Oxygen isotope ratio measurements in CO(2) by means of a continuous-wave quantum cascade laser at 4.3 mum

A mid-infrared laser spectrometer was developed for simultaneous high-precision (18)O/(16)O and (17)O/(16)O isotope ratio measurements in carbon dioxide. A continuous-wave, liquid-nitrogen cooled, distributed feedback quantum cascade laser, working at a wavelength of 4.3 microm, was used to probe (12)C(16)O(2), (16)O(12)C(18)O, and (16)O(12)C(17)O lines at ~2311.8 cm(-1). High sensitivity was achieved by means of wavelength modulation spectroscopy with second-harmonic detection. The experimental reproducibility in the short and long terms was deeply investigated through the accurate analysis of a large number of spectra. In particular, we found a short term precision of 0.5 per thousand and 0.6 per thousand, respectively, for (18)O/(16)O and (17)O/(16)O isotope ratios. The occurrence of systematic deviations is also discussed.     

First real-time measurement of the evolving 2H/1H ratio during water evaporation from plant leaves

We have studied the temporal behaviour of the deuterium isotope ratio of water vapour emerging from a freshly cut plant leaf placed in a dry nitrogen atmosphere. The leaf material was placed directly inside the sample gas cell of the stable isotope ratio infrared spectrometer. At the reduced pressure ( approximately 40 mbar) inside the cell, the appearance of water evaporating from the leaf is easily probed by the spectrometer, as well as the evolving isotope ratios, with a precision of about 1 per thousand. The demonstration experiment we describe measures the 2H/1H isotope ratio only, but the experiment can be easily extended to include the 18O/16O and 17O/16O isotope ratios. Plant leaf water isotope ratios provide important information towards quantification of the different components in the ecosystem water and carbon dioxide exchange.     

A laser extraction/combustion technique for in situ delta(13)C analysis of organic and inorganic materials

A CO(2) laser extraction system is described for in situ delta(13)C analysis of organic and inorganic materials. Carbonaceous compounds volatilized by the laser are quantitatively converted to CO(2) gas by a combustion furnace mounted after the sample chamber. Gases produced by the laser and combustion processes are swept by helium carrier gas and separated by a packed gas chromatography column prior to their introduction to an isotope ratio monitoring mass spectrometer. A sample of lentil bean was analyzed at a spatial resolution of 200 μm and yielded delta(13)C values with precision of +/- 0.3 per thousand. The accuracy of delta(13)C measurements was better than +/- 0.5 per thousand from NBS 22 (mineral oil), USGS 24 (graphite), and IAEA CO-1 (calcium carbonate). Copyright 1999 John Wiley & Sons, Ltd.     

Assessment of the amount of body water in the Red Knot (Calidris canutus): an evaluation of the principle of isotope dilution with 2H, (17)O, and (18)O as measured with laser spectrometry and isotope ratio mass spectrometry

We have used the isotope dilution technique to study changes in the body composition of a migratory shorebird species (Red Knot, Calidris canutus) through an assessment of the amount of body water in it. Birds were quantitatively injected with a dose of water with elevated concentrations of 2H, (17)O, and (18)O. Thereafter, blood samples were taken and distilled. The resulting water samples were analysed using an isotope ratio mass spectrometry (for 2H and (18)O only) and a stable isotope ratio infrared laser spectrometry (2H, (17)O, and (18)O) to yield estimates of the amount of body water in the birds, which in turn could be correlated to the amount of body fat. Here, we validate laser spectrometry against mass spectrometry and show that all three isotopes may be used for body water determinations. This opens the way to the extension of the doubly labelled water method, used for the determination of energy expenditure, to a triply labelled water method, incorporating an evaporative water loss correction on a subject-by-subject basis or, alternatively, the reduction of the analytical errors by statistically combining the (17)O and (18)O measurements.     

The position dependent 15N enrichment of nitrous oxide in the stratosphere.

