Negative Thermal Ionization Mass Spectrometry Research Articles

Nanoscale Os isotopic quantification of Wadi Tayin dunite platinum group minerals by atom probe tomography

Serpentinites record processes that redistribute major and trace elements between mantle and crust. Platinum Group Elements (PGEs) are trace elements in serpentinites hosted in sulphides and alloys. Alloys are challenging to find, and most analytical techniques lack the spatial resolution to analyse them; hence, this research adopts automatic mineral mapping technique to detect PGM grains in a sample from the Wadi Tayin (Oman) peridotite and uses atom probe tomography, a nanoscale quantitative analytical technique, to analyse the grain. This work applies and assesses the applicability of atom probe tomography to measure the 187Os/188Os isotopic ratio of natural alloys and uses the ratio to constrain the source of Os. A novel algorithm is used to automatically determine the number of counts of the 187Os and 188Os peaks, to calculate the isotopic 187Os/188Os ratio and the analytical uncertainty. The 187Os/188Os ratio is 0.126 ± 0.003, consistent with the isotopic composition reported by literature in the dunite of the Main Mantle Section of the Wadi Tayin ophiolite.The analytical uncertainty is one order of magnitude higher than conventional bulk rock techniques, such as negative-thermal ionisation mass spectrometry (N-TIMS) and inductively coupled plasma mass spectrometry (ICP-MS). However, the precision is sufficient to conclude that the non-radiogenic 187Os/188Os ratio is compatible with a mantle origin for the alloy. Decreasing whole-rock Re with increasing LOI and the overprinting of magmatic pentlandite by magnetite demonstrate that progressive serpentinisation may have modified the Re budget. The results indicate that atom probe tomography can analyse 187Os/188Os ratio quantitatively in micron-sized natural alloys and provide insights into natural processes.

Platinum-rhenium-osmium isotope systematics of chromitite and native osmium from the Guli Massif (Maimecha-Kotui Province, Russia)

To gain insight into the source of ore material, this study presents the first Re-Os and Pt-Os isotopic and highly siderophile element (HSE) abundance data for chromitite and osmium minerals from the Guli massif of ultramafic, alkaline rocks and carbonatites located in the Maimecha-Kotui province, Polar Siberia. The study utilized a number of analytical techniques, including electron microprobe analysis, negative thermal ionization mass spectrometry (N-TIMS) and high pressure asher digestion and an isotope dilution-inductively coupled plasma-mass spectrometry. The HSE concentrations in chromitite samples range from 191 to 866 ppb with a predominance of Ir-group platinum-group elements (PGE) (Os, Ir and Ru) over Pt-group PGE (Rh, Pt and Pd) and Re, which is consistent with their platinum-group mineral control (i. e., Os-Ir alloys and laurite, RuS2) within the chromitite. The Re-Os and Pt-Os isotope data indicate that the HSE budget of the chromitite and osmium minerals from the Guli massif was largely controlled by that of the mantle source, which evolved with long-term near-chondritic Re/Os and Pt/Os ratios; this source is within the range of those for the majority of komatiite and abyssal peridotite sources.

Probing the 186Os/188Os Precision Barrier: New Recommended Values for the DROsS Reference Material and an Assessment of Mixed 1011 and 1012 Ω Amplifier Arrays

