Multi-collector Inductively Coupled Plasma Mass Spectrometry Research Articles

Solution and laser ablationMC-ICP-MS lead isotope analysis of gold

A new approach is presented for the analysis of lead isotopes in gold using solution and laser ablation multi-collector inductively-coupled plasma mass-spectrometry (MC-ICP-MS). Repeat analyses of two gold reference materials (RAuGP3: 34.1 ± 0.5 ppm Pb and RAuGP5: 129 ± 4 ppm Pb, from SPEX CertiPrep Ltd) yield an external reproducibility of 20. Instrumental mass bias is corrected by standard sample bracketing, using NIST SRM981 for solution analyses and NIST SRM610 for laser ablation analyses. Both techniques agree within error, and they have been further demonstrated through the analyses of both geological and archaeological samples: multiple grains from two Irish alluvial gold deposits and samples of two Early Bronze Age Irish gold lunulae. The applicability of these techniques to studies concerning the sources and age of gold mineralisation, and the identification of gold sources exploited in antiquity, is highlighted.

A Simple and Fast Separation Method of Fe Employing Extraction Resin for Isotope Ratio Determination by Multicollector ICP-MS

A new, simple and fast separation method for Fe using an extraction chromatographic resin, Aliquat 336 (commercially available as TEVA resin) has been developed. A one milliliter column containing 0.33 mL TEVA resin on 0.67 mL CG-71C was used.Iron was adsorbed with 6mol·L-1 HCl + H2O2 on TEVA resin, and recovered with 2 mol·L-1HNO3. The recovery yield and total blank were 93.5 ± 6.5% and 6 ng, respectively. Theseparation method is simple, and takes < 2 hours. For evaluation of the Fe separation, Fe isotope ratios were measured by a double-spike method employing multicollector inductively coupled plasma source mass spectrometry (MC-ICP-MS) with repeatability of 0.06‰ (SD) for the standard solution and ~0.05‰ for the silicate samples. Therefore, the column chemistry developed in this study is a viable option for Fe isotope ratio measurement by MC-ICP-MS.

Isotope ratio mapping by means of laser ablation-single collector-ICP-mass spectrometry: Zn tracer studies in thin sections of Daphnia magna

In this paper, the possibilities of LA-single collector-ICP-mass spectrometry for obtaining isotope ratio images of thin sections of Daphnia magna specimens exposed to isotopically enriched Zn tracers were evaluated. Zn was selected considering its importance in ecotoxicological studies. All aspects of the analytical methodology deployed were carefully studied and optimized for obtaining the best isotope ratio precision and accuracy. The development of this methodological approach consisted of: (i) evaluation of the performance of two different medium mass resolution exit slits, a conventional one providing a mass resolution of m/Δm ≈ 4000 and triangular shaped peaks, and another (wider) one, offering a lower mass resolution (m/Δm ≈ 2000), but providing enhanced ion transmission and flat-topped peaks; (ii) the use of a wet plasma for improving plasma robustness; (iii) thorough optimization of the ablation conditions (such as laser spot size, repetition rate, scan speed and laser fluence) and data acquisition parameters (such as scan mode, the number of nuclides monitored, mass window, the number of samples per peak and dwell times); and (iv) adequate data treatment including the use of the moving average of 5 individual values. With the methodology developed, isotope ratio images with 30 micrometer spatial resolution scale were obtained for exposed and unexposed Daphnia magna individuals. The typical overall uncertainty of the measurements was approximately 5% RSD for the 66Zn/64Zn and 68Zn/64Zn ratios, which can be considered as satisfactory taking into account that the amount of Zn present per 30 μm × 30 μm pixel only reached a few picograms in the most favourable cases (giving rise to 10 000–20 000 counts per s for 64Zn). These results open the possibility for tracer studies with stable isotopes to be carried out using single collector instrumentation, a much more widespread analytical technique than multi-collector ICP-MS.

