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USGS2002

δ 13 C and δ 18 O isotopic composition of CaCO 3 measured by continuous flow isotope ratio mass spectrometry: statistical evaluation and verification by application to Devils Hole core DH-11 calcite

A new method was developed to analyze the stable carbon and oxygen isotope ratios of small samples (400 ± 20 µg) of calcium carbonate. This new method streamlines the classical phosphoric acid/calcium carbonate (H 3 PO 4 /CaCO 3 ) reaction method by making use of a recently available Thermoquest-Finnigan GasBench II preparation device and a Delta Plus XL continuous flow isotope ratio mass spectrometer. Conditions for which the H 3 PO 4 /CaCO 3 reaction produced reproducible and accurate results with minimal error had to be determined. When the acid/carbonate reaction temperature was kept at 26 °C and the reaction time was between 24 and 54 h, the precision of the carbon and oxygen isotope ratios for pooled samples from three reference standard materials was ≤0.1 and ≤0.2 per mill or ‰, respectively, although later analysis showed that materials from one specific standard required reaction time between 34 and 54 h for δ 18 O to achieve this level of precision. Aliquot screening methods were shown to further minimize the total error. The accuracy and precision of the new method were analyzed and confirmed by statistical analysis. The utility of the method was verified by analyzing calcite from Devils Hole, Nevada, for which isotope-ratio values had previously been obtained by the classical method. Devils Hole core DH-11 recently had been re-cut and re-sampled, and isotope-ratio values were obtained using the new method. The results were comparable with those obtained by the classical method with correlation = +0.96 for both isotope ratios. The consistency of the isotopic results is such that an alignment offset could be identified in the re-sampled core material, and two cutting errors that occurred during re-sampling then were confirmed independently. This result indicates that the new method is a viable alternative to the classical reaction method. In particular, the new method requires less sample material permitting finer resolution and allows automation of some processes resulting in considerable time savings.

Rapid Communications in Mass Spectrometry↗
USGS2021

USGS44, a new high-purity calcium carbonate reference material for δ13C measurements

Rationale The stable carbon isotopic ( δ 13 C) reference material (RM) LSVEC Li 2 CO 3 has been found to be unsuitable for δ 13 C standardization work because its δ 13 C value increases with exposure to atmospheric CO 2 . A new CaCO 3 RM, USGS44, has been prepared to alleviate this situation. Methods USGS44 was prepared from 8 kg of Merck high-purity CaCO 3 . Two sets of δ 13 C values of USGS44 were determined. The first set of values was determined by online combustion, continuous-flow (CF) isotope-ratio mass spectrometry (IRMS) of NBS 19 CaCO 3 ( δ 13 C VPDB = +1.95 milliurey (mUr) exactly, where mUr = 0.001 = 1‰), and LSVEC Li 2 CO 3 ( δ 13 C VPDB = −46.6 mUr exactly), and normalized to the two-anchor δ 13 C VPDB-LSVEC isotope-delta scale. The second set of values was obtained by dual-inlet (DI)-IRMS of CO 2 evolved by reaction of H 3 PO 4 with carbonates, corrected for cross contamination, and normalized to the single-anchor δ 13 C VPDB scale. Results USGS44 is stable and isotopically homogeneous to within 0.02 mUr in 100-μg amounts. It has a δ 13 C VPDB-LSVEC value of −42.21 ± 0.05 mUr. Single-anchor δ 13 C VPDB values of −42.08 ± 0.01 and −41.99 ± 0.02 mUr were determined by DI-IRMS with corrections for cross contamination. Conclusions The new high-purity, well-homogenized calcium carbonate isotopic reference material USGS44 is stable and has a δ 13 C VPDB-LSVEC value of −42.21 ± 0.05 mUr for both EA/IRMS and DI-IRMS measurements. As a carbonate relatively depleted in 13 C, it is intended for daily use as a secondary isotopic reference material to normalize stable carbon isotope delta measurements to the δ 13 C VPDB-LSVEC scale. It is useful in quantifying drift with time, determining mass-dependent isotopic fractionation (linearity correction), and adjusting isotope-ratio-scale contraction. Due to its fine grain size (smaller than 63 μm), it is not suitable as a δ 18 O reference material. A δ 13 C VPDB-LSVEC value of −29.99 ± 0.05 mUr was determined for NBS 22 oil.

