Doubly Labelled Water analysis: preparation, memory correction, calibration and quality assurance for δ2H and δ18O measurements over four orders of magnitudes

The absolute δ18 O value for SLAP with respect to VSMOW reveals a much lower value than previously established

RATIONALE

SLAP is one of the two calibration materials for the isotopic water scale. By consensus the established δ18 O value is -55.5‰, although several expert laboratories measure significantly more negative δ18 OSLAP values. The real δ18 OSLAP value as such does not influence the isotopic water scale; however, knowledge of the size of isotopic scale contraction in stable isotope measurements is vital for second-order isotopes. This study describes the quantification of δ18 OSLAP with respect to δ18 OVSMOW .

METHODS

SLAP-like water was quantitatively mixed with highly 18 O-enriched water to mimic VSMOW. The 18 O concentration was determined using an electron ionization quadrupole mass spectrometer. The isotopic composition of the SLAP-like and VSMOW-like waters was measured using an optical spectrometer, alongside original VSMOW and SLAP.

RESULTS

This study resulted in a much more negative δ18 O value for SLAP than expected. The averaged outcome of seven independent experiments is δ18 OSLAP  = -56.33 ± 0.03‰. There is a large discrepancy between the actual isotopic measurements of even the most carefully operating isotope laboratories and the true δ18 O value.

CONCLUSIONS

Although this finding as such does not influence the use of the VSMOW-SLAP scale, it raises the intriguing question of what we actually measure with our instruments and why even a fully corrected measurement can be so far off. Our result has consequences for issues like the transfer of δ18 O from and to the VPDB scale, various fractionation factors, and Δ17 O. The absolute 18 O abundance for SLAP was calculated as (1887.98 ± 0.43) × 10-6 based on the absolute 18 O abundance of VSMOW and the presented δ18 OSLAP in this paper.

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.

Equilibrium isotope fractionation factors of H exchange between steam and soil clay fractions

RATIONALE

Steam equilibration overcomes the problem of the traditional measurements of H isotope compositions, which leave an arbitrary amount of adsorbed water in the sample, by controlling for the entire exchangeable H pool, including adsorbed water and hydroxyl-H. However, the use of steam equilibration to determine nonexchangeable stable H isotope compositions in environmental media (expressed as δ² H_n values) by mathematically eliminating the influence of exchangeable H after sample equilibration with waters of known H-isotopic composition requires the knowledge of the equilibrium isotope fractionation factor between steam-H and exchangeable H of the sample (α_ex-w ), which is frequently unknown.

METHODS

We developed a new method to determine the α_ex-w values for clay minerals, topsoil clay fractions, and mica by manipulating the contributions of exchangeable H to the total H pool via different degrees of post-equilibration sample drying. We measured the δ² H values of steam-equilibrated mineral and soil samples using elemental analyzer-pyrolysis-isotope ratio mass spectrometry.

RESULTS

The α_ex-w values of seven clay minerals ranged from 1.071 to 1.140, and those of 19 topsoil clay fractions ranged from 0.885 to 1.216. The α_ex-w value of USGS57 biotite, USGS58 muscovite, and of cellulose was 0.965, 0.871, and 1.175, respectively. The method did not work for kaolinite, because its small exchangeable H pool did not respond to the selected drying conditions. Structurally different mineral groups such as two- and three-layer clay minerals or mica showed systematically different α_ex-w values. The α_ex-w value of the topsoil clay fractions correlated with the soil clay content (r = 0.63, P = 0.004), the local mean annual temperature (r = 0.68, P = 0.001), and the δ² H values of local precipitation (r = 0.72, P < 0.001), likely to reflect the different clay mineralogy under different weathering regimes.

CONCLUSIONS

Our new α_ex-w determination method yielded realistic results in line with the few previously published values for cellulose. The determined α_ex-w values were similar to the widely assumed values of 1.00-1.08 in the literature, suggesting that the adoption of one of these values in steam equilibration approaches is appropriate.

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.