We present high precision negative ion thermal ionisation mass spectrometry (N‐TIMS) Os isotope measurement results for the DROsS isotope reference material (iCRM), to investigate the limits on the precision of TIMS‐based 186Os/188Os results. We used analytical conditions previously highlighted to optimise precision, present a new flexible data processing protocol, and measured 184Os intensities on a Faraday Cup equipped with an amplifier using a 1012 Ω resistor. Despite a measurement procedure that minimised uncertainty contributions from counting statistics and Johnson‐Nyquist noise, the intermediate measurement precision of our approach does not significantly improve on previous high precision Os isotope measurements, with the exception of 184Os/188Os. This is probably due to uncertainties in measured amplifier gain factors, which are greater when using mixed arrays of 1011 and 1012 Ω resistors than when using 1011 Ω resistors alone, though Faraday Cup deterioration could also contribute. We propose that multi‐dynamic Os isotope measurements could largely eliminate both of these uncertainties. Our 184Os/188Os measurement results are the most precise yet, yielding 184Os/188Os = 0.0013036 ± 0.0000007 (2s, n = 38). Additionally, we average our data with published data to recommend the following isotope ratios for DROsS: 186Os/188Os = 0.1199319 ± 0.0000024, 187Os/188Os = 0.1609227 ± 0.0000022, 189Os/188Os = 1.219709 ± 0.000010, 190Os/188Os = 1.983793 ± 0.000011.

A Combined Re-Os and Pt-Os Isotope and HSE Abundance Study of Ru-Os-Ir Alloys from the Kunar and Unga Placer Deposits, the Taimyr Peninsula, Polar Siberia

In order to provide further insights into the origin of Ru-Os-Ir alloys, this study presents new highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundance and 187Re-187Os and 190Pt-186Os isotope data for detrital grains of native Ru-Os-Ir alloys in placer deposits of the Kunar and Unga Rivers, which display a close spatial association with the Kunar dunite–harzburgite complex in the northern part of the Taimyr Peninsula in the Polar Siberia. The study utilized electron microprobe analysis, negative thermal ionization mass-spectrometry (N-TIMS) and laser ablation multiple-collector inductively coupled plasma mass-spectrometry (LA MC-ICP-MS). The primary nature of the Ru-Os-Ir alloys is supported by the occurrence of euhedral inclusions of high-Mg olivine (Fo92–93) that fall within the compositional range of mantle olivine. The LA MC-ICP-MS data show similar average initial 187Os/188Os and γ187Os(740 Ma) values for PGM assemblages from the Kunar and Unga deposits of 0.1218 ± 0.0010, −0.18 ± 0.85, and 0.1222 ± 0.0025, +0.10 ± 2.1, respectively. These values are identical, within their respective uncertainties, to the initial 187Os/188Os value of the Ru-Os-Ir alloy grain measured by N-TIMS (0.1218463 ± 0.0000015, γ187Os(740 Ma) = −0.1500 ± 0.0012). The combined 187Re-187Os isotopic data for all studied grains (γ187Os(740 Ma) = −0.02 ± 1.6) indicate evolution of the Kunar and Unga mantle sources with a long-term chondritic 187Re/188Os ratio of 0.401 ± 0.030. In contrast to the 187Os/188Os data, the initial 186Os/188Os value of 0.1198409 ± 0.0000012 (µ186Os(740 Ma) = +34 ± 10) obtained for the same Ru-Os-Ir alloy grain by N-TIMS is suprachondritic and implies evolution of the Kunar and Unga mantle source(s) with a long-term suprachondritic 190Pt/188Os ratio of 0.00247 ± 0.00021. This value is ~40% higher than the average chondritic 190Pt/188Os ratio of 0.00180 and indicates long-term enrichment of the Kunar source in Pt over Os. Establishing the source of this enrichment requires further investigation.

High-precision measurements of Mo isotopes by N-TIMS

We have developed a new high-precision analytical method for measuring molybdenum (Mo) isotopes by negative thermal ionization mass spectrometry (N-TIMS). This method represents an improvement over existing methods, thanks to a new chemical separation method that yields a better purification of the Mo fraction. In addition, we have used a multi-dynamic method for data collection with three different magnet settings to avoid being sensitive to drifts in Faraday cup efficiencies. Our method resulted in a more reliable thermal emission of MoO3- for a given run and we have obtained Mo isotope ratios measured on a series of 26 standards with the following reproducibilities (2 SD): ±6.1, ±5.8, ±2.9, and ±9.6 ppm for 94Mo/96Mo, 95Mo/96Mo, 97Mo/96Mo, and 100Mo/96Mo, respectively. This represents an improvement by a factor of 2–3 compared with previously published methods, using N-TIMS or MC-ICPMS. This new analytical method will have applications in the field of cosmochemistry (nucleosynthetic anomalies), environmental geochemistry or nuclear forensics (mass-independent fractionation).

