Temperature dependencies of high-temperture reduction on conversion products and their isotopic signatures

High-temperature reduction (HTR) is widely used for oxygen and hydrogen isotope determination. Decomposition of cellulose, sucrose and polyethylene foil by HTR is quantitative for temperatures around 1450°C. For lower reaction temperature production of CO2, water and the deposition of carbon inside the reactor are significant and thus the element of interest for isotopic analysis is split into different pools, leading to isotope fractionation. After reduction of cellulose or sucrose at 1125°C less than 60% of the oxygen is found as CO, which is monitored with the isotope ratio mass spectrometer to determine the δ18O value. The remaining oxygen is unevenly distributed between CO2 and H2O, preferentially as CO2. Raising the reaction temperature to 1425°C yields almost quantitative conversion of oxygen into CO and results in a 3‰ more positive δ18O value. Similarly, only 40–50% of the carbon of cellulose and sucrose is transformed into CO in the HTR reactor at 1125°C. This is far from the stoichiometric expected value of 83% for quantitative carbon transfer for cellulose and 92% for sucrose. Of the carbon 40–50% is deposited in the reactor and the remainder can be found as CO2. Based on the comparison of carbon isotope results from HTR and those obtained from combustion, we hypothesize that CO produced during the HTR originates partly from sample carbon and glassy carbon. A combined combustion and HTR carbon isotope determination may provide an insight into the intramolecular carbon distribution of organic substances. These results suggest that HTR should be carried out at temperatures above 1450°C to make sure that fractionations associated with the reduction process are minimal. If this is not possible frequent calibration is required using reference materials of the same structure as the sample