Continuous-flow Isotope Ratio Mass Spectrometry Research Articles

Managing argon interference during measurements of 18O/16O ratios in O2 by continuous-flow isotope ratio mass spectrometry

Monitoring changes in stable oxygen isotope ratios in molecular oxygen allows for studying many fundamental processes in bio(geo)chemistry and environmental sciences. While the measurement of ^{18}O/^{16}O ratios of mathrm {O}_{2} in gaseous samples can be carried out conveniently and from extracting moderately small aqueous samples for analyses by continuous-flow isotope ratio mass spectrometry (CF-IRMS), oxygen isotope signatures, updelta ^{18}O, could be overestimated by more than 6permille because of interferences from argon in air. Here, we systematically evaluated the extent of such Ar interferences on ^{18}O/^{16}O ratios of mathrm {O}_{2} for measurements by gas chromatography/IRMS and GasBench/IRMS and propose simple instrumental modifications for improved Ar and mathrm {O}_{2} separation as well as post-measurement correction procedures for obtaining accurate updelta ^{18}O. We subsequently evaluated the consequences of Ar interferences for the quantification of O isotope fractionation in terms of isotope enrichment factors, upepsilon _{mathrm {O}}, and ^{18}O kinetic isotope effects (^{18}O KIEs) in samples where mathrm {O}_{2} is consumed and Ar:mathrm {O}_{2} ratios increase steadily and substantially over the course of a reaction. We show that the extent of O isotope fractionation is overestimated only slightly and that this effect is typically smaller than uncertainties originating from the precision of updelta ^{18}O measurements and experimental variability. Ar interferences can become more relevant and bias upepsilon _{mathrm {O}} values by more than 2permille in aqueous samples where fractional mathrm {O}_{2} conversion exceeds 90%. Practically, however, such samples would typically contain less than 25 upmuM of mathrm {O}_{2} at ambient temperature, an amount that is close to the method detection limit of ^{18}O/^{16}O ratio measurement by CF-IRMS.Graphical abstract

Systematics of smectite hydrogen-isotope composition: Structural hydrogen versus adsorbed water

Atmospheric water adsorbed onto clay mineral surfaces, of smectite group minerals in particular, can contaminate hydrogen released from structural hydroxyl groups during hydrogen-isotope analysis. Interlayer cation composition and cation hydration enthalpy, in particular, can affect the magnitude of this excess hydrogen yield and hence the hydrogen isotope composition (δ2H) of smectites. To evaluate this problem, δ2H of different cation-saturated (Ca2+, Na+, K+), dried forms of six smectite standards from the Clay Minerals Society Source Clays were measured using a modified sample drying and on-line High-Temperature-Conversion-Elemental-Analysis (TCEA) Continuous-Flow-Isotope-Ratio-Mass-Spectrometry (CF-IRMS) protocol. More negative interlayer cation hydration enthalpies (Ca2+ > Na+ > K+) led to higher residual adsorbed water contents, which produced poorer δ2H reproducibility for the determination of smectite hydroxyl hydrogen. The lowest adsorbed water contribution and the most reproducible and possibly accurate δ2H for smectite hydroxyl hydrogen was obtained for K-saturated smectites dried for both 4 and 24 h at 220 °C prior to isotopic analysis and transferred to a “Zero-Blank” autosampler in ≤2.5 min. This approach provided the lowest measurement error for hydroxyl δ2H and facilitated much greater sample throughput than classical methods for smectitic clays. This study proposes a protocol for hydroxyl δ2H determination in smectitic clay minerals, and reveals the effect of H-isotope fractionation of adsorbed water during sample preparation.

