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.     

Carbon and oxygen isotope microanalysis of carbonate

Technical modification of the conventional method for the δ¹³C and δ¹⁸O analysis of 10–30 µg carbonate samples is described. The CO₂ extraction is carried out in vacuum using 105% phosphoric acid at 95°C, and the isotopic composition of CO₂ is measured in a helium flow by gas chromatography/isotope ratio mass spectrometry (GC/IRMS). The feed‐motion of samples to the reaction vessel provides sequential dropping of only the samples (without the sample holder) into the acid, preventing the contamination of acid and allowing us to use the same acid to carry out very large numbers of analyses. The high accuracy and high reproducibility of the δ¹³C and δ¹⁸O analyses were demonstrated by measurements of international standards and comparison of results obtained by our method and by the conventional method. Our method allows us to analyze 10 µg of the carbonate with a standard deviation of ±0.05‰ for δ¹³C and δ¹⁸O. The method has been used successfully for the analyses of the oxygen and carbon isotopic composition of the planktonic and benthic foraminifera in detailed palaeotemperature reconstructions of the Okhotsk Sea. 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.     

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.     

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.     

A dynamic soil chamber system coupled with a tunable diode laser for online measurements of δ13C, δ18O,and efflux rate of soil‐respired CO2

High frequency observations of the stable isotopic composition of CO₂ effluxes from soil have been sparse due in part to measurement challenges. We have developed an open‐system method that utilizes a flow‐through chamber coupled to a tunable diode laser (TDL) to quantify the rate of soil CO₂ efflux and its δ¹³C and δ¹⁸O values (δ¹³C_R and δ¹⁸O_R, respectively). We tested the method first in the laboratory using an artificial soil test column and then in a semi‐arid woodland. We found that the CO₂ efflux rates of 1.2 to 7.3 µmol m^?2 s^?1 measured by the chamber‐TDL system were similar to measurements made using the chamber and an infrared gas analyzer (IRGA) (R² = 0.99) and compared well with efflux rates generated from the soil test column (R² = 0.94). Measured δ¹³C and δ¹⁸O values of CO₂ efflux using the chamber‐TDL system at 2 min intervals were not significantly different from source air values across all efflux rates after accounting for diffusive enrichment. Field measurements during drought demonstrated a strong dependency of CO₂ efflux and isotopic composition on soil water content. Addition of water to the soil beneath the chamber resulted in average changes of +6.9 µmol m^?2 s^?1, ?5.0‰, and ?55.0‰ for soil CO₂ efflux, δ¹³C_R and δ¹⁸O_R, respectively. All three variables initiated responses within 2 min of water addition, with peak responses observed within 10 min for isotopes and 20 min for efflux. The observed δ¹⁸O_R was more enriched than predicted from temperature‐dependent H₂O‐CO₂ equilibration theory, similar to other recent observations of δ¹⁸O_R from dry soils (Wingate L, Seibt U, Maseyk K, Ogee J, Almeida P, Yakir D, Pereira JS, Mencuccini M. Global Change Biol. 2008; 14: 2178). The soil chamber coupled with the TDL was found to be an effective method for capturing soil CO₂ efflux and its stable isotope composition at high temporal frequency. Published in 2010 by John Wiley & Sons, Ltd.     

CO2 isotope analyses using large air samples collected on intercontinental flights by the CARIBIC Boeing 767

Analytical details for ¹³C and ¹⁸O isotope analyses of atmospheric CO₂ in large air samples are given. The large air samples of nominally 300 L were collected during the passenger aircraft‐based atmospheric chemistry research project CARIBIC and analyzed for a large number of trace gases and isotopic composition. In the laboratory, an ultra‐pure and high efficiency extraction system and high‐quality isotope ratio mass spectrometry were used. Because direct comparison with other laboratories was practically impossible, the extraction and measurement procedures were tested in considerable detail. Extracted CO₂ was measured twice vs. two different working reference CO₂ gases of different isotopic composition. The two data sets agree well and their distributions can be used to evaluate analytical errors due to isotope measurement, ion corrections, internal calibration consistency, etc. The calibration itself is based on NBS‐19 and also verified using isotope analyses on pure CO₂ gases (NIST Reference Materials (RMs) and NARCIS CO₂ gases). The major problem encountered could be attributed to CO₂‐water exchange in the air sampling cylinders. This exchange decreased over the years. To exclude artefacts due to such isotopic exchange, the data were filtered to reject negative δ¹⁸O(CO₂) values. Examples of the results are given. Copyright © 2009 John Wiley & Sons, Ltd.     