Rapid Communications in Mass Spectrometry↗
USGS2018

Seeking excellence: An evaluation of 235 international laboratories conducting water isotope analyses by isotope-ratio and laser-absorption spectrometry

Rationale Water stable isotope ratios ( δ 2 H and δ 18 O values) are widely used tracers in environmental studies; hence, accurate and precise assays are required for providing sound scientific information. We tested the analytical performance of 235 international laboratories conducting water isotope analyses using dual-inlet and continuous-flow isotope ratio mass spectrometers and laser spectrometers through a water isotope inter-comparison test. Methods Eight test water samples were distributed by the IAEA to international stable isotope laboratories. These consisted of a core set of five samples spanning the common δ -range of natural waters, and three optional samples (highly depleted, enriched, and saline). The fifth core sample contained unrevealed trace methanol to assess analyst vigilance to the impact of organic contamination on water isotopic measurements made by all instrument technologies. Results For the core and optional samples ~73 % of laboratories gave acceptable results within 0.2 ‰ and 1.5 ‰ of the reference values for δ 18 O and δ 2 H, respectively; ~27 % produced unacceptable results. Top performance for δ 18 O values was dominated by dual-inlet IRMS laboratories; top performance for δ 2 H values was led by laser spectrometer laboratories. Continuous-flow instruments yielded comparatively intermediate results. Trace methanol contamination of water resulted in extreme outlier δ -values for laser instruments, but also affected reactor-based continuous-flow IRMS systems; however, dual-inlet IRMS δ -values were unaffected. Conclusions Analysis of the laboratory results and their metadata suggested inaccurate or imprecise performance stemmed mainly from skill- and knowledge-based errors including: calculation mistakes, inappropriate or compromised laboratory calibration standards, poorly performing instrumentation, lack of vigilance to contamination, or inattention to unreasonable isotopic outcomes. To counteract common errors, we recommend that laboratories include 1–2 'known' control standards in all autoruns; laser laboratories should screen each autorun for spectral contamination; and all laboratories should evaluate whether derived d- excess values are realistic when both isotope ratios are measured. Combined, these data evaluation strategies should immediately inform the laboratory about fundamental mistakes or compromised samples.

Rapid Communications in Mass Spectrometry↗
USGS2015

LIMS for Lasers 2015 for achieving long-term accuracy and precision of δ2H, δ17O, and δ18O of waters using laser absorption spectrometry

Rationale Although laser absorption spectrometry (LAS) instrumentation is easy to use, its incorporation into laboratory operations is not easy, owing to extensive offline manipulation of comma-separated-values files for outlier detection, between-sample memory correction, nonlinearity ( δ -variation with water amount) correction, drift correction, normalization to VSMOW-SLAP scales, and difficulty in performing long-term QA/QC audits. Methods A Microsoft Access relational-database application, LIMS (Laboratory Information Management System) for Lasers 2015, was developed. It automates LAS data corrections and manages clients, projects, samples, instrument-sample lists, and triple-isotope ( δ 17 O, δ 18 O, and δ 2 H values) instrumental data for liquid-water samples. It enables users to (1) graphically evaluate sample injections for variable water yields and high isotope-delta variance; (2) correct for between-sample carryover, instrumental drift, and δ nonlinearity; and (3) normalize final results to VSMOW-SLAP scales. Results Cost-free LIMS for Lasers 2015 enables users to obtain improved δ 17 O, δ 18 O, and δ 2 H values with liquid-water LAS instruments, even those with under-performing syringes. For example, LAS δ 2 H VSMOW measurements of USGS50 Lake Kyoga (Uganda) water using an under-performing syringe having ±10 % variation in water concentration gave +31.7 ± 1.6 ‰ (2-σ standard deviation), compared with the reference value of +32.8 ± 0.4 ‰, after correction for variation in δ value with water concentration, between-sample memory, and normalization to the VSMOW-SLAP scale. Conclusions LIMS for Lasers 2015 enables users to create systematic, well-founded instrument templates, import δ 2 H, δ 17 O, and δ 18 O results, evaluate performance with automatic graphical plots, correct for δ nonlinearity due to variable water concentration, correct for between-sample memory, adjust for drift, perform VSMOW-SLAP normalization, and perform long-term QA/QC audits easily. Published in 2015. This article is a U.S. Government work and is in the public domain in the USA.

