Improved Performance for the Analysis of Aromatics in Gasoline by ASTM Method D5769 Using the Agilent 5973 inert Gas Chromatography/Mass Spectrometry System

Importance of the Topic

The precise quantification of benzene, toluene and higher aromatics in gasoline is essential for regulatory compliance, engine performance optimization and environmental protection. ASTM Method D5769, which combines gas chromatography with mass spectrometry (GC/MS), provides a robust framework for determining aromatic content between 0.1 and 42 weight percent.

Study Objectives and Overview

This application study evaluates the performance gains achieved by integrating an extended linearity electron ionization source into the Agilent 5973 inert GC/MS platform. The goals were to simplify method setup, ensure reliable linear calibration across the full concentration range and meet or exceed D5769 criteria for resolution, sensitivity and spectral integrity.

Methodology

All experiments followed the exact chromatographic and mass spectrometric parameters defined in ASTM D5769. Calibration standards containing 23 target aromatics and four deuterated internal standards (benzene-d6, toluene-d9, ethylbenzene-d10, naphthalene-d8) were prepared at five concentration levels from 0.1 to 42 weight percent. Data acquisition used characteristic quantitation ions for each compound. Performance tests included chromatographic resolution, signal-to-noise at low levels, mass spectral integrity and linearity of calibration curves.

Used Instrumentation

• Agilent 5973 inert GC/MS system with standard electron ionization (EI) source
• Optional extended linearity EI source kit (Ultra large aperture drawout plate)
• Agilent 6890N gas chromatograph with capillary inlet
• Agilent 7683 automatic liquid sampler (ALS) with nanoliter adapter
• HP-1 capillary column, 60 m × 0.25 mm id, 1 µm film thickness

Main Results and Discussion

Chromatographic resolution between critical pairs (e.g., 1-methyl-2-ethylbenzene vs. 1,3,5-trimethylbenzene) consistently exceeded the D5769 requirement of 2.0, achieving values above 6.5. The signal-to-noise ratio at 0.01 mass % for 1,4-diethylbenzene exceeded 20 (vs. minimum 5). Mass spectral integrity tests for 1,2,3-trimethylbenzene at 3 mass % demonstrated compliance with ion abundance criteria. The key benefit of the extended linearity source was evident in reconstructed ion chromatograms: a 10 mass % toluene standard showed ion source saturation and flat-topped peaks with the standard source but yielded well-shaped, unsaturated peaks after installing the extended linearity kit. As a result, calibration curves that previously failed linearity specifications now met the ASTM requirement across the entire concentration range without complex compensatory measures.

Benefits and Practical Applications

• Eliminates time-consuming custom tuning and specialized inlet or split modifications
• Facilitates straightforward five-point calibrations for percent-level analytes
• Improves throughput by reducing method complexity and setup time
• Ensures reliable analytical results for quality control and regulatory reporting in refineries and testing laboratories

Future Trends and Potential Applications

Advances in ion source design will continue to expand the linear dynamic range of GC/MS systems, enabling direct quantitation of matrix-rich, high-concentration samples. Integration of smart autotuning routines, adaptive split control and real-time linearity diagnostics may further simplify complex analytical methods. Broader adoption of extended linearity sources can also benefit petrochemical, environmental and food safety applications where high-level analyte determination is required.

Conclusion

The Agilent 5973 inert GC/MS equipped with an extended linearity EI source kit fully satisfies ASTM D5769 performance requirements for aromatic analysis in gasoline. By mitigating ion source saturation, it streamlines method validation and operation, delivering accurate, reproducible results across a wide concentration range without additional compensatory techniques.

References

1. ASTM International. Annual Book of ASTM Standards, Vol. 05.03, D4928–D5950, Petroleum Products and Lubricants (III). West Conshohocken, PA, 2020.
2. Leibrand, R. J. Latest Enhancements to the GC/MS Analysis of Gasoline by ASTM Method D-5769. Agilent Technologies Application Note, 5966-2798E, 1999.