Determination of the triple oxygen isotopic composition of tropospheric ozone in terminal positions using a multistep nitrite‐coated filter‐pack system

The triple oxygen isotopic composition (Δ17 O) of tropospheric ozone (O3 ) is a useful tracer for identifying the source and is essential for clarifying the atmospheric chemistry of oxidants. However, the single nitrite-coated filter method is inaccurate owing to the nitrate blank produced through the reaction of nitrite and oxygen compounds other than O3 . A multistep nitrite-coated filter-pack system is newly adopted to transfer the O-atoms in terminal positions of O3 to nitrite on each filter to determine the Δ17 O of O3 in terminal positions (denoted as Δ17 O(O3 )term ). The NO3 - produced by this reaction is chemically converted into N2 O, and continuous-flow isotope ratio mass spectrometry (CF-IRMS) is used to determine the oxygen isotopic compositions. The reciprocal of the NO3 - quantities on the nitrite-coated filters in each sample showed a strong linear relationship with Δ17 O of NO3 - . Using the linear relation, we corrected the changes in Δ17 O of NO3 - on the filters. We verified the accuracy of the new method through the measurement of artificial O3 with known Δ17 O(O3 )term value that had been determined from the changes in Δ17 O of O2 . The Δ17 O(O3 )term of tropospheric O3 was in agreement with previous studies. We accurately determined the δ18 O and Δ17 O values of tropospheric O3 by blank correction using our new method. Measurements of Δ17 O(O3 )term of the ambient troposphere showed 1.1 ± 0.7‰ diurnal variations between daytime (higher) and nighttime (lower) due likely to the formation of the temperature inversion layer at night.

High-precision measurement of phenylalanine and glutamic acid δ15 N by coupling ion-exchange chromatography and purge-and-trap continuous-flow isotope ratio mass spectrometry.

Nitrogen isotopic compositions (δ15 N) of source and trophic amino acids (AAs) are crucial tracers of N sources and trophic enrichments in diverse fields, including archeology, astrobiochemistry, ecology, oceanography, and paleo-sciences. The current analytical technique using gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) requires derivatization, which is not compatible with some key AAs. Another approach using high-performance liquid chromatography-elemental analyzer-IRMS (HPLC/EA/IRMS) may experience coelution issues with other compounds in certain types of samples, and the highly sensitive nano-EA/IRMS instrumentations are not widely available. We present a method for high-precision δ15 N measurements of AAs (δ15 N-AA) optimized for canonical source AA-phenylalanine (Phe) and trophic AA-glutamic acid (Glu). This offline approach entails purification and separation via high-pressure ion-exchange chromatography (IC) with automated fraction collection, the sequential chemical conversion of AA to nitrite and then to nitrous oxide (N2 O), and the final determination of δ15 N of the produced N2 O via purge-and-trap continuous-flow isotope ratio mass spectrometry (PT/CF/IRMS). The cross-plots of δ15 N of Glu and Phe standards (four different natural-abundance levels) generated by this method and their accepted values have a linear regression slope of 1 and small intercepts demonstrating high accuracy. The precisions were 0.36‰-0.67‰ for Phe standards and 0.27‰-0.35‰ for Glu standards. Our method and the GC/C/IRMS approach produced equivalent δ15 N values for two lab standards (McCarthy Lab AA mixture and cyanobacteria) within error. We further tested our method on a wide range of natural sample matrices and obtained reasonable results. Our method provides a reliable alternative to the current methods for δ15 N-AA measurement as IC or HPLC-based techniques that can collect underivatized AAs are widely available. Our chemical approach that converts AA to N2 O can be easily implemented in laboratories currently analyzing δ15 N of N2 O using PT/CF/IRMS. This method will help promote the use of δ15 N-AA in important studies of N cycling and trophic ecology in a wide range of research areas.

Measurement of oxygen and hydrogen isotopic ratios of speleothem fluid inclusion water using Picarro

