A New Approach to ASTM D3606 for the Analysis of Benzene and Toluene in Gasoline using Mid-Column Backflush and Hydrogen Carrier with the Agilent 7890B GC

Importance of the Topic

Gasoline analysis for benzene and toluene content is critical for regulatory compliance and public health. Conventional ASTM D3606 methods using packed and TCEP columns face challenges with modern ethanol-blended fuels, including resolution issues and retention time instability. Implementing a capillary mid-column backflush approach enhances robustness and reliability in aromatic quantitation.

Objectives and Study Overview

This work presents a modified ASTM D3606 protocol on an Agilent 7890B gas chromatograph equipped with capillary columns and a mid-column backflush configuration. Key aims include:

• Developing single- and dual-channel GC setups for benzene and toluene determination in finished gasoline.
• Evaluating hydrogen as a carrier gas alternative to helium.
• Assessing method performance in ethanol-containing samples.

Methodology and Instrumentation

An Agilent 7890B GC was configured with a 30 m×0.25 mm HP-1 precolumn and a 30 m×0.25 mm HP-INNOWax analytical column. A capillary flow technology (CFT) splitter at the mid-column enabled precise backflush via an Aux electronic pressure control module. Hydrogen carrier gas flowed isothermally at 80 °C. Sample introduction used a multimode inlet (MMI) in split mode with 100:1 ratio. Calibration standards comprised seven levels of benzene (0.06–5.00 vol%) and toluene (0.50–20.0 vol%) in isooctane, with 2-butanol (4 vol%) as internal standard. Safety venting directed AUX and MMI bleed lines externally.

Main Results and Discussion

Calibration curves exhibited excellent linearity for benzene (R² = 0.9995) and toluene (R² = 0.9997). Chromatograms demonstrated clear separation of benzene and toluene from co-eluting hydrocarbons and ethanol interference. Dual-channel monitoring allowed real-time visualization of precolumn elution and optimized backflush timing using isooctane/n-nonane mixtures. Repeatability tests on 87, 89, and 93 octane gasoline samples yielded ≤2% variation in measured concentrations.

Benefits and Practical Applications

• Enhanced retention time stability compared to packed or TCEP columns.
• Reliable separation of aromatics from ethanol and high-boiling interferences.
• Cost and speed advantages of hydrogen carrier gas.
• Dual-channel configuration streamlines method development and quality control.

Future Trends and Potential Applications

Adoption of capillary backflush techniques is expected to expand into broader fuel analyses and other complex matrices. Development of software wizards for automated backflush timing will facilitate routine use. A forthcoming revision of ASTM D3606 may formally incorporate hydrogen-carrier and wax column configurations for enhanced method performance.

Conclusion

The mid-column backflush GC method using HP-1 and HP-INNOWax capillary columns with hydrogen carrier on the Agilent 7890B offers a robust and reproducible alternative to traditional D3606 analysis. It effectively resolves benzene and toluene in ethanol-blended gasoline, improving method stability and compliance.

References

1. ASTM D3606-10; Standard Test Method for Determination of Benzene and Toluene in Finished Motor and Aviation Gasoline by Gas Chromatography; ASTM International; 2010.