Analysis of the hydrogen and oxygen stable isotope ratios of beverage waters without prior water extraction using isotope ratio infrared spectroscopy

Hydrogen (δ²H) and oxygen (δ¹⁸O) stable isotope analysis is useful when tracing the origin of water in beverages, but traditional analytical techniques are limited to pure or extracted waters. We measured the isotopic composition of extracted beverage water using both isotope ratio infrared spectroscopy (IRIS; specifically, wavelength‐scanned cavity ring‐down spectroscopy) and isotope ratio mass spectrometry (IRMS). We also analyzed beer, sodas, juices, and milk ‘as is’ using IRIS. For IRIS analysis, four sequential injections of each sample were measured and data were corrected for sample‐to‐sample memory using injections (a) 1‐4, (b) 2‐4, and (c) 3‐4. The variation between δ²H and δ¹⁸O values calculated using the three correction methods was larger for unextracted (i.e., complex) beverages than for waters. The memory correction was smallest when using injections 3‐4. Beverage water δ²H and δ¹⁸O values generally fit the Global Meteoric Water Line, with the exception of water from fruit juices. The beverage water stable isotope ratios measured using IRIS agreed well with the IRMS data and fit 1:1 lines, with the exception of sodas and juices (δ²H values) and beers (δ¹⁸O values). The δ²H and δ¹⁸O values of waters extracted from beer, soda, juice, and milk were correlated with complex beverage δ²H and δ¹⁸O values (r = 0.998 and 0.997, respectively) and generally fit 1:1 lines. We conclude that it is possible to analyze complex beverages, without water extraction, using IRIS although caution is needed when analyzing beverages containing sugars, which can clog the syringe and increase memory, or alcohol, a known spectral interference. Copyright © 2010 John Wiley & Sons, Ltd.     

The hydrological response of heavy clay grassland soils to rainfall in south‐west England using δ2H

Stable isotopes of water have been previously used in catchment studies to separate rain‐event water from pre‐event groundwater. However, there are a lack of studies at the smaller scale looking at the separation of event water from pre‐event water. This is particularly relevant for heavy clay soil systems through which the movement of water is uncertain but is thought to be rainwater‐dominated. The data presented here were collected at a rural site in the south‐west of England. The historic rainfall at the site was isotopically varied but similar to the global meteoric water line, with annual weighted means of ?37‰ for δ²H and ?5.7‰ for δ¹⁸O and with no seasonal variation. Drainage was sampled from the inter‐flow (surface runoff + lateral through‐flow) and drain‐flow (55 cm deep mole drains) pathways of two 1 ha lysimeters during two rainfall events, which had δ²H values of ?68‰ and ?92‰, respectively. The δ²H values of the lysimeter drainage water suggest that there was no contribution of event water during the first, small discharge (Q) event; however, the second larger event did show isotopic variation in δ²H values negatively related to Q indicating that rainwater was contributing to Q. A hydrograph separation indicated that only 49–58% of the inter‐flow and 18–25% of the drain‐flow consisted of event water. This was surprising given that these soil types are considered retentive of soil water. More work is needed on heavy clay soils to understand better the nature of water movement from these systems. 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.     

On the calibration of continuous,high‐precision δ18O and δ2H measurements using an off‐axis integrated cavity output spectrometer

