Continuous-flow isotope analysis of the deuterium/hydrogen ratio in atmospheric hydrogen

A convenient method is described for analyzing the deuterium/hydrogen (D/H) ratio of atmospheric molecular hydrogen (H₂) based on mass spectrometric isotope-ratio monitoring. The method requires small amounts of air (∼300 mL STP), is operated on-line, and comprises four steps: (1) the condensation of the air matrix at ∼40 K; (2) the collection of the non-condensed components of the air sample (H₂, Ne, He, and traces of N₂) in a 5 Å molecular sieves pre-concentration trap at ∼63 K; (3) gas chromatographic purification of H₂ in a flow of He; and (4) quantification of the D/H ratio in an isotope-ratio mass spectrometer. The precision of the determination of the D/H ratio is better than 2‰, which is comparable to, or better than, that obtained by conventional duel-inlet off-line analysis. There are, however, discrepancies relative to the D/H ratios determined by conventional duel-inlet analysis. This is due to differences in peak shape between reference and sample air, depending on the amount of H₂ injected. Consequently, calibration runs are required. After the calibration of the system, we obtained an accuracy of 1.5‰, so that the accumulated uncertainty is estimated to be less than 4‰. The method also allows determination of the H₂ concentration, with an uncertainty estimated to be 2%.