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This study analyzed the isotopic profiles of four aragonitic shells of Scrobicularia plana in conjunction with measured seawater temperatures and salinities. Comparison of [delta].sup.18O.sub.SHELL with expected values revealed fractionation of [delta].sup.18O in near equilibrium with the ambient environment. Growth cessation occurred between November and March. Carbonate deposition stopped when temperatures were, Author(s): Sílvia Santos [sup.1] , Joana F. M. F. Cardoso [sup.1] [sup.2] , Valeska Borges [sup.1] , Rob Witbaard [sup.1] , Pieternella C. Luttikhuizen [sup.1] , Henk W. van der [...]

A novel approach to obtain a fractionation free [sup.11]B/[sup.10]B isotope ratio based on oxygen isotopes determined in situ from the same filament loading by N-TIMS is described. The method uses only a few nanograms of B to produce B[O.sub.2.sup.-] ions. First, the oxygen isotopes are determined at a lower filament temperature using Re[O.sub.4.sup.-] ions and employing [sup.187]Re/[sup.185]Re for internal normalization. Subsequently, the filament temperature is increased to get sufficient B[O.sub.2.sup.-] ions and predetermined [sup.18]O/[sup.16]O isotopes from the same filament loading is used to correct for boron mass fractionation. The validity of the method has been demonstrated by analyzing a NIST-SRM-951 boron isotopic certified standard, two synthetic B mixtures, and two coral reference materials. An average analytical precision of 0.6[per thousand] (n = 6) has been demonstrated. This is an important and crucial step forward in making the application of B[O.sub.2.sup.-] ions by N-TIMS routine in coral, foraminifera, and other samples where only limited amounts of boron are available. This new method does not require any additional effort in loading or in carrying out the mass spectrometric analysis but eliminates the need of assuming a fixed [sup.18]O/[sup.16]O ratio and thus provides higher accuracy for applications in paleo-oceanography, geochemistry, and cosmo-chemistry.

We use the GENESIS atmospheric general circulation model (GCM) with water isotopic transport and fractionation capabilities to quantify the influence of atmospheric C[O.sub.2], sea level, and elevation of the Western Cordillera on the [[delta].sup.18]O of middle Cretaceous precipitation. The model predicts a systematic increase of nearly 3 [per thousand] in the [[delta].sup.18]O of North American precipitation due to warming associated with an increase in C[O.sub.2] from 2 to 12 times pre-industrial levels. In contrast, the specification of lowstand conditions and a high ancestral Western Cordillera reduces the [[delta].sup.18]O of North American precipitation locally by as much as 6%0 and 8 [per thousand]. We compare the simulated [[delta].sup.18]O of precipitation with the [[delta].sup.18]O of paleosol siderite spherules and find good agreement only when the model includes lowstand conditions and a high ancestral Western Cordillera. Our results imply either that Cretaceous high-latitude paleosol [[delta].sup.18]O was influenced by orographic precipitation and the Western Interior Seaway, or that the GCM's hydrological cycle is deficient at high pC[O.sub.2]. Additional paleosol data are needed to resolve this issue. Keywords: paleoclimate, Cretaceous, siderite, [[delta].sup.18]O, general circulation model.

Isotope variability due to natural processes provides important information for studying a variety of complex natural phenomena from the origins of a particular sample to the traces of biochemical reaction mechanisms. These measurements require high-precision determination of isotope ratios of a particular element involved. Isotope ratio mass spectrometers (IRMS) are widely employed tools for such a high-precision analysis, which have some limitations. This work aims at overcoming the limitations inherent to IRMS by estimating the elemental isotopic abundance from the experimental isotopic distribution. In particular, a computational method has been derived that allows the calculation of [sup.13]C/[sup.12]C ratios from the whole isotopic distributions, given certain caveats, and these calculations are applied to several cases to demonstrate their utility. The limitations of the method in terms of the required number of ions and S/N ratio are discussed. For high-precision estimates of the isotope ratios, this method requires very precise measurement of the experimental isotopic distribution abundances, free from any artifacts introduced by noise, sample heterogeneity, or other experimental sources.

