Detailed Hydrocarbon Analysis Featuring - Rtx-DHA Columns

Importance of the Topic

Detailed analysis of hydrocarbons and oxygenates in gasoline is critical for feedstock evaluation, refining process control, and quality assurance in spark-ignition fuels. Precise quantification of individual components supports regulatory compliance, process optimization, and improved fuel performance.

Objectives and Study Overview

This document evaluates the performance of Restek’s Rtx-DHA-100 gas chromatography columns under ASTM D6730-01 and related methods. It aims to:

• Validate column reproducibility and inertness for polar oxygenates and hydrocarbons.
• Assess resolution of critical compound pairs in detailed hydrocarbon analysis.
• Demonstrate run-time improvements using hydrogen as carrier gas.
• Compare performance against competing PIONA columns.

Methodology and Instrumentation

• Column configuration:
  • Main analytical column: 100 m × 0.25 mm ID, 0.50 µm crossbond dimethyl polysiloxane (Rtx-DHA-100).
  • Tuning column: 2–5 m × 0.25 mm ID, 1.00 µm diphenyl/dimethyl polysiloxane (Rtx-5 DHA).
• Carrier gases: Helium (2.2 mL/min, 28 cm/s) and Hydrogen (3.62 mL/min, 55 cm/s).
• Detection: Flame ionization detector at 250–300 °C.
• GC system: Agilent 7890B.
• Injector: Split mode (150:1) with deactivated inlet liners.
• Temperature program: Subambient start (5–35 °C) followed by ramping to 200–300 °C.

Main Results and Discussion

• Column Efficiency and Inertness:
  • C5 efficiency: 613,600 theoretical plates (ASTM requirement 400,000–550,000).
  • tert-Butanol skewness: 1.25 (acceptable 1.00–5.00).
  • Retention factor (k'): 0.489 (target 0.45–0.50).
• Resolution of Critical Pairs:
  • tert-Butanol/2-methylbutene-2 resolution: 5.60 (minimum 3.25).
  • All 19 ASTM-specified critical pairs resolved per D6730-01.
• Run-Time Reduction:
  • Hydrogen carrier gas decreased analysis time by 33–51% compared to helium.
  • Optimized method with hydrogen: ~72 min vs. 146 min standard.
• Competitor Comparison:
  • Rtx-DHA-100 outperformed four competing PIONA columns in efficiency, peak shape, and resolution.
  • Competitors A and B failed ASTM criteria; C and D met minimal requirements but with lower efficiency.

Benefits and Practical Applications

• Guaranteed column-to-column reproducibility for routine DHA workflows.
• Improved resolution of oxygenates enhances quantification accuracy.
• Reduced analysis times boost laboratory throughput.
• Compatibility with ASTM D6729, D6730, D6733, D5134, D5501, and CAN/CGSB methods.

Future Trends and Opportunities

• Expanded use of hydrogen carrier gas to shorten cycle times without compromising performance.
• Advances in stationary phase technology for greater selectivity and thermal stability.
• Integration with automated sample handling and data analytics platforms for high-throughput DHA.
• Potential coupling with mass spectrometry for enhanced compound identification.

Conclusion

Restek’s Rtx-DHA-100 columns deliver superior performance for detailed hydrocarbon analysis, meeting or exceeding ASTM and CGSB specifications. Their high efficiency, inertness, and rapid run capabilities make them a valuable choice for fuel quality testing, process control, and regulatory compliance.

References

• ASTM D6729, D6730-01, D6733, D5134, D5501 methods for detailed hydrocarbon analysis.
• CAN/CGSB 3.0 No. 14.3-99 standard.