High-temperature reversed-phase liquid chromatography coupled to isotope ratio mass spectrometry

Compound‐specific isotope analysis (CSIA) by liquid chromatography coupled to isotope ratio mass spectrometry (LC/IRMS) has until now been based on ion‐exchange separation. In this work, high‐temperature reversed‐phase liquid chromatography was coupled to, and for the first time carefully evaluated for, isotope ratio mass spectrometry (HT‐LC/IRMS) with four different stationary phases. Under isothermal and temperature gradient conditions, the column bleed of XBridge C₁₈ (up to 180 °C), Acquity C₁₈ (up to 200 °C), Triart C₁₈ (up to 150 °C), and Zirchrom PBD (up to 150 °C) had no influence on the precision and accuracy of δ¹³C measurements, demonstrating the suitability of these columns for HT‐LC/IRMS analysis. Increasing the temperature during the LC/IRMS analysis of caffeine on two C₁₈ columns was observed to result in shortened analysis time. The detection limit of HT‐RPLC/IRMS obtained for caffeine was 30 mg L^–1 (corresponding to 12.4 nmol carbon on‐column). Temperature‐programmed LC/IRMS (i) accomplished complete separation of a mixture of caffeine derivatives and a mixture of phenols and (ii) did not affect the precision and accuracy of δ¹³C measurements compared with flow injection analysis without a column. With temperature‐programmed LC/IRMS, some compounds that coelute at room temperature could be baseline resolved and analyzed for their individual δ¹³C values, leading to an important extension of the application range of CSIA. Copyright © 2011 John Wiley & Sons, Ltd.     

An automated method for ‘clumped‐isotope’ measurements on small carbonate samples

Clumped‐isotope geochemistry deals with the state of ordering of rare isotopes in molecules, in particular with their tendency to form bonds with other rare isotopes rather than with the most abundant ones. Among its possible applications, carbonate clumped‐isotope thermometry is the one that has gained most attention because of the wide potential of applications in many disciplines of earth sciences. Clumped‐isotope thermometry allows reconstructing the temperature of formation of carbonate minerals without knowing the isotopic composition of the water from which they were formed. This feature enables new approaches in paleothermometry. The currently published method is, however, limited by sample weight requirements of 10–15 mg and because measurements are performed manually. In this paper we present a new method using an automated sample preparation device coupled to an isotope ratio mass spectrometer. The method is based on the repeated analysis (n = 6–8) of 200 µg aliquots of sample material and completely automated measurements. In addition, we propose to use precisely calibrated carbonates spanning a wide range in Δ₄₇ instead of heated gases to correct for isotope effects caused by the source of the mass spectrometer, following the principle of equal treatment of the samples and standards. We present data for international standards (NBS 19 and LSVEC) and different carbonates formed at temperatures exceeding 600°C to show that precisions in the range of 10 to 15 ppm (1 SE) can be reached for repeated analyses of a single sample. Finally, we discuss and validate the correction procedure based on high‐temperature carbonates instead of heated gases. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Oxygen isotope analysis of phosphate: improved precision using TC/EA CF‐IRMS