The ¹⁸O and ²H of water vapor serve as powerful tracers of hydrological processes. The typical method for determining water vapor δ¹⁸O and δ²H involves cryogenic trapping and isotope ratio mass spectrometry. Even with recent technical advances, these methods cannot resolve vapor composition at high temporal resolutions. In recent years, a few groups have developed continuous laser absorption spectroscopy (LAS) approaches for measuring δ¹⁸O and δ²H which achieve accuracy levels similar to those of lab‐based mass spectrometry methods. Unfortunately, most LAS systems need cryogenic cooling and constant calibration to a reference gas, and have substantial power requirements, making them unsuitable for long‐term field deployment at remote field sites. A new method called Off‐Axis Integrated Cavity Output Spectroscopy (OA‐ICOS) has been developed which requires extremely low‐energy consumption and neither reference gas nor cryogenic cooling. In this report, we develop a relatively simple pumping system coupled to a dew point generator to calibrate an ICOS‐based instrument (Los Gatos Research Water Vapor Isotope Analyzer (WVIA) DLT‐100) under various pressures using liquid water with known isotopic signatures. Results show that the WVIA can be successfully calibrated using this customized system for different pressure settings, which ensure that this instrument can be combined with other gas‐sampling systems. The precisions of this instrument and the associated calibration method can reach ～0.08‰ for δ¹⁸O and ～0.4‰ for δ²H. Compared with conventional mass spectrometry and other LAS‐based methods, the OA‐ICOS technique provides a promising alternative tool for continuous water vapor isotopic measurements in field deployments. Copyright © 2009 John Wiley & Sons, Ltd.     

An online method combining a Gasbench II with continuous flow isotope ratio mass spectrometry to determine the content and isotopic compositions of minor amounts of carbonate in silicate rocks

An online method using continuous flow isotope ratio mass spectrometry (CF‐IRMS) interfaced with a Gasbench II device was established to analyze carbon and oxygen isotopic compositions and to estimate the content of minor amounts of carbonate in silicate rocks. The mixtures of standard materials and high‐purity quartz are firstly used to calibrate different quantities of carbonate in silicates. The results suggest that the accuracy and precision of the online analysis are both better than those obtained using an offline method. There is a positive correlation between the carbonate weight and the Mass44 ion beam intensity (or peak area). When the weight of carbonate in the mixtures is greater than 70 µg (equal to ～1800 mV Mass44 ion beam intensity), the δ¹³C and δ¹⁸O values of samples usually have accuracy and precision of ±0.1‰ and ±0.2‰ (1σ), respectively. If the weight is less than 70 µg, some limitations (e.g., not perfectly linear) are encountered that significantly reduce the accuracy and precision. The measured δ¹⁸O values are systematically lower than the true values by ?0.3 to ?0.7‰; the lower the carbonate content, the lower the measured δ¹⁸O value. For samples with lower carbonate content, the required phosphoric acid doses are higher and more oxygen isotope exchanges with the water in the phosphoric acid. To guarantee accurate results with high precision, multiple analyses of in‐house standards and an artificial MERCK sample with δ¹³C values from ?35.58 to 1.61‰ and δ¹⁸O from 6.04 to 18.96‰ were analyzed simultaneously with the unknown sample. This enables correction of the measured raw data for the natural sample based on multiple‐point normalization. The results indicate that the method can be successfully applied to a range of natural rocks. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydrogen and oxygen isotopes of water from inclusions in minerals: design of a new crushing system and on‐line continuous‐flow isotope ratio mass spectrometric analysis

An analytical line for stable isotope analyses of water recovered from fluid inclusions in minerals was built and successfully tested. The line is based on the principle of continuous‐flow analysis of water via high‐temperature reduction on glassy carbon. It includes a custom‐designed set of high‐efficiency crushers and a cryo‐focusing cell. This paper provides details of the line design and discusses strategies for line conditioning and mitigation of memory effects. The line allows measurements of hydrogen and oxygen isotopes during a single acquisition. The precision of the analyses depends on the amount of water released from the inclusions. The best results are obtained for samples containing at least 0.1–0.2 µL (0.06–0.11 µmol) H₂O. For such samples precision is better than 1.5‰ for δD and 0.5‰ for δ¹⁸O (1σ). Smaller amounts of water can be measured but at lower precision. Analyses of modern calcite formed under stable conditions in a deep cave allowed assessment of the accuracy of the analyses. The δD values measured in fluid inclusions of this working standard match the δD value of the parent water, and the oxygen isotope values agree within ca. 0.5‰. This indicates that fluid inclusions trapped in calcite at near‐ambient temperatures (e.g. speleothems and low‐temperatures phreatic calcite) faithfully preserve the original isotopic composition of the parent waters. Copyright © 2009 John Wiley & Sons, Ltd.     