Re‐Os Isotopic Age of Molybdenite of the Jingren Deposit and its Mineralogical Significance of Magnetite, Pyrite and Chalcopyrite

AbstractThe Jingren deposit is part of the Qimantage metallogenic belt within the eastern Kunlun orogenic belt, the largest metallogenic belt in Qinghai Province, northwestern China. Exploration data show that the metal resources of the Jingren deposit are greater than 93000 t in a mining area of 76.15 km2, which indicates significant exploration potential in the near future. Three W–E‐trending faults, F1‐3, dominate the extension of the mineralization zone, which consists of chalcopyrite, pyrite, magnetite, galena, sphalerite, and molybdenite as well as bismuth‐bearing minerals. The deposit contains a large amount of late Triassic intrusive rocks, however, previous research did not reach a consensus on the timing or the origin of the mineralization owing to a lack of geochronological data and poor exposure conditions. In the present study, Re‐Os isotopic dating from six molybdenite samples collected from a borehole of the granodiorite in the Jingren deposit using negative thermal ionization mass spectrometry (NTIMS) showed 187Re and 187Os concentrations of 0.26–4.40 ppm and 1.03–16.46 ppb, respectively, with an initial 187Os/188Os value of 0.06 ± 0.19. This proves that the Jingren deposit has a metallogenic age of (225 ± 4) Ma and is the product of united mineralization of the Qimantage metallogenic belt and that the Jingren deposit might actually be an Indosinian metallogeny. In addition, the Re content of these samples, at 0.42 ppm to 7.00 ppm shows that the mineralization was derived mainly from a crustal source. Furthermore, electron probe microanalysis (EPMA) conducted on chalcopyrite obtained from 22 metallic mineral samples revealed (Fe + Cu)/S ratios of 1.801–1.947 with an average of 1.852, which is lower than the ideal value (1.875). Besides, the main ore body formed in a relatively higher temperature environment than the surrounding rocks in the Jingren deposit. These data indicate that the Jingren deposit formed in a metallogenic environment at lower temperature. Moreover, according to the TiO2‐Al2O3‐(MgO + MnO) and TiO2‐Al2O3‐MgO genetic classification diagram for magnetite, the Jingren deposit most likely belongs to the skarn family. In addition, the Co‐Ni‐As genetic classification diagram of the pyrite indicates sedimentary and skarn genetic characteristics.

Search forαdecay of naturally occurring osmium nuclides accompanied byγquanta

A search for $\alpha$ decay of naturally occurring osmium isotopes to the lowest excited levels of daughter nuclei has been performed by using an ultra-low-background Broad-Energy Germanium $\gamma$-detector with a volume of 112 cm$^3$ and an ultra-pure osmium sample with a mass of 118 g at the Gran Sasso National Laboratory of the INFN (Italy). The isotopic composition of the osmium sample has been measured with high precision using Negative Thermal Ionisation Mass Spectrometry. After 15851 h of data taking with the $\gamma$-detector no effect has been detected, and lower limits on the $\alpha$ decays were set at level of $\lim T_{1/2}\sim 10^{15}-10^{19}$ yr. The limits for the $\alpha$ decays of $^{184}$Os and $^{186}$Os to the first excited levels of daughter nuclei, $T_{1/2}(^{184}$Os$)\geq 6.8\times10^{15}$ yr and $T_{1/2}(^{186}$Os$)\geq3.3\times10^{17}$ yr (at 90% C.L.), exceed the present theoretical estimates of the decays half-lives. For $^{189}$Os and $^{192}$Os also decays to the ground states of the daughter nuclei were searched for due to the instability of the daughter nuclides relative to $\beta$ decay.

