High-precision Isotope Ratio Mass Research Articles

High-Precision Measurements of 33S and 34S Fractionation during SO2 Oxidation Reveal Causes of Seasonality in SO2 and Sulfate Isotopic Composition

This study presents high-precision isotope ratio-mass spectrometric measurements of isotopic fractionation during oxidation of SO2 by OH radicals in the gas phase and H2O2 and transition metal ion catalysis (TMI-catalysis) in the aqueous phase. Although temperature dependence of fractionation factors was found to be significant for H2O2 and TMI-catalyzed pathways, results from a simple 1D model revealed that changing partitioning between oxidation pathways was the dominant cause of seasonality in the isotopic composition of sulfate relative to SO2. Comparison of modeled seasonality with observations shows the TMI-catalyzed oxidation pathway is underestimated by more than an order of magnitude in all current atmospheric chemistry models. The three reactions showed an approximately mass-dependent relationship between (33)S and (34)S. However, the slope of the mass-dependent line was significantly different to 0.515 for the OH and TMI-catalyzed pathways, reflecting kinetic versus equilibrium control of isotopic fractionation. For the TMI-catalyzed pathway, both temperature dependence and (33)S/(34)S relationship revealed a shift in the rate-limiting reaction step from dissolution at lower temperatures to TMI-sulfite complex formation at higher temperatures. 1D model results showed that although individual reactions could produce Δ(33)S values between -0.15 and +0.2‰, seasonal changes in partitioning between oxidation pathways caused average sulfate Δ(33)S values of 0‰ throughout the year.

Natural Carbon Isotope Abundance of Plasma Metabolites and Liver Tissue Differs between Diabetic and Non-Diabetic Zucker Diabetic Fatty Rats

Background‘You are what you eat’ is an accurate summary for humans and animals when it comes to carbon isotope abundance. In biological material, natural13C/12C ratio is subject to minute variations due to diet composition (mainly from ingestion of C3 and C4 metabolism plants) and to the discrimination between ‘light’ and ‘heavy’ isotopes during biochemical reactions (isotope effects and isotopic fractionation).Methodology/Principal FindingsCarbon isotopic abundance was measured in ZDF (fa/+) and ZDF (fa/fa), (lean and obese-diabetic rats respectively) fed the same diet. By analysing plasma metabolites (glucose and non-esterified fatty acids), breath and liver tissue by high-precision isotope ratio mass spectrometry, we demonstrate for the first time statistically distinguishable metabolic carbon isotope abundance between ZDF (fa/+) and ZDF (fa/fa) rats based on plasma glucose, palmitic, oleic, linoleic, arachidonic acids and bulk analysis of liver tissue (P<0.005) resulting into clear isotopic fingerprints using principal component analysis. We studied the variation of isotopic abundance between both groups for each metabolite and through the metabolic pathways using the precursor/product approach. We confirmed that lipids were depleted in 13C compared to glucose in both genotypes. We found that isotopic abundance of linoleic acid (C18: 2n-6), even though both groups had the same feed, differed significantly between both groups. The likely reason for these changes between ZDF (fa/+) and ZDF (fa/fa) are metabolic dysregulation associated with various routing and fluxes of metabolites.Conclusion/SignificanceThis work provides evidence that measurement of natural abundance isotope ratio of both bulk tissue and individual metabolites can provide meaningful information about metabolic changes either associated to phenotype or to genetic effects; irrespective of concentration. In the future measuring the natural abundance δ13C of key metabolites could be used as endpoints for studying in vivo metabolism, especially with regards to metabolic dysregulation, and development and progression of metabolic diseases.