Calcite‐CO2 mixed into CO2‐free air: a new CO2‐in‐air stable isotope reference material for the VPDB scale

In order to generate a reliable and long‐lasting stable isotope ratio standard for CO₂ in samples of clean air, CO₂ is liberated from well‐characterized carbonate material and mixed with CO₂‐free air. For this purpose a dedicated acid reaction and air mixing system (ARAMIS) was designed. In the system, CO₂ is generated by a conventional acid digestion of powdered carbonate. Evolved CO₂ gas is mixed and equilibrated with a prefabricated gas comprised of N₂, O₂, Ar, and N₂O at close to ambient air concentrations. Distribution into glass flasks is made stepwise in a highly controlled fashion. The isotopic composition, established on automated extraction/measurement systems, varied within very small margins of error appropriate for high‐precision air‐CO₂ work (about ±0.015‰ for δ¹³C and ±0.025‰ for δ¹⁸O). To establish a valid δ¹⁸O relation to the VPDB scale, the temperature dependence of the reaction between 25 and 47°C has been determined with a high level of precision. Using identical procedures, CO₂‐in‐air mixtures were generated from a selection of reference materials; (1) the material defining the VPDB isotope scale (NBS 19, δ¹³C = +1.95‰ and δ¹⁸O = ?2.2‰ exactly); (2) a local calcite similar in isotopic composition to NBS 19 (‘MAR‐J1’, δ¹³C = +1.97‰ and δ¹⁸O = ?2.02‰), and (3) a natural calcite with isotopic compositions closer to atmospheric values (‘OMC‐J1’, δ¹³C = ?4.24‰ and δ¹⁸O = ?8.71‰). To quantitatively control the extent of isotope‐scale contraction in the system during mass spectrometric measurement other available international and local carbonate reference materials (L‐SVEC, IAEA‐CO‐1, IAEA‐CO‐8, CAL‐1 and CAL‐2) were also processed. As a further control pure CO₂ reference gases (Narcis I and II, NIST‐RM 8563, GS19 and GS20) were mixed with CO₂‐free synthetic air. Independently, the pure CO₂ gases were measured on the dual inlet systems of the same mass spectrometers. The isotopic record of a large number of independent batches prepared over the course of several months is presented. In addition, the relationship with other implementations of the VPDB‐scale for CO₂‐in‐air (e.g. CG‐99, based on calibration of pure CO₂ gas) has been carefully established. The systematic high‐precision comparison of secondary carbonate and CO₂ reference materials covering a wide range in isotopic composition revealed that assigned δ‐values may be (slightly) in error. Measurements in this work deviate systematically from assigned values, roughly scaling with isotopic distance from NBS 19. This finding indicates that a scale contraction effect could have biased the consensus results. The observation also underlines the importance of cross‐contamination errors for high‐precision isotope ratio measurements. As a result of the experiments, a new standard reference material (SRM), which consists of two 5‐L glass flasks containing air at 1.6 bar and the CO₂ evolved from two different carbonate materials, is available for distribution. These ‘J‐RAS’ SRM flasks (‘Jena‐Reference Air Set’) are designed to serve as a high‐precision link to VPDB for improving inter‐laboratory comparability. a Copyright © 2005 John Wiley & Sons, Ltd.     

Characterization of Vegetables by Stable Isotopic and Elemental Signatures

ABSTRACT

109 Romanian parsley, celery and parsnip root, cucumber, vegetable marrow, onion, and pepper samples were characterized by their stable isotope ratios and elemental concentrations in comparison to imported varieties. Organic and conventional agriculture practices were also compared as well as a differentiation between eggplant and peppers grown in greenhouses and those in the field. δ²H values were between ?70.8 and 5.6‰, while the δ¹⁸O values were between ?9.0 and 4.1‰. The δ¹⁵N values for white onions were from 2.8 to 11.1‰. For Romanian organic onions, the carbon isotopic values were lower (?29.2 to ?28.2‰) compared to conventional onions from other countries (?27.8‰ to ?23.8%). Linear discriminant analysis was used to characterize agricultural practices and the geographic origin of the vegetables.     