Precise Measurement of Tellurium Isotope Ratios in Terrestrial Standards Using a Multiple Collector Inductively Coupled Plasma Mass Spectrometry.

Precise tellurium (Te) isotope ratio measurement using mass spectrometry is a challenging task for many decades. In this paper, Te isotope ratio measurements using multi-collector inductively coupled plasma mass spectrometry (MC–ICP–MS) in terrestrial Te standards have been reported. Newly developed Faraday cup with 1012 Ω resistor is used to measure low abundance 120Te, whereas the 1011 Ω resistor is used to measure other Te isotopes. The relative standard deviation obtained for Te isotope ratio measurement by Faraday cups of 120Te/128Te [0.002907(05)], 122Te/128Te [0.079646(10)], 123Te/128Te [0.027850(07)], 125Te/128Te [0.221988(09)], 126Te/128Te [0.592202(20)], and 130Te/128Te [1.076277(30)] were 0.140%, 0.014%, 0.026%, 0.005%, 0.004%, and 0.004%, respectively. The measured isotope ratio results are compared with previous results obtained by thermal ionization mass spectrometry (TIMS), negative thermal ionization mass spectrometry (N–TIMS), and MC–ICP–MS, showing an improvement in the precision about one order of magnitude for 120Te/128Te ratio. The present study shows better precision for Te isotope ratios compared to earlier studies.

Chemical Procedures for Rhenium Extraction from Geological Samples: Optimising the Anion Resin Bead Clean‐up Step

Rhenium–osmium geochronometry for samples with low Re and complex matrices requires improved Re extraction methods. Here, we investigate plausible controls on efficiency and efficacy of Re extraction during our anion resin bead purification. Four different protocols are compared, each isolating a single variable to test. Rhenium concentrations for solutions at each step of each protocol document differences in chemical recovery/yield. The negative‐thermal ionisation mass spectrometry (N‐TIMS) signal intensity serves as a proxy for Re yield and purity. These data document correlations between the N‐TIMS signal intensity and (a) the duration of anion resin bead conditioning prior to loading with Re‐bearing solution, and (b) both duration and strength of nitric acid used during rinsing of the Re‐loaded anion resin bead. The optimal protocol improved Re signal intensity around fourteen times compared with our current Re extraction protocol, an aggregate of 2.4 times improvement in chemical recovery (yield) and 5.8 times improvement in emission efficiency (purity). Repeated N‐TIMS isotopic measurements on our in‐house Re standard solution (1407) verify that our optimal protocol‐3 does not fractionate Re isotopes. The improved anion resin bead method considerably lowers the Re detection limit and allows Re‐Os isotopic analysis of picogram‐level Re hosted in geological samples with complex matrices.

Refinement of the Micro‐Distillation Technique for Isotopic Analysis of Geological Samples with pg‐Level Osmium Contents

In recent years, the 187Re–187Os isotope system has been increasingly used to study samples containing very small quantities of Os. For such samples, optimisation of measurement procedures is essential to minimise the loss of Os before mass spectrometric measurements. Micro‐distillation is a necessary purification step that is applied after the main Os chemical separation procedure, prior to Os isotope ratio measurements by negative‐thermal ionisation mass spectrometry (N‐TIMS). However, unlike the other separation steps, this procedure has not yet been optimised for small samples. In this study, we present a refined micro‐distillation method that achieved higher yields and allowed high‐precision R(187Os/188Os) expressed as 187Os/188Os measurements for small‐sized geological samples that contain only a few pg Os. The Os recovery in the micro‐distillation step was tested by changing the operating conditions including heating time and temperature, and amounts of oxidant and reductant. Recoveries were measured by the isotope dilution ICP‐MS method after the addition of 190Os‐enriched spike solution. We found that the most critical factor controlling the chemical yield of Os during micro‐distillation is the extent of dilution of the reductant (HBr) by H2O evaporated from the oxidant. A refined micro‐distillation method, in which the amount of oxidant solution is reduced from the conventional method, achieved an improved chemical yield of Os (~ 90%). This refined method was applied to the measurement of 187Os/188Os by N‐TIMS of varying test portions of the geological reference material BIR‐1a. The resulting 187Os/188Os ratios of BIR‐1a matched the literature data, with propagated uncertainties of 0.2, 1.1 and 11% digested sample quantities containing 150, 10 and 1 pg of Os, respectively.

