Instrumental Isotopic Fractionation Research Articles

δ238U of Coal Reference Materials Determined by MC‐ICP‐MS

Uranium (U) associated with coal can be an important source of U and result in environmental pollution during coal combustion. In this study, we developed a method for measurement of U isotope ratios in coals using multiple‐collector inductively coupled plasma‐mass spectrometry. The 233U‐236U double‐spike was utilised to calibrate the instrumental isotopic fractionation. High‐pressure bomb and dry ashing were adopted to digest the coal samples. The δ238UCRM‐145 values obtained from the two different digestion procedures were in good agreement. The δ238UCRM‐145 of seven coal and one fly ash reference materials are reported. Furthermore, the results of fly ash, bottom ash and feed coal samples reveal that the combustion processes lead to relatively small U isotopic fractionation between the samples within the same coal‐fired power plant, indicating that U isotope data can be used as a tracer for heavy metal pollution resulting from coal combustion. The U isotope measurement method of coal established in this study provides technical support to understand the behaviour of U during coal formation and combustion.

Uncertainty evaluation in reference material characterization by double isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS)

Double isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) is one of the most important measurement methods in the establishment of measurement standards for inorganic analysis. However, there is still no good consensus on approaches for uncertainty evaluation in double ID-ICP-MS. Particularly, approaches to treating the instrumental isotopic fractionation (IIF) phenomenon and correlation between uncertainty sources are diverse and often unclear in the literature. Here, an approach for uncertainty evaluation in double ID-ICP-MS measurements is described. Using three certified reference materials (CRMs), it is shown that the IIF effects cancel out and should not be included in the uncertainty budget. Further, a consistent way of considering correlated sources of uncertainty to prevent double counting of uncertainties is described in detail along with an example of measuring the mass fraction of magnesium in an oyster powder CRM.

The determination of the isotopic composition of strontium

If the process of instrumental isotopic fractionation in thermal-ionization mass spectrometry is modeled in terms of Rayleigh's distillation, the values of the isotopic ratios of strontium in a sample material can be calculated by distributions of measured ratios obtained by instrumental runs in which the process of isotopic fractionation in the sample reservoir more closely fitted the modeled behavior. Such runs can be recognized because are known, in the modeled conditions, the values of the slopes of distributions of measured values of isotopic ratios which are characterized by an equivalent difference in nominal mass between the isotope at the numerator and the isotope at the denominator (distributions 88Sr/87Sr vs. 87Sr/86Sr, and 88Sr/86Sr vs. 86Sr/84Sr). The instrumental runs that more closely fitted the theoretical conditions are characterized by the lowest deviations between the observed value of the slope of the distribution and the respective theoretical value. This model of calculation was used to determine the true/initial values of the isotopic ratios of strontium in the NIST SRM 987 by two independent sets of instrumental runs. In the first set the 86Sr, 87Sr and 88Sr peaks were sampled, and the 87Sr/86Sr ratio and the 88Sr/87Sr ratio were the ratios characterized by an equivalent difference in nominal mass between the isotope at the numerator and the isotope at the denominator. In the second set, the 84Sr, 86Sr and 88Sr peaks were sampled, and the 86Sr/84Sr ratio and the 88Sr/86Sr ratio were the ratios characterized by an equivalent difference in nominal mass. The first set of instrumental runs gives calculated true/initial 88Sr/86Sr values in the range 8.275 ± 0.019, and 87Sr/86Sr values in the range 0.7060 ± 0.0008 (errors are 1σ). The second set gives 88Sr/86Sr values in the range 8.249 ± 0.019 and 84Sr/86Sr in the range 0.05734 ± 0.00017 (1σ).

Determination of the isotopic composition of lutetium using MC-ICPMS.

In this study, we report the first independent measurements of lutetium isotopic composition using multi-collector ICPMS in four commercial lutetium materials, including a new NRC candidate lutetium isotopic reference material, LUIS-1. The regression model was used to correct for the instrumental isotopic fractionation (mass bias) using NIST SRM 989 isotopic rhenium as the primary calibrator. The regression model is based on a short 15-min measurement sessions at varying ICP plasma power. Isotope ratio of R176/175 = 0.026553(11)k = 1, corresponding isotopic abundances of x176 = 0.025866(11)k = 1, x175 = 0.974134(11)k = 1, and an atomic weight of Ar(Lu) = 174.966693(13)k = 1 were obtained for lutetium in LUIS-1. Uncertainty estimation was performed using a mixture of modeling approaches in accordance with the JCGM 100:2008 "Guide to the Expression of Uncertainty in Measurement" and its Supplement 1. The relative contribution of the rhenium primary standard to the combined uncertainty of 176Lu/175Lu isotope ratio was 15%.

