Measurement of δ18O, δ17O, and 17O-excess in water by off-axis integrated cavity output spectroscopy and isotope ratio mass spectrometry

The stable isotope hydrology of Sable Island, Nova Scotia, Canada with implications for evaluating the water budget of wild horses

We investigated the stable isotope hydrology of Sable Island, Nova Scotia, Canada over a five year period from September, 2017 to August, 2022. The δ2H and δ18O values of integrated monthly precipitation were weakly seasonal and ranged from -66 to -15 ‰ and from -9.7 to -1.9 ‰, respectively. Fitting these monthly precipitation data resulted in a local meteoric water line (LMWL) defined by: δ2H = 7.22 ± 0.21 · δ18O + 7.50 ± 1.22 ‰. Amount-weighted annual precipitation had δ2H and δ18O values of -36 ± 11 ‰ and -6.1 ± 1.4 ‰, respectively. Deep groundwater had more negative δ2H and δ18O values than mean annual precipitation, suggesting recharge occurs mainly in the winter, while shallow groundwater had δ2H and δ18O values more consistent with mean annual precipitation or mixing of freshwater with local seawater. Surface waters had more positive values and showed evidence of isolation from the groundwater system. The stable isotopic compositions of plant (leaf) water, on the other hand, indicate plants use groundwater as their source. Fog had δ2H and δ18O values that were significantly more positive than those of local precipitation, yet had similar 17O-excess values. δ2H values of horsehair from 4 individuals lacked seasonality, but had variations typical to those of precipitation on the island. Differences in mean δ2H values of horsehair were statistically significant and suggest variations in water use may exist between spatially disparate horse communities. Our results establish an important initial framework for ongoing isotope studies of feral horses and other wildlife on Sable Island.

A comprehensive analysis of submarine groundwater discharge and nutrient fluxes in the Bohai Sea, China

Groundwater discharge and associated nutrient fluxes in the Bohai Sea, China has attracted great attention, but most studies lacked high spatial resolution for the whole sea. As the largest semi-enclosed sea in China, the Bohai Sea is confronted with strong environmental pollution problems such as eutrophication induced by terrestrial nutrient inputs. However, the role of SGD has not been evaluated well for the whole Bohai Sea. In this study, stable isotopes (hydrogen and oxygen), radioactive isotope (228Ra), salinity, and temperature were combined to trace the diluted seawater. Mass balances of 228Ra, oxygen isotope, and salinity were used to quantify SGD and nutrient fluxes to the Bohai Sea. The estimated submarine fresh groundwater discharge (SFGD) and SGD to the Bohai Sea were (6.0 ± 0.5) × 109 and (2.7 ± 1.6) × 1011 m3 a-1, respectively. SFGD represents 10 % to 11 % of the total river discharge and SGD is about 2 to 8 folds of the total river discharge to the sea. Moreover, SGD derived dissolved nutrients to the Bohai Sea were (4.8 ± 4.0) × 1010 mol a-1 for dissolved inorganic nitrogen, (1.9 ± 1.7) × 1010 mol a-1 for dissolved inorganic phosphorus, and (6.7 ± 5.5) × 1010 mol a-1 for silicon. These nutrient inputs were about 10 to 20 folds of the total riverine inputs. Overall, this study underscores the importance of evaluating SGD to better understand the terrestrial imported nutrients in regional scale.

A protocol for distilling animal body water from biological samples and measuring oxygen and hydrogen stable isotopes via cavity ring-down spectroscopy

The application of stable isotope analysis (SIA) to the fields of ecology and animal biology has rapidly expanded over the past three decades, particularly with regards to water analysis. SIA now provides the opportunity to monitor migration patterns, examine food webs, and assess habitat changes in current and past study systems. While carbon and nitrogen SIA of biological samples have become common, analyses of oxygen or hydrogen are used more sparingly despite their promising utility for tracing water sources and animal metabolism. Common ecological applications of oxygen or hydrogen SIA require injecting enriched isotope tracers. As such, methods for processing and analyzing biological samples are tailored for enriched tracer techniques, which require lower precision than other techniques given the large signal-to-noise ratio of the data. However, instrumentation advancements are creating new opportunities to expand the applications of high-throughput oxygen and hydrogen SIA. To support these applications, we update methods to distill and measure water derived from biological samples with consistent precision equal to, or better than, ± 0.1 ‰ for δ17O, ± 0.3 ‰ for δ18O, ± 1 ‰ for δ2H, ± 2 ‰ for d-excess, and ± 15 per meg for Δ17O.

Isotope effects accompanying δ2 H, δ18 O and δ17 O analyses of aqueous saline solutions using cavity ring-down laser spectroscopy

RATIONALE

The presence of substantial amounts of dissolved salts creates serious difficulties in isotope analyses of water samples using conventional isotope ratio mass spectrometry. Although nowadays laser-based instruments are increasingly used for this purpose, a comprehensive assessment of isotope effects associated with direct analyses of aqueous saline solutions using this technology is lacking.

METHODS

Here we report the results of laboratory experiments aimed at quantifying isotope effects associated with direct, δ2 H, δ18 O and δ17 O analyses of single-salt solutions and double-salt mixtures prepared with a water of known isotopic composition. Three single-salt solutions (NaCl, CaCl2 and MgSO4 ) and two double-salt mixtures (NaCl + CaCl2 and NaCl + MgSO4 ) were prepared and investigated for a wide range of molalities. The triple-isotope composition of the prepared solutions was analysed with the aid of a Picarro L2140-i Cavity Ring-Down Spectroscopy analyser.

RESULTS

The NaCl and CaCl2 solutions revealed small negative salt effects, independent of molality and comparable with measurement uncertainty. The MgCl2 solution showed the highest salt effects, reaching saturated solution ca. +2.7‰ (2 H), -3.5‰ (18 O) and -1.7‰ (17 O). Salt effects for the double-salt mixtures generally mirrored the effects observed for the single-salt solutions. The observed salt effects are discussed in the context of processes occurring during the injection of the salt solutions into the vaporizer unit of the CRDS analyser.

CONCLUSIONS

The presented study has demonstrated feasibility of direct, triple-isotope analyses of aqueous salt solutions using a Picarro L2140-i CRDS analyser for a broad range of salinities up to saturated conditions. Large uncertainties of 17 O-excess determinations for solutions forming hydrated salts preclude the use of this parameter for interpretation purposes.

Global and local meteoric water lines for δ17O/δ18O and the spatiotemporal distribution of Δ'17O in Earth's precipitation

Recently, δ17O and its excess (Δ'17O) have become increasingly significant "triple-oxygen-isotope" indicators of distinctive hydrological processes in hydrology and climatology. This situation mirrors the research regarding δ18O and δ2H in the 1960s towards a solid theoretical base and a surge in application examples and field studies worldwide. Currently, systematic global measurements for δ17O in precipitation are still lacking. As a result, attempts have been made to define a Global δ17O/δ18O Meteoric Water Line (GMWL), often by using regional or local datasets of varying systematicity. Different definitions of the global reference slope (λref) for determining Δ'17O values have been proposed, by ongoing debate around a proposed consensus value of 0.528. This study used worldwide samples archived in the IAEA Global Network of Isotopes in Precipitation (GNIP) to (a) derive a δ17O/δ18O GMWL based on four-year monthly records from 66 GNIP stations, (b) formulate local δ17O/δ18O meteoric water lines (LMWL) for these stations' areas, and (c) evaluate regional and seasonal variations of Δ'17O in precipitation. The GMWL for δ17O/δ18O was determined to be δ'17O = 0.5280 ± 0.0002 δ'18O + 0.0153 ± 0.0013, in keeping with the consensus value. Furthermore, our results suggested that using a line-conditioned 17O-excess is a viable alternative over the global λref in the context of regional hydrology and paleoclimatology interpretations; however, without challenging the global λref as such.

A simple method for rapid removal of the memory effect in cavity ring-down spectroscopy water isotope measurements

RATIONALE

The accuracy determined in the routine analysis of water isotopes (δ17 O, δ18 O, δ2 H) using cavity ring-down spectroscopy is greatly affected by the memory effect (ME), a sample-to-sample carryover that biases measurements. This study aims to develop a simple method that rapidly removes the ME.

