GC-APCI IMS of Diesel

Importance of the Topic

Diesel fuel value and regulatory compliance hinge on its sulfur content. Sulfur heteroatoms in diesel lead to harmful emissions and catalyst poisoning, so refineries employ hydrodesulfurization to reduce sulfur levels. Profiling aromatic sulfur compounds such as benzothiophenes and dibenzothiophenes is essential for optimizing desulfurization processes and meeting stringent environmental standards.

Objectives and Study Overview

This study demonstrates the coupling of gas chromatography with atmospheric pressure chemical ionization (GC-APCI), ion mobility spectrometry (IMS), and high-resolution mass spectrometry (Q-TOF) to profile sulfur species in complex diesel matrices. Two diesel samples—before and after hydrodesulfurization—were compared to assess the method’s ability to separate and identify aromatic sulfur compounds against a dense hydrocarbon background.

Methodology and Instrumentation

Sample Preparation:

• Processed and unprocessed diesel diluted 1:10 (v/v) in isooctane
• No further cleanup or derivatization required

Instrumental Setup:

• Gas chromatograph: Agilent 7890B GC with split/splitless inlet
• Interface: Agilent GC-APCI
• Separation column: Agilent J&W DB-5ms, 30 m×0.25 mm, 0.25 µm film
• Carrier gas: Helium, constant flow 1.3 mL/min
• GC oven program: 50 °C (2 min) → 250 °C at 10 °C/min (10 min) → 300 °C at 15 °C/min (10 min)
• Ion mobility Q-TOF: Agilent 6560 IMS Q-TOF operated in positive ion mode
• IMS drift gas: Nitrogen, low‐field DC separation
• Mass range: high‐resolution TOF with sub-ppm mass accuracy
• Software: Agilent MassHunter Data Acquisition and Qualitative, mMass for Kendrick mass defect analysis

Main Results and Discussion

Extra Dimension of Separation:
The addition of IMS to GC-APCI Q-TOF provided gas‐phase separation based on ion size, charge, and shape, effectively removing column bleed and hydrocarbon background. Heat maps revealed hundreds of components, and targeted regions isolated sulfur‐containing ions.

Dibenzothiophene Profiling:
Kendrick mass defect plots identified homologous series of alkylated and nonalkylated dibenzothiophenes. IMS spectra separated isobaric interferences, allowing accurate mass spectra extraction of each species. Comparison of pre‐ and post‐hydrodesulfurization samples showed a shift toward lower molecular weights and the disappearance of key sulfur peaks after treatment.

Hydrocarbon Series and Isomer Resolution:
Kendrick plots revealed hydrocarbon homologous series differing by CH2 units and two‐hydrogen increments. IMS maps between m/z 178 and 232 separated dozens of isomers, including dimethylbenzenes and ethylbenzenes, within narrow GC peaks.

ASTM Standard Analysis:
Analysis of a multicomponent ASTM mixture at ~4.4 min retention demonstrated over ten coeluting compounds in the 100–125 Da range. IMS extended peak capacity and unveiled isomeric complexity that conventional GC‐MS could not resolve.

Benefits and Practical Applications

• Enhanced specificity: IMS clean‐up reduces chemical noise and column bleed.
• Improved resolution: Orthogonal separation enables isobaric and isomeric discrimination.
• Minimal sample prep: Direct dilution simplifies workflow and reduces analysis time.
• High-throughput potential: Fast IMS duty cycle and TOF acquisition match narrow GC peaks.
• Refinery optimization: Detailed sulfur profiling supports hydrodesulfurization process control and fuel quality assurance.

Future Trends and Potential Uses

Advancements in IMS resolving power and machine‐learning-driven data processing will further enhance complex matrix analysis. Hybrid LC/GC-IMS-MS workflows may extend applications to biofuels, petrochemical streams, and environmental monitoring. Integration with automated sample introduction and real-time process analytical technology (PAT) systems will drive high-throughput screening in industrial settings.

Conclusion

The GC-APCI IMS Q-TOF platform offers a robust solution for profiling sulfur compounds in diesel, combining chromatographic, mobility, and high-resolution mass separations. This multidimensional approach resolves isobaric interferences, identifies homologous series, and distinguishes treatment effects, delivering actionable insights for fuel quality control and regulatory compliance.

References

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2. Jody, C. M. et al. Conformational Ordering of Biomolecules in the Gas Phase: Nitrogen Collision Cross Sections Measured on a Prototype High Resolution Drift Tube Ion Mobility-Mass Spectrometer. Anal. Chem. 2014, 86, 2107–2116.
3. Kendrick, E. A Mass Scale Based on CH2 = 14.0000 for High-Resolution Mass Spectrometry of Organic Compounds. Anal. Chem. 1963, 35, 2146–2154.
4. Hughey, C. A. et al. Kendrick Mass Defect Spectroscopy: A Compact Visual Analysis for Ultrahigh-Resolution Broadband Mass Spectra. Anal. Chem. 2001, 73, 4676–4681.