A speleothem fluid inclusion extraction and isotope analysis system was established based on a vacuum extraction device and a Picarro L2140-i wavelength scanned cavity ring-down spectroscopy (WS-CRDS) water isotope analyzer. WS-CRDS allows to measure the stable oxygen and hydrogen isotope composition ( δ 18O and δ D) of small water samples simultaneously, which is faster, simpler, and requires less maintenance compared to currently used continuous-flow isotope ratio mass spectrometry (CF-IRMS). The precision of water isotopic analysis utilizing the WS-CRDS is often superior to that of CF-IRMS. Therefore, the use of Picarro for isotope analysis has rapidly gained popularity. The oxygen isotopic composition of speleothem calcite ( δ 18Oc) is one of the main proxies of speleothem-based paleoclimate reconstructions, but the interpretation of δ 18Oc remains complex as it can be influenced by many factors. Speleothem fluid inclusions contain the characteristics of cave drip water. The δ 18O and δ D values of fluid inclusion water provide direct information about paleoprecipitation, and can be further used to reconstruct paleotemperature when combined with δ 18Oc. The present paper contains a detailed description of the system as we have designed it, and provides an introduction to the system’s performance. The vacuum extraction device, connected to the Picarro analyzer, is designed to release the water trapped in speleothem fluid inclusions by pneumatically-operated crushing of discrete speleothem calcite chips. The whole device is kept at a temperature of 120°C to ensure the total and instantaneous vaporization of the water without isotopic fractionation. The vaporized water is carried by a flow of high-purity nitrogen to a 40 mL volume for homogenization, and is then introduced into the Picarro analyzer for δ 18O and δ D analysis. A syringe injection unit was added to the vacuum extraction device to be able to measure standard water, with δ 18O and δ D values similar to the speleothem samples, before and during measurements. To verify the precision and accuracy of the system, the same standard water samples were measured with an automated Picarro L2140-i set-up that includes an autosampler and a Picarro evaporation unit. The δ 18O and δ D values of standard waters as measured by both systems are comparable. Using the crushing device, speleothem samples from the East Asian Monsoon region yield paired δ 18O/ δ D values within ±0.5‰ of the Global Meteoric Water Line (GMWL), indicating no isotopic fractionation during the measurements. The precision is a function of the water amount released and of the isotopic composition of the sample. The reproducibility of crushed speleothem samples, whose water vapor concentration is between 2500 and 50000 ppm, is 0.5‰ for δ 18O and 2‰ for δ D (1 SD), similar to results obtained from other published measuring systems.

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.

Hair cortisol and stable carbon and nitrogen isotope ratios in barbary macaques (Macaca sylvanus) from Gibraltar.

Prolonged exposure to the stress hormone cortisol can precipitate a catabolic state in animals resulting in a negative nitrogen balance and increased excreted nitrogen, possibly affecting stable nitrogen and carbon isotope ratios in animal tissues. Here we test the hypothesis that stress, as measured by hair cortisol, is a significant contributor to variation in δ15 N and δ13 C values in hair. The results of the present study have important implications for the use of stable carbon and nitrogen isotope ratios measured in animal and human tissues to make inferences regarding feeding ecology and diet. Samples containing roughly 10-20 hairs (approx. 4-6 cm long) were collected from 49 free ranging macaque monkeys (Macaca sylvanus) in Gibraltar. Hair samples were analyzed for stable carbon and nitrogen isotopes by continuous-flow isotope-ratio mass spectrometry. Cortisol extracted from matched hair samples was measured using gas chromatography/mass spectrometry. Cortisol and stable isotope ratios were compared statistically using nonparametric Kruskal-Wallis and Mann-Whitney U tests, as well as Spearman correlation. Correlation analysis revealed that although hair δ15 N and δ13 C values were significantly correlated with each other, neither was correlated with cortisol concentrations. Cortisol concentrations were not correlated with age-adjusted body mass index. Age category was correlated with δ13 C values but not with δ15 N values. The results of our study suggest that the cumulative stress experienced by these macaques under typical social and environmental conditions is not a significant contributor to variation in δ15 N and δ13 C values, nor does it have a significant effect on relative body mass.

Do artificial structures affect the feed of Patella caerulea limpets?

Do artificial structures affect the feed of Patella caerulea limpets?

Impact of nitrate contamination on the analysis of carbon and oxygen isotopes in carbonate and a low-temperature phosphoric acid digestion approach for online sample preparation.

High-temperature phosphoric acid digestion combined with continuous-flow isotope ratio mass spectrometry (CF-IRMS) is one of the standard methods for the determination of carbon and oxygen isotopes in carbonate. However, the routine purification protocol has not been tested by samples with nitrate contamination. Here we show that the standard protocol is not capable of measuring such samples with satisfactory accuracy and precision. We tested the low-temperature (25°C) phosphoric acid digestion method combined with CF-IRMS for the measurement of natural carbonate samples with ~1% nitrate concentration and nitrate-doped carbonate standards. We also recorded the Raman spectra of the gases extracted from the nitrate-doped carbonate standards to identify the gas species that affect the results. The accuracy and precision of δ18 O values for nitrate-doped carbonate standards digested at 72°C are much poorer than routine measurements. For the measurements of doped standards reacted at 25°C, the accuracy and precision of the δ13 C and δ18 O values are indistinguishable from normal results. Raman spectra demonstrate that NO2 generated from high-temperature digestion is the possible interference species for oxygen isotope measurement. Low-temperature digestion is a reliable method for the measurement of carbon and oxygen isotopes in nitrate-contaminated carbonate samples because the formation of NO2 can be largely reduced. This approach can be applied to some nitrate-rich speleothems, evaporites and other samples with nitrate contamination.

