Automatic Quantitative Analysis of Total Aromatics and Oxygenates in Gasoline Samples Using Comprehensive Two-Dimensional Gas Chromatography (GCxGC) and Time-of-Flight Mass Spectrometry (TOFMS)

Significance of the Topic

This study addresses the critical need for rapid and accurate quantification of aromatic and oxygenate compounds in gasoline, driven by stringent environmental regulations and fuel quality requirements. Monitoring components such as benzene, toluene, ethylbenzene, xylenes (BTEX) and oxygenates ensures compliance with the US Clean Air Act amendments and supports efforts to reduce toxic and ozone-forming emissions.

Objectives and Study Overview

The primary goal was to demonstrate a single 10-minute analytical workflow using comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GCxGC-TOFMS) for simultaneous quantitative determination of total aromatics and oxygenates in gasoline. The study evaluates method performance, linearity, coelution resolution, and application to commercial fuel samples.

Methodology and Instrumentation

The GCxGC system consisted of an Agilent 6890 GC with a LECO thermal modulator. A nonpolar DB-5 column (10 m × 0.18 mm × 0.18 µm) served as the first dimension, and a polar DB-WAX column (1 m × 0.1 mm × 0.1 µm) as the second. Oven programs were ramped from low initial temperatures to 180 °C at 10 °C/min in both dimensions, with a 5 s modulation period and 0.8 s hot pulse. Helium was used as carrier gas at 1 mL/min. The Pegasus 4D TOFMS acquired data at 200 Hz over m/z 20–400 with electron impact ionization at 70 eV.

Calibration employed two standard mixtures: 11 oxygenates (e.g., methanol, MTBE, ethanol, butanol isomers) and 23 hydrocarbons (including BTEX, C8–C12 aromatics, naphthalenes) with appropriate deuterated internal standards. Five concentration levels in triplicate were analyzed, using quantitation masses following ASTM methods D5599 and D5769. ChromaTOF software performed automated peak finding, deconvolution, library matching, and quantitation.

Main Results and Discussion

Chromatograms presented as two-dimensional contour plots showed well-organized chemical class separation by boiling point (first dimension) and polarity (second dimension). Despite coelution challenges, TOFMS spectral continuity enabled reliable deconvolution of overlapping peaks, exemplified by MTBE in the alkane region. All 36 target analytes exhibited excellent linearity (R2 ≥ 0.99). Application to three commercial gasoline samples (87 and 93 octane) yielded detailed aromatic profiles, with total aromatics ranging from ~20 % to >27 % by weight and no detectable oxygenates above calibration limits.

Benefits and Practical Applications

• Single-run quantitation of both aromatics and oxygenates in under 10 minutes enhances laboratory throughput.
• High peak capacity of GCxGC coupled with fast TOFMS acquisition ensures accurate detection and quantitation, even for coeluting isomers.
• Automated data processing reduces manual intervention, improving reproducibility and suitability for QA/QC and regulatory monitoring.

Future Trends and Opportunities

Advances in GCxGC-TOFMS may include higher modulation speeds, improved column technologies, and enhanced data analytics incorporating machine learning for deconvolution. Miniaturized or portable GCxGC systems could enable on-site fuel quality testing. Expansion to other complex matrices such as biofuels, petrochemicals, and environmental samples will broaden application scope.

Conclusion

The GCxGC-TOFMS approach demonstrated here achieves rapid, robust, and automated quantitation of 36 fuel-relevant analytes in a single analysis, meeting ASTM requirements and environmental regulations. The combined power of two-dimensional separation and high-speed TOFMS with advanced software deconvolution establishes a reliable platform for routine gasoline analysis.

References

• ASTM Method D5599: Determination of Oxygenates in Gasoline.
• ASTM Method D5769: Determination of Aromatics in Gasoline.