Isotope Delta Research Articles

Final report on CCQM-K167: carbon isotope delta measurements of vanillin

Main textThe CCQM Isotope Ratio Working Group (IRWG) determined that an additional key comparison of carbon isotope delta measurements was required to capture the progress of this field. Vials containing 0.25 mg of vanillin were prepared at NRC, and evaluated for bottle-to-bottle homogeneity prior to distribution to the eight participating institutes. Participants were able to choose any suitable method and reference materials for carbon isotope delta measurements, and report a carbon isotope delta value and the associated uncertainty, and analysis details. To determine the key comparison reference value (KCRV) and its associated uncertainty, the NRC in collaboration with the NIST, developed a multivariate Bayesian random laboratory effects model, which also incorporates the uncertainty due to bottle-to-bottle homogeneity and any correlations between the reported results that arise due to the use of common reference materials. The KCRV for this study was determined to be -25.833 ‰ relative to the VPDB, with associated uncertainty of 0.028 ‰, and expanded uncertainty of 0.056 ‰ (k=2). All the results reported by the participants in this study were considered equivalent to the KCRV.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Final report on pilot study CCQM-P211: carbon isotope delta measurements of vanillin

Main textThis pilot study was conducted in parallel to the key comparison CCQM-K167. Vials containing 0.25 mg of vanillin were prepared at NRC and distributed to four participating institutes. Institutes could choose any suitable reference materials and methodology to perform carbon isotope delta measurements. Participants reported analysis details, and a carbon isotope delta value and associated uncertainty for the vanillin sample. Each of the carbon isotope delta results of vanillin were compared to the KCRV established from CCQM-K167, and metrological compatibility to the KCRV was determined. Three expert laboratories participated in this study, and their reported results serve as a benchmark to compare the performance of all other participants to these world-class institutes.To reach the main text of this paper, click on Final Report.The final report has been peer-reviewed and approved for publication by the CCQM.

Calculation and Interpretation of Inter-laboratory Variation in Isotope Delta (δ) Values Using Real Interpretative Differences

Building on applications in anthropology, the use of stable isotope ratio analysis of human bone collagen to investigate an individual’s life history is becoming more commonplace in the forensic science community. Careful consideration of the resultant isotope delta (δ) values, particularly in regard to their accuracy and reliability, is paramount when introducing isotope data into the U.S. court system. In this study, we use a simple framework to calculate real interpretative difference (RID) values for collagen (“col”) and assess isotope data comparability for sample analysis (RIDanalysis) as well as sample preparation/analysis combined (RIDcombined). The RIDcombined values of 0.59‰ for δ13Ccol and 0.91‰ for δ15Ncol are similar to more complex, published calculations of inter-laboratory variability in the stable isotope analysis of skeletal remains, but they are easier to calculate and intuitively elegant. The RIDcombined as well as RIDanalysis values presented here allow users to examine multiple sources of inter-laboratory isotopic variation (preparation, analysis, and both together) in a two-step process whereby a RID value is constructed and then tested. Implementation of this RID approach will provide surety for the legal and research communities in forensic applications of stable isotope ratio analysis.

Copper, Zinc and Lead Isotopic Delta Values and Isotope Ratios of Various Geological and Biological Reference Materials

Copper, Zn and Pb isotope ratios are important tracers for understanding geological and biological processes. We present optimised column separation methods using an anion exchange resin for Cu and Zn, and a Pb‐specific resin for Pb, and provide Cu, Zn and Pb isotope data for thirty‐three commercially available geological and biological reference materials (RMs) using MC‐ICP‐MS. Our measurement procedures can be applied to various environmental samples. The Cu, Zn and Pb yields were higher than 99.2%, and isotopic fractionation did not occur during column separation. Copper and Zn delta values of AGV‐2, BCR‐2, BIR‐1a, BHVO‐2, GSP‐2, NOD‐A‐1, NOD‐P‐1, DOLT‐4, BCR‐414, BCR‐279 and NIST SRM 1573a were in good agreement with previously reported values. The intermediate measurement precision for Cu and Zn delta values was better than ± 0.02‰ (2s). The δ65CuNIST976, δ66ZnJMCand206Pb/207Pb values determined in this study ranged from −0.19 to +0.73‰, −0.96 to +1.00‰ and 1.1172 to 1.2263, respectively. The δ66ZnJMCvariability (Δ66Znmax–min) in this study was relatively larger than that of δ65CuNIST976variability (Δ65Cumax–min). A combination of Cu, Zn and Pb isotopes can be used as potential environmental tracers to track the origins of these elements in the environment as well as to understand their influence on organisms.

