Continuous Flow Isotope Ratio Mass Research Articles

Isotopic (δ13C, δ18O) Analysis of Small Amounts of Carbonate in Silicate Rocks by the Continuous Flow Isotope Ratio Mass Spectrometry Method

An experimental study of the main factors affecting the accuracy of oxygen and carbon isotopic analysis in carbonates dispersed in silicate matrix is carried out. Artificial 1, 2, 5, and 10% mixtures of quartz with carbonates with different isotopic parameters (KH-2, Ko, MCA-8) were analyzed by continuous flow isotope ratio mass spectrometry (CF IRMS). It is established that, in addition to the influence of the instrumental nonlinearity, the results are affected by two factors: trace amounts of CO2, constantly present in the system (the blank effect) and the presence of chemically neutral silicate particles (the matrix effect). The blank effect depends on the isotopic parameters of the sample and has very little influence on the estimated carbonate content in the rock. The matrix effect, on the contrary, strongly affects the estimated carbonate content, and produces the isotopic shift towards underestimated contents of heavy 13C and 18O isotopes. It is shown that this effect is related to the processes occurring near the CO2–acid–quartz interface, which are accompanied by kinetic fractionation of carbon and oxygen isotopes. Both effects are dependent on the amount of silicate matrix in the system and most clearly manifested during analysis of carbonate-poor rocks. When the carbonate content in the rock is about 1–2%, deviations from the true δ13C and δ18O values can reach the first ppm, while carbonate content obtained by chromatographic peak calibration can be underestimated by 20–40%.

Isotopic (δ13C, δ18O) Analysis of Small Amounts of Carbonate in Silicate Rocks by the Continuous Flow Isotope Ratio Mass Spectrometry Method

An experimental study of the main factors affecting the accuracy of oxygen and carbon isotopic analysis in carbonates dispersed in silicate matrix is carried out. Artificial 1, 2, 5, and 10% mixtures of quartz with carbonates with different isotopic parameters (KH-2, Ko, MCA-8) were analyzed by continuous flow isotope ratio mass spectrometry (CF IRMS). It is established that, in addition to the influence of the instrumental nonlinearity, the results are affected by two factors: trace amounts of CO2, constantly present in the system (the blank effect) and the presence of chemically neutral silicate particles (the matrix effect). The blank effect depends on the isotopic parameters of the sample and has very little influence on the estimated carbonate content in the rock. The matrix effect, on the contrary, strongly affects the estimated carbonate content, and produces the isotopic shift towards underestimated contents of heavy 13C and 18O isotopes. It is shown that this effect is related to the processes occurring near the CO2–acid–quartz interface, which are accompanied by kinetic fractionation of carbon and oxygen isotopes. Both effects are dependent on the amount of silicate matrix in the system and most clearly manifested during analysis of carbonate-poor rocks. When the carbonate content in the rock is about 1–2%, deviations from the true δ13C and δ18O values can reach the first ppm, while carbonate content obtained by chromatographic peak calibration can be underestimated by 20–40%.

Triple oxygen stable isotope analysis of nitrite measured using continuous flow isotope ratio mass spectrometry

Oxygen stable isotopes (i.e., 16O, 17O, 18O) of nitrite (NO2−) are useful for investigating chemical processes and sources contributing to this important environmental contaminant and nutrient. However, it remains difficult to quantify the oxygen isotope compositions of NO2− due to the lack of internationally recognized NO2− reference materials with a well-known Δ(17O) value. Here we have adopted a combination of methodologies to develop a technique for measuring Δ(17O) of NO2− by reducing nitrate (NO3−) materials with internationally recognized Δ(17O) values to NO2− using activated cadmium catalyzed by chloride in a basic solution while conserving Δ(17O). The NO3− reference materials reduced to NO2− and sample NO2− unknowns are converted to N2O using sodium azide/acetic acid reagent and decomposed to O2 by passing through a heated gold tube and introduced into a continuous flow isotope ratio mass spectrometer for analysis at m/z 32, 33, and 34 for Δ(17O) quantification.The adapted method involves the following main points:•NO3− reference materials with internationally recognized oxygen isotope composition are reduced to NO2− under high pH conditions that conserve Δ(17O) values.•The NO3− reference materials reduced to NO2− and sample NO2− with unknown Δ(17O) values are reduced to N2O using chemical methods involving sodium azide/acetic acid.•The product N2O is extracted, purified, decomposed to O2, and analyzed for its isotope composition using a continuous flow isotope ratio mass spectrometer for Δ(17O) quantification. The Δ(17O) of NO2− samples are calibrated with respect to the NO3− reference materials with known Δ(17O) values.

