page_title: Hydrogen isotope biogeochemistry
section_title: Offline combustion and reduction
hierarchical_section_title: Hydrogen isotope biogeochemistry / Measurement techniques / Offline combustion and reduction
caption_attribution_description: English: Schematic diagram of TC/EA-IRMS
context_page_description: Hydrogen isotope biogeochemistry is the scientific study of biological, geological, and chemical processes in the environment using the distribution and relative abundance of hydrogen isotopes. There are two stable isotopes of hydrogen, protium ¹H and deuterium ²H, which vary in relative abundance on the order of hundreds of permil. The ratio between these two species can be considered the hydrogen isotopic fingerprint of a substance. Understanding isotopic fingerprints and the sources of fractionation that lead to variation between them can be applied to address a diverse array of questions ranging from ecology and hydrology to geochemistry and paleoclimate reconstructions. Since specialized techniques are required to measure natural hydrogen isotope abundance ratios, the field of hydrogen isotope biogeochemistry provides uniquely specialized tools to more traditional fields like ecology and geochemistry.
context_section_description: The conversion to simple molecules (i.e. H₂ for hydrogen) is required prior to IRMS measurement for stable isotopes. This is due to several reasons with regard to hydrogen: 
  organic molecules and some inorganic ones (e.g. CO₂ + H₂O) can have proton-exchange reactions with ion source of mass spectrometer and produce the products such as  and  that cannot be distinguished;
  isotope effects due to ionization and transmission in the mass spectrometer can vary with different molecular forms. It would require standards in every different molecular form that is being measured, which is not convenient.
  The classical offline preparation for the conversion is combustion over CuO at > 800 °C in sealed quartz tubes, followed by the isolation of resulting water and the reduction to H₂ over hot metal at 400 ~1000 ᵒC on a vacuum line. The produced gas is then directly injected into the dual-inlet mass spectrometer for measurement. The metals used for the reduction to H₂ includes U, Zn, Cr, Mg and Mn, etc. U and Zn had been widely used since the 1950s until Cr was successfully employed in the late 1990s.
  The offline combustion/reduction has the highest accuracy and precision for hydrogen isotope measurement without limits for sample types. The analytical uncertainty is typically 1~2‰ in δD. Thus it is still being used today when highest levels of precision are required. However, the offline preparation procedure is very time-consuming and complicated. It also requires large sample size (several 10² mg). Thus the online preparation based on combustion/reduction coupled with the subsequent continuous flow-IRMS (CF-IRMS) system has been more commonly used nowadays. Chromium reduction or high temperature conversion are the dominant online preparation methods for the detection of hydrogen isotope by IRMS.
caption: Schematic diagram of TC/EA-IRMS and the principle of hydrogen isotope detection by TC/EA[115]
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