Double Spike Technique Research Articles

Precise measurement of Sm isotopes by TIMS using double spike technique

Precise measurement of Sm isotopes by TIMS using double spike technique

Isotope fractionation of cadmium in lunar material

The double spike technique has been used to measure the isotope fractionation and elemental abundance of Cd in nine lunar samples, the Brownfield meteorite and the Columbia River Basalt BCR-1, by thermal ionisation mass spectrometry. Lunar soil samples give a tightly grouped set of positive isotope fractionation values of between + 0.42% and + 0.50% per mass unit. Positive isotope fractionation implies that the heavy isotopes are enhanced with respect to those of the Laboratory Standard. A vesicular mare basalt gave zero isotope fractionation, indicating that the Cd isotopic composition of the Moon is identical to that of the Earth. A sample of orange glass from the Taurus-Littrow region gave a negative isotope fractionation of − 0.23 ± 0.06% per mass unit, presumably as a result of redeposition of Cd from the Cd-rich vapour cloud associated with volcanism. Cadmium is by far the heaviest element to show isotope fractionation effects in lunar samples. The volatile nature of Cd is of importance in explaining these isotope fractionation results. Although a number of mechanisms have been postulated to be the cause of isotope fractionation of certain elements in lunar soils, we believe that the most likely mechanisms are ion and particle bombardment of the lunar surface.

Precise isotope analysis of nanogram-level Pb for natural rock samples without use of double spikes

A simple technique has been developed for the precise analysis of lead isotope in natural rock samples by thermal ionization mass spectrometry (TIMS). Two-stage column chromatography, using 100 and 10 μl columns, was used to minimize the amounts of impurities in separated lead samples. This dramatically improved the reproducibility of mass fractionation during mass spectrometry using an ion emitter made of a mixture of silicic acid and phosphoric acid. This improvement made it possible to precisely determine the Pb isotopic compositions of very small sample sizes, employing “zero-time correction” for mass discrimination, without requiring a double-spike technique. Using the present method, analytical reproducibility of 208Pb/ 204Pb of 0.02% and 0.06% (2 σ) was attained for 100 and 1 ng of Pb, respectively, separated from natural rock samples. Furthermore, we obtained a reproducibility of 0.06% (2 σ) for 208Pb/ 204Pb for 10 ng of Pb separated from GSJ JP-1 (peridotite), in which the Pb concentration was 0.09 ppm. The measured isotope compositions of USGS standard rocks AGV-1 and BCR-1 were comparable with the published values using the double-spike technique. These observations suggest that our simple technique is reliable in terms of both accuracy and precision for the determination of the Pb isotopic compositions of natural rock samples irrespective of rock chemistry and sample sizes from 1 to 100 ng of Pb.

A novel approach to double-spike Pb–Pb dating of carbonate rocks: examples from Neoproterozoic sequences in southern Africa

Application of the 207Pb– 204Pb double-spiking technique to correct for mass-fractionation led to marked improvement in the quality of Pb isotope data and resulting ages obtained on low-grade metamorphosed Neoproterozoic carbonate rocks from the Pan-African Gariep and Saldania Belts in southern Namibia and South Africa. Carbonate rock powders were dissolved using 1 N acetic acid. Residue fractions were separated from the supernatant (carbonate fractions) and analysed separately. Marbles from the lower Pickelhaube Formation, Gariep Belt, yielded distinctly different 207Pb/ 206Pb isochron ages of 728±32 (MSWD=1.5) and 545±13 Ma (MSWD=7.4) for the carbonate and residue fractions, respectively. The former is interpreted as dating early diagenesis, whereas the latter is in excellent agreement with the timing of regional metamorphism. In contrast, both the carbonate and residue fractions from the Bloeddrif Member, Holgat Formation, Gariep Belt, and from the Kombuis Member, Matjies River Formation, Saldania Belt, yielded indistinguishable 207Pb/ 206Pb isochron ages. The isotope data from both fractions combined define an isochron age for the Bloeddrif Member marbles of 555±28 (MSWD=22.8). A whole rock isochron age of 553±30 Ma (MSWD=14.3) is given by the weighed sums of separately analysed carbonate and residue fractions data of the Kombuis Member. Both ages are interpreted to reflect early diagenesis. Our novel approach to double-spike 207Pb/ 206Pb dating of carbonate rocks highlights not only the importance of dating carbonate and residue fractions separately, but also led to a major revision of stratigraphic correlation between the Gariep and Saldania Belts.