The FIRMS Network’s PT scheme: What can be learned about inter-laboratory performance?

Demonstrating comparability of isotope-delta values between different laboratories by participation in inter-laboratory exercises (ILEs) is a vital component of method validation. Since its inception, the Forensic Isotope Ratio Mass Spectrometry (FIRMS) Network has provided non-accredited ILEs to its members. Provision of an accredited proficiency testing (PT) scheme to ISO/IEC 17043:2010 was not possible until the administration of the scheme was outsourced to an accredited PT provider—in this case LGC Standards. LGC’s FIRMS PT scheme has been operating since 2013 and in 2015 it was included in their scope of accreditation to ISO/IEC 17043:2010 and it remains the only such accredited PT for stable isotopic analyses.Each round of the PT scheme contains two materials that are of interest to forensic scientists. Participants report up to four isotope delta values for each material and the results of the PT scheme are interpreted using z- or z′-scores calculated using a standard deviation of performance assessment (SDPA) that was initially derived from the submitted data, but now have fixed values.This paper presents and discusses results for the FIRMS PT scheme, including inter-laboratory comparability over time not just for individual laboratories but also for particular matrices that have been analysed in multiple rounds. The FIRMS PT scheme has also distributed the same physical material as earlier FIRMS ILEs providing a longer-term view. A number of interesting findings are presented highlighting the value of PT participation together with a discussion of the status of forensic bio-element stable isotope ratio analysis in the light of these outcomes.

USGS44, a new high-purity calcium carbonate reference material for δ13 C measurements.

RationaleThe stable carbon isotopic (δ 13C) reference material (RM) LSVEC Li2CO3 has been found to be unsuitable for δ 13C standardization work because its δ 13C value increases with exposure to atmospheric CO2. A new CaCO3 RM, USGS44, has been prepared to alleviate this situation.MethodsUSGS44 was prepared from 8 kg of Merck high‐purity CaCO3. Two sets of δ 13C 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 CaCO3 (δ 13CVPDB = +1.95 milliurey (mUr) exactly, where mUr = 0.001 = 1‰), and LSVEC Li2CO3 (δ 13CVPDB = −46.6 mUr exactly), and normalized to the two‐anchor δ 13CVPDB‐LSVEC isotope‐delta scale. The second set of values was obtained by dual‐inlet (DI)‐IRMS of CO2 evolved by reaction of H3PO4 with carbonates, corrected for cross contamination, and normalized to the single‐anchor δ 13CVPDB scale.ResultsUSGS44 is stable and isotopically homogeneous to within 0.02 mUr in 100‐μg amounts. It has a δ 13CVPDB‐LSVEC value of −42.21 ± 0.05 mUr. Single‐anchor δ 13CVPDB values of −42.08 ± 0.01 and −41.99 ± 0.02 mUr were determined by DI‐IRMS with corrections for cross contamination.ConclusionsThe new high‐purity, well‐homogenized calcium carbonate isotopic reference material USGS44 is stable and has a δ 13CVPDB‐LSVEC value of −42.21 ± 0.05 mUr for both EA/IRMS and DI‐IRMS measurements. As a carbonate relatively depleted in 13C, it is intended for daily use as a secondary isotopic reference material to normalize stable carbon isotope delta measurements to the δ 13CVPDB‐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 18O reference material. A δ 13CVPDB‐LSVEC value of −29.99 ± 0.05 mUr was determined for NBS 22 oil.

