Gas Chromatographic Analysis of Diesel Fuel Dilution for In-Service Motor Oil Using ASTM Method D7593

Importance of Diesel Fuel Dilution Analysis

Diesel fuel contamination of in-service motor oil is a critical diagnostic parameter that signals potential engine wear or impending mechanical failure. Rapid and reliable quantification of diesel dilution enables maintenance teams to optimize service intervals, prevent catastrophic engine damage, and maintain performance efficiency.

Study Objectives and Overview

This study demonstrates the implementation of ASTM Method D7593 using an Agilent 7890 Series gas chromatograph equipped with a post-column backflush Capillary Flow Technologies device. The main objective was to validate a four‐minute GC procedure for accurate measurement of diesel fuel content in used engine oil, ensuring compliance with ASTM accuracy and precision requirements.

Methodology

• Sample Injection: 0.1 µL injection volume with carbon disulfide syringe washes to prevent carryover.
• Column Separation: DB-1 capillary column (15 m × 0.25 mm, 0.25 µm) at 3.5 mL/min helium carrier flow.
• Backflush Procedure: Automatic reversal of flow at 1.45 minutes to purge residual oil matrix using a 2‐way splitter and auxiliary EPC control.
• Calibration: Four commercially prepared standards (0, 2, 5, 10 % w/w diesel in 75 cSt base oil) run in duplicate, yielding an R² of 0.9993.

Used Instrumentation

• Gas Chromatograph: Agilent 7890 Series with split/splitless inlet and flame ionization detector.
• Autosampler: Agilent 7650 ALS with 0.5 µL autosampler syringe.
• Backflush Device: CFT 2-way splitter with make-up gas and deactivated fused silica restrictor.
• Column Oven: Programmed to 225 °C hold during separation, then ramped to 350 °C to elute heavy matrix.

Main Results and Discussion

• Calibration Performance: Linear response across 0–10 % w/w standards exceeded ASTM requirement (R² ≥ 0.993).
• Sample Precision: Five in-service oil samples analyzed 10 times each showed diesel contents from 3.1 % to 34.0 % w/w with RSDs of 0.64–1.45 %.
• Long-Term Robustness: 500 injections of a 2 % w/w standard over 10 days delivered average recovery of 1.99 % w/w and RSD of 1.35 %, confirming system stability.
• Backflush Efficiency: Baseline comparisons before and after 550 total injections demonstrated complete removal of heavy oil matrix from the column.

Benefits and Practical Applications

The rapid four-minute analysis reduces instrument downtime and increases sample throughput. High precision and robustness make this approach suitable for routine quality assurance, engine maintenance programs, and regulatory compliance in industrial and field laboratories.

Future Trends and Potential Applications

Ongoing developments may integrate automated backflush timing adjustments, expanded chromatographic columns for broader fuel range detection, and coupling GC data with predictive maintenance algorithms powered by machine learning.

Conclusion

The ASTM D7593 method implemented on the Agilent 7890 GC with CFT backflush provides a fast, accurate, and robust solution for monitoring diesel dilution in motor oils. The procedure meets stringent precision requirements and offers a high-throughput workflow for industrial laboratories.

References

• ASTM D7593-14, Standard Test Method for Determination of Fuel Dilution for In-Service Engine Oils by Gas Chromatography, ASTM International, West Conshohocken, PA, 2014.