Isotope analysis of strontium by multicollector inductively-coupled plasma mass spectrometry: High-precision combined measurement of 88Sr/86Sr and 87Sr/86Sr isotope ratios

The paper describes a new high-precision method for the simultaneous precise determination of 88Sr/86Sr and 87Sr/86Sr ratios in a single portion of a geological sample by multicollector inductively coupled mass spectrometry (MC-ICP-MS). The isotope analysis is carried out with mass bias effect correction by a combination of internal normalization to the standard Zr-isotope ratio and bracketing standard method (external normalization). Our results for geochemical IAPSO and BCR-1 standard samples are in a good agreement with the published data. The reproducibility of the 88Sr/86Sr ratio varies from ±0.015 to ±0.05‰ (depending on the sample features) and, together with the analysis accuracy, is superior to the previously reported methods of MC-ICP-MS analysis. Still ahead is only double spike thermal ionization mass spectrometry with its ±0.02‰ reproducibility. However, the new method allows the simultaneous determination of 88Sr/86Sr and 87Sr/86Sr ratios and its productivity is higher by 5 to 6 times. On the other hand, in sample preparation, it is necessary to strive for at least 95% Sr yield from the chromatographic column; otherwise the sorption-desorption process may lead to a 0.6‰ 88Sr/86Sr ratio bias relative to the true value.

High-precision simultaneous determination of 147Sm/144Nd and 143Nd/144Nd ratios in Sm–Nd mixtures using multi-collector inductively coupled plasma mass spectrometry and its comparison to isotope dilution analysis

This work demonstrates, for the first time, the feasibility and capability of the high-precision simultaneous determination of 147Sm/144Nd and 143Nd/144Nd ratios in Sm–Nd mixtures using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) without Nd and Sm separation and without the use of costly enriched spikes. Mass discrimination was exponentially corrected by applying the natural constant of 146Nd/144Nd ratio as an internal standard, after correction of the isobaric interference of 144Sm on 144Nd using interference-free 147Sm/149Sm ratio for Sm mass fractionation, without assuming identical mass bias of Nd and Sm. The accuracy and precision of the present protocol, obtained from replicate analyses of various types of Sm–Nd mixtures encompassing a wide range of Sm/Nd (ca. 0.1–1.0) or 147Sm/144Nd ratios (ca. 0.06–0.62) was found to be comparable to the classic isotope dilution (ID) method. The present method is characterized by a higher sample throughput compared to the ID method, and shows great potential for the simultaneous determination of 147Sm/144Nd and 143Nd/144Nd ratios in real geological samples.

High-noble measurement of 107Ag/109Ag in native silver and gold by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS)

High-noble measurement of 107Ag/109Ag in native silver and gold by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS)

Separation and Precise Measurement of Lithium Isotopes in Three Reference Materials Using Multi Collector‐Inductively Coupled Plasma Mass Spectrometry

AbstractLithium separation technique for three reference materials has been established together with precise determination of lithium isotope using a Neptune multi collector‐inductively coupled plasma mass spectrometry (MC‐ICP‐MS). The solutions of lithium element standard reference materials, potassium, calcium, sodium, magnesium and iron single element, were used to evaluate analytical methods applied. Three separate stages of ion‐exchange chromatography were carried out using organic cation‐exchange resin (AG 50W‐X8). Lithium was enriched for the three stages using different eluants, which are 2.8 M HCl, 0.15 M HCl and 0.5 M HCl in 30% ethanol, respectively. The columns for the first and second stages are made of polypropylene, and those for the third stage are made of quartz. Total reagent volume for the entire chemical process was 35 mL for three reference materials. The recovery yielded for the three stages is 98.9–101.2% with an average of 100.0%, 97.6–101.9% with an average of 99.9%, and 99.8–103.3% with an average of 100.6%, respectively. The precision of this technique is conservatively estimated to be ±0.72–1.04‰ (2σ population), which is similar to the precision obtained by different authors in different laboratories with MC‐ICP‐MS. The δ7Li values (7Li/6Li relative to the IRMM‐016 standard) determined for andesite (AGV‐2) and basalt (BHVO‐2) are 5.68‰ (n= 18), 4.33‰ (n= 18), respectively. The δ7Li value (7Li/6Li relative to the L‐SVEC standard) determined for IRMM‐016 is –0.01‰ (n= 15). All these analytical results are in good agreement with those previously reported. In addition, the results for the same kinds of samples analyzed at the MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, are consistent with those obtained at the Plasma Laboratory, University of Maryland, within analytical uncertainty. According to these experiment results, it is concluded that this proposed procedure is a suitable method for determining the lithium isotopic composition of natural samples.