Inter-laboratory test for oxygen and hydrogen stable isotope analyses of geothermal fluids: Assessment of reservoir fluid compositions.

Knowledge of the accuracy and precision for oxygen (δ18 O values) and hydrogen (δ2 H values) stable isotope analyses of geothermal fluid samples is important to understand geothermal reservoir processes, such as partial boiling-condensation and encroachment of cold and reinjected waters. The challenging aspects of the analytical techniques for this specific matrix include memory effects and higher scatter of delta values with increasing total dissolved solids (TDS) concentrations, deterioration of Pt-catalysts by dissolved/gaseous H2 S for hydrogen isotope equilibration measurements and isotope salt effects that offset isotope ratios determined by gas equilibration techniques. An inter-laboratory comparison exercise for the determination of the δ18 O and δ2 H values of nine geothermal fluid samples was conducted among eleven laboratories from eight countries (CeMIEGeo2017). The delta values were measured by dual inlet isotope ratio mass spectrometry (DI-IRMS), continuous flow IRMS (CF-IRMS) and/or laser absorption spectroscopy (LAS). Moreover, five of these laboratories analyzed an additional sample set at least one month after the analysis period of the first set. Statistical evaluation of all the results was performed to obtain the expected isotope ratios of each sample, which were then subsequently used in deep reservoir fluid composition calculations. The overall analytical precisions of the measurements were ± 0.2‰ for δ18 O values and ± 2.0‰ for δ2 H values within the 95% confidence interval. The measured and calculated δ18 O and δ2 H values of water sampled at the weir box, separator and wellhead of geothermal wells suggest the existence of hydrogen and oxygen isotope-exchange equilibrium between the liquid and vapor phases at all sampling points in the well. Thus, both procedures for calculating the isotopic compositions of the deep geothermal reservoir fluid - using either the analytical data of the liquid phase at the weir box together with those of vapor at the separator or the analytical data of liquid and vapor phases at the separator -are equally valid.

Seawater DIC analysis: The effects of blanks and long‐term storage on measurements of concentration and stable isotope composition

AbstractDissolved inorganic carbon (DIC) concentration and stable isotope composition measurements (δ13C) are central to the study of biological and chemical processes in the ocean. We examined a small volume (1 mL) method using continuous‐flow isotope ratio mass spectrometry (CF‐IRMS) for making both measurements in a single sample run. Prior exploration of this approach has yielded variable results. Effects of concentration, isotopic composition, storage time, and the gas used to flush the sample tubes were tested using standard solutions. Multiple isotopic composition standards permitted calculation of an apparent blank that develops due to slow rates of gas exchange through the septum. A blank correction is applied to the δ13C measurements, giving precision and accuracy better than 0.1‰ after 6 months of storage and ∼ 0.2‰ after 18 months of storage. CF‐IRMS concentration measurements on field samples collected near Tatoosh Island, off the Pacific coast of Washington, U.S.A., were compared to independent measurements (Dickson, CDQC Laboratory, UCSD) on the same water, the latter approach involving collection of 500 mL of seawater fixed with mercuric chloride. Concentration measurements by CF‐IRMS on seawater samples (precision ∼ 1–2%) are correctable to the Dickson results, though scatter in this relation exceeds analytical precision. The DIC and δ13C measurements on coastal seawater, in conjunction with salinity measurements, indicated strong effects of biological activity on carbon dynamics relative to possible effects of mixing water masses. In sum, this small volume methodology has application to field situations and laboratory experiments where larger volumes are logistically challenging and storage space is limited.

Precise carbon isotopic ratio analyses of micro amounts of carbonate and non-carbonate in basalt using continuous-flow isotope ratio mass spectrometry.