Oxygen isotope values of biogenic apatite have long demonstrated considerable promise for paleothermometry potential because of the abundance of material in the fossil record and greater resistance of apatite to diagenesis compared to carbonate. Unfortunately, this promise has not been fully realized because of relatively poor precision of isotopic measurements, and exceedingly small size of some substrates for analysis. Building on previous work, we demonstrate that it is possible to improve precision of δ¹⁸O_PO4 measurements using a ‘reverse‐plumbed’ thermal conversion elemental analyzer (TC/EA) coupled to a continuous flow isotope ratio mass spectrometer (CF‐IRMS) via a helium stream [Correction made here after initial online publication]. This modification to the flow of helium through the TC/EA, and careful location of the packing of glassy carbon fragments relative to the hot spot in the reactor, leads to narrower, more symmetrically distributed CO elution peaks with diminished tailing. In addition, we describe our apatite purification chemistry that uses nitric acid and cation exchange resin. Purification chemistry is optimized for processing small samples, minimizing isotopic fractionation of PO₄^?3 and permitting Ca, Sr and Nd to be eluted and purified further for the measurement of δ⁴⁴Ca and ⁸⁷Sr/⁸⁶Sr in modern biogenic apatite and ¹⁴³Nd/¹⁴⁴Nd in fossil apatite. Our methodology yields an external precision of ± 0.15‰ (1σ) for δ¹⁸O_PO4. The uncertainty is related to the preparation of the Ag₃PO₄ salt, conversion to CO gas in a reversed‐plumbed TC/EA, analysis of oxygen isotopes using a CF‐IRMS, and uncertainty in constructing calibration lines that convert raw δ¹⁸O data to the VSMOW scale. Matrix matching of samples and standards for the purpose of calibration to the VSMOW scale was determined to be unnecessary. Our method requires only slightly modified equipment that is widely available. This fact, and the demonstrated improvement in precision, should help to make apatite paleothermometry far more accessible to paleoclimate researchers. Copyright © 2009 John Wiley & Sons, Ltd.     

Adaptation of continuous‐flow cavity ring‐down spectroscopy for batch analysis of δ13C of CO2 and comparison with isotope ratio mass spectrometry

Measurements of δ¹³C in CO₂ have traditionally relied on samples stored in sealed vessels and subsequently analyzed using magnetic sector isotope ratio mass spectrometry (IRMS), an accurate but expensive and high‐maintenance analytical method. Recent developments in optical spectroscopy have yielded instruments that can measure δ¹³CO₂ in continuous streams of air with precision and accuracy approaching those of IRMS, but at a fraction of the cost. However, continuous sampling is unsuited for certain applications, creating a need for conversion of these instruments for batch operation. Here, we present a flask (syringe) adaptor that allows the collection and storage of small aliquots (20–30 mL air) for injection into the cavity ring‐down spectroscopy (CRDS) instrument. We demonstrate that the adaptor's precision is similar to that of traditional IRMS (standard deviation of 0.3‰ for 385 ppm CO₂ standard gas). In addition, the concentration precision (±0.3% of sample concentration) was higher for CRDS than for IRMS (±7% of sample concentration). Using the adaptor in conjunction with CRDS, we sampled soil chambers and found that soil‐respired δ¹³C varied between two different locations in a piñon‐juniper woodland. In a second experiment, we found no significant discrimination between the respiration of a small beetle (~5 mm) and its diet. Our work shows that the CRDS system is flexible enough to be used for the analysis of batch samples as well as for continuous sampling. This flexibility broadens the range of applications for which CRDS has the potential to replace magnetic sector IRMS. Copyright © 2011 John Wiley & Sons, Ltd.     

New isotope ratio mass spectrometric method of precise δ37Cl determinations

The most precise method of chlorine isotope analysis described to date is based on the isotope ratio mass spectrometry (IRMS) of chlorine quantitatively converted into chloromethane, CH₃Cl. This gas can be produced from several chlorine‐containing compounds and analyzed by IRMS. However, the mass spectrum of chloromethane is rather complicated and the ratio of the most abundant ions (mass‐52/mass‐50) differs from the ³⁷Cl/³⁵Cl isotope ratio. This difference becomes significant when the δ exceeds 10‰. Moreover, the electron ionization source yields approximately 80% of all the ionic species at the useful masses 50 and 52. To overcome these drawbacks, we have devised a negative ion mass spectrometer which retains all the best features of IRMS, including a dual‐inlet system with changeover valve, dual collector assembly and CH₃Cl gas as analyte. In the modified ion source we have replaced the ionization chamber with an electron beam by a metal tube with a hot metal filament inside it. Within this tube the ³⁵Cl^? and ³⁷Cl^? ions are produced with an efficiency dependent on the filament material and its temperature. No other ionic species were found in the mass spectrum except of traces at masses 26 and 28 at ppm levels, probably due to the formation of CN^? and CO^?. The minimal amount of Cl used in our method is of the order of 5 µmol (3 mg AgCl) and the precision is better than 0.005‰ (1σ). 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.     

