Standardization for ¹³ C-¹³ C clumped isotope analysis by the fluorination method

Area of Science:

• Isotope Ratio Mass Spectrometry
• Analytical Chemistry
• Organic Geochemistry

Background:

• ¹³C-¹³C isotopologues of C₂ molecules are measured via fluorination to hexafluoroethane (C₂F₆).
• Standardization on an absolute reference scale is needed for accurate isotopic analysis.

Purpose of the Study:

• To standardize the fluorination method for measuring ¹³C-¹³C isotopologues on an absolute reference scale.
• To quantify and correct for isotopic scrambling during the measurement process.

Main Methods:

• Preparation of physical mixtures of ¹³C-¹³C-labeled and natural ethanol.
• Measurement of enriched ethanol samples using the fluorination method.
• Quantification of isotopologue scrambling during dehydration, fluorination, and ionization.

Main Results:

• Measured ∆¹³C¹³C values were ~20% lower than expected.
• Ion source scrambling was minimal (0.5-2%), suggesting scrambling occurred during dehydration or fluorination.
• An empirical transfer function (∆¹³C¹³C_CSC = λ × ∆¹³C¹³C) was determined with λ = 1.25 ± 0.01 for ethanol/ethene.

Conclusions:

• Isotopic scrambling during ion source ionization is negligible for natural sample analysis (<0.04‰).
• Observed isotopic reordering likely occurred during ethene fluorination, not ethanol dehydration or ion source ionization.
• The developed fluorination method, using the empirical transfer function, enables accurate determination of actual ∆ values.