Continuous-flow isotope ratio mass spectrometry (CF-IRMS) is a specialized technique used to quickly analyze very small amounts of sample. We have used CF-IRMS to assess the influences of sample weight and relative carbon content on the accuracy and precision of the δ13 C values of micro amounts of carbonate and non-carbonate in silicate rocks. The analytical work was performed on a Gasbench II (GB) sample preparation device and on an Elemental Analyzer (EA), which were both interfaced to CF-IRMS instruments. Potential silicate matrix effects on the carbon isotopic analyses were investigated by measuring mixtures of calibrated carbon reference materials and quartz powder. The calibration lines, established by the measured raw values and the known values of three reference materials mixed with quartz powder, were used to calibrate the δ13 C values of basalt samples from eastern China. The δ13 C values measured by GB-CF-IRMS of one national carbonate reference material, GBW04416, deviate slightly from the known value for approximately 20-70μg of carbonate contained in 4.5-mL vials; the smaller the sample size, the lower the measured δ13 C values. External precision better than 0.1‰ (1σ, n=26) is achieved at a signal intensity for mass 44 of between 868 and 1614mV, corresponding to a sample weight of 30.8-50.2μg, whereas it is reduced to 0.27‰ (1σ, n=34) at a signal intensity between 519 and 1614mV, corresponding to a sample weight of 21.1-50.2μg. In the EA-CF-IRMS experiments for non-carbonate carbon, at high carbon concentration (greater than 800ppm) and at optimum sample weights, the accuracy and precision are both better than 0.2‰. For carbon concentrations less than 500ppm, the measured δ13 C values deviate from the average by up to -1.2‰ and the precision is 0.74‰. The measured δ13 C values decrease substantially at lower carbon concentration and higher sample weights, and poorer precision is obtained. Suggestions are made to measure repeatedly the same carbon concentration of sample and reference materials in order to obtain not only reproducible, but also accurate carbon isotope ratios.

Differentiated correction on the signal intensity dependence of GasBench II-IRMS from blank effect and instrument nonlinear effect

Continuous-Flow Isotope Ratio Mass Spectrometry (CF-IRMS) coupled with a GasBench II was designed to rapidly analyze δ13C in carbonate or CO2 in air. The dependence of measured δ13C (δ13Cm) on signal intensity (peak area or peak height) was the main error source resulting from blank effect of samples and/or instrument itself effect. To determine the reasons and origins of signal intensity dependence, eight signal intensity gradients for two carbonate (δ13C values were −49.64 and 1.61‰, respectively) and a gas mixture of 0.3% CO2 in He (δ13C=−19.32‰) standards were made respectively. The results showed that there was obvious signal intensity dependence, in particular when sample amount (peak area) was smaller. Given that the dependence was caused by blank effect and/or instrument nonlinear effect, without considering the blank effect, the signal intensity dependence could not be eliminated directly by the instrument nonlinear correction (empirical equation) methods However, it could be eliminated by blank correction (indirect subtraction method). In practice, blank effect should be corrected first, and then the instrument nonlinear effect.

New biotite and muscovite isotopic reference materials, USGS57 and USGS58, for δ2H measurements–A replacement for NBS 30