Mixed‐mode chromatography/isotope ratio mass spectrometry

Liquid chromatography coupled to molecular mass spectrometry (LC/MS) has been a standard technique since the early 1970s but liquid chromatography coupled to high-precision isotope ratio mass spectrometry (LC/IRMS) has only been available commercially since 2004. This development has, for the first time, enabled natural abundance and low enrichment delta(13)C measurements to be applied to individual analytes in aqueous mixtures creating new opportunities for IRMS applications, particularly for the isotopic study of biological molecules. A growing number of applications have been published in a range of areas including amino acid metabolism, carbohydrates studies, quantification of cellular and plasma metabolites, dietary tracer and nucleic acid studies. There is strong potential to extend these to new compounds and complex matrices but several challenges face the development of LC/IRMS methods. To achieve accurate isotopic measurements, HPLC separations must provide baseline-resolution between analyte peaks; however, the design of current liquid interfaces places severe restrictions on compatible flow rates and in particular mobile phase compositions. These create a significant challenge on which reports associated with LC/IRMS have not previously focused. Accordingly, this paper will address aspects of chromatography in the context of LC/IRMS, in particular focusing on mixed-mode separations and their benefits in light of these restrictions. It aims to provide an overview of mixed-mode stationary phases and of ways to improve high aqueous separations through manipulation of parameters such as column length, temperature and mobile phase pH. The results of several practical experiments are given using proteogenic amino acids and nucleosides both of which are of noted importance in the LC/IRMS literature. This communication aims to demonstrate that mixed-mode stationary phases provide a flexible approach given the constraints of LC/IRMS interface design and acts as a practical guide for the development of new chromatographic methods compatible with LC/IRMS applications.

Plasma appearance of labeled β-carotene, lutein, and retinol in humans after consumption of isotopically labeled kale

The bioavailability of carotenoids from kale was investigated by labeling nutrients in kale with 13C, feeding the kale to seven adult volunteers, and analyzing serial plasma samples for labeled lutein, beta-carotene, and retinol. Ingested doses of labeled carotenoids were 34 micromol for beta-carotene and 33 micromol for lutein. Peak plasma concentrations, areas under the plasma concentration-time curves (AUCs), and percentages of dose recovered at peak plasma concentrations were calculated. Average peak plasma concentrations were 0.38, 0.068, and 0.079 microM for [13C]lutein, [13C]beta-carotene, and [13C]retinol, respectively. Average AUC values (over 28 days) were 42.8, 13.6, 13.2 microM h for [13C]lutein, [13C]beta-carotene, and [13C]retinol, respectively. Percentages of dose recovered at peak plasma concentrations were 3.6, 0.7, and 0.7% for [13C]lutein, [13C]beta-carotene, and [13C]retinol, respectively. A positive relationship was observed between baseline plasma retinol levels and [13C]retinol plasma response. It is possible that this relationship was mediated either through some aspect of beta-carotene absorption or via the common pathways of metabolism for postdose and endogenous retinoid.

The development of multiple collector mass spectrometry for isotope ratio measurements

Multiple collector mass spectrometry has enabled significant advances in our understanding of geological, biological, nuclear, and physical processes in terrestrial and extra-terrestrial environments. The development of improved mass spectrometers and more efficient and universal ionization techniques is driven by the requirement to extract the highest quality information from the smallest amount of sample possible. Important milestones include the refinement of the variable multiple collector system, improvements in current amplifier technology to achieve ultimate precision and accuracy, increased abundance sensitivity, tailored ion optics using zoom optics and increased ion optical magnification to meet the special requirements of multiple collection, increased signal-to-noise using multiple ion counting devices, and the integration of a high efficiency ionization inductively coupled plasma source. New applications are on the horizon in many fields in addition to the geological and cosmological origins of high precision isotope ratio mass spectrometry. This is made possible by in situ sampling of samples at high spatial resolution using laser ablation techniques as well as online coupling of chromatographic devices to study elemental isotope effects in isolated molecular species. This paper describes and discusses the instrumental development of multiple collector isotope ratio technology and highlights some emerging applications.

Sourcing Organic Compounds Based on Natural Isotopic Variations Measured by High Precision Isotope Ratio Mass Spectrometry

AbstractFor Abstract see ChemInform Abstract in Full Text.