Laser ablation isotope ratio mass spectrometry for enhanced sensitivity and spatial resolution in stable isotope analysis

Stable isotope analysis permits the tracking of physical, chemical, and biological reactions and source materials at a wide variety of spatial scales. We present a laser ablation isotope ratio mass spectrometry (LA‐IRMS) method that enables δ¹³C measurement of solid samples at 50 µm spatial resolution. The method does not require sample pre‐treatment to physically separate spatial zones. We use laser ablation of solid samples followed by quantitative combustion of the ablated particulates to convert sample carbon into CO₂. Cryofocusing of the resulting CO₂ coupled with modulation in the carrier flow rate permits coherent peak introduction into an isotope ratio mass spectrometer, with only 65 ng carbon required per measurement. We conclusively demonstrate that the measured CO₂ is produced by combustion of laser‐ablated aerosols from the sample surface. We measured δ¹³C for a series of solid compounds using laser ablation and traditional solid sample analysis techniques. Both techniques produced consistent isotopic results but the laser ablation method required over two orders of magnitude less sample. We demonstrated that LA‐IRMS sensitivity coupled with its 50 µm spatial resolution could be used to measure δ¹³C values along a length of hair, making multiple sample measurements over distances corresponding to a single day's growth. This method will be highly valuable in cases where the δ¹³C analysis of small samples over prescribed spatial distances is required. Suitable applications include forensic analysis of hair samples, investigations of tightly woven microbial systems, and cases of surface analysis where there is a sharp delineation between different components of a sample. Copyright © 2011 John Wiley & Sons, Ltd.     

A system for high‐quality CO2 isotope analyses of air samples collected by the CARIBIC Airbus A340‐600

In 2007, JRC‐IRMM began a series of atmospheric CO₂ isotope measurements, with the focus on understanding instrumental effects, corrections as well as metrological aspects. The calibration approach at JRC‐IRMM is based on use of a plain CO₂ sample (working reference CO₂) as a calibration carrier and CO₂‐air mixtures (in high‐pressure cylinders) to determine the method‐related correction under actual analytical conditions (another calibration carrier, in the same form as the samples). Although this approach differs from that in other laboratories, it does give a direct link to the primary reference NBS‐19‐CO₂. It also helps to investigate the magnitude and nature for each of the instrumental corrections and allows for the quantification of the uncertainty introduced. Critical tests were focused on the instrumental corrections. It was confirmed that the use of non‐symmetrical capillary crimping (an approach used here to deal with small samples) systematically modifies δ¹³C(CO₂) and δ¹⁸O(CO₂), with a clear dependence on the amount of extracted CO₂. However, the calibration of CO₂‐air mixtures required the use of the symmetrical dual‐inlet mode. As a proof of our approach, we found that δ¹³C(CO₂) on extracts from mixtures agreed (within 0.010‰) with values obtained from the ‘mother’ CO₂ used for the mixtures. It was further found that very low levels of hydrocarbons in the pumping systems and the isotope ratio mass spectrometry (IRMS) instrument itself were critical. The m/z 46 values (consequently the calculated δ¹⁸O(CO₂) values) are affected by several other effects with traces of air co‐trapped with frozen CO₂ being the most critical. A careful cryo‐distillation of the extracted CO₂ is recommended. After extensive testing, optimisation, and routine automated use, the system was found to give precise data on air samples that can be traced with confidence to the primary standards. The typical total combined uncertainty in δ¹³C(CO₂) and δ¹⁸O(CO₂) on the VPDB‐CO₂ scale, estimated on runs of CO₂‐air mixtures, is ±0.040‰ and 0.060‰ (2‐σ values). Inter‐comparison with MPI‐BGC resulted in a scale discrepancy of a similar magnitude. Although the reason(s) for this discrepancy still need to be understood, this basically confirms the approach of using specifically prepared CO₂‐air mixtures as a calibration carrier, in order to achieve scale unification among laboratories. As important practical application and as a critical test, JRC‐IRMM took part in the passenger aircraft‐based global monitoring project CARIBIC ( http://www.caribic‐atmospheric.com ). In this way, reliable CO₂ isotope data for the tropopause region and the free troposphere were obtained. From June 2007 to January 2009, approximately 500 CARIBIC air samples have been analysed. Some flights demonstrated a compact correlation of both δ¹³C(CO₂) and δ¹⁸O(CO₂) with respect to CO₂ concentration, demonstrating mixing of tropospheric and stratospheric air masses. These excellent correlations provide an independent, realistic data quality check. Copyright © 2009 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.     