High-precision and high-resolution carbonate 230Th dating by MC-ICP-MS with SEM protocols

To facilitate the measurement of U–Th isotopic compositions suitable for high-precision and high-resolution 230Th dating of coral and speleothem carbonates, secondary electron multiplier (SEM) protocol techniques for multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) have been developed. The instrumental sensitivities are 1–2%, with a precision of ±1–2‰ (2σ) for abundance determination of 50–200fg 234U (1–4ng 238U) or 230Th. This method features chemistry refinements, improvements to procedural and instrumental blanks, spectral inference reductions, and careful consideration of non-linear SEM behavior. Measurement consistency of this MC-ICP-MS combined with previous mass spectrometric results on U–Th standards and a variety of carbonates demonstrates the validity of the SEM protocol method. For fossil corals, a routine U–Th isotopic determination at permil-level precision requires only 10–50mg of carbonate. As little as 200mg of young coral with an age of less than 20yr can be dated with a precision of ±0.3–0.8yr. About 20–200mg speleothem samples with sub-ppm-to-ppm U are required to earn a 5‰ precision on ages from 5 to 100kyr. Requirement of small sample size, 10–100s mg carbonate, can permit high temporal resolution to date speleothems with slow growth rates, i.e., 1–10mm/kyr. This high-precision 230Th chronology is critical to accurately establish age models, date events and splice geochemical proxy time series records from multiple samples in the fields of paleoclimatology and paleoceanography. The U–Th isotopic determination techniques described here can also be applied to different environmental samples, such as waters, rocks, and sediments.

In-situ Pb isotope analysis of Fe–Ni–Cu sulphides by laser ablation multi-collector ICPMS: New insights into ore formation in the Sudbury impact melt sheet

Laser-ablation (LA) multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) is ideally suited to in situ determination of isotope ratios in sulphide minerals. Using samples of magmatic sulphide ore from the Sudbury impact structure, we test LA-MC-ICPMS analytical protocols that aim to meet a range of analytical challenges in the analysis of Pb isotopes. These include: potential matrix sensitive isotopic fractionation; interferences on Pb isotopes; low melting points of many sulphide minerals; the availability of standards.Magmatic sulphides of wide ranging mineralogy (pyrrhotite, pentlandite, chalcopyrite, pyrite and sphalerite) were analysed for Pb isotopic composition, using the silicate glass NIST SRM 610 as an external standard to correct for instrumental mass-fractionation. Despite matrix sensitive melting and re-deposition around ablation pits, several lines of evidence indicate that all analyses are accurate, within typical analytical uncertainties of 0.003–2% (2σ), and that the defined approach is insensitive to compositional diversity in sample matrix: (a) laser ablation and dissolution based measurements of sulphide powders are in agreement; (b) analyses from each sample define isochron ages within uncertainty of the known crystallization age (1850Ma); (c) the results of sulphide measurements by laser ablation are consistent with age-corrected feldspar analyses from the same samples.The results have important implications for ore formation in Sudbury. The Pb isotope data regressions are consistent with age corrected feldspar analyses from each respective sample, which together with time integrated Th/U ratios that match whole rock values (3.1, 4.0 and 6.1 for the Worthington, Copper Cliff and Parkin Offset Dykes, respectively) indicate chemical equilibrium between the silicate and sulphide systems during ore formation. The sulphides within each respective sample have indistinguishable model initial Pb isotope ratios (207Pb/204Pbm), irrespective of mineralogy or texture, indicating a common origin for ores within each of three different Offset Dykes. Furthermore, variations between Offset Dykes (e.g., 207Pb/204Pbm=15.514±0.012, 15.399±0.009 and 15.275±0.003) show that the ores have differing crustal sources on previously unrecognized scales. Mass balance considerations, particularly for MgO, Ni and Cu, indicate that the spatial distribution of mafic target rocks played a significant role in controlling the mineralization potential in different parts of the melt sheet.