Determination of the isotopic composition of hafnium using MC-ICPMS

Despite the numerous important applications of hafnium isotopes in geological science, and the advances in multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS), hafnium still lacks calibrated measurements of its isotope ratios and, in turn, isotopic abundances and atomic weight. In this study, we determined the isotopic composition of hafnium in four commercial hafnium reagents, including a commonly used hafnium standard (JMC-475) and a NRC candidate isotopic reference material (HALF-1) by MC-ICPMS. The state-of-the-art regression model with NIST SRM 989 isotopic rhenium as calibrator was used to correct instrumental isotopic fractionation without reliance on other hafnium standards, normalizing isotope ratios, or exponential mass-bias fractionation model.

Silver isotope analysis of gold nuggets: An appraisal of instrumental isotope fractionation effects and potential for high-resolution tracing of placer gold

We describe a technique for high-precision analysis of 109Ag/107Ag ratios in natural and archaeological gold (1.0–120 mg/g Ag) with particular emphasis on the control of Ag yield and effects of instrumental mass bias. After dissolution of mg-sized gold samples in aqua regia and conversion to chlorides, a miniature column filled with the anion exchange resin AG@1-X8 was used for removal of Au from the sample solutions. In a second miniature column made of the Triskem TBP resin Ag was converted from the chloride to the nitrate form. Isotopic analyses on the MC-ICP-MS employed the combination of Pd-doping and standard bracketing and considering the measurements of replicate dissolutions and chromatographic separations for SRM978a and CEZAg reference solutions, the combined analytical uncertainty (2 s) of the analytical method is better than 0.016‰. A solution prepared from several gold nuggets was characterised isotopically (CEZAg with δ109Ag = 0.043 ± 0.015‰) for use as a silver-in‑gold reference material in Ag isotope studies of natural and processed gold.Results for gold nuggets from three continents indicate a wide variation in Ag concentrations (5 to 123 mg/g) and this is matched by an equally wide range in isotopic compositions (δ109Ag −0.58 to +0.83‰). This is larger than the variation found so far in in native gold from primary deposits (−0.42 to +0.5‰). Although small isotopic effects to lower δ109Ag during strong Ag loss are possible within a single grain, most analysed nuggets are internally isotopically homogeneous. This suggests preservation of the primary δ109Ag in the supergene environment and that gold was sourced from several distinct gold deposits in the catchment. Recent studies, however, indicate that even single primary deposits may be isotopically heterogeneous implying that Ag isotope fractionation is caused by numerous deposition-dissolution cycles in hydrothermal systems. Fine-grained detrital gold from three placers along the Rhine river in Germany show only small differences in δ109Ag (−0.005 ± 0.047 to +0.124 ± 0.007‰, despite being distributed along 480 km of river length. This may indicate thorough mixing of gold grain populations during transport.The small sample size required for Ag isotope work on gold opens the way for detailed micro-sampling approaches. These may be used to further examine the potential for within-grain isotopic variability related to fluviatile processing and can be used to correlate the composition of the placers with that of grains from primary sources.

Determination of the Isotopic Composition of Osmium Using MC-ICPMS.

Despite its widespread applications in geology, all osmium isotope ratio measurements are either uncalibrated or rely on the veracity of the uncalibrated 1937 Nier values by adopting them as normalizing constants typically in conjunction with an exponential mass bias correction model. In this study, isotope ratios of osmium were determined in six commercial osmium materials, including the DROsS standard and a new NRC isotopic osmium reference material OSIS-1, by MC-ICPMS. We use a previously optimized and validated regression mass bias correction model to correct instrumental isotope fractionation effects which does not rely either on Nier's values or on a strictly mass-dependent behavior of the isotopes. Deviations from mass-dependent fractionation (mass independent fractionation) were observed for osmium isotopes in MC-ICPMS with the most dramatic effect occurring for 187Os, wherein, on average, close to half-percent bias in the isotope ratio 187Os/188Os was observed as a result of imposing Russell's law.

