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

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.

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.

17 O-excess and d-excess of atmospheric water vapor measured by cavity ring-down spectrometry: Evidence of a matrix effect and implication for the calibration procedure

RATIONALE

Producing robust high-frequency time series of raw atmospheric water vapor isotope data using laser spectrometry requires accurate calibration. In particular, the chemical composition of the analyzed sample gas can cause isotope bias. This study assesses the matrix effect on calibrated δ¹⁷ O, δ¹⁸ O, δ² H, ¹⁷ O-excess, and d-excess values of atmospheric water vapor.

METHODS

A Picarro L2140-i cavity ring-down spectrometer with an autosampler and a vaporizer is used to analyze δ¹⁷ O, δ¹⁸ O, δ² H, ¹⁷ O-excess, and d-excess of two water standards. Isotope data obtained using synthetic air and dry ambient air as carrier gas at water mixing ratios ranging from 2000 to 30 000 ppmv are compared. Based on the results, atmospheric water vapor measurements are calibrated. The expected precision is estimated by Monte Carlo simulation.

RESULTS

The dry air source strongly impacts raw isotope values of the two water standards but has no effect on the mixing ratio dependency functions. When synthetic air is used, δ¹⁷ O, δ¹⁸ O, and ¹⁷ O-excess of calibrated atmospheric water vapor are overestimated by 0.6‰, 0.7‰, and 217 per meg, respectively, whereas δ² H and d-excess are underestimated by 1.5‰ and 7.3‰. Optimum precisions for the calibrated δ¹⁷ O, δ¹⁸ O, δ² H, ¹⁷ O-excess, and d-excess values and 12 min integration time are 0.02‰, 0.03‰, 0.4‰, 14 per meg, and 0.4‰, respectively.

CONCLUSIONS

Regarding the obtained results, recommendations for the calibration of atmospheric water vapor isotope measurements are presented. The necessity to use dry ambient air as dry air source when running the standards for calibration is pointed out as a prerequisite for accurate atmospheric water vapor ¹⁷ O-excess and d-excess measurements.

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.

Improving memory effect correction to achieve high-precision analysis of δ17 O, δ18 O, δ2 H, 17 O-excess and d-excess in water using cavity ring-down laser spectroscopy

RATIONALE

The precision obtained in routine isotope analysis of water (δ¹⁷ O, δ¹⁸ O, δ² H, ¹⁷ O-excess and d-excess values) using cavity ring-down spectroscopy is usually below the instrument specifications provided by the manufacturer. This study aimed at reducing this discrepancy, with particular attention paid to mitigating the memory effect (ME).

METHODS

We used a Picarro L2140i analyzer coupled with a high-precision A0211 vaporizer and an A0325 autosampler. The magnitude and duration of the ME were estimated using 24 series of 50 successive injections of samples with contrasting compositions. Four memory correction methods were compared, and the instrument performance was evaluated over a 17-month period of routine analysis, using two different run architectures.

RESULTS

The ME remains detectable after the 30th injection, implying that common correction procedures only based on the last preceding sample need to be revised. We developed a new ME correction based on the composition of several successive samples, and designed a run architecture to minimize the magnitude of the ME. The standard deviation obtained from routine measurement of a quality assurance water sample over a seven-month period was 0.015‰ for δ¹⁷ O, 0.023‰ for δ¹⁸ O, 0.078‰ for δ² H, 0.006‰ for ¹⁷ O-excess and 0.173‰ for d-excess. In addition, we provided the first δ¹⁷ O and ¹⁷ O-excess values for the GRESP certified reference material.

CONCLUSIONS

This study demonstrates the long-term persistence of the ME, which is often overlooked in routine analysis of natural samples. As already evidenced when measuring labelled water, it calls for consideration of the compositions of several previous samples to obtain an appropriate correction, a prerequisite to achieve high-precision data.

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.

Continuous and simultaneous measurement of triple-oxygen and hydrogen isotopes of liquid and vapor during evaporation experiments

RATIONALE

Oxygen and hydrogen isotopes are important tools for studying the modern and past hydrological cycle. Previous evaporation experiments used episodic measurement of liquid and/or vapor or did not measure all isotopologues of water. Here, we describe an evaporation experimental system that allows all isotopologues of liquid and water vapor to be measured simultaneously and near-continuously at high precision using cavity ring-down laser spectroscopy (CRDS).

METHODS

Evaporating liquid is periodically sampled from a closed recirculating loop by a syringe pump that delivers a constant supply of water to the vaporizer, achieving a water vapor concentration of 20,000 ppmV H₂ O (±132, 1σ). Vapor is sampled directly from the evaporation chamber. Isotope ratios are measured simultaneously with a Picarro L2140-i CRDS instrument.