Reproducible measurements of the δ2H composition of non-exchangeable hydrogen in complex organic materials using the UniPrep2 online static vapour equilibration and sample drying system

Non-exchangeable hydrogen-isotope (δ2Hn) measurements of complex organic samples are used in forensics to determine sample authenticity, traceability, and provenance. However, δ2Hn assays of organics are usually complicated by uncontrolled “exchangeable hydrogen” and residual moisture contamination; hence, δ2Hn assays are persistently incomparable amongst laboratories. We introduce a revised technical solution (UniPrep2) to control hydrogen-isotope exchange and for robust online sample drying and vapour equilibration. The UniPrep2 device is coupled to a high-temperature thermochemical elemental analyser and continuous-flow isotope-ratio mass spectrometer. This technical solution empowers isotope analysts to:•Conduct reproducible controlled vapour equilibrations of complex organic samples and standards to determine the δ2Hn values by controlling hydrogen-isotope exchange.•Conduct online vacuum-oven evacuation with extensive helium drying without exposure to air to reabsorb or exchange hydrogen with ambient water vapour.The protocol describes the operation of the UniPrep2 device and the step-by-step procedures needed to obtain accurate and precise δ2Hn values for a wide range of organic sample types. Two analytical approaches are described in detail; the Dual-Vapour Equilibration (DVE) approach, intended for determining δ2Hn for a complex organic environmental sample where matrix equivalent H isotope reference materials are not available, and the Comparative Equilibration (CE) approach, which is intended for routine high-throughput analyses of complex organic samples where at least two matrix-equivalent organic isotope reference materials with consensus δ2Hn values are being used. These standard operating procedures are envisioned to be a sound basis for advancing hydrogen-isotope analysis for different organic environmental matrices and studies.

DEPEND study protocol: early detection of patients with pancreatic cancer – a pilot study to evaluate the utility of faecal elastase-1 and 13C-mixed triglyceride breath test as screening tools in high-risk individuals

IntroductionPancreatic cancer (PC) is the fifth leading cause of cancer-related death in the UK. The incidence of PC is increasing, with little or no improvement in overall survival and the...

Organic matter removal for continuous flow isotope ratio mass spectrometry analysis of carbon and oxygen isotope compositions of calcite or dolomite in organic‐rich samples

Abstractδ13C and δ18O values of carbonate in organic‐rich samples such as soils, biofilms, and lake sediments have been scarcely used so far in paleo‐environmental, biomineralization, or diagenetic studies because organic matter in high proportions is suspected to alter carbonate isotope analysis. Yet, with the improvement of analytical capabilities and in particular the use of CF‐IRMS, this may not be an issue anymore. To evaluate this, δ13C and δ18O values of calcite or dolomite mixed in various proportions with yeast (used here as a model for immature organic matter) were measured. The results indicate that measurements of calcite or dolomite δ13C and δ18O by CF‐IRMS require organic matter removal only when its weight proportion exceeds that of carbonates. Yeast generated an unidentified molecule during phosphoric acid digestion at 25°C (none at 80°C), which shifted the carbonate δ18O values for yeast proportions higher than 50%. It also generated CO2, but with a noticeable shift in δ13C and δ18O values only at 80°C (none at 25°C) and for yeast proportions higher than 75%. Three methods of organic matter removal were tested: the well‐established NaOCl and H2O2 wet treatments as well as low temperature plasma ashing (LTA) so far seldom examined. LTA removed the isotope shifts, although imperfectly for yeast proportions of 95%, without carbonate dissolution. NaOCl efficiently removed the isotope shifts in all cases but dissolved part of the calcite and dolomite. H2O2 treatment caused severe dissolution of calcite and dolomite while worsening the δ13C and δ18O shifts and should thus be avoided.

