Determination of Aromatic Hydrocarbons in Diesel Fuel by LC-RID According to ASTM D6591

Importance of the Topic

Accurate quantification of aromatic hydrocarbons in diesel fuel is essential for assessing fuel quality, combustion behavior, and ensuring compliance with environmental regulations. The ASTM D6591 method provides a reliable HPLC-RID approach to determine mono-, di-, and tri-plus aromatics, replacing older dye-based procedures and offering improved robustness for middle distillates.

Objectives and Overview of the Study

This application note demonstrates the use of a Shimadzu Prominence HPLC system to analyze aromatic content in diesel fuel according to ASTM D6591. Key goals include achieving adequate resolution between analytes, verifying calibration linearity, and assessing method precision and applicability to real-world samples.

Methodology and Instrumentation

The analysis employs a Shim-pack GIST NH2 analytical column with guard column and n-heptane as the mobile phase. A six-port two-position valve is used to backflush tri-plus aromatic compounds after di-aromatic elution. Analytical conditions include a 0.8 mL per minute flow rate, 3 microliter injections, and column and detector temperatures set at 35 degrees Celsius.

Instrumentation

• Shimadzu Prominence HPLC with LC-20AD pump, DGU-20A3R degasser, SIL-20AC autosampler, CTO-20A column oven with valve, and RID-20A refractive index detector
• Shim-pack GIST NH2 column 250 mm x 4.6 mm ID 3 µm with 10 mm x 4.0 mm guard
• ASTM D6591 standard kit containing cyclohexane, o-xylene, dibenzothiophene, and 9-methylanthracene

Main Results and Discussion

System resolution standard injections showed clear separation: cyclohexane at 4.35 minutes, o-xylene at 5.88 minutes, dibenzothiophene at 8.89 minutes, and 9-methylanthracene at 9.81 minutes. The resolution between cyclohexane and o-xylene exceeded 8.9, surpassing the ASTM requirement of 5. Calibration curves for each aromatic compound were linear with r2 values of 0.99999. Precision tests on the mid-level standard yielded retention time RSDs below 0.13 % and area RSDs below 1.5 %. Analysis of a commercial diesel sample confirmed the method’s applicability, providing distinct peaks for saturated compounds, mono-, di-, and tri-plus aromatics.

Benefits and Practical Applications

• High resolution and rapid analysis reduce runtime and solvent usage
• Eliminates reliance on fluorescent dyes, increasing method robustness
• Single instrument configuration complies with both ASTM D6591 and D6379
• Suitable for quality control laboratories monitoring fuel composition

Future Trends and Potential Applications

Advancements may include integration with mass spectrometry or UV detectors for enhanced sensitivity, development of greener mobile phases, online and in-line monitoring systems, application to biofuel blends, and the use of chemometric tools to refine aromatic profiling in complex matrices.

Conclusion

The presented HPLC-RID method delivers precise and reproducible determination of mono-, di-, and tri-plus aromatic hydrocarbons in diesel fuels. By leveraging backflushing and an optimized column configuration, laboratories can achieve faster turnaround and maintain compliance with industry standards using a single HPLC system.

References

• ASTM D1319-18 Standard Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption
• ASTM D6591-18 Standard Test Method for Determination of Aromatic Hydrocarbon Types in Middle Distillates High Performance Liquid Chromatography Method with Refractive Index Detection
• ASTM D6379-11 Standard Test Method for Determination of Aromatic Hydrocarbon Types in Aviation Fuels and Petroleum Distillates High Performance Liquid Chromatography Method with Refractive Index Detection