The position dependent 15N fractionation of nitrous oxide (N2O), which cannot be obtained from mass spectrometric analysis on molecular N2O itself, can be determined with high precision using isotope ratio mass spectrometry on the NO+ fragment that is formed on electron impact in the source of an isotope ratio mass spectrometer. Laboratory UV photolysis experiments show that strong position dependent 15N fractionations occur in the photolysis of N2O in the stratosphere, its major atmospheric sink. Measurements on the isotopic composition of stratospheric N2O indeed confirm the presence of strong isotope enrichments, in particular the difference in the fractionation constants for 15N14NO and 14N15NO. The absolute magnitudes of the fractionation constants found in the stratosphere are much smaller, however, than those found in the lab experiments, demonstrating the importance of dynamical and also additional chemical processes like the reaction of N2O with O(1D).     

Routine analysis by high precision gas chromatography/mass selective detector/isotope ratio mass spectrometry to 0.1 parts per mil

Stable isotope methods are potentially quite useful for validating natural or enhanced mineral degradation of contaminants. For this reason, a continuous flow gas chromatograph (GC), isotope ratio mass spectrometer (IRMS) has been coupled with a quadrupole mass selective detector (MSD) to allow simultaneous mass spectral and stable carbon isotope ratio data to be obtained from a single chromatographic analysis. This allows the target contaminant and any extra-cellular degradation intermediates to be both qualified and quantified. Previously acceptable limits of precision (0.3 parts per mil) are undesirable given the small fractionation observed during aerobic degradation. To further understand the fate of organic contaminants and to gain information about the metabolic degradative pathway employed by a microorganism, routine isotopic analyses on a range of analytes have been performed. Quantities of sample producing mass-44 ion beam signal (I(44)) of 2 x 10(-10) to 1 x 10(-8) A were analysed. When the IRMS was tuned for high sensitivity, ion source nonlinearities were overcome by peak height correction from an algorithm that was produced using known isotopic standards of varying concentrations. This led to sample accuracy of <0.01 per thousand and sample precision of 0.1 per thousand. Copyright 1999 John Wiley & Sons, Ltd.     

Oxygen and nitrogen isotopic composition of nitrate in commercial fertilizers,nitric acid,and reagent salts

Nitrate is a key component of synthetic fertilizers that can be beneficial to crop production in agro-ecosystems, but can also cause damage to natural ecosystems if it is exported in large amounts. Stable isotopes, both oxygen and nitrogen, have been used to trace the sources and fate of nitrate in various ecosystems. However, the oxygen isotope composition of synthetic and organic nitrates is poorly constrained. Here, we present a study on the N and O isotope composition of nitrate-based fertilizers. The δ¹⁵N values of synthetic and natural nitrates were 0?±?2?‰ similar to the air N₂ from which they are derived. The δ¹⁸O values of synthetic nitrates were 23?±?3?‰, similar to air O₂, and natural nitrate fertilizer δ¹⁸O values (55?±?5?‰) were similar to those observed in atmospheric nitrate. The Δ¹⁷O values of synthetic fertilizer nitrate were approximately zero following a mass-dependent isotope relationship, while natural nitrate fertilizers had Δ¹⁷O values of 18?±?2?‰ similar to nitrate produced photochemically in the atmosphere. These narrow ranges of values can be used to assess the amount of nitrate arising from fertilizers in mixed systems where more than one nitrate source exists (soil, rivers, and lakes) using simple isotope mixing models.     