Pd–Ag chronology of volatile depletion, crystallization and shock in the Muonionalusta IVA iron meteorite and implications for its parent body

Muonionalusta, a Group IVA iron meteorite, was analyzed for its 107Pd–107Ag isotope systematics by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in order to better constrain the initial Solar System abundance of 107Pd and to provide high resolution chronology of the evolution of its parent body. Six metal samples from Muonionalusta yield Ag abundances between 0.1012 and 1.461ng/g, 107Ag/109Ag between 1.131 and 1.805, with 108Pd/109Ag ratios of 2201 to 52,300. The metal Pd/Ag and Ag isotopic data are correlated with a slope corresponding to a 107Pd/108Pd of (2.15±0.30)×10−5. If the Pd–Ag and Pb–Pb isotope systems closed at the same time in Muonionalusta, i.e., 2–3Ma after CAI formation, then an initial Solar System ratio of 107Pd/108Pd=(2.8±0.5)×10−5 can be inferred. One troilite sample contains 8.478ng/g Ag and has a 107Ag/109Ag ratio of 1.0833; its Pd concentration is 205.2ng/g corresponding to a low 108Pd/109Ag of 13.56. The Pd–Ag results for this troilite plot near the extrapolation of the line passing through the metal points and define an initial 107Ag/109Ag that is substantially higher than chondritic, indicating that Muonionalusta formed from a precursor with high Pd/Ag. Pd and Ag concentrations in Muonionalusta metal suggest fractional crystallization from a source having Pd/Ag>4500, but the initial Ag isotopic composition for Muonionalusta troilite limits the duration of the high Pd/Ag to an interval of ≤0.6Ma before cooling to closure of the Pd–Ag system. This result suggests that depletion of Ag and other volatile elements occurred shortly before accretion and cooling of the IVA parent body, and may have been associated with violent disruption of a progenitor to the IVA parent. Another troilite sample, its chromite inclusions and adjacent metal were isotopically homogenized locally after 107Pd had decayed, possibly by a later episode of shock >50Ma after Solar System formation.

Evolution of the South African mantle — A case study of garnet peridotites from the Finsch diamond mine (Kaapvaal craton); part 1: Inter-mineral trace element and isotopic equilibrium

A thorough assessment of inter-mineral equilibrium is essential for the understanding of trace element partitioning and also for the interpretation of isotopic data. Here we investigated high temperature (~1200°C and 6GPa) garnet peridotite xenoliths from the Kaapvaal craton (Finsch mine, South Africa), with exceptionally well equilibrated mineral major element compositions, for their trace element and isotopic inter-mineral equilibrium. Trace element compositions for all major mineral phases, i.e. olivine, orthopyroxene (opx), clinopyroxene (cpx) and garnet, were analysed by laser ablation inductively coupled plasma mass spectrometry (ICP-MS). Garnet, cpx and opx of selected samples were analysed for their Sm–Nd and Lu–Hf isotope compositions by multi collector ICP-MS.Most important mineral characteristics include: a) olivines from most samples are enriched in high-field-strength elements relative to other incompatible trace elements. Their lithium content correlates negatively with Mg#, indicating a depletion signature; b) all other minerals are depleted in heavy and middle rare earth elements (H- and M-REE) and enriched in light REE and large ion lithophile elements. This implies a complex history of depletion and metasomatic overprint for the Finsch cratonic mantle; c) orthopyroxene has similarly shaped trace element patterns as cpx, with one to two orders of magnitude lower abundances; and d) both, garnet and cpx, display variable, mostly positive εHf coupled with moderate variations in εNd.Trace element partitioning between garnet/cpx, cpx/opx and garnet/opx, displays only a weak pressure and temperature dependency. However, equilibrium partitioning of most trace elements between garnet and cpx shows a strong compositional dependency, i.e. on the Cr- (and Ca-) content of the garnets. Garnet–cpx partition coefficients follow a second grade polynomial correlation with Cr2O3 of garnet, whereby high chromium garnets (Cr2O3>6wt.%) have generally higher partition coefficients for all trace elements than low Cr garnets. Potentially, enhanced trace element compatibility in high-Cr garnets may be related to the formation of a knorringite component and its influence on the garnet crystal structure. However, some elements, especially the quadrivalent Zr and Hf and the small trivalent HREE, show increased scatter indicating deviation from complete inter-mineral equilibrium.Garnet–cpx Sm–Nd isochrons of all analysed samples are consistent with the age of the Finsch kimberlite eruption (~118Ma). This further demonstrates complete isotopic equilibration of Sm and Nd between garnet and cpx for these mantle samples. In contrast to Sm–Nd, the Lu–Hf garnet–cpx isochrons display an age range between 60 and 570Ma, indicating Lu–Hf disequilibrium. Most probably slow diffusion of Hf and Lu, in the absence of a melt or fluid, resulted in incomplete Lu–Hf equilibration and the preservation of an “old” radiogenic Hf in garnet and cpx.

