H2 clumped isotope measurements at natural isotopic abundances

Rationale 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 (delta D) has been used to investigate its atmospheric budget; here we add a new observable, the clumped isotopic signature Delta DD, to the tools that can be used to study the global cycle of H-2. Methods A method for determining Delta DD 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/Delta M approximate to 6000), which is sufficient for HD+ and DD+ to be distinguished from H-3(+) and H2D+, respectively. The method involves sequential measurement of isotopologues, and DD is measured using an ion counter. For verification, catalytic Delta DD equilibration experiments were performed at temperatures of up to 850 degrees C. Results The typical precision obtained for Delta DD is 2-6 parts per thousand, 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 Delta DD on temperature as expected form theoretical calculations. Conclusions We have established a method for determining Delta DD 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.