Automated Clean-up for Mineral Oil (Hydrocarbon Oil Index) Analysis using the Agilent 7696A Sample Prep WorkBench

Importance of the Topic

Environmental contamination by petroleum hydrocarbons poses significant risks to water quality and human health. Accurate determination of mineral oil (hydrocarbon oil index) in water is essential for regulatory compliance and environmental monitoring. Traditional liquid–liquid extraction (LLE) and clean-up steps are labor intensive and time consuming. Automating key steps can improve throughput, reproducibility and safety.

Objectives and Study Overview

This application note describes the automation of the drying and dispersive solid-phase extraction (d-SPE) clean-up steps for hydrocarbon oil index analysis on the Agilent 7696A Sample Prep WorkBench. The goal is to integrate sample preparation with fast GC-FID analysis, using low thermal mass (LTM) technology to achieve high throughput without compromising recovery or reproducibility.

Methodology and Instrumentation

Sample preparation workflow:

• 900 mL water sample extracted with 50 mL hexane (LLE).
• Extract concentrated to ~1–1.5 mL by nitrogen blow-down.
• Automated clean-up on 7696A WorkBench:
  
  • Add 500 µL extract to vial containing 20 mg anhydrous Na₂SO₄; mix 1 min at 2 000 rpm.
  • Transfer 350 µL dried extract to vial with 30 mg Florisil; mix 1 min at 2 000 rpm.
  • Transfer 150 µL cleaned extract to final vial for GC analysis.

Used Instrumentation:

• Agilent 7696A Sample Prep WorkBench (500 µL and 250 µL syringes).
• Agilent 7890 GC with SSl inlet, LTM II oven and FID detector.
• Column: DB-5HT, 0.32 mm × 10 m, 0.10 µm film thickness.
• Injection: 1 µL splitless (0.4 min purge delay) at 350 °C.
• Carrier: Helium, 9 mL/min constant flow.
• Oven program: 40 °C (0.5 min) → 200 °C/min → 240 °C → 100 °C/min → 340 °C (0.5 min); total 3 min analysis time.
• FID gases: H₂ 40 mL/min, air 400 mL/min; temperature 340 °C.

Main Results and Discussion

Automated clean-up performance was evaluated using hydrocarbon standards and spiked water samples:

• Reproducibility for n-alkanes C10, C20, C30 and C40: RSD < 1% (C40 RSD ~2.5%).
• Mineral oil recovery: 99–101% for C10–C30, ~86% for C40; overall recovery >95% in water samples.
• Stearyl stearate removal (simulating polar interferences): average recovery 2–3%, well below 5% threshold.
• Chromatographic profiles before and after clean-up show complete removal of late-eluting interferences while preserving hydrocarbon distribution.

Benefits and Practical Applications

Automation of the drying and d-SPE clean-up on the 7696A Sample Prep WorkBench offers:

• High throughput by reducing manual handling and integrating with fast GC-FID.
• Excellent reproducibility and recovery for regulatory hydrocarbon oil index analysis.
• Efficient removal of co-extracted polar compounds with minimal adsorbent and solvent use.
• Scalability for routine environmental monitoring and quality control laboratories.

Future Trends and Applications

Emerging directions include:

• Further miniaturization and on-line coupling of sample prep modules with GC or GC-MS.
• Adoption of greener solvents and sorbents to reduce environmental footprint.
• Expansion to other matrices (soil, sludge, biota) and broader hydrocarbon fractions.
• Integration with high-resolution mass spectrometry for speciation of petroleum contaminants.

Conclusion

The Agilent 7696A Sample Prep WorkBench effectively automates the critical drying and dispersive SPE clean-up steps for mineral oil determination in water. Coupled with fast LTM-GC-FID, the method delivers high recoveries, excellent precision and rapid analysis times, making it well suited for environmental testing and industrial QA/QC.

References

1. International standard ISO 9377-2, Water Quality, Determination of hydrocarbon oil index, part 2: Method using solvent extraction and gas chromatography, 2000.
2. F. David, K. Jacq and R.L. Firor, High Throughput Mineral Oil Analysis by GC-FID Using the Agilent Low Thermal Mass (LTM II) System, 9/2011, Agilent Technologies publication 5990-9104EN.