An insect isoscape of UK and Ireland.

The study of insect migration is problematic due to the small size of insects. Stable isotope analysis can be used to elucidate movement, either by geographic assignment of location of a species, or by simply distinguishing migrant from resident populations. There are few isoscapes of any kind in the UK/Ireland available for interrogation. Thus, I have measured stable isotope ratios (of H, C, N and S) of 299 individuals of the non-migratory Brimstone moth (Opisthograptis luteolata) collected from 93 locations around the UK and Ireland by citizen scientists. After removing lipids, stable isotope ratios were measured by continuous flow isotope ratio mass spectrometry, using either a conventional elemental analyser (C, N and S) or a high-temperature, thermal conversion elemental analyser in reductive mode. Maps (isoscapes) were constructed that illustrate the stable isotope spatial distribution of this insect. These are the first isoscapes of H, C, N and S of biological samples covering both UK and Ireland. The insect isoscape patterns can be explained from what we know of moth diet, climate and geology. Sulfur isotopes may be of particular use for distinguishing individuals from areas of unique geology. Isoscape patterns may (with care) predict isotope compositions of other, herbivorous, non-aquatic, chitinous taxa. Such isoscapes, when extended beyond the UK and Ireland, would provide a useful tool to elucidate insect migration.

New Ag3 PO4 comparison material for stable oxygen isotope analysis.

RationaleA silver phosphate reference material (Ag3PO4) for the measurement of stable oxygen isotope compositions is much needed; however, it is not available from the authorities distributing reference materials. This study aims to fill this gap by calibrating a new Ag3PO4 stable isotope comparison material produced by the University of Natural Resources and Life Sciences (BOKU).MethodsAliquots of Ag3PO4 were distributed to four laboratories who frequently measure the δ 18O value in Ag3PO4; the University of Natural Resources and Life Sciences (BOKU), the University of Western Australia (UWA), the University of Helsinki (UH), and the Helmholtz Centre for Environmental Research (UFZ). The instruments used to perform the measurements were high‐temperature conversion elemental analysers coupled with continuous flow isotope ratio mass spectrometers. The working gas δ 18O value was set to 0‰ and the normalization was done by a three‐point linear regression using the reference materials IAEA‐601, IAEA‐602, and NBS127.ResultsThe mean δ 18O value of the new BOKU Ag3PO4 comparison material on the VSMOW‐SLAP scale is 13.71‰ and the combined uncertainty is estimated as ±0.34‰. This estimated uncertainty is within the range typical for comparison materials of phosphates and sulphates. Consistent results from the different laboratories probably derived from similar instrumentation, and use of the same reference materials and normalization procedure. The matrix effect of the different reference materials used in this study was deemed negligible.ConclusionsThe BOKU Ag3PO4 can be used as an alternative comparison material for stable oxygen isotope analysis and is available for stable isotope research laboratories to facilitate calibration.

Nitrogen stable isotope turnover and discrimination in lizards.

Nitrogen stable isotope ratio (δ15 N) processes are not well described in reptiles, which limits reliable inference of trophic and nutrient dynamics. In this study we detailed δ15 N turnover and discrimination (Δ15 N) in diverse tissues of two lizard species, and compared these results with previously published carbon data (δ13 C) to inform estimates of reptilian foraging ecology and nutrient physiology. We quantified 15 N incorporation and discrimination dynamics over 360 days in blood fractions, skin, muscle, and liver of Sceloporus undulatus and Crotaphytus collaris that differed in body mass. Tissue samples were analyzed on a continuous flow isotope ratio mass spectrometer. Δ15 N for plasma and red blood cells (RBCs) ranged between +2.7 and +3.5‰; however, skin, muscle, and liver did not equilibrate, hindering estimates for these somatic tissues. 15 N turnover in plasma and RBCs ranged from 20.7 ± 4 to 303 ± 166 days among both species. Comparison with previously published δ13 C results for these same samples showed that 15 N and 13 C incorporation patterns were uncoupled, especially during winter when hibernation physiology could have played a role. Our results provide estimates of 15 N turnover rates and discrimination values that are essential to using and interpreting isotopes in studies of diet reconstruction, nutrient allocation, and trophic characterization in reptiles. These results also suggest that somatic tissues can be unreliable, while life history shifts in nutrient routing and metabolism potentially cause 15 N and 13 C dynamics to be decoupled.