METHODS

We developed a method, designed for the Picarro L2140-i, that removes the ME by injecting small amounts of water with an extreme isotopic value ("kick") in the opposite direction of the ME. We conducted 11 experiments to identify the optimal kick for pairs of isotopically enriched and depleted samples. Once quantified, the optimal kick was used to create an ME-free, unbiased calibration curve, which was verified using international and internal lab standards.

RESULTS

Our kick method removes the ME very efficiently in half the time it takes for experiments without a kick. The optimal number of kick injections required to minimize stabilization time between standards of different compositions is three injections of δ2 H ≈ -1000‰ water per a 100‰ difference between standards. Three runs of routine measurements using the kick method resulted in uncertainties of 0.03‰, 0.2‰, and 5 permeg for δ18 O, δ2 H, and 17 O-excess, respectively.

CONCLUSIONS

This study demonstrates a new method for rapidly removing the ME. Our kick protocol is a readily available, cheap, and efficient approach to reduce instrumental bias and improve measurement accuracy.

Disentangling nitrate pollution sources and apportionment in a tropical agricultural ecosystem using a multi-stable isotope model

Fertilizers increase agricultural productivity and farmers' income. However, intensive agriculture frequently overuses fertilizers, which in turn can contaminate surface and groundwater. In this study, hydrochemical and multi-isotope (δ¹⁵N_NO3, δ¹⁸O_NO3 and δ¹⁸O_H2O) data have been combined to identify nitrate pollution sources in Ghana's Densu River Basin, trace the Nitrogen (N) biogeochemical processes in the basin and apportion the contribution of each pollution source. Surface water NO₃^- ranged from 0.3 to 10.6 mg/L (as N), while groundwater NO₃^- ranged from 0.9 to 34 mg/L. Hierarchical cluster analysis classified the water samples into three spatial categories: upstream, midstream, and downstream, reflecting river and land use patterns. The multi-isotope model considered five primary NO₃^- sources: atmospheric deposition, manure/sewage, NH₄⁺ in fertilizers, other NO₃^- based fertilizers and soil N. Nitrification was identified as the major biogeochemical process upstream, whereas mixing of sources and denitrification dominate the midstream to downstream sections of the basin. Nitrate source apportioning using a MixSIAR model reveal that N fertilizers (40 %) and soil N (34 %) contribute the most to nitrate pollution upstream of the river. From the midstream to downstream sections, manure/sewage (43 %) become the dominant nitrate source, reflecting the transition from agriculture to peri-urban and urban land use. This study has shown that soil erosion and runoff contribute to nitrate pollution in the Densu River, at levels comparable to N fertilizers, and groundwater across the basin is impacted mainly by manure/sewage. The multi-isotope analyses allowed the partitioning of N sources in other ways not possible using only classical hydrochemical methods.

Seasonal Variations in Triple Oxygen Isotope Ratios of Precipitation in the Western and Central United States

Triple oxygen isotope ratios Δ ' 17 O offer new opportunities to improve reconstructions of past climate by quantifying evaporation, relative humidity, and diagenesis in geologic archives. However, the utility of Δ ' 17 O in paleoclimate applications is hampered by a limited understanding of how precipitation Δ ' 7 O values vary across time and space. To improve applications of Δ ' 17 O , we present δ 18 O , d-excess, and Δ ' 17 O data from 26 precipitation sites in the western and central United States and three streams from the Willamette River Basin in western Oregon. In this data set, we find that precipitation Δ ' 17 O tracks evaporation but appears insensitive to many controls that govern variation in δ 18 O , including Rayleigh distillation, elevation, latitude, longitude, and local precipitation amount. Seasonality has a large effect on Δ ' 17 O variation in the data set and we observe higher seasonally amount-weighted average precipitation Δ ' 17 O values in the winter (40 ± 15 per meg [± standard deviation]) than in the summer (18 ± 18 per meg). This seasonal precipitation Δ ' 17 O variability likely arises from a combination of sub-cloud evaporation, atmospheric mixing, moisture recycling, sublimation, and/or relative humidity, but the data set is not well suited to quantitatively assess isotopic variability associated with each of these processes. The seasonal Δ ' 17 O pattern, which is absent in d-excess and opposite in sign from δ 18 O , appears in other data sets globally; it showcases the influence of seasonality on Δ ' 17 O values of precipitation and highlights the need for further systematic studies to understand variation in Δ ' 17 O values of precipitation.

Balancing precision and throughput of ..17O and .÷...17O analysis of natural waters by Cavity Ringdown Spectroscopy

δ ¹⁷O and Δ'¹⁷O are emerging tracers increasingly used in isotope hydrology, climatology, and biochemistry. Differentiating small relative abundance changes in the rare ¹⁷O isotope from the strong covariance with ¹⁸O imposes ultra-high precision requirements for this isotope analysis. Measurements of δ ¹⁷O by Cavity Ringdown Spectroscopy (CRDS) are attractive due to the ease of sample preparation, automated throughput, and avoidance of chemical conversions needed for isotope-ratio mass spectrometry. However, the CRDS approach requires trade-offs in measurement precision and uncertainty. In this protocol document, we present the following:•New analytical procedures and a software tool for conducting δ ¹⁷O and Δ'¹⁷O measurements by CRDS.•Outline a robust uncertainty framework for Δ'¹⁷O determinations.•Description of a CRDS performance framework for optimizing throughput, instrumental stability, and Δ'¹⁷O measurement precision and accuracy.

Nitrate source apportionment and risk assessment: A study in the largest ion-adsorption rare earth mine in China

Nitrate (NO₃^-) pollution in water bodies has received widespread attention, but studies on nitrogen transformation and pollution risk assessment are still limited, especially in rare earth mining areas. In this study, surface and groundwater samples were collected from the largest rare earth mining site in southern China, and analyzed for the hydrochemical and stable isotopic characteristics. The results showed that the NO₃^- concentrations ranged from 1.61 to 453.11 mg/L, with 35% of surface water and 53.3% of groundwater samples exceeding the WHO standard (i.e., 50 mg/L). Health risk assessment showed that 31.4% of the water samples had a moderate to high non-carcinogenic risk, and the high-risk areas were concentrated in rare earth mining regions. Additionally, adults were more vulnerable to the non-carcinogenic health risks than children. The high variability of δ¹⁵N-NO₃^- (from -6.43 to 17.09‰) and δ¹⁸O-NO₃^- (from -7.91 to 22.79‰) showed that NO₃^- was influenced by multiple nitrogen sources and transformation processes. Hydrochemistry and isotopic evidence further indicated that NO₃^- was primarily influenced by nitrification and hydraulic connection between surface and groundwater. The results of the Bayesian mixing model showed that about 70% of NO₃^- originated from mine drainage and soil N in the rare earth mining area, while more than 90% of NO₃^- originated from fertilizer, soil N, and manure and sewage in rural and urban areas in the middle and downstream. This study suggests reducing anthropogenic nitrogen discharge (e.g., leaching agents and fertilizer inputs) as the primary means of NO₃^- pollution control with biogeochemical processes (e.g., denitrification) to further reduce its pollution.

Stable hydrogen and oxygen isotope abundance of major bottled water brands sold in the United Kingdom

Bottled water in the UK has a ∼20 % share of the soft drinks market with a sales value of >£1.5 billion. Bottled water is susceptible to fraud and it is important to characterise the chemical signature of aquifers used by the bottled water industry. Measuring ¹⁸O/¹⁶O and ²H/¹H ratios in bottled water is one important step in fraud prevention and aquifer characterisation as these ratios in groundwater tend to be stable or change very slowly through time. Here we characterise the isotopic signature of 30 brands of bottled water sold in the UK. The average δ¹⁸O of bottled waters is -7.4 and -48.4 for δ²H. This isotopic composition is closely related to that of the annual rainfall and follows latitudinal and longitudinal gradients which combine to explain 77 % of the δ¹⁸O variance.