Recalculation of stable isotope expressions for HCNOS: EasyIsoCalculator.

The stable HCNOS isotope compositions can be reported as (a) the isotope ratio of two stable isotopes (R); (b) the isotope delta value (δ); and (c) the atom fraction of the isotopes (x). Recalculations between these different expressions are needed frequently and require the use of the absolute isotope ratio for the zero points of the stable isotope delta scales (Rstd). The inconsistent use of Rstd values may lead to a discrepancy in recalculated results. We summarised the recalculation procedures between different expressions of the stable isotope compositions and introduced a user-friendly EasyIsoCalculator that allows the recalculation between the main expressions of isotope compositions. We mathematically and empirically evaluated the possible inconsistencies in reporting of the stable isotope data due to the use of different Rstd and different normalisation methods. The recalculation between δ-values and other expressions of the stable isotope compositions always involves the use of Rstd. The choice of Rstd will have a significant influence on the recalculated values. The use of different Rstd values has a significant influence also on the normalisation of raw values but only when the normalisation is conducted versus the working standard gas value, causing discrepancy, e.g. for δ(13C/12C) up to ~ 0.3 ‰. Differences in the selection of Rstd value may lead to significant differences among different laboratories. The uncertainty in the calculations originates primarily from the uncertainty in the Rstd determination; however, it is lower than the discrepancy arising from the inconsistent use of Rstd. Consistent use of the same Rstd values is required to eliminate the unnecessary discrepancy if different data sets are recalculated from delta value to other expressions.

Absolute isotope ratios defining isotope scales used in isotope ratio mass spectrometers and optical isotope instruments.

The isotope delta is calculated from the isotope ratio of a sample and the absolute isotope ratio of the zero reference point defining each stable isotope international scale (Rstd ). Therefore, Rstd requires accurate determination. However, the literature contains a large number of Rstd values, and selection of different Rstd may lead to inconsistency in reporting and recalculating stable isotope results. We reviewed Rstd used in the proprietary software provided by the manufacturers of stable isotope instruments commonly employed for analyses of stable HCNOS compositions. We compared the Rstd values and assessed the potential implications of using different Rstd and the normalization versus tank working gas standard for consistency in reporting stable isotope results. Different Rstd values are used by different manufacturers of stable isotope analytical instruments. For R(2 H/1 H)VSMOW two different but very similar values are used, 0.00015575 and 0.00015576; for R(13 C/12 C)VPDB three different values are used, 0.0111802, 0.0112372 and 0.01118028; and for R(15 N/14 N)Air-N2 two values, 0.0036782 and 0.0036765, are used. All manufacturers are using the same value for R(18 O/16 O)VSMOW , 0.00200520, but three different values for R(18 O/16 O)VPDB , 0.002067200, 0.00208835 and 0.002088349. For R(34 S/32 S)VCDT four different Rstd are used, 0.0441509, 0.0441626, 0.044162589 and 0.0441520597. The use of different Rstd values may lead to differences in the isotope delta values obtained if the normalization versus working standard gas is applied. For the range of Rstd used in proprietary software, the potential differences are lowest for oxygen (< 0.002 ‰) and nitrogen (< 0.001 ‰), and highest for carbon (0.107 to 0.112 ‰) and sulfur (0.023 ‰). Evaluation of the existing Rstd values and recommendations for the best estimates are highly desirable to ensure worldwide consistency in stable isotope data reporting.

