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Rationale Subambient pCO2 has persisted across the major Phanerozoic ice ages, including the entire late Cenozoic (ca 30 Ma to present). Stable isotope analysis of plant-derived organic matter is used to infer changes in pCO2 and climate in the geologic past, but a growth chamber that can precisely control environmental conditions, including pCO2 and δ13 C value of CO2 (δ13 CCO2 ) at subambient pCO2 , is lacking. Methods We designed and built five identical chambers specifically for plant growth under stable subambient pCO2 (ca 100 to 400 ppm) and δ13 CCO2 conditions. We tested the pCO2 and δ13 CCO2 stability of the chambers both with and without plants, across two 12-hour daytime experiments and two extended 9-day experiments. We also compared the temperature and relative humidity conditions among the chambers. Results The average δ13 CCO2 value within the five chambers ranged from -18.76 to -19.10‰; the standard deviation never exceeded 0.14‰ across any experiment. This represents better δ13 CCO2 stability than that achieved by all previous chamber designs, including superambient pCO2 chambers. Every pCO2 measurement (n = 1225) was within 5% of mean chamber values. The temperature and relative humidity conditions differed by no more than 0.4°C and 1.6%, respectively, across all chambers within each growth experiment. Conclusions This growth chamber design extends the range of pCO2 conditions for which plants can be grown for δ13 C analysis of their tissues at subambient levels. This new capability allows for careful isolation of environmental effects on plant 13 C discrimination across the entire range of pCO2 experienced by terrestrial land plants.

RATIONALE Accurate online analysis of the δ18O values of nitrogen-bearing organic compounds is of interest to several emergent fields, including ecology, forensics and paleontology. During online analysis, high-temperature conversion (HTC) of nitrogen-bearing organics produces N2 gas which creates isobaric interference with the isotopic measurement. Specifically, N2 reacts with trace amounts of oxygen in the mass spectrometer source to form 14N16O (m/z 30), which prevents accurate evaluation of the sample 12C18O peak (m/z 30). METHODS We present an alternative system to the conventional HTC, which uses a nickel-catalyzed (“NiCat”) reduction furnace to convert HTC-produced CO into CO2, allowing for δ18O measurement using signal intensities at m/z 44 and 46. RESULTS This system yields identical δ18O values for nitrogen-doped and undoped sucrose and cellulose compounds up to molar yield ratios of N2:CO = 0.22. In contrast, our conventional HTC system configured to factory recommendations with the stock gas chromatography (GC) column produced a discrepancy of ~5‰ between nitrogen-doped and undoped samples. CONCLUSIONS Because of its ability to eliminate isobaric interference, the NiCat system is a viable alternative to conventional HTC for δ18O measurement, and can be constructed from relatively inexpensive and readily available materials. As an additional advantage, the CO2 analyte produced by NiCat may be cryofocused, to allow for oxygen-isotope determinations on very small amounts of sample substrate. Copyright © 2012 John Wiley & Sons, Ltd.

The measurement of the oxygen stable isotope content in organic compounds has applications in many fields, ranging from paleoclimate reconstruction to forensics. Conventional High-Temperature Conversion (HTC) techniques require >20 µg of O for a single δ18O measurement. Here we describe a system that converts the CO produced by HTC into CO2 via reduction within a Ni-furnace. This CO2 is then concentrated cryogenically, and 'focused' into the isotope ratio mass spectrometry (IRMS) source using a low-flow He carrier gas (6–8 mL/min). We report analyses of benzoic acid (C7H6O2) reference materials that yielded precise δ18O measurement down to 1.3 µg of O, suggesting that our system could be used to decrease sample requirement for δ18O by more than an order of magnitude. Copyright © 2010 John Wiley & Sons, Ltd.