Precise microsampling of poorly laminated speleothems for U-series dating

One of the principal reasons why speleothems are recognised as important palaeoclimate archives is their suitability for accurate and precise uranium-series (U-series) age determination. Sampling speleothem sections for U-series dating is straightforward in most cases because visible growth layers are preserved. However, this is not always the case, and here we describe a sampling strategy whereby growth layers are resolved from trace-element images produced by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We apply this method to a section of an Italian subaqueous speleothem (CD3) that lacks persistent visible growth layering.Trace-element imaging revealed growth layers that are strongly non-planar in their geometry owing to the speleothem's pronounced euhedral crystal terminations. The most prominent trace-element layers were first digitized as x, y vector contours. We then interpolated these in the growth-axis direction to generate a series of contour lines at ∼250-μm increments. The coordinates of these contours were used to guide the sampling via a computerised micromilling lathe. This produced a total of 22 samples for U-series dating by multi-collector ICP-MS. The dating results returned ages in correct stratigraphic order within error. Close inspection of the U-series data and the derived depth–age model suggests that the main source of model-age uncertainty is unrelated to the contour sampling but instead more associated with how closely spaced the model ages are in time, i.e. the model age density. Comparisons between stable oxygen and carbon isotope profiles derived from aliquots of the dating samples and two other stable isotope profiles from CD3 spanning the same time period compare very favourably. Taken together, this suggests that our trace-element contouring method provides a reliable means for extracting samples for dating (and other geochemical analyses), and can be applied to similar speleothems lacking visible growth layering.

Isotope fractionation during ion beam formation in multi-collector inductively coupled plasma mass spectrometry

Instrumental mass discrimination in multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) was investigated with respect to influences after ion extraction from the plasma and during ion beam formation. The ions were implanted in targets which were subsequently subjected to spatially (radially) resolved isotopic analysis via laser ablation MC-ICP-MS or to bulk isotopic composition via conventional solution nebulization MC-ICP-MS. For the elements Cd, Pb and U, the isotope ratios show a strong variation as a function of distance from the center, whereas for lighter elements such as Li or Ni, such variation could not be detected by this approach, due to low signal-to-background ratios. The bulk isotope ratio of Li was found to be fractionated by 9–12% with respect to that in the sample at the earliest accessible location of the ion beam. Heavier elements show less isotope fractionation down to ≈0.4% for Pb and U. The combination of bulk and spatially resolved data suggests a combination of coulombic repulsion and scattering effects as main contributors to instrumental mass discrimination.

Isotope Measurement of Uranium at Ultratrace Levels Using Multicollector Inductively Coupled Plasma Mass Spectrometry

Mass spectrometric analysis was carried out using multicollector inductively coupled plasma mass spectrometry(MC-ICP-MS) for the precise and accurate determination of the isotope ratios of ultratrace levels of uranium dissolved in 3%HNO3. We used the certified reference material (CRM) 112-A at a trace level of 100 pg/mL for the uranium isotopic measurement. Multiple collectors were utilized for the simultaneous measurement of uranium isotopes to reduce the signal uncertaintydue to variations in the ion beam intensity over time. Mass bias correction was applied to the measured U isotopes to improve theprecision and accuracy. Furthermore, elemental standard solution with certified values of platinum, iridium, gold, and thalliumdissolved in 3% HNO3 were analyzed to investigate the formation rates of the polyatomic ions of Ir40Ar+, Pt40Ar+, Tl40Ar+,Au40Ar+ for the concentration range of 50–400 pg/mL. Those polyatomic ions have mass-to-charge ratios in the 230–245 m/zregion that it would contribute to the increase of background intensity of uranium, thorium, plutonium, and americium isotopes. The effect of the polyatomic ion interference on uranium isotope measurement has been estimated.

