Determination of Aromatic Hydrocarbons in Petroleum Middle Distillates with the Agilent 1260 Infinity Binary HPLC System with RID Detection According to IP391(2000)/ASTM D6591

Significance of the Topic

The determination of aromatic hydrocarbons in petroleum middle distillates is critical for fuel quality control, environmental compliance and engine performance. Aromatic content influences cetane number, combustion efficiency and pollutant formation. Reliable, standardized analytical methods such as IP391(2000)/ASTM D6591 enable laboratories and refineries to quantify mono-, di- and poly-aromatics with confidence and reproducibility.

Objectives and Study Overview

This application note evaluates the performance of the Agilent 1260 Infinity Binary HPLC system coupled with a refractive index detector (RID) for the analysis of aromatic hydrocarbons in middle distillates. Key aims include:

• Demonstrating method equivalence to IP391(2000)/ASTM D6591 and related ASTM protocols
• Establishing calibration, detection limits and precision for mono-aromatics (MAH), di-aromatics (DAH) and tri-aromatics (TRI+)
• Assessing applicability to real diesel samples

Methodology and Instrumentation

Sample preparation involves dilution of 0.9–1.1 g of distillate to 10 mL with n-heptane and filtration.
Chromatographic conditions:

• Column: Agilent ZORBAX NH2 (4.6×250 mm, 5 µm) with NH2 guard
• Mobile phase: isocratic heptane at 1.0 mL/min
• Column temperature: 30 °C; RID temperature: 35 °C
• Injection: 20 µL standard mode with heptane needle wash
• Valve switching: backflush of TRI+ fraction after elution of DAH according to calculated timing equation

Instrumentation suite:

• Agilent 1260 Infinity Binary Pump with degasser
• 1260 Infinity Autosampler with thermostat
• 1260 Infinity Thermostatted Column Compartment
• 1260 Infinity Refractive Index Detector
• Agilent OpenLAB CDS ChemStation

Main Results and Discussion

Calibration: four standards covering 1–50 mg/mL cyclohexane, 0.5–40 mg/mL MAH/DAH, 0.1–4 mg/mL TRI+ showed linearity coefficients >0.9998.
Limits of detection around 1 µg/mL for aromatic compounds (signal-to-noise ratio 3).
Precision: retention time RSD ≤0.06 %; area RSD ≤0.16 % (n=5).
Chromatograms demonstrated clear separation of non-aromatics, MAH, DAH and TRI+ fractions. Real sample analysis identified ~4.3 mg/mL TRI+ hydrocarbons in diesel oil.

Benefits and Practical Applications

This normal-phase RID method offers:

• Compliance with established international standards
• High sensitivity and precision for quality assurance
• Minimal solvent complexity and straightforward sample prep
• Compatibility with existing HPLC infrastructure

Applications include petroleum quality control, environmental monitoring and regulatory reporting.

Future Trends and Opportunities

Emerging directions in aromatic hydrocarbon analysis may include:

• Ultra-high-pressure liquid chromatography for faster separations
• Advanced detectors (mass spectrometry, UV-vis diode array) for improved selectivity
• Green solvent selection and micro-scale methods to reduce waste
• Automated sample handling and data analytics integration
• Online process monitoring within refineries

Conclusion

The Agilent 1260 Infinity Binary HPLC system with RID detection provides a robust, reproducible and sensitive approach for the determination of mono-, di- and poly-aromatic hydrocarbons in petroleum middle distillates. Results meet or exceed IP391(2000)/ASTM D6591 performance criteria, enabling reliable routine analysis in laboratory and industrial settings.

References

• IP391(2000)/ASTM D6591 Standard Test Method for Determination of Aromatic Hydrocarbon Types in Middle Distillates by HPLC with RID
• ASTM-P-0135 System Performance Standard
• ASTM-P-0136 Calibration Standard Set
• Agilent Application Note 5991-3172EN
• Agilent Application Note 5991-3171EN