Effects of GC temperature and carrier gas flow rate on on‐line oxygen isotope measurement as studied by on‐column CO injection

Although deemed important to δ¹⁸O measurement by on‐line high‐temperature conversion techniques, how the GC conditions affect δ¹⁸O measurement is rarely examined adequately. We therefore directly injected different volumes of CO or CO–N₂ mix onto the GC column by a six‐port valve and examined the CO yield, CO peak shape, CO–N₂ separation, and δ¹⁸O value under different GC temperatures and carrier gas flow rates. The results show the CO peak area decreases when the carrier gas flow rate increases. The GC temperature has no effect on peak area. The peak width increases with the increase of CO injection volume but decreases with the increase of GC temperature and carrier gas flow rate. The peak intensity increases with the increase of GC temperature and CO injection volume but decreases with the increase of carrier gas flow rate. The peak separation time between N₂ and CO decreases with an increase of GC temperature and carrier gas flow rate. δ¹⁸O value decreases with the increase of CO injection volume (when half m/z 28 intensity is <3 V) and GC temperature but is insensitive to carrier gas flow rate. On average, the δ¹⁸O value of the injected CO is about 1‰ higher than that of identical reference CO. The δ¹⁸O distribution pattern of the injected CO is probably a combined result of ion source nonlinearity and preferential loss of C¹⁶O or oxygen isotopic exchange between zeolite and CO. For practical application, a lower carrier gas flow rate is therefore recommended as it has the combined advantages of higher CO yield, better N₂–CO separation, lower He consumption, and insignificant effect on δ¹⁸O value, while a higher‐than‐60 °C GC temperature and a larger‐than‐100 µl CO volume is also recommended. When no N₂ peak is expected, a higher GC temperature is recommended, and vice versa. Copyright © 2015 John Wiley & Sons, Ltd.     

A simplified method of preparing phosphoric acid for stable isotope analyses of carbonates

In order to produce CO₂ for stable isotope analyses (δ¹⁸O and δ¹³C), carbonate samples are commonly digested in phosphoric acid. The acid recipe here presented is based on phase shifting crystalline orthophosphoric acid of pro‐analysis quality to a liquid state through heating, followed by pre‐vacuum treatment during a start‐up procedure before mass analyses for common acid bath preparation, or adding a small amount of phosphoric pentoxide for single drop equipments, respectively. This methodology results in a final acid concentration of 104%. Copyright © 2005 John Wiley & Sons, Ltd.     

Determination of δ18O in soils: measuring conditions and a potential application

The stable oxygen isotope signature (δ¹⁸O) of soil is expected to be the result of a mixture of components within the soil with varying δ¹⁸O signatures. Thus, the δ¹⁸O of soils should provide information about the soil's substrate, especially about the relative contribution of organic matter versus minerals. As there is no standard method available for measuring soil δ¹⁸O, the method for the measurement of single components using a high‐temperature conversion elemental analyser (TC/EA) was adapted. We measured δ¹⁸O in standard materials (IAEA 601, IAEA 602, Merck cellulose) and soils (organic and mineral soils) in order to determine a suitable pyrolysis temperature for soil analysis. We consider a pyrolysis temperature suitable when the yield of signal intensity (intensity of mass 28 per 100 µg) is at a maximum and the acquired raw δ¹⁸O signature is constant for the standard materials used and when the quartz signal from the soil is still negligible. After testing several substances within the temperature range of 1075 to 1375°C we decided to use a pyrolysis temperature of 1325°C for further measurements. For the Urseren Valley we have found a sequence of increasing δ¹⁸O signatures from phyllosilicates to upland soils, wetland soils and vegetation. Our measurements show that the δ¹⁸O values of upland soil samples differ significantly from wetland soil samples. The latter can be related to the changing mixing ratio of the mineral and organic constituents of the soil. For wetlands affected by soil erosion, we have found intermediate δ¹⁸O signatures which lie between typical signatures for upland and wetland sites and give evidence for the input of upland soil material through erosion. Copyright © 2008 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.     

