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

Importance of the Topic

The measurement of compound-specific sulfur isotope ratios, particularly δ34S, in complex mixtures such as crude oil provides critical information on the geological origin of hydrocarbons, diagenetic and thermal alteration pathways, and paleoenvironmental redox conditions. By coupling gas chromatography with multicollector ICP-MS, it becomes possible to resolve individual sulfur-bearing compounds and assess isotopic fractionation that cannot be discerned through bulk analyses alone.

Objectives and Study Overview

This work demonstrates the use of a Thermo Scientific Trace 1310 GC coupled via a GCI 300 Interface to a Neptune XT MC-ICP-MS for sulfur isotopic speciation in crude oils. The main goals were to optimize GC-MC-ICP-MS parameters for compound separation and isotope ratio stability, calibrate δ34S values against international standards, and apply the method to four crude oil samples from diverse reservoirs to assess both source signatures and evidence of thermochemical sulfate reduction.

Methodology and Instrumentation

Sample Preparation

• Four crude oils (Bryan Mount, Basrah Light, Saudi Light, Saudi Medium) were diluted 1:200 in hexane and filtered through 20 μm PTFE.
• An internal standard (3-hexylthiophene) was spiked at 1 μL per 1,000 μL of sample for drift correction.
• Five replicate injections were performed for each oil.

Chromatography and Mass Spectrometry

• A temperature-controlled transfer line (GCI 300 Interface) and T-insert enabled SF6 introduction for plasma tuning.
• GC oven was programmed from 100 °C to 300 °C at 10 °C/min on a 30 m × 0.25 mm × 0.25 μm column with He as carrier gas.
• MC-ICP-MS operated at medium resolution (Δm/m ≈ 5,000) with 1.5 L/min Ar and SF6/He tune gas to suppress O2+ interferences.
• Faraday cup configuration monitored 32S, 33S, and 34S with 131 ms integration time over a 21 min run.

Used Instrumentation

• Thermo Scientific Trace 1310 Gas Chromatograph
• Thermo Scientific GCI 300 Interface with T-insert for reference gas introduction
• Thermo Scientific Neptune XT Multicollector ICP-MS
• PTFE syringe filters and hexane solvent for sample preparation

Main Results and Discussion

Eleven major sulfur-containing peaks were resolved in each crude oil, with dibenzothiophene unambiguously identified and other peaks tentatively assigned based on retention time. Calibration against EA-IRMS-determined δ34SVCDT values of four standards yielded an external normalization curve (R2 = 0.997). Correcting for isotope ratio drift via an internal standard approach ensured reproducible δ34S measurements (2σ typically <1‰). Differences up to 3‰ between low- and high-molecular-weight fractions indicated partial thermochemical sulfate reduction, but values remained below thresholds for extensive reduction. Distinct δ34S fingerprints of dibenzothiophene allowed discrimination among the four oils.

Practical Benefits and Applications

• Enables molecular-level characterization of sulfur isotope composition in petroleum, supporting forensic and reservoir studies.
• Provides evidence of paleoenvironmental processes such as thermochemical sulfate reduction.
• Offers improved compound identification in complex hydrocarbon matrices compared to bulk isotope analysis.

Future Trends and Opportunities

Advances in interface design and high-resolution ICP-MS will further reduce interferences and enhance sensitivity for trace sulfur species. Integration with two-dimensional GC or soft-ionization techniques could expand speciation to a broader range of sulfur compounds. Machine learning-based data processing may streamline isotope drift correction and peak identification in increasingly complex samples.

Conclusion

The GC-MC-ICP-MS configuration using a GCI 300 Interface successfully determined compound-specific δ34S values in crude oils with high precision and reliability. The approach differentiates oil sources and detects subtle isotopic shifts associated with thermochemical reactions, demonstrating its value for petroleum geochemistry, environmental forensics, and industrial QA/QC.

Reference

• R. R. Seal, Rev. Mineral. Geochemistry, 2006, 61, 633–677.
• A. Amrani, A. L. Sessions and J. F. Adkins, Anal. Chem., 2009, 81, 9027–9034.
• S. Li et al., Org. Geochem., 2015, 78, 1–22.
• Z. Gvirtzman et al., Geochim. Cosmochim. Acta, 2015, 167, 144–161.
• P. F. Greenwood et al., in Principles and Practice of Analytical Techniques in Geosciences, RSC, 2015, pp. 285–312.
• E. M. Krupp and O. F. X. Donard, Int. J. Mass Spectrom., 2005, 242, 233–242.
• T. Hirata, Y. Hayano and T. Ohno, J. Anal. At. Spectrom., 2003, 18, 1283.
• J.-I. Kimura et al., J. Anal. At. Spectrom., 2016, 31, 790–800.
• A. von Quadt et al., J. Anal. At. Spectrom., 2016, 31, 658–665.