First use of triply labelled water analysis for energy expenditure measurements in mice

The doubly labelled water (DLW) method is widely used to determine energy expenditure. In this work, we demonstrate the addition of the third stable isotope, ¹⁷O, to turn it into triply labelled water (TLW), using the three isotopes measurement of optical spectrometry. We performed TLW (²H, ¹⁸O and¹⁷O) measurements for the analysis of the CO₂ production (r_CO2) of mice on different diets for the first time. Triply highly enriched water was injected into mice, and the isotope enrichments of the distilled blood samples of one initial and two finals were measured by an off-axis integrated cavity output spectroscopy instrument. We evaluated the impact of different calculation protocols and the values of evaporative water loss fraction. We found that the dilution space and turnover rates of ¹⁷O and ¹⁸O were equal for the same mice group, and that values of r_CO2 calculated based on ¹⁸O–²H, or on ¹⁷O–²H agreed very well. This increases the reliability and redundancy of the measurements and it lowers the uncertainty in the calculated r_CO2 to 3% when taking the average of two DLW methods. However, the TLW method overestimated the r_CO2 compared to the indirect calorimetry measurements that we also performed, much more for the mice on a high-fat diet than for low-fat. We hypothesize an extra loss or exchange mechanism with a high fractionation for ²H to explain this difference.

Faster and more precise isotopic water analysis of discrete samples by predicting the repetitions’ asymptote instead of averaging last values

Water stable isotope analysis using Cavity Ring-Down Spectroscopy (CRDS) has a strong between-sample memory effect. The classic approach to correct this memory effect is to inject the sample at least 6 times and ignore the first two to three injections. The average of the remaining injections is then used as measured value. This is in many cases insufficient to completely compensate the memory effect. We propose a simple approach to correct this memory effect by predicting the asymptote of consecutive repeated injections instead of averaging over them. The asymptote is predicted by fitting a y=ax+b relation to the sample repetitions and keeping b as measured value. This allows to save analysis time by doing less injections while gaining precision. We provide a Python program applying this method and describe the steps necessary to implement this method in any other programming language. We also show validation data comparing this method to the classical method of averaging over the last couple of injections. The validation suggests a gain in time of a factor two while gaining in precision at the same time. The method does not have any specific requirements for the order of analysis and can therefore also be applied to an existing set of analyzes in retrospect.•

We fit a simple y=ax+b relation to the sample repetitions of Picarro L2130-i isotopic water analyzer, in order to keep the asymptote (b) as measured value instead of using the average over the last couple of measurements.

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This allows a higher precision in the measured value with less repetitions of the injection saving precious time during analysis.

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We provide a sample code using Python, but generally this method is easy to implement in any automated data treatment protocol.

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.

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.

Validation of saliva and urine use and sampling time on the doubly labelled water method to measure energy expenditure, body composition and water turnover in male and female cats

Less invasive protocols are necessary to study energy expenditure (EE) of cats living in homes for expressing their normal living conditions. The present study compared sampling times and the use of saliva, urine and blood to measure 2H and 18O to apply the doubly labelled water method. In the first study, four cats were used to evaluate the enrichment (2, 4, 6, 7 and 8 h) and elimination (2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 d) of 2H and 18O (subcutaneously injected). The maximum enrichment was after 5 h (R2 0·82) of injection, with an Ln linear elimination of both isotopes (P < 0·001; R2 0·99). The results of EE were similar, regardless of the sampling time used (P = 0·999). In the second study, seven male cats and seven female cats were used. Before and after isotope injection (5 h, 7 d, 10 d and 14 d), blood, saliva and urine were collected. Isotope enrichment was lower in urine (P < 0·05) and at the similar level in blood and saliva. Isotope elimination was similar for all fluids (P < 0·473). The EE calculated with blood and saliva was similar but higher for urine (P = 0·015). According to Bland-Altman statistics, blood and saliva presented low bias and high correlation (P < 0·001), but this was not observed for urine (P = 0·096). Higher EE was observed for male cats (384 (se 39) kJ/kg0·67 per d) than for female cats (337 (se 34) kJ/kg0·67 per d; P < 0·05). The sampling time for the method is flexible, and saliva can be used as a substitute for blood.

Demonstration of a memory calibration method in water isotope measurement by laser spectroscopy

RATIONALE

Measuring δ¹⁸ O and δ² H values in water using wavelength-scanned cavity ring down spectroscopy (WS-CRDS) requires multiple injections of up to six (and sometimes eight or more) of one sample to remove the memory effect, which decreases the sample throughput and increases the consumables cost. Thus, improved methods for removing the memory effect are required.