An injection method for measuring the carbon isotope content of soil carbon dioxide and soil respiration with a tunable diode laser absorption spectrometer

We present a novel technique in which the carbon isotope ratio (δ¹³C) of soil CO₂ is measured from small gas samples (<5 mL) injected into a stream of CO₂‐free air flowing into a tunable diode laser absorption spectrometer (TDL). This new method extends the dynamic range of the TDL to measure CO₂ mole fractions ranging from ambient to pure CO₂, reduces the volume of sample required to a few mL, and does not require field deployment of the instrument. The measurement precision of samples stored for up to 60 days was 0.23‰. The new TDL method was applied with a simple gas well sampling technique to obtain and measure gas samples from shallow soil depth increments for CO₂ mole fraction and δ¹³C analysis, and subsequent determination of the δ¹³C of soil‐respired CO₂. The method was tested using an artificial soil system containing a controlled CO₂ source and compared with an independent method using the TDL and an open soil chamber. The profile and chamber estimates of δ¹³C of an artificially produced CO₂ flux were consistent and converged to the δ¹³C of the CO₂ source at steady state, indicating the accuracy of both methods under controlled conditions. The new TDL method, in which a small pulse of sample is measured on a carrier gas stream, is analogous for the TDL technique to the development of continuous‐flow configurations for isotope ratio mass spectrometry. While the applications presented here are focused on soil CO₂, this new TDL method could be applied in a number of situations requiring measurement of δ¹³C of CO₂ in small gas samples with ambient to high CO₂ mole fractions. Copyright © 2010 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.     

Comparison of the silver nitrate and bacterial denitrification methods for the determination of nitrogen and oxygen isotope ratios of nitrate in surface water

Nitrogen (N) and oxygen (O) isotope ratios of NO are often used to trace dominant NO pollution sources in water. Both the silver nitrate (AgNO₃) method and the bacterial denitrification method are frequently used analytical techniques to determine δ¹⁵N‐ and δ¹⁸O‐NO in aqueous samples. The AgNO₃ method is applicable for freshwater and requires a concentration of 100–200 µmol of NO for isotope determination. The bacterial denitrification method is applicable for seawater and freshwater and for KCl extracts of soils with a NO concentration as low as 1 µmol. We have carried out a thorough method comparison using 42 real surface water samples having a wide range of δ¹⁵N‐ and δ¹⁸O‐NO values and NO concentrations. Various correction pairs using three international references and blanks were used to correct raw δ¹⁵N‐ and δ¹⁸O‐NO values. No significant difference between the corrected data was observed when using various correction pairs for each analytical method. Both methods also showed excellent repeatability with high intraclass correlation coefficients (ICC). The ICC of the AgNO₃ method was 0.992 for δ¹⁵N and 0.970 for δ¹⁸O. The ICC of the bacterial denitrification method was 0.995 for δ¹⁵N and 0.954 for δ¹⁸O. Moreover, a positive linear relationship with a high correlation coefficient (r ≥ 0.88) between the two methods was found for δ¹⁵N‐ and δ¹⁸O‐NO. The comparability of the methods was assessed by the Bland‐Altman technique using 95% limits of agreement. The average difference between results obtained by the bacterial denitrification and the AgNO₃ method for δ¹⁵N was ?1.5‰ with 95% limits of agreement ?3.6 and +0.5‰. For δ¹⁸O this was +2.0‰, with 95% limits of agreement ?3.3 and +7.3‰. We found that for δ¹⁵N and for δ¹⁸O, 97% of the differences fell within these 95% limits of agreement. In conclusion, the AgNO₃ and the bacterial denitrification methods are highly correlated and statistically interchangeable. Copyright © 2010 John Wiley & Sons, Ltd.     