Total evaporation technique for high-accuracy isotopic analysis of isotopically enriched molybdenum by negative thermal ionization mass spectrometry

The isotopic composition of isotopically enriched materials is essential for isotope dilution mass spectrometry and calibrated mass spectrometry, which are considered to be potential primary methods/authority methods of the highest metrological quality for the analysis of elemental content or isotopic compositions. In this study, a total evaporation (TE) technique was developed for accurate determination of isotopic compositions for seven enriched molybdenum materials with the utilization of negative ion-thermal ionization mass spectrometry (NTIMS). By using La(NO3)3 as an activator, the sample was efficiently ionized into for NTIMS measurement. To achieve highly precise results, the oxygen isotope interference in on Mo isotopic analysis was corrected by using the IUPAC recommended isotopic compositions for atmospheric O2. Various sources of uncertainty to TE-NTIMS, such as measurement repeatability, oxide interference correction, and the ion loss before and after measurement were discussed with respect to the determination of their uncertainty contribution and their influence on the results. As a result, the major isotope abundance for each enriched Mo material were determined as (k = 2): x(92Mo) = 0.974 69 ± 11 for enriched Mo-92, x(94Mo) = 0.920 26 ± 11 for enriched Mo-94, x(95Mo) = 0.965 50 ± 20 for enriched Mo-95, x(96Mo) = 0.967 59 ± 12 for enriched Mo-96, x(97Mo) = 0.942 06 ± 13 for enriched Mo-97, x(98Mo) = 0.984 56 ± 11 for enriched Mo-98, and x(100Mo) = 0.993 655 ± 12 for enriched Mo-100, respectively. These measurement results have been compared to those determined by a multi-collector inductively coupled plasma mass spectrometry via the mathematical iteration method in our previous work. Identical major isotope-abundance values were achieved within their combined standard uncertainties, indicating that the TE technique is a powerful and alternative tool for accurate determination of the isotopic composition for isotopically enriched materials. The potential influence owing to the variations of O isotopic compositions during NTIMS measurements were also investigated by applying the ‘In-run O’ isotope-abundance values reported in recent literatures for O isotope interference correction. The corrected Mo isotope abundances of either the major or the minor isotopes were in good agreement with that corrected by using the IUPAC atmospheric O2 isotopic compositions.

More than ten percent ionization efficiency for Tc measurement by negative thermal ionization mass spectrometry

Pt filaments with Ba(OH)2 as the ionization activator were highlighted for the measurement of Tc by N-TIMS.

Defining the Potential of Nanoscale Re‐Os Isotope Systematics Using Atom Probe Microscopy

