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Abstract:
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 ( 300mL 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 5A° 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%.