Ab-initio thermophysical properties and isotopic separative effects in Mg[Cr.sub.2][O.sub.4] were computed with the CRYSTAL03 code. All calculations were carried out within the LCAO approach (Linear Combination of Atomic Orbitals) by using the Hartree-Fock and the hybrid B3LYP density functional methods. Vibrational frequencies have been computed at the long-wavelength limit corresponding to the center of the Brillouin zone (k [left arrow] 0). Since thermodynamic functions depend on the phonon dispersion relation, the ab-initio based vibrational analysis of the optical modes has been coupled with the semiempirical Kieffer model for the acoustic modes, which is based on a sine wave dispersion equation. All computed magnitudes (zero point energy, internal energy, Helmholtz free energy, optical vibrational modes, thermal corrections to internal energy, enthalpy and Gibbs free energy, thermal pressure, bulk modulus and its P and T derivatives, entropy, isochoric and isobaric heat capacity, and isotopic separative effects) are consistent with direct observations and/or reasonable estimates, within the accuracy of the method. The computed separative effects are similar to those observed in other [Cr.sup.3+]-bearing substances. The crystal field seems to have no influence on the separative effect for an isotopomeric couple with respect to an isolated gaseous couple with [Cr.sup.3+] as central cation. Keywords: Ab-initio thermochemistry, picrochromite, Cr-isotopes, Cr-fractionation, vibrational properties, thermal physics

The mechanism(s) by which sex specific differences in regional body fat distribution develop are not known. We assessed the effects of a high-fat (HF) meal on fatty acid oxidation and uptake into regional fat depots using isotopic tracers and adipose biopsies, Thirty men (BMI 23.6 [+ or -] 0.3 kg/[m.sup.2]) and 29 women (BMI 22.4 [+ or -] 0.3 kg/[m.sup.2]) received a meal containing [[sup.3]H]triolein. Twelve of the men and 13 of the women received an additional 80 g of triolein in the meal (HF) and the remainder received a normal-fat (NF) meal. Adipose tissue lipoprotein lipase (LPL) activity was measured in the fed and fasted state. After 24 h, meal fatty acid uptake into subcutaneous adipose tissue was assessed. The efficiency of meal fat uptake into upper body subcutaneous fat was similar in both sexes, but women had a greater leg fat uptake, especially in response to a HF meal (P < 0.0001). A correlation between fed-state LPL activity and meal fat uptake was found in both upper and lower body fat (P < 0.0001, r = 0.69). These studies show that, in times of net fat storage, women preferentially increase uptake in leg adipose tissue, and this is likely mediated by fed-state LPL activity. lipoprotein lipase; isotopic fat tracers; triolein; fat biopsies

Abstract: Enriched stable mercury (Hg) isotopes were added to four 10 m diameter enclosures in Lake 239 at the Experimental Lakes Area to increase inorganic Hg loading. Our main objectives [...]