Rapid Communications in Mass Spectrometry↗
USGS2003

Oxygen isotopes in nitrate: New reference materials for 18O:17O:16O measurements and observations on nitrate-water equilibration

Despite a rapidly growing literature on analytical methods and field applications of O isotope-ratio measurements of NO 3 − in environmental studies, there is evidence that the reported data may not be comparable because reference materials with widely varying δ 18 O values have not been readily available. To address this problem, we prepared large quantities of two nitrate salts with contrasting O isotopic compositions for distribution as reference materials for O isotope-ratio measurements: USGS34 (KNO 3 ) with low δ 18 O and USGS35 (NaNO 3 ) with high δ 18 O and ‘mass-independent’ δ 17 O. The procedure used to produce USGS34 involved equilibration of HNO 3 with 18 O-depleted meteoric water. Nitric acid equilibration is proposed as a simple method for producing laboratory NO 3 − reference materials with a range of δ 18 O values and normal (mass-dependent) 18 O: 17 O: 16 O variation. Preliminary data indicate that the equilibrium O isotope-fractionation factor ( α ) between [NO 3 − ] and H 2 O decreases with increasing temperature from 1.0215 at 22°C to 1.0131 at 100°C. USGS35 was purified from the nitrate ore deposits of the Atacama Desert in Chile and has a high 17 O: 18 O ratio owing to its atmospheric origin. These new reference materials, combined with previously distributed NO 3 − isotopic reference materials IAEA-N3 (=IAEA-NO-3) and USGS32, can be used to calibrate local laboratory reference materials for determining offset values, scale factors, and mass-independent effects on N and O isotope-ratio measurements in a wide variety of environmental NO 3 − samples. Preliminary analyses yield the following results (normalized with respect to VSMOW and SLAP, with reproducibilities of ±0.2–0.3‰, 1 σ ): IAEA-N3 has δ 18 O = +25.6‰ and δ 17 O = +13.2‰; USGS32 has δ 18 O = +25.7‰; USGS34 has δ 18 O = −27.9‰ and δ 17 O = −14.8‰; and USGS35 has δ 18 O = +57.5‰ and δ 17 O = +51.5‰.

Rapid Communications in Mass Spectrometry↗
USGS2004

Using dual-bacterial denitrification to improve δ15N determinations of nitrates containing mass-independent 17O

The bacterial denitrification method for isotopic analysis of nitrate using N 2 O generated from Pseudomonas aureofaciens may overestimate δ 15 N values by as much as 1–2‰ for samples containing atmospheric nitrate because of mass-independent 17 O variations in such samples. By analyzing such samples for δ 15 N and δ 18 O using the denitrifier Pseudomonas chlororaphis , one obtains nearly correct δ 15 N values because oxygen in N 2 O generated by P. chlororaphis is primarily derived from H 2 O. The difference between the apparent δ 15 N value determined with P. aureofaciens and that determined with P. chlororaphis , assuming mass-dependent oxygen isotopic fractionation, reflects the amount of mass-independent 17 O in a nitrate sample. By interspersing nitrate isotopic reference materials having substantially different δ 18 O values with samples, one can normalize oxygen isotope ratios and determine the fractions of oxygen in N 2 O derived from the nitrate and from water with each denitrifier. This information can be used to improve δ 15 N values of nitrates having excess 17 O. The same analyses also yield estimates of the magnitude of 17 O excess in the nitrate (expressed as Δ 17 O) that may be useful in some environmental studies. The 1- σ uncertainties of δ 15 N, δ 18 O and Δ 17 O measurements are ±0.2, ±0.3 and ±5‰, respectively.

Rapid Communications in Mass Spectrometry↗
USGS2008

Effects of nitrate and water on the oxygen isotopic analysis of barium sulfate precipitated from water samples