Stable C isotope data of southern mixed-grass prairie vegetation from Oklahoma, United States.

Stable carbon isotopic data (δ13C) of 41 individual plant species was collected from long-term grazed and ungrazed pastures in Oklahoma, USA. These data can serve as a library of stable carbon isotope values for Southern mixed-grass prairie species. Seventeen warm-season (C4) and twenty-four cool-season (C3) plants were identified and collected from the United States Department of Agriculture (USDA) Southern Plains Experimental Range (SPER). Plant samples were dried at 55°C, and ground finely. The δ13C isotopic compositions were determined using a Europa Scientific automated nitrogen carbon analyzer (ANCA/NT) with a Solid/Liquid Preparation Module (Dumas combustion sample preparation system) coupled to a Europa 20–20 Stable isotope analyzer continuous flow isotope ratio mass spectrometer (Sercon Ltd, formerly Europa Scientific Ltd., Crewe, England). These data can be used as end members in isotopic mixing models or in paleoecology to correlate soil ages with plant species composition. Data from plant species provides information about soil organic carbon sequestration and possible long-term climate change.

Analysis of Chemical Composition of Gases from Rock Samples by Pulsed Discharge Chromatography Combined Mass Spectrometry

Understanding the chemical composition of gases trapped in rocks is important in a wide variety of earth science studies and gases are often extracted from rock samples and analyzed. Gas chromatography (GC) and mass spectrometry (MS) are the most frequently used analytical techniques for measuring gases extracted from rock samples, but these techniques have rarely been coupled except for gas chromatography-mass spectrometry (GC-MS) or Continuous flow isotope ratio mass spectrometry (CF-IRMS). A thermal conductivity detector (TCD) is most frequently used in GC because of its ability to detect all gases, but has limited sensitivity and thus cannot accurately detect trace gases in some rock samples. To resolve this issue, we built a new device that tests gases extracted from rock samples by GC and MS together, and employs a pulsed discharge detector (PDD) with GC analyses rather than a TCD because PDD is capable of detecting most gases and exhibits higher sensitivity than that of TCD. In this device, a rock sample can be heated or crushed in vacuum to release gases. A diaphragm gauge is then used to measure the pressure of the gases released from the rock samples. Four kinds of samples were tested using this new device: serpentinized peridotites from Inner Mongolia, volcanic rocks from Wudalianchi, shale rocks from Sichuan basin and reservoir rocks from Songliao basin. The chemical contents of H2, CH4, H2O, CO, N2, C2H4, C2H6, O2, H2S, Ar, CO2, C3H8, COS and SO2 could be measured simultaneously.

Nitrogen Mass Fraction and Stable Isotope Ratios for Fourteen Geological Reference Materials: Evaluating the Applicability of Elemental Analyser Versus Sealed Tube Combustion Methods

Thirteen commercially available silicate reference materials (RM) and one in‐house reference material, eleven of which have no previously published values, were analysed for nitrogen mass fraction and isotopic ratios with an Elemental Analyser (EA), and a Sealed Tube Combustion line, coupled to a continuous flow isotope ratio mass spectrometer (IRMS). These materials ranged from < 10 μg g−1 to 1% m/m nitrogen mass fractions and δ15N of −0.5 to +19.8‰. Existing nitrogen RM BHVO‐2, MS#5 and SGR‐1b were used to assess the accuracy of the data from the sealed tube combustion line, which was found to be in good agreement with existing published values. In contrast, the EA‐IRMS failed to fully liberate nitrogen from all silicate rocks and minerals (achieving a mean of 44 ± 10% nitrogen yield) resulting in kinetic fractionation of isotope values by −1.4‰ on average. Therefore, sealed tube combustion is better suited for analyses of silicate‐bound nitrogen. The EA worked reliably for organic samples, but care should be taken when using the EA for silicate nitrogen research. Moving forward, it is recommended that BHVO‐2, Biotite‐Fe, FK‐N and UB‐N be used as quality control materials as they appear to be most reproducible in terms of nitrogen mass fraction (relative error < 10%, 1s), and isotopic composition (< 0.6‰, 1s).

