM signals
• Limit of confirmation corresponded to 6 mg/kg MOSH for motor oil, 22 mg/kg for vaseline, 30 mg/kg for vacuum pump oil
• Hopanes detected in heavily contaminated samples; traces found near confirmation limit in some EVOO

Benefits and Practical Applications of the Method

• Single analytical run provides both quantitative (FID) and qualitative (MS) data
• Enhanced specificity for MOSH/MOAH without extensive offline clean-up
• Reliable quantification supports regulatory monitoring and quality control in food laboratories

Future Trends and Potential Applications

• Development of certified MOSH/MOAH standards for improved calibration
• Integration with automated data processing and artificial intelligence for peak deconvolution
• Extension to other food matrices (e.g., baked goods, dairy)
• Miniaturized or greener chromatographic approaches to reduce solvent usage
• Routine implementation in industrial QA/QC and regulatory surveillance

Conclusion

The proposed LC–GC–FID/QqQ MS method, leveraging combined silica and silver-ion columns, delivers accurate separation and quantification of MOSH and MOAH in edible oils. Dual detection ensures comprehensive data collection, while hopane marker analysis confirms contamination origin, making this approach highly suitable for advanced food safety monitoring.

Reference

• Zoccali M., Purcaro G., Mondello L. Determination of Mineral Oil Saturated and Aromatic Hydrocarbons in Edible Oil by Liquid–liquid–gas Chromatography with Dual Detection. Shimadzu Technical Report C146-E310, 2016.