Independence of alpine meadow plants from soil organic matter for nitrogen supply: evidence from stable nitrogen isotopes

Stable nitrogen isotope signatures of major sources of mineral nitrogen (mineralization of soil organic nitrogen, biological N(2) fixation by legumes, annual precipitation and plant litter decomposition) were measured to relatively define their individual contribution to grass assimilation at the Haibei Alpine Meadow Ecosystem, Qinghai, China. The results indicated that delta(15)N values (-2.40 per thousand to 0.97 per thousand) of all grasses were much lower than those of soil organic matter (3.4+/-0.18 per thousand) and mineral nitrogen (ammonium and nitrate together, 7.8+/-0.57 per thousand). Based on the patterns of stable nitrogen isotopes, soil organic matter (3.4+/-0.18 per thousand), biological N(2) fixation (0 per thousand), and precipitation (-6.34+/-0.24 per thousand) only contributed to a small fraction of nitrogen requirements of grasses, but plant litter decomposition (-1.31+/-1.01 per thousand) accounted for 67 %.     

Nitrogen-15 fractionation in the thermal decomposition of nitrous oxide of natural isotopic composition

The 15N fractionation in the thermal decomposition of nitrous oxide (N2O) of natural isotopic composition has been investigated in quartz reaction vessel in the temperature interval 888-1073 K. The formulas relating the observed experimentally 15N fractionations with the primary 15N kinetic isotope effect, (k14/k15)p for 14N15N16O, and secondary 15N kinetic isotope effect, (k14/k15)s for 15N14N16O, have been derived. The experimentally estimated 15N kinetic isotope effects have been compared with the primary and secondary 15N kinetic isotope effects calculated with the absolute rate theory formulations applied to linear three atom molecules. A good agreement was found for the primary 15N kinetic isotope effect, (k14/k15)p, in the temperature interval 888-1007 K. But at 1073 K the decompositions of N2O, accompanied by NO (nitric oxide) formation proceed with a twice times smaller primary kinetic isotope effect, (k14/k15)p of 1.0251 +/- 0.0009, only, suggesting the nonlinear transition state structures with participation of the fourth external atom at high temperature decompositions of nitrous oxide. The nitrogen isotope effects determined in this study correlate well with nitrogen isotope fractionations observed in the natural biological, earth and atmospheric processes.     

Box-modeling of bone and tooth phosphate oxygen isotope compositions as a function of environmental and physiological parameters

A time-dependent box model is developed to calculate oxygen isotope compositions of bone phosphate as a function of environmental and physiological parameters. Input and output oxygen fluxes related to body water and bone reservoirs are scaled to the body mass. The oxygen fluxes are evaluated by stoichiometric scaling to the calcium accretion and resorption rates, assuming a pure hydroxylapatite composition for the bone and tooth mineral. The model shows how the diet composition, body mass, ambient relative humidity and temperature may control the oxygen isotope composition of bone phosphate. The model also computes how bones and teeth record short-term variations in relative humidity, air temperature and delta18O of drinking water, depending on body mass. The documented diversity of oxygen isotope fractionation equations for vertebrates is accounted for by our model when for each specimen the physiological and diet parameters are adjusted in the living range of environmental conditions.     

ConFlo III - an interface for high precision delta(13)C and delta(15)N analysis with an extended dynamic range

A newly developed interface coupling a CHN combustion device (elemental analyser 'EA') to an isotope ratio mass spectrometer is described and evaluated. The purpose of the device is to extend the dynamic range of delta(13)C and delta(15)N analysis from less than 2 orders of magnitude to more than 3 orders of magnitude. Carbon isotope ratio measurements of atropine as a model compound have been performed analysing between 1 μg to 5 mg C with acceptable to excellent precision (0.6 to 0.06 per thousand, delta-notation). The correction due to the blank signal is critical for sample amounts smaller than 4 μg C. The maximum sample weight is determined by the combustion capacity of the EA. Larger sample amounts are measured using dilution of a small part of the EA effluent with helium. The dilution mechanism works virtually free of isotope fractionation. Copyright 1999 John Wiley & Sons, Ltd.     