Naturally enriched 13C breath test as a carbohydrate intake marker in obese women

The 13C breath test has been proposed as a potential biomarker for the evaluation of metabolism and carbohydrate intake. To study the dynamics of the use of substrates in human metabolism is especially important for glucose metabolism in obese people. This study aimed to verify the use of 13C in a naturally enriched corn glucose liquid meal through the breath test as a potential marker of energy metabolism in women with a body mass index (BMI) above 35 kg /m2. Twenty-five woman (13 non-obese; 12 obese) participated in the study. The breath samples were collected in tubes prior to consumption of the standard liquid meal and at 30-min intervals for 6 h and analyzed in a continuous flow isotope ratio mass spectrometer. The curves of breath isotopic enrichment were adjusted by the Boltzmann sigmoidal regression model, and the t test was used to evaluate differences between the means of the curves. It was found a faster metabolic turn-over of the 13C-glucose in obese women. The breath test using naturally enriched stable isotopes has potential to be used as an energy metabolism marker in subjects with different BMI.

Isotopic discrimination between carrion and elytra clippings of lab-reared American burying beetles (Nicrophorus americanus): Implications for conservation and evaluation of feeding relationships in the wild.

Differences in stable isotope composition between an animal and its diet are quantified by experimentally derived diet-tissue discrimination factors. Appropriate discrimination factors between consumers and prey are essential for interpreting stable isotope patterns in ecological studies. While available for many taxa, these values are rarely estimated for organisms within the carrion food web. We used a controlled-diet stable isotope feeding trial to quantify isotopic diet-tissue discrimination factors of carbon (δ13 C values) and nitrogen (δ15 N values) from laboratory-reared Nicrophorus americanus raised on carrion. We used exoskeleton samples of beetle elytra (wing covers) to determine diet-tissue discrimination factors using a continuous flow isotope ratio mass spectrometer equipped with an elemental analyzer. We also measured the isotopic compositions of five species of co-occurring, wild-caught burying beetles and evaluated feeding relationships. We found differences in stable carbon discrimination between carrion sources (mammalian and avian) and lab-reared beetles, but no difference in stable nitrogen discrimination. Values for δ13 C did not differ among wild-caught burying beetle species, but values for δ15 N were significantly different for the three species with overlapping breeding seasons. Furthermore, wild-caught burying beetles within our study area do not appear to use avian carrion resources to rear their young. This study informs future interpretation of stable isotope data for insects within the carrion food web. In addition, these results provide insight into carrion resources used by co-occurring burying beetle species in situ. We also demonstrated that independent of adult food type, the larval food source has a significant impact on the isotopic signatures of adult beetles, which can be estimated using a minimally invasive elytra clipping.

Advances in the use of isotopes in geochemical exploration: instrumentation and applications in understanding geochemical processes

Among the emerging techniques to detect the real footprint of buried ore deposits is isotope tracing. Novel and automated preparation systems such as continuous flow isotope ratio mass spectrometry, off-axis integrated cavity output spectroscopy for isotopic compositions of selected molecules, multi-collector inductively coupled-plasma mass spectrometry (ICP-MS), triple quadrupole ICP-MS, laser ablation ICP-MS, and a multitude of inline preparation systems have facilitated the use of isotopes as tracers in mineral exploration, as costs for isotope analyses have decreased and the time required for the analyses has improved. In addition, the isotope systems being used have expanded beyond the traditional light stable and Pb isotopes to include a multitude of elements that behave differently during processes that promote the mobilization of elements during both primary and secondary dispersion. Isotopes are also being used to understand barren areas that lack a critical process to form an ore deposit and to reveal precise redox mechanisms. The goal is to be able to use isotopes to reflect a definitive process that occurs in association with the deposit and not in barren systems, and then to relate these to something that is easier to measure, namely elemental concentrations. As new generations of exploration and environmental scientists are becoming more comfortable with the application of isotopes to effectively trace processes involved in geoscience, and new technologies for rapid and inexpensive analyses of isotopes are continually being developed, novel applications of isotope tracing are becoming more mainstream. Thematic collection: This article is part of the Exploration 17 collection available at: https://www.lyellcollection.org/cc/exploration-17

