Determination of Aromatic Hydrocarbon Types in Middle Distillates with the Alliance HPLC System and RI Detection According to ASTM D6591 (IP548)

Significance of the Topic

The distribution of aromatic hydrocarbons in middle distillates such as diesel fuel directly influences combustion quality, exhaust emissions and compliance with stringent environmental regulations that limit total and polynuclear aromatics content.

Objectives and Study Overview

This study demonstrates the application of the Waters Alliance HPLC System with refractive index detection to quantify mono-, di-, tri+- and polycyclic aromatic hydrocarbons in diesel fuels and petroleum distillates in line with ASTM D6591 criteria. It also highlights adaptability for mono- and di-aromatic analysis in aviation fuels following ASTM D6379.

Methodology

The method follows ASTM D6591 high-performance liquid chromatography under normal-phase conditions using heptane as mobile phase at 1 mL/min, 10 µL injection volume and 30 °C for both column and detector. A two-position six-port valve enables forward flush to elute non-aromatics, mono- and diaromatics, then backflush to release tri+ aromatics as a sharp peak. Calibration employed standards of cyclohexane, o-xylene, 1-methylnaphthalene, phenanthrene, dibenzothiophene and 9-methylanthracene.

Used Instrumentation

• Waters Alliance HPLC System
• Waters 2414 Refractive Index Detector
• Waters Two-Position Six-Port Switching Valve
• Waters Spherisorb Amino (NH2) column and guard cartridge
• Empower 3 Chromatography Data System

Main Results and Discussion

Baseline separation of all system performance standard (SPS) compounds was achieved with resolution exceeding ASTM requirements, particularly between o-xylene and cyclohexane. Calibration curves for key aromatics showed linearity with r2 > 0.999. Precision tests on mid-level standards returned retention time RSDs < 0.05 % and area RSDs < 0.5 %. Analysis of commercial diesel diluted in heptane confirmed clear elution windows for each aromatic class suitable for routine quantification.

Benefits and Practical Applications

• Broad solvent compatibility supports diverse normal-phase separations
• Tool-free maintenance of consumables reduces downtime
• Widely referenced amino column chemistry ensures reproducible performance
• Centralized method management, data integrity and electronic sign-off via Empower 3
• Scalable software options for individual laboratories up to enterprise installations

This workflow is ideally suited for regulatory compliance testing and quality control in petrochemical and fuel production settings.

Future Trends and Potential Uses

Emerging developments include integration of automated sample preparation, in-line and multi-dimensional detection strategies, adoption of greener solvent systems, coupling with mass spectrometry for enhanced selectivity, and expansion into high-throughput laboratory environments to meet evolving regulatory demands.

Conclusion

The Waters Alliance HPLC System paired with RI detection and a switching valve configuration not only satisfies but surpasses the ASTM D6591 performance criteria, providing robust, precise and highly linear quantification of aromatic hydrocarbon classes in middle distillates.

References

1. ASTM D6591–19, Standard Test Method for Determination of Aromatic Hydrocarbon Types in Middle Distillates – High Performance Liquid Chromatography Method with Refractive Index Detection.
2. Waters Application Note 720006977EN (2020): Determination of Aromatic Hydrocarbon Types in Aviation Fuels and Petroleum Distillates with the Alliance HPLC System and RI Detection According to ASTM D6379.