Discrepancies between isotope ratio infrared spectroscopy and isotope ratio mass spectrometry for the stable isotope analysis of plant and soil waters

The use of isotope ratio infrared spectroscopy (IRIS) for the stable hydrogen and oxygen isotope analysis of water is increasing. While IRIS has many advantages over traditional isotope ratio mass spectrometry (IRMS), it may also be prone to errors that do not impact upon IRMS analyses. Of particular concern is the potential for contaminants in the water sample to interfere with the spectroscopy, thus leading to erroneous stable isotope data. Water extracted from plant and soil samples may often contain organic contaminants. The extent to which contaminants may interfere with IRIS and thus impact upon data quality is presently unknown. We tested the performance of IRIS relative to IRMS for water extracted from 11 plant species and one organic soil horizon. IRIS deviated considerably from IRMS for over half of the samples tested, with deviations as large as 46‰ (δ²H) and 15.4‰ (δ¹⁸O) being measured. This effect was reduced somewhat by using activated charcoal to remove organics from the water; however, deviations as large as 35‰ (δ²H) and 11.8‰ (δ¹⁸O) were still measured for these cleaned samples. Interestingly, the use of activated charcoal to clean water samples had less effect than previously thought for IRMS analyses. Our data show that extreme caution is required when using IRIS to analyse water samples that may contain organic contaminants. We suggest that the development of new cleaning techniques for removing organic contaminants together with instrument‐based software to flag potentially problematic samples are necessary to ensure accurate plant and soil water analyses using IRIS. Copyright © 2010 John Wiley & Sons, Ltd.     

Detailed assessment of isotope ratio infrared spectroscopy and isotope ratio mass spectrometry for the stable isotope analysis of plant and soil waters

As an alternative to isotope ratio mass spectrometry (IRMS), the isotope ratio infrared spectroscopy (IRIS) approach has the advantage of low cost, continuous measurement and the capacity for field‐based application for the analysis of the stable isotopes of water. Recent studies have indicated that there are potential issues of organic contamination of the spectral signal in the IRIS method, resulting in incorrect results for leaf samples. To gain a more thorough understanding of the effects of sample type (e.g., leaf, root, stem and soil), sample species, sampling time and climatic condition (dry vs. wet) on water isotope estimates using IRIS, we collected soil samples and plant components from a number of major species at a fine temporal resolution (every 2 h for 24–48 h) across three locations with different climatic conditions in the Heihe River Basin, China. The hydrogen and oxygen isotopic compositions of the extracted water from these samples were measured using both an IRMS and an IRIS instrument. The results show that the mean discrepancies between the IRMS and IRIS approaches for δ¹⁸O and δD, respectively, were: –5.6‰ and ?75.7‰ for leaf water; –4.0‰ and ?23.3‰ for stem water; –3.4‰ and ?28.2‰ for root water; ?0.5‰ and –6.7‰ for xylem water; –0.06‰ and ?0.3‰ for xylem flow; and ?0.1‰ and 0.3‰ for soil water. The order of the discrepancy was: leaf > stem ≈ root > xylem > xylem flow ≈ soil. In general, species of the same functional types (e.g., woody vs. herbaceous) within similar habitats showed similar deviations. For different functional types, the differences were large. Sampling at nighttime did not remove the observed deviations. Copyright © 2011 John Wiley & Sons, Ltd.     

Identification of sources and production processes of bottled waters by stable hydrogen and oxygen isotope ratios

Bottled water is a food product that considerably depends on the environment from which it originates, not only at the place where it is produced, but predominantly on the conditions in the recharge area of the wells captured for bottling. According to their source and the bottling process, bottled waters can be divided into natural and artificially sparkling waters, still and flavoured waters. These waters originate from various parts of the hydrological cycle and their natural origin is reflected in their hydrogen and oxygen stable isotopic compositions (delta(2)H and delta(18)O). A total of 58 domestic and foreign brands and 16 replicates of bottled waters, randomly collected on the Slovene market in September 2004, were analysed for delta(2)H and delta(18)O. The isotopic composition varied between -83 per thousand and -46 per thousand with an average of -66 per thousand for hydrogen, and between -11.9 per thousand and -7.5 per thousand with an average of -9.6 per thousand for oxygen. This investigation helped (1) to determine and test the classification of bottled waters, (2) to determine the natural origin of bottled water, and (3) to indicate differences between the natural and production processes. The production process may influence the isotopic composition of flavoured waters and artificially sparkling waters. No such modification was observed for still and natural sparkling waters. The methods applied, together with hydrological knowledge, can be used for the authentication of bottled waters for regulatory and consumer control applications.     

