Simultaneous determination of δ15N and δ18O of N2O and δ13C of CH4 in nanomolar quantities from a single water sample

We have developed a rapid, sensitive, and automated analytical system to simultaneously determine the concentrations and stable isotopic compositions (δ¹⁵N, δ¹⁸O, and δ¹³C) of nanomolar quantities of nitrous oxide (N₂O) and methane (CH₄) in water, by combining continuous‐flow isotope‐ratio mass spectrometry and a helium‐sparging system to extract and purify the dissolved gases. Our system, which is composed of cold traps and a capillary gas chromatograph that use ultra‐pure helium as the carrier gas, achieves complete extraction of N₂O and CH₄ in a water sample and separation among N₂O, CH₄, and the other component gases. The flow path following exit from the gas chromatograph was periodically changed to pass the gases through the combustion furnace to convert CH₄ and the other hydrocarbons into CO₂, or to bypass the combustion furnace for the direct introduction of eluted N₂O into the mass spectrometer, for determining the stable isotopic compositions through monitoring the ions of m/z 44, 45, and 46 of CO and N₂O⁺. The analytical system can be operated automatically with sequential software programmed on a personal computer. Analytical precisions better than 0.2‰ and 0.3‰ and better than 1.4‰ and 2.6‰ were obtained for the δ¹⁵N and δ¹⁸O of N₂O, respectively, when more than 6.7 nmol and 0.2 nmol of N₂O, respectively, were injected. Simultaneously, analytical precisions better than 0.07‰ and 2.1‰ were obtained for the δ¹³C of CH₄ when more than 5.5 nmol and 0.02 nmol of CH₄, respectively, were injected. In this manner, we can simultaneously determine stable isotopic compositions of a 120 mL water sample with concentrations as low as 1.7 nmol/kg for N₂O and 0.2 nmol/kg for CH₄. Copyright © 2010 John Wiley & Sons, Ltd.     

Online high‐precision δ2H and δ18O analysis in water by pyrolysis

A method for online simultaneous δ²H and δ¹⁸O analysis in water by high‐temperature conversion is presented. Water is injected by using a syringe into a high‐temperature carbon reactor and converted into H₂ and CO, which are separated by gas chromatography (GC) and carried by helium to the isotope ratio mass spectrometer for hydrogen and oxygen isotope analysis. A series of experiments was conducted to evaluate several issues such as sample size, temperature and memory effects. The δ²H and δ¹⁸O values in multiple water standards changed consistently as the reactor temperature increased from 1150 to 1480°C. The δ¹⁸O in water can be measured at a lower temperature (e.g. 1150°C) although the precision was relatively poor at temperatures <1300°C. Memory effects exist for δ²H and δ¹⁸O between two waters, and can be reduced (to <1%) with proper measures. The injection of different amounts of water may affect the isotope ratio results. For example, in contrast to small injections (100 nL or less) from small syringes (e.g. 1.2 µL), large injections (1 µL or more) from larger syringes (e.g. 10 µL) with dilution produced asymmetric peaks and shifts of isotope ratios, e.g. 4‰ for δ²H and 0.4‰ for δ¹⁸O, probably resulting from isotope fractionation during dilution via the ConFlo interface. This method can be used to analyze nanoliter samples of water (e.g. 30 nL) with good precision of 0.5‰ for δ²H and 0.1‰ for δ¹⁸O. This is important for geosciences; for instance, fluid inclusions in ancient minerals may be analyzed for δ²H and δ¹⁸O to help understand the formation environments. Copyright © 2009 John Wiley & Sons, Ltd.     

Minimization of sample requirement for δ18O in benzoic acid

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.     

Calibration of δ17O and δ18O of international measurement standards – VSMOW,VSMOW2, SLAP,and SLAP2

Due to exhaustion of the two primary calibration materials, Vienna Standard Mean Ocean Water (VSMOW) and Standard Light Antarctic Precipitation (SLAP), two replacement materials, VSMOW2 and SLAP2, were created with isotopic compositions as close as possible to the original standards in their D/H and ¹⁸O/¹⁶O ratios. Measurements of the δ¹⁷O composition constitute therefore an appropriate independent check of the achieved isotopic adjustment. Aliquots from ampoules of VSMOW, VSMOW2, SLAP, and SLAP2 were fluorinated by BrF₅ and analyzed using a dual‐inlet Delta E mass spectrometer. VSMOW2 and SLAP2 were found to be indistinguishable from VSMOW and SLAP, respectively, in their δ¹⁷O and δ¹⁸O values within measurement uncertainties. This result is a confirmation of the successful isotopic matching of VSMOW2 and SLAP2 to their predecessors. Further checks of the δ¹⁷O value of SLAP2 seem desirable. Copyright © 2010 John Wiley & Sons, Ltd.     

β‐CdC2O4

Crystals of an­hydrous cadmium oxalate, β‐[Cd(C₂O₄)], have been synthesized hydro­thermally and the crystal structure solved using single‐crystal X‐ray diffraction data. The Cd and oxalate ions lie about independent inversion centres. The structure consists of a three‐dimensional framework built from sheets of cadmium octahedra linked together by oxalate groups.     