Box-modeling of bone and tooth phosphate oxygen isotope compositions as a function of environmental and physiological parameters*

A time-dependent box model is developed to calculate oxygen isotope compositions of bone phosphate as a function of environmental and physiological parameters. Input and output oxygen fluxes related to body water and bone reservoirs are scaled to the body mass. The oxygen fluxes are evaluated by stoichiometric scaling to the calcium accretion and resorption rates, assuming a pure hydroxylapatite composition for the bone and tooth mineral. The model shows how the diet composition, body mass, ambient relative humidity and temperature may control the oxygen isotope composition of bone phosphate. The model also computes how bones and teeth record short-term variations in relative humidity, air temperature and δ¹⁸O of drinking water, depending on body mass. The documented diversity of oxygen isotope fractionation equations for vertebrates is accounted for by our model when for each specimen the physiological and diet parameters are adjusted in the living range of environmental conditions.     

Isotopic characteristics of meteoric waters in the Belgrade region

The stable isotope composition of hydrogen (delta(2)H) and oxygen (delta(18)O) in monthly precipitation and river water (Sava River and Danube) samples in the Belgrade area gathered between 1992 and 2005 are determined. The local meteoric water line delta(2)H=7.8 (+/-0.2) delta(18)O+7.3(+/-1.6) (r(2)=0.98, n=60, sigma=0.52) for the whole period of observation is close to the global meteoric water line. The amount-weighted mean delta(2)H and delta(18)O values of precipitation were-65+/-27 per thousand and-9.4+/-3.4 per thousand, respectively. Good correlation between delta(18)O values (r approximately >0.67) and ambient temperature and relative humidity was obtained. Stream-water data ranged from-94 to-60 per thousand for delta(2)H and from-11.0 to approximately 5.7 per thousand for delta(18)O with highly statistically significant difference (p>0.01) between the Sava River and the Danube. In addition, the isotopic compositions of local precipitation and adjacent river water at monitoring sites were compared. Obtained data will give an opportunity to improve the knowledge of mixing stream water and local groundwater, and assessment of potential groundwater risks and pressures in the Belgrade basin.     

On-line systems for continuous water and gas isotope ratio measurements

New continuous on-line techniques for water and air extracted from ice cores are developed. Water isotope ratio determination on any of the water phases (water vapour, water, ice) is of great relevance in different research fields, such as climate and paleoclimate studies, geological surveys, and hydrological studies. The conventional techniques for water isotopes are available in different layouts but all of them are rather time-consuming. Here we report new fast on-line techniques that process water as well as ice samples. The analysis time is only approximately 5 min per sample which includes equilibration and processing. Measurement precision and accuracy are better than 0.1 per thousand and 1 per thousand for delta18O and deltaD, respectively, comparable to conventional techniques. The new on-line techniques are able to analyze a wide range of aqueous samples. This allows, for the first time, to make continuous isotope measurements on ice cores. Similarly, continuous and fast analysis of aqueous samples can be of great value for hydrological, geological and perhaps medical applications.Furthermore, a new technique for the on-line analysis of air isotopes extracted from ice cores is developed. This technique allows rapid analyses with high resolution of the main air components nitrogen, oxygen, and argon. Measurement precision is comparable to precisions obtained by conventional techniques. It is now possible to measure delta15N and delta18O(atm) over entire ice cores helping to synchronize chronologies, to assess gas age-ice age differences, and to calibrate the paleothermometry for rapid temperature changes. This new on-line air extraction and analyzing technique complements the water methods in an ideal way as it separates the air from the melt-water of an ice sample. The remaining water waste flux can directly be analyzed by the water methods.     

A modified technique for the preparation of SO2 from sulphates and sulphides for sulphur isotope analyses

A modified technique for the conversion of sulphates and sulphides to SO2 with the mixture of V2O5-SiO2 for sulphur isotopic analyses is described. This technique is more suitable for routine analysis of large number of samples. Modification of the reaction vessel and using manifold inlet system allows to analyse up to 24 samples every day. The modified technique assures the complete yield of SO2, consistent oxygen isotope composition of the SO2 gas and reproducibility of delta34S measurements being within 0.10 per thousand. It is observed, however, oxygen in SO2 produced from sulphides differs in delta18O with respect to that produced from sulphates.     