Determination of low‐level (sub‐microgram) sulfur concentrations by isotope dilution multi‐collector inductively couple plasma mass spectrometry using a 33S spike and internal normalization for mass bias correction

The certification of sulfur (S) in Standard Reference Materials™ by the National Institute of Standards and Technology (NIST) has been exclusively performed using isotope dilution thermal ionization mass spectrometry (ID-TIMS). The ID-TIMS measurement method is limited in its capability for low concentration measurements (<1 µg/g) due to the blank associated with the chemical reduction procedure (≈0.2 S µg). Newly developed materials and applications, such as biofuels made from soy and nanomedicine, pose a challenge to the ID-TIMS technique because of their very low concentrations (<<1 µg/g) of S. As described here, a measurement technique with low S blanks is essential for low-level S measurements. An isotope dilution (ID) multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) method combined with an internal normalization approach for mass bias correction has been used to determine low-level S concentrations in synthetically prepared mixtures using a (33)S-enriched spike material. Dilute sulfur solutions (1 µg S/g) were prepared from NIST SRM 3154 (Sulfate Spectrometric Solution) to test the capability of this technique for measuring very low-level S concentrations. The concentration results for each solution were 0.983 ± 0.004 µg/g (95% CI, n = 2, k = 2), 1.006 ± 0.005 µg/g (95% CI, n = 2, k = 2), and 0.999 ± 0.003 µg/g (95% CI, n = 2, k = 2), in excellent agreement with the gravimetric determination, deviating less than 0.35% and suggesting the technique can yield unbiased and accurate results. The blanks averaged 13 ± 0.0017 ng S (1s). The data results provide a clear indication that the ID-MC-ICP-MS method for the determination of low-level S concentrations is feasible. The more than one order of magnitude reduction of the blanks suggests that it is a better alternative to the ID-TIMS method for very low S materials such as are encountered in biofuels and some biochemical species. Published 2012. This article is a US Government work and is in the public domain in the USA.

A new measurement of the half-life of 166mHo

The work presented here is a new and precise measurement of the half-life of 166mHo by determining the activity concentration, using an ionisation chamber calibrated for this nuclide, and measuring the number of 166mHo atoms using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Since the isotope 166Er interferes with the mass spectrometric measurement, Er has to be eliminated from the 166mHo radioactive solution. The elimination was achieved using ion-exchange chromatography with the cation exchange resin Dowex AG 50W-X8 and 2-Hydroxybutanoic acid as the mobile phase. After a first transit through the chromatographic column, the purified 166mHo eluate was spiked with natural Er to get a resulting Er isotopic composition close to that of natural Er at better than 99.5%, and then it underwent two further separations to eliminate the Er. The activity concentration of this Er-free radioactive 166mHo solution was measured in our reference ionisation chamber calibrated for this nuclide by means of the 4πβ(PC)-γ and 4πβ(PS)-4πγ coincidence techniques and integral counting with a well-type NaI(Tl) detector and Monte Carlo efficiencies. An aliquot of this standardized solution was sent to the Paul Scherrer Institute (PSI) for mass concentration determination using an isotope dilution MC-ICP-MS approach. The mass concentration of 166mHo in this solution was determined with 0.25% relative standard uncertainty. This value was corroborated by two other independent measurements. The new half-life of 166mHo, 1132.6(39) years (k=1), is compatible with the value determined in 1965, but is 5.6% shorter and about 43 times more precise.

High-precision and accurate determinations of neodymium isotopic compositions at nanogram levels in natural materials by MC-ICP-MS

A comparatively simple, rapid, low-blank, high-precision and reproducible technique for Nd isotope analysis (143Nd/144Nd) of trace amounts (1–5 ng) of Nd by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) was developed and is reported here. The technique utilizes a combination of a high-sensitivity desolvator (Aridus II or APEX-IR) and high-efficiency Jet-sample/X-skimmer cones, together with high-yield pre-concentration and column chemistry, to achieve optimal conditions for Nd isotope determination in natural materials with a variety of Nd-depleted matrices. This new method was validated by analyzing a series of international reference materials (RMs), which span a wide range of the Nd isotopic composition and cover different sample matrices in order to provide a useful benchmark for future studies. The long-term precision of 143Nd/144Nd (±0.000016, 2SD) for a sample size of ca. 1.25 ng Nd is a factor of 5 better than the existing high-sensitivity MC-ICP-MS methods using <2 ng of Nd and is comparable to the best TIMS methods. Replicate analyses of the RMs, including JNdi-1 (0.512116 ± 0.000016, 2SD), AGV-1 (0.512794 ± 0.000013, 2SD), BHVO-1 (0.512992 ± 0.000015, 2SD), Nod-P-1 (0.512430 ± 0.000015, 2SD) and two sets of synthetic solutions for seawater and calcium carbonate matrices, are in excellent agreement with the published values, demonstrating the robustness of the present protocol for natural samples with different matrices. A depth profile of the seawater Nd isotopes near the Bermuda Atlantic Time-series Study station was also analyzed for comparison, and is identical to the reported seawater profile in this region. Applying this improved technique, we are able to generate high-precision and reproducible Nd isotope data for Nd-depleted natural materials, such as seawater, continental runoff, benthic fluxes at continental margins, and marine biogenic carbonate, which can provide pertinent and complementary information about the marine Nd cycle and past ocean circulation.