Stable hydrogen and oxygen isotope ratios of bottled waters of the world

Bottled and packaged waters are an increasingly significant component of the human diet. These products are regulated at the regional, national, and international levels, and determining the authenticity of marketing and labeling claims represents a challenge to regulatory agencies. Here, we present a dataset of stable isotope ratios for bottled waters sampled worldwide, and consider potential applications of such data for regulatory, forensic and geochemical standardization applications. The hydrogen and oxygen isotope ratios of 234 samples of bottled water range from -147 per thousand to +15 per thousand and from -19.1 per thousand to +3.0 per thousand, respectively. These values fall within and span most of the normal range for meteoric waters, indicating that these commercially available products represent a source of waters for use as laboratory working standards in applications requiring standardization over a large range of isotope ratios. The measured values of bottled water samples cluster along the global meteoric water line, suggesting that bottled water isotope ratios preserve information about the water sources from which they were derived. Using the dataset, we demonstrate how bottled water isotope ratios provide evidence for substantial evaporative enrichment of water sources prior to bottling and for the marketing of waters derived from mountain and lowland sources under the same name. Comparison of bottled water isotope ratios with natural environmental water isotope ratios demonstrates that on average the isotopic composition of bottled water tends to be similar to the composition of naturally available local water sources, suggesting that in many cases bottled water need not be considered as an isotopically distinct component of the human diet. Our findings suggest that stable isotope ratios of bottled water have the power to distinguish ultimate (e.g., recharge) and proximal (e.g., reservoir) sources of bottled water and constitute a potential tool for use in the regulatory monitoring of water products.     

Reducing and correcting for contamination of ecosystem water stable isotopes measured by isotope ratio infrared spectroscopy

Concern exists about the suitability of laser spectroscopic instruments for the measurement of the (18)O/(16)O and (2)H/(1)H values of liquid samples other than pure water. It is possible to derive erroneous isotope values due to optical interference by certain organic compounds, including some commonly present in ecosystem-derived samples such as leaf or soil waters. Here we investigated the reliability of wavelength-scanned cavity ring-down spectroscopy (CRDS) (18)O/(16)O and (2)H/(1)H measurements from a range of ecosystem-derived waters, through comparison with isotope ratio mass spectrometry (IRMS). We tested the residual of the spectral fit S(r) calculated by the CRDS instrument as a means to quantify the difference between the CRDS and IRMS δ-values. There was very good overall agreement between the CRDS and IRMS values for both isotopes, but differences of up to 2.3‰ (δ(18)O values) and 23‰ (δ(2)H values) were observed in leaf water extracts from Citrus limon and Alnus cordata. The S(r) statistic successfully detected contaminated samples. Treatment of Citrus leaf water with activated charcoal reduced, but did not eliminate, δ(2)H(CRDS) - δ(2)H(IRMS) linearly for the tested range of 0-20% charcoal. The effect of distillation temperature on the degree of contamination was large, particularly for δ(2)H values but variable, resulting in positive, negative or no correlation with distillation temperature. S(r) and δ(CRDS) - δ(IRMS) were highly correlated, in particular for δ(2)H values, across the range of samples that we tested, indicating the potential to use this relationship to correct the δ-values of contaminated plant water extracts. We also examined the sensitivity of the CRDS system to changes in the temperature of its operating environment. We found that temperature changes ≥4 °C for δ(18)O values and ≥10 °C for δ(2)H values resulted in errors larger than the CRDS precision for the respective isotopes and advise the use of such instruments only in sufficiently temperature-stabilised environments.     

Spectral analysis software improves confidence in plant and soil water stable isotope analyses performed by isotope ratio infrared spectroscopy (IRIS)

