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He H.,CAS Institute of Geology and Geophysics | Zhu R.,CAS Institute of Geology and Geophysics | Saxton J.,Nu Instruments Ltd.
Physics of the Earth and Planetary Interiors | Year: 2011

Noble gas isotopic ratios provide unique and important constraints for the evolution of the subcontinental mantle. We present one-step crushing noble gas isotope data from corundum and co-existing mantle xenoliths and megacrystals from the Changle volcanic fields in the middle part of the Tan-Lu Fault Zone, eastern North China Craton. The 3He/4He ratio is 7.6-8.3 times the atmospheric ratio (Ra) in corundum, indicating contributions of fluids from convective asthenosphere. The 3He/4He ratio in titanomagnetite megacrystals ranges from 5.8 to 6.5Ra, suggesting possible U-Th contamination in the crystal lattice, with the 3He/4He ratio of host magma being higher than 6.5Ra. The olivine and opx in lherzolite show high helium contents and homogeneous isotopic helium ratios (6.9-7.0Ra). The cpx in wehrlite show significantly higher helium contents and slightly higher helium ratio (7.6Ra) than co-existing olivine (7.0-7.5Ra). Combined with isotope correlation diagrams, the systematic difference in 4He abundance, the 3He/4He and 4He/40Ar* ratios of cpx, opx and olivine in pyroxenite, lherzolite and wehrlite suggest comprehensive refertilization of lithospheric mantle in the eastern North China Craton. © 2011 Elsevier B.V.

Young E.D.,University of California at Los Angeles | Rumble D.,Geophysical Laboratory | Freedman P.,Nu Instruments Ltd | Mills M.,Nu Instruments Ltd
International Journal of Mass Spectrometry | Year: 2016

We describe a unique and novel isotope ratio mass spectrometer (IRMS), the Panorama, developed explicitly for high-mass-resolution analysis of isotopologue ratios of gas samples. The double-focussing instrument routinely operates at a mass resolving power of 40,000 with a maximum useful MRP of ∼80,000. The instrument achieves this exceptional MRP for a multi-collector using a Matsuda ion optical design with an ESA radius of 1018mm and a magnetic sector radius of 800mm. Collectors comprise 9 Faraday cups and a single channel of ion counting each with continuously variable collector slits. First results demonstrate both accuracy and precision comparable to, and in some cases, surpassing, other gas-source multi-collector IRMS instruments for singly-substituted species. For example, accurate bulk D/H and 13C/12C for methane gas measured with CH4 as the analyte are measured simultaneously with internal precision of 0.02-0.04‰ (1 std error) and ∼0.006‰ (1 se), respectively. Ion counting with continuous rebalancing of sample and standard gases permits high-precision measurements of rare, multiply-substituted isotopologues with relative abundances as small as ∼0.1ppm. In the case of methane, both 13CH3D/12CH4 and 12CH2D2/12CH4 ratios are measured with precision of ∼0.1‰ and ∼0.5‰, respectively. Accuracy of the multiply-substituted species measurements is demonstrated using isotope ratio mixing experiments. The ability to measure both δ13CH3D and δCH2D2 (‰ variations relative to the stochastic reference frame) provides heretofore unmatched capabilities to identify kinetic reaction pathways, isotope fractionation during transport, mixing, as well as temperatures of formation for methane gas. The high-resolution instrument can be used for a wide variety of applications. For example, it easily resolves 36Ar+ from 18O18O+ for oxygen bond-ordering studies. It also easily resolves 14N16O+ from 15N15N+ for measurements of the doubly-substituted N2 species. © 2016 Elsevier B.V.

Abouchami W.,Max Planck Institute for Chemistry | Abouchami W.,University of Munster | Galer S.J.G.,Max Planck Institute for Chemistry | De Baar H.J.W.,Netherlands Institute for Sea Research | And 8 more authors.
Geochimica et Cosmochimica Acta | Year: 2014

We present depth profiles of Cd isotopes and concentrations from the Southern Ocean at four stations in the Atlantic sector along the Greenwich Meridian (47°S to 68°S) located across the main Antarctic frontal zones and productivity belt. The vertical profiles of Cd concentration typically show low values in surface waters, elevated values at intermediate depths, reflecting remineralization of sinking particulate organic matter, and constant values in deep waters. The surface-to-deep isotopic gradient shows "heavy" Cd isotope signatures in the mixed surface layer, becoming more pronounced northward, with values up to e{open}112/110Cd of around +4.1 in the Subantarctic sector of the Southern Ocean. Deep Antarctic waters display a uniform and "light" ε112/110Cd of +1.18±0.38 and Cd concentrations of 0.761±0.101nmol/kg (n=23, 2SD). Intermediate waters are characterized by ε112/110Cd lying between those of surface and deep waters, with a constant value of about +0.8 in the High Nutrient Low Chlorophyll sector and a notably higher value of +2.3 in the Subantarctic sector. The Cd isotope fractionation in the Southern Ocean closely follows a simple closed-system Rayleigh model, in which biological uptake of Cd imparts the ε112/110Cd signature to the surface layer while that of deep waters is determined by the flux of regenerated isotopically-light Cd from sinking organic matter from the surface ocean and the degree of mixing of distinct water masses. The vertical gradient documented for Cd isotopes and nutrient ratios, along with the meridional gradient in surface waters, highlights the important role played by upwelling in the Southern Ocean in closing the meridional overturning circulation via the export of Antarctic intermediate and mode waters which have a distinctive chemical (low Cd:P) and Cd isotope ("heavy") signature. The combined Cd-Zn isotope systematics provide evidence for a strong link between the magnitude of biological Cd stable isotope fractionation and Zn availability in the contrasted nutrient and ecological regimes of the Southern Ocean. Substitution of Cd for Zn in the enzyme carbonic anhydrase appears to be the driving mechanism for Cd isotope fractionation in the Antarctic Circumpolar Current, while an "excess-uptake" mechanism seems to predominate in the Weddell Gyre. Our study highlights some of the complexities of the biogeochemical cycling of Cd in the oceans. Nevertheless, systematic variations in Cd isotopic compositions with water mass distribution in the Southern Ocean suggest that Cd isotopes could, with some caveats, be useful tracers of changes in past nutrient utilization and deep water circulation. © 2013 Elsevier Ltd.