The behavior of multivalent elements such as Fe, Cr, V, and Eu in magmatic systems reflects the [fo.sub.2] of the environment. In particular, Eu behavior in pyroxene from basaltic systems has been demonstrated to be an effective measure of [fo.sub.2]. We selected two nearly isochemical lunar pigeonite basalts (15058, 15499), a lunar high-Ti basalt (75035), and Pasamonte (representing asteroid 4 Vesta, an unequilibrated eucrite), to explore other potential variables that may affect this indicator of [fo.sub.2]. All of these basalts crystallized at an [fo.sub.2] of approximately IW-1, yet they experienced different cooling and crystallization histories and their pyroxenes exhibit a wide range of compositional trajectories within the pyroxene quadrilateral. There are several variables that influence the Eu/Eu * recorded in pyroxene that may compromise the determination of [fo.sub.2]. Previous experimental studies show that pyroxene composition influences the ability of pyroxene to accommodate REE and fractionate [Eu.sup.2+] from [Eu.sup.3+]. We demonstrate that in addition to the influence of Ca in the M2 site, the Al content in the pyroxene and its influence on coupled substitutions will also influence the fractionation of [Eu.sup.2+] from [Eu.sup.3+]. For example, the coupled substitution [sup.T][Si.sup.4+] + [M.sup.2][R.sup.2+] [right arrow] [sup.T][Al.sup.3+] + [sup.M2][REE.sup.3+] may accommodate [REE.sup.3+] in preference to [Eu.sup.2+], which is too large. Different pyroxene growth surfaces will incorporate [Eu.sup.2+] and [Eu.sup.3+] differently due to differences in growth rate, Al content, and site configuration. In consort with the pyroxene composition, fractionation of [Eu.sup.2+] from [Eu.sup.3+] will be aided by the Al content of the basaltic melt, which increases the activities of network-forming components such as Ca[Al.sub.2][O.sub.4] and Fe[Al.sub.2][O.sub.4] in the melt during pyroxene crystallization. The Al content will result in changing the partitioning behavior of [Eu.sub.3+] while having very little effect on [Eu.sup.2+]. Melt composition, the appearance of plagioclase on the liquidus, and the kinetics of plagioclase crystallization are influenced by cooling rate. Data from the four basalts selected also suggest that [Eu.sup.2+]/[Eu.sup.3+] in the melt remains buffered even with extreme differences in cooling rate and plagioclase crystallization kinetics. Unexpectedly, many of these same variables affect the substitution of multivalent V. If the [fo.sub.2] determined from Eu behavior in pyroxene is not placed within a petrologic and crystal-chemical context, errors of 1 to 2 log units may result. The influence of these variables may be reduced by using multiple, co-crystallizing phases (i.e., plagioclase and pyroxene) and ratioing [D.sub.Eu] to adjacent REE ([D.sub.Sm.Gd]). Keywords: Eu valence, V valence, pyroxene, oxybarometer, Moon, 4 Vesta

To study Li isotopic fractionation during granite differentiation and late-stage pegmatite evolution, Li isotopic compositions and concentrations have been measured for the S-type Harney Peak Granite, the spatially associated Tin Mountain pegmatite, and possible metasedimentary source rocks in the Black Hills, South Dakota. The Harney Peak Granite is isotopically heterogeneous, with [[delta].sup.7]Li varying from -3.1 to +6.6. The [[delta].sup.7]Li values of Proterozoic metasedimentary rocks that are possible sources of the Harney Peak Granite range from -3.1 to +2.5 and overlap with post-Archean shales and the Harney Peak Granite. For the granite suite, there is no correlation between [[delta].sup.7]Li and elements indicative of degrees of granite differentiation (Si[O.sub.2], Li, Rb, etc.). The Li isotopic composition of the Harney Peak Granite, therefore, appears to reflect the source composition. Minerals from the zoned Tin Mountain pegmatite have extremely high Li contents and heavier Li isotopic compositions than the granite or surrounding Black Hills metasedimentary rocks. The heavier compositions may reflect Li isotopic fractionation resulting from extensive crystal-melt fractionation. Lithium concentrations decrease in the order: spodumene (~3.7 wt%), muscovite (0.2 to 2.0 wt%), plagioclase (100-1100 ppm), quartz (30-140 ppm). Plagioclase, muscovite, and spodumene in all zones display a relatively narrow range in [[delta].sup.7]Li of +7.9 to +11.4. In contrast, quartz is isotopically heavier and more variable (+14.7 to + 21.3), with [[delta].sup.7]Li showing an inverse correlation with Li concentration. This correlation reflects the mixing of isotopically heavy Li in quartz and lighter Li in fluid inclusions, as documented by fluid inclusion compositions ([[delta].sup.7]Li = +8.1 to +13.4 and Li of 280 to 3960 ppm). Extrapolation of this trend to an estimated intrinsic Li concentration in quartz of +21. The large difference in [[delta].sup.7]Li between quartz and other minerals may reflect [sup.7]Li preference for less highly coordinated sites, which have higher bond-energies (i.e., the two- or fourfold site in quartz vs. higher coordination number sites in other minerals). Comparison of the Li isotopic composition of fluid inclusions with that of the wall zone of the Tin Mountain pegmatite suggests ~4[per thousand] isotopic fractionation during fluid exsolution, which agrees with the results derived from studies of hydrothermal alteration of basalts. Keywords: Stable isotopes, igneous petrology, pegmatites, fluid phase, lithium, Harney Peak Granite, Tin Montain pegmatite, isotope fractionation

Abstract: Precise U-Pb monazite and [sup.40]Ar/[sup.39]Ar hornblende ages have been obtained from three locations in the high-grade Archean core of the Wind River Range, Wyoming. Monazites from metapelites in the [...]