RESULTS

For liquid measurements, Allan variance analysis indicates an optimum data collection window of 34 min for oxygen isotopes and 27 min for hydrogen isotopes. During these periods, the mean standard error is ±0.0081‰ for δ¹⁷ O values, ±0.0081‰ for δ¹⁸ O values, and ±0.019‰ for δ² H values. For the derived parameters ¹⁷ O-excess and d-excess, the standard error of the mean is 5.8 per meg and 0.07‰, respectively. For the vapor phase a 12.5 min data window for all isotopologues results in a mean standard error of ±0.012‰ for δ¹⁷ O values, ±0.011‰ for δ¹⁸ O values, and ±0.023‰ for δ² H values. For the derived parameters, the standard error of the mean is 9.2 per meg for ¹⁷ O-excess and 0.099‰ for d-excess. These measurements result in consistently narrow 95% confidence limits for the slopes of ln(δ¹⁷ O + 1) vs ln(δ¹⁸ O + 1) and ln(δ² H + 1) vs ln(δ¹⁸ O + 1).

CONCLUSIONS

The experimental method permits measurement of fractionation of triple-oxygen and hydrogen isotopes of evaporating water under varying controlled conditions at high precision. Application of this method will be useful for testing theoretical models of evaporation and conducting experiments to simulate evaporation and isotopic equilibration in natural systems.

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.

A multimillion-year-old record of Greenland vegetation and glacial history preserved in sediment beneath 1.4 km of ice at Camp Century

Understanding the history of the Greenland Ice Sheet (GrIS) is critical for determining its sensitivity to warming and contribution to sea level; however, that history is poorly known before the last interglacial. Most knowledge comes from interpretation of marine sediment, an indirect record of past ice-sheet extent and behavior. Subglacial sediment and rock, retrieved at the base of ice cores, provide terrestrial evidence for GrIS behavior during the Pleistocene. Here, we use multiple methods to determine GrIS history from subglacial sediment at the base of the Camp Century ice core collected in 1966. This material contains a stratigraphic record of glaciation and vegetation in northwestern Greenland spanning the Pleistocene. Enriched stable isotopes of pore-ice suggest precipitation at lower elevations implying ice-sheet absence. Plant macrofossils and biomarkers in the sediment indicate that paleo-ecosystems from previous interglacial periods are preserved beneath the GrIS. Cosmogenic ²⁶Al/¹⁰Be and luminescence data bracket the burial of the lower-most sediment between <3.2 ± 0.4 Ma and >0.7 to 1.4 Ma. In the upper-most sediment, cosmogenic ²⁶Al/¹⁰Be data require exposure within the last 1.0 ± 0.1 My. The unique subglacial sedimentary record from Camp Century documents at least two episodes of ice-free, vegetated conditions, each followed by glaciation. The lower sediment derives from an Early Pleistocene GrIS advance. ²⁶Al/¹⁰Be ratios in the upper-most sediment match those in subglacial bedrock from central Greenland, suggesting similar ice-cover histories across the GrIS. We conclude that the GrIS persisted through much of the Pleistocene but melted and reformed at least once since 1.1 Ma.

Relating Δ17O Values of Animal Body Water to Exogenous Water Inputs and Metabolism

The dynamics of animal body water and metabolism are integral aspects of biological function but are difficult to measure, particularly in free-ranging individuals. We demonstrate a new method to estimate inputs to body water via analysis of Δ17O, a measure of 17O/16O relative to 18O/16O. Animal body water is primarily a mixture of drinking or food water (meteoric water; Δ17O ≈ 0.030 per mille [‰]) and metabolic water synthesized from atmospheric oxygen (Δ17O ≈ –0.450‰). Greater drinking or food water intake should increase Δ17O toward 0.030‰, whereas greater metabolic rate should decrease Δ17O toward –0.450‰. We found that wild mammal Δ17O values generally increased with body mass, consistent with both a decline in mass-specific metabolic rate and an increase in water intake. Captive mouse (Peromyscus maniculatus) Δ17O values were higher than predicted but exhibited the expected relative change based on metabolic rate and water intake. Measurements of Δ17O may enable novel ecophysiological studies.

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.

Recent summer warming in northwestern Canada exceeds the Holocene thermal maximum

Eastern Beringia is one of the few Western Arctic regions where full Holocene climate reconstructions are possible. However, most full Holocene reconstructions in Eastern Beringia are based either on pollen or midges, which show conflicting early Holocene summer temperature histories. This discrepancy precludes understanding the factors that drove past (and potentially future) climate change and calls for independent proxies to advance the debate. We present a ~13.6 ka summer temperature reconstruction in central Yukon, part of Eastern Beringia, using precipitation isotopes in syngenetic permafrost. The reconstruction shows that early Holocene summers were consistently warmer than the Holocene mean, as supported by midges, and a thermal maximum at ~7.6–6.6 ka BP. This maximum was followed by a ~6 ka cooling, and later abruptly reversed by industrial-era warming leading to a modern climate that is unprecedented in the Holocene context and exceeds the Holocene thermal maximum by +1.7 ± 0.7 °C.

Traditional precipitation isotope archives (e.g., ice cores) are fundamental to our knowledge of past climate but limited to glaciated locales. Here the authors show that pore ice in relict permafrost holds equal promise as a proxy and use it to provide insights on the Holocene summer climate history of northwestern Canada.

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.