The advent of continuous-flow isotope-ratio mass spectrometry (CF-IRMS) coupled with a high temperature conversion (HTC) system enabled faster, more cost effective, and more precise δ2H analysis of hydrogen-bearing solids. Accurate hydrogen isotopic analysis by on-line or off-line techniques requires appropriate isotopic reference materials (RMs). A strategy of two-point calibrations spanning δ2H range of the unknowns using two RMs is recommended. Unfortunately, the supply of the previously widely used isotopic RM, NBS 30 biotite, is exhausted. In addition, recent measurements have shown that the determination of δ2H values of NBS 30 biotite on the VSMOW-SLAP isotope-delta scale by on-line HTC systems with CF-IRMS may be unreliable because hydrogen in this biotite may not be converted quantitatively to molecular hydrogen. The δ2HVSMOW-SLAP values of NBS 30 biotite analyzed by on-line HTC systems can be as much as 21mUr (or ‰) too positive compared to the accepted value of −65.7mUr, determined by only a few conventional off-line measurements. To ensure accurate and traceable on-line hydrogen isotope-ratio determinations in mineral samples, we here propose two isotopically homogeneous, hydrous mineral RMs with well-characterized isotope-ratio values, which are urgently needed. The U.S. Geological Survey (USGS) has prepared two such RMs, USGS57 biotite and USGS58 muscovite. The δ2H values were determined by both glassy carbon-based on-line conversion and chromium-based on-line conversion, and results were confirmed by off-line conversion. The quantitative conversion of hydrogen from the two RMs using the on-line HTC method was carefully evaluated in this study. The isotopic compositions of these new RMs with 1-σ uncertainties and mass fractions of hydrogen are:USGS57 (biotite)δ2HVSMOW-SLAP=−91.5±2.4mUr (n=24)Mass fraction hydrogen=0.416±0.002% (n=4)Mass fraction water=3.74±0.02% (n=4)USGS58 (muscovite)δ2HVSMOW-SLAP=−28.4±1.6mUr (n=24)Mass fraction hydrogen=0.448±0.002% (n=4)Mass fraction water=4.03±0.02% (n=4).These δ2HVSMOW-SLAP values encompass typical ranges for solid unknowns of crustal and mantle origin and are available to users for recommended two-point calibration.

Thermal Heating Induced Fractionation Effect on δ15N Measurements (Using Continuous Flow Isotope Ratio Mass-Spectrometry) for Samples Containing Lower N Contents

Simultaneous measurements of stable isotopes of Nitrogen (N), Carbon (C) and Sulfur (S) in a single aliquot of sample using stable isotope mass spectrometry coupled with elemental analyzer (EA) in a continuous flow mode has provided added advantages for stable isotopic data community scientifically as well as economically. For NCS isotopic measurements, certain configurational changes were adapted in the EA setup such as (1) introduction of ‘purge and trap columns’ for adsorbing and desorbing analyte CO2 and SO2, and (2) introduction of in-line ring heater to cover for the entire passage between combustion and reduction reactor to prevent any condensation of analyte gases. These modifications were made to achieve necessary accuracy and precision NCS isotopic measurements. However, the presence of in-line ring heater appears to have an influence on δ15N measurements especially for samples containing lower N amounts (≤2 μmoles). We compared δ15N measurements in both NCS and NC modes by reconfiguring same EA (Vario Pyrocube) connected with Isoprime mass spectrometer with wide range variability in N content. We observed a decreasing pattern in δ15N values with increasing N contents (till the true δ15N value is approached), which is an opposite trend compared to that typically observed with the NC mode of EA setup. We surmise thermal heating of the passage from the combustion and reduction reactors is most likely responsible for the observed pattern. Measured δ15N values for samples containing lower N (≤2 μmoles), however, could be corrected using appropriate correction procedure.

Seals and sea lions are what they eat, plus what? Determination of trophic discrimination factors for seven pinniped species

Mixing models are a common method for quantifying the contribution of prey sources to the diet of an individual using stable isotope analysis; however, these models rely upon a known trophic discrimination factor (hereafter, TDF) that results from fractionation between prey and animal tissues. Quantifying TDFs in captive animals is ideal, because diet is controlled and the proportional contributions and isotopic values of all prey items are known. To calculate TDFs for the Hawaiian monk seal, northern elephant seal, bearded seal, ringed seal, spotted seal, harbor seal, and California sea lion, we obtained whiskers, serum, plasma, red blood cells, and prey items from nine captive individuals. We obtained δ(13) C and δ(15) N values using continuous-flow isotope-ratio mass spectrometry. The average δ(13) C and δ(15) N values from bulk and lipid-corrected prey from the diet were subtracted from the δ(13) C and δ(15) N values of each blood and whisker sample to calculate tissue-specific TDFs for each individual (∆(13) C or ∆(15) N). The ∆(13) C values ranged from +1.7 to +3.2‰ (bulk prey) and from +0.8 to +1.9‰ (lipid-corrected prey) for the various blood components, and from +3.9 to +4.6‰ (bulk prey) or +2.6 to +3.9‰ (lipid-corrected prey) for whiskers. The ∆(15) N values ranged from +2.2 to +4.3‰ for blood components and from +2.6 to +4.0‰ for whiskers. The TDFs tended to group by tissue, with whiskers having greater ∆(13) C values than blood components. In contrast, the ∆(15) N values were greater in serum and plasma than in red blood cells and whiskers. By providing the first TDF values for five seal species (family Phocidae) and one otariid species (family Otariidae), our study facilitates more accurate mixing models for these species. These values are particularly important for critically endangered Hawaiian monk seals and the three Arctic seal species (bearded, ringed, and spotted) that are faced with a rapidly changing environment.

