Isotopic Abundance Measurements Research Articles

Possibilities and Scope of the Double Isotope Effect Method in the Elucidation of Mechanisms of Enzyme Catalyzed Reactions

Kinetic isotope effects on enzyme catalyzed reactions are indicative for the first irreversible in a sequence of individual steps. Hints on the relative velocities of other steps can only be obtained from the partitioning factor R and its dependence on external reaction conditions. In general, the experimental data needed are obtained from isotope abundance measurements in a defined position of the substrate or product as a function of turnover. This method does not reveal events dealing with neighbour atoms or preceeding the main isotope sensitive step. In the method presented here, the analytical measurement is extended to the second atom involved in a bond fission of formation (“Double Isotope Effect Method”). It is shown that the additional results obtained support the identification of the main isotopically sensitive step and its relative contribution to the overall reaction rate, the identification of other kinetically significant steps and the differentiation between stepwise and concerted reaction mechanisms. The method and its advantages are demonstrated on reactions comprising C–N-bond splitting (urease and arginase reaction), C–C-bond fission (reactions catalyzed by pyruvate-dehydrogenase, pyruvate-fonniate-lyase and lactate-oxidase), C–O-bond formation (ribulose-bisphosphate-oxygenase reaction), and N–O-bond fission (nitrate- and nitrite-reductase reactions).

Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines

Methane-hydrogen gas seeps with mantle-like C and noble gas isotopic characteristics issue from partially serpentinized ultramafic rocks in the Zambales ophiolite, Philippines. New measurements of noble gas and 14C isotope abundances, rock/mixed-volatile equilibrium calculations, and previous chemical and isotopic data suggest that these reduced gases are products of periodotite hydration. The gas seeps are produced in rock-dominated zones of serpentinization, and similar gases may be ubiquitous in ultramafic terranes undergoing serpentinization.

Measurement of isotopic abundances in collected stratospheric ozone samples

Enrichment of heavy O3 isotopes has been measured in collected stratospheric samples. A balloon‐borne cryogenic sampler was used to gather six O3 samples between 26 and 35 km in three flights. Subsequent laboratory mass spectrometer analysis of rare O3 isotopes at both mass 49 and 50 has resulted in more precise measurements than have previously been reported with in situ and ground‐based techniques. In one flight, 50O3 was enriched by 12–16% and 49O3 by 9–11%, both increasing with altitude. In the remaining two flights, the isotope enrichment was nearly mass‐independent at 8–9%. The enrichments in O3 at mass 50 are less than the large 40% value observed in some stratospheric measurements but similar to 49O3 and 50O3 fractionations produced in laboratory‐generated ozone.

Measurements of natural concentrations of 129I in uranium ores by accelerator mass spectrometry

An accelerator mass spectrometry system is described and utilized for measurements of 129I concentrations in natural and environmental samples. We report here on measurements of 129I isotopic abundances in iodine reagents and in iodine of mineral origin and of 129I concentrations in uranium ores of different origins. The 129I isotopic abundances for two measured contemporary iodine reagents and for iodine from a deep underground brine are 1.3 × 10 −13 and about 4 × 10 −14, respectively. 129I U ratios in the range 10 −13–10 −12 are measured and compared to a simple model of 129I production by spontaneous and induced fission of uranium. No clear correlation with the uranium concentrations or residence times is observed.

Mass spectrometry in geochemistry: An established tool in the earth sciences

Thermal ionization mass spectrometry is an established and important tool in the earth sciences. Precise measurements of radiogenic isotope abundances have allowed the development of useful geochronometers and petrogenetic tracers. Further advances in isotope geochemistry will, in part, depend on the capability of analyzing smaller and smaller samples. Techniques highly suitable for small sample/high resolution analysis such as SIMS or AMS will therefore become increasingly more important in the earth sciences. Nevertheless, because of continued instrumentation improvements in conventional mass spectrometry, and the probable high costs and lower accessibility of SIMS/AMS techniques, conventional techniques will remain of major importance in the foreseeable -future.