Nicotine,acetanilide and urea multi‐level 2H‐, 13C‐ and 15N‐abundance reference materials for continuous‐flow isotope ratio mass spectrometry

Accurate determinations of stable isotope ratios require a calibration using at least two reference materials with different isotopic compositions to anchor the isotopic scale and compensate for differences in machine slope. Ideally, the δ values of these reference materials should bracket the isotopic range of samples with unknown δ values. While the practice of analyzing two isotopically distinct reference materials is common for water (VSMOW‐SLAP) and carbonates (NBS 19 and L‐SVEC), the lack of widely available organic reference materials with distinct isotopic composition has hindered the practice when analyzing organic materials by elemental analysis/isotope ratio mass spectrometry (EA‐IRMS). At present only L‐glutamic acids USGS40 and USGS41 satisfy these requirements for δ¹³C and δ¹⁵N, with the limitation that L‐glutamic acid is not suitable for analysis by gas chromatography (GC). We describe the development and quality testing of (i) four nicotine laboratory reference materials for on‐line (i.e. continuous flow) hydrogen reductive gas chromatography‐isotope ratio mass‐spectrometry (GC‐IRMS), (ii) five nicotines for oxidative C, N gas chromatography‐combustion‐isotope ratio mass‐spectrometry (GC‐C‐IRMS, or GC‐IRMS), and (iii) also three acetanilide and three urea reference materials for on‐line oxidative EA‐IRMS for C and N. Isotopic off‐line calibration against international stable isotope measurement standards at Indiana University adhered to the ‘principle of identical treatment’. The new reference materials cover the following isotopic ranges: δ²H_nicotine ?162 to ?45‰, δ¹³C_nicotine ?30.05 to +7.72‰, δ¹⁵N_nicotine ?6.03 to +33.62‰; δ¹⁵N_acetanilide +1.18 to +40.57‰; δ¹³C_urea ?34.13 to +11.71‰, δ¹⁵N_urea +0.26 to +40.61‰ (recommended δ values refer to calibration with NBS 19, L‐SVEC, IAEA‐N‐1, and IAEA‐N‐2). Nicotines fill a gap as the first organic nitrogen stable isotope reference materials for GC‐IRMS that are available with different δ¹⁵N values. Comparative δ¹³C and δ¹⁵N on‐line EA‐IRMS data from 14 volunteering laboratories document the usefulness and reliability of acetanilides and ureas as EA‐IRMS reference materials. Published in 2009 by John Wiley & Sons, Ltd.     

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) a 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 b and SLAP reference waters. These materials were used to calibrate the other isotopic reference materials in this study, which yielded:

Comparison of secondary ion mass spectrometry and micromilling/continuous flow isotope ratio mass spectrometry techniques used to acquire intra‐otolith δ18O values of wild Atlantic salmon (Salmo salar)

The chemical signals in the sequential layers of fish otoliths have the potential to provide fisheries biologists with temporal and spatial details of migration which are difficult to obtain without expensive tracking methods. Signal resolution depends, however, on the extraction technique used. We compared the use of mechanical micromilling and continuous flow isotope ratio mass spectrometry (CF‐IRMS) methods with secondary ion mass spectrometry (SIMS) to obtain δ¹⁸O profiles from otoliths of wild Atlantic salmon (Salmo salar) and used these to corroborate the time of freshwater emigration of the juvenile with macroscopic patterns within the otolith. Both techniques showed the transition occurring at the same visible feature on the otolith, allowing future analyses to easily identify the juvenile (freshwater) versus adult (marine) life‐stages. However, SIMS showed a rapid and abrupt transition whereas micromilling provided a less distinct signal. The number of samples that could be obtained per unit area sampled using SIMS was 2 to 3 times greater than that when using micromilling/CF‐IRMS although the δ¹⁸O values and analytical precisions (～0.2‰) of the two methods were comparable. In addition, SIMS δ¹⁸O results were used to compare otolith aragonite values with predicted values calculated using various isotope fractionation equations. Copyright © 2010 John Wiley & Sons, Ltd.     