BaSO 4 precipitated from mixed salt solutions by common techniques for SO isotopic analysis may contain quantities of H 2 O and NO that introduce errors in O isotope measurements. Experiments with synthetic solutions indicate that δ 18 O values of CO produced by decomposition of precipitated BaSO 4 in a carbon reactor may be either too low or too high, depending on the relative concentrations of SO and NO and the δ 18 O values of the H 2 O, NO , and SO . Typical δ 18 O errors are of the order of 0.5 to 1‰ in many sample types, and can be larger in samples containing atmospheric NO , which can cause similar errors in δ 17 O and Δ 17 O. These errors can be reduced by (1) ion chromatographic separation of SO from NO , (2) increasing the salinity of the solutions before precipitating BaSO 4 to minimize incorporation of H 2 O, (3) heating BaSO 4 under vacuum to remove H 2 O, (4) preparing isotopic reference materials as aqueous samples to mimic the conditions of the samples, and (5) adjusting measured δ 18 O values based on amounts and isotopic compositions of coexisting H 2 O and NO . These procedures are demonstrated for SO isotopic reference materials, synthetic solutions with isotopically known reagents, atmospheric deposition from Shenandoah National Park, Virginia, USA, and sulfate salt deposits from the Atacama Desert, Chile, and Mojave Desert, California, USA. These results have implications for the calibration and use of O isotope data in studies of SO sources and reaction mechanisms.

Rapid Communications in Mass Spectrometry↗
USGS2009

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) 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 ( δ 18 O of +78.91‰) and two barium sulfates (one depleted and one enriched in 18 O) were prepared and calibrated relative to VSMOW2 and SLAP reference waters. These materials were used to calibrate the other isotopic reference materials in this study, which yielded: Reference material δ 18 O and estimated combined uncertainty IAEA-602 benzoic acid +71.28 ± 0.36‰ USGS35 sodium nitrate +56.81 ± 0.31‰ IAEA-NO-3 potassium nitrate +25.32 ± 0.29‰ IAEA-601 benzoic acid +23.14 ± 0.19‰ IAEA-SO-5 barium sulfate +12.13 ± 0.33‰ NBS 127 barium sulfate +8.59 ± 0.26‰ VSMOW2 water 0‰ IAEA-600 caffeine −3.48 ± 0.53‰ IAEA-SO-6 barium sulfate −11.35 ± 0.31‰ USGS34 potassium nitrate −27.78 ± 0.37‰ SLAP water −55.5‰ The seemingly large estimated combined uncertainties arise from differences in instrumentation and methodology and difficulty in accounting for all measurement bias. They are composed of the 3-fold standard errors directly calculated from the measurements and provision for systematic errors discussed in this paper. A primary conclusion of this study is that nitrate samples analyzed for δ 18 O should be analyzed with internationally distributed isotopic nitrates, and likewise for sulfates and organics. Authors reporting relative differences of oxygen-isotope ratios ( δ 18 O) of nitrates, sulfates, or organic material should explicitly state in their reports the δ 18 O values of two or more internationally distributed nitrates (USGS34, IAEA-NO-3, and USGS35), sulfates (IAEA-SO-5, IAEA-SO-6, and NBS 127), or organic material (IAEA-601 benzoic acid, IAEA-602 benzoic acid, and IAEA-600 caffeine), as appropriate to the material being analyzed, had these reference materials been analyzed with unknowns. This procedure ensures that readers will be able to normalize the δ 18 O values at a later time should it become necessary. The high-temperature reduction technique for analyzing δ 18 O and δ 2 H is not as widely applicable as the well-established combustion technique for carbon and nitrogen stable isotope determination. To obtain the most reliable stable isotope data, materials should be treated in an identical fashion; within the same sequence of analyses, samples should be compared with working reference materials that are as similar in nature and in isotopic composition as feasible.

Rapid Communications in Mass Spectrometry↗
USGS2010

Novel silver tubing method for quantitative introduction of water into high temperature conversion systems for stable hydrogen and oxygen isotopic measurements

A new method to seal water in silver tubes for use in a TC/EA reduction unit using a semi-automated sealing apparatus can yield reproducibilities (1 standard deviation) of δ 2 H and &delta 18 O measurements of 1.0 ‰ and 0.06 ‰, respectively. These silver tubes containing reference waters may be preferred for calibration of H- and O-bearing materials analyzed with a TC/EA reduction unit. The new sealing apparatus employs a computer controlled stepping motor to produce silver tubes identical in length. The reproducibility of mass of water sealed in tubes (in a range of 200 to 400 µg) can be as good as 1 percent. Although silver tubes sealed with reference waters are robust and can be shaken or heated to 110 °C with no loss of integrity, they should not be frozen because the expansion during the phase transition of water to ice will break the cold seals and all water will be lost. They should be shipped in insulated containers. This new method eliminates air inclusions and isotopic fractionation of water associated with the loading of water into capsules using a syringe. The method is also more than an order of magnitude faster than preparing water samples in ordinary Ag capsules. Nevertheless, some laboratories may prefer loading water into silver capsules because expensive equipment is not needed, but they are cautioned to apply the necessary corrections for evaporation, back exchange with laboratory atmospheric moisture, and blank.