A new filter supplement for isotope ratio measurements

A new filter system has been constructed for mass spectrometric measurements of isotopic abundances with low mass cross contamination. This filter system reduces the number of scattered ions of incorrect masses which reach the Faraday collector cups. The filter consists of two electrostatic quadrupoles followed by a variable slit system. The filter system has been tested as an addition to the Finnigan mass spectrometers MAT 260 and MAT 261, where a three to four times improved abundance sensitivity could be obtained.

Fast-Pulse Detection for Isotopic Abundance Determination By Resonance Ionization, Time-Of-Flight Mass Spectrometry

ABSTRACT A linear time-of-flight mass spectrometer with static electrode voltages was developed for isotopic abundance measurements and showed good resolving power (620) for atomic ions. Positive ions, produced by multiphoton excitations, were detected by a microchannel plate detector that featured low noise and good pulse height distribution. Signal pulses were too fast for precise quantification by currently available transient digitizers. A gated pulse counter, which used AND logic circuitry to direct pulses for different isotopes into separate channels of a counter, was developed for quantification of the signal pulses and results for isotopic ratio measurements showed good precision, 0.3% to 1% relative standard deviation. The time-of-flight mass spectrometer with gated pulse counter was particularly well matched to pulsed laser ionization, because it could detect all isotopes of the selected element ionized in each laser pulse, and it did not impose a limit on the laser pulse repetition rate.

Noble gases, 81Kr [sbnd]Kr ages, and 10Be of chrondrites from China

A comprehensive study of the cosmic-ray exposure history of five ordinary chondrites from China was carried out using measurements of the noble gas isotopic abundances and 10Be concentrations. The following average cosmic-ray exposure ages, based on cosmogenic 21Ne and on 81Kr Kr dating were obtained: Zhaodong (L4) — 15.7 ± 3.0 m.y., Nan Yang Pao (L6) — 48 ± 10.0 m.y., Guangrao (L6) — 16.8 ± 3.5 m.y., and Lunan (H6) — 26.7 ± 5.0 m.y. The H5 chondrite Zaoyang was exposed for only 0.90 ± 0.12 m.y. to galactic cosmic rays as calculated from the 10Be activity and from the low amounts of cosmic-ray-produced noble gases. The Zhaodong chondrite contains large amounts of 80Kr and 82Kr produced by neutron capture of bromine. From the high slowing down density for neutrons we derive a preatmospheric mass of more than 1800 kg for this meteorite.

Isotopic analyses based on the mass spectrum of carbon dioxide.

Measurements of carbon and oxygen isotopic abundances are commonly based on the mass spectrum of carbon dioxide, but analysis of that spectrum is not trivial because three isotope ratios (17O/16O, 18O/16O, and 13C/12C) must be determined from only two readily observable ion-current ratios (45/44 and 46/44). Here, approaches to the problem are reassessed in the light of new information regarding the distribution of oxygen isotopes in natural samples. It is shown that methods of calculation conventionally employed can lead to systematic errors in the computed abundance of 13C and that these errors may be related to incorrect assessment of the absolute abundance of 17O. Further, problems arising during the analysis of samples enriched by admixture of 18O-labeled materials are discussed, and it is shown (i) that serious inaccuracies arise in the computed abundance of 17O and 13C if methods of calculation conventionally employed in the analysis of natural materials are applied to material labeled with 18O but (ii) that computed fractional abundances of 18O are always within 0.4% of the correct result. Methods for exact calculation of two isotope ratios when the third is known are presented and discussed, and a more exact approach to the computation of all three isotope ratios in natural materials is given.

Determination of K-factors for isotope abundance measurements of uranium and plutonium by thermal ionisation mass spectrometry

K-factors (= certified isotope ratio/observed isotope ratio) are determined for the isotope abundance measurements of uranium and plutonium by thermal ionisation mass spectrometry. An mdf of 0.07% and 0.18% per mass unit differing by a factor of about 3, is obtained for uranium and plutonium, respectively, employing double rhenium filament assembly in the ion source and Faraday cup as the detector using the presently available isotopic reference materials of uranium and plutonium.