Stable isotope variations of dew under three different climates

As a supplementary or the only water source in dry regions, dew plays a critical role in the survival of organisms. The new hydrological tracer ¹⁷O-excess, with almost sole dependence on relative humidity, provides a new way to distinguish the evaporation processes and reconstruct the paleoclimate. Up to now, there is no published daily dew isotope record on δ²H, δ¹⁸O, δ¹⁷O, d-excess, and ¹⁷O-excess. Here, we collected daily dew between July 2014 and April 2018 from three distinct climatic regions (i.e., Gobabeb in the central Namib Desert with desert climate, Nice in France with Mediterranean climate, and Indianapolis in the central United States with humid continental climate). The δ²H, δ¹⁸O, and δ¹⁷O of dew were simultaneously analyzed using a Triple Water Vapor Isotope Analyzer based on Off-Axis Integrated Cavity Output Spectroscopy technique, and then d-excess and ¹⁷O-excess were calculated. This report presents daily dew isotope dataset under three climatic regions. It is useful for researchers to use it as a reference when studying global dew dynamics and dew formation mechanisms.

Measurement(s)	stable isotope variation • dew
Technology Type(s)	water vapour isotope analysis
Factor Type(s)	climate • temporal interval
Sample Characteristic - Environment	desert climate • Mediterranean climate • humid continental climate
Sample Characteristic - Location	Namib Desert • Nice • Indianapolis

Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.19070114

Progress and challenges in dual- and triple-isotope (δ18 O, δ2 H, Δ17 O) analyses of environmental waters: An international assessment of laboratory performance

RATIONALE

Stable isotope analyses of environmental waters (δ² H, δ¹⁸ O) are an important assay in hydrology and environmental research with rising interest in δ¹⁷ O, which requires ultra-precise assays. We evaluated isotope analyses of six test water samples for 281 laboratory submissions measuring δ² H and δ¹⁸ O along with a subset analyzing δ¹⁷ O and Δ¹⁷ O by laser spectrometry and isotope ratio mass spectrometry (IRMS).

METHODS

Six test waters were distributed to laboratories spanning a wide δ range of natural waters for δ² H, δ¹⁸ O and δ¹⁷ O and Δ¹⁷ O. One sample was a blind duplicate to test reproducibility and claims of analytical precision.

RESULTS

Results showed that ca 83% of the submissions produced acceptable δ¹⁸ O and δ² H results within 0.2‰ (mUr) and 1.6‰ of the benchmark values, respectively. However, 17% of the submissions gave questionable to unacceptable results. A blind duplicate revealed many laboratories reported overly optimistic precision, and many could not replicate within their claimed precision. For Δ¹⁷ O, dual-inlet results for IRMS using quantitative O₂ conversion were accurate and highly precise, but the results for laser spectrometry ranged by ca 200 per meg (μUr) for each sample, with ca 70% unable to replicate the duplicate to their claimed Δ¹⁷ O precision. One complicating factor is the lack of certified primary reference waters for δ¹⁷ O.

CONCLUSIONS

No single factor or combination of factors was identifiable for poor or good performance, and underperformance came from issues like data normalization including inadequate memory and drift corrections, compromised working reference materials and underperforming instrumentation. We recommend isotope laboratories include high and low δ value controls of known isotope composition in each run. Progress in Δ¹⁷ O analyses by laser spectrometry requires extraordinary proof of performance claims and would benefit from the development of adoptable and systematic advanced data processing procedures to correct for memory and drift.

Mapping salinization and trace element abundance (including As and other metalloids) in the groundwater of north-central Mexico using a double-clustering approach

This study aimed to determine the reliability of the double-clustering method to understand the spatial association and distribution of major and minor constituents in the groundwater of an arid endorheic basin in central Mexico (Comarca Lagunera Region). The results of the double-clustering approach were compared with well-known spatial statistics such as spatial autocorrelations (Moran index) and the local indicator of spatial association (LISA). Fifty-five groundwater samples were collected from diverse wells within the basin, and the major ions, metalloids, and trace elements were determined. Overall, the double-clustering analysis was an effective tool for identifying lithogenic/anthropogenic processes occurring in the basin and for establishing zones with high or low abundance of major ions and trace elements, even where processes affecting the groundwater quality were spatially dispersed. Although 89% of the samples showed As higher than the threshold value of 10 μg/L proposed by the World Health Organization for drinking water, both the double-clustering and LISA analyses identified As hotspots in the alluvial aquifer, where the extraction of deeper and warmer groundwater might promote the concomitant release of the metalloids As, Sb, and Ge and the trace elements V and W. Similarly, both statistical analyses identified mountainous sectors where the weathering of silicates and carbonates plays a key role in the abundance of HCO₃^-, Ga, and Ba. However, the LISA analysis failed to identify hotspots of carbonate-derived elements such as Ca, Mg, Sr, and U and silicate-derived elements such as Ca, Mg, K, Sr, Rb, Cs, Pb, Ni, and Y. Otherwise, the double-clustering analysis clearly defined high- and low-concentration zones for all these elements in the study region. Unlike the LISA analysis, the double-clustering approach was also successful in determining alluvial areas with high concentrations of Si and Ti and areas where the concentrations of Na, Cl^-, SO₄^2-, NO₃^-, B, Li, Cu, Re, and Se in groundwater were elevated, increasing the groundwater salinity. Overall, this study demonstrated that the double-clustering is an easy-to-apply approach, capable of visualizing disperse zones where specific anthropogenic processes may threaten the groundwater quality.

Aged soils contribute little to contemporary carbon cycling downstream of thawing permafrost peatlands

Vast stores of millennial-aged soil carbon (MSC) in permafrost peatlands risk leaching into the contemporary carbon cycle after thaw caused by climate warming or increased wildfire activity. Here we tracked the export and downstream fate of MSC from two peatland-dominated catchments in subarctic Canada, one of which was recently affected by wildfire. We tested whether thermokarst bog expansion and deepening of seasonally thawed soils due to wildfire increased the contributions of MSC to downstream waters. Despite being available for lateral transport, MSC accounted for ≤6% of dissolved organic carbon (DOC) pools at catchment outlets. Assimilation of MSC into the aquatic food web could not explain its absence at the outlets. Using δ¹³ C-Δ¹⁴ C-δ¹⁵ N-δ² H measurements, we estimated only 7% of consumer biomass came from MSC by direct assimilation and algal recycling of heterotrophic respiration. Recent wildfire that caused seasonally thawed soils to reach twice as deep in one catchment did not change these results. In contrast to many other Arctic ecosystems undergoing climate warming, we suggest waterlogged peatlands will protect against downstream delivery and transformation of MSC after climate- and wildfire-induced permafrost thaw.

Determining nitrate and sulfate pollution sources and transformations in a coastal aquifer impacted by seawater intrusion-A multi-isotopic approach combined with self-organizing maps and a Bayesian mixing model

Over the past few decades, the La Paz aquifer system in Baja California Sur, Mexico, has been under severe pressure due to overexploitation for urban water supply and agriculture; this has caused seawater intrusion and deterioration in groundwater quality. Previous studies on the La Paz aquifer have focused mainly on seawater intrusion, resulting in limited information on nitrate and sulfate pollution. Therefore, pollution sources have not yet been identified sufficiently. In this study, an approach combining hydrochemical tools, multi-isotopes (δ²H_H2O, δ¹⁸O_H2O, δ¹⁵N_NO3, δ¹⁸O_NO3, δ³⁴S_SO4, δ¹⁸O_SO4), and a Bayesian isotope mixing model was used to estimate the contribution of different nitrate and sulfate sources to groundwater. Results from the MixSIAR model revealed that seawater intrusion and soil-derived sulfates were the predominant sources of groundwater sulfate, with contributions of ~43.0% (UI₉₀ = 0.29) and ~42.0% (UI₉₀ = 0.38), respectively. Similarly, soil organic nitrogen (~81.5%, UI₉₀ = 0.41) and urban sewage (~12.1%, UI₉₀ = 0.25) were the primary contributors of nitrate pollution in groundwater. The dominant biogeochemical transformation for NO₃^- was nitrification. Denitrification and sulfate reduction were discarded due to the aerobic conditions in the study area. These results indicate that dual-isotope sulfate analysis combined with MixSIAR models is a powerful tool for estimating the contributions of sulfate sources (including seawater-derived sulfate) in the groundwater of coastal aquifer systems affected by seawater intrusion.

