Isotopic disproportionation during hydrogen isotopic analysis of nitrogen-bearing organic compounds

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.

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.

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 H₂and 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.

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 chromium-containing solid residues are formed quantitatively.