Intercalibration of Mg Isotope Delta Scales and Realisation of SI Traceability for Mg Isotope Amount Ratios and Isotope Delta Values

The continuous improvement of analytical procedures using multi‐collector technologies in ICP‐mass spectrometry has led to an increased demand for isotope standards with improved homogeneity and reduced measurement uncertainty. For magnesium, this has led to a variety of available standards with different quality levels ranging from artefact standards to isotope reference materials certified for absolute isotope ratios. This required an intercalibration of all standards and reference materials, which we present in this interlaboratory comparison study. The materials Cambridge1, DSM3, ERM‐AE143, ERM‐AE144, ERM‐AE145, IRMM‐009 and NIST SRM 980 were cross‐calibrated with expanded measurement uncertainties (95% confidence level) of less than 0.030‰ for the δ25/24Mg values and less than 0.037‰ for the δ26/24Mg values. Thus, comparability of all magnesium isotope delta (δ) measurements based on these standards and reference materials is established. Further, ERM‐AE143 anchors all magnesium δ‐scales to absolute isotope ratios and therefore establishes SI traceability, here traceability to the SI base unit mole. This applies especially to the DSM3 scale, which is proposed to be maintained. With ERM‐AE144 and ERM‐AE145, which are product and educt of a sublimation–condensation process, for the first time a set of isotope reference materials is available with a published value for the apparent triple isotope fractionation exponent θapp, the fractionation relationship ln α(25/24Mg)/ln α(26/24Mg).

Oxygen isotopic fractionation in rat bones as a result of consuming thermally processed water – bioarchaeological applications

Abstract Stable isotope analyses of oxygen are used in anthropology for such purposes as determination of origin of individuals, tracking migration routes or dynamics of human community relocation. The methodology related to oxygen isotope analysis has been founded on the relationship between its isotopic composition within phosphate groups of bone tissue (δ18Op) in individuals being analysed and the water consumed by such individuals (δ18Ow). Such a relationship has been observed in many species of mammals, including humans. However, the influence of culinary practices on the isotopic delta values of apatite phosphates of individuals has not yet been researched. The present study, which was conducted using laboratory rats, is an investigation of the influence of the thermal processing of water drank by such rats on the isotopic composition (δ18Op) of bone apatite. Increasing the value of the isotopic composition of water by about 6.1 ‰ during boiling resulted in an increase in the oxygen isotopic value δ18Op of rats drinking the water by about 4 ‰ (29%). It can be expected that regular consumption of heavily isotopic drinks and foods by humans may cause the δ18Op of individuals to exceed the range of isotopic environmental variability, even by a few per mille.

Seasonal and Interannual Variations in Nitrogen Availability and Particle Export in the Northwestern North Pacific Subtropical Gyre

AbstractTime series of particulate nitrogen isotope delta (PN‐δ15N) and flux of trapped particles at 200 m were determined in the northwestern North Pacific subtropical gyre between 2010 and 2014 in order to examine temporal variations in mixed layer nitrate availability and downward particle export. Lower PN‐δ15N (&lt;4‰) seen every winter corresponded to convective nitrate supply triggering phytoplankton blooms. Greater particle fluxes were associated with higher CaCO3 concentrations (mass fraction &gt;50%), implying that new nitrogen supply stimulated predominantly coccolithophore growth and downward transport of coccoliths. Even in the stratified summer of 2011, a δ15N drop associated with a spike in particulate flux was found and was attributed to the uplift of nitrate‐rich deep waters due to a mesoscale cyclonic eddy passing the mooring station. Total nitrate input sustains 86–93% of annual new production in this region while nitrogen fixation accounts for the rest. Trapped particles also showed that the winter δ15N decrease appeared earlier in the 2011–2012 blooms than those in 2013–2014, coinciding with 8% higher CaCO3 concentrations and a 40% lower particulate organic carbon to inorganic carbon export ratio, R(POC:PIC). This reflected stronger convective mixing and hence larger nutrient supply in 2011–2012, caused by larger ocean heat loss related to winter monsoon intensity. Such interannual change of winter R(POC:PIC) can affect CO2 uptake rate in the northwestern North Pacific subtropical gyre where anthropogenic CO2 accumulates in subtropical mode waters.

Consistent Differences in Field Leaf Water-Use Efficiency among Soybean Cultivars.