Little is known about the isotopic composition of formaldehyde in the atmosphere, a chemical intermediate in hydrocarbon oxidation. Here, we present a promising new method to analyze the carbon ([[delta].sup.13]C) and hydrogen ([delta]D) isotopic composition of atmospheric formaldehyde. The direct isotopic analytical technique described uses continuous-flow gas chromatography--isotope ratio mass spectrometry, which provides flexibility for either isotopic analysis without correction for derivative functional groups. Current levels of precision of measurement are [+ or -]1.1 and [+ or -] 50[per thousand] (1[sigma]) for [[delta].sup.13]C and [delta]D analyses, respectively. Concentration of formaldehyde in ambient air is also determined, coincident with isotopic measurement, to a precision of [+ or -] 15%. The method has the required sensitivity for analyses of formaldehyde in urban air on relatively small volume grab samples of whole air (10-70L STP), potentially providing high temporal resolution. This is particularly advantageous for studying formaldehyde given its short lifetime and large variability in the atmosphere.

A classic problem in analytical chemistry has been determination of individual components in a mixture without availability of the pure individual components. Measurement of the distribution of isotopomers in a labeled compound or mixture of labeled compounds is an example of this problem that is commonly encountered when stable isotopically labeled metabolites are used to determine in vivo kinetics and metabolism. We present a method that uses the measured mass spectral data of the unlabeled material to represent any and all combinations of isotopomer variations of that material and to determine abundances of these isotopomers. Although examples of the method are presented for gas chromatography/mass spectrometry, the method is applicable to any type of mass spectrometry data. The method also accounts for errors induced by mass spectrometer ionization and resolution effects. To demonstrate this method, we determined the isotopomer distributions of samples of [sup.13]C-labeled leucine and glucose for both highly enriched isotopomers and labeled isotopomers present in low abundance against a natural isotopic abundance background. The method accurately and precisely determined isotopomer identity and abundance in the labeled materials without adding noise or error that was not inherent in the original mass spectral data. In examples shown here, isotopomer uncertainties were calculated with relative standard errors of < 1% from good quality mass spectral data.

A description is provided on the debates surrounding the foreign isotope program inside the US Atomic Energy Commission that shipped useful amounts of short-lived isotope phosphotrus-32 to a sanatorium in Trieste and in the US Congress. Supportive discussions state that it was an important instrument for projecting a positive image of America and for consolidating its alliance with friendly nations in the early years of cold war.

The installation for the plasma separation process (IPSP), using the ion cyclotron resonance (ICR) heating method, is discussed. It is shown that requirements having to do with spatial uniformity and time stability of the magnetic field can be met by means of precision manufacture and mechanical adjustment of solenoid coils.

The influences of charge exchange on ion collection of laser isotope separation of uranium are evaluated. Parametric study performed in terms of electric amplitude, ion density, and electron temperature revealed that the ratio of charge exchanged ions depends largely on collection time.

Research is presented describing the isotope effect study of the base-promoted HF elimination mechansim of 4-fluoro-4(4'-nitrophenyl)butan-2-one.

The isotope effects of gadolinium in Gd-EDTA ligand exchange system were studied by means of ion exchange chromatography. The separation coefficients of gadolinium isotopes, (epsilon), and the local enrichment factors, (beta), were calculated from the observed isotopic ratios at the front and rear boundaries of the gadolinium adsorption band. Clear mass independent anomalies were observed in the isotope effects of (super 155)Gd and (super 157)Gd. The relation between the isotope effects of gadolinium isotopes, studied by the three-isotope plot and the separation coefficient methods, and the mass of gadolinium isotopes was found to be related to the change in the mean square radius of the nuclear charge distribution parameter, (r(super 2)), of these isotopes, which suggests that the nucleus shape and size highly affect the gadolinium isotope effects in chemical exchange reactions.