METHODS

We calculated the memory coefficients by sequential WS-CRDS measurement of two lab standard waters with isotopic differences, and used them to establish calibration equations. We then used these equations to correct the measured δ¹⁸ O and δ² H values by removing the memory effect, instead of using multiple injections in the routine daily measurements.

RESULTS

By using this method, the number of injections per sample was reduced to one. The reproducibility (one standard deviation) of the δ¹⁸ O and δ² H values obtained for quality control sample was less than 0.05‰ and 0.5‰ for an annual average, respectively.

CONCLUSIONS

By measuring the memory coefficients and establishing the calibration equations, a highly effective method was developed for determining the δ¹⁸ O and δ² H values of water, which could significantly improve sample throughput for liquid water dual isotope measurement without sacrificing the precision.

Validity of Absolute Intake and Nutrient Density of Protein, Potassium, and Sodium Assessed by Various Dietary Assessment Methods: An Exploratory Study

It is suggested that nutrient densities are less affected by measurement errors than absolute intake estimates of dietary exposure. We compared the validity of absolute intakes and densities of protein (kJ from protein/total energy (kJ)), potassium, and sodium (potassium or sodium (in mg)/total energy (kJ)) assessed by different dietary assessment methods. For 69 Dutch subjects, two duplicate portions (DPs), five to fifteen 24-h dietary recalls (24 hRs, telephone-based and web-based) and two food frequency questionnaires (FFQs) were collected and compared to duplicate urinary biomarkers and one or two doubly labelled water measurements. Multivariate measurement error models were used to estimate validity coefficients (VCs) and attenuation factors (AFs). This research showed that group bias diminished for protein and sodium densities assessed by all methods as compared to the respective absolute intakes, but not for those of potassium. However, the VCs and AFs for the nutrient densities did not improve compared to absolute intakes for all four methods; except for the AF of sodium density (0.71) or the FFQ which was better than that of the absolute sodium intake (0.51). Thus, using nutrient densities rather than absolute intakes does not necessarily improve the performance of the DP, FFQ, or 24 hR.

Comparing saliva and urine samples for measuring breast milk intake with the 2H oxide dose-to-mother technique among children 2-4 months old

Saliva and urine are the two main body fluids sampled when breast milk intake is measured with the ²H oxide dose-to-mother technique. However, these two body fluids may generate different estimates of breast milk intake due to differences in isotope enrichment. Therefore, we aimed to assess how the estimated amount of breast milk intake differs when based on saliva and urine samples and to explore whether the total energy expenditure of the mothers is related to breast milk output. We used a convenience sample of thirteen pairs of mothers and babies aged 2–4 months, who were exclusively breastfed and apparently healthy. To assess breast milk intake, we administered doubly labelled water to the mothers and collected saliva samples from them, while simultaneously collecting both saliva and urine from their babies over a 14-d period. Isotope ratio MS was used to analyse the samples for ²H and ¹⁸O enrichments. Mean breast milk intake based on saliva samples was significantly higher than that based on urine samples (854·5 v. 812·8 g/d, P = 0·029). This can be attributed to slightly higher isotope enrichments in saliva and to a poorer model fit for urine samples as indicated by a higher square root of the mean square error (14·6 v. 10·4 mg/kg, P = 0·001). Maternal energy expenditure was not correlated with breast milk output. Our study suggests that saliva sampling generates slightly higher estimates of breast milk intake and is more precise as compared with urine and that maternal energy expenditure does not influence breast milk output.