Previous studies have demonstrated the potential for large errors to occur when analyzing waters containing organic contaminants using isotope ratio infrared spectroscopy (IRIS). In an attempt to address this problem, IRIS manufacturers now provide post-processing spectral analysis software capable of identifying samples with the types of spectral interference that compromises their stable isotope analysis. Here we report two independent tests of this post-processing spectral analysis software on two IRIS systems, OA-ICOS (Los Gatos Research Inc.) and WS-CRDS (Picarro Inc.). Following a similar methodology to a previous study, we cryogenically extracted plant leaf water and soil water and measured the δ(2)H and δ(18)O values of identical samples by isotope ratio mass spectrometry (IRMS) and IRIS. As an additional test, we analyzed plant stem waters and tap waters by IRMS and IRIS in an independent laboratory. For all tests we assumed that the IRMS value represented the "true" value against which we could compare the stable isotope results from the IRIS methods. Samples showing significant deviations from the IRMS value (>2σ) were considered to be contaminated and representative of spectral interference in the IRIS measurement. Over the two studies, 83% of plant species were considered contaminated on OA-ICOS and 58% on WS-CRDS. Post-analysis, spectra were analyzed using the manufacturer's spectral analysis software, in order to see if the software correctly identified contaminated samples. In our tests the software performed well, identifying all the samples with major errors. However, some false negatives indicate that user evaluation and testing of the software are necessary. Repeat sampling of plants showed considerable variation in the discrepancies between IRIS and IRMS. As such, we recommend that spectral analysis of IRIS data must be incorporated into standard post-processing routines. Furthermore, we suggest that the results from spectral analysis be included when reporting stable isotope data from IRIS.     

Influence of dietary composition on the carbon, nitrogen, oxygen and hydrogen stable isotope ratios of milk

The stable isotope ratios ((13)C/(12)C, (15)N/(14)N, (18)O/(16)O, D/H) of animal feed and milk were investigated, considering cows stabled in two farms and fed with diets made up of different kinds of C(3) plants and different amounts of maize. Maize was characterised by delta(13)C, delta(18)O and deltaD values significantly higher than those of the C(3) plants, while, for the C(3) plants, Festuca arudinacea had significantly higher content of (13)C and (15)N. The delta(13)C and delta(18)O values of the overall diet and the delta(13)C of milk casein and lipids were shown to be significantly correlated with the percentage of maize in the animal diet. On the other hand, the delta(18)O values of milk water and the delta(18)O, deltaD and delta(15)N values of casein were shown to be only slightly influenced by the amount of maize in the feed, being probably more closely correlated with the geo-climatic and pedological characteristics of the area of origin and with the presence of fresh plant or silage in the ration. The delta(13)C value of casein was shown to be a suitable parameter for evaluating the amount of maize in the diet: each 10% increase in the maize content corresponded to a shift of 0.7 per thousand to 1.0 per thousand in the delta(13)C of casein. A threshold value of -23.5 per thousand for delta(13)C in milk casein, above which it is not possible to exclude the presence of maize in the diet, was suggested. The results obtained could be useful for determining mislabelling of dairy products declared to have been produced by pastured animals or of PDO cheeses with an established amount of maize in the diet and for verifying the unpermitted addition of exogenous components to milk.     

Fast high-precision on-line determination of hydrogen isotope ratios of water or ice by continuous-flow isotope ratio mass spectrometry

A new fast high-precision on-line technique is described for the determination of hydrogen isotope ratios of water by continuous-flow mass spectrometry. For the first time H(2)/H(2)O-equilibration using a platinum catalyst has been used in a fully continuous process. A significant reduction in the H(2)/H(2)O-equilibration time is achieved by a complete vaporization of the water and by increasing the exchange temperature to 100 degrees C. The analysis time is only approximately 5 min/sample which includes equilibration and processing. Measurement precision and accuracy are better than 1 per thousand and sample consumption is only approximately 5 microL. This new technique allows the measurement of a wide range of aqueous samples either in a semi-continuous way (discrete samples are injected one after another) or in a fully continuous way. This allows us, for the first time, to make continuous measurements of ice cores.     

Tracking human travel using stable oxygen and hydrogen isotope analyses of hair and urine

The stable oxygen and hydrogen isotope compositions of organic samples are increasingly being used to investigate patterns of animal migration. Relatively few studies have applied these techniques to modern humans, despite a variety of potential forensic applications. We analyzed drinking water and food at two geographic locations, East Greenbush, New York (USA) and Fairbanks, Alaska (USA), with different delta(18)O and deltaD values for precipitation and tap water. Foods varied widely in measured delta(18)O and deltaD values, but not systematically by purchase location. We measured delta(18)O and deltaD values of scalp hair from five residents at each location. We used a mixing model to establish the proportion of oxygen and hydrogen in head hair derived from drinking water ( approximately 27% and approximately 36%, respectively). Finally, we analyzed the delta(18)O and deltaD values of facial hair and urine from a subject who traveled from Fairbanks to East Greenbush, on to the UK and back to Fairbanks. Urine delta(18)O and deltaD values responded immediately and strongly to travel-related change in drinking water, and were well described by a single-pool turnover model. Beard hair delta(18)O values tracked changes in urine delta(18)O closely, and oscillated between the values for the resident populations in both locations. In contrast, beard hair deltaD values did not track changes in urine deltaD as well, and retained a signature of the traveler's permanent residence. Our findings show that the delta(18)O and deltaD values of urine and facial hair (specifically delta(18)O) can provide a record of the geographical movements of humans.     