Organic contamination detection for isotopic analysis of water by laser spectroscopy

RATIONALE

Hydrogen and oxygen stable isotope ratios (δ² H, δ¹⁷ O, and δ¹⁸ O values) are commonly used tracers of water. These ratios can be measured by isotope ratio infrared spectroscopy (IRIS). However, IRIS approaches are prone to errors induced by organic compounds present in plant, soil, and natural water samples. A novel approach using ¹⁷ O-excess values has shown promise for flagging spectrally contaminated plant samples during IRIS analysis. A systematic assessment of this flagging system is needed to prove it useful.

METHODS

Errors induced by methanol and ethanol water mixtures on measured IRIS and isotope ratio mass spectrometry (IRMS) results were evaluated. For IRIS analyses both liquid- and vapour-mode (via direct vapour equilibration) methods are used. The δ² H, δ¹⁷ O, and δ¹⁸ O values were measured and compared with known reference values to determine the errors induced by methanol and ethanol contamination. In addition, the ¹⁷ O-excess contamination detection approach was tested. This is a post-processing detection tool for both liquid and vapour IRIS triple-isotope analyses, utilizing calculated ¹⁷ O-excess values to flag contaminated samples.

RESULTS

Organic contamination induced significant errors in IRIS results, not seen in IRMS results. Methanol caused larger errors than ethanol. Results from vapour-IRIS analyses had larger errors than those from liquid-IRIS analyses. The ¹⁷ O-excess approach identified methanol driven error in liquid- and vapour-mode IRIS samples at levels where isotope results became unacceptably erroneous. For ethanol contaminated samples, a mix of erroneous and correct flagging occurred with the ¹⁷ O-excess method. Our results indicate that methanol is the more problematic contaminant for data corruption. The ¹⁷ O-excess method was therefore useful for data quality control.

CONCLUSIONS

Organic contamination caused significant errors in IRIS stable isotope results. These errors were larger during vapour analyses than during liquid IRIS analyses, and larger for methanol than ethanol contamination. The ¹⁷ O-excess method is highly sensitive for detecting narrowband (methanol) contamination error in vapour and liquid analysis modes in IRIS.

Spectroscopic investigation of hydrogen and triple-oxygen isotopes in atmospheric water vapor and precipitation during Indian monsoon season

Water vapor, the most important greenhouse gas in the atmosphere, has four natural stable isotopologues (H₂¹⁶O, H₂¹⁷O, H₂¹⁸O and HD¹⁶O), and their isotopic compositions can be used as hydrological tracers. But the underlying processes and pattern-dynamics of the isotopic compositions of atmospheric water vapor and precipitation in response to various meteorological conditions during monsoon season in a tropical hot and humid region is poorly understood. Here, we present results of H and triple-O-isotopes of water in precipitation and atmospheric water vapor during monsoon season exploiting high-resolution integrated cavity output spectroscopy technique. We observed a distinct temporal variation of the isotopic compositions of water at different phases of the monsoon. The diurnal patterns of the isotopic variations were influenced by the local meteorological factors such as temperature, relative humidity and amount of precipitation. We also investigated the monsoonal dynamics of the second-order isotopic parameters, so-called d-excess and ¹⁷O-excess along with the influence of local meteorological factors on isotopic variations to improve our understanding of the underlying isotopic fractionation processes. Consequently, our results provide a unique isotopic-fingerprint dataset of rainwater and atmospheric water vapor for a tropical region and thus shed a new light on hydrological and meteorological processes in the atmosphere.

Review on Applications of 17O in Hydrological Cycle

The triple oxygen isotopes (16O, 17O, and 18O) are very useful in hydrological and climatological studies because of their sensitivity to environmental conditions. This review presents an overview of the published literature on the potential applications of 17O in hydrological studies. Dual-inlet isotope ratio mass spectrometry and laser absorption spectroscopy have been used to measure 17O, which provides information on atmospheric conditions at the moisture source and isotopic fractionations during transport and deposition processes. The variations of δ17O from the developed global meteoric water line, with a slope of 0.528, indicate the importance of regional or local effects on the 17O distribution. In polar regions, factors such as the supersaturation effect, intrusion of stratospheric vapor, post-depositional processes (local moisture recycling through sublimation), regional circulation patterns, sea ice concentration and local meteorological conditions determine the distribution of 17O-excess. Numerous studies have used these isotopes to detect the changes in the moisture source, mixing of different water vapor, evaporative loss in dry regions, re-evaporation of rain drops during warm precipitation and convective storms in low and mid-latitude waters. Owing to the large variation of the spatial scale of hydrological processes with their extent (i.e., whether the processes are local or regional), more studies based on isotopic composition of surface and subsurface water, convective precipitation, and water vapor, are required. In particular, in situ measurements are important for accurate simulations of atmospheric hydrological cycles by isotope-enabled general circulation models.

Triple-isotope calibration of in-house water standards supplemented by determination of 17O content of USGS49-50 reference materials using cavity ring-down laser spectrometry

The procedure of calibrating in-house water standards suitable for routine analyses of triple-isotope composition of water samples using Picarro L2140-i CRDS analyser is presented and discussed. Such standards are indispensable for achieving and maintaining high quality of isotope analyses of water in terms of their precision and accuracy. A set of seven different water standards consisting of three in-house standards and four secondary standards commercially available was calibrated against VSMOW2/SLAP2 primary reference materials. The calibrated standards cover a wide range of isotopic composition, with δ values ranging from close to zero to the values comparable with SLAP2. The apparent consistency of the calibrated values of δ²H, δ¹⁸O and d-excess with corresponding certified values for commercially available USGS47-50 standards and the consistency of the calibrated values of δ¹⁷O and Δ¹⁷O with its literature values for USGS47-48 standards confirm the high quality of the performed calibration. Moreover, the calibration exercise allowed to obtain δ¹⁷O and Δ¹⁷O values for USGS49 and USGS50 standards, not reported so far.

Isotope ratio infrared spectroscopy analysis of water samples without memory effects

RATIONALE

Since their introduction more than a decade ago, isotope ratio infrared spectroscopy (IRIS) systems have rapidly become the standard for oxygen (δ¹⁸ O) and hydrogen (δ² H) isotope analysis of water samples. An important disadvantage of IRIS systems is the well-documented sample-to-sample memory effect, which requires each sample to be analyzed multiple times before the desired accuracy is reached, lengthening analysis times and driving up the costs of analyses.

METHODS

We present an adapted set-up and calculation protocol for fully automated analysis of water samples using a Picarro L2140-i cavity ring-down spectroscopy instrument. The adaptation removes memory effects by use of a continuously moisturized nitrogen carrier gas. Water samples of 0.5 μL are measured on top of the water vapor background, after which isotope ratios are calculated by subtraction of the background from the sample peaks.

RESULTS

With this new technique, single injections of water samples have internal precisions (1σ) below 0.05‰ for δ¹⁸ O values and 0.1‰ for δ² H values, regardless of the isotope ratio of the previous sample. Precision is worse, however, when the isotope difference between the sample and background water is too large (i.e., exceeding approximately 9‰ for δ¹⁸ O values and 70‰ for δ² H values). Isotope ratios show negligible drift across the four weeks within which the experiments were performed. The single-injection 1σ precision for ¹⁷ O excess (Δ'¹⁷ O) determined with this method is 60 per meg.

CONCLUSIONS

Our experiments demonstrate that by removing sample-to-sample memory effects with a moisturized carrier gas, the time for measurement of δ¹⁸ O and δ² H values using an IRIS system can be reduced markedly without compromising the analytical precision and accuracy. Thorough replication is needed to achieve sufficiently low uncertainties for Δ'¹⁷ O.