Short-term, but not long-term, increased daytime workload leads to decreased night-time energetics in a free-living song bird

Reproduction is energetically expensive and to obtain sufficient energy, animals can either alter their metabolic system over time to increase energy intake (increased-intake hypothesis) or reallocate energy from maintenance processes (compensation hypothesis). The first hypothesis predicts a positive relationship between basal metabolic rate (BMR) and energy expenditure (DEE) because of the higher energy demands of the metabolic system at rest. The second hypothesis predicts a trade-off between different body functions, with a reduction of the BMR as a way to compensate for increased daytime energetic expenditure. We experimentally manipulated the workload of wild pied flycatchers by adding or removing chicks when chicks were 2 and 11 days old. We then measured the feeding frequency (FF), DEE and BMR at day 11, allowing us to assess both short- and long-term effects of increased workload. The manipulation at day 2 caused an increase in FF when broods were enlarged, but no response in DEE or BMR, while the manipulation at day 11 caused an increase in FF, no change in DEE and a decrease in BMR in birds with more chicks. Our results suggest that pied flycatchers adjust their workload but that this does not lead to a higher BMR at night (no support for the increased-intake hypothesis). In the short term, we found that birds reallocate energy with a consequent reduction of BMR (evidence for the compensation hypothesis). Birds thus resort to short-term strategies to increase energy expenditure, which could explain why energy expenditure and hard work are not always correlated in birds.

The Importance of Isotopic Turnover for Understanding Key Aspects of Animal Ecology and Nutrition

Stable isotope-based methods have proved to be immensely valuable for ecological studies ranging in focus from animal movements to species interactions and community structure. Nevertheless, the use of these methods is dependent on assumptions about the incorporation and turnover of isotopes within animal tissues, which are oftentimes not explicitly acknowledged and vetted. Thus, the purpose of this review is to provide an overview of the estimation of stable isotope turnover rates in animals, and to highlight the importance of these estimates for ecological studies in terrestrial, freshwater, and marine systems that may use a wide range of stable isotopes. Specifically, we discuss 1) the factors that contribute to variation in turnover among individuals and across species, which influences the use of stable isotopes for diet reconstructions, 2) the differences in turnover among tissues that underlie so-called ‘isotopic clocks’, which are used to estimate the timing of dietary shifts, and 3) the use of turnover rates to estimate nutritional requirements and reconstruct histories of nutritional stress from tissue isotope signatures. As we discuss these topics, we highlight recent works that have effectively used estimates of turnover to design and execute informative ecological studies. Our concluding remarks suggest several steps that will improve our understanding of isotopic turnover and support its integration into a wider range of ecological studies.

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.

A low TSH profile predicts olanzapine-induced weight gain and relief by adjunctive topiramate in healthy male volunteers

Second generation antipsychotics, like olanzapine (OLZ), have become the first line drug treatment for patients with schizophrenia. However, OLZ treatment is often associated with body weight (BW) gain and metabolic derangements. Therefore, the search for prospective markers for OLZ's negative side effects as well as adjunctive treatments to inhibit these has been of major interest. The aim of this study was to investigate in healthy male volunteers (age: 36 ± 11 years; BW: 84 ± 12 kg; BMI=25.5 ± 2.5) whether adjunctive topiramate (TPM) administration opposes OLZ-induced weight gain over the course of 14 days treatment. In addition, we investigated behavioral, endocrine and metabolic characteristics as underlying and potentially predictive factors for weight regulation and/or metabolic derangements associated with OLZ and TPM treatment. While adjunctive TPM indeed reduced OLZ-induced weight gain (P<0.05, Mann-Whitney U), behavioral/metabolic/endocrine characteristics of OLZ treatment were not affected by TPM. Using multiple regression analysis, BW gain was the key factor explaining metabolic disturbances (e.g., plasma insulin- LDL interaction: P<0.01, R(2)=.320), and cumulative food intake during treatment was the best denominator of BW gain (P<0.01, R(2)=.534). Neither TPM treatment, nor its circulating levels, contributed to variation observed in ΔBW. In a second multiple regression analysis, we observed that a low baseline thyrotropin profile (TSHAUC) before the start of drug treatment was associated with an increase in ΔBW over the course of drug treatment (P<0.05, R(2)=.195). Adding TSHAUC as covariate revealed that adjunctive TPM treatment did attenuate OLZ induced BW gain (P<0.05, ANCOVA). Further exploration of the circulating thyroid hormones revealed that individuals with a low plasma TSH profile were also those that were most sensitive to adjunctive TPM treatment blocking OLZ-induced ΔBW gain. Others have shown that OLZ-induced BW gain is associated with improvement in brief psychiatric rating scores (BPRS); adjunctive TPM treatment may be a solution specifically for those subjects susceptible to OLZ-induced rapid weight gain who-on a therapeutic level-benefit most of OLZ treatment.