The Position Dependent 15N Enrichment of Nitrous Oxide in the Stratosphere

Abstract

The position dependent ¹⁵N fractionation of nitrous oxide (N₂O), which cannot be obtained from mass spectrometric analysis on molecular N₂O itself, can be determined with high precision using isotope ratio mass spectrometry on the NO⁺ fragment that is formed on electron impact in the source of an isotope ratio mass spectrometer. Laboratory UV photolysis experiments show that strong position dependent ¹⁵N fractionations occur in the photolysis of N₂O in the stratosphere, its major atmospheric sink. Measurements on the isotopic composition of stratospheric N₂O indeed confirm the presence of strong isotope enrichments, in particular the difference in the fractionation constants for ¹⁵N¹⁴NO and ¹⁴N¹⁵NO. The absolute magnitudes of the fractionation constants found in the stratosphere are much smaller, however, than those found in the lab experiments, demonstrating the importance of dynamical and also additional chemical processes like the reaction of N₂O with O(¹D).     

Barium isotope fractionation during the experimental transformation of aragonite to witherite and of gypsum to barite,and the effect of ion (de)solvation

In this study, we present the experimental results for stable barium (Ba) isotope fractionation (¹³⁷Ba/¹³⁴Ba) during the transformation of aragonite (CaCO₃) and gypsum (CaSO₄·2H₂O) in Ba-bearing aqueous solution to witherite (BaCO₃) and barite (BaSO₄), respectively. The process was studied at three temperatures between 4 and 60?°C. In all cases, the transformation leads to a relative enrichment of the lighter ¹³⁴Ba isotope in the solid compared to the aqueous solution, with ^137/134Ba enrichment factors between –0.11 and ?0.17?‰ for BaCO₃, and –0.21 and –0.26?‰ for BaSO₄. The corresponding mass-dependent ^138/134Ba enrichment factors are ?0.15 to –0.23?‰ for BaCO₃, and –0.28 to –0.35?‰ for BaSO₄. The magnitude of isotope fractionation is within the range of recent reports for witherite and barite formation, as well as trace Ba incorporation into orthorhombic aragonite, and no substantial impact of temperature can be found between 4 and 80?°C. In previous studies, ion (de)solvation has been suggested to impact both the crystallization process of Ba-bearing solids and associated Ba isotope fractionation. Precipitation experiments of BaSO₄ and BaCO₃ using an methanol-containing aqueous solution indicate only a minor effect of ion and crystal surface (de)solvation on the overall Ba isotope fractionation process.     

A STEM/EELS method for mapping iron valence ratios in oxide minerals

The valence state of iron in minerals has useful applications in the geosciences for estimating redox conditions during mineral formation or re-equilibration. STEM/EELS techniques offer the advantage over other methods of being able to measure Fe valence with very high spatial resolution across mineral grains and grain boundaries. We have modified an EELS method for point analyses of iron valence ratios (Fe(3+)/SigmaFe) making it possible to generate line scans and maps of Fe valence. We demonstrate this method with measurements at an interface between iron-bearing oxides in a finely intergrown sample of magnetite and ilmenite. The STEM/EELS method is based on a calibrated relationship between Fe(3+)/SigmaFe and the relative intensities of the Fe L(3) and L(2) white lines in core energy-loss spectra for oxide and silicate minerals. Our method overcomes problems of energy drift in spectrum images by aligning energy-loss edges at a fixed energy position prior to background removal. An automated routine for batch processing of core loss spectra, including additional background removal and calculation of Fe L(3)/L(2) intensity ratios, allows for rapid Fe(3+)/SigmaFe determinations of multiple point analyses or spectrum images and the preparation of Fe valence maps, with an analytical error of +/-0.05 to +/-0.09 in the Fe(3+)/SigmaFe measurements.