Selenium isotope ratios as indicators of selenium sources and oxyanion reduction

Selenium stable isotope ratio measurements should serve as indicators of sources and biogeochemical transformations of Se. We report measurements of Se isotope fractionation during selenate reduction, selenite sorption, oxidation of reduced Se in soils, and Se volatilization by algae and soil samples. These results, combined with previous work with Se isotopes, indicate that reduction of soluble oxyanions is the dominant cause of Se isotope fractionation. Accordingly, Se isotope ratios should be useful as indicators of oxyanion reduction, which can transform mobile species to forms that are less mobile and less bioavailable. Additional investigations of Se isotope fractionation are needed to confirm this preliminary assessment. We have developed a new method for measurement of natural Se isotope ratio variation which requires less than 500 ng Se per analysis and yields ±0.2‰ precision on 80Se/ 76Se. A double isotope spike technique corrects for isotopic fractionation during sample preparation and mass spectrometry. The small minimum sample size is important, as Se concentrations are often below 1 ppm in solids and 1 μg/L in fluids. The Se purification process is rapid and compatible with various sample matrices, including acidic rock or sediment digests.

Optimal double and triple spiking for high precision lead isotopic measurement

Conventional lead isotope ratio measurements by thermal ionization mass spectrometry (TIMS) are currently limited to external precisions of ∼0.1% (2 σ) due to variations in instrumental mass discrimination, the magnitude of which can only be assessed using external standard or duplicate measurements. The goal of the double-spike technique, by contrast, is to circumvent this fractionation problem entirely by determining the mass discrimination factor ϵ directly for each sample. The unknown factor can be obtained after performing a second measurement on a sample aliquot to which a double spike of known, calibrated composition has been added. Practical applications of the double-spike technique up until now have used a mixed 207 Pb– 204 Pb tracer with considerable success, but there appears to have been little or no attempt to optimize the composition of the tracer used. An optimal tracer composition must fulfil two criteria: (1) the ratio of the internal, measured errors to a posteriori errors—the error magnification factors—should be as small as possible; (2) the propagated errors should not be strongly influenced by the spike/sample ratio of the second, mixture run—the latter is important since sample size cannot, in general, be rigorously controlled in advance. Of equal importance to the tracer composition itself is the choice of the three mass-balance equation set used to solve for ϵ; geometrically, this is equivalent to defining a mixing line in a particular 3D isotope space. Potential spike compositions were examined in four such 3D isotope spaces, corresponding to cases where 208 Pb, 207 Pb, 206 Pb and 204 Pb in turn are common, denominator isotopes of the ratios on the three orthogonal axes. Within each isotope space there exist three possible double-spike families. Tracers composed of three stable isotopes of lead—so-called `triple spikes'—are also considered, of which there are three families per isotope space. These spike compositions were evaluated for a typical common lead composition, at first using geometric methods, and then refined by determining error magnification factors using an ion statistical error model. It is shown that the 207 Pb– 204 Pb double spike used in previous studies, where ϵ is determined in 208,207,206 Pb/ 204 Pb isotope space, is one of the least suitable to use. Optimal tracer compositions for determining ϵ appear to be restricted to two families: The first is a family of 207 Pb– 204 Pb– 206 Pb triple spikes with 207 Pb/ 204 Pb of around unity and the isotope space has 206 Pb as the denominator isotope; the second family consists of 204 Pb– 207 Pb double spikes, solved in 208,206,204 Pb/ 207 Pb isotope space. Both of these families yield error magnification factors on a posteriori, corrected 206 Pb/ 204 Pb ratios as low as ∼1.2, and are less than 2 for all but highly underspiked and overspiked mixture compositions.

Evaluation of a sequential extraction method for determining actinide fractionation in soils and sediments

A sequential chemical extraction method for the determination of the geochemical fractionation of Am, Pu, and U was evaluated rigorously on a single marine sediment standard ( IAEA-135). Partitioning of actinides was operationally defined for five reagent fractions and stable element analyses (Al, Ca, Fe, Mn, Ti, etc.) were performed to assist in establishing the phase specificity of the extractions. The method produces, in general, results that agree to within a one standard deviation interval for replicate measurements for each fraction. Actinide readsorption, examined by a double-spiking technique, was found to be significant in some cases. Ethylenediamine-tetraacetic acid (EDTA) was examined as a hold-back reagent to lessen the effect of readsorption but seriously compromised the phase selectivity of the extractants. In addition, the use of NaOCl for the destruction of organic matter was found to dissolve carbonates out of sequence and is not recommended for carbonate-bearing materials.