Improved method for simultaneous determination of the carbon isotopic composition and concentration of atmospheric CO2 using CF-IRMS

Determination of the concentration and isotopic composition of atmospheric CO2 is essential for the identification of sources and sinks of anthropogenic CO2 and the monitoring of various emission reduction techniques, such as geological CO2 storage. A customized and convenient method was proposed to determine the carbon isotopic composition and concentration of CO2 of air samples simultaneously using a continuous flow-isotope ratio mass spectrometer (CF-IRMS) interfaced with a GasBench II (Thermo Fisher Scientific, Bremen, Germany). Convenient sampling and isotopic analysis were achieved using 12-mL Exetainer vials with open tops, screw caps, and rubber septa for both air sampling and isotopic analysis. The proposed method differs from previously reported methods in several aspects, including the convenience of sampling and analysis, and high accuracy and precision despite small sample consumption. Combined with the Keeling plot technique, this method allows accurate determination of the isotope composition of atmospheric CO2. The proposed technique can be applied in various studies involving the isotopic composition and concentration of CO2, such as diagnosing cancer, tracing of anthropogenic CO2 and monitoring of leakage from subsurface CO2 storage.

Refined protocol for δ13 C analysis of calcite and dolomite in carbonate mixture samples.

The carbon isotope composition of carbonates is widely used in petrology and paleoenvironments as a proxy for carbon sources and environmental parameters during mineral formation or late diagenesis. However, most carbonate minerals in nature are complicated mixtures (i.e., coexisting calcite and dolomite). Precise separation of individual signals in mixed carbonate minerals is essential to obtain information regarding these indexes because of the diverse isotopic behavior of different minerals. The conditions for pretreating calcite-dolomite mixtures to determine each of their δ13 C values were optimized with a series of experiments varying the purging and reaction conditions using an isotope ratio mass spectrometer equipped with an online GasBench II interface. Sequential phosphoric acid reactions of pure calcite and dolomite were performed to measure the characteristics of single minerals under different temperatures and reaction times. Artificial mixtures with varied proportions of calcite and dolomite were used to verify the precision of the proposed method. The optimized pretreatment conditions for δ13 C analysis of calcite in the mixture involved purging the sample bottle using helium and then reacting it with 99% H3 PO4 for 100 min in a cold water bath at 2°C. The optimized pretreatment conditions for δ13 C analysis of dolomite in the mixture involved purging the sample bottle using helium gas after δ13 Ccal measurement and reacting it with H3 PO4 on a heating plate for 10 h at 45°C. This study introduced a workflow with ultimate precision for the selective δ13 C measurements of calcite-dolomite mixtures combining a GasBench II with a continuous flow isotope ratio mass spectrometer.

A rapid throughput technique to isolate pyrogenic carbon by hydrogen pyrolysis for stable isotope and radiocarbon analysis