Simultaneous δ2H and δ18O analyses of water inclusions in halite with off‐axis integrated cavity output spectroscopy

Stable isotope compositions of ancient halite fluid inclusions have been recognized to be valuable tools for reconstructing past environments. Nevertheless, in order to better understand the genesis of halite deposits, it could be of great interest to combine both δ²H and δ¹⁸O measurements of the water trapped as inclusions in the defects of the mineral lattice. We developed a method combining off‐axis integrated cavity output spectroscopy (OA‐ICOS) connected on line with a modified elemental analyzer (EA‐OA‐ICOS) to perform those measurements. The first step was to test the method with synthetic halite crystals precipitated in the laboratory from isotopically calibrated waters. Water isotopic signatures have been measured with conventional techniques, equilibration for δ¹⁸O and chromium reduction for δ²H. Then, we modified and optimized a conventional EA to connect it online with an OA‐ICOS instrument for H₂O measurements. The technique is first evaluated for calibrated free water samples. The technique is also evaluated for salt matrix effect, accuracy, and linearity for both isotopic signatures. Then, the technique is used to measure simultaneously δ²H and δ¹⁸O values of halite water inclusions precipitated from the evaporation experiments. Data generated with this new technique appeared to be comparable with those inferred from prior off‐line technique studies. The advantages offered by the OA‐ICOS technique are the simultaneous acquisition of both isotopic ratios and the substantial reduction of data acquisition time and sample aliquot size. Natural halite samples have been analyzed with this method. Natural halite samples as old as Precambrian have also been analyzed with this method.     

Compound‐specific δ18O analyses of neutral sugars in soils using gas chromatography–pyrolysis–isotope ratio mass spectrometry: problems,possible solutions and a first application

Although gas chromatography–pyrolysis–isotope ratio mass spectrometry (GC‐Py‐IRMS) has allowed us to make online compound‐specific δ¹⁸O measurements for about the last ten years, this technique has hardly been applied. We tested different pyrolysis reactor designs using standards (vanillin, ethylvanillin, a fatty acid methyl ester and alkanes) in order to optimize the GC‐Py‐IRMS δ¹⁸O measurements. The method was then applied to methylboronic acid (MBA) sugar derivatives (pentoses, 6‐deoxyhexoses and hexoses). Plant‐ and microbial‐derived monosaccharides were extracted hydrolytically from litter and topsoils before derivatization. The measured δ¹⁸O values of samples and co‐analyzed reference material were first drift‐corrected by use of regularly discharged pulses of CO reference gas. Secondly, they were corrected for the amount dependence of the δ¹⁸O values. Thirdly, the δ¹⁸O values were calibrated using the reference material (principle of ‘Identical Treatment’), and, finally, a correction was applied by taking the hydrolytically introduced and water‐exchangeable oxygen atoms into account. Our results suggest that the δ¹⁸O values of plant‐derived monosaccharides in litter reflect the climatic conditions of the last year, whereas δ¹⁸O values of the respective topsoils reflect the averaged climate signal of the last decades or even centuries. This demonstrates the high potential of the method for palaeoclimate reconstructions. Copyright © 2009 John Wiley & Sons, Ltd.     

The Positional δ(18O) Values of Extracted and Synthetic Vanillin

The positional δ(¹⁸O) values of vanillin ( 1 ) of different origins have been determined from the global values of 2‐methoxy‐4‐methylphenol ( 4 ), obtained from 1 upon Clemensen reduction, and of 3‐methylanisole ( 5 ), obtained from 4 by removal of the phenolic O‐atom. By these means, it is possible to differentiate samples of 1 of synthetic origin from those extracted from Vanilla plants or produced from lignin by chemical oxidation. The main difference between the samples derived from guaiacol and those possessing the aromatic moiety of natural origin is in the enrichment values of the O‐atoms at C(3) and C(4), while the extractive materials from the pods are distinguished from the product from lignin on the basis of the carbonyl oxygen δ(¹⁸O) values, ranging from +25.5 and +26.2 in the natural material to +19.7‰ in the lignan‐based sample. The values for the phenolic O‐atom vary from +8.9 and +12‰ of the synthetic materials to +6.5, +5.3, and +6.3‰, respectively, of the sample from lignin and the two samples from Vanilla pods,whereas the MeO O‐atoms show the following values for the same compounds: −2.9, −3.2, +3.5, +3.1, and +2.3‰, respectively. This study indicates the significance of the positional δ(¹⁸O) values of polyoxygenated compounds for the definition of their origin.     

Reactivity of η-, γ-, and α-Al2O3 for ZnAl2O4 Formation

The rate of ZnAl₂O₄ formation was measured for η-, γ-; and α- Al₂O₃ in order to distinguish the reactivity of them. The reactivity decreased as follows: η- > γ- > α-Al₂O₃. The reaction rate fitted to Jander's equation and the activation energies calculated were 33, 47 and 113 Kcal/mol for η-, γ- and α-Al₂O₃ systems, respectively. These differences are explained by an assumption that η- and γ-Al₂O₃ resulted in a ZnAl₂O₄ with imperfect spinel structure, but α-Al₂O₃ gave the perfect spinel structure. This assumption is based on the theoretical consideration of the activation energy needed for the diffusion-controlled reaction and date of lattice constant of each ZnAl₂O₄ obtained from three aluminas. The fact that η-Al₂O₃ shows very high reactivity compared with that of γ-Al₂O₃ was found to be explained on the basis of Jander's equation, a comparison of specific surface area and the defect structures of the aluminas.