Measurement of extremely (2) H-enriched water samples by laser spectrometry: application to batch electrolytic concentration of environmental tritium samples

RATIONALE

Natural water samples artificially or experimentally enriched in deuterium ((2) H) at concentrations up to 10,000 ppm are required for various medical, environmental and hydrological tracer applications, but are difficult to measure using conventional stable isotope ratio mass spectrometry.

METHODS

Here we demonstrate that off-axis integrated cavity output (OA-ICOS) laser spectrometry, along with (2) H-enriched laboratory calibration standards and appropriate analysis templates, allows for low-cost, fast, and accurate determinations of water samples having δ(2) HVSMOW-SLAP values up to at least 57,000 ‰ (~9000 ppm) at a processing rate of 60 samples per day.

RESULTS

As one practical application, extremely (2) H-enriched samples were measured by laser spectrometry and compared to the traditional (3) H Spike-Proxy method in order to determine tritium enrichment factors in the batch electrolysis of environmental waters. Highly (2) H-enriched samples were taken from different sets of electrolytically concentrated standards and low-level (<10 TU) IAEA inter-comparison tritium samples, and all cases returned accurate and precise initial low-level (3) H results.

CONCLUSIONS

The ability to quickly and accurately measure extremely (2) H-enriched waters by laser spectrometry will facilitate the use of deuterium as a tracer in numerous environmental and other applications. For low-level tritium operations, this new analytical ability facilitated a 10-20 % increase in sample productivity through the elimination of spike standards and gravimetrics, and provides immediate feedback on electrolytic enrichment cell performance. Copyright © 2016 John Wiley & Sons, Ltd.

Total energy expenditure assessed by salivary doubly labelled water analysis and its relevance for short-term energy balance in humans

RATIONALE

The doubly labelled water (DLW) method is a stable isotopic technique for measuring total energy expenditure (TEE). Saliva is the easiest sampling fluid for assessing isotopic enrichments, but blood is considered superior because of its rapid exchange with body water. Therefore, we compared a large range of isotopic enrichments in saliva and blood, and related TEE in subjects with their ad libitum total energy intake (TEI). The relevance of these parameters to body weight and fat change over an 8-day interval was also assessed.

METHODS

Thirty subjects underwent DLW analysis over either 8 or 14 days, during which time initial and final blood and saliva enrichments were compared. TEI was assessed by dieticians over the 8-day period only. Isotope ratio mass spectrometry was used for the measurement of δ(2)H and δ(18)O values.

RESULTS

No discrepancies were observed between sampling fluids over a wide range of enrichments. During the 8-day period, average TEI exceeded TEE by ~5% or less. Using saliva as sampling fluid, TEI and TEI-TEE, but not TEE, were positively correlated to body weight change. TEI-TEE and physical activity EE (AEE), but not TEI, correlated, respectively, positively and negatively to changes in fat mass.

CONCLUSIONS

The DLW method in humans can be reliably applied using saliva as sampling fluid. TEI-TEE as well as AEE contributes significantly to changes in fat mass over an 8-day period.

Energy Expenditure and Metabolic Changes of Free-Flying Migrating Northern Bald Ibis

Many migrating birds undertake extraordinary long flights. How birds are able to perform such endurance flights of over 100-hour durations is still poorly understood. We examined energy expenditure and physiological changes in Northern Bald Ibis Geronticus eremite during natural flights using birds trained to follow an ultra-light aircraft. Because these birds were tame, with foster parents, we were able to bleed them immediately prior to and after each flight. Flight duration was experimentally designed ranging between one and almost four hours continuous flights. Energy expenditure during flight was estimated using doubly-labelled-water while physiological properties were assessed through blood chemistry including plasma metabolites, enzymes, electrolytes, blood gases, and reactive oxygen compounds. Instantaneous energy expenditure decreased with flight duration, and the birds appeared to balance aerobic and anaerobic metabolism, using fat, carbohydrate and protein as fuel. This made flight both economic and tolerable. The observed effects resemble classical exercise adaptations that can limit duration of exercise while reducing energetic output. There were also in-flight benefits that enable power output variation from cruising to manoeuvring. These adaptations share characteristics with physiological processes that have facilitated other athletic feats in nature and might enable the extraordinary long flights of migratory birds as well.