Measurement of δ18O and δ2H of water and ethanol in wine by Off-Axis Integrated Cavity Output Spectroscopy and Isotope Ratio Mass Spectrometry

There are two officially approved methods for stable isotope analysis for wine authentication. One describes δ18O measurements of the wine water using Isotope Ratio Mass Spectrometry (IRMS), and the other one uses Deuterium-Nuclear Magnetic Resonance (2H-NMR) to measure the deuterium of the wine ethanol. Recently, off-axis integrated cavity output (laser) spectroscopy (OA-ICOS) has become an easier alternative to quantify wine water isotopes, thanks to the spectral contaminant identifier (SCI). We utilized an OA-ICOS analyser with SCI to measure the δ18O and δ2H of water in 27 wine samples without any pre-treatment. The OA-ICOS results reveal a wealth of information about the growth conditions of the wines, which shows the advantages to extend the official δ18O wine water method by δ2H that is obtained easily from OA-ICOS. We also performed high-temperature pyrolysis and chromium reduction combined with IRMS measurements to illustrate the “whole wine” isotope ratios. The δ18O results of OA-ICOS and IRMS show non-significant differences, but the δ2H results of both methods differ much more. As the δ2H difference between these two methods is mainly caused by ethanol, we investigated the possibility to deduce deuterium of wine ethanol from this difference. The results present large uncertainties and deviate from the obtained 2H-NMR results. The deviation is caused by the other constituents in the wine, and the uncertainty is due to the limited precision of the SCI-based correction, which need to improve to obtain the 2H values of ethanol as alternative for the 2H-NMR method.

High-precision methane isotopic abundance analysis using near-infrared absorption spectroscopy at 100 Torr

A near-infrared methane (CH₄) sensor system for carbon isotopic abundance analysis was developed based on laser absorption spectroscopy (LAS). For good thermal stability, two CH₄ absorption lines with a similar low-state energy level were selected to realize relative weak temperature dependence. Wavelet denoising (WD) was employed for a pre-treatment of the direct absorption spectral (DAS) signal to perform a preliminary suppression of high-frequency noise. Due to the abnormal ¹³CH₄ profile caused by superimposition of multiple lines, two statistical analysis algorithms including linear regression and neural network prediction were respectively employed on the retrieval of molecule fractions instead of the traditionally used standard absorption line fitting method. Performance assessment and a comparison between the two methods were carried out. Compared with the concentration deducing method based on the maximum absorbance in rough data, the linear regression and the neural network prediction obtained a sensitivity enhancement by ∼2 times and ∼10 times, respectively. A simultaneous measurement of pressure and concentration was performed using the neural network, which indicated a good potential of the technique for multi-parameter analysis using a single LAS-based sensor system.

Estimation of nitrate pollution sources and transformations in groundwater of an intensive livestock-agricultural area (Comarca Lagunera), combining major ions, stable isotopes and MixSIAR model

The identification of nitrate (NO₃^-) sources and biogeochemical transformations is critical for understanding the different nitrogen (N) pathways, and thus, for controlling diffuse pollution in groundwater affected by livestock and agricultural activities. This study combines chemical data, including environmental isotopes (δ²H_H2O, δ¹⁸O_H2O, δ¹⁵N_NO3, and δ¹⁸O_NO3), with land use/land cover data and a Bayesian isotope mixing model, with the aim of reducing the uncertainty when estimating the contributions of different pollution sources. Sampling was taken from 53 groundwater sites in Comarca Lagunera, northern Mexico, during 2018. The results revealed that the NO₃^- (as N) concentration ranged from 0.01 to 109 mg/L, with more than 32% of the sites exceeding the safe limit for drinking water quality established by the World Health Organization (10 mg/L). Moreover, according to the groundwater flow path, different biogeochemical transformations were observed throughout the study area: microbial nitrification was dominant in the groundwater recharge areas with elevated NO₃^- concentrations; in the transition zones a mixing of different transformations, such as nitrification, denitrification, and/or volatilization, were identified, associated to moderate NO₃^- concentrations; whereas in the discharge area the main process affecting NO₃^- concentrations was denitrification, resulting in low NO₃^- concentrations. The results of the MixSIAR isotope mixing model revealed that the application of manure from concentrated animal-feeding operations (∼48%) and urban sewage (∼43%) were the primary contributors of NO₃^- pollution, whereas synthetic fertilizers (∼5%), soil organic nitrogen (∼4%), and atmospheric deposition played a less important role. Finally, an estimation of an uncertainty index (UI90) of the isotope mixing results indicated that the uncertainties associated with atmospheric deposition and NO₃^--fertilizers were the lowest (0.05 and 0.07, respectively), while those associated with manure and sewage were the highest (0.24 and 0.20, respectively).

Triple isotope variations of monthly tap water in China

Tap water isotopic compositions could potentially record information on local climate and water management practices. A new water isotope tracer 17O-excess became available in recent years providing additional information of the various hydrological processes. Detailed data records of tap water 17O-excess have not been reported. In this report, monthly tap water samples (n = 652) were collected from December 2014 to November 2015 from 92 collection sites across China. The isotopic composition (δ2H, δ18O, and δ17O) of tap water was analyzed by a Triple Water Vapor Isotope Analyzer (T-WVIA) based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) technique and two second-order isotopic variables (d-excess and 17O-excess) were calculated. The geographic location information of the 92 collection sites including latitude, longitude, and elevation were also provided in this dataset. This report presents national-scale tap water isotope dataset at monthly time scale. Researchers and water resource managers who focus on the tap water issues could use them to probe the water source and water management strategies at large spatial scales.

An agent-based model that simulates the spatio-temporal dynamics of sources and transfer mechanisms contributing faecal indicator organisms to streams. Part 1: Background and model description

A new Model for the Agent-based simulation of Faecal Indicator Organisms (MAFIO) is developed that attempts to overcome limitations in existing faecal indicator organism (FIO) models arising from coarse spatial discretisations and poorly-constrained hydrological processes. MAFIO is a spatially-distributed, process-based model presently designed to simulate the fate and transport of agents representing FIOs shed by livestock at the sub-field scale in small (<10 km²) agricultural catchments. Specifically, FIO loading, die-off, detachment, surface routing, seepage and channel routing are modelled on a regular spatial grid. Central to MAFIO is that hydrological transfer mechanisms are simulated based on a hydrological environment generated by an external model for which it is possible to robustly determine the accuracy of simulated catchment hydrological functioning. The spatially-distributed, tracer-aided ecohydrological model EcH₂O-iso is highlighted as a possible hydrological environment generator. The present paper provides a rationale for and description of MAFIO, whilst a companion paper applies the model in a small agricultural catchment in Scotland to provide a proof-of-concept.

Real-time measurement of CO2 isotopologue ratios in exhaled breath by a hollow waveguide based mid-infrared gas sensor

A hollow waveguide (HWG) based mid-infrared gas sensor using a 2.73 µm distributed feedback (DFB) laser was developed for simultaneously measuring the concentration changes of the three isotopologues ¹³CO₂, ¹²CO₂, and ¹⁸OC¹⁶O in exhaled breath by direct absorption spectroscopy, and then determining the ¹³CO₂/¹²CO₂ isotope ratio (δ¹³C) and ¹⁸OC¹⁶O/¹²CO₂ isotope ratio (δ¹⁸O). The HWG sensor showed a fast response time of 3 s. Continuous measurement of δ¹³C and δ¹⁸O in the standard CO₂ sample with known isotopic ratios for ∼2 h was performed. Precisions of 2.20‰ and 1.98‰ for δ¹³C and δ¹⁸O respectively at optimal integration time of 734 s were estimated from Allan variance analysis. Accuracy of -0.49‰ and -1.20‰ for δ¹³C and δ¹⁸O, respectively, were obtained with comparison to the values of the reference standard. The Kalman filtering method was employed to improve the precision and accuracy of the HWG sensor while maintaining high time resolution. Precision of 5.45‰ and 4.88‰ and the accuracy of 0.21‰ and -1.13‰ for δ¹³C and δ¹⁸O, respectively, were obtained at the integration time of 0.54 s with the application of Kalman filtering. The concentrations of ¹²CO₂, ¹³CO₂ and ¹⁸OC¹⁶O in breath cycles were measured and processed by Kalman filtering in real time. The measured values of δ¹⁸O and δ¹³C in exhaled breath were estimated to be -21.35‰ and -33.64‰, respectively, with the integration time of 1 s. This study demonstrates the ability of the HWG sensor to obtain δ¹³C and δ¹⁸O values in breath samples and its potential for immediate respiratory monitoring and disease diagnosis.