An experimental study of kinetic isotope effects at 300 K for the reactions of Zr with C2H4, C2D4, C3H6 and C3D6 is presented. The lack of a measurable deuterium isotope effect in the 300 K rate constant eliminates direct insertion of Zr into a CH bond as the first stage in the reaction of Zr with ethylene or propylene.

In this paper, the variation of isotope concentration along a vacuum arc centrifuge is investigated for a two species plasma. Using the fluid equations of motion for both isotope species, together with continuity, the spatial dependence of the isotope concentration and separation factor is numerically solved. As well as comparison with experimental data detailing the spatial evolution of the separation profile for a Cu/Ni plasma, the important physical processes occurring in the plasma column as separation takes place are identified. Index Terms - Arc discharges, isotope separation, magnetic confinement, plasma devices, plasma properties, rotating bodies.

The magnetic isotope effect was successfully used to enrich germanium-73 using the hydrogen abstraction reaction of triplet benzophenone with triethylgermane in an SDS micellar solution. The relaxation mechanism explains the magnetic field dependence of 73Ge enrichment. Enrichment of 73Ge at zero magnetic field was at 0.5% for the cage product (diphenyl(triethylgermyl)methanol). As magnetic field strength increased, enrichment also rose to a maximum of 5.3% at 500 G. This gradually declined at higher magnetic fields to 1.0% at 10 kG.

The pressure dependence of deuterium-hydrogen (D-H) fractionation in water to 500 ° C and 200 megapascals has been calculated from high-temperature, high-pressure spectroscopic data. Pressure effects have a maximum at the critical temperature of water (20 per mil between 22 and 200 megapascals). Even larger effects are predicted for vaporlike densities from molecular dynamics simulations and molecular orbital calculations. Pressure effects explain many of the large discrepancies in published mineral-water D-H fractionation curves. Possible applications to natural examples include mineral-water isotope geobarometry., The stable isotopes of oxygen and hydrogen have been the most commonly used geochemical tracers in the study of both fossil and modern fluid-rock interaction processes for more than 30 [...]

A new, completely automated gas chromatography technique has been developed to separate the different gaseous compounds produced during underground coal gasification for their 13C/12C and D/H isotope ratio measurements. The technique was designed for separation and collection of [H.sub.2], CO, C[O.sub.2], [H.sub.2]O, [H.sub.2]S, C[H.sub.4], and heavier hydrocarbons. These gaseous compounds are perfectly separated by the gas-phase chromatograph and quantitatively sent to seven combustion and collection lines. [H.sub.2], CO, C[H.sub.4], and heavier hydrocarbons are quantitatively oxidized to C[O.sub.2] and/or [H.sub.2]O. The isotopic analyses are performed by the sealed-tube method. The zinc method is used for reduction of both water and [H.sub.2]S to hydrogen for D/H analysis. Including all preparation steps, the reproducibility of isotope abundance values, for a quantity higher than or equal to 0.1 mL of individual components in a mixture (5 mL of gases being initially injected in the gas chromatograph), is [+ or -]0.1[per thousand] for [Delta]13[C.sub.PDB] and [+ or -]6[per thousand] for [Delta][D.sub.SMOW].

This paper introduces an ultrahigh-speed algorithm for calculating isotope distributions from molecular formulas, elemental isotopic masses, and elemental isotopic abundances. For a given set of input data (molecular formula, elemental isotopic masses, and elemental isotopic abundances), and assuming round-off error to be negligible, the new algorithm rigorously produces isotope distributions whose mean and standard deviation are 'correct' in the sense that an error-free algorithm would produce a distribution having the same mean and standard deviation. The peak heights are also 'correct' in the sense that the height of each nominal isotope peak from the ultra-high-speed calculation equals the integrated peak area of the corresponding nominal isotope peak from an exact calculation. As a consequence of these properties, the algorithm generally places isotope peaks within millidaltons of their true centroids. The method uses Fourier transform methods and relates closely to two other recently introduced algorithms. The suite of capabilities provided by these three algorithms is sufficient to solve an extremely wide range of problems requiring isotope distribution simulation.