87Sr/86Sr isotope ratio measurements by laser ablation multicollector inductively coupled plasma mass spectrometry: Reconsidering matrix interferences in bioapatites and biogenic carbonates

This study is dedicated to the systematic investigation of the effect of interferences on Sr isotopic analyses in biological apatite and carbonate matrices using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC ICP-MS). Trends towards higher 87Sr/86Sr ratios for LA-MC ICP-MS compared to solution-nebulization based MC ICP-MS when analysing bioapatite matrices (e.g. human teeth) and lower ratios in case of calcium carbonates (e.g. fish ear stones) were observed. This effect can be related to the presence of significant matrix-related interferences such as molecular ions (e.g. (40Ca-31P-16O)+, (40Ar-31P-16O)+, (42Ca-44Ca)+, (46Ca40Ar)+) as well as in many cases concomitant atomic ions (e.g. 87Rb+, 174Hf2+). Direct 87Sr/86Sr ratio measurements in Ca-rich samples are conducted without the possibility of prior sample separation, which can be accomplished routinely for solution-based analysis. The presence of Ca-Ar and Ca-Ca molecular ion interferences in the mass range of Sr isotopes is shown using the mass resolving capabilities of a single collector inductively coupled plasma sector field mass spectrometer operated in medium mass resolution when analysing bioapatites and calcium carbonate samples.The major focus was set on analysing human tooth samples, fish hard parts and geological carbonates. Potential sources of interferences were identified and corrected for. The combined corrections of interferences and adequate instrumental isotopic fractionation correction procedures lead to accurate data even though increased uncertainties have to be taken into account. The results are discussed along with approaches presented in literature for data correction in laser ablation analysis.

The performance of single and multi-collector ICP-MS instruments for fast and reliable 34S/32S isotope ratio measurements.

The performance and validation characteristics of different single collector inductively coupled plasma mass spectrometers based on different technical principles (ICP-SFMS, ICP-QMS in reaction and collision modes, and ICP-MS/MS) were evaluated in comparison to the performance of MC ICP-MS for fast and reliable S isotope ratio measurements. The validation included the determination of LOD, BEC, measurement repeatability, within-lab reproducibility and deviation from certified values as well as a study on instrumental isotopic fractionation (IIF) and the calculation of the combined standard measurement uncertainty. Different approaches of correction for IIF applying external intra-elemental IIF correction (aka standard-sample bracketing) using certified S reference materials and internal inter-elemental IIF (aka internal standardization) correction using Si isotope ratios in MC ICP-MS are explained and compared. The resulting combined standard uncertainties of examined ICP-QMS systems were not better than 0.3–0.5% (uc,rel), which is in general insufficient to differentiate natural S isotope variations. Although the performance of the single collector ICP-SFMS is better (single measurement uc,rel = 0.08%), the measurement reproducibility (>0.2%) is the major limit of this system and leaves room for improvement. MC ICP-MS operated in the edge mass resolution mode, applying bracketing for correction of IIF, provided isotope ratio values with the highest quality (relative combined measurement uncertainty: 0.02%; deviation from the certified value: <0.002%).

Evaluation strategies and uncertainty calculation of isotope amount ratios measured by MC ICP-MS on the example of Sr.

This paper critically reviews the state-of-the-art of isotope amount ratio measurements by solution-based multi-collector inductively coupled plasma mass spectrometry (MC ICP-MS) and presents guidelines for corresponding data reduction strategies and uncertainty assessments based on the example of n(87Sr)/n(86Sr) isotope ratios. This ratio shows variation attributable to natural radiogenic processes and mass-dependent fractionation. The applied calibration strategies can display these differences. In addition, a proper statement of uncertainty of measurement, including all relevant influence quantities, is a metrological prerequisite. A detailed instructive procedure for the calculation of combined uncertainties is presented for Sr isotope amount ratios using three different strategies of correction for instrumental isotopic fractionation (IIF): traditional internal correction, standard-sample bracketing, and a combination of both, using Zr as internal standard. Uncertainties are quantified by means of a Kragten spreadsheet approach, including the consideration of correlations between individual input parameters to the model equation. The resulting uncertainties are compared with uncertainties obtained from the partial derivatives approach and Monte Carlo propagation of distributions. We obtain relative expanded uncertainties (Urel; k = 2) of n(87Sr)/n(86Sr) of < 0.03 %, when normalization values are not propagated. A comprehensive propagation, including certified values and the internal normalization ratio in nature, increases relative expanded uncertainties by about factor two and the correction for IIF becomes the major contributor.Electronic supplementary materialThe online version of this article (doi:10.1007/s00216-015-9003-9) contains supplementary material, which is available to authorized users.

