Speciation and δ34S analysis of volatile organic compounds in crude oil by GC-MC-ICP-MS

Importance of the Topic

Sulfur is one of the most abundant elements in crude oil and its redox transformations lead to significant isotopic fractionation. Compound-specific δ34S analysis provides insight into the origin of oil, the geological history of reservoirs, and processes such as thermochemical sulfate reduction that affect hydrocarbon quality.

Objectives and Study Overview

This study demonstrates the coupling of a gas chromatograph (Thermo Scientific Trace 1310 GC) to a multicollector ICP-MS (Thermo Scientific Neptune XT) via the GCI 300 Interface for speciation and δ34S determination of volatile sulfur compounds in crude oil samples. Four crude oils of different geological provenance were analyzed to evaluate instrument performance and isotope ratio stability.

Methodology

Sample Preparation:

• Crude oils (Bryan Mount, Basrah Light, Saudi Light, Saudi Medium) diluted 1:200 in hexane and filtered through 20 µm PTFE.
• Internal standard (3-hexylthiophene) spiked at 1 µL per 1 mL sample.
• Five replicate injections per oil.

GC-MC-ICP-MS Conditions:

• Trace 1300 GC with GCI 300 Interface and semi-demountable torch with T-insert for SF6 tuning gas.
• Column: 30 m × 0.25 mm × 0.25 µm; oven program 100 °C to 300 °C at 10 °C/min.
• Injection: 1 µL splitless, PTV inlet from 80 °C to 300 °C.
• Carrier gas He at 3 mL/min; transfer line at 290 °C.

ICP-MS Tuning and Data Acquisition:

• Neptune XT at medium resolution (∆m/m ≈ 5 000) to resolve O2 interferences.
• RF power 1200 W; Ar flow 1.5 L/min; SF6 in He (3 mL/min) for tuning.
• Isotopes L3–32S, L1–33S, H1–34S measured with 131 ms integration over a 21 min run.
• Isotope ratio drift corrected using a linear model based on signal slope.

Instrumentation Used

• Thermo Scientific Trace 1310/1300 Gas Chromatograph
• GCI 300 Transfer Interface
• Thermo Scientific Neptune XT Multicollector ICP-MS
• Thermo Scientific EA IsoLink IRMS System for EA-IRMS calibration
• In-house “GC Neptune Evaluation” software on the Shiny platform

Main Results and Discussion

Eleven major sulfur peaks were consistently detected in each oil and identified by retention time against external standards. Calibration with four compounds (δ34SVCDT by EA-IRMS) produced a linear normalization (R2 = 0.997). Compound δ34S values ranged from approximately –12‰ to +6‰, reflecting both oil source and molecular structure. Dibenzothiophene δ34SVCDT differed among oils by more than 2‰, while no large shifts indicated major thermochemical sulfate reduction. Within each sample, δ34S variation across peaks correlated with compound class (thiophenes vs. sulfides).

Benefits and Practical Applications

Compound-specific sulfur isotope analysis by GC-MC-ICP-MS enables:

• Source fingerprinting of crude oil and petroleum products.
• Assessment of diagenetic or thermal processes in reservoirs.
• Enhanced speciation of complex sulfur matrices for quality control.

Future Trends and Applications

Advances may include higher-throughput drift correction algorithms, extension to heavier sulfur-containing biomarkers, integration with high-resolution organic MS for structural elucidation, and broader application to environmental sulfur cycling studies.

Conclusion

The Thermo Scientific Trace GC–Neptune XT MC-ICP-MS system, coupled via the GCI 300 Interface, provides robust, reproducible δ34S measurements of individual volatile sulfur compounds in crude oil. This approach offers valuable geochemical and forensic insights into petroleum origin and alteration.

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