H₂ clumped isotope measurements at natural isotopic abundances

International audienceRationale: Molecular hydrogen (H 2) is an important gas for atmospheric chemistry, and an indirect greenhouse gas due to its reaction with OH. The isotopic composition of H 2 (δD) has been used to investigate its atmospheric budget; here we add a new observable, the clumped isotopic signature ΔDD, to the tools that can be used to study the global cycle of H 2.Methods: A method for determining ΔDD in H 2 was developed using the high-resolution MAT 253-Ultra isotope ratio mass spectrometer (Thermo Fisher). The HH, HD and DD abundances are quantified at medium resolution (M/ΔM ≈ 6000), which is sufficient for HD + and DD + to be distinguished from H 3 + and H 2 D + , respectively. The method involves sequential measurement of isotopologues, and DD is measured using an ion counter. For verification, catalytic ΔDD equilibration experiments were performed at temperatures of up to 850°C.Results: The typical precision obtained for ΔDD is 2-6‰, close to the theoretical counting statistics limit, and adequate for detecting the expected natural variations. Compatibility and medium-term reproducibility are consistent with the precision values. The method was validated using temperature equilibration experiments, which showed a dependence of ΔDD on temperature as expected form theoretical calculations.Conclusions: We have established a method for determining ΔDD in H 2 at natural isotopic abundances, with a precision that is adequate for observing the expected variations in atmospheric and other natural H 2. This method opens the road to new research on the natural H 2 cycle