Analysis of the Stable Isotope Ratios (18O/16O, 17O/16O, and D/H) in Glacier Water by Laser Spectrometry

A compact isotope ratio sensor based on laser absorption spectroscopy at 2.7 μm was developed for high precision and simultaneous measurements of the D/H, ¹⁸O/¹⁶O and ¹⁷O/¹⁶O isotope ratios in glacier water. Measurements of the oxygen and hydrogen isotope ratios in glacier water demonstrate a 1σ precision of 0.3‰ for δ¹⁸O, 0.2‰ for δ¹⁷O, and 0.5‰ for δ²H, respectively. The δ values of the working standard glacier water obtained by the calibrated sensor system is basically identical to the IRMS measurement results with a very high calibration accuracy from 0.17‰ to 0.75‰. Preliminary results on the reproducibility measurements display a standard deviation of 0.13‰ for δ¹⁸O, 0.13‰ for δ¹⁷O, and 0.64‰ for δ²H, respectively.

Improved accuracy and precision of water stable isotope measurements using the direct vapour equilibration method

RATIONALE

A method to measure the δ² H and δ¹⁸ O composition of pore water in soil samples using direct vapour equilibration and laser spectrometry was first described in 2008, and was rapidly adopted. Here, we describe an improved setup to measure pore water δ² H and δ¹⁸ O values through direct vapour equilibration with a laser spectrometer, combining a liquid and a vapour mode for water isotope analyses, and resulting in improved accuracy.

METHODS

We first tested new gas sampling bags as part of the equilibration protocol. Then, to assess measurement accuracy, vapour samples from equilibrated liquid waters of known isotope composition were measured in the liquid mode of the analyser using the new setup as well as the manufacturer's vapour mode. Various modes of preparing liquid water standards, namely equilibration, nebulisation, and vapourisation, were tested to determine the best calibration in terms of accuracy. Finally, the proposed modified liquid setup was validated by analysing water vapour equilibrated from soil pore water of a known composition.

RESULTS

The δ² H and δ¹⁸ O measurements were found to be more accurate by the modified liquid mode than by the factory-setup vapour mode. The strong and non-linear dependence of measured δ² H and δ¹⁸ O values on H₂ O concentration in vapour mode, especially at concentrations equal to the vapour pressure saturation typically found in laboratories, is problematic for corrections. Regarding calibration and standards, the use of two equilibrated liquid water standards was found to best calibrate measurements in the modified liquid setup. Finally, the modified liquid mode setup and its calibration, as described here, were shown to be appropriate for soil pore water analysis.

CONCLUSIONS

The proposed modified setup results in more precise δ² H and δ¹⁸ O soil pore water values than the usual protocols. An average standard deviation of 0.04‰ for δ¹⁸ O values and 0.3‰ for δ² H values, based on 228 soil sample analyses, was obtained.

17 O-excess as a detector for co-extracted organics in vapor analyses of plant isotope signatures

RATIONALE

The stable isotope compositions of hydrogen and oxygen in water (δ² H and δ¹⁸ O values) have been widely used to investigate plant water sources, but traditional isotopic measurements of plant waters are expensive and labor intensive. Recent work with direct vapor equilibration (DVE) on laser spectroscopy has shown potential to side step limitations imposed by traditional methods. Here, we evaluate DVE analysis of plants with a focus on spectral contamination introduced by organic compounds. We present ¹⁷ O-excess as a way of quantifying organic compound interference in DVE.

METHODS

We performed isotopic analysis using the δ² H, δ¹⁸ O and δ¹⁷ O values of water on an Off-Axis Integrated Cavity Output Spectroscopy (IWA-45EP OA-ICOS) instrument in vapor mode. We used a set of methanol (MeOH) and ethanol (EtOH) solutions to assess errors in isotope measurements. We evaluated how organic compounds affect the ¹⁷ O-excess. DVE was used to measure the isotopic signatures in natural plant material from Pinus banksiana, Picea mariana, and Larix laricina, and soil from boreal forest for comparison with solutions.

RESULTS

The ¹⁷ O-excess was sensitive to the presence of organic compounds in water. ¹⁷ O-excess changed proportionally to the concentration of MeOH per volume of water, resulting in positive values, while EtOH solutions resulted in smaller changes in the ¹⁷ O-excess. Soil samples did not show any spectral contamination. Plant samples were spectrally contaminated on the narrow-band and were enriched in ¹ H and ¹⁶ O compared with source water. L. laricina was the only species that did not show any evidence of spectral contamination. Xylem samples that were spectrally contaminated had positive ¹⁷ O-excess values.

CONCLUSIONS

¹⁷ O-excess can be a useful tool to identify spectral contamination and improve DVE plant and soil analysis in the laboratory and in situ. The ¹⁷ O-excess flagged the presence of MeOH and EtOH. Adding measurement of δ¹⁷ O values to traditional measurement of δ² H and δ¹⁸ O values may shed new light on plant water analysis for source mixing dynamics using DVE.

Maximizing precision and accuracy of the doubly labeled water method via optimal sampling protocol, calculation choices, and incorporation of 17O measurements

BACKGROUND/OBJECTIVES

The doubly labeled water (DLW) method is the gold standard methodology for determination of free-living, total daily energy expenditure (TEE). However, there is no single accepted approach for either the sampling protocols (daily vs. two-point, in which samples are collected after dosing and at the end of the measurement period) or the calculations used in the determination of the rate of carbon dioxide production (rCO₂) and TEE. Moreover, fluctuations in natural background abundances introduce error in the calculation of rCO₂ and TEE. The advent of new technologies makes feasible the possibility of including additional isotope measures (¹⁷O) to account for background variation, which may improve accuracy.

SUBJECTS/METHODS

Sixteen subjects were studied for 7 consecutive days in a whole-room indirect calorimeter (IC) with concurrent measurement of TEE by DLW. Daily urine samples were obtained and isotope ratios were determined using off-axis integrated cavity output spectroscopy (OA-ICOS).

RESULTS

We determined the best combination of approaches for estimating dilution spaces and elimination rates and calculated average daily volume of carbon dioxide production (VCO₂) using six different published equations. Using this best combination, multi-point fitting of isotope elimination rates using the daily urine samples substantially improved the average precision (4.5% vs. 6.0%) and accuracy (-0.5% vs. -3.0%) compared with the two-point method. This improvement may partly reflect the less variable day-to-day chamber measurements of energy expenditure. Utilizing ¹⁷O measurements to correct for errors due to background isotope fluctuations provided additional but minor improvements in precision (4.2% vs. 4.5%) and accuracy (0.2% vs. 0.5%).

CONCLUSIONS

This work shows that optimizing sampling and calculation protocols can improve the accuracy and precision of DLW measurements.

High-precision measurements of δ2H, δ18O and δ17O in water with the aid of cavity ring-down laser spectroscopy

A thorough evaluation of measurement uncertainty together with control of short-term and long-term precision of measurements should be a basis of any successful quality assurance/quality control (QA/QC) strategy aimed at maintaining a high quality of the analytical process. Here we present the results of a comprehensive assessment of the analytical performance of a Picarro L2140-i CRDS laser spectrometer analysing δ²H, δ¹⁸O and δ¹⁷O in water. The assessment is based on results obtained during 15 months of continuous operation of this instrument (February 2017 to May 2018). The short-term precision of measured and derived quantities was 0.11, 0.036, 0.028, 0.23 ‰ and 11 per meg, for δ²H, δ¹⁸O, δ¹⁷O, d-excess and Δ¹⁷O, respectively, and is comparable to the precision reported by the manufacturer. The long-term precision of the L2140-i, defined as standard uncertainty of the time series of 153 analyses of a laboratory standard conducted throughout 15 months, was roughly two times lower (0.24, 0.053, 0.038, 0.37 ‰ and 21 per meg, for δ²H, δ¹⁸O, δ¹⁷O, d-excess and Δ¹⁷O). In-depth assessment of the measurement uncertainty of a single analysis revealed that assigned uncertainty of the calibration standards is an important component of the uncertainty budget, especially in case of δ²H analysis.

