Determination of Aromatic Content in Diesel Fuel According to ASTM D5186

Importance of the Topic

The concentration of aromatic hydrocarbons in diesel fuel critically influences combustion quality, engine performance and exhaust emissions.
Aromatic content affects fuel cetane number and environmental compliance under regulations such as those from EPA and CARB.
Reliable quantification of monoaromatic and polynuclear aromatic fractions is essential for fuel quality control and regulatory adherence.

Study Objectives and Overview

This application study aimed to evaluate the Agilent 1260 Infinity Analytical SFC system equipped with a SIM flame ionization detector (FID) for rapid, cost-effective measurement of aromatic content in diesel fuels according to ASTM D5186.
The goal was to demonstrate compliance with accuracy, precision and linearity criteria defined in the method and to propose a viable alternative to normal-phase HPLC approaches ASTM D1319 and ASTM D2425.

Methodology and Instrumentation

Instrumentation comprised an Agilent 1260 Infinity Analytical SFC Control Module, binary pump, thermostatted column compartment, HTC PAL autosampler and a SIM FID.
Columns used include YMC-PACK-SIL (250×4.5 mm, 5 μm) or Agilent ZORBAX RX-SIL (4.6×250 mm, 5 μm).
Mobile phase A: supercritical CO2 (99.995 %).
Purge phase B: hexane via pump channel B to continuously clean the backpressure regulator.
SFC operated at 1.8 mL/min, column temperature 25 °C, injection volume 0.5 μL.
FID conditions: 300 °C, H2 50 mL/min, air 500 mL/min, N2 make-up 50 mL/min.

Key Results and Discussion

FID sensitivity exceeded detection requirements down to 0.1 mass % toluene in hexadecane.
Chromatographic resolution between hexadecane and toluene (RNM) reached >10 (ASTM ≤4), and between tetrahydronaphthalene and naphthalene (RMD) exceeded 4 (ASTM ≤2).
Retention time reproducibility for performance mixture components was consistently <0.3 % RSD.
Relative response factors for toluene, THN and naphthalene were within ±10 % of theoretical carbon response.
Linearity assessment using neat diesel and 1:1, 1:3 dilutions yielded aromatic content within ASTM repeatability limits.
Overlay of multiple diesel sample injections demonstrated high repeatability and reliability.

Benefits and Practical Applications

The integrated SFC-FID approach fully meets ASTM D5186 criteria and offers faster analysis and reduced solvent consumption compared to normal-phase HPLC.
Its robustness, precision and simplified maintenance via continuous backpressure regulator purging make it ideal for routine fuel QA/QC and process control in petrochemical facilities.

Future Trends and Potential Applications

Automation and data integration with CDS software will streamline method deployment.
Expansion to other fuel matrices and trace contaminants, coupling with mass spectrometry for detailed speciation, and development of portable SFC-FID systems for on-site monitoring are promising directions.

Conclusion

The Agilent 1260 Infinity Analytical SFC system enhanced with a SIM flame ionization detector provides a validated, efficient, and cost-effective solution for determining aromatic and polynuclear aromatic content in diesel fuels according to ASTM D5186.

References

1. ASTM D5186-03 (2009) Standard Test Method for Determination of Aromatic Content and Polynuclear Aromatic Content of Diesel Fuels and Aviation Turbine Fuels by Supercritical Fluid Chromatography.
2. ASTM D1319-14 Standard Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption.
3. ASTM D2425-04 (2009) Standard Test Method for Hydrocarbon Types in Middle Distillates by Mass Spectrometry.