Analysis of quantization error in high-precision continuous-flow isotope ratio mass spectrometry

High-precision isotope ratio mass spectrometry (IRMS) systems are equipped with digitizers that deliver effective maximum digitization depths of 16 to 24 bits; however, there are no analyses of the proper board depth required to retain high precision in continuous-flow techniques. We report an experimental and theoretical evaluation of quantization error in continuous-flow IRMS (CF-IRMS). CO 2 samples (100 pmol–30 nmol) were injected into a gas chromatography combustion IRMS system (GC–CIRMS). The analog signal was digitized by high precision, 24-bit ADC boards at 10 Hz, and was post-processed to simulate 12, 14, and 16-bit data sets. δ 13 C pdb values were calculated for all data sets by the conventional “summation” method or by curve-fitting the chromatographic peaks to the exponentially modified Gaussian (EMG) function. Benchmarks of S.D.( δ 13 C pdb ) = 0.3, 0.6, and 1.0‰ were considered to assess precision. In the presence of significant quantization noise, curve-fitting required several-fold less CO 2 than the summation method to reach a given benchmark. We derived an equation to describe the theoretical limitations of precision for the summation method as a function of CO 2 admitted to the source and the step size of the boards. Theory was in close agreement with the observed lower limit of precision for the simulated 16-bit data set. Curve-fitting achieved a precision of S.D. <0.3‰ for injections 20-fold smaller than summation for CO 2 samples collected on an IRMS with 16-bit resolution. By mitigating the impact of quantization noise, curve-fitting expands the dynamic range within a single run to include lower analyte levels, and effectively reduces the need for high pumping capacities and high precision ADC boards.

Sourcing Organic Compounds Based on Natural Isotopic Variations Measured by High Precision Isotope Ratio Mass Spectrometry

Sourcing Organic Compounds Based on Natural Isotopic Variations Measured by High Precision Isotope Ratio Mass Spectrometry

High-Precision Isotope Ratio Mass Spectrometry and Stable Isotope Precursors for Tracer Studies in Cell Culture

The use of stable isotope-labeled tracers is demonstrated in an in vitro system with analysis by high-precision isotope ratio mass spectrometry (IRMS), using n-3 long-chain polyunsaturated fatty acid (LCP) biosynthesis from [U-13C]18:3n-3 (18:3n-3*) in Y79 human retinoblastoma cells as a model system. The cells were cultured as a suspension in RPMI 1640 medium supplemented with 15% fetal calf serum at 37°C with 5% CO2 in air. They were harvested by sedimentation and cell lipids were extracted to determine the presence of 18:3n-3* metabolites using gas chromatography–combustion (GCC)–IRMS. As the dose of 18:3n-3* was systematically increased from treatment to treatment, the atom percent excess and the amounts of biosynthesized LCP* increased, while the percentage dose in each n-3 LCP* remained constant. Cultures incubated with 0.5 μmol (10 μM) of albumin-bound 18:3n-3, composed of 18:3n-3* diluted 1/60 or 1/100 with natural abundance 18:3n-3, yielded products with enrichments about 1.5 at.% excess (δ13CPDB < 1500‰), which is optimal for high-precision measurements. Kinetics in Y79 cells incubated with 18:3n-3* showed that n-3 LCP* incorporation increased over time; 18:3n-3*, 20:5n-3*, 22:5n-3*, and 22:6n-3* were detected at all time points with the 1/60 dilution. These data document experimental parameters for optimal stable isotope use and IRMS detection for in vitro tracer methodology.

HF-W Chronometry and Inner Solar System Accretion Rates

Models for the mechanisms of accretion of the terrestrial planets are re-examined using the experimental technique of high-precision isotope ratio mass spectrometry of tungsten (W). The decay of 182Hf to 182W (via 182Ta) provides a new kind of radiometric chronometer of planet formation processes. Hafnium and W, the parent and daughter trace elements, are highly refractory; however, Hf is lithophile and strongly partitioned into the silicate portion of a planet, whereas W is moderately siderophile and preferentially partitioned into a coexisting metallic phase. More than 90% of terrestrial W has gone into the Earth’s core during its formation. The residual silicate portion, the Earth’s primitive mantle, has a Hf/W ratio in the range 10–40, an order of magnitude higher than chondritic (~1.3). Tungsten isotopic data for the Earth and the Moon suggest that we can date a major event of planet formation: The Moon formed about 50 Myrs after the start of the solar system, providing strong support for the Giant Impact Theory of lunar origin. Recent simulations of this event imply that the Earth was probably only half formed at the time. From this we can deduce the planetary accretion rate. Tungsten isotope data for Mars provide evidence of a much shorter accretion interval, perhaps as little as 10 Myrs, but the rates for the Earth over the same time interval could have been comparable. The large W isotopic heterogeneities on Mars could only have been produced within the first 30 Myrs of the solar system. Large-scale mixing, e.g. from convective overturn, as is thought to drive the Earth’s plates, must be absent from Mars.

