Comprehensive inter-laboratory calibration of reference materials for δ18O versus VSMOW using various on-line high-temperature conversion techniques

Internationally distributed organic and inorganic oxygen isotopic reference materials have been calibrated by six laboratories carrying out more than 5300 measurements using a variety of high-temperature conversion techniques (HTC) in an evaluation sponsored by the International Union of Pure and Applied Chemistry (IUPAC). To aid in the calibration of these reference materials, which span more than 125‰, an artificially enriched reference water (δ18O of +78.91‰) and two barium sulfates (one depleted and one enriched in 18O) were prepared and calibrated relative to VSMOW2 and SLAP reference waters. These materials were used to calibrate the other isotopic reference materials in this study. The seemingly large estimated combined uncertainties arise from differences in instrumentation and methodology and difficulty in accounting for all measurement bias. They are composed of the 3-fold standard errors directly calculated from the measurements and provision for systematic errors discussed in this paper. A primary conclusion of this study is that nitrate samples analyzed for δ18O should be analyzed with internationally distributed isotopic nitrates, and likewise for sulfates and organics. Authors reporting relative differences of oxygen-isotope ratios (δ18O) of nitrates, sulfates, or organic material should explicitly state in their reports the δ18O values of two or more internationally distributed nitrates (USGS34, IAEA-NO-3, and USGS35), sulfates (IAEA-SO-5, IAEA-SO-6, and NBS 127), or organic material (IAEA-601 benzoic acid, IAEA-602 benzoic acid, and IAEA-600 caffeine), as appropriate to the material being analyzed, had these reference materials been analyzed with unknowns. This procedure ensures that readers will be able to normalize the δ18O values at a later time should it become necessary. The high-temperature reduction technique for analyzing δ18O and δ2H is not as widely applicable as the well-established combustion technique for carbon and nitrogen stable isotope determination. To obtain the most reliable stable isotope data, materials should be treated in an identical fashion; within the same sequence of analyses, samples should be compared with working reference materials that are as similar in nature and in isotopic composition as feasible.

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