High intrinsic water-use efficiency (WUEi), the ratio of leaf photosynthesis to stomatal conductance, may be a useful trait in adapting crops to water-limited environments. In soybean, cultivar differences in stomatal response to vapor pressure deficit have not consistently translated into differences in WUEi in the field. In this study, six cultivars of soybeans previously shown to differ in WUEi in indoor experiments were grown in the field in Beltsville, Maryland, and tested for mid-day WUEi on nine clear days during the mid-seasons of two years. Measurement dates were chosen for diverse temperatures, and air temperatures ranged from 21 to 34 °C on the different dates. Air saturation deficits for water vapor ranged from 0.9 to 2.2 kPa. Corrected carbon isotope delta values for 13C (CID) were determined on mature, upper canopy leaves harvested during early pod filling each year. WUEi differed among cultivars in both years and the differences were consistent across measurement dates. Correlations between mean WUEi and CID were not significant in either year. It is concluded that consistent cultivar differences in WUEi exist in these soybean cultivars under field conditions, but that carbon isotope ratios may not be useful in identifying them because of cultivar differences in mesophyll conductance.

Three certified sugar reference materials for carbon isotope delta measurements.

For isotope delta analysis, it is preferable to have at least two matrix-matched reference materials whose isotope delta values encompass those of the samples to be analyzed. The National Research Council Canada (NRC) has developed three sugar Certified Reference Materials (CRMs), BEET-1 (beet sugar), GALT-1 (galactose), and FRUT-1 (fructose), to be collectively used for carbon isotope delta measurements in sugars, and other organic materials. All materials were homogenized and packaged in glass ampules. All three sugar materials were analyzed at the NRC using elemental analyzer/isotope ratio mass spectrometry (EA/IRMS). Six additional laboratories also provided EA/IRMS measurements. Data from all laboratories were re-normalized using three international secondary reference materials (IAEA-CH-6, USGS40, and USGS62) included as blind samples in the inter-laboratory comparison, thus providing added quality control and robustness to the study. Re-normalized carbon isotope delta values from each laboratory were combined using a random laboratory effects statistical model with accounting of the correlations between the laboratory results due to the use of the same reference materials for calibration. The consensus δ(13 C) values and combined standard uncertainties which include effects due to characterization, homogeneity, and stability for BEET-1, GALT-1, and FRUT-1 are -26.02(7) ‰, -21.41(6) ‰, and -10.98(5) ‰, respectively, on the VPDB scale. Three new δ(13 C) sugar CRMs (BEET-1, GALT-1, and FRUT-1) were developed and are available from NRC. These three CRMs can be utilized as a set for daily δ(13 C) scale normalization of sugar-based or other organic materials in order to produce reliable δ(13 C) measurements.

Scale distortion from pressure baselines as a source of inaccuracy in triple‐isotope measurements

Isotope ratio measurements have become extremely precise in recent years, with many approaching parts-per-million (ppm) levels of precision. However, seemingly innocuous errors in signal baselines, which exist only when gas enters the instrument, might lead to significant errors. These "pressure-baseline" (PBL) offsets may have a variety of origins, such as incoherent scattering of the analyte, isobaric interferences, or electron ablation from the walls of the flight tube. They are probably present in all but ultra-high-resolution instruments, but their importance for high-precision measurements has not been investigated. We derive the governing equations for the PBL effect. We compare the oxygen triple-isotope composition of gases on three different mass spectrometers before and after applying a correction for PBLs to determine their effects. We also compare the composition of atmospheric O2 with that of several standard minerals (San-Carlos Olivine and UWG-2) on two high-precision mass spectrometers and compare those results with the differences reported in the literature. We find that PBLs lead to stretching or compression of isotopic variations. The scale distortion is non-mass-dependent, affecting the accuracy of triple-isotope covariations. The governing equations suggest that linear stretching corrections using traditional isotopic delta values (e.g., δ18 O) are rigorous for PBL-induced errors in pure gases. When the reference and sample gases are not comparable in composition or purity, however, a different correction scheme may be required. These non-mass-dependent errors are systematic and may have influenced previous measurements of triple-isotope covariations in natural materials. Accurate measurements of isotopic variations are essential to biogeochemistry and for testing theoretical models of isotope effects. PBLs are probably ubiquitous, contributing to the interlaboratory disagreements in triple-isotope compositions of materials differing greatly in δ18 O values. Moreover, they may lead to inaccurate determination of triple-isotope compositions and fractionation factors, which has implications for isotopic studies in hydrology and biogeochemistry.