An isotopic perspective on mass bias and matrix effects in multi-collector inductively-coupled-plasma mass spectrometry

Lead and Tl isotope intensities and ratios were measured in radial and axial plasma profiles using multi-collector inductively-coupled-plasma mass spectrometry (MC-ICP-MS). Signal profiles for 204Pb, 206Pb, 207Pb and 208Pb display mass dependent radial and axial distributions in the plasma. In radial profiles lighter isotopes exhibit greater dispersal from the plasma axis than heavier isotopes, in agreement with ICP-MS elemental observations. However, signal maxima (Imax) for heavier Pb isotopes occur closer to the load coil than Imax for lighter Pb isotopes, the reverse of ICP-MS elemental observations. These mass dependent distributions predict that the radial dispersion and axial Imax locations for 203Tl and 205Tl isotopes should occur interspersed with the values for simultaneously measured Pb isotopes. Instead however, their values are appropriate for masses greater that 208Pb, indicating that the observed profiles are not the product of a single stage mass dependent process and that other element properties also effect the distributions. Isotope ratio (and thus mass bias) variation with sampling depth reflects both the mass dependent separation of the numerator and denominator isotope Imax along the axis of the plasma and the distance along the axis of the plasma over which the isotope signals are distributed. Addition of matrix to the plasma significantly reduces the variability of these two characteristics suggesting that more accurate correction of instrumental mass bias and more accurate normalization of sample ratios to standard values could be achieved by addition of a common matrix to samples and standards.

Multiple linear regression and on-line ion exchange chromatography for alternative Rb–Sr and Nd–Sm MC-ICP-MS isotopic measurements

A mathematical procedure of multiple linear regression in combination with on-line liquid chromatography was applied for the measurement of Sr and Nd isotope ratios by multicollector ICP-MS in the presence of isobaric isotopes. The separation of Rb and Sr and the separation of Nd and Sm were accomplished by ion exchange chromatography with large volume injection. This mode of injection allowed wide flat-topped chromatographic peaks to be obtained while maintaining chromatographic resolution and avoiding sample dilution. The application of the multiple linear regression procedure to the raw chromatographic data allowed the determination of the isotope ratios of Sr or Nd without interferences from isobaric isotopes. For Sr measurements, the raw 87Sr/86Sr isotope ratio precision (0.006 to 0.010%) was similar to that of continuous sample introduction (ca. 0.003%) at the same concentration level (50 ng g−1). The mathematical procedure was first evaluated for the cation exchange separation of Rb and Sr. Strontium isotope ratios, obtained using NIST SRM 987, were in agreement with the certified values (bias ca. 0.01%). For the case of Nd and Sm, their chromatographic separation was accomplished by anion exchange using EDTA as mobile phase. Neodymium eluted at 12.8 minutes while samarium eluted at 17.5 minutes, showing flat-topped chromatographic peaks and maintaining baseline neodymium–samarium separation. Precise neodymium and samarium isotope ratios suitable for Nd–Sm dating could be measured by MC-ICP-MS using the proposed multiple linear regression procedure using only one cup configuration.

Observations of Large Mass-Independent Fractionation Occurring in MC-ICPMS: Implications for Determination of Accurate Isotope Amount Ratios

Multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) suffers large bias in isotope amount ratio determinations which has to be properly accounted for. The choice of the proper discrimination model is crucial. Over the last few decades, the exponential mass-bias correction model (Russell's law) has become a standard curriculum in isotope amount ratio measurements. In nature, however, isotopic fractionation that deviates significantly from the exponential model has been known for a long time. Recently, such fractionation was also observed in MC-ICPMS. This phenomenon is termed mass-independent fractionation. In this study, significant departure from the mass-dependent fractionation model is reported for germanium and lead with the most dramatic occurring for germanium-73 and lead-204 isotopes wherein, on average, close to a half percent bias was evidenced from the Russell's law.