Saxton J.M.,Nu Instruments Ltd.
Chemical Geology | Year: 2015

Most noble gas analyses are made in static mode when instrument volume is minimized to maintain adequate sensitivity. This makes the building of large instruments to obtain high resolving power impracticable. A method is presented which makes improved use of the available resolving power to remove isobaric interferences, which may be used on multicollector instruments. By arranging that the target mass position on a minor isotope (e.g. 36Ar), from which the interference must be removed, coincides with the approximately 50% point on the side of a major isotope (e.g. 40Ar), it is possible both to set the mass accurately and to verify the mass position and stability during measurements. The peak top of the major isotope is measured in a separate mass step. Calibration measurements are necessary, using different relative amounts of target/interference, to assess residual tailing to the measurement position and also the relative efficiency at the extreme edge of the target peak. The method is demonstrated by using it to obtain 36Ar measurements free of H35Cl. With samples containing 4×10-15 to 3×10-14mol of 40Ar, 36Ar/40Ar was measured, without HCl interference, to a 1σ precision of 0.5%, only slightly worse than counting statistics. This is potentially useful for 40Ar/39Ar dating, where 36Ar is used to correct for trapped air, and may be particularly significant for smaller or younger samples. © 2015 Elsevier B.V.

Mushtaq S.,London Metropolitan University | Steers E.B.M.,London Metropolitan University | Churchill G.,Nu Instruments Ltd. | Barnhart D.,Nu Instruments Ltd. | And 3 more authors.
Spectrochimica Acta - Part B Atomic Spectroscopy | Year: 2016

We report results of comprehensive studies using the Nu Instruments Astrum high-resolution glow discharge mass spectrometer (GD-MS) and optical emission spectrometry (OES) to investigate the relative importance of discharge mechanisms, such as Penning ionization (PI) and asymmetric charge transfer (ACT), at low-power/low-pressure discharge conditions. Comparison of the ratios of the ion signals of each constituent element to that of the plasma gas shows that for oxygen, the ratio in krypton is more than ten times higher than in argon (oxygen ground state ions are produced by Kr-ACT). For many elements, the ratios are very similar but that for tungsten is higher with krypton, while for iron, the reverse holds. These effects are linked to the arrangement of ionic energy levels of the elements concerned and the resulting relative importance of ACT and PI. The GD-MS and GD-OES results have shown that the ACT process can play an important role as the ionization mode in low-power/low-pressure discharges. However, OES results have shown that the magnitude of change in spectral intensities of elements studied are dependent on the discharge conditions. © 2016 Elsevier B.V. All rights reserved.

Cottle J.M.,University of California at Santa Barbara | Burrows A.J.,Nu Instruments Ltd | Kylander-Clark A.,University of California at Santa Barbara | Freedman P.A.,Nu Instruments Ltd | Cohen R.S.,Nu Instruments Ltd
Journal of Analytical Atomic Spectrometry | Year: 2013

This contribution describes the analytical performance resulting from modifications to the source of a Nu Instruments 'Nu Plasma' high resolution Multi Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS). Key advances in the 'enhanced sensitivity' (HR-ES) setup include changes to the geometry of the skimmer and sampler cones, re-design of the interface pumping system and modifications to the source lens. Comparison of both solution and laser-ablation data acquired on the same instrument before and after the modifications indicates that the sensitivity is improved by 80-125% in solution mode and 70-95% in laser-ablation mode. Analyses of standard solutions and reference zircon suggest no loss in accuracy. For laser ablation, the practical result is that for a given ablated volume, the HR-ES modification increases the precision with which individual isotopic measurements (e.g. U-Th/Pb, Hf) can be made. Conversely, for a required precision it reduces the amount of material needed by ∼2×, thereby significantly increasing the spatial resolution of in situ isotope MC-ICP-MS measurements. © 2013 The Royal Society of Chemistry.

Nu Instruments Ltd | Date: 2010-02-03

An approach to extending the dynamic range of the detector of a mass spectrometer is described. In one embodiment, in the case of high intensity beams, means are provided to deflect the ion beam, after the collector slit (1), on to an attenuator (4), which may be a grid or an array of small holes, through which only a small fraction of the ion beam reaches the ion detector (6). Use of an array of holes ensures that the recorded signal is insensitive to the distribution of ions within the beam. The beam passes directly to a detector if the signal is of low intensity.

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