Rapid Communications in Mass Spectrometry↗
USGS2010

Applying the silver-tube introduction method for thermal conversion elemental analyses and a new δ 2 H value for NBS 22 oil

The δ 2 H VSMOW–SLAP value of total hydrogen of the international measurement standard NBS 22 oil was determined by a new method of sealing water in silver tubes for use in a thermal conversion elemental analysis (TC/EA) reduction unit. The isotopic fractionation of water due to evaporation is virtually non-existent in this silver-tube method. A new value for the δ 2 H VSMOW–SLAP of NBS 22 oil, calibrated with isotopic reference waters, was determined to be −116.9 ± 0.8‰ (1σ and n = 31).

Rapid Communications in Mass Spectrometry↗
USGS2010

A counter-intuitive approach to calculating non-exchangeable 2 H isotopic composition of hair: treating the molar exchange fraction f E as a process-related rather than compound-specific variable

Hair is a keratinous tissue that incorporates hydrogen from material that an animal consumes but it is metabolically inert following synthesis. The stable hydrogen isotope composition of hair has been used in ecological studies to track migrations of mammals as well as for forensic and archaeological purposes to determine the provenance of human remains or the recent geographic life trajectory of living people. Measurement of the total hydrogen isotopic composition of a hair sample yields a composite value comprised of both metabolically informative, non-exchangeable hydrogen and exchangeable hydrogen, with the latter reflecting ambient or sample preparation conditions. Neither of these attributes is directly measurable, and the non-exchangeable hydrogen composition is obtained by estimation using a commonly applied mathematical expression incorporating sample measurements obtained from two distinct equilibration procedures. This commonly used approach treats the fraction of exchangeable hydrogen as a mixing ratio, with a minimal procedural fractionation factor assumed to be close or equal to 1. Instead, we propose to use full molar ratios to derive an expression for the non-exchangeable hydrogen composition explicitly as a function of both the procedural fractionation factor α and the molar hydrogen exchange fraction f E . We apply these derivations in a longitudinal study of a hair sample and demonstrate that the molar hydrogen exchange fraction f E should, like the procedural fractionation factor α, be treated as a process-dependent parameter, i.e. a reaction-specific constant. This is a counter-intuitive notion given that maximum theoretical values for the molar hydrogen exchange fraction f E can be calculated that are arguably protein-type specific and, as such, f E could be regarded as a compound-specific constant. We also make some additional suggestions for future approaches to determine the non-exchangeable hydrogen composition of hair and the use of standards.

Rapid Communications in Mass Spectrometry↗
USGS2011

Improved online δ18O measurements of nitrogen‐ and sulfur‐bearing organic materials and a proposed analytical protocol

It is well known that N 2 in the ion source of a mass spectrometer interferes with the CO background during the δ 18 O measurement of carbon monoxide. A similar problem arises with the high‐temperature conversion (HTC) analysis of nitrogenous O‐bearing samples (e.g. nitrates and keratins) to CO for δ 18 O measurement, where the sample introduces a significant N 2 peak before the CO peak, making determination of accurate oxygen isotope ratios difficult. Although using a gas chromatography (GC) column longer than that commonly provided by manufacturers (0.6 m) can improve the efficiency of separation of CO and N 2 and using a valve to divert nitrogen and prevent it from entering the ion source of a mass spectrometer improved measurement results, biased δ 18 O values could still be obtained. A careful evaluation of the performance of the GC separation column was carried out. With optimal GC columns, the δ 18 O reproducibility of human hair keratins and other keratin materials was better than ±0.15 ‰ (n = 5; for the internal analytical reproducibility), and better than ±0.10 ‰ (n = 4; for the external analytical reproducibility).