Practical Application of Lead Triple Spiking for Correction of Instrumental Mass Discrimination

The external precision of Pb isotope ratio measurements by TIMS is currently limited to ~0.1% due to variations in instrumental mass discrimination. It was shown by Dodson (1963) that the unknown mass discrimination factor could be obtained from a second measurement on an aliquot of a sample to which a double spike has been added. From the three isotope ratios measured on the two runs it is straightforward to solve for the three unknowns the respective fractionation factors of the two runs and the spike/sample ratio. Practical applications of the Pb double spike technique have used a mixed 2~176 tracer in the ratio ~9:1 (e.g. Hamelin et al. , 1985; Woodhead et al., 1995). These studies have demonstrated that a factor of up to -3 improvement in external precision can be obtained compared with good conventional, uncorrected measurements. Further, any systematic errors arising from matrix effects are effectively eliminated using the technique (Woodhead et al., 1995). Slightly more favourable results have been obtained by using a double spike composed of the non-natural isotopes 2~176 (Todt et al., 1996), which has the added advantage that only one mass spectrometric run is required; a disadvantage, ignoring the cost of these spikes, though, is that isobaric interferences on these masses must be continually monitored and corrected for during measurement. A systematic survey of stable isotope tracer compositions, axis combinations and mathematical solutions to this problem was recently undertaken by Galer (1997). From this study, there appears to be an optimal composition to use as a spike to correct samples of common Pb for instrumental mass fractionation. This spike composition (designated here as TS) is unusual in that it is a mixture of three stable isotopes of lead 2~176176 rather than just two, 2~176 used in previous studies. The error magnification factor 7 on the 2~176 ratio (i.e. the bias-corrected error relative to the measured, unspiked internal error) was estimated by Galer (1997) using a realistic error propagation model based upon the ion statistical uncertainties on the two runs and the TS composition. For the optimized TS composition, 7 lies at ~1.2 over a large range in spike/sample ratios, becoming larger than 2.0 only when the spike/sample ratio is less than 0.05 or greater than 3.8. This implies that when preparing the TS-sample mixture, the quantity of Pb in the sample need only be known to within a factor of ~40 either way to avoid an unfavourable 7 > 2. This behaviour is in sharp contrast to that of the 2~176 double spikes that have been used previously, which never have 7 < 2 for any spike/ sample ratio. Such double spikes as these, even in the most favourable cases, require internal precisions on the individual runs a factor of ~2 better to achieve the same final precision as the TS in effect, measurement times (or sample sizes) would need to be a factor of ~4 larger. In order to see how such an optimal triple spike would perform in practise, a tracer of TS composition was prepared and subjected to a number of tests to evaluate its performance. All Pb isotopic measurements were performed in static multi-collection mode on a Finnigan MAT-261. Inter-cup efficiency biases were determined by simplex minimization from measurements of the NBS-982 equal-atom Pb standard. The TS itself was calibrated by joint runs of the triple spike alone and mixtures in varying proportions with the NBS-982 standard the mass bias-corrected TS isotopic composition can then be obtained by solving the equations with the TS as the 'unknown' (cf. Hamelin et al. , 1985). Thus, any isotopic compositions determined using the TS are traceable to the NBS-982 Pb isotopic composition of Todt et al. (1996) which, in turn, was normalized to a 2~176 ratio of 1.00016. The first test of the triple spike was measurement of the NBS-981 common lead standard, whose isotopic composition has recently been redetermined by Todt et al. (1996). Samples of NBS-981 were mixed with TS in a large range of spike/sample ratios and combined with unspiked runs to yield fractionation-corrected ratios for NBS-981. To improve in-run

Application of the `double spike' technique to Pb-isotope geochronology

New Pb-isotope data are presented for three suites of carbonate-bearing sedimentary rocks from the Archaean and Proterozoic of Western Australia. The measured data are corrected for instrumental mass fractionation using both conventional and double-spike techniques, with the results contrasted on isochron diagrams. Compared with the data corrected using conventional methods, the double-spike-corrected data show a significant improvement in all cases: the isochrons produced are more tightly constrained, resulting in lower uncertainties in the age estimates obtained and marked improvements in MSWD values. While highlighting the enormous potential for the direct dating of carbonate sequences throughout the geological record, this study demonstrates the improvements possible using double-spike correction for mass fractionation—an issue pertinent to all aspects of Pb-isotope geochronology.

