GC-IRMS: Assessment of precision and accuracy of carbon isotope fingerprints measurements in natural gas

Significance of the Topic

The precise and accurate determination of carbon isotope fingerprints in natural gas is fundamental for geochemical tracing, source differentiation, and environmental forensics. Insights into isotope ratios help to distinguish biogenic from thermogenic gas origins and to understand subsurface processes affecting gas composition.

Objectives and Study Overview

This study evaluates the precision and accuracy of compound specific carbon isotope analysis of methane, ethane and propane in natural gas reference materials. It aims to establish traceability to international scales and to validate the performance of a GC-IRMS system for routine laboratory standardization.

Methodology and Instrumentation

Natural gas mixtures distributed by the US Geological Survey (USGS HCG-1, HCG-2, HCG-3) were analyzed. Separation of hydrocarbons and CO₂ was achieved by gas chromatography. Key instrumental components include:

• Thermo Scientific GC IsoLink II Conversion Interface for quantitative combustion to CO₂
• Thermo Scientific DELTA V Isotope Ratio Mass Spectrometer
• Thermo Scientific ConFlo IV Universal Interface
• CP PoraPlot Q column (27.5 m × 0.32 mm, 10 μm)
• Split injection at defined ratios and temperature program from 40 °C to 240 °C

Main Results and Discussion

Typical chromatograms demonstrated clear separation of methane, ethane and propane. Composition values for USGS standards ranged from 1 % to 94 % for methane, 0 % to 43 % for ethane and 0 % to 11 % for propane. Measured δ13C values closely matched expected values for all three reference materials, exhibiting standard deviations below 0.2 ‰. A linear correlation between measured and expected δ13C confirmed high accuracy and stability across varying molecular compositions.

Benefits and Practical Applications

Reliable carbon isotope fingerprints enable consistent interlaboratory comparisons and reduce uncertainties in geochemical interpretation. Standardized reference materials support quality assurance in academic research, industry exploration and environmental investigations.

Future Trends and Applications

Advancements may include extension to heavier hydrocarbons, automation of calibration routines, integration with hydrogen isotope analysis and application to unconventional gas reservoirs and carbon sequestration monitoring.

Conclusion

The validated GC-IRMS workflow delivers precise and accurate carbon isotope data for natural gas analysis. Use of international reference materials ensures traceability and fosters confidence in geochemical and forensic studies.

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