Heike Geilmann
Tyler B. Coplen
Haiping Qi
Matthias Gehre
Arndt Schimmelmann
Willi A. Brand
Sreejesh Nair
2015
<div id="rcm7174-sec-0001" class="section">
<h4>Rationale</h4>
<div class="para">
<p>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 (H<sub>2</sub>) has been suspected as a possible cause of incomplete H<sub>2</sub> yield and hydrogen isotopic fractionation.</p>
</div>
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<div id="rcm7174-sec-0002" class="section">
<h4>Methods</h4>
<div class="para">
<p>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 H<sub>2</sub> yields. The technique also was optimized to provide acceptable sample throughput.</p>
</div>
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<div id="rcm7174-sec-0003" class="section">
<h4>Results</h4>
<div class="para">
<p>The classical HTC reaction of a number of selected compounds exhibited H<sub>2</sub> yields from 60 to 90 %. Yields close to 100 % were measured for the experiments with the chromium-enhanced reactor. The <i>δ</i><sup>2</sup>H 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 H<sub>2</sub>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 H<sub>2</sub> and the observed hydrogen isotopic result, reflecting isotopic fractionation.</p>
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<div id="rcm7174-sec-0004" class="section">
<h4>Conclusions</h4>
<div class="para">
<p>The classical HTC technique to produce H<sub>2</sub> 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 H<sub>2</sub> being substantially shifted to more negative <i>δ</i><sup>2</sup>H values. The reaction can be understood as an initial disproportionation leading to H<sub>2</sub> and HCN with the HCN-hydrogen systematically enriched in <sup>2</sup>H by more than 50 ‰. In the reaction of HCN with chromium, H<sub>2</sub> and chromium-containing solid residues are formed quantitatively.</p>
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application/pdf
10.1002/rcm.7174
en
Wiley
Isotopic disproportionation during hydrogen isotopic analysis of nitrogen-bearing organic compounds
article