Validation of deuterium and oxygen18 in urine and saliva samples from children using on‐line continuous‐flow isotope ratio mass spectrometry

The doubly labelled water method is valuable for measuring energy expenditure in humans. It usually involves blood or urine sampling, which might be difficult in neonates and children with cerebral palsy or other disabilities. We therefore aimed to validate a method making use of saliva samples analyzed by automated thermal conversion elemental analyzer in combination with isotope ratio mass spectrometry (TC‐EA/IRMS). The subjects received labelled water orally and urine and saliva samples were collected and analyzed. Deuterium as well as oxygen¹⁸ was measured in one single run using a peak jump method. Excellent linearity was found for measurement of enrichments of deuterium (R² = 0.9999) and oxygen¹⁸ (R² = 0.9999). The intra‐assay precision and the inter‐assay precision of the measurement of two standards were good for both deuterium and oxygen¹⁸. The variation between urine and saliva samples was small (4.83% for deuterium and 2.33% for oxygen¹⁸ n = 40). Saliva sampling is to be preferred, therefore, as it can be easily collected and is non‐invasive. Moreover, its time of production is almost exactly known. The TC‐EA/IRMS method is a good alternative to the more laborious off‐line IRMS measurements. 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.     

Membrane permeation continuous‐flow isotope ratio mass spectrometry for on‐line carbon isotope ratio determination

Gaseous membrane permeation (MP) technologies have been combined with continuous‐flow isotope ratio mass spectrometry for on‐line δ¹³C measurements. The experimental setup of membrane permeation‐gas chromatography/combustion/isotope ratio mass spectrometry (MP‐GC/C/IRMS) quantitatively traps gas streams in membrane permeation experiments under steady‐state conditions and performs on‐line gas transfer into a GC/C/IRMS system. A commercial polydimethylsiloxane (PDMS) membrane sheet was used for the experiments. Laboratory tests using CO₂ demonstrate that the whole process does not fractionate the C isotopes of CO₂. Moreover, the δ¹³C values of CO₂ permeated on‐line give the same isotopic results as off‐line static dual‐inlet IRMS δ¹³C measurements. Formaldehyde generated from aqueous formaldehyde solutions has also been used as the feed gas for permeation experiments and on‐line δ¹³C determination. The feed‐formaldehyde δ¹³C value was pre‐determined by sampling the headspace of the thermostated aqueous formaldehyde solution. Comparison of the results obtained by headspace with those from direct aqueous formaldehyde injection confirms that the headspace sampling does not generate isotopic fractionation, but the permeated formaldehyde analyzed on‐line yields a ¹³C enrichment relative to the feed δ¹³C value, the isotopic fractionation being 1.0026 ± 0.0003. The δ¹³C values have been normalized using an adapted two‐point isotopic calibration for δ¹³C values ranging from ?42 to ?10‰. The MP‐GC/C/IRMS system allows the δ¹³C determination of formaldehyde without chemical derivatization or additional analytical imprecision. Copyright © 2009 John Wiley & Sons, Ltd.     

Kinetic 12C/13C isotope fractionation by invertase: evidence for a small in vitro isotope effect and comparison of two techniques for the isotopic analysis of carbohydrates

