Detailed hydrocarbon analysis (DHA) using ASTM method D6729 & D6729 Appendix X2

Significance of the Topic

Detailed hydrocarbon analysis (DHA) is critical for evaluating gasoline composition by separating and quantifying individual hydrocarbon classes. It supports quality control in spark ignition fuels, guiding production efficiency and regulatory compliance.

Objectives and Study Overview

This study compares the performance of helium and hydrogen as carrier gases in DHA following ASTM D6729 and its Appendix X2. The aim is to assess analysis time reduction and chromatographic resolution when replacing helium with on-site generated hydrogen.

Methodology and Instrumentation

Sample and carrier gas setup:

• Analyte: Commercial gasoline in the C1–C13 range focusing on PONA composition.
• Carrier gases: High-purity helium cylinder vs. unfiltered hydrogen from a Peak Scientific Precision Trace generator.

Chromatographic conditions:

• Column: 100 m×0.25 mm, 0.5 µm 100% dimethylpolysiloxane (J&W).
• Injector: Split 250:1, 0.2 µL injection, 280 °C.
• Oven program with three temperature ramps customized to each carrier gas.
• Detector: Flame ionization detector on an Agilent 7890A GC.

Main Results and Discussion

Analysis time:

• n-Pentadecane eluted at 125 min with helium and under 74 min with hydrogen.

PONA quantification:

• Overall weight percentages for paraffins, aromatics, naphthenes and olefins showed differences below 0.5% between gases.

Critical separations:

• Improved resolution of 1-methylcyclopentene/benzene with hydrogen despite faster run times.
• Toluene and 2,3,3-trimethylpentane co-eluted under hydrogen, indicating a need for method tuning.
• Tridecane and 1-methylnaphthalene were equally resolved with both gases.

Benefits and Practical Applications

Switching to hydrogen carrier gas offers:

• Significant reduction in analysis time, increasing laboratory throughput and profitability.
• Comparable quantitative PONA results while maintaining industry-required separations.
• Mitigated helium supply constraints and cost volatility by on-site hydrogen generation.

Future Trends and Applications

Emerging directions include:

• Further optimization of oven programs for complete resolution under hydrogen.
• Expansion of hydrogen-based DHA to other fuel types and chain lengths.
• Integration of advanced detectors and data processing for deeper hydrocarbon profiling.

Conclusion

The study demonstrates that hydrogen generated on-site can replace helium in DHA methods to yield faster runs without compromising critical chromatographic separations or quantitative accuracy, aligning with ASTM standards.

Reference

1. ASTM D6729-01 Standard Test Method for Determination of Individual Components in Spark Ignition Engine Fuels.
2. ASTM D6729-01 Appendix X2: Hydrocarbon Data Using Hydrogen Carrier Gas.