Lessons learned from inter-laboratory studies of carbon isotope analysis of honey

Forensic application of carbon isotope ratio measurements of honey and honey protein to investigate the degree of adulteration with high fructose corn syrup or other C4 plant sugars is well established. These measurements must use methods that exhibit suitable performance criteria, particularly with regard to measurement uncertainty and traceability – low levels of adulteration can only be detected by methods that result in suitably small measurement uncertainties such that differences of 1‰ or less can be reliably detected. Inter-laboratory exercises are invaluable to assess the state-of-the art of measurement capabilities of laboratories necessary to achieve such performance criteria. National and designated metrology institutes from a number of countries recently participated in an inter-laboratory assessment (CCQM-K140) of stable carbon isotope ratio determination of bulk honey. The same sample material was distributed to a number of forensic isotope analysis laboratories that could not participate directly in the metrological comparison. The results from these studies have demonstrated that the majority of participants provided isotope delta values with acceptable performance metrics; that all participants ensured traceability of their results; and that where measurement uncertainties were reported; these were fit-for-purpose. A number of the forensic laboratories only reported precision rather than full estimates of measurement uncertainty and this was the major cause of the few instances of questionable performance metrics. Reporting of standard deviations in place of measurement uncertainties is common practice outside metrology institutes and the implications for interpretations of small differences in isotopic compositions are discussed. The results have also highlighted a number of considerations that are useful for organisers of similar inter-laboratory studies in the future.

Source and fate of nitrate in contaminated groundwater systems: Assessing spatial and temporal variations by hydrogeochemistry and multiple stable isotope tools

Intensive farming usually imply a degradation of groundwater resources worldwide. In particular, nitrate concentrations exceeding the 50 mg L−1 limit established for drinking water pose the human health at risk. Therefore, assessing the impact of farming on groundwater, in terms of space and time, is of fundamental importance for policy decision makers and land managers. This study was aimed at assessing the nitrate source and fate in groundwater by combining hydrogeochemical and isotopic tools. The study area is located in the coastal plain of Arborea (Italy), a nitrate vulnerable zone (NVZ) due to intensive farming and animal husbandry (28,000 bovine livestock units). This area represents Mediterranean environments where groundwater resources are of relevant importance.In order to assess the present level of groundwater contamination and evaluate temporal variations, 6 hydrogeochemical surveys were carried out bimonthly at 13 sampling sites located in an area of 6 km2. Additional samples were collected in specific surveys (82 water samples in total). The physical-chemical parameters, nitrogen species concentrations, major and minor components were determined, together with the boron, hydrogen, oxygen, nitrogen, and sulfur isotopic delta values.Results showed that groundwater samples were of meteoric origin, as indicated by the δ2H and δ18OH2O values. The groundwater showed near-neutral pH (6.8–7.9) and different values of redox potential (0.2 ÷ 0.5 V), dissolved oxygen (2 ÷ 6 mg L−1), electrical conductivity (0.8 ÷ 2.1 mS cm−1) and chemical composition (sodium-chloride ÷ calcium-bicarbonate). Nitrate was not homogeneously distributed in groundwater, being observed a large range of concentrations, from <1 up to 162 mg L−1. The above differences reflected the variability of groundwater circulation at small scale, which in turn controlled the interaction of water with different sediments (sands and/or clays). The shallow wells (about 5 m depth), screened in groundwater interacting mainly with sands, showed marked variations under the monitoring period, with nitrate peaks reflecting high leaching of nitrate in correspondence of fertilization and irrigation periods. The deeper wells (15–37 m depth) showed high to moderate nitrate when screened in sandy aquifer, whereas they had very low nitrate and relatively high ammonium (up to 1.8 mg L−1) when clay layers were intercepted.Trends of δ15N and δ18ONO3 values in the nitrate of shallow groundwater were related to the nitrate concentration observed over the monitored period. This dual isotope systematic showed a likely source of nitrate in groundwater from either manure or sewage. The δ11B signature coupled to δ15N values clearly identified the manure as the predominant source of nitrate in the shallow and deep groundwater at Arborea. Relative enrichments in heavy nitrogen coupled to high concentrations of nitrate in groundwater were mainly attributed to volatilization processes occurring during the storage of animal wastes prior to application on the soil. Mixing of groundwater with seawater was not recognized, whereas mixing between shallow and deep groundwater may have occurred locally.Natural attenuation of nitrate contamination was observed in the deep groundwater interacting with lagoon clays rich in organic matter. Heterotrophic denitrification processes were highlighted by relatively high δ15N, δ18ONO3, δ34S and δ18OSO4 values in association with low SO42−/Cl− and high HCO3−/SO42− molar ratios observed in the groundwater with low concentration of nitrate.Results of this study showed that site-specific investigations are required for designing the best practices aimed at preserving groundwater resources under Mediterranean conditions. The spreading of animal waste on soils affects groundwater systems and likely extends over long time, strongly depending on the time lag of nutrient transport from source areas to receptor wells. Therefore, adequate monitoring of groundwater quality is required in areas of intensive farming.