Atom probe microscopy (APM) is a relatively new in situ tool for measuring isotope fractions from nanoscale volumes (< 0.01 μm3). We calculate the theoretical detectable difference of an isotope ratio measurement result from APM using counting statistics of a hypothetical data set to be ± 4δ or 0.4% (2s). However, challenges associated with APM measurements (e.g., peak ranging, hydride formation and isobaric interferences), result in larger uncertainties if not properly accounted for. We evaluate these factors for Re‐Os isotope ratio measurements by comparing APM and negative thermal ionisation mass spectrometry (N‐TIMS) measurement results of pure Os, pure Re, and two synthetic Re‐Os‐bearing alloys from Schwander et al. (2015, Meteoritics and Planetary Science, 50, 893) [the original metal alloy (HSE) and alloys produced by heating HSE within silicate liquid (SYN)]. From this, we propose a current best practice for APM Re‐Os isotope ratio measurements. Using this refined approach, mean APM and N‐TIMS 187Os/189Os measurement results agree within 0.05% and 2s (pure Os), 0.6–2% and 2s (SYN) and 5–10% (HSE). The good agreement of N‐TIMS and APM 187Os/189Os measurements confirms that APM can extract robust isotope ratios. Therefore, this approach permits nanoscale isotope measurements of Os‐bearing alloys using the Re‐Os geochronometer that could not be measured by conventional measurement principles.

Determination of Osmium Concentration and Isotope Composition at Ultra-low Level in Polar Ice and Snow.

Here we use two chemical separation procedures to determine exceptionally low Os concentrations (∼10-15 g g-1) and Os isotopic composition in polar snow/ice. Approximately 50 g of meltwater is spiked with 190Os tracer solution and frozen at -20 °C in quartz-glass ampules. A mixture of H2O2 and HNO3 is then added, and the sample is heated to 300 °C at 100 bar. This allows tracer Os to be equilibrated with the sample as all Os species are oxidized to OsO4. The resulting OsO4 is separated using either distillation (Method-I) or solvent-extraction (Method-II), purified, and measured using negative thermal ionization mass spectrometry (N-TIMS). A new technique is presented that minimizes Re and Os blanks of the Pt filaments used in N-TIMS. We analyze snow collected from Summit, Greenland during 2009, 2014, and 2017. We find that the average Os concentration of the snow is 0.459 ± 0.018 (95% C.I.) fg g-1 corresponding to an Os flux of 0.0579 ± 0.0023 (95% C.I.) fmol cm-2 yr-1. The average R(187Os/188Os) ratio of the Summit snow is 0.264 ± 0.026 (95% C.I.). Assuming that the volcanic source is negligible, the average ratio indicates that about 0.0518 ± 0.0040 (95% C.I.) fmol cm-2 yr-1 of Os is of cosmic derivation, corresponding to an accretion rate of extra-terrestrial Os to the Earth of 264 ± 21 mol yr-1.

Diamond ages from Victor (Superior Craton): Intra-mantle cycling of volatiles (C, N, S) during supercontinent reorganisation

The central Superior Craton hosts both the diamondiferous 1.1 Ga Kyle Lake and Jurassic Attawapiskat kimberlites. A major thermal event related to the Midcontinent Rift at ca. 1.1 Ga induced an elevated geothermal gradient that largely destroyed an older generation of diamonds, raising the question of when, and how, the diamond inventory beneath Attawapiskat was formed. We determined Re–Os isotope systematics of sulphides included in diamonds from Victor by isotope dilution negative thermal ionisation mass spectrometry in order to obtain insights into the age and nature of the diamond source in the context of regional tectonothermal evolution. Regression of the peridotitic inclusion data (n=14 of 16) yields a 718 ± 49 Ma age, with an initial 187Os/188Os ratio of 0.1177 ± 0.0016, i.e. depleted at the time of formation (γOs −3.7 ± 1.3). Consequently, Re depletion model ages calculated for these samples are systematically overestimated. Given that reported 187Os/188Os in olivine from Attawapiskat xenoliths varies strongly (0.1012–0.1821), the low and nearly identical initial Os of sulphide inclusions combined with their high 187Re/188Os (median 0.34) suggest metasomatic formation from a mixed source. This was likely facilitated by percolation of amounts of melt sufficient to homogenise Os, (re)crystallise sulphide and (co)precipitate diamond; that is, the sulphide inclusions and their diamond host are synchronous if not syngenetic.The ∼720 Ma age corresponds to rifting beyond the northern craton margin during Rodinia break-up. This suggests mobilisation of volatiles (C, N, S) and Os due to attendant mantle stretching and metasomatism by initially oxidising and S-undersaturated melts, which ultimately produced lherzolitic diamonds with high N contents compared to older Kyle Lake diamonds. Thus, some rift-influenced settings are prospective with respect to diamond formation. They are also important sites of hidden, intra-lithospheric volatile redistribution that can be revealed by diamond studies. Later emplacement of the Attawapiskat kimberlites, linking the carbon cycle to the surface, was associated with renewed disturbance during passage of the Great Meteor Hotspot. Lherzolitic diamond formation from oxidising small-volume melts may be the expression of an early and deep stage of the lithospheric conditioning required for the successful eruption of kimberlites, which complements the late and shallow emplacement of volatile-rich metasomes after upward displacement of a redox freezing front.