Isotopologue fractionation during N2O production by fungal denitrification

Identifying the importance of fungi to nitrous oxide (N₂O) production requires a non‐intrusive method for differentiating between fungal and bacterial N₂O production such as natural abundance stable isotopes. We compare the isotopologue composition of N₂O produced during nitrite reduction by the fungal denitrifiers Fusarium oxysporum and Cylindrocarpon tonkinense with published data for N₂O production during bacterial nitrification and denitrification. The fractionation factors for bulk nitrogen isotope values for fungal denitrification were in the range −74.7 to −6.6‰. There was an inverse relationship between the absolute value of the fractionation factors and the reaction rate constant. We interpret this in terms of variation in the relative importance of the rate constants for diffusion and enzymatic reduction in controlling the net isotope effect for N₂O production during fungal denitrification. Over the course of nitrite reduction, the δ¹⁸O values for N₂O remained constant and did not exhibit a relationship with the concentration characteristic of an isotope effect. This probably reflects isotopic exchange with water. Similar to the δ¹⁸O data, the site preference (SP; the difference in δ¹⁵N between the central and outer N atoms in N₂O) was unrelated to concentration during nitrite reduction and, therefore, has the potential to act as a conservative tracer of production from fungal denitrification. The SP values of N₂O produced by F. oxysporum and C. tonkinense were 37.1 ± 2.5‰ and 36.9 ± 2.8‰, respectively. These SP values are similar to those obtained in pure culture studies of bacterial nitrification but quite distinct from SP values for bacterial denitrification. The large magnitude of the bulk nitrogen isotope fractionation and the δ¹⁸O values associated with fungal denitrification are distinct from bacterial production pathways; thus multiple isotopologue data holds much promise for resolving bacterial and fungal production. Our work further provides insight into the role that fungal and bacterial nitric oxide reductases have in determining site preference during N₂O production. Copyright © 2008 John Wiley & Sons, Ltd.     

Continuous analysis of δ18O and δD values of water by diffusion sampling cavity ring‐down spectrometry: a novel sampling device for unattended field monitoring of precipitation,ground and surface waters

A novel sampling device suitable for continuous, unattended field monitoring of rapid isotopic changes in environmental waters is described. The device utilises diffusion through porous PTFE tubing to deliver water vapour continuously from a liquid water source for analysis of δ¹⁸O and δD values by Cavity Ring‐Down Spectrometry (CRDS). Separation of the analysed water vapour from non‐volatile dissolved and particulate contaminants in the liquid sample minimises spectral interferences associated with CRDS analyses of many aqueous samples. Comparison of isotopic data for a range of water samples analysed by Diffusion Sampling‐CRDS (DS‐CRDS) and Isotope Ratio Mass Spectrometry (IRMS) shows significant linear correlations between the two methods allowing for accurate standardisation of DS‐CRDS data. The internal precision for an integration period of 3 min (standard deviation (SD) = 0.1 ‰ and 0.3 ‰ for δ¹⁸O and δD values, respectively) is similar to analysis of water by CRDS using an autosampler to inject and evaporate discrete water samples. The isotopic effects of variable air temperature, water vapour concentration, water pumping rate and dissolved organic content were found to be either negligible or correctable by analysis of water standards. The DS‐CRDS system was used to analyse the O and H isotope composition in short‐lived rain events. Other applications where finely time resolved water isotope data may be of benefit include recharge/discharge in groundwater/river systems and infiltration‐related changes in cave drip water. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Offline oxygen isotope analysis of organic compounds with high N:O