The Δ&amp;lt;sup&amp;gt;17&amp;lt;/sup&amp;gt;O and &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O values of atmospheric nitrates simultaneously collected downwind of anthropogenic sources – implications for polluted air masses

Abstract. There are clear motivations for better understanding the atmospheric processes that transform nitrogen (N) oxides (NOx) emitted from anthropogenic sources into nitrates (NO3-), two of them being that NO3- contributes to acidification and eutrophication of terrestrial and aquatic ecosystems, and particulate nitrate may play a role in climate dynamics. For these reasons, oxygen isotope delta values (δ18O, Δ17O) are frequently applied to infer the chemical pathways leading to the observed mass-independent isotopic anomalies from interaction with 17O-rich ozone (O3). Recent laboratory experiments suggest that the isotopic equilibrium between NO2 (the main precursor of NO3-) and O3 may take long enough under certain field conditions that nitrates may be formed near emission sources with lower isotopic values than those formed further downwind. Indeed, previously published field measurements of oxygen isotopes in NO3- in precipitation (wNO3-) and in particulate (pNO3-) samples suggest that abnormally low isotopic values might characterize polluted air masses. However, none of the air studies have deployed systems allowing collection of samples specific to anthropogenic sources in order to avoid shifts in isotopic signature due to changing wind directions, or separately characterized gaseous HNO3 with Δ17O values. Here we have used a wind-sector-based, multi-stage filter sampling system and precipitation collector to simultaneously sample HNO3 and pNO3-, and co-collect wNO3-. The nitrates are from various distances (&lt;1 to &gt;125 km) downwind of different anthropogenic emitters, and consequently from varying time lapses after emission. The separate collection of nitrates shows that the HNO3 δ18O ranges are distinct from those of w- and pNO3-. Interestingly, the Δ17O differences between pNO3- and HNO3 shift from positive during cold sampling periods to negative during warm periods. The low pNO3-Δ17O values observed during warm periods may partly derive from the involvement of 17O-depleted peroxy radicals (RO2) oxidizing NO during that season. Another possibility is that nitrates derive from NOx that has not yet reached isotopic equilibrium with O3. However, these mechanisms, individually or together, cannot explain the observed pNO3 minus HNO3 isotopic changes. We propose differences in dry depositional rates, faster for HNO3, as a mechanism for the observed shifts. Larger proportions of pNO3- formed via the N2O5 pathway would explain the opposite fall–winter patterns. Our results show that the separate HNO3, wNO3- and pNO3- isotopic signals can be used to further our understanding of NOx oxidation and deposition. Future research should investigate all tropospheric nitrate species as well as NOx to refine our understanding of nitrate distribution worldwide and to develop effective emission reduction strategies.