Consistent predictable patterns in the hydrogen and oxygen stable isotope ratios of animal proteins consumed by modern humans in the USA

Published datasets of proteinaceous animal tissues suggest that co‐variation between amino acid hydrogen (δ²H) and oxygen (δ¹⁸O) isotope ratios is a common feature in systems where isotopic variation is driven by geographic or temporal variation in the δ²H and δ¹⁸O values of environmental water. This has led to the development of models relating tissue δ²H and δ¹⁸O values to those of water, with potential application in a number of fields. However, the strength and ubiquity of the influence of environmental water on protein isotope ratios across taxonomic groups, and thus the relevance of predictive models, is an open question. Here we report strong co‐variation of δ²H and δ¹⁸O values across a suite of terrestrial and aquatic animal meats purchased in American food markets, including beef, poultry (chicken and turkey), chicken eggs, pork, lamb, freshwater fish, and marine fish. Significant isotope co‐variation was not found for small collections of marine bivalves and crustaceans. These results imply that isotopic signals from environmental water were propagated similarly through most of the diverse natural and human‐managed foodwebs represented by our samples. Freshwater fish had the largest variation in δ²H and δ¹⁸O values, with ranges of 121 ‰ and 19.2 ‰, respectively, reflecting the large isotopic variation in environmental freshwaters. In contrast marine animals had the smallest variation for both δ²H (7 ‰ range, crustaceans) and δ¹⁸O (3.0 ‰ range, bivalves) values. Known‐origin beef samples demonstrated direct relationships between the variance of environmental water isotope ratios and that of collected meats. Copyright © 2011 John Wiley & Sons, Ltd.     

Reliable determination of oxygen and hydrogen isotope ratios in atmospheric water vapour adsorbed on 3A molecular sieve

The isotope ratio of atmospheric water vapour is determined by wide-ranging feedback effects from the isotope ratio of water in biological water pools, soil surface horizons, open water bodies and precipitation. Accurate determination of atmospheric water vapour isotope ratios is important for a broad range of research areas from leaf-scale to global-scale isotope studies. In spite of the importance of stable isotopic measurements of atmospheric water vapour, there is a paucity of published data available, largely because of the requirement for liquid nitrogen or dry ice for quantitative trapping of water vapour. We report results from a non-cryogenic method for quantitatively trapping atmospheric water vapour using 3A molecular sieve, although water is removed from the column using standard cryogenic methods. The molecular sieve column was conditioned with water of a known isotope ratio to 'set' the background signature of the molecular sieve. Two separate prototypes were developed, one for large collection volumes (3 mL) and one for small collection volumes (90 microL). Atmospheric water vapour was adsorbed to the column by pulling air through the column for several days to reach the desired final volume. Water was recovered from the column by baking at 250 degrees C in a dry helium or nitrogen air stream and cryogenically trapped. For the large-volume apparatus, the recovered water differed from water that was simultaneously trapped by liquid nitrogen (the experimental control) by 2.6 per thousand with a standard deviation (SD) of 1.5 per thousand for delta(2)H and by 0.3 per thousand with a SD of 0.2 per thousand for delta(18)O. Water-vapour recovery was not satisfactory for the small volume apparatus.     

The stable hydrogen and oxygen isotope variation of water stored in polyethylene terephthalate (PET) bottles

A set of bottled waters from a single natural spring distributed worldwide in polyethylene terephthalate (PET) bottles has been used to examine the effects of storage in plastic polymer material on the isotopic composition (delta18O and delta2H values) of the water. All samples analyzed were subjected to the same packaging procedure but experienced different conditions of temperature and humidity during storage. Water sorption and the diffusive transfer of water and water vapor through the wall of the PET bottle may cause isotopic exchange between water within the bottle and water vapor in air near the PET-water interface. Changes of about +4 per thousand for delta2H and +0.7 per thousand for delta18O have been measured for water after 253 days of storage within the PET bottle. The results of this study clearly indicate the need to use glass bottles for storing water samples for isotopic studies. It is imperative to transfer PET-bottled natural waters to glass bottles for their use as calibration material or potential international working standards.     