Stable isotope variations of daily precipitation from 2014-2018 in the central United States

Stable isotopes of hydrogen and oxygen (δ²H, δ¹⁸O and δ¹⁷O) serve as powerful tracers in hydrological investigations. To our knowledge, daily precipitation isotope record especially ¹⁷O-excess is rare in the mid-latitudes. To fill such knowledge gap, daily precipitation samples (n=446) were collected from June 2014 to May 2018 in Indianapolis, Indiana, U.S. A Triple Water Vapor Isotope Analyzer (T-WVIA) based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) technique was used to concurrently measure precipitation isotopic variations (δ²H, δ¹⁸O and δ¹⁷O). Meanwhile, ¹⁷O-excess and d-excess as second-order isotopic variables were calculated to provide additional information on precipitation formation and transport mechanisms. This study presents a four-year daily precipitation isotope dataset for mid-latitudes, and makes it available to researchers around the world who may use it as a reference for site comparisons and for assessing global hydrological models.

Triple oxygen isotope analysis of nitrate using isotope exchange cavity ringdown laser spectroscopy

RATIONALE

Triple oxygen isotopes (¹⁶ O/¹⁷ O/¹⁸ O) in nitrate are a valuable tool to ascertain the pathways of nitrate formation in the atmosphere and the fate of nitrate in ecosystems. Here we present a new method for determining Δ¹⁷ O values in nitrates, based on nitrate-water isotope equilibration (IE) and subsequent isotopic analysis of water using cavity ringdown laser spectroscopy (CRDS).

METHODS

Nitrate oxygen (O-NO₃ ^- ) is equilibrated with water oxygen (O-H₂ O) at low pH and 80°C. Subsequently, the δ¹⁷ O and δ¹⁸ O values of equilibrated water are determined by CRDS, scaled to V-SMOW and V-SLAP and calibrated against nitrate standards (USGS-34, USGS-35 and IAEA-NO3). We provide isotopic measurements of synthetic and natural nitrates and a direct inter-lab comparison with the classic method of thermal-decomposition of nitrate followed by isotope ratio mass spectrometry of O₂ (TD-IRMS).

RESULTS

For synthetic NaNO₃ , the precision (1SD) of the IE-CRDS method is 0.8‰ for δ¹⁷ O values, 1.7‰ for δ¹⁸ O values and 0.2‰ for Δ¹⁷ O values when using an O-NO₃ ^- /O-H₂ O ratio greater than 0.0114 ± 0.0001 (e.g. 12 μmol of NO₃ ^- in 50 μL of acid solution). For natural samples, after purification of nitrates by column chemistry and reprecipitation as AgNO₃ , the precision is better than 1.8‰ for δ¹⁷ O values, 3.2‰ for δ¹⁸ O values and 1‰ for Δ¹⁷ O values. IE-CRDS and TD-IRMS yield Δ¹⁷ O values within the analytical errors of the two methods.

CONCLUSIONS

The IE-CRDS method for determining Δ¹⁷ O values in nitrates utilizes a user-friendly and relatively cheaper benchtop analytical instrument, representing an alternative to IRMS-based methods for certain applications.

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

RATIONALE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Stable isotope compositions (δ2H, δ18O and δ17O) of rainfall and snowfall in the central United States

Stable isotopes of hydrogen and oxygen (δ²H, δ¹⁸O and δ¹⁷O) can be used as natural tracers to improve our understanding of hydrological and meteorological processes. Studies of precipitation isotopes, especially ¹⁷O-excess observations, are extremely limited in the mid-latitudes. To fill this knowledge gap, we measured δ²H, δ¹⁸O and δ¹⁷O of event-based precipitation samples collected from Indianapolis, Indiana, USA over two years and investigated the influence of meteorological factors on precipitation isotope variations. The results showed that the daily temperature played a major role in controlling the isotope variations. Precipitation experienced kinetic fractionation associated with evaporation at the moisture source in the spring and summer and for rainfall, while snowfall, as well as precipitation in the fall and winter, were mainly affected by equilibrium fractionation. The ¹⁷O-excess of both rainfall and snowfall were not affected by local meteorological factors over the whole study period. At the seasonal scale, it was the case only for the spring. Therefore, ¹⁷O-excess of rainfall, snowfall and the spring precipitation could be considered as tracers of evaporative conditions at the moisture source. This study provides a unique precipitation isotope dataset for mid-latitudes and provides a more mechanistic understanding of precipitation formation mechanisms in this region.

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

RATIONALE

Water stable isotope ratios (δ² H and δ¹⁸ 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 δ¹⁸ O and δ² H, respectively; ~27 % produced unacceptable results. Top performance for δ¹⁸ O values was dominated by dual-inlet IRMS laboratories; top performance for δ² 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.

Validation of the doubly labeled water method using off-axis integrated cavity output spectroscopy and isotope ratio mass spectrometry

When the doubly labeled water (DLW) method is used to measure total daily energy expenditure (TDEE), isotope measurements are typically performed using isotope ratio mass spectrometry (IRMS). New technologies, such as off-axis integrated cavity output spectroscopy (OA-ICOS) provide comparable isotopic measurements of standard waters and human urine samples, but the accuracy of carbon dioxide production (V̇co₂) determined with OA-ICOS has not been demonstrated. We compared simultaneous measurement V̇co₂ obtained using whole-room indirect calorimetry (IC) with DLW-based measurements from IRMS and OA-ICOS. Seventeen subjects (10 female; 22 to 63 yr) were studied for 7 consecutive days in the IC. Subjects consumed a dose of 0.25 g H₂¹⁸O (98% APE) and 0.14 g ²H₂O (99.8% APE) per kilogram of total body water, and urine samples were obtained on days 1 and 8 to measure average daily V̇co₂ using OA-ICOS and IRMS. V̇co₂ was calculated using both the plateau and intercept methods. There were no differences in V̇co₂ measured by OA-ICOS or IRMS compared with IC when the plateau method was used. When the intercept method was used, V̇co₂ using OA-ICOS did not differ from IC, but V̇co₂ measured using IRMS was significantly lower than IC. Accuracy (~1-5%), precision (~8%), intraclass correlation coefficients ( R = 0.87-90), and root mean squared error (30-40 liters/day) of V̇co₂ measured by OA-ICOS and IRMS were similar. Both OA-ICOS and IRMS produced measurements of V̇co₂ with comparable accuracy and precision compared with IC.

Development of an isotopic stream index connecting physiographic characteristics of montane catchments

This study is to develop an isotopic catchment-effect index (CEI) connecting the physiographic characteristics of stream catchments. A CEI, describing the extent of difference in stable water isotopic compositions (δ values) between stream water and local precipitation at any given sampling site, can help in judging whether water resource management should be focused on upstream regions of streams or local hydrology issues. To establish the isotopic CEI, this study measured δ values of stream water and derived δ¹⁸ O of local precipitation based on regional isotopic altitude gradient at montane catchments of various sizes. Results indicate that the CEI is strongly related to catchment physiographic characteristics, such as length of main stream, mean area, mean elevation, perimeter, and slope. These characteristics are considered important indices of streamflow. Based on mathematical regression modeling describing the relationships between CEI and respective physiographic factors, CEI values can predict respective physiographic factors and vice versa. Moreover, according to the multiple equations derived in this study, catchments of larger size and steeper slope give elevated CEI values while greater stream length reduces the CEI's value. A greater CEI value indicates that local stream water is principally sourced from upstream reaches rather than contributions from local precipitation. In addition, CEI values are greater in winter than in summer resulting from monsoon effect. Consequently, this study establishes CEI as a useful descriptor of the physiographic characteristics of catchments.

Isotopic composition in precipitation and groundwater in the northern mountainous region of the Central Valley of Costa Rica

The linkage between precipitation and recharge is still poorly understood in the Central America region. This study focuses on stable isotopic composition in precipitation and groundwater in the northern mountainous region of the Central Valley of Costa Rica. During the dry season, rainfall samples corresponded to enriched events with high deuterium excess. By mid-May, the Intertropical Convergence Zone poses over Costa Rica resulting in a depletion of ¹⁸O/¹⁶O and ²H/H ratios. A parsimonious four-variable regression model (r²= 0.52) was able to predict daily δ¹⁸O in precipitation. Air mass back trajectories indicated a combination of Caribbean Sea and Pacific Ocean sources, which is clearly depicted in groundwater isoscape. Aquifers relying on Pacific-originated recharge exhibited a more depleted pattern, whereas recharge areas relying on Caribbean parental moisture showed an enrichment trend. These results can be used to enhance modelling efforts in Central America where scarcity of long-term data limits water resources management plans.

