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Abstract:
RATIONALE: High-precision hydrogen isotope ratio analysis of nitrogen-bearing organic materials using
high-temperature conversion (HTC) techniques has proven troublesome in the past. Formation of reaction products other
than molecular hydrogen (H2) has been suspected as a possible cause of incomplete H2 yield and hydrogen isotopic
fractionation.
METHODS: The classical HTC reactor setup and a modified version including elemental chromium, both operated at
temperatures in excess of 1400 °C, have been compared using a selection of nitrogen-bearing organic compounds,
including caffeine. A focus of the experiments was to avoid or suppress hydrogen cyanide (HCN) formation and to reach
quantitative H2 yields. The technique also was optimized to provide acceptable sample throughput.
RESULTS: The classical HTC reaction of a number of selected compounds exhibited H2 yields from 60 to 90 %. Yields
close to 100 % were measured for the experiments with the chromium-enhanced reactor. The δ2H values also were
substantially different between the two types of experiments. For the majority of the compounds studied, a highly
significant relationship was observed between the amount of missing H2 and the number of nitrogen atoms in the
molecules, suggesting the pyrolytic formation of HCN as a byproduct. A similar linear relationship was found between
the amount of missing H2 and the observed hydrogen isotopic result, reflecting isotopic fractionation.
CONCLUSIONS: The classical HTC technique to produce H2 from organic materials using high temperatures in the
presence of glassy carbon is not suitable for nitrogen-bearing compounds. Adding chromium to the reaction zone
improves the yield to 100 % in most cases. The initial formation of HCN is accompanied by a strong hydrogen isotope
effect, with the observed hydrogen isotope results on H2 being substantially shifted to more negative δ2H values. The
reaction can be understood as an initial disproportionation leading to H2 and HCN with the HCN-hydrogen
systematically enriched in 2H by more than 50 ‰. In the reaction of HCN with chromium, H2 and chromiumcontaining
solid residues are formed quantitatively.