The equilibrium iron isotope fractionation between lower mantle minerals and metallic iron at core-mantle boundary conditions can be evaluated from the high-pressure [sup.57.Fe] partial vibrational density of states determined by synchrotron inelastic nuclear resonant x-ray scattering spectroscopy using a diamond anvil. Ferropericlase [(Mg,Fe)O] and (Fe,Mg)Si[O.sub.3]--post-perovskite are enriched in heavy iron isotopes relative to metallic iron at ultrahigh pressures, as opposed to the equilibrium iron isotope fractionation between these compounds at low pressure. The enrichment of Earth and Moon basalts in heavy iron isotopes relative to those from Mars and asteroid Vesta can be explained by the equilibrium iron isotope fractionation during the segregation of Earth's core and the assumption that Earth was already differentiated before the Moon-forming 'giant impact.'

This work describes the atomic route to laser isotope separation. This is a process which uses intense pulsed lasers to photoionize one isotopic species of a chemical element, after which these ions are extracted electromagnetically. The paper describes only the isotopic enrichment of uranium for nuclear fuel cycles. It makes brief mention of the traditional cascade processess used at present, and then turns to the atomic physics of laser photoionization. The principles of the laser isotope separation process, important spectroscopic experiments and considerations which determine the design of a plant are described. This review concentrates on the laser ionization process, few details of vaporization of uranium metal or the extraction of ions are given.

The chiral selectivity of a commercially available heparin affinity column was investigated for chloroquine. The (+) enantiomer eluted first, which is consistent with results reported previously using heparin as a chiral additive in capillary zone electrophoresis. The effects of pH, ionic strength, and organic modifier on the enantiodiscrimination were explored. A combination of electrostatic and hydrophobic interactions seems to play an important role in the enantiodiscrimination exhibited by this novel phase.

The formation of the anion radicals of naphthalene and 18-crown-6 in metallic sodium mirror-containing tetrahydrofuran (THF) and dimethoxyethane (DME) solutions leads to the separation of sodium (Na) 22 and 23 isotopes. The reaction leaves a bulk of the radioactive sodium in the anion radical solution. The presence of a more exposed sodium cation results in a 1. 30 +- 0.02 isotopic separation factor in THF without the addition of crown ethers. However, the substitution of THF with DME brings the separation factor to 1.02 +- 0.02 which is within the experimental error of unity.

The isotope-ratio and several other properties of lunar metals are measured and analyzed to explain the formation of the Moon. The findings reveal that Moon was actually formed much after the predicted time that is after the separation of the Earth's core from its mantle.

The technique for disentangling the effects of genome G+C content and isotope incorporation on DNA buoyancy density is discussed.

A novel 15N2-DNA stable isotope probing (15N2-DNA-SIP) method was implemented for the identification of free-living diazotrophs in soil responsible for nitrogen fixation in situ.

The basis of stable isotope fractination as it applies to elements like Fe, Mo, Zn, Cu and Tl in paleoceanography is described. The metal stable isotopes have potential to throw light on temporal and spatial changes in both metal cycling and ocean oxygenation through time and these can be used as paleoredox proxies.

The transport of nitrogen in vitreous silica is studied by using gas phase analysis and isotopic labeling of nitrogen. The results have concluded that nitrogen diffuses both in molecular and atomic forms with approximately 15 percent atomic nitrogen transport at 900 degree Celsius.

The experimental and field data that document a progressive increase in the hydrogen isotopic (D/H) ratio of most organic hydrogen at the bulk and molecular levels is reviewed. It is suggested that the transfer of hydrogen from water to organic is the most important mechanism leading to those changes.

The history of the field as well as all of the various applications since the first report of the mass-independent isotope effect is reviewed. There are numerous new frontiers open utilizing mass-independent isotopic fractionations and compositions as a highly specific probe of nature with advances in the ability to measure nanomolar-sized samples and to perform isotope ratio measurements.

A report on the IR data for monomeric beta-hairpin molecules with isotopic labeling correlated with ab initio theoretical modeling is discussed. Isotopic labeling of peptides can provide site-specific information, even with the use of low-resolution spectroscopic techniques.