A thoroughly validated spreadsheet for calculating isotopic abundances (2H, 17O, 18O) for mixtures of waters with different isotopic compositions

RATIONALE

Oxygen and hydrogen stable isotopes are widely used tracers for studies on naturally occurring and laboratory mixtures of isotopically different waters. Although the mixing calculations are straightforward to perform, there are ample possibilities to make mistakes, especially when dealing with a large number of mixed fluids. To facilitate isotope mixing calculations and to avoid computational mistakes, a flexible tool to carry out these calculations is in demand.

METHODS

We developed, in three independent efforts, spreadsheets to carry out the mixing calculations for a combination of waters with different isotopic compositions using the isotope mass balance equation. We validated our calculations by comparison of the results of the three spreadsheets for a large number of test calculations. For all the cases, we obtained identical results down to the 12(th) to 14(th) significant digit.

RESULTS

We present a user-friendly, thoroughly validated spreadsheet for calculating (2) H, (17) O and (18) O stable isotopic abundances and respective isotope delta values for mixtures of waters with arbitrary isotopic compositions. The spreadsheet allows the mixing of up to 10 different waters, of which up to five can be specified using their isotopic abundances and up to five others using their isotope delta values. The spreadsheet is implemented in Microsoft Excel and is freely available from our research groups' websites.

CONCLUSIONS

The present tool will be applicable in the production and characterization of singly and doubly labeled water (DLW) mother solutions, the analysis of isotope dilution measurements, the deduction of unknown isotope values of constituents for mixtures of natural waters, and many other applications.

Measurement of Whole-Body CO2 Production in Birds Using Real-Time Laser-Derived Measurements of Hydrogen (δ(2)H) and Oxygen (δ(18)O) Isotope Concentrations in Water Vapor from Breath

The doubly labeled water (DLW) method is commonly used to measure energy expenditure in free-living wildlife and humans. However, DLW studies involving animals typically require three blood samples, which can affect behavior and well-being. Moreover, measurement of H (δ(2)H) and O (δ(18)O) isotope concentrations in H2O derived from blood using conventional isotope ratio mass spectrometry is technically demanding, time-consuming, and often expensive. A novel technique that would avoid these constraints is the real-time measurement of δ(2)H and δ(18)O in the H2O vapor of exhaled breath using cavity ring-down (CRD) spectrometry, provided that δ(2)H and δ(18)O from body H2O and breath were well correlated. Here, we conducted a validation study with CRD spectrometry involving five zebra finches (Taeniopygia guttata), five brown-headed cowbirds (Molothrus ater), and five European starlings (Sturnus vulgaris), where we compared δ(2)H, δ(18)O, and rCO2 (rate of CO2 production) estimates from breath with those from blood. Isotope concentrations from blood were validated by comparing dilution-space estimates with measurements of total body water (TBW) obtained from quantitative magnetic resonance. Isotope dilution-space estimates from δ(2)H and δ(18)O values in the blood were similar to and strongly correlated with TBW measurements (R(2) = 0.99). The (2)H and (18)O (ppm) in breath and blood were also highly correlated (R(2) = 0.99 and 0.98, respectively); however, isotope concentrations in breath were always less enriched than those in blood and slightly higher than expected, given assumed fractionation values between blood and breath. Overall, rCO2 measurements from breath were strongly correlated with those from the blood (R(2) = 0.90). We suggest that this technique will find wide application in studies of animal and human energetics in the field and laboratory. We also provide suggestions for ways this technique could be further improved.