Quantitative evaluation of carbon isotopic fractionation during reversed-phase high-performance liquid chromatography

The fractionation of 13C during low-performance preparative LC and high-performance LC is reported quantitatively for methyl palmitate and using high-precision isotope ratio mass spectrometry (IR-MS). For both preparative and high-performance analytical columns, 13C enrichment is about 7‰ greater than the parent starting material, drops sharply in the first section of the peak and then settles to a value about 1‰ below that of the starting material. Recycling over a single HPLC column did not induce greater fractionation. These results emphasize the importance of quantitative peak collection for high-precision IR-MS studies, particularly the first part of the peak where the isotope ratio changes rapidly.

12] Assessing metabolism of β-[13C]carotene using high-precision isotope ratio mass spectrometry

This chapter describes a stable isotope tracer method based on the use of highly enriched β-[U-13C]carotene and high-precision isotope ratio mass spectrometry (IRMS). Biological applications of IRMS are reviewed in the chapter. In gas chromatography combustion (GCC)–IRMS, combusted analytes are detected as the CO2+ produced in a high-sensitivity tight ion source. Each carbon atom of the analyte molecule has an independent chance of ionization and therefore detection limits are related to moles of analyte carbon, rather than to moles of analyte as in gas chromatography/mass chromatography (GC/MS). The ionization probability and molecular ion stability of CO2 is constant for all analytes. For these reasons, compounds of low ionization efficiency, low stability, and high molecular weight tend to be detected with high sensitivity by GCC–IRMS compared to conventional organic GC/MS. GCC–IRMS permits use of low doses typical of daily β-carotene intake, which do not perturb endogenous pool sizes of β-carotene or retinol. β-[13C]carotene is clearly evident in plasma by 3 hr after a dose of 2 mg β-[U-13C]carotene. In contrast, no detectable increase in concentration of labeled βC could be observed prior to 5 hr postdose with 40 mg β-carotene-d8 uring organic mass spectrometry.

Chapter 3 Experimental Approaches To The Study Of β-Carotene Metabolism: Potential of A13C Tracer Approach To Modeling β-Carotene Kinetics In Humans

This chapter reviews the biological relevance of β-carotene (βC) to human health. Previous approaches to model quantitative aspects of βC absorption and metabolism are also discussed. In addition, the chapter discusses the potential merits of the development of a 13 C-based stable tracer approach, and its uses for βC modeling in humans. The development and application of mathematical and compartmental kinetic models, using stable isotope tracer techniques, offers the advantages of (1) safely studying βC metabolism in humans, (2) providing predictive information regarding βC biokinetics, and (3) aiding in the evaluation of the role(s) of βC in various biological responses. Stable tracer approaches to the study of βC metabolism in humans offer several advantages over the use of unlabeled or radioactive βC. β-Carotene enriched with 13 C, coupled with the use of gas chromatography-combustion- high precision isotope ratio mass spectrometry, constitutes a safe and sensitive approach that requires only small doses of βC typical of daily dietary intake and that do not perturb endogenous pool sizes.