Dinonyl phthalate as a dopant for CR-39 in cosmic ray studies

A recent investigation of the properties of CR-39 detectors demonstrated that the track response can be altered by the addition of heavy phthalic acid esters prior to the polymerisation process. Continuation of these studies employing fast iron ions at the Berkeley BEVALAC and using dinonyl phthalate as a CR-39 dopant, indicates a reduction in sensitivity combined with improved uniformity and resolution. A confirmation of these properties in very large area detectors would be a welcome development for cosmic ray elemental and isotope abundance measurements.

Mass Spectrometry: Analytical Capabilities and Potentials

The mass range of mass spectrometers has been extended by almost an order of magnitude in the past decade, ionization procedures have been introduced which allow ionic, nonvolatile compounds to be examined, and new capabilities have been achieved through the successful integration of separation and analysis techniques. In combination with other techniques, mass spectrometry has been used in biological and environmental research to characterize constituents of mixtures, including those present in trace amounts; in metabolic profiling, where high throughput and large dynamic range are important; and in protein structure determinations. Measurements of stable isotope abundances by mass spectrometry have been used in enzymology, studies of photosynthesis, and carbon dating. Outside the area of chemical analysis, mass spectrometry has been used to study gas-phase acidities and basicities and to study organic reaction mechanisms in the gas phase. Trends in mass spectrometry include multidimensional experiments, use of ionization methods, direct analysis without extensive sample preparation, and the development of advanced instrumentation including an ion trap and an inductively coupled plasma mass spectrometer. It is likely that mass spectrometry will come to be much more widely used and that data will increasingly be other than conventional mass spectra.

A METHOD FOR THE IDENTIFICATION AND ELIMINATION OF CONTAMINATION DURING CARBON ISOTOPIC ANALYSES OF EXTRATERRESTRIAL SAMPLES

The study of carbon abundance and isotopic composition in extraterrestrial samples is fraught with problems related to contamination in the terrestrial environment and during sample handling. A stepped combustion method is described which demonstrates that progress can be made towards resolving the indigenous species from contamination which for the most part burns at low temperature (< 425 ± 25 ±C). The proposed method is not applicable to samples which have indigenous phases burning at low temperatures e.g. the C1 and C2 carbonaceous chondrites. A number of examples where its application is possible are given.Even meteorites collected immediately after their fall, such as Allende, contain a proportion of extraneous carbon which has deleterious effects on any bulk estimate of isotopic composition. “Falls” which have spent a considerable time in museum collections and “finds” (other than Antarctic samples) can be considered as grossly contaminated. Bulk isotope and carbon abundance measurements in the literature for most samples having less than 1 wt% C are thus of questionable value. Antarctic samples have much less contamination of an organic nature but all seem to contain a weathering component which can be easily recognised and hence disregarded in estimates of bulk composition. Stepped combustion, applied to an Apollo 11 lunar soil which has not been specially stored and which now contains, due to contamination, nearly twice as much carbon as when originally collected, can still afford data closely resembling those obtained from the sample when it was first returned to Earth.

A general circulation model study of atmospheric carbon monoxide

The carbon monoxide cycle is studied by incorporating the known and hypothetical sources and sinks in a tracer model that uses the winds generated by a general circulation model. Photochemical production and loss terms, which depend on OH radical concentrations, are calculated in an interactive fashion. The computed global distribution and seasonal variations of CO are compared with observations to obtain constraints on the distribution and magnitude of the sources and sinks of CO, and on the tropospheric abundance of OH. The simplest model that accounts for available observations requires a low latitude plant source of about 1.3×1015 g yr−1, in addition to sources from incomplete combustion of fossil fuels and oxidation of methane. The globally averaged OH concentration calculated in the model is 7×105 cm−3. Models that calculate globally averaged OH concentrations much lower than our nominal value are not consistent with the observed variability of CO. Such models are also inconsistent with measurements of CO isotopic abundances, which imply the existence of plant sources.