Isotope ratio analysis of carbon and nitrogen by elemental analyser continuous flow isotope ratio mass spectrometry (EA-CF-IRMS) without the use of a reference gas

A new approach to normalize carbon and nitrogen isotope ratios measured by EA-CF-IRMS without using a reference gas for correction of instrumental drifts and instrumental isotope fractionation.

Instrumental isotope fractionation in multiple-collector icp-ms

A new law is proposed to account for the decrease of instrumental isotope fractionation in MC-ICP-MS with increasing transmission.

Strontium isotope analysis of curved tooth enamel surfaces by laser-ablation multi-collector ICP-MS

Laser ablation multi-collector ICP-MS (LA-MC-ICP-MS) is increasingly applied to measure the strontium isotope composition (87Sr/86Sr) of fossil or modern tooth samples in order to address questions about mobility. Recently, concerns have been raised that an analytical bias due to instrumental isotopic mass fractionation might be introduced when this method is used to sample across naturally curved tooth enamel surfaces.We address this concern by reanalyzing data that were originally produced using 750μm linear LA-MC-ICP-MS scans on the external, slightly curved surfaces of fossil hominin teeth (Australopithecus and Paranthropus) from South Africa. By re-integrating the first and last 1/3 of the data along the original 750μm linear scans, we compared 87Sr/86Sr results produced by sections of each scan with varying degrees of laser focus. The results show no evidence of any analytical scatter between strontium isotope values for the different sections of the curved tooth surfaces and results for the different sections of each linear scan agree well within the precision of the method (external 2σ ± 0.0004). Furthermore, each analytical session was bracketed by analysis of a curved rodent (Otomys sp.) tooth in-house standard with up to ±100μm vertical change along the 750μm linear scan. Long-term, average LA-MC-ICP-MS 87Sr/86Sr results for one such Otomys tooth standard (0.72989±0.00029, n=71) agree well with the solution MC-ICP-MS analysis of the same tooth (0.72976±0.00002). New LA-MC-ICP-MS 87Sr/86Sr ratios, using both a 213nm and solid-state 193nm laser ablation system, for sets of consecutive 750μm linear scans along the curved, outer tooth enamel surface on another single Otomys tooth in-house standard agree within ±0.0004 regardless of progressively more extreme vertical change during analysis (±10μm to ±900μm).These results are similar to LA-MC-ICP-MS Sr isotope analyses, using the same instrumentation and methodology, of an in-house, polished, flat clinopyroxene mineral standard (LA: 0.70482±0.00029, n=155; solution: 0.70495±0.00003).We conclude that, provided the stable 86Sr/88Sr ratio is used in conjunction with the exponential law to constantly correct for instrumental isotopic mass fractionation during acquisition, curved tooth enamel surfaces represent no impediment to accurate LA-MC-ICP-MS strontium isotope analyses.

Accurate Measurement of Isotope Amount Ratios of Lead in Bronze with Multicollector Inductively Coupled Plasma Mass Spectrometry

Isotope amount ratios of lead in a bronze sample have been successfully determined using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Matrix separation conditions were tested and optimized using ion exchange chromatography with anion-exchange resin, AG1-X8, and sequential elution of the 0.5 M HBr and 7 M HNO3 to separate lead from very high contents of copper and tin in bronze matrix. Mercury was also removed efficiently in the optimized separation condition. The instrumental isotope fractionation of lead in the MC-ICP-MS measurement was corrected by the external stan- dard sample bracketing method using an external standard, NIST SRM 981 lead common isotope ratio standard followed by cor- rection of procedure blank to obtain reliable isotope ratios of lead. The isotope ratios, 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb/ 204 Pb, and 208 Pb/ 206 Pb, of lead were determined as 18.0802 ± 0.0114, 15.5799 ± 0.0099, 38.0853 ± 0.0241, and 2.1065 ± 0.0004, respec- tively, and the determined isotope ratios showed good agreement with the reference values of an international comparison for the same sample within the stated uncertainties