Quantitative isotopic measurements of gas-phase alcohol mixtures using a broadly tunable swept external cavity quantum cascade laser

Isotopic quantification of gas-phase mixtures is performed using a swept external cavity quantum cascade laser and broadband infrared spectral analysis.

A City-wide Investigation of the Isotopic Distribution and Source of Tap Waters for Forensic Human Geolocation Ground-truthing

Human geolocation is prefaced on the accuracy of the geographic precision of mapped isotopic values for drinking water. As most people live in cities, it becomes important to understand city water supplies and how the isotopic values uniquely reflect that city. This study investigated the isotopic distribution of δ² H and δ¹⁸ O from sourced tap waters that were collected from across the Metro Vancouver (MV) area (n = 135). The results revealed that the isotopic values reflect their water sources with a range of 5.3‰ for δ¹⁸ O_tap and 29.3‰ for δ² H_tap for MV. The results indicate that individual cities need higher resolution studies to determine their tap water isotopic ranges, and a good understanding of the water supply network itself for human geolocation work. With an extended high-resolution understanding of each city, human tissue may be compared with more certainty for geolocation.

Stable isotopes in atmospheric water vapor and applications to the hydrologic cycle

The measurement and simulation of water vapor isotopic composition has matured rapidly over the last decade, with long-term datasets and comprehensive modeling capabilities now available. Theories for water vapor isotopic composition have been developed by extending the theories that have been used for the isotopic composition of precipitation to include a more nuanced understanding of evaporation, large-scale mixing, deep convection, and kinetic fractionation. The technologies for in-situ and remote sensing measurements of water vapor isotopic composition have developed especially rapidly over the last decade, with discrete water vapor sampling methods, based on mass spectroscopy, giving way to laser spectroscopic methods and satellite- and ground-based infrared absorption techniques. The simulation of water vapor isotopic composition has evolved from General Circulation Model (GCM) methods for simulating precipitation isotopic composition to sophisticated isotope-enabled microphysics schemes using higher-order moments for water- and ice-size distributions. The incorporation of isotopes into GCMs has enabled more detailed diagnostics of the water cycle and has led to improvements in its simulation. The combination of improved measurement and modeling of water vapor isotopic composition opens the door to new advances in our understanding of the atmospheric water cycle, in processes ranging from the marine boundary layer, through deep convection and tropospheric mixing, and into the water cycle of the stratosphere. Finally, studies of the processes governing modern water vapor isotopic composition provide an improved framework for the interpretation of paleoclimate proxy records of the hydrological cycle.

Water vapor δ(2) H, δ(18) O and δ(17) O measurements using an off-axis integrated cavity output spectrometer - sensitivity to water vapor concentration, delta value and averaging-time

RATIONALE

High-precision analysis of atmospheric water vapor isotope compositions, especially δ(17) O values, can be used to improve our understanding of multiple hydrological and meteorological processes (e.g., differentiate equilibrium or kinetic fractionation). This study focused on assessing, for the first time, how the accuracy and precision of vapor δ(17) O laser spectroscopy measurements depend on vapor concentration, delta range, and averaging-time.

METHODS

A Triple Water Vapor Isotope Analyzer (T-WVIA) was used to evaluate the accuracy and precision of δ(2) H, δ(18) O and δ(17) O measurements. The sensitivity of accuracy and precision to water vapor concentration was evaluated using two international standards (GISP and SLAP2). The sensitivity of precision to delta value was evaluated using four working standards spanning a large delta range. The sensitivity of precision to averaging-time was assessed by measuring one standard continuously for 24 hours.

RESULTS

Overall, the accuracy and precision of the δ(2) H, δ(18) O and δ(17) O measurements were high. Across all vapor concentrations, the accuracy of δ(2) H, δ(18) O and δ(17) O observations ranged from 0.10‰ to 1.84‰, 0.08‰ to 0.86‰ and 0.06‰ to 0.62‰, respectively, and the precision ranged from 0.099‰ to 0.430‰, 0.009‰ to 0.080‰ and 0.022‰ to 0.054‰, respectively. The accuracy and precision of all isotope measurements were sensitive to concentration, with the higher accuracy and precision generally observed under moderate vapor concentrations (i.e., 10000-15000 ppm) for all isotopes. The precision was also sensitive to the range of delta values, although the effect was not as large compared with the sensitivity to concentration. The precision was much less sensitive to averaging-time than the concentration and delta range effects.

CONCLUSIONS

The accuracy and precision performance of the T-WVIA depend on concentration but depend less on the delta value and averaging-time. The instrument can simultaneously and continuously measure δ(2) H, δ(18) O and δ(17) O values in water vapor, opening a new window to better understand ecological, hydrological and meteorological processes. Copyright © 2016 John Wiley & Sons, Ltd.

Routine high-precision analysis of triple water-isotope ratios using cavity ring-down spectroscopy

RATIONALE

Water isotope analysis for δ(2) H and δ(18) O values via laser spectroscopy is routine for many laboratories. While recent work has added the δ(17) O value to the high-precision suite, it does not follow that researchers will routinely obtain high precision (17) O excess (Δ(17) O). We demonstrate the routine acquisition of high-precision δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values using a commercially available laser spectroscopy instrument.

METHODS

We use a Picarro L2140-i cavity ring-down spectroscopy analyzer with discrete liquid injections into an A0211 vaporization module by a Leap Technologies LC PAL autosampler. The instrument is run in two modes: (1) as recommended by the manufacturer (default mode) and (2) after modifying select default settings and using alternative data types (advanced mode). Reference waters analyzed over the course of 15 months while running unknown samples are used to assess system performance.

RESULTS

The default mode provides precision for δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values that may be sufficient for many applications. When using the advanced mode, we reach a higher level of precision for δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values (0.4 mUr, 0.04 mUr, 0.07 mUr, 0.5 mUr, and 8 μUr, respectively, where mUr = 0.001 = ‰, and μUr = 10(-6) ) in a shorter amount of time and with fewer syringe actuations than in the default mode. The improved performance results from an increase in the total integration time for each injected water pulse.

CONCLUSIONS

Our recommended approach for routine δ(2) H, δ(17) O, δ(18) O, d and Δ(17) O measurements with the Picarro L2140-i is to make use of conditioning vials, use fewer injections (5 per vial) with greater pulse duration (520 seconds (s) per injection) and use only the first 120 s for δ(2) H measurements and all 520 s for δ(17) O and δ(18) O measurements. Although the sample throughput is 10 unknowns per day, our optimal approach reduces the number of syringe actuations, the effect of memory, and the total analysis time, while improving precision relative to the default approach. Copyright © 2016 John Wiley & Sons, Ltd.

Inter- and intraindividual correlations of background abundances of (2)H, (18)O and (17)O in human urine and implications for DLW measurements

BACKGROUND/OBJECTIVES

The method of choice for measuring total energy expenditure in free-living individuals is the doubly labeled water (DLW) method. This experiment examined the behavior of natural background isotope abundance fluctuations within and between individuals over time to assess possible methods of accounting for variations in the background isotope abundances to potentially improve the precision of the DLW measurement.

SUBJECTS/METHODS

In this work, we measured natural background variations in (2)H, (18)O and (17)O in water from urine samples collected from 40 human subjects who resided in the same geographical area. Each subject provided a urine sample for 30 consecutive days. Isotopic abundances in the samples were measured using Off-Axis Integrated Cavity Output Spectroscopy.

RESULTS

Autocorrelation analyses demonstrated that the background isotopes in a given individual were not temporally correlated over the time scales of typical DLW studies. Using samples obtained from different individuals on the same calendar day, cross-correlation analyses demonstrated that the background variations of different individuals were not correlated in time. However, the measured ratios of the three isotopes (2)H, (18)O and (17)O were highly correlated (R(2)=0.89-0.96).

CONCLUSIONS

Although neither specific timing of DLW water studies nor intraindividual comparisons were found to be avenues for reducing the impact of background isotope abundance fluctuations on DLW studies, strong inter-isotope correlations within an individual confirm that use of a dosing ratio of 8‰:1‰ (0.6 p.p.m.: p.p.m.) optimizes DLW precision. Theoretical implications for the possible use of (17)O measurements within a DLW study require further study.