Although the advantages of online δ¹⁸O analysis of organic compounds make its broad application desirable, researchers have encountered NO⁺ isobaric interference with CO⁺ at m/z 30 (e.g. ¹⁴N¹⁶O⁺, ¹²C¹⁸O⁺) when analyzing nitrogenous substrates. If the δ¹⁸O value of inter‐laboratory standards for substrates with high N:O value could be confirmed offline, these materials could be analyzed periodically and used to evaluate δ¹⁸O data produced online for nitrogenous unknowns. To this end, we present an offline method based on modifications of the methods of Schimmelmann and Deniro (Anal. Chem. 1985; 57: 2644) and Sauer and Sternberg (Anal. Chem. 1994; 66: 2409), whereby all the N₂ from the gas products of a chlorinated pyrolysis was eliminated, resulting in purified CO₂ for analysis via a dual‐inlet isotope ratio mass spectrometry system. We evaluated our method by comparing observed δ¹⁸O values with previously published or inter‐laboratory calibrated δ¹⁸O values for five nitrogen‐free working reference materials; finding isotopic agreement to within ±0.2‰ for SIGMA® cellulose, IAEA‐CH3 cellulose (C₆H₁₀O₅) and IAEA‐CH6 sucrose (C₁₂H₂₂O₁₁), and within ±1.8‰ for IAEA‐601 and IAEA‐602 benzoic acids (C₇H₆O₂). We also compared the δ¹⁸O values of IAEA‐CH3 cellulose and IAEA‐CH6 sucrose that was nitrogen‐'doped' with adenine (C₅H₅N₅), imidazole (C₃H₄N₂) and 2‐aminopyrimidine (C₄H₅N₃) with the undoped δ¹⁸O values for the same substrates; yielding isotopic agreement to within ±0.7‰. Finally, we provide an independent analysis of the δ¹⁸O value of IAEA‐600 caffeine (C₈H₁₀N₄O₂), previously characterized using online systems exclusively, and discuss the reasons for an average 1.4‰ enrichment in δ¹⁸O observed offline relative to the consensus online δ¹⁸O value. Copyright © 2010 John Wiley & Sons, Ltd.     

Inter-laboratory calibration of new silver orthophosphate comparison materials for the stable oxygen isotope analysis of phosphates

Stable oxygen isotope compositions (δ¹⁸O values) of two commercial and one synthesized silver orthophosphate reagents have been determined on the VSMOW scale. The analyses were carried out in three different laboratories: lab (1) applying off‐line oxygen extraction in the form of CO₂ which was analyzed on a dual inlet and triple collector isotope ratio mass spectrometer, while labs (2) and (3) employed an isotope ratio mass spectrometer coupled to a high‐temperature conversion/elemental analyzer (TC/EA) where Ag₃PO₄ samples were analyzed as CO in continuous flow mode. The δ¹⁸O values for the proposed new comparison materials were linked to the generally accepted δ¹⁸O values for Vennemann's TU‐1 and TU‐2 standards as well as for Ag₃PO₄ extracted from NBS120c. The weighted average δ¹⁸O_VSMOW values for the new comparison materials UMCS‐1, UMCS‐2 and AGPO‐SCRI were determined to be + 32.60 (± 0.12), + 19.40 (± 0.12) and + 14.58 (± 0.13)‰, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Discrepancies between isotope ratio infrared spectroscopy and isotope ratio mass spectrometry for the stable isotope analysis of plant and soil waters