Simultaneous δ15N, δ13C and δ34S measurements of low‐biomass samples using a technically advanced high sensitivity elemental analyzer connected to an isotope ratio mass spectrometer

Conventional simultaneous CNS stable isotope abundance measurements of solid samples usually require high sample amounts, up to 1 mg carbon, to achieve exact analytical results. This rarely used application is often impaired by high C:S element ratios when organic samples are analyzed and problems such as incomplete conversion into sulphur dioxide occur during analysis. We introduce, as a technical innovation, a high sensitivity elemental analyzer coupled to a conventional isotope ratio mass spectrometer, with which CNS‐stable isotope ratios can be determined simultaneously in samples with low carbon content (<40 µg C corresponding to ～100 µg dry weight). The system includes downsized reactors, a temperature program‐controlled gas chromatography (GC) column and a cryogenic trap to collect small amounts of sulphur dioxide. This modified application allows for highly sensitive measurements in a fully automated operation with standard deviations better than ±0.47‰ for δ¹⁵N and δ³⁴S and ±0.12‰ for δ¹³C (n = 127). Samples collected from one sampling site in a Baltic fjord within a short time period were measured with the new system to get a first impression of triple stable isotope signatures. The results confirm the potential of using δ³⁴S as a stable isotope tracer in combination with δ¹⁵N and δ¹³C measurements to improve discrimination of food sources in aquatic food webs. Copyright © 2009 John Wiley & Sons, Ltd.     

Consistent predictable patterns in the hydrogen and oxygen stable isotope ratios of animal proteins consumed by modern humans in the USA

Published datasets of proteinaceous animal tissues suggest that co‐variation between amino acid hydrogen (δ²H) and oxygen (δ¹⁸O) isotope ratios is a common feature in systems where isotopic variation is driven by geographic or temporal variation in the δ²H and δ¹⁸O values of environmental water. This has led to the development of models relating tissue δ²H and δ¹⁸O values to those of water, with potential application in a number of fields. However, the strength and ubiquity of the influence of environmental water on protein isotope ratios across taxonomic groups, and thus the relevance of predictive models, is an open question. Here we report strong co‐variation of δ²H and δ¹⁸O values across a suite of terrestrial and aquatic animal meats purchased in American food markets, including beef, poultry (chicken and turkey), chicken eggs, pork, lamb, freshwater fish, and marine fish. Significant isotope co‐variation was not found for small collections of marine bivalves and crustaceans. These results imply that isotopic signals from environmental water were propagated similarly through most of the diverse natural and human‐managed foodwebs represented by our samples. Freshwater fish had the largest variation in δ²H and δ¹⁸O values, with ranges of 121 ‰ and 19.2 ‰, respectively, reflecting the large isotopic variation in environmental freshwaters. In contrast marine animals had the smallest variation for both δ²H (7 ‰ range, crustaceans) and δ¹⁸O (3.0 ‰ range, bivalves) values. Known‐origin beef samples demonstrated direct relationships between the variance of environmental water isotope ratios and that of collected meats. Copyright © 2011 John Wiley & Sons, Ltd.     

Comparison of different mass spectrometry techniques in the measurement of L‐[ring‐13C6]phenylalanine incorporation into mixed muscle proteins

Precise measurement of low enrichment of stable isotope labeled amino‐acid tracers in tissue samples is a prerequisite in measuring tissue protein synthesis rates. The challenge of this analysis is augmented when small sample size is a critical factor. Muscle samples from human participants following an 8 h intravenous infusion of L‐[ring‐¹³C₆]phenylalanine and a bolus dose of L‐[ring‐¹³C₆]phenylalanine in a mouse were utilized. Liquid chromatography tandem mass spectrometry (LC/MS/MS), gas chromatography (GC) MS/MS and GC/MS were compared to the GC‐combustion‐isotope ratio MS (GC/C/IRMS), to measure mixed muscle protein enrichment of [ring‐¹³C₆]phenylalanine enrichment. The sample isotope enrichment ranged from 0.0091 to 0.1312 molar percent excess. As compared with GC/C/IRMS, LC/MS/MS, GC/MS/MS and GC/MS showed coefficients of determination of R² = 0.9962 and R² = 0.9942, and 0.9217 respectively. However, the precision of measurements (coefficients of variation) for intra‐assay are 13.0%, 1.7%, 6.3% and 13.5% and for inter‐assay are 9.2%, 3.2%, 10.2% and 25% for GC/C/IRMS, LC/MS/MS, GC/MS/MS and GC/MS, respectively. The muscle sample sizes required to obtain these results were 8 µg, 0.8 µg, 3 µg and 3 µg for GC/C/IRMS, LC/MS/MS, GC/MS/MS and GC/MS, respectively. We conclude that LC/MS/MS is optimally suited for precise measurements of L‐[ring‐¹³C₆]phenylalanine tracer enrichment in low abundance and in small quantity samples. Copyright © 2013 John Wiley & Sons, Ltd.