Isotope fractionations during sulfate reduction by natural bacterial populations were measured in seven different marine sediments and compared with the isotopic composition of solid-phase sulfides in the same sediments. The measured fractionations during sulfate reduction could explain only between 41% and 85% of the [sup.34]S depletion in the sedimentary sulfides. This result directly demonstrates that the depletion of [sup.34]S in solid sulfides is an expression of the combined activity of sulfate-reducing organisms with additional fractionations accumulated during the oxidative part of the sulfur cycle. The only known process to significantly augment the fractionations created during sulfate reduction is the microbial disproportionation of the intermediate sulfur compounds: elemental sulfur, thiosulfate, and sulfite. In a simple model, we show how important each of these disproportionation processes could be in generating the isotopic composition of sedimentary sulfides. The sulfate reduction rates in the sediments studied varied by a factor of 1000 and did not correlate with the fractionation during sulfate reduction. By contrast, a correlation was observed between sulfate-reduction rate and the extent of [sup.34]S depletion into sedimentary sulfides, the most [sup.34]S-depleted sulfides found in sediments supporting the lowest rates of sulfate reduction. Thus, the fractionations imposed during the disproportionating processes are better expressed in sediments with low sulfate-reduction rates. The direct phototrophic oxidation of sulfide to sulfate, with minimal fractionation, may have been important in the sediments that had a high sulfate-reduction rate. Keywords: marine sediments, microorganisms, sulfides, sulfates, rates, oxidation.

Exotic Nuclei and Their Decay Eighty-one elements, from hydrogen to bismuth, occur naturally in stable form on Earth, and two more--thorium and uranium--appear with sufficiently long radioactive lifetimes that they [...]

Kinetic isotope effects are powerful tools in the determination of transition-state geometries. To examine the enzymatic rearrangement of chorismate mutase, the sensitive remote label method is used to measure the primary heavy atom isotope effects at C(1) on chorismate, site of bond formation, and ether oxygen, site of bond cleavage. Results suggest that the use of other chorismate derivatives will allow further refinement of the structure of the enzymatic transition state.

Pressure is one of the key variables in Earth science, ranging from an extremely small fraction of 1 atm in the upper atmosphere (~1 Pa or ~[10.sup.-5] atm) to probably [...]

Mass spectrometric analysis of isotopic mixtures is difficult when isotopes of different elements possess nearly the same mass and cannot be resolved. A new strategy, in which gaseous reagents induce selective 'chemical shifts' in the mass spectrum, is proposed for resolving such mass interferences. This application of ion-molecule chemistry is demonstrated for the separations of 187[Re.sup.+] and 187[Os.sup.+] and of 186[Os.sup.+] and 186[W.sup.+].

The goal of quantitative proteomics is to examine the expression levels of all of the proteins in a biological system and recognize those that change as a function of some stimulus. Quantification is now frequently based on derivatization of peptides with isotopically distinguishable labeling agents. This study examines the extent to which isotopic forms of peptides having the same amino acid sequence are resolved by reversed-phase chromatography and assesses the degree to which resolution of these isotopically different forms of a peptide impact quantification. Three derivatizing agents were examined, the [d.sub.0] and [d.sub.3] forms of N-acetoxysuccinimide, the [d.sub.0] and [d.sub.4] forms of succinic anhydride, and the [d.sub.0] and [d.sub.8] forms of the commercial ICAT reagent. Peptide mixtures from control and experimental samples were derivatized individually, mixed, subjected to reversed-phase chromatography, and analyzed by ESI-MS. When partial resolution of the isotopic forms of a peptide occurs, the largest error in assessing the true isotope ratio in a sample occurs when sampling at the extremes of a peak. Early in the elution of a peak, the sample will be enriched in the deuterated species, whereas the opposite is true at the tailing edge of a peak. Acetylated peptides showed the lowest degree of separation. Resolution of the deuterated and nondeuterated forms in this case was 0.023. This amounts to slightly over a 1-s difference in their peak maxima and can cause a typical error of [+ or -]6% at the leading and tailing edges of a peak. In contrast, resolution of the deuterated and nondeuterated forms of the ICAT reagent were calculated to be 0.45. This means that in a peak of l-min width (W1/2), the peak maxima will vary by ~ 30 s, and measurement errors of-83 and +500% can occur at the leading and tailing edges of a peak. It is concluded that resolution of isotopic forms of a peptide can cause substantial quantification errors in quantitative proteomics.