Biological control of calcium isotopic abundances in the global calcium cycle

Measurements of 44Ca/ 40Ca, expressed as δ 44Ca, were made on igneous rocks and on shell and bone material from modern organisms to investigate the magnitude and origins of calcium isotopic fractionation in nature. The results document a span of 4‰ in δ 44Ca, measured with the double spike technique to a precision of ±0.15‰. Volcanic rocks, including basalt and rhyolite, show little variability and cluster near δ 44Ca = 0±0.2. Systematic analysis of biological samples indicates that biological processing of calcium discriminates against heavy isotopes, and that biological fractionation is the primary generator of calcium isotopic fractionation in nature. Preliminary data suggest that calcium becomes isotopically lighter as it moves through food chains. Calcium carbonate shells of marine microorganisms and deep-sea carbonate ooze have δ 44Ca about 1.0ℵ. lower than seawater; this fractionation causes seawater to be enriched in heavy calcium (δ 44Ca = +0.9) relative to igneous rocks. Marine organisms consequently are isotopically heavier than their terrestrial counterparts at similar trophic level. The calcium isotopic composition of living and fossil organisms may record information on diet and environment.

The isotopic composition of germanium in terrestrial samples

The isotopic composition of germanium was measured in a range of terrestrial materials using solid-source mass spectrometry. An ion-enhancing gel loading technique was developed to enable measurable ion beams to be produced. Within the limits of experimental error, no variations in isotopic abundances were found in any of the reagents or minerals analysed. The extent of mass discrimination was determined using the double spike technique. Good agreement is obtained between the measured isotope abundances and other published data apart from linear mass fractionation effects.

Precise lead isotope measurements by the double spike technique: A reconsideration

A reappraisal of the “Double Spike Method” for correction of the mass discrimination in lead isotope analysis is presented in view of the possible efficiency of this method, especially as related to the improved performance of the new multi-collector mass spectrometers. First, the effects of mass discrimination during lead isotope analysis, using the silica-gel phosphoric acid method, are discussed in order to verify the validity of the linear discrimination law. In this discussion, we give evidence of a slight bias of the isotopic composition of the SRM 981 NBS lead standard, commonly used for mass discrimination calibrations. A rigorous way to deal with the propagation of the within-run statistical errors on the precision of the double spike correction is then presented. This allows the optimization of the parameters of the method (spike isotopic composition and precision, spiking proportion) and the calculation of the respective error associated with each corrected isotopic ratio. The perturbation of the double spike correction due to analytical contamination, which has caused previous failure in the application of the method, is avoided by preparation of the mixture between the spike and the sample at the very end of the chemical separation procedure. Using a single collector mass spectrometer, examples of application on lead standards and on a set of samples suggest a possible improvement in the precision by a factor of 3 compared to the usual method of fractionation normalization and confirm the usefulness of the method with the new generation of mass spectrometers.

Absolute isotope abundances of tin

The so-called double-spike technique has been combined with isotope ratio measurements by thermal-ionization mass spectrometry to determine the absolute isotope abundances of natural tin. The experimental conditions for the mass-spectrometric analysis are given, and the procedures for preparation of the double-spike solution and of mixtures of this solution with natural tin solutions described. From this work it is possible to propose a new value for the atomic weight of natural tin: 118.710±0.007.

Evaluation of lutetium for volumetric calibration by isotope dilution mass spectrometry

Through the choice of a suitable element, the double spike technique of isotope dilution offers a viable method for determining the quantities of fissile material in holding tanks. Lutetium was found to meet all the requirements for this determination, and a procedure for volumetric calibrations with lutetium is described. Results are presented for the expected and actual gravimetrically determined values for the concentration and weight for various samples indicating a precision of approx. 0.1%.

Sr isotopic fractionation in Allende chondrules: A reflection of solar nebular processes

True relative Sr isotopic compositions, determined by the double-spike technique, are reported for 8 olivine chondrules from Allende and a single chondrule from Richardton. The Richardton chondrule has an Sr composition identical with the whole meteorite, but the Allende chondrules are up to 1.4‰ per mass unit light-isotope enriched, closely similar to Ca-Al inclusions (CAI) from the same individual stone. The correspondence of the patterns for chondrules and CAI suggests that both groups of objects derived their fractionated Sr in similar ways. The lack of any detectable non-linear Sr isotopic anomaly in the objects suggests that their Sr compositions did not have some exotic or extrasolar origin, but were derived from normal solar system Sr by mass fractionation. The consistent light-Sr enrichment of Allende objects may be explained by several schemes, and all are heavily model-dependent. Most plausible to the author is that the CAI and chondrules derived their fractionated Sr from a region of the nebula made isotopically light by partial kinetic mass separation of elements in the vapour phase. Later, the solid objects may have moved to an isotopically more normal region, where the Allende matrix accreted.

