Speciation and δ34S analysis of volatile organic compounds in crude oil by GC-MC-ICP-MS using the Thermo Scientific GCI 300 Interface

Significance of the topic

The isotopic composition of sulfur in volatile organic compounds provides insight into the source and alteration history of crude oil. By measuring δ34S values at the compound level, analysts can distinguish geological origins and detect thermochemical sulfate reduction processes that occurred during petroleum formation.

Objectives and Study Overview

This study aimed to evaluate the coupling of a gas chromatograph to a multicollector ICP-MS for speciation and δ34S analysis of individual sulfur-bearing compounds in crude oil. Four representative crude oils were analyzed to demonstrate method performance and to compare isotopic patterns among samples.

Methodology

Crude oil samples were diluted in hexane, filtered, and spiked with an internal standard, 3-hexylthiophene. A 1 μL injection was performed in splitless mode. Chromatographic separation was achieved on a 30 m GC column with a temperature program from 100 °C to 300 °C. Carrier gas flow and PTV conditions were optimized to produce sharp peaks for sulfur compounds.

Used Instrumentation

• Gas Chromatograph: Thermo Scientific Trace 1310 GC
• Interface: Thermo Scientific GCI 300 transfer line
• Mass Spectrometer: Thermo Scientific Neptune Plus multicollector ICP-MS
• Tuning Gas: SF6 in helium introduced via a T-insert for stable sulfur signal

Main Results and Discussion

Eleven sulfur-containing peaks were resolved in each crude oil chromatogram. External standards calibrated by EA-IRMS established a δ34S normalization curve. Medium resolution mode prevented O2 interferences and supported accurate isotope ratio measurement. Replicate analyses showed precision within 0.5‰ for most compounds. Differences of 2–3‰ between benzo- and dibenzothiophenes were seen in all samples, indicating limited thermochemical sulfate reduction. The δ34S of dibenzothiophene was distinct for each crude, enabling sample discrimination.

Benefits and Practical Applications

• Provides compound‐specific sulfur isotope data for oil fingerprinting
• Reveals paleoenvironmental redox processes impacting petroleum
• Supports quality control and source authentication in petroleum analysis

Future Trends and Opportunities

Advances may include automated data correction for isotope ratio drift and expanded libraries of compound-specific δ34S. Integration with high-resolution chromatography and further miniaturization of interfaces could broaden applications to refined products and environmental monitoring.

Conclusion

The GC-MC-ICP-MS configuration using the GCI 300 interface and Neptune Plus demonstrated robust, reproducible δ34S measurements of volatile sulfur compounds in crude oil. Compound-specific isotope data successfully distinguished sample origins and provided indicators of thermochemical sulfate reduction.

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