A high precision isotope ratio mass spectrometry method for measuring the [formula omitted] ratio of air

Studies of the distribution of O 2 in air will inform us about critical problems in the global carbon cycle which are not readily accessed by other measurements, including the rate of seasonal net production in the oceans on a hemispheric scale, the rate at which the oceans are taking up anthropogenic CO 2 and the net rate of change of the continental biomass. In this paper, we outline a method for measuring the O 2 N 2 ratio of air to a standard error of ± 6 per meg (± 0.006%.) for a sample analyzed in quadruplicate, corresponding to ± 1.2 ppm V O 2 in air out of 210,000. The method involves measuring the ratio of 16O 2 to 15N 14N by isotope ratio mass spectrometry. Potential and actual problems with this method include fractionation as sample and reference gases are introduced to the mass spectrometer, mass spectrometric nonlinearity, effects of imbalance of sample and reference ion currents on the measured isotopic ratio, isobaric interferences at masses 28 and 29 due to the formation of CO + from CO 2 in the source and zero enrichments. We discuss the magnitude of errors introduced by these factors and outline the relevant corrections. The ultimate mass spectrometric uncertainty is about ±2 per meg (±0.4 ppmV) for a 1 h instrumental analysis. Overall precision is currently limited by fractionation as sample and reference gases are introduced into the mass spectrometer. A considerable improvement in precision may be possible.

Study of beta-carotene metabolism in humans using 13C-beta-carotene and high precision isotope ratio mass spectrometry.

Study of beta-carotene metabolism in humans using 13C-beta-carotene and high precision isotope ratio mass spectrometry.

The Absolute Isotopic Composition and Atomic Weight of Terrestrial Nickel.

Twenty-nine samples of high-purity nickel metals, reagent salts and minerals, collected from worldwide sources, have been examined by high-precision isotope ratio mass spectrometry for their nickel isotopic composition. These materials were compared directly with SRM 986, certified isotopie standard for nickel, using identical measurement techniques and the same instrumentation. This survey shows no statistically significant variations among the samples investigated, indicating that the certified atomic weight and associated uncertainty for SRM 986 is applicable to terrestrial nickel samples. The atomic weight calculated for SRM 986 is 58.69335±0.00015 [2]. The currently recommended IUPAC value for terrestrial nickel is 58.69±0.01.

Solvent exchange of buried water and hydrogen exchange of peptide NH groups hydrogen bonded to buried waters in bovine pancreatic trypsin inhibitor

Solvent exchange of 18O-labeled buried water in bovine pancreatic trypsin inhibitor (BPTI), trypsin, and trypsin-BPTI complex is measured by high-precision isotope ratio mass spectrometry. Buried water is labeled by equilibration of the protein in 18O-enriched water. Protein samples are then rapidly dialyzed against water of normal isotope composition by gel filtration and stored. The exchangeable 18O label eluting with the protein in 10-300 s is determined by an H2O-CO2 equilibration technique. Exchange of buried waters with solvent water is complete before 10-15 s in BPTI, trypsin, and BPTI-trypsin, as well as in lysozyme and carboxypeptidase measured as controls. When in-exchange dialysis and storage are carried out at pH greater than or equal to 2.5, trypsin-BPTI and trypsin, but not free BPTI, have the equivalent of one 18O atom that exchanges slowly (after 300 s and before several days). This oxygen is probably covalently bound to a specific site in trypsin. When in-exchange dialysis and storage are carried out at pH 1.1, the equivalent of three to seven 18O atoms per molecule is associated with the trypsin-BPTI complex, apparently due to nonspecific covalent 18O labeling of carboxyl groups at low pH. In addition to 18O exchange of buried waters, the hydrogen isotope exchange of buried NH groups H bonded to buried waters was also measured. Their base-catalyzed exchange rate constants are on the order of NH groups that in the crystal are exposed to solvent (static accessibility greater than 0) and hydrogen-bonded main chain O, and their pH min is similar to that for model compounds. The pH dependence of their exchange rate constants suggests that direct exchange with water may significantly contribute to their observed exchange rate.

Isotope ratio mass spectrometer instrumentation and application to organic matter contained in recent sediments

The distribution of the stable carbon isotopes in aquatic organisms and sediments reveals that plants which derive their carbon from compounds dissolved in water are enriched in 13C with respect to plants which derive their carbon from atmospheric CO 2. The organic material of sediments reflect the isotopic composition of those organisms which contribute to the organic detritus. Sediments from south Texas bays, the Bahama banks and British Honduras reef were studied. Modifications to a high precision isotope ratio mass spectrometer are described which make it possible to assay either CO 2 or SO 2 samples interchangeably without deleterious memory effects.