High precision Mg isotope measurements of meteoritic samples by secondary ion mass spectrometry

The possibility of establishing an accurate relative chronology of the early solar system events based on the decay of short-lived 26Al to 26Mg (half-life of 0.72 Myr) depends on the level of homogeneity (or heterogeneity) of 26Al and Mg isotopes. However, this level is difficult to constrain precisely because of the very high precision needed for the determination of isotopic ratios, typically of ±5 ppm. In this study, we report for the first time a detailed analytical protocol developed for high precision in situ Mg isotopic measurements (25Mg/24Mg and 26Mg/24Mg ratios, as well as 26Mg excess) by MC-SIMS. As the data reduction process is critical for both accuracy and precision of the final isotopic results, factors such as the Faraday cup (FC) background drift and matrix effects on instrumental fractionation have been investigated. Indeed these instrumental effects impacting the measured Mg-isotope ratios can be as large or larger than the variations we are looking for to constrain the initial distribution of 26Al and Mg isotopes in the early solar system. Our results show that they definitely are limiting factors regarding the precision of Mg isotopic compositions, and that an under- or over-correction of both FC background instabilities and instrumental isotopic fractionation leads to important bias on δ25Mg, δ26Mg and Δ26Mg values (for example, olivines not corrected for FC background drifts display Δ26Mg values that can differ by as much as 10 ppm from the truly corrected value). The new data reduction process described here can then be applied to meteoritic samples (components of chondritic meteorites for instance) to accurately establish their relative chronology of formation.

Sulfur four isotope NanoSIMS analysis of comet‐81P/Wild 2 dust in impact craters on aluminum foil C2037N from NASA’s Stardust mission

Abstract– We present NanoSIMS four‐isotope S analyses of 24 comet Wild 2 dust impact residues in craters on aluminum foil C2037N returned by NASA’s Stardust mission. Except for one sample, all impact residues have normal S isotopic compositions within 2σ uncertainties of at least two S isotope ratios. This implies that most S‐rich Wild 2 dust impactors formed in the solar system. Instrumental isotope fractionation due to sample topography is the main contribution to our analytical uncertainty. One impact crater residue shows small anomalies of δ33S = −57 ± 17‰, and δ34S = −41 ± 17‰ (1σ uncertainties). Although this could be simply a statistical outlier or the fingerprint of a chemical isotope fractionation it is also possible that the observed anomaly results from the mixture of a cometary FeS particle with a small (150 nm diam.) presolar FeS supernova grain. This would translate into a presolar sulfide abundance of approximately 200 ppm.

A simplified calculation of power-broadened linewidths, with application to resonance ionization mass spectrometry

I present a simple derivation of the power-broadened linewidth of an optical transition in a two-level atom. The novelty of the approach lies in its flexibility, as the approach described here can be applied to any spectral lineshape, corresponding to either homogeneous or inhomogeneous broadening mechanisms. For a Lorentzian profile, I recover the same dependence of linewidth on laser power that one calculates from the optical Bloch equations. The present treatment makes explicit that power broadening arises because of the saturability of the atomic response to intense lasers, rather than to explicitly quantum mechanical aspects of the laser–atom interaction, all of which are ignored. I discuss the utility of power broadening in resonance ionization mass spectrometry, where it is an important means of mitigating unwanted instrumental isotope fractionation.

Mn/Cr relative sensitivity factors for synthetic calcium carbonate measured with a NanoSIMS ion microprobe

calcite by spot analysis mode decreased with time (and depth of crater). This may be interpreted as due to charge-up of the deep crater formed by primary ion bombardment because of poor conductivity of carbonate. The relative Mn/Cr sensitivity of carbonate measured by spot analysis mode decreased with time. It may be necessary to use a time-dependent relative sensitivity factor for chronological studies of meteoritic carbonates. The time-averaged RSF for the calcite spot analysis is significantly lower than that for San Carlos olivine. The instrumental isotopic fractionation of the 53 Cr/ 52 Cr ratio measured with the NanoSIMS 50 seems to be different between the calcite and San Carlos olivine.