Identifying migratory Salmo trutta using carbon and nitrogen stable isotope ratios

Many Salmo trutta populations consist of non-anadromous (freshwater-resident) brown trout and anadromous (sea-run migratory) sea trout. Although adult brown trout and sea trout can usually be identified using differences in size and body colouration, it is not possible to easily identify eggs/alevins as the progeny of brown trout or sea trout. In this study we show that delta(13)C and delta(15)N, measured using a continuous flow isotope ratio mass spectrometer (CF-IRMS), can accurately identify fish eggs as the progeny of freshwater-resident (delta(13)C(egg) = -25.7 +/- 1.9 per thousand,delta(15)N(egg) = 9.2 +/- 1.8 per thousand) or migratory (delta(13)C(egg) = -19.9 +/- 1.1 per thousand, delta(15)N(egg) = 14. 3 +/- 1.5 per thousand) adult female Salmo trutta. Case studies show that stable isotope analysis is a more reliable technique for distinguishing anadromous adult fish than differentiation using morphological characteristics. For example, stable isotope analysis of brown trout from Loch Eck, Scotland, revealed that some individuals possessed delta(13)C and delta(15)N signatures indicative of marine feeding despite visual identification as freshwater-resident fish. It is most likely that these fish are misidentified sea trout although it possible that these fish may be brown trout that have adopted an estuarine feeding strategy to avoid interspecific competition for food within Loch Eck with salmon, powan and Arctic charr. Most stable isotope studies of fish ecology use terminal tissue sampling to provide sufficient biological material for isotopic analysis; however, our study suggests that adipose fin tissue could provide a comparable measure of delta(13)C and delta(15)N. Such a strategy would be invaluable when studying the trophic ecology or migration patterns of fish of high conservation value.     

Tracing the source of bottled water using stable isotope techniques

The Indian Summer Monsoon (ISM) precipitation recharges ground water aquifers in a large portion of the Indian subcontinent. Monsoonal precipitation over the Indian region brings moisture from the Arabian Sea and the Bay of Bengal (BoB). A large difference in the salinity of these two reservoirs, owing to the large amount of freshwater discharge from the continental rivers in the case of the BoB and dominating evaporation processes over the Arabian Sea region, allows us to distinguish the isotopic signatures in water originating in these two water bodies. Most bottled water manufacturers exploit the natural resources of groundwater, replenished by the monsoonal precipitation, for bottling purposes. The work presented here relates the isotopic ratios of bottled water to latitude, moisture source and seasonality in precipitation isotope ratios. We investigated the impact of the above factors on the isotopic composition of bottled water. The result shows a strong relationship between isotope ratios in precipitation (obtained from the GNIP data base)/bottled water with latitude. The approach can be used to predict the latitude at which the bottled water was manufactured. The paper provides two alternative approaches to address the site prediction. The limitations of this approach in identifying source locations and the uncertainty in latitude estimations are discussed. Furthermore, the method provided here can also be used as an important forensic tool for exploring the source location of bottled water from other regions.     

Effects of chemical lipid extraction and arithmetic lipid correction on stable isotope ratios of fish tissues

For accurate interpretation of fish trophodynamics from carbon stable isotope data it is necessary to extract tissue lipids. This is because lipid content varies within and among tissues in both space and time, and because lipids are 13C-depleted relative to proteins. However, lipid extraction may affect delta15N, thus requiring costly and time-consuming separation of delta13C and delta15N analyses. These problems have prompted the development of arithmetic correction techniques for delta13C, but the techniques and their underlying assumptions have not been systematically tested. This study compared the effects of lipid extraction and arithmetic correction techniques on delta13C and delta15N of European sea bass (Dicentrarchus labrax) tissues. Following Folch lipid extraction from muscle and liver, there was a mean increase in delta15N of 0.77 per thousand, but enrichment varied with lipid content such that effects on delta15N were hard to predict. Changes in delta13C and C:N between untreated and lipid-extracted samples reflected the quantity of lipid removed. The arithmetic correction techniques of mass balance and lipid correction were sensitive to the C:N of the lipid-extracted tissue and to the assumed depletion of lipid delta13C relative to protein delta13C. However, the mass balance approach was appropriate for the mathematical correction of bulk tissue data in most circumstances, provided that the C:N of lipid-extracted tissue could be determined for a small proportion of samples. Application of mass balance arithmetic correction can lead to significant time and cost savings in trophodynamic studies, because the majority of delta13C and delta15N analyses would not need to be run separately.