The natural ¹³C/¹²C isotope composition (δ¹³C) of plants and organic compounds within plant organs is a powerful tool to understand carbon allocation patterns and the regulation of photosynthetic or respiratory metabolism. However, many enzymatic fractionations are currently unknown, thus impeding our understanding of carbon trafficking pathways within plant cells. One of them is the ¹²C/¹³C isotope effect associated with invertases (EC 3.2.1.26) that are cornerstone enzymes for Suc metabolism and translocation in plants. Another conundrum of isotopic plant biology is the need to measure accurately the specific δ¹³C of individual carbohydrates. Here, we examined two complementary methods for measuring the δ¹³C value of sucrose, glucose and fructose, that is, off‐line high‐performance liquid chromatography (HPLC) purification followed by elemental analysis and isotope ratio mass spectrometry (EA‐IRMS) analysis, and gas chromatography‐combustion (GC‐C)‐IRMS. We also used these methods to determine the in vitro ¹²C/¹³C isotope effect associated with the yeast invertase. Our results show that, although providing more variable values than HPLC～EA‐IRMS, and being sensitive to derivatization conditions, the GC‐C‐IRMS method gives reliable results. When applied to the invertase reaction, both methods indicate that the ¹²C/¹³C isotope effect is rather small and it is not affected by the use of heavy water (D₂O). Copyright © 2009 John Wiley & Sons, Ltd.     

Direct analysis of carbon isotope variability in albumins by liquid flow-injection isotope ratio mass spectrometry

We demonstrate the high precision C isotopic analysis of a series of purified albumins by liquid chromatography-combustion isotope ratio mass spectrometry (IRMS) by using direct aqueous liquid injection. Albumins from 18 species and albumens from chicken and turkey egg were obtained from a commercial source and shown to be of > 98% purity by capillary zone electrophoresis and high-performance liquid chromatography. One microliter of an aqueous protein solution with a total of < 40-pmol protein (2. 5 µg), which contained about 150-nmol C, was injected directly into a flowing stream of high-performance liquid chromatography grade water. The solution passed through a pneumatic nebulizer, was sprayed onto a moving wire, passed through a drying oven, and was combusted in a furnace. After the water of combustion was removed, the resulting CO₂ gas was directed to a high precision IRMS instrument operated in continuous flow mode. The average precision across the 20 samples analyzed was SD(δ ¹³C)=0.45%., and the average accuracy was δ¹³C < 0.4%. compared to aliquots analyzed by conventional preparation by using combustion tubes and dual inlet analysis. The observed isotope ratio range was about ?22.5%. < δ ¹³C_PDB < ?16%. as expected for modern materials from a natural source. These results demonstrate rapid, high precision, and accurate C isotopic analysis of untreated macromolecules in an aqueous stream by liquid source IRMS.     

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.     

99Tc NMR determination of the oxygen isotope content in 18O‐enriched water

⁹⁹Tc NMR has been suggested as an original method of evaluating the content of oxygen isotopes in oxygen‐18‐enriched water, a precursor for the production of radioisotope fluorine‐18 used in positron emission tomography. To this end, solutions of NH₄TcO₄ or NaTcO₄ (up to 0.28 mol/L) with natural abundance of oxygen isotopes in virgin or recycled ¹⁸O‐enriched water have been studied by ⁹⁹Tc NMR. The method is based on ¹⁶O/¹⁷O/¹⁸O intrinsic isotope effects in the ⁹⁹Tc NMR chemical shifts, and the statistical distribution of oxygen isotopes in the coordination sphere of TcO₄⁻ and makes it possible to quantify the composition of enriched water by measuring the relative intensities of the ⁹⁹Tc NMR signals of the Tc¹⁶O_4−n¹⁸O_n⁻ isotopologues. Because the oxygen exchange between TcO₄⁻ and enriched water in neutral and alkaline solutions is characterized by slow kinetics, gaseous HCl was bubbled through a solution for a few seconds to achieve the equilibrium distribution of oxygen isotopes in the Tc coordination sphere without distortion of the oxygen composition of the water. Pertechnetate ion was selected as a probe due to its high stability in solutions and the significant ⁹⁹Tc NMR shift induced by a single ¹⁶O→¹⁸O substitution (−0.43 ± 0.01 ppm) in TcO₄⁻ and spin coupling constant ¹J(⁹⁹Tc–¹⁷O) (131.46 Hz) favourable for the observation of individual signals of Tc¹⁶O_4−n¹⁸O_n⁻ isotopologues.     