Rapid Communications in Mass Spectrometry↗
USGS2011

Guidelines and recommended terms for expression of stable-isotope-ratio and gas-ratio measurement results

To minimize confusion in the expression of measurement results of stable isotope and gas-ratio measurements, recommendations based on publications of the Commission on Isotopic Abundances and Atomic Weights of the International Union of Pure and Applied Chemistry (IUPAC) are presented. Whenever feasible, entries are consistent with the Système International d'Unités, the SI (known in English as the International System of Units), and the third edition of the International Vocabulary of Basic and General Terms in Metrology (VIM, 3rd edition). The recommendations presented herein are approved by the Commission on Isotopic Abundances and Atomic Weights and are designed to clarify expression of quantities related to measurement of isotope and gas ratios to ensure that quantity equations instead of numerical value equations are used for quantity definitions. Examples of column headings consistent with quantity calculus (also called the algebra of quantities) and examples of various deprecated usages connected with the terms recommended are presented.

Rapid Communications in Mass Spectrometry↗
USGS2011

The effects of α-cellulose extraction and blue-stain fungus on retrospective studies of carbon and oxygen isotope variation in live and dead trees†

Tree-ring carbon and oxygen isotope ratios from live and recently dead trees may reveal important mechanisms of tree mortality. However, wood decay in dead trees may alter the δ 13 C and δ 18 O values of whole wood obscuring the isotopic signal associated with factors leading up to and including physiological death. We examined whole sapwood and α-cellulose from live and dead specimens of ponderosa pine ( Pinus ponderosa ), one-seed juniper ( Juniperous monosperma ), piñon pine ( Pinus edulis ) and white fir ( Abies concolor ), including those with fungal growth and beetle frass in the wood, to determine if α-cellulose extraction is necessary for the accurate interpretation of isotopic compositions in the dead trees. We found that the offset between the δ 13 C or δ 18 O values of α-cellulose and whole wood was the same for both live and dead trees across a large range of inter-annual and regional climate differences. The method of α-cellulose extraction, whether Leavitt-Danzer or Standard Brendel modified for small samples, imparts significant differences in the δ 13 C (up to 0.4‰) and δ 18 O (up to 1.2‰) of α-cellulose, as reported by other studies. There was no effect of beetle frass or blue-stain fungus ( Ophiostoma ) on the δ 13 C and δ 18 O of whole wood or α-cellulose. The relationships between whole wood and α-cellulose δ 13 C for ponderosa, piñon and juniper yielded slopes of ~1, while the relationship between δ 18 O of whole wood and α-cellulose was less clear. We conclude that there are few analytical or sampling obstacles to retrospective studies of isotopic patterns of tree mortality in forests of the western United States.

Rapid Communications in Mass Spectrometry↗
USGS2002

Oxygen isotope corrections for online δ34S analysis

Elemental analyzers have been successfully coupled to stable-isotope-ratio mass spectrometers for online measurements of the δ34S isotopic composition of plants, animals and soils. We found that the online technology for automated δ34S isotopic determinations did not yield reproducible oxygen isotopic compositions in the SO2 produced, and as a result calculated δ34S values were often 1–3‰ too high versus their correct values, particularly for plant and animal samples with high C/S ratio. Here we provide empirical and analytical methods for correcting the S isotope values for oxygen isotope variations, and further detail a new SO2-SiO2 buffering method that minimizes detrimental oxygen isotope variations in SO2.

Rapid Communications in Mass Spectrometry↗
USGS2014

Isotopic nitrogen in fecal fiber as an indicator of winter diet in caribou and muskoxen

RATIONALE: The ratios of stable nitrogen isotopes (δ 15 N values) in excreta have been used to examine aspects of trophic and nutritional ecology across taxa. Nitrogen fractions in feces of herbivores include endogenous (e.g., sloughed intestinal cells, unresorbed digestive secretions, and microbial debris) and dietary sources. For animals such as large herbivores, that have diets and feces with high concentrations of indigestible fiber, endogenous 15 N may constrain the use of fecal δ 15 N values to estimate dietary δ 15 N values and reconstruct diets. METHODS: We compared two techniques (detergent and detergent-free) to isolate fractions of plant fibers in the forages of caribou ( Rangifer tarandus ) and muskoxen ( Ovibos moschatus ); estimated the discrimination factors between the δ 15 N values of fecal fiber residues and of the diets of captive animals; and used the more effective isotopic tracer of dietary δ 15 N values to examine the relationships between the δ 15 N values of fecal residues and diet composition in several populations of wild caribou and muskoxen throughout North America in winter. RESULTS: The detergent-based approach contaminated the fractions of plant fibers in forages and feces with 14 N, whereas the detergent-free method was a good proxy to estimate δ 15 N values of plant fibers (r 2 = 0.92) and provided a better estimate of the fecal-fiber to diet discrimination factor for both species (caribou = 3.6‰; muskoxen = 2.8‰). In wild populations, the δ 15 N values of fecal fibers reflected diet composition in muskoxen (adjusted R 2 = 0.43) but not caribou (adjusted R 2 = 0.06). CONCLUSIONS: Contamination from detergent residues prohibited the use of detergent extraction in isolating forage 15 N from endogenous 15 N in the feces of herbivores. Although δ 15 N values in fecal fibers can be used to track dietary δ 15 N values in wild herbivores, discrimination between fecal extracts and diet may vary with the contribution of endogenous nitrogen (N), and, therefore, residual endogenous 15 N in feces may limit dietary reconstructions from fecal δ 15 N values for some large herbivores.