Notes on the origin of copromacrinite based on nitrogen functionalities and δ13C and δ15N determined on samples from the Peach Orchard coal bed, southern Magoffin County, Kentucky

This paper represents the first attempt to show, by means other than just petrographic ones, that one type of macrinite, herein designated copromacrinite, may result from macrofauna feces. For that purpose a combination of coal petrography, X-ray photoelectron spectroscopy, and elemental-analysis continuous-flow isotope ratio mass spectrometry methods were used to determine nitrogen functionalities and δ13C and δ15N compositions in 1) vitrinite-rich, 2) fusinite+semifusinite-rich, and 3) macrinite-rich (with a possible coprolitic origin) samples of the high volatile A bituminous Peach Orchard coal (Bolsovian; Middle Pennsylvanian) from Magoffin County, Kentucky. There were no significant differences between pyridinic-N and quaternary-N abundance in the three samples, however, pyrrolic-N was higher (~54%) in the macrinite-rich sample than in the other two samples (~38%). The data suggest that pyridinic-N and quaternary-N are independent of maceral group composition and that pyrrolic-N is dependent on maceral composition (fusinite+semifusinite versus macrinite). δ13C values obtained for bulk and demineralized coal of the vitrinite- and fusinite+semifusinite-rich samples are similar with δ13C values of −24.80±0.01‰ VPDB and −24.61±0.09‰ VPDB for bulk samples and −24.81±0.07‰ VPDB and −24.52±0.04‰ VPDB for demineralized samples. These values are within the expected range for vitrinite-rich samples and the slightly higher δ13C value of the fusinite+semifusinite-rich sample is expected as δ13C values for inertinite are higher than for vitrinite. However, there was a significant shift to a lower δ13C value (−26.80±0.01‰ VPDB for the bulk sample value) for the macrinite-rich sample. Because the samples are basically isorank, and δ13C (and δ15N) shifts do not occur during maturation until anthracite rank, the difference may be related to the presence or composition of the macrinite within the sample which lacks heat-effect indicators, such as devolatilization vacuoles and distorted pores. δ15N values are also similar for bulk and demineralized coal of the vitrinite- and fusinite+semifusinite-rich samples, and the bulk values were heavier in this samples (3.07±0.03‰ Air and 2.92±0.10‰ Air, respectively), and much lighter (−2.83±0.09‰ Air) for the macrinite-rich sample.The study of Peach Orchard coal samples using reflected-light microscopy, isotopic composition, and nitrogen-forms analyses revealed that the macrinite-rich sample contains macrinite with coprolitic features (e.g. oxidation rind, mix of undigested palynomorphs, frequent and randomly located funginite, agglutination pulp of semifusinite reflectance, internal lack of bedding fabric, and suggestion of structures resulting from intestines and stomach walls), more pyrrolic-N (~16%), and lower δ13C (~2‰ VPDB) and δ15N (~4‰ Air) values than the vitrinite and semifusinite+fusinite rich samples. These findings suggest that the maceral macrinite has multiple origins based on petrography and measurable chemical differences between the macrinite, vitrinite, and semifusinite+fusinite fractions within the coal. Assuming that copromacrinite observed is an excretion then the anomalies observed may result from the symbiotic relations between the macrofauna (e.g. cockroaches) and microbiota during the digestive processes, and the nitrogen balance mechanisms inside macrofauna body.

A new organic reference material, l-glutamic acid, USGS41a, for δ(13) C and δ(15) N measurements - a replacement for USGS41.

