2 H-fractionations during the biosynthesis of carbohydrates and lipids imprint a metabolic signal on the δ2 H values of plant organic compounds.

Hydrogen (H) isotope ratio (δ2 H) analyses of plant organic compounds have been applied to assess ecohydrological processes in the environment despite a large part of the δ2 H variability observed in plant compounds not being fully elucidated. We present a conceptual biochemical model based on empirical H isotope data that we generated in two complementary experiments that clarifies a large part of the unexplained variability in the δ2 H values of plant organic compounds. The experiments demonstrate that information recorded in the δ2 H values of plant organic compounds goes beyond hydrological signals and can also contain important information on the carbon and energy metabolism of plants. Our model explains where 2 H-fractionations occur in the biosynthesis of plant organic compounds and how these 2 H-fractionations are tightly coupled to a plant's carbon and energy metabolism. Our model also provides a mechanistic basis to introduce H isotopes in plant organic compounds as a new metabolic proxy for the carbon and energy metabolism of plants and ecosystems. Such a new metabolic proxy has the potential to be applied in a broad range of disciplines, including plant and ecosystem physiology, biogeochemistry and palaeoecology.

13C and 18O fractionation effects on open splits and on the ion source in continuous flow isotope ratio mass spectrometry.

Measurements of carbon and oxygen isotopes of CO(2) by continuous flow isotope ratio mass spectrometry are widely used in environmental studies and climate change research. Yet, there are remaining problems associated with the reproducibility of measurements, in particular when high precision is required and/or the amount of sample material is limited. Isotopic fractionations in open splits and nonlinear effects occurring in the mass spectrometer due to different sample amounts alter the results. In this study, we discuss the influence and the origin of these two effects and propose procedures for preventing their impact. Fractionation in the open split can be related to diffusion of CO(2) and can lead to shifted delta-values when measuring a sample gas against a reference gas injected via different open splits. We present a method, where such fractionations can be minimized by adjusting either the position of the capillaries or the flow rates involved or both. The nonlinear peak area dependence of delta(13)C measurements for small sample sizes can be explained by adsorption/desorption processes in the ionization chamber or its vicinity. For constant amplitudes, the magnitude of the nonlinearity only depends on the amount of CO(2) entering the ion source. This nonlinearity can be eliminated by a small additional flux of a conditioning gas fed to the mass spectrometer. The best results were obtained when using carbon monoxide. For the adsorption process in the mass spectrometer we found a fractionation factor of 0.982 +/- 0.005 for delta(13)C and 1.002 +/- 0.004 for delta(18)O.

Temperature dependencies of high-temperature 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 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.