On-Line Hydrogen-Isotope Measurements of Organic Samples Using Elemental Chromium: An Extension for High Temperature Elemental-Analyzer Techniques

Matthias Gehre, Julian Renpenning, Tetyana Gilevska, Haiping Qi, Tyler B. Coplen, Harro A. J. Meijer, Willi A. Brand, Arndt Schimmelmann

The high temperature conversion (HTC) technique using an elemental analyzer with glassy carbon tube and filling is a widely used method for hydrogen isotopic analysis of water and many solid and liquid organic samples (TC/EA) with analysis by isotope-ratio mass spectrometry (IRMS). However, the TC/EA IRMS method may produce inaccurate δ2H results, with values deviating by more than 20 mUr (milliurey = 0.001 = 1 ‰) from the true value for some materials. We describe a single-oven chromium-filled elemental analyzer system coupled to an IRMS for improved measurements of hydrogen isotopic compositions of organic substances (Cr EA method). Differences between the techniques due to matrix effects or the presence of heteroatoms are discussed. Several types of reactions and experimental setups have been evaluated for quantitative conversion of organically bound hydrogen to the molecular hydrogen (H2) analyte. Formation of hydrogen-bearing by-products was experimentally verified. Hetero-atoms like nitrogen or chlorine (and other halogens) can form HCN or HCl (HX) under TC/EA conditions and can cause isotopic fractionation when the hydrogen yield is significantly lower than 100 %. In contrast, hot chromium maximizes the yield of molecular hydrogen in a helium carrier gas by irreversibly and quantitatively scavenging all reactive elements except hydrogen. To overcome handling problems with water as the principal calibration anchor for hydrogen isotopic measurements, we have employed an effective and simple strategy using reference waters sealed in silver tube segments. These crimped silver tubes can be employed in both the Cr-EA and TC/EA techniques. They simplify considerably the normalization of hydrogen-isotope measurement data to the VSMOW-SLAP scale, and their use improves accuracy of the data by eliminating evaporative loss and associated isotopic fractionation while handling water as a bulk sample. The Cr EA technique expands the analytical possibilities for on-line hydrogen-isotope measurements of organic samples significantly. This method yielded reproducibility values (1-sigma) for δ2H measurements on water and caffeine samples of better than 1.0 mUr and 0.5 mUr, respectively. The calibration of organic samples, commonly having high δ2H values, will benefit from the availability of suitably 2H enriched reference waters, extending the VSMOW-SLAP scale above zero.