Temperature dependencies of high‐temperature reduction on conversion products and their isotopic signatures

Abstract

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 degrees C. For lower reaction temperature production of CO(2), 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 degrees C less than 60% of the oxygen is found as CO, which is monitored with the isotope ratio mass spectrometer to determine the delta(18)O value. The remaining oxygen is unevenly distributed between CO(2) and H(2)O, preferentially as CO(2). Raising the reaction temperature to 1425 degrees C yields almost quantitative conversion of oxygen into CO and results in a 3 per thousand more positive delta(18)O value. Similarly, only 40-50% of the carbon of cellulose and sucrose is transformed into CO in the HTR reactor at 1125 degrees 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 CO(2). 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 degrees 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.

Responses of carbon and oxygen stable isotopes in rice grain (Oryza sativa L.) to an increase in air temperature during grain filling in the Japanese archipelago

“…Pyrolysis was performed in a ceramic tube of glassy carbon at 1,450°C under a continuous flow of He at 100 ml min À1 . The gas obtained was separated from residual gases by molecular sieving through a 5-Å column (length, 3 m) at 50°C (Werner et al 1996;Bo¨hlke et al 2003;Leuenberger and Filot 2007 Table 1). The maximum air temperature was negatively correlated with rice yield (Fig.…”

Routine hydrogen isotope measurement of cellulose nitrate by high‐temperature pyrolysis – reference materials and precision

“…About one‐sixth of the carbon in the cellulose nitrate molecule is deposited as elemental carbon within the reactor 15. The amount of the trace gases CO 2 and H 2 O depends on the reaction temperature and on the gas flow conditions in the reactor 16. While temperature can be considered as constant for different measurement runs, changes in the gas flow conditions may occur due to variations in the reactor packing after cleaning and refilling the reactor.…”

“…This is either due to low operating temperatures15 or to a fixed technical setup which allows the injection of only one sample type during a single run (either liquid or solid). Furthermore, the utilization of high‐temperature pyrolysis for oxygen and carbon isotope analysis showed that reasonable results are only obtained if a proper calibration is applied 15, 16. Such a calibration requires appropriate reference materials whose δ 2 H values can most reliably be normalized to the V‐SMOW/SLAP scale if direct alternating measurement of reference waters and organic materials is possible.…”

Climate signals in carbon and oxygen isotope ratios ofPinus cembratree‐ring cellulose from the Călimani Mountains, Romania

“…After being encapsulated in silver, 0.2 mg (±10%) of α‐cellulose was pyrolized over glassy carbon at 1450°C and simultaneous measurements of oxygen and carbon isotope ratios (δ 13 C and δ 18 O) were performed (Leuenberger and Filot, ; Loader and Waterhouse, ) using a ThermoQuest TCEA interfaced with a Thermo Delta V Advantage IRMS. The isotopic ratios are reported in per mil (‰) relative to the Vienna Standard Mean Ocean Water (VSMOW) for oxygen, and Vienna Pee Dee Belemnite (VPDB) for carbon (Coplen, ), respectively, using the traditional δ (delta) notation.…”