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

RATIONALE

The C- C isotopologues of C molecules have recently been measured using a fluorination method. The C compound is first fluorinated into hexafluoroethane (C F ), and its 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 C F .

METHODS

We prepared physical mixtures of C- 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 ∆ C C values, we quantified the extent to which isotopologues were scrambled during dehydration, fluorination, and ionization in the ion source.

RESULTS

The measured ∆ C C value was approximately 20% lower than that expected from the amount of C- 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 ∆ C 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 C and C molecules. Thus, we obtained the empirical transfer function ∆ C C  = λ × ∆ C 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.