Determination of Rhenium in Geological Samples by Isotope Dilution–Multicollector–Inductively Coupled Plasma-Mass Spectrometry with Novel Chromatographic Separation

ABSTRACTA method was developed to determine rhenium contents in geological samples using multicollector–inductively coupled plasma-mass spectrometry (MC–ICP-MS) and extraction with an ion-exchange resin. Samples were digested in Carius tubes and osmium was converted into volatile OsO4, which was purified by distillation and microdistillation. The purified Os contents and isotopic ratios were determined using negative thermal ionization mass spectrometry. After the distillation of Os, the samples were treated with HF, then 1.2 M HCl, and loaded on ion-exchange resin columns. Re was eluted using 0.75 M HNO3 and directly determined by MC–ICP-MS. This method was validated using a series of reference materials and the analytical Re data are consistent with the literature values. This method precision (relative standard deviation) ranged from 0.8 to 6%. The procedural blank and detection limit (3σ) of Re were 1.1 pg and 0.5 pg/g (for a sample size of 2g), respectively. These results indicate that the proposed method can be applied to determine trace Re in geological samples. Using Carius tube digestion combined with HF desilicification and redissolution, the Re and Os contents found in the soluble and insoluble phases of several reference materials indicated that the distributions of Re and Os were homogeneous and heterogeneous, respectively.

Simultaneous measurement of Re–Os and S isotopic compositions of sulfur-bearing minerals using a Carius tube digestion-based N-TIMS and MC-ICP-MS approach

This study reports an improved procedure for the simultaneous determination of Re–Os and S isotopic compositions of sulfur-bearing minerals using negative thermal ionization mass spectrometry (N-TIMS) and multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), respectively.

A comparison using Faraday cups with 1013 Ω amplifiers and a secondary electron multiplier to measure Os isotopes by negative thermal ionization mass spectrometry.

According to the Johnson-Nyquist noise equation, the value of electron noise is proportional to the square root of the resistor value. This relationship gives a theoretical improvement of 100 in the signal/noise ratio by going from 1011 Ω to 1013 Ω amplifiers for Faraday detection in thermal ionization mass spectrometry (TIMS). We measured Os isotopes using static Faraday cups with 1013 Ω amplifiers in negative thermal ionization mass spectrometry (NTIMS) and compared the results with those obtained with 1011 Ω amplifiers and by peak-hopping on a single secondary electron multiplier (SEM). We analysed large loads of Os (1 μg) at a range of intensities of 187 OsO3 (0.02-10 mV) in addition to small loads of Os (5-500 pg) to compare the results of the three methods. Using 1013 Ω amplifiers, the long-term reproducibility determined from Merck Os was 187 Os/188 Os = 0.1211 ± 0.0086 and 0.120229 ± 0.000034 at 0.02 mV and 10 mV of 187 OsO3 intensities. Meanwhile, the analysed JMC Os loadings of 5 and 500 pg showed 187 Os/188 Os = 0.10669 ± 0.00036 and 0.106807 ± 0.000023. In comparison, the values measured by the SEM were 187 Os/188 Os = 0.10704 ± 0.00056 and 0.10690 ± 0.00013. All errors are in 2 standard deviation (SD). Both the accuracy and the precision determined using the 1013 Ω amplifiers and the SEM are identical when the Os amounts are within 10-50 pg. However, the former analysis time can be shortened by approximately two-thirds. The SEM measurement is still the most precise method for Os amounts <10 pg, but the analyses using 1013 Ω amplifiers suggest they are significantly better than the SEM for Os amounts >50 pg.