Rayleigh's distillation law and linear hypothesis of isotope fractionation in thermal ionization mass spectrometry

The relationship which exists between Rayleigh's distillation law and linear models of instrumental isotopic fractionation in thermal ionization mass spectrometry is shown. If the process of isotope fractionation in the mass spectrometer source occurs in terms of a Rayleigh's distillation, and, within the range of mass of isotopes of the element, the vapor/residue distribution coefficient is a linear function of mass with a slope which is sufficiently small in absolute value, then the linear hypothesis of isotope fractionation is fulfilled. The model shows that the fractionation factor per amu, defined as the instantaneous difference between the measured and true values of the isotope ratio, per unit of measured/true value and per unit of mass difference between the two isotopes which define the ratio, can be interpreted as a function of two parameters: the residual mass fraction of the sample on the filament, and the rate of change of the distribution coefficient with mass. These two parameters can be calculated and, in particular, the value of the residual mass fraction of the sample when the measured values of the isotopic ratios coincide with the actual values can be calculated as a function of the rate of change with mass of the distribution coefficient. A linear model of instrumental isotopic fractionation can be derived from the exponential hypothesis of fractionation, which can be also interpreted in terms of a Rayleigh's distillation process, but where mass is an exponential function of the distribution coefficient. Experimental results of instrumental isotopic fractionation (up to 1% amu −1) of strontium in NIST standard reference material 987, loaded as a nitrate on a single tungsten filament, can be interpreted in terms of the linear models of isotope fractionation (and therefore of Rayleigh's distillation law) within experimental error. They show: (i) changes in the vapor/residue distribution coefficient with mass in the range −0.006 to −0.004 amu −1; (ii) approximately constant rates of sample consumption in the range of residual mass fraction from ∼1 to ∼0.3–0.25, which are between 0.05 and 0.13% min −1; (iii) values of the residual mass fraction of the sample, when the measured values of the isotopic ratios coincide with the true ones, between 0.3668 and 0.3671, which correspond to sample consumption of 63.3%. Since the linear hypothesis of fractionation is fulfilled, the values of isotopic ratios of strontium in the standard material can be determined. The global weighted averages of the weighted averages of the results obtained in eleven runs in which 86Sr, 87Sr and 88Sr peaks were sampled are as follows: 86Sr/ 88Sr = 0.119445 ± 0.000053, 87Sr/ 86Sr = 0.71016 ± 0.00019, and 87Sr/ 88Sr = 0.084826 ± 0.000040.

Isotope fractionation studies of molybdenum

Mass spectrometric studies of the isotopic composition of molybdenum have become an active area of research in stable isotope geochemistry, biogeochemistry and cosmochemistry. The redox chemistry of Mo, together with its proclivity for covalent bonding, indicates its importance in isotope fractionation studies such as palaeoceanography. The measurement of the magnitude of isotope fractionation of Mo in natural systems is a challenging task, in that natural fractionation has to be carefully distinguished from chemical and instrumental isotope fractionation. An ion exchange chemical separation procedure has been developed with high efficiency and low blank, to ensure that the isobaric elements Zr and Ru are removed from the samples before mass spectrometric analysis. The isotope fractionation resulting from this procedure is 0.14‰ per u. The isotopic composition of Mo of a Laboratory Standard has been measured by positive and negative thermal ionization mass spectrometry (P-TIMS and N-TIMS, respectively), to give an isotope fractionation of 6.4‰ and 0.5‰ per u, respectively, with respect to the absolute isotope abundances of Mo. In both cases the lighter isotopes are enhanced with respect to the heavier isotopes. An ascorbic acid activator has enabled the sensitivity of P-TIMS to be improved as compared to traditional methods. The same experiment was repeated using a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS) to give an isotope fractionation of approximately 17.0‰ per u. In this case the heavier isotopes are enhanced with respect to the lighter isotopes. The strengths and weaknesses of these three mass spectrometric techniques are evaluated. We conclude that MC-ICP-MS is the optimum mass spectrometric method for accurately measuring the isotope fractionation of Mo in natural materials, provided chemical and instrumental isotope fractionation can be resolved from naturally induced isotope fractionation. There is an urgent need for an internationally accepted calibrated reference material to be developed. Our Laboratory Standard has recently been calibrated at Curtin University by measurements of gravimetric mixtures of two enriched isotopes of Mo, to obtain the absolute isotope abundances of Mo. This Laboratory Standard can be made available to those laboratories in which isotopic studies of Mo are executed, to enable meaningful inter-laboratory comparisons to be conducted.