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

Stable isotopes of water have long been used to improve understanding of the hydrological cycle, catchment hydrology, and polar climate. Recently, there has been increasing interest in measurement and use of the less-abundant (17)O isotope in addition to (2)H and (18)O. Off-axis integrated cavity output spectroscopy (OA-ICOS) is demonstrated for accurate and precise measurements δ(18)O, δ(17)O, and (17)O-excess in liquid water. OA-ICOS involves no sample conversion and has a small footprint, allowing measurements to be made by researchers collecting the samples. Repeated (514) high-throughput measurements of the international isotopic reference water standard Greenland Ice Sheet Precipitation (GISP) demonstrate the precision and accuracy of OA-ICOS: δ(18)OVSMOW-SLAP = -24.74 ± 0.07‰ (1σ) and δ(17)OVSMOW-SLAP = -13.12 ± 0.05‰ (1σ). For comparison, the International Atomic Energy Agency (IAEA) value for δ(18)OVSMOW-SLAP is -24.76 ± 0.09‰ (1σ) and an average of previously reported values for δ(17)OVSMOW-SLAP is -13.12 ± 0.06‰ (1σ). Multiple (26) high-precision measurements of GISP provide a (17)O-excessVSMOW-SLAP of 23 ± 10 per meg (1σ); an average of previously reported values for (17)O-excessVSMOW-SLAP is 22 ± 11 per meg (1σ). For all these OA-ICOS measurements, precision can be further enhanced by additional averaging. OA-ICOS measurements were compared with two independent isotope ratio mass spectrometry (IRMS) laboratories and shown to have comparable accuracy and precision as the current fluorination-IRMS techniques in δ(18)O, δ(17)O, and (17)O-excess. The ability to measure accurately δ(18)O, δ(17)O, and (17)O-excess in liquid water inexpensively and without sample conversion is expected to increase vastly the application of δ(17)O and (17)O-excess measurements for scientific understanding of the water cycle, atmospheric convection, and climate modeling among others.

High-saturated fat-sucrose feeding affects lactation energetics in control mice and mice selectively bred for high wheel-running behavior

Feeding a diet high in fat and sucrose (HFS) during pregnancy and lactation is known to increase susceptibility to develop metabolic derangements later in life. A trait for increased behavioral activity may oppose these effects, since this would drain energy from milk produced to be made available to the offspring. To investigate these interactions, we assessed several components of behavioral energetics during lactation in control mice (C) and in mice of two lines selectively bred for high wheel-running activity (S1, S2) subjected to a HFS diet or a low-fat (LF) diet. Energy intake, litter growth, and milk energy output at peak lactation (MEO; assessed by subtracting maternal metabolic rate from energy intake) were elevated in HFS-feeding dams across all lines compared with the LF condition, an effect that was particularly evident in the S dams. This effect was not preceded by improved lactation behaviors assessed between postnatal days 1 and 7 (PND 1-7). In fact, S1 dams had less high-quality nursing, and S2 dams showed poorer pup retrieval than C dams during PND 1-7, and S dams had generally higher levels of physical activity at peak lactation. These data demonstrate that HFS feeding increases MEO underlying increased litter and pup growth, particularly in mice with a trait for increased behavioral physical activity.

Validity of the doubly labeled water method for estimating CO2 production in mice under different nutritional conditions

The doubly labeled water (DLW) technique is used to assess metabolic rate (MR) in free-living conditions. We investigated whether differences in the nutritional and body adiposity status affect validity of the assessment of CO2 production (rCO2) by the DLW technique. To serve this purpose, we compared calculated rCO2 by the DLW method to actual CO2 production concomitantly measured in an indirect calorimetry setup over a 3-day period in mice fed with a low-fat (LF) diet or an obesogenic high-fat/high-sucrose (HF) diet. To uncover a potential effect of body composition on DLW accuracy, the HF-fed group was further subdivided in a diet-induced obesity-prone (DIO) and diet-induced obesity-resistant (DR) group. Furthermore, we assessed the influence of different sampling protocols, duration, and methodology of calculation. An excellent match was found between rCO2 assessed by the two methods in the LF-fed mice (least discrepancy -0.5 ± 1.1%). In contrast, there was a consistent overestimation of rCO2 by the DLW technique in the HF-fed animals compared with actual CO2 production independent from body mass gain (least discrepancy DR +15.9 ± 2.2%, DIO +18.5 ± 3.2%). The least discrepancies were found when two-pool model equations and the intercept method were used to calculate the body water pool. Furthermore, the HF group presented different equilibration kinetics of (2)H and (18)O and a lower dilution space ratio between the two. We recommend particular caution when using the DLW method for MR assessment in HF-fed animals and potentially humans because of the overestimation of rCO2.