Ca isotope fractionation on the Earth and other solar system materials

It is demonstrated that differences in the 40Ca 44Ca ratio due to mass dependent isotope fractionation in nature are clearly resolvable to a level of 0.5%. This precision is obtained 1. (a) by using the double spike technique; 2. (b) by using a mass-dependent law for correction of instrumental mass fractionation; and 3. (c) by eliminating fractionation effects identified as due to differential elution of isotopes through ion exchange resins. We have determined the following uniform Ca isotopic composition after removing small natural fractionation effects: 40Ca 44Ca = 47.153 ± 3 , 42Ca 44Ca = 0.31221 ± 2 , 43Ca 44Ca = 0.06486 ± 1 , 46Ca 44Ca = 0.00152 ± 1 , 48Ca 44Ca = 0.08871 ± 2 , where the errors correspond to the last figures shown. This yields an atomic weight of 40.076 ± 0.001. The data indicate the absence in the studied samples of detectable, distinct nuclear components in Ca similar to those observed for oxygen. In the samples studied, there is a distinct but small degree of Ca isotope fractionation. Overlapping ranges of fractionation of 2.5%. for 40Ca 44Ca (four atomic mass units) are observed in meteorites, lunar, and terrestrial samples. Means by which isotope fractionation could arise for Ca are discussed, but the small range of effects and the lack of systematic variations do not permit at present the identification of the mechanisms responsible for the fractionation observed in the suite of samples. Ca in the biological cycle does not show fractionation effects larger than observed for non-biogenic samples. In contrast to these results, we have observed large effects of up to 13%. for industrially purified Ca.

The isotopic composition of cadmium in terrestrial minerals

The isotopic composition of cadmium in eight terrestrail minerals, and a Laboratory Standard has been measured by solid source mass spectrometry. Within experimental errors, no natural variations in isotopic abundances were found. The extent of mass discrimination was determined by using the double spike technique. The absolute isotopic abundances of cadmium as determined in this work for 106Cd, 108Cd, 110Cd, 111Cd, 112Cd, 113Cd, 114Cd and 116Cd are 1.25%, 0.894%, 12.51%, 12.81%, 24.13%, 12.22%, 28.71% and 7.47% respectively. Because of the higher precision, these abundances should replace the currently accepted values. We would also recommend that a value of 112.410± 0.005 should be used for the atomic weight of cadmium, rather than the currently accepted figure of 112.40.

Mass discrimination in the measurement of lead isotope reference samples.

This paper presents a study of the adjustment of published values for lead isotope reference materials by the double spike technique. The objective was to learn, through hindsight, about the characteristics of common analytical errors in lead isotope analyses. The conclusion is that the great majority of such errors are substantially reduced by the double spike technique. The results emphasize the precision with which the isotope ratios of the Broken Hill and California Institute of Technology reference materials are known.

Propagation of experimental errors in lead isotope ratio measurements using the double spike method

Computer simulation of random errors in the measurements for the double spike technique applied to Pb isotope analyses, indicates that normalized error multiplication factors for the calculated values of isotope ratios lie in the range 2 to 3. The optimum isotopic composition of the tracer is about 207 Pb 204 Pb = 2.0 and the optimum ratio of amount of tracer to amount of unknown is about P = 1.0. Determination of errors in the fractionation factor alone does not adequately describe the ultimate errors in the calculated isotope ratios except under special circumstances. Uncertainty in the actual composition of the tracer is closely proportional to uncertainty in the unknown composition. The purity of the spike is unimportant at least as long as the separated 207Pb and 204Pb isotopes are greater than 90% pure.

The isotopic composition of lead in iron meteorites

Lead isotopic composition and lead concentration were measured in four troilite samples from Canyon Diablo, two troilites from Toluca, and single troilites from Bogou, Baquedano, Gibeon, Grant and Staunton. Uranium concentrations were measured in four samples; they ranged from 2.6 to 25 ppb. Observed isotopic compositions were corrected for fractionation by the double spike technique. The lead compositions ranged from primitive compositions to modern radiogenic compositions. It is concluded that the radiogenic lead found in iron meteorites is due to terrestrial contamination. New values recommended for the primordial lead ratios are Pb 206 Pb 204 = 9.346 , Pb 207 Pb 204 = 10.218 , and Pb 208 Pb 204 = 28.96 .