Alaska↗
USGS2014

Caution on the use of NBS 30 biotite for hydrogen-isotope measurements with on-line high-temperature conversion systems

RATIONALE The supply of NBS 30 biotite is nearly exhausted. During measurements of NBS 30 and potential replacements, reproducible δ 2 H VSMOW-SLAP values could not be obtained by three laboratories using high-temperature conversion (HTC) systems. The cause of this issue has been investigated using the silver-tube technique for hydrogen-isotope measurements of water. METHODS The δ 2 H VSMOW-SLAP values of NBS 30 biotite, other biotites, muscovites, and kaolinite with different particle sizes, along with IAEA-CH-7 polyethylene, and reference waters and NBS 22 oil that were sealed in silver-tube segments, were measured. The effect of absorbed water on mineral surfaces was investigated with waters both enriched and depleted in 2 H. The quantitative conversion of hydrogen from biotite into gaseous hydrogen as a function of mass and particle size was also investigated. RESULTS The δ 2 H VSMOW-SLAP values of NBS 30 obtained by three laboratories were as much as 21 ‰ too high compared with the accepted value of −65.7 ‰, determined by conventional off-line measurements. The experiments showed a strong correlation between grain size and the δ 2 H VSMOW-SLAP value of NBS 30 biotite, but not of biotites with lower iron content. The δ 2 H VSMOW-SLAP values of NBS 30 as a function of particle size show a clear trend toward −65.7 ‰ with finer grain size. CONCLUSIONS Determination of the δ 2 H VSMOW-SLAP values of hydrous minerals and of NBS 30 biotite by on-line HTC systems coupled to isotope-ratio mass spectrometers may be unreliable because hydrogen in this biotite may not be converted quantitatively into molecular hydrogen. Extreme caution in the use and interpretation of δ 2 H VSMOW-SLAP on-line measurements of hydrous minerals is recommended.

Rapid Communications in Mass Spectrometry↗
USGS2025

Correction to “Stable isotope reference materials and scale definitions – outcomes of the 2024 IAEA experts meeting”

This article corrects the following: Camin, F., Besic, D., Brewer, P., Allison, C., Coplen, T., Dunn, P., Gehre, M., Gröning, M., Meijer, H.-A.-J, Hélie, J.-F., Iacumin, P., Kraft, R., Krajnc, B., Kümmel, S., Lee, S., Meija, J., Mester, Z., Mohn, J., Moossen, H., Qi, H., Skrzypek, G., Sperlich, P., Viallon, J., Wassenaar, L. and Wielgosz, R. (2025). Stable Isotope Reference Materials and Scale Definitions—Outcomes of the 2024 IAEA Experts Meeting. Rapid Commun Mass Spectrom, 39: e10018, https://doi.org/10.1002/rcm.10018. In the above article, the text of note 3 in Section 4.2 is incorrect. The correct text is shown below: 3. Whilst limited amounts of the original VSMOW and SLAP scale-defining reference materials may still be available, to enable continuous wide distribution of RMs for VSMOW-SLAP scale realization (normalization, calibration) the scale-maintaining reference materials VSMOW2 and SLAP2 are available. VSMOW2 and SLAP2 have δ(18O) values identical to VSMOW and SLAP, respectively, but with an uncertainty with respect to VSMOW and SLAP. We apologize for this error. The CIAAW-IUPAC endorsed all isotope delta scale definitions appearing in Camin et al. (2025) and agreed to publish the current and all future updates and corrections to these definitions on its website (www.ciaaw.org).

Rapid Communications in Mass Spectrometry↗