The use of isotope ratio infrared spectroscopy (IRIS) for the stable hydrogen and oxygen isotope analysis of water is increasing. While IRIS has many advantages over traditional isotope ratio mass spectrometry (IRMS), it may also be prone to errors that do not impact upon IRMS analyses. Of particular concern is the potential for contaminants in the water sample to interfere with the spectroscopy, thus leading to erroneous stable isotope data. Water extracted from plant and soil samples may often contain organic contaminants. The extent to which contaminants may interfere with IRIS and thus impact upon data quality is presently unknown. We tested the performance of IRIS relative to IRMS for water extracted from 11 plant species and one organic soil horizon. IRIS deviated considerably from IRMS for over half of the samples tested, with deviations as large as 46‰ (δ²H) and 15.4‰ (δ¹⁸O) being measured. This effect was reduced somewhat by using activated charcoal to remove organics from the water; however, deviations as large as 35‰ (δ²H) and 11.8‰ (δ¹⁸O) were still measured for these cleaned samples. Interestingly, the use of activated charcoal to clean water samples had less effect than previously thought for IRMS analyses. Our data show that extreme caution is required when using IRIS to analyse water samples that may contain organic contaminants. We suggest that the development of new cleaning techniques for removing organic contaminants together with instrument‐based software to flag potentially problematic samples are necessary to ensure accurate plant and soil water analyses using IRIS. Copyright © 2010 John Wiley & Sons, Ltd.     

Compound‐specific hydrogen isotope analysis of 1,2‐dichloroethane: potential for delineating source and fate of chlorinated hydrocarbon contaminants in groundwater

A Delta Plus XL continuous flow gas chromatography/high‐temperature conversion‐isotope ratio mass spectrometer system (GC‐TC‐IRMS) with a liquid nitrogen trap installed at the end of the micropyrolysis oven was used to measure hydrogen isotope (δ²H) values of 1,2‐dichloroethane (1,2‐DCA). The 1,2‐DCA δ²H values were within uncertainty of the δ²H value for the same 1,2‐DCA analyzed using off‐line sample preparation and conventional dual inlet mass spectrometry, verifying that this system can accurately measure 1,2‐DCA δ²H values. After 71 reproducible and accurate 1,2‐DCA δ²H measurements had been obtained, the standard deviation on the mean of the cumulative 1,2‐DCA δ²H measurements was greater than ±5‰. The cumulative load of chlorine at this point was ～5.5 × 10^?6 moles, which may be the limit to the quantity of chlorine that can be input before the reproducibility of 1,2‐DCA δ²H measurements is compromised. This study is the first to our knowledge to demonstrate a method for obtaining accurate and reproducible compound‐specific δ²H values for chlorinated hydrocarbons at dissolved concentrations typical of field conditions. Paired δ²H and δ¹³C values suggest that dual parameter isotopic measurements can distinguish between different contaminant sources, as well as providing additional constraints on degradation pathways and contaminant remediation. Copyright © 2007 John Wiley & Sons, Ltd.     

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.     

Determination of the stable carbon isotopic compositions of 2‐methyltetrols in ambient aerosols from the Changbai Mountains

Isoprene is one of the most important non‐methane hydrocarbons (NMHCs) in the troposphere: it is a significant precursor of O₃ and it affects the oxidative state of the atmosphere. The diastereoisomeric 2‐methyltetrols, 2‐methylthreitol and 2‐methylerythritol, are marker compounds of the photooxidation products of atmospheric isoprene. In order to obtain valuable information on the δ¹³C value of isoprene in the atmosphere, the stable carbon isotopic compositions of the 2‐methyltetrols in ambient aerosols were investigated. The 2‐methyltetrols were extracted from filter samples and derivatized with methylboronic acid, and the δ¹³C values of the methylboronate derivatives were determined by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). The δ¹³C values of the 2‐methyltetrols were then calculated through a simple mass balance equation between the 2‐methyltetrols, methylboronic acid and the methylboronates. The δ¹³C values of the 2‐methyltetrols in aerosol samples collected at the Changbai Mountain Nature Reserves in eastern China were found to be ?24.66 ± 0.90‰ and ?24.53 ± 1.08‰ for 2‐methylerythritol and 2‐methylthreitol, respectively. Based on the measured isotopic composition of the 2‐methyltetrols, the average δ¹³C value of atmospheric isoprene is inferred to be close to or slightly heavier than ?24.66‰ at the collection site during the sampling period. Copyright © 2010 John Wiley & Sons, Ltd.