High-precision analysis of 182W/184W and 183W/184W by negative thermal ionization mass spectrometry: Per-integration oxide corrections using measured 18O/16O

Here we describe a new analytical technique for the high-precision measurement of 182W/184W and 183W/184W using negative thermal ionization mass spectrometry (N-TIMS). We improve on the recently reported method of Trinquier et al. (2016), which described using Faraday cup collectors coupled with amplifiers utilizing 1013Ω resistors to continuously monitor the 18O/16O of WO3− and make per-integration oxide corrections. In our study, we report and utilize a newly measured oxygen mass fractionation line, as well as average 17O/16O and 18O/16O, which allow for more accurate per-integration oxide interference corrections. We also report a Faraday cup and amplifier configuration that allows 18O/16O to be continuously monitored for WO3− and ReO3−, both of which are ionized during analyses of W using Re ribbon. The long-term external precision of 182W/184W is 5.7ppm and 3.7ppm (2SD) when mass bias corrected using 186W/184W and 186W/183W, respectively. For 183W/184W mass bias is corrected using 186W/184W, yielding a long-term external precision of 6.6ppm. An observed, correlated variation in 182W/184W and 183W/184W, when mass bias corrected using 186W/184W, is most likely the result of Faraday cup degradation over months-long intervals.

Single laboratory comparison of MC-ICP-MS and N-TIMS boron isotope analyses in marine carbonates

Measurements of boron isotopes (δ11B) in marine carbonates are an important tool for reconstructing past ocean acidity and atmospheric carbon dioxide levels, but are challenging to obtain due to low boron concentrations and the presence of only two stable boron isotopes. Previous studies have observed differences in absolute δ11B values of marine carbonates measured via negative thermal ionization mass spectrometry (N-TIMS) and multicollector inductively coupled mass spectrometry (MC-ICP-MS) that are relatively large compared to magnitudes of δ11B variability in paleoceanographic studies. Here we investigate the cause of these δ11B offsets and implications for paleo-pH reconstructions by performing N-TIMS and MC-ICP-MS measurements within the same laboratory on a suite of eighteen homogenized samples, including inorganic calcites grown under controlled pH conditions and Quaternary planktic foraminifera. Results show that δ11B values between both measurement techniques are highly correlated (R2>0.99), but calcite δ11B values are ~0.5 to 2.7‰ lower when measured by MC-ICP-MS compared to N-TIMS. The δ11B offset between techniques cannot be attributed to procedural blank contribution, but the magnitude of offset moderately correlates with indicators of sample chemical composition, including B/Ca, Mg/Ca and δ18O (R2=0.39 to 0.50). N-TIMS δ11B does not change with calcium addition to a biogenic carbonate sample, suggesting that varying calcium abundance between samples and standards is not the reason for N-TIMS “matrix effects”. Correlation between δ18O and the δ11B offset is particularly apparent for calcites (R2=0.71); the origin of this relationship is not clear, but is not likely due to 17O interference. Although identifying the origin of measurement offsets requires further study, we demonstrate that such offsets do not affect the validity of δ11B data: The sensitivity of δ11B in inorganic calcite to pH is the same when measured by either technique, and paleo-pH reconstructed from δ11B measurements in planktic foraminifera is equivalent within uncertainty when a technique-specific constant offset is applied. Our results strongly indicate that δ11B measurements from both measurement techniques (N-TIMS and MC-ICP-MS) can be employed for paleo-pH reconstructions with equal confidence.