SIDER: an R package for predicting trophic discrimination factors of consumers based on their ecology and phylogenetic relatedness

Stable isotope mixing models (SIMMs) are an important tool used to study species' trophic ecology. These models are dependent on, and sensitive to, the choice of trophic discrimination factors (TDF) representing the offset in stable isotope delta values between a consumer and their food source when they are at equilibrium. Ideally, controlled feeding trials should be conducted to determine the appropriate TDF for each consumer, tissue type, food source, and isotope combination used in a study. In reality however, this is often not feasible nor practical. In the absence of species‐specific information, many researchers either default to an average TDF value for the major taxonomic group of their consumer, or they choose the nearest phylogenetic neighbour for which a TDF is available. Here, we present the SIDER package for R, which uses a phylogenetic regression model based on a compiled dataset to impute (estimate) a TDF of a consumer. We apply information on the tissue type and feeding ecology of the consumer, all of which are known to affect TDFs, using Bayesian inference. Presently, our approach can estimate TDFs for two commonly used isotopes (nitrogen and carbon), for species of mammals and birds with or without previous TDF information. The estimated posterior probability provides both a mean and variance, reflecting the uncertainty of the estimate, and can be subsequently used in the current suite of SIMM software. SIDER allows users to place a greater degree of confidence on their choice of TDF and its associated uncertainty, thereby leading to more robust predictions about trophic relationships in cases where study‐specific data from feeding trials is unavailable. The underlying database can be updated readily to incorporate more stable isotope tracers, replicates and taxonomic groups to further increase the confidence in dietary estimates from stable isotope mixing models, as this information becomes available.

Uncertainty evaluation in normalization of isotope delta measurement results against international reference materials.

Isotope delta measurements are normalized against international reference standards. Although multi-point normalization is becoming a standard practice, the existing uncertainty evaluation practices are either undocumented or are incomplete. For multi-point normalization, we present errors-in-variables regression models for explicit accounting of the measurement uncertainty of the international standards along with the uncertainty that is attributed to their assigned values. This manuscript presents framework to account for the uncertainty that arises due to a small number of replicate measurements and discusses multi-laboratory data reduction while accounting for inevitable correlations between the laboratories due to the use of identical reference materials for calibration. Both frequentist and Bayesian methods of uncertainty analysis are discussed.

Stable carbon isotope labelled carbon dioxide as tracer gas for air change rate measurement in a ventilated single zone

Carbon dioxide (CO2) has often been used as tracer gas for measurement of the air change rate λ (h−1) in buildings. In such measurements, a correction is required for the presence of indoor CO2, which commonly consists of atmospheric CO2 mixed with human respired CO2. Here, 13C isotope-labelled CO2 was employed as tracer gas, and cavity ring-down spectroscopy (CRDS) was used for simultaneous measurement of the two isotope analogues 12CO2 and 13CO2. This enabled the simultaneous measurement of the 13CO2 tracer gas, with correction for background 13CO2, and the concentration of indoor CO2, allowing for presence of occupants. The background correction procedure assumes that the isotope delta of the background indoor CO2 equals δB = −19‰, based on the prior information that the carbon isotope ratio RB = 13C/12C of all carbon in the bio-geosphere of earth is in the interval 0.010900 <RB < 0.011237. Evidence supported that λ could be accurately measured, using the new 13CO2 tracer method, even when the background 13CO2 concentration varied during the measurement time interval, or when the actual δB value differed from the assumed value. The measurement uncertainty for λ was estimated at 3%. Uncertainty in λ due to uncertainty in RB, uRB(λ), was estimated to increase with a decreasing amount of 13CO2 tracer. This indicated that at least 4 ppm tracer must be used, in order to obtain uRB(λ)/λ < 2%. The temporal resolution of the λ measurement was 1.25/λ h.