The carbon isotopic composition of atmospheric methane

Recent measurements of the carbon isotopic abundance of atmospheric methane give a value of δ(13C/12C) = −47.0 ± 0.3‰, which is significantly different from earlier values measured by others between 1950 and 1970. The isotopic composition of various sources and the possibility of a temporal trend are discussed. The average methane concentration measured was 1.66 ppm, which is in accord with the increase in concentration since 1965–1970 observed in other studies.

Quantitation of Methadone Eenantiomers in Humans Using Stable Isotope-labeled [2H3]-, [2H5]-, and [2H8]methadone

A new technique for simultaneous stereoselective kinetic studies of methadone enantiomers was developed using three deuterium-labeled forms of methadone and GLC-chemical-ionization mass spectrometry. A racemic mixture (1:1) of (R)-(-)-[2H5]methadone (l-form) and (S)-(+)-[2H3]methadone (d-form) was administered orally in place of a single daily dose of unlabeled (±)-[2H8]methadone in longterm maintenance patients. Racemic (±)-[2H8] methadone was used as an internal standard for the simultaneous quantitation of [2H0]-, [2H3]-, and [2H5] methadone in plasma and urine. A newly developed extraction procedure, using a short, disposable C18 reversed-phase cartridge and improved chemical-ionization procedures employing ammonia gas, resulted in significant reduction of the background impurities contributing to the ions used for isotopic abundance measurements. These improvements enabled the measurement of labeled plasma methadone levels for 120 hr following a single dose. This methodology was applied to the study of methadone kinetics in two patients; in both patients, the analgesically active l-enantiomer of the drug had a longer plasma elimination half-life and a smaller area under the plasma disappearance curve than did the inactive d-form.

The mass spectrum of glycine

AbstractIrreproducibilities in the measurements of isotopic abundance in 15N labelled glycine have been related to the complex nature of chemical processes occurring during the evaporation of glycine prior to ionization.

The effect of trace gases on neodymium ion formation

During measurements of Nd isotope abundances, significant ionization suppression and enhancement effects have been observed and correlated with the presence of oxygen and water vapour in the ion source region of a solid source mass spectrometer. These effects are of particular interest at present because many laboratories are developing the SmNd technique for use in geochronological studies.

Cadmium isotope fractionation in fractions of two H3 chondrites

Measurements of Cd isotope abundances reveal anomalies of up to 2.8% and 3.1% for the 116Cd/ 106Cd ratio in separates of the unequilibrated (H3) chondrites Tieschitz and Brownfield. Samples of chondrules, matrix and whole rock from Tieschitz yield a correlation between isotopic fractionation and Cd concentration, implying a two-component mixture of unfractionated and fractionated Cd. A correlation is also observed in mineral concentrates prepared from Brownfield, but it is not as definite. We favour the interpretation that Tieschitz has fractionated Cd uniformly distributed throughout the meteorite with a non-uniformly distributed unfractionated Cd component. The trend observed in the Brownfield data suggests a non-uniform distribution of the fractionated component, possibly brought about by terrestrial weathering processes.

Measurements of Isotopic Abundances in Interstellar Clouds

While an examination of the available data reveals some seemingly contradictory results, a general framework having the following outlines can be put forward:1. With the exception of the two galactic center sources SgrA and SgrB, the relative isotopic abundances exhibited by the giant molecular clouds in our Galaxy exhibit few, if any, significant variations from the values obtained by averaging the data from all these sources.2. The 13C/12C and 14N/15N abundance ratios are ∼130% and ∼150%, respectively, of their terrestrial values throughout the galactic plane and somewhat higher, ∼300%, near the galactic center.3. The 16O/18O and 17O/18O abundance ratios are ∼130% and ∼160%, respectively, of their terrestrial values throughout the Galaxy, although the former may be somewhat lower near the galactic center.4. The S and Si isotopes have generally terrestrial abundances.