Group-type analysis of Crude Oil using GC/FI-TOFMS - Determination of average molecular weights by group-type analysis

Significance of the Topic

Analysis of molar mass distributions in crude oil is critical for petroleum refining, quality control and research. The average molecular weight of hydrocarbon classes influences processing behavior and product specifications.

Objectives and Study Overview

• Develop a method to determine average molecular weights of different hydrocarbon groups in crude oil using GC/FI-TOFMS.
• Apply group-type analysis software to classify hydrocarbons by saturation level and evaluate molecular weight parameters.

Methodology

Fifty microliters of crude oil were fractionated by C18 solid phase extraction with sequential elution in hexane, toluene and chloroform/methanol. The combined extract was dried and redissolved in hexane. Gas chromatography was performed on an Agilent 6890N with a DB-5ms column (30 m x 0.25 mm, 0.25 μm) using a temperature program from 50°C to 280°C at 15°C/min. A split injection (1:200) at 280°C and helium carrier gas at 1 mL/min ensured separation of hydrocarbon constituents. Field ionization mass spectrometry was conducted on a JMS-T100GC AccuTOF GC equipped with a FI source at –10 kV cathode voltage. Mass spectra were acquired from m/z 35–500 at 0.3 s/spectrum and summed over the chromatographic profile for analysis.

Instrumentation Used

• JMS-T100GC AccuTOF GC (JEOL Ltd.)
• Agilent 6890N Gas Chromatograph
• DB-5ms Capillary Column
• C18 Solid Phase Extraction Cartridge
• Polymerix™ Group-Type Analysis Software (Sierra Analytics)

Results and Discussion

Summed FI mass spectra displayed predominant molecular ions with minimal fragmentation, enabling clear assignment of hydrocarbon series by degree of unsaturation 0–4. Group-type analysis yielded number average (Mn), weight average (Mw), z average (Mz) molecular weights and dispersity for paraffinic, naphthenic, olefinic and aromatic classes. The method demonstrated reliable differentiation of hydrocarbon groups and provided quantitative molecular weight distributions in a single analysis.

Benefits and Practical Applications

• Soft ionization preserves molecular ion integrity for accurate mass determination.
• Combined GC separation and FI-TOFMS supports detailed compositional analysis of complex mixtures.
• Applicable to petroleum quality control, refining optimization and petrochemical research.

Future Trends and Applications

Advances may include integration with high-resolution and tandem mass spectrometry for structural elucidation, expansion of mass range and development of automated data processing pipelines. Potential applications extend to environmental monitoring of hydrocarbons, polymer analysis and heavy oil characterization.

Conclusion

GC/FI-TOFMS coupled with group-type analysis software offers a robust approach for determining molecular weight distributions of hydrocarbon classes in crude oil. The method provides rapid, detailed insights essential for petroleum science and industry.

References

• JEOL Ltd., MS Data Sheet No. 040, 2006.
• Sierra Analytics, Polymerix Group-Type Analysis Software.