An Improved Flame Photometric Detector for the Analysis of Dibenzothiophenes in Diesel, Distillates, and Feedstocks Using the Agilent 7890B Series GC

Importance of the Topic

The distribution of sulfur compounds, particularly dibenzothiophenes and their alkylated derivatives, plays a critical role in refining processes and environmental compliance. Accurate sulfur speciation informs hydrotreating catalyst optimization, ensures low-sulfur fuel production, and aids regulatory adherence.

Study Objectives and Overview

This work evaluates the performance of an Agilent 7890B GC equipped with a high-temperature Flame Photometric Detector (FPD+) for determination of benzothiophene-class sulfur compounds in diesel, light cycle oil (LCO), and cracked gas oils. A Capillary Flow Technology (CFT) Deans switch configuration implements two-dimensional GC separations with heart-cutting to improve selectivity and minimize detector quenching.

Methodology and Used Instrumentation

This method combines a nonpolar first-dimension column (HP-1 or DB-1) with a midpolar DB-17HT second-dimension column. Key features include:

• Agilent 7890B Gas Chromatograph with Multimode Inlet (MMI) operated in split or backflush mode
• CFT Deans switch for heart-cutting and backflush capabilities
• FPD+ detector with independent transfer line (up to 400 °C) and emission block (150 °C) zones, offering 2.5 pg S/sec sensitivity
• 7693A autosampler, constant flow carrier gases, and temperature-programmed GC runs

Main Results and Discussion

Calibration of 4,6-dimethyldibenzothiophene exhibited a quadratic FPD response linearized by square-root transformation (R2 = 0.993). Analysis of highway diesel revealed C2–C3 dibenzothiophenes contributing ~1.5 ppm sulfur in the 18–24 min heart-cut window. Two LCO samples displayed distinct distributions: LCO1 contained complex C1–C4 alkyl DBTs, whereas LCO2 showed lower overall levels dominated by dibenzothiophene and 4-methyl DBT. Cracked gas oil evaluated with combined heart-cut/backflush mode highlighted heavy sulfur species (C3–C4 DBTs) and demonstrated protection of analytical columns from high-boiling fractions.

Benefits and Practical Applications

• High-temperature FPD+ operation extends analysis to heavy fuels and feeds without quenching
• CFT Deans switch heart-cutting/backflush preserves column integrity and shortens run times
• Enhanced selectivity enables precise profiling of alkylated dibenzothiophenes for process optimization and QA/QC

Future Trends and Opportunities

Emerging directions include coupling high-temperature sulfur detection with comprehensive two-dimensional GC, integration with mass spectrometry or vacuum ultraviolet detectors, and broader application to other heteroatom classes (nitrogen/phosphorus). Advances in detector design may further improve sensitivity and robustness for ultra-trace speciation in complex matrices.

Conclusion

The Agilent 7890B GC with FPD+ and CFT Deans switch delivers a robust, sensitive, and high-temperature platform for detailed sulfur speciation in diesel, LCO, and heavy feedstocks. Its combined heart-cut/backflush approach ensures rapid, accurate profiles of alkylated dibenzothiophenes, supporting refining process optimization and regulatory compliance.

References

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