18O/16O ratio measurements of inorganic and organic materials by elemental analysis-pyrolysis-isotope ratio mass spectrometry continuous-flow techniques

We have used a high‐precision, easy, low‐cost and rapid method of oxygen isotope analysis applied to various O‐bearing matrices, organic and inorganic (sulfates, nitrates and phosphates), whose ¹⁸O/¹⁶O ratios had already been measured. It was first successfully applied to ¹⁸O analyses of natural and synthetic phosphate samples. The technique uses high‐temperature elemental analysis–pyrolysis (EA‐pyrolysis) interfaced in continuous‐flow mode to an isotope ratio mass spectrometry (IRMS) system. Using the same pyrolysis method we have been able to generate a single calibration curve for all those samples showing pyrolysis efficiencies independent of the type of matrix pyrolysed. We have also investigated this matrix‐dependent pyrolysis issue using a newly developed pyrolysis technique involving 'purge‐and‐trap' chromatography. As previously stated, silver phosphate being a very stable material, weakly hygroscopic and easily synthesized with predictable ¹⁸O/¹⁶O values, could be considered as a good candidate to become a reference material for the determination of ¹⁸O/¹⁶O ratios by EA‐pyrolysis‐IRMS. Copyright © 2011 John Wiley & Sons, Ltd.     

Strong anion‐exchange liquid chromatography coupled with isotope ratio mass spectrometry using a Liquiface interface

The introduction of liquid chromatography coupled with isotope ratio mass spectrometry (LC/IRMS) as an analytical tool for the measurement of isotope ratios in non‐volatile analytes has somewhat simplified the analytical cycle from sample collection to analysis mainly due to the avoidance of the extensive sample processing and derivatisation that were necessary for gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Here we test the performance of coupling strong anion exchange to IRMS using only the second commercially available interface; the Liquiface. The system was modified from installation specification to improve peak resolution in the interface and maintain peak separation from the column to the mass spectrometer. The system performance was assessed by the determination of sensitivity, accuracy and precision attained from carbohydrate separations. The system performed satisfactorily after modifications, resulting in maintenance of peak resolution from column to mass spectrometer. The sensitivity achieved suggested that ～150 ng carbon could be analysed with acceptable precision (<0.3‰). Accuracy was maintained in the interface as determined by correlation with offline techniques, resulting in regression coefficient of r² = 0.98 and a slope of 0.99. The average precision achieved for the separation of seven monosaccharides was 0.36‰. The integration of a carbonate removal device limited the effect of background carbon perturbations in the mass spectrometer associated with eluent gradients, and the coupling of strong anion‐exchange chromatography with IRMS was successfully achieved using the Liquiface. Copyright © 2010 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.     

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.     

Bis­(di‐μ‐acetato‐aqua{2‐[N‐ethyl‐N‐(2‐hydroxy‐4‐methyl­benzyl)­amino­methyl]‐1‐methyl­benz­imid­az­ole}­nickel)­nickel(II) tetra­hy­drate

The previously unknown title compound, tetra‐μ‐ace­tato‐1:2κ²O;1:2κ²O:O′;­2:3κ²O;­2:3κ²O:O′‐di­aqua‐1κO,3κO‐bis­(μ‐2‐{[N‐ethyl‐N‐(2‐hy­droxy‐5‐methylbenzyl)­am­ino]­methyl}‐1‐methyl‐1H‐benz­imid­az­ole)‐1κ³N³,N,O:2κO;3κ³N³,N,O:2κO‐tri­nickel(II) tetra­hy­drate, [Ni₃(C₁₈H₂₂N₃O)₂(C₂H₃O₂)₄(H₂O)₂]·­4H₂O, (I), is a centrosymmetric linear trinuclear nickel(II) complex, where the Ni atoms are in an octahedral coordination and the ligand heteroatoms act so as to model amino acid residues.