Rapid, reliable isolation of Pyrogenic Carbon (PyC; char, soot, black carbon; biochar) for determination of stable carbon isotope (δ13 C) composition and radiocarbon (14 C) dating is needed across multiple fields of research in geoscience, environmental science and archaeology. Many current techniques do not provide reliable isolation from contaminating organics and/or are relatively time consuming to employ. Hydrogen pyrolysis (HyPy) does provide reliable isolation of PyC but the current methodology is time consuming. We explored the potential for subjecting multiple samples to HyPy analysis by placing up to nine individual samples in custom designed borosilicate sample vessels in a single reactor run. We tested for cross contamination between samples in the same run using materials with highly divergent radiocarbon activities (~0.04 to 116.3 pMC), δ13 C values (-11.9 to -26.5‰) and labile carbon content. We determined 14 C/13 C by accelerator mass spectrometry and δ13 C values by elemental analyser coupled to continuous flow isotope ratio mass spectrometer. Very small but measurable transfer between samples of highly divergent isotope composition was detectable. Where samples are of broadly similar composition, this cross contamination is considered negligible with respect to measurement uncertainty. Where samples are of divergent composition, it was found that placing a sample vessel loaded with silica mesh adsorbent between samples eliminated measurable cross-contamination in all cases for both 14 C/13 C and δ13 C values. It is possible to subject up to seven samples to HyPy in the same reactor run for determination of radiocarbon content and δ13 C value without diminishing the precision or accuracy of the results. This approach enables an increase in sample throughput of 300-600%. HyPy process background values are consistently lower than the nominal laboratory process background for quartz tube combustion in the NERC Radiocarbon Laboratory, indicating that HyPy may also be advantageous as a relatively 'clean' radiocarbon pretreatment method.

Isotopic niche partitioning in two sympatric howler monkey species.

Ecological similarity between species can lead to interspecific trophic competition. However, when ecologically similar species coexist, they may differ in foraging strategies and habitat use, which can lead to niche partitioning. As the body tissues of consumers contain a stable isotope signature that reflects the isotopic composition of their diet, stable isotope analysis is a useful tool to study feeding behavior. We measured the isotopic niche width, which is a proxy for trophic niche width, of mantled (Alouatta palliata) and black (A. pigra) howler monkeys. Specifically, studied populations in allopatry and sympatry to assess whether these species showed niche partitioning. Between 2008 and 2012, we collected hair samples from 200 subjects (113 black and 87 mantled howler monkeys) and used continuous flow isotope ratio mass spectrometry to estimate δ13 C and δ15 N. We described the isotopic niche width of each species in allopatry and sympatry with the Bayesian estimation of the standard ellipse areas. In allopatry, isotopic niche width and isotopic variation were similar in both species. In sympatry, black howler monkeys had a significantly broader isotopic niche, which was mainly determined by high δ15 N values, and included the majority of mantled howler monkeys' isotopic niche. The isotopic niche of mantled howler monkeys did not differ between sympatry and allopatry. The coexistence of these ecologically similar species may be linked to trophic niche adjustments by one species, although the particular features of such adjustments (e.g., dietary, spatial, or sensory partitioning) remain to be addressed.

Sample acidification has a predictable effect on isotopic ratios of particulate organic matter along the Chilean coast.

Stable isotopes of carbon and nitrogen have proved to be valuable tools for researchers working across the different subfields of ecology. However, the chemical pretreatment of samples prior to analytical determination of stable isotope ratios can influence the results, and therefore conclusions regarding the ecology of the taxon or system under study. Here, we determined the effect of vapor acidification with concentrated HCl on the δ13 C and δ15 N values of particulate organic matter (POM), which are commonly used as baselines for studies of trophic ecology, or to understand oceanographic patterns. Samples of marine POM were obtained along a large-scale latitudinal gradient (ca 3000 km) along the Chilean coast, along with a range of oceanographic variables thought to potentially influence inorganic carbon at each sampling location. A random subset of 50 samples was divided into two parts: one acidified by HCl fumigation treatment, and the other acting as a control. We compared paired differences in δ13 C and δ15 N values measured by continuous flow isotope ratio mass spectrometry and used a model selection approach to examine which oceanographic factor best explained shifts in values following acid treatment. Acidification resulted in statistically significant reductions in both δ13 C and δ15 N values, but the effect was relatively small. The model that best explained the differences between acidified and non-acidified δ13 C values included depth, salinity and sea-surface temperature at the sampling point. A regression of acidified on non-acidified δ13 C values shows that the treatment effect was strongest on samples more depleted in 13 C. The differences between δ13 C and δ15 N values in acidified and non-acidified samples are linear and predictable. This implies that the nature of the POM and its possible alteration during the acid treatment are important factors that support the reliable determination of the values of δ13 C and δ15 N of POM.