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

RATIONALE

The ¹³ C-¹³ C isotopologues of C₂ molecules have recently been measured using a fluorination method. The C₂ compound is first fluorinated into hexafluoroethane (C₂ F₆ ), and its ¹³ C-isotopologues are subsequently measured using a conventional isotope ratio mass spectrometer. Here, we present an approach for standardizing the fluorination method on an absolute reference scale by using isotopically enriched C₂ F₆ .

METHODS

We prepared physical mixtures of ¹³ C-¹³ C-labeled ethanol and natural ethanol. The enriched ethanol samples were measured using the recently developed fluorination method. Based on the difference between the calculated and measured ∆¹³ C¹³ C values, we quantified the extent to which isotopologues were scrambled during dehydration, fluorination, and ionization in the ion source.

RESULTS

The measured ∆¹³ C¹³ C value was approximately 20% lower than that expected from the amount of ¹³ C-¹³ C ethanol. The potential scrambling in the ion source was estimated to be 0.5-2%, which is lower than the observed isotopic reordering. Therefore, isotopic reordering may have occurred during either dehydration or fluorination.

CONCLUSIONS

For typical analysis of natural samples, scrambling in the ion source can only change the ∆¹³ C¹³ C value by less than 0.04‰, which is lower than the current analytical precision (±0.07‰). Therefore, the observed isotopic reordering may have occurred during the fluorination of ethene through the scrambling of isotopologues of ethene but not in the ion source of the mass spectrometer or during the dehydration of ethanol, given the small amount of C₁ and C₃₊ molecules. Thus, we obtained the empirical transfer function ∆¹³ C¹³ C_CSC  = λ × ∆¹³ C¹³ C with a λ value of 1.25 ± 0.01 for ethanol/ethene and 1.00 for ethane. Using the empirical transfer function, the developed fluorination method can provide actual differences in ∆ values.

A fluorination method for measuring the 13 C-13 C isotopologue of C2 molecules

RATIONALE

Doubly substituted isotope species ("clumped" isotopes) can provide insights into the biogeochemical history of a molecule, including its temperature of formation and/or its (bio)synthetic pathway. Here, we propose a new fluorination method for the measurement of ¹³ C-¹³ C species in C₂ molecules using a conventional isotope ratio mass spectrometer. Target molecules include ethane, ethene and ethanol.

METHODS

¹³ C-¹³ C isotope species in C₂ molecules were measured as C₂ F₆ using a conventional isotope ratio mass spectrometer. Ethane and ethene are directly fluorinated to C₂ F₆ . Ethanol is measured after dehydration to ethene and subsequent fluorination of the latter. The method enables the measurement of the Δ¹³ C¹³ C values normalized against a reference working standard.

RESULTS

The reproducibility of the whole protocol, including chemical modification steps and measurement of C₂ F₆ isotopologues, is better than ±0.14‰ for all the compounds. Ethane from natural gas samples and biologically derived ethanol show a narrow range of Δ¹³ C¹³ C values, varying from 0.72‰ to 0.90‰. In contrast, synthetic ethanol as well as putative abiotic ethane show Δ¹³ C¹³ C values significantly different from this range with values of 1.14‰ and 0.25‰, respectively.

CONCLUSIONS

The method presented here provides alternative means of measuring ¹³ C-¹³ C species to that using high-resolution mass spectrometry. Preliminary data from natural and synthetic molecules re-emphasizes the potential of ¹³ C clumped isotope species as a (bio)marker.