The widely used l-glutamic acid isotopic reference material USGS41, enriched in both (13) C and (15) N, is nearly exhausted. A new material, USGS41a, has been prepared as a replacement for USGS41. USGS41a was prepared by dissolving analytical grade l-glutamic acid enriched in (13) C and (15) N together with l-glutamic acid of normal isotopic composition. The δ(13) C and δ(15) N values of USGS41a were directly or indirectly normalized with the international reference materials NBS 19 calcium carbonate (δ(13) CVPDB = +1.95 mUr, where milliurey = 0.001 = 1 ‰), LSVEC lithium carbonate (δ(13) CVPDB = -46.6 mUr), and IAEA-N-1 ammonium sulfate (δ(15) NAir = +0.43 mUr) and USGS32 potassium nitrate (δ(15) N = +180 mUr exactly) by on-line combustion, continuous-flow isotope-ratio mass spectrometry, and off-line dual-inlet isotope-ratio mass spectrometry. USGS41a is isotopically homogeneous; the reproducibility of δ(13) C and δ(15) N is better than 0.07 mUr and 0.09 mUr, respectively, in 200-μg amounts. It has a δ(13) C value of +36.55 mUr relative to VPDB and a δ(15) N value of +47.55 mUr relative to N2 in air. USGS41 was found to be hydroscopic, probably due to the presence of pyroglutamic acid. Experimental results indicate that the chemical purity of USGS41a is substantially better than that of USGS41. The new isotopic reference material USGS41a can be used with USGS40 (having a δ(13) CVPDB value of -26.39 mUr and a δ(15) NAir value of -4.52 mUr) for (i) analyzing local laboratory isotopic reference materials, and (ii) quantifying drift with time, mass-dependent isotopic fractionation, and isotope-ratio-scale contraction for isotopic analysis of biological and organic materials. Published in 2016. This article is a U.S. Government work and is in the public domain in the USA.

An analytical system for stable isotope analysis on carbon monoxide using continuous-flow isotope-ratio mass spectrometry

Abstract. A fully automated system for the determination of δ13C and δ18O in atmospheric CO has been developed. CO is extracted from an air sample and converted into carbon dioxide (CO2) using the Schütze reagent. The isotopic composition is determined with an isotope-ratio mass spectrometer (IRMS) technique. The entire system is continuously flushed with high-purity helium (He), the carrier gas. The blank signal of the Schütze reagent is ~ 4 nmol mol−1, or 1–3 % of the typical sample size. The repeatability is 0.1 ‰ for δ13C and 0.2 ‰ for δ18O. The peak area allows for simultaneous determination of the mole fraction with an analytical repeatability of ~ 0.7 nmol mol−1 for 100 mL of ambient air (185.4 nmol mol−1 of CO). An automated single measurement is performed in only 18 min, and the achieved time efficiency (and small volume of sample air) allows for repetitive measurements practically.

Accuracy of δ(18)O isotope ratio measurements on the same sample by continuous-flow isotope-ratio mass spectrometry.

The doubly labeled water method is considered the reference method to measure energy expenditure. Conventional mass spectrometry requires a separate aliquot of the same sample to be prepared and analyzed separately. With continuous-flow isotope-ratio mass spectrometry, the same sample could be analyzed sequentially for both (2)H and (18)O content and thus minimize sample requirement, reduce analytical cost, and avoid memory effect. The (2)H contents of 197 urine samples collected from 22 doubly labeled water studies were determined using a Thermo Delta V Advantage continuous-flow isotope-ratio mass spectrometer. The (18)O content of these samples was measured either using a separate aliquot of the same sample using a VG Isogas gas-isotope-ratio mass spectrometer or using the same sample following the (2)H measurements on a Thermo Delta V continuous-flow isotope-ratio instrument. The δ(18)O values using the same aliquot of samples were accurate to 0.18 ± 2.61‰ (mean difference ± standard deviation (SD); 95% CI, -0.18 to 0.55‰; P = 0.33) compared with the values based on the standard conventional method. Bland and Altman pair-wise comparison also yielded a bias of 0.18‰ with a 95% limit of agreement between -4.94 and 5.30‰. The study demonstrated that continuous-flow isotope-ratio mass spectrometry is capable of producing accurate (18)O measurements on the same sample after (2)H measurements. The method greatly reduces the analytical cost and sample size requirement and could